Biological Foundations of Mental Health PDF

Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 2 From the dynamic synapse to synaptopathies - Part 1 of 4 Dr Deepak Srivastava Senior Lecturer, Basic and Clinical Neuroscience, King’s College London Slide 3: Hello, and welcome to this...

Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 2 From the dynamic synapse to synaptopathies - Part 1 of 4 Dr Deepak Srivastava Senior Lecturer, Basic and Clinical Neuroscience, King’s College London Slide 3: Hello, and welcome to this lecture entitled, ‘From the dynamics synapse to synaptopathies’. My name is Deepak Srivastava and I am the head of the neuronal circuitry and neurodevelopmental disorders research group here at the Institute of Psychiatry, Psychology and Neuroscience, King's College London. In this lecture, we will focus on understanding the function of synapses in the healthy brain. In particular, we will focus on tiny dendritic protrusions that decorate dendrites, which are known as 'dendritic spines'. Dendritic spines are the site for the majority of excitatory synapses in the mammalian brain. We'll explore the basic function of these structures, as well as the overall structure and what they contain. We will go on to examine how dendritic spines make synaptic connections and how these synaptic connections can be fine-tuned by a number of physiological stimuli. Finally, we'll explore the evidence that indicates that abnormal dendritic spine function is connected with mental illnesses and how studying genetic risk factors associated with mental illnesses can tell us how dendritic spine dysfunction may contribute to the emergence of disease. Slide 4: In this part of the lecture, we'll explore the basic function of synapses, the structure and content of dendritic spines, and talk about two processes known as 'spinogenesis' and 'synaptogenesis'. Slide 5: Synapses are the site where synaptic communication occurs by the transfer of chemical messages between cells within the central nervous system. The importance of correct synaptic communication lies in the knowledge of the important functions that synaptic communication is responsible for. This includes cognitive function, including executive and more complex functions such as social behaviours, personality, learning and memory, motor behaviours amongst others. Synaptic communication can occur between sensory organs and neurons, between neurons and neurons, as well as from neurons to target organs. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 1. Typically, the flow of information occurs only in one direction – from the pre-synaptic neuron to the postsynaptic neuron. And, finally, there is increasing evidence that disruption of synapse number, and/or function is strongly linked with brain dysfunction. In the diagram here, we can see a cartoon of a post-synaptic neuron in grey. You can see its cell body – or 'soma' – and its dendrites, that emerge out of it. The dendrites are where this neuron will receive information and, thus, dictates the receptive field of the post-synaptic neuron. That is, the size of the dendritic arbor is critical in determining how many pre-synaptic cells it can connect with. We can also see several bundles of blue axons, which are part of the pre-synaptic neuron – the arrow shows the direction of information flow. Basically, information flows along the axons until they reach the synapse. The information is then transferred across the synapse to the post-synaptic neuron. This neuron then collates the information and then decides whether or not to send this information to the next neuron through the generation of an action potential that is sent along its axon. Slide 6: Within the mammalian brain, synapses can be classified in three different ways. You have axodendritic or axospino synapses. This is where the axon of the pre-synaptic neuron synapses with the post-synaptic cell along its dendrite or on dendritic protrusions known as dendritic spines. These synapses account for the vast majority of synapses in the brain and can be excitatory, inhibitory or neuromodulatory. You also have axosomatic synapses. These are synapses that occur on the cell body, or 'soma', of the postsynaptic cell. These are typically inhibitory or neuromodulatory. Finally, you have axoaxonic synopses. This is where the pre-synaptic axon synapses directly on the axon of a post-synaptic cell and, thereby, controls the amount of information flow along the axon of the post-synaptic neuron. For the remainder of this lecture, we will focus on axodendritic or axospino synapses. Slide 7: As previously mentioned, a lot of synapses occur on highly specialised dendritic protrusions, known as 'dendritic spines'. Here, on the right, we have an example of a pyramidal neuron located in layer 5 of the mouse frontal cortex. You can see that it has a very typified morphology. There is a cell body, or soma, at the bottom and, then, projecting to the top of the cortex – or the pia of the cortex – you can see a primary dendrite. It is this typified structure that is quintessential of pyramidal neurons that are found within the cortex. If we now zoom into the dendrite of one of these neurons, we can see that it is decorated by these funny little protrusions that come off the dendrites. These protrusions are known as dendritic spines. And what we do know is that dendritic spines form the post-synaptic compartment of synapses and that they are the site where the majority of excitatory synapses occur within the mammalian forebrain. In this cartoon of an excitatory synapse, we can see in the pre-synaptic terminal where the synaptic vesicles containing neurotransmitters reside. Once an action potential arrives at the pre-synaptic terminal, the synaptic vesicles move to the synaptic membrane, fuse with the membrane and release their neurotransmitter into the synaptic cleft. On the other side of the synaptic cleft, we have a dendritic spine, which is typified by its spine neck and spine head. Within the spine head, you have the post-synaptic density – or PSD for short – which contains a large number of proteins, including the neurotransmitter receptors. It is these receptors that receive the information Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 2. from the pre-synaptic neuron, in the form of neurotransmitters, and then translates these signals into a response in the post-synaptic cell. Slide 8: One question you may have is, 'why have dendritic spines?' Firstly, as these structures are where the majority of excitatory synapses occur, they increase the surface area and thus the potential number of synaptic connections a post-synaptic neuron can make. Secondly, it is emerging that dendritic spines can compartmentalise, both electrical and biochemical signals from the rest of the cell. What this means is that dendritic spines can filter – or even amplify – signals, both biochemical as well as electrical, before allowing them to pass into the rest of the cell and, thus, influence the output of the neuron. In order to do this, dendritic spines have developed their specialised shapes, but they also contain a vast number of proteins. These include receptors – such as glutamate receptors; adhesion proteins – that physically connect pre- and post-synapses together; scaffold proteins – such as PSD95, that organises the PSD and proteins within dendritic spines. A major component of dendritic spines is F-actin – it is the rearrangement of Factin that allows dendritic spines to change shape. We'll explore this concept in more detail later. It should also be noted that dendritic spines have a number of organelles within them, such as the endoplasmic reticulum and polyribosomes – these are required for the production of new proteins. They also contain mitochondria, which provide the fuel needed for many processes. Slide 9: Over recent years, we have really begun to develop an appreciation of the important role that dendritic spines play in normal brain function. For example, during early brain development, dendritic spines can be seen to emerge out of dendrites and to search at the surrounding neuropil for an appropriate pre-synaptic partner. Once it finds the appropriate pre-synaptic partner, it can make a synaptic connection. It is thought to be one of the ways that neural circuits or neural networks can be formed, and it is the basis by which wiring within the brain occurs. Interestingly, a number of signals, including synaptic activity as well as neuromodulating signals, can also cause dendritic spines to change shape and size as well as to increase or decrease in number. In this cartoon, we can see that synaptic activity – such as long-term potentiation – seen here in red, causes the existing dendritic spine to increase in its size, but also causes a new dendritic spine to emerge. This spine has the potential to form a synapse. And, overall, this has led us to the emerging theme that synaptic connectivity within neural circuits – or neural networks – can be remodelled and, thus, that wiring within the brain can be refined. Importantly, the changes in synaptic connectivity can occur in a bi-directional manner. Slide 10 Here, again, we just have a cartoon of a neural circuit or neural network. And, simply put, a physiological stimulus – such as synaptic activity – can cause either a change in the number or the shape of dendritic spines. This can either lead to an increase or decrease in either the number or the strength of synaptic connections. Moreover, it is these changes and synaptic connectivity – driven in part by changes in dendritic spine, shape or number– that are thought to be essential for normal brain function. Slide 11: So, how do neurons make synapses? There have been several different models by which synapses can be formed. The prevailing model that is used is dependent on the time of development as well as the region of the Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 3. brain where this process is occurring. I would like to focus on one model of synapse formation that is thought to be the prevalent mechanism that occurs during development and within the adult forebrain. This model, known as the Filopodial model, can be easily broken down into two events: 'spinogenesis' and 'synaptogenesis'. In this model, the axon and dendrites of the pre- and post-synaptic neurons have already been established. At this point, you can see a pre-synaptic terminal on the axon. In this model, the dendrite creates a dendritic protrusion known as a filopodia. This is a very long protrusion that is very dynamic – that is, it moves around the surrounding neuropil very quickly and can appear and disappear very quickly. Filipodia do not have a discernible head structure and do not contain the proteins necessary to create a synaptic connection. For example, these protrusions do not have a post-synaptic density – or 'PSD' – and they do not contain neurotransmitter receptors. The Filipodia then searches the surrounding neuropil looking for an appropriate pre-synaptic partner – which is known as 'target selection'. Slide 12: Once a pre- and post-synaptic cell have identified each other as partners, the next stage is known as 'synaptogenesis'. The initial step of this is known as 'synapse assembly'. Here, several key synaptic proteins are recruited to the nascent dendritic protrusion. These proteins include NMDA receptors, the scaffold protein, 'PSD95', and a number of adhesion proteins. One of the main roles of the adhesion proteins is to physically connect the pre- and post-synaptic side of the synapse together. It is the recruitment of these synaptic proteins that signals the change of the filipodia into a dendritic spine. At this stage, the nascent dendritic spine has a defined head with a PSD and contains the key elements, like NMDA receptors, that would allow synaptic communication to occur. However, these connections are weak and considered to be unstable. The next step is known as synapse stabilisation. This is where synaptic activity induces the recruitment of more adhesion molecules to further stabilise the dendritic spines, as well as NMDA receptors and other synaptic proteins to establish these pre- and post-synaptic structures as fully functional synaptic connections. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 4. Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 2 From the dynamic synapse to synaptopathies - Part 2 of 4 Dr Deepak Srivastava Senior Lecturer, Basic and Clinical Neuroscience, King’s College London Slide 3: In this part of the lecture, we will now focus on the function of dendritic spines and discuss how the structure of dendritic spines is thought to be linked with the functional properties of synapses. Slide 4: Dendritic spines come in a myriad of shapes and sizes. The shape and size of a dendritic spine can tell you a lot about its function. Here, we have a serial electron microscopy reconstruction of a small part of dendrite from a hippocampal neuron. What you will hopefully be able to see is that dendritic spines can be large and small in size, and even long and short. So, what is the consequence of having dendritic spines with different morphologies? Well, in this image, we can also see the size of the synaptic connection, which is shown in red. This nicely shows you that larger dendritic spines typically have larger synaptic connections, whereas smaller or thinner spines have much smaller synaptic connections. Slide 5: Dendritic spine shape is also intimately linked with its function. For example, in this study, the authors have labelled the dendrite of a neuron with green fluorescent protein, or GFP, to show the shape of the cell. They have also immunostained the cell for GluA1-containing AMPA receptors, as a proxy for measuring synaptic strength. More GluA1-containing AMPA receptors would indicate the stronger synapses. Hopefully, what you can see is that larger spines – shown here by these red arrows – contain a lot of GluA1containing AMPA receptors. Whilst, on the other hand, much smaller or thinner spines – shown here by the yellow arrows – have much smaller amounts of GluA1-containing AMPA receptors. What this tells us is that larger dendritic spines not only contain more GluA1-containing AMPA receptors but that they are more likely to have bigger responses to a glutamate or synaptic activity whereas thinner or smaller dendritic spines, that have less AMPA receptors, are likely to have smaller responses to glutamate or synaptic activity. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 1. This was very nicely shown by a study carried out by Matsuzaki and colleagues in 2001. Here, the authors performed a very elegant study to show that larger dendritic spines have much stronger responses to glutamate whereas smaller dendritic spines have a smaller response to glutamate. In this study, the authors have recorded excitatory post-synaptic currents in hippocampal neurons. The hippocampal neurons are bathed in cage glutamate – that is, glutamate that is inactive unless you shine a particular wavelength of laser on it, allowing it to become active. The authors were able to uncage glutamate at very specific sites, such as directly over a dendritic spine – this means that you can activate only the AMPA receptors within a specific dendritic spine. Thus, the authors uncaged glutamate over dendritic spines with different sizes and, at the same time, recorded the excitatory postsynaptic current that it resulted. In the left image, we can see four spines labelled 'A', 'B', 'C' and 'D' – all of which have different sizes. In the middle and right panels, we can see that the measured, post-synaptic current induced by the uncaging – in these images, yellow and red colours – indicate a larger response whereas darker and blue colours indicate a smaller post-synaptic current. What the authors found was that if they uncaged glutamate over the spine labelled 'A', that the current was much larger than if they uncaged glutamate over spines 'C' and 'D'. Taken together, these studies demonstrate that larger dendritic spines typically contain more AMPA receptors and generate larger excitatory post-synaptic currents as compared to dendritic spines with smaller size. Thus, indicating that dendritic spine structure is linked to synaptic function. Slide 6: So, do dendritic spines change shape in response to different stimuli? In short, yes, they can. As we discussed earlier, physiological stimuli – such as changes in synaptic activity – can change the number and strength of synaptic connections. This also leads to a change in dendritic spine size. For example, if we were to induce a long-term potentiation- or LTP-like stimulus, we can see that dendritic spines can actually increase in size. Conversely, if we were to induce a long-term depression- or LTD-like stimulus, we can see that dendritic spines actually shrink in size. The ability of dendritic spines to change size in response to stimulation is known as 'structural plasticity' and is thought that this process plays an essential role in the encoding of information. Slide 7: If structural plasticity does play a central role in the encoding of information – based on our understanding that larger dendritic spines have more AMPA receptors and, thus, make stronger synaptic connections – one would expect that following a LTP-like stimulus that not only would dendritic spines change in size but also the amount of AMPA receptors would also increase. In 2006, Kopec and colleagues tested this idea. What they did was to monitor the amount of GluA1-containing AMPA receptors in dendritic spines, before and after the induction of LTP. Here, the authors induced LTP using a chemical approach and, thus, have labelled this 'chemically-induced long-term potentiation' or 'cLTP'. What the authors did was to monitor both the size of the dendritic spines, by measuring spine volume, as well as the amount of AMPA receptors within dendritic spines. This was done by making hippocampal neurons express a red fluorescent protein, to outline the morphology of the cell, and to express GluA1-containing AMPA receptors that would link to a special form of GFP, that only fluoresces when the receptor is expressed at the surface of synapses. What this means was that the authors could easily monitor the size of dendritic spines whilst simultaneously measuring the amount of synaptic and, thus, active AMPA receptors within dendritic spines. The authors then monitored both the size of dendritic spines as well as the amount of AMPA receptors in dendritic spines 30 minutes before and up to 80 minutes after the induction of this chemical LTP. What the authors found was that, as expected, the induction of chemical LTP caused dendritic spines to increase in size. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 2. This can be seen in the left-hand image. In addition, the authors found that the amount of GluA1 in dendritic spines also increased following the induction of chemical LTP. This can be seen in the middle panel where an increase in the amount of GluA1 within dendritic spines is shown by an increase in the amount of yellow and red colours. These data are summarised in the graph on the right and it shows that as spine size increases – shown by the red line – the amount of AMPA receptors also increases. Ultimately, what this tells us is that as dendritic spines change size in response to stimulation, the amount of AMPA receptor also changes. Thus, demonstrating that structural and functional plasticity are linked. Slide 8: So, to summarise, what we have shown here is that physiological stimuli – such as long-term potentiation (LTP) or long-term depression (LTD) – can not only change the number of dendritic spines but can also result in a change in the size of dendritic spines. This results in a concurrent change in the amount of AMPA receptors within these dendritic spines, which underlies the changes in synaptic strength that is observed. Thus, structural and functional plasticity are coordinated and can be changed, resulting in refinement of neuronal circuitry. This process is, therefore, thought to be essential for normal brain function. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 3. Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 2 From the dynamic synapse to synaptopathies - Part 3 of 4 Dr Deepak Srivastava Senior Lecturer, Basic and Clinical Neuroscience, King’s College London Slide 3: In this section, we will investigate the evidence indicating that abnormal dendritic spine function may be linked with a range of mental illnesses. Slide 4: In the last two sections, we have focused on the role of dendritic spines in the healthy brain. Now, I would like to explore the potential contribution of dendritic spine dysfunction in disease. There is now increasing evidence that dendrites and dendritic spine morphology may be affected in a range of brain disorders. For example, here we have a cartoon of the dendritic arbour as seen in normal neurons and those seen in disorders such as autism spectrum disorders (ASD) or schizophrenia. As you can see that in a range of different disorders, the overall dendritic arbour seems to be simplified as compared to that seen in a normal neuron or healthy neuron. Similarly, in studies where researchers have examined the post-mortem brains of individuals with different mental health issues, such as autism or schizophrenia, we can see that there is an abnormal number of dendritic spines compared to healthy or controlled individuals. For example, in patients with autism, you can see that there seems to be an increase in the number of dendritic spines, compared to healthy or controlled patients. Conversely, if we look at the number of dendritic spines of neurons found in the brains of patients with schizophrenia, you can see that there seems to be a reduction in the number of dendritic spines as compared to healthy individuals. Slide 5: This has led to the overall idea that aberrant dendritic architecture or abnormal dendritic spine density could result in altered neuronal network or circuitry and, thus, wiring – which could, ultimately, result in cognitive deficits that are seen in brain disorders, such autism spectrum disorders and schizophrenia. However, a major issue with relying in post mortem studies, to help identify underlying causes of disease, is that we do not know whether the observed deficits, such as impaired dendritic arbours or altered dendritic spine numbers, are a Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 1. cause of the disease or have been caused by the disease progression. Indeed, the post-mortem tissues have been taken from individuals at the end of their life and after they have likely suffered from the disease for a long time. As such, a number of factors – such as chronic exposure to drugs – may have influenced the observed phenotypes. Slide 6: So, what evidence is there that dysfunction of dendritic spines may contribute to disease? Well, if we look at typical neurodevelopment, we can see that dendritic growth and dendritic spine morphogenesis and, thus, synapse formation occurs early on in life. If we examine when specific disease symptoms occur, we can see that they coincide with critical periods of synapse formation. For example, symptoms associated with autism spectrum disorders emerge during early childhood, a period when there is increased spine and synapse formation. Current research indicates that an increase in the number of dendritic spines, occurring early on in this disorder, may contribute to the symptoms. Symptoms associated with schizophrenia, on the other hand, typically emerge around adolescence or early adulthood. This also coincides with a period when there is a refinement of synaptic connections. This is typified by pruning of synaptic connections. One theory is that an increase in synapse elimination during this period may contribute to the emergence of schizophrenic symptoms. Slide 7: However, perhaps the most compelling evidence that dendritic spine dysfunction may play an important role in the emergence of disease, lies in recent large-scale studies investigating the underlying genetic causes for neurodevelopmental and psychiatric disorders. These studies have identified a large number of de novo protein coding mutations. That is, genetic variance that would cause a change in the sequence of specific proteins that are associated with risk of developing diseases – these include disorders such as intellectual disability, epilepsy and autism spectrum disorders, all of which have an early onset, as well as disorders like schizophrenia and bipolar disorder, which have a late onset. Interestingly, if we compare these de novo protein coding variants with the proteome of human post-synaptic density – or the 'PST' – as a proxy for what proteins are present at synapses, we find that there is a large overlap. What this strongly indicates is that many of the de novo protein coding mutations associated with various neurodevelopmental and psychiatric disorders occur in proteins that are found at synapses. This strongly supports the idea that dysfunction at synapses and, in turn, dendritic spines play an important role in the emergence of disease. Slide 8: Indeed, if we examine which genes have been implicated with disease in more detail, we can start to see that many of these genes, not only encode for proteins that localise to dendritic spines, but also have critical roles in dendritic spine formation, maintenance and remodelling. In this image, we can see several classes of synaptic proteins, all of which have been implicated with disease. These include adhesion proteins, scaffold proteins and glutamate receptors, all of which we have discussed earlier in this lecture as having critical roles in the basic function of dendritic spines. In addition to this, we also find a number of signalling molecules as well as voltage-gated calcium channels as being implicated. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 2. These examples help to build a picture indicating that alterations in the function of some or many of these proteins, could easily result in dysfunction of dendritic spines and, thus, impact synaptic communication and connectivity. Right now, there is a lot of work that is going on, trying to understand how these aberrant structures occur and, moreover, if it is possible to reverse or stop these deficits from occurring. And it is with this that we hope we will be able to treat different mental illnesses. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 3. Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 2 From the dynamic synapse to synaptopathies - Part 4 of 4 Dr Deepak Srivastava Senior Lecturer, Basic and Clinical Neuroscience, King’s College London Slide 3: In the final part of this lecture, I would like to explore synaptic deficits in schizophrenia and one way we can study genetic mutations and link them with abnormal dendritic spine function. Slide 4: I would like to now focus on how we believe dysfunction of dendritic spines may contribute to mental illnesses. In order to explore this question, I want to focus on schizophrenia. This mental illness is a highly complex disorder. Indeed, schizophrenia is a chronic disease that significantly impacts the psychological and the social and cognitive functioning. It affects approximately 1% of the population. At the clinical level, it is described as having positive symptoms – such as hallucinations and delusion, negative symptoms – blunted affect, avolition, asociability – as well as thought disorders. Working memory deficits or other cognitive deficits seem to be incorporated into these thought disorders. Most importantly, schizophrenia is a heterogeneous disorder. That is, the symptoms that one patient displays may be very different to what another patient experiences. Slide 5: Current treatments for schizophrenia rely on the use of antipsychotic drugs such as haloperidol, olanzapine and clozapine. These drugs are particularly good at addressing the positive symptoms that are associated with schizophrenia in the majority of patients. However, about a fourth of patients are non-responsive to this type of drug treatment. In addition to this, antipsychotics have little impact on the negative symptoms seen in schizophrenia as well as on the thought disorders for cognitive deficits associated with this disorder. This has major implications, in terms of the functional recovery of the patient, as it seems that the severity of the negative and cognitive symptoms of schizophrenia that seem to be most associated with the functional recovery of the patient. Furthermore, there are a number of severe side effects including sedation and weight gain as well as even motor deficits, which, again, seem to have a negative impact on patient functional recovery. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 1. There are a number of other approaches to treating schizophrenia, such as behavioural treatments, including cognitive behavioural therapy. This is an approach that has been used as an adjunct to antipsychotic drug treatment and can be effective in reducing relapse and resistant symptoms. However, these behavioural therapies have little impact on the negative and cognitive symptoms that are associated with schizophrenia and, therefore, they have little impact on the patient's functional recovery. Slide 6: So, how do we go about trying to understand what may be causing the negative and cognitive deficits in this disorder? And how can we make more effective and safer therapies for this disorder? Well, one thing we can do is to start looking at the neuropathology of the disorder. So, what do we know about the neuropathology of the disorder? Well, actually, we don't know very much, mostly because there are many inconsistencies between studies. However, what is agreed upon by many is that there are reductions in the grey matter or patients compared to unaffected individuals. Here we have structural MRI images of brains of healthy individuals or those who are suffering from schizophrenia. And we can see that there seems to be a difference in the overall volume of the brain of schizophrenic patients as compared to the healthy individuals. In addition to this, EEG and MEG studies have suggested that there is a dysfunction in neuronal network function in schizophrenic patients. And, as we've seen already before, post mortem human studies suggest that there may be a reduction in the number of dendritic spines in patients with schizophrenia as those compared to healthy individuals. Slide 7: The cause or causes of schizophrenia is likely to be multifaceted and likely involves a range of genetic as well as environmental factors. Each of these factors are unlikely to cause the disease by itself. But, a combination of both genetic and environmental factors would likely increase the chance of an individual developing the disease. While a number of environmentals have been linked with an increased risk of developing schizophrenia, there is also a very strong genetic component to this disease. This topic will be covered in more detail elsewhere in this course. But for now, what I would like to highlight is that the genetic landscape of schizophrenia is highly complex. Genetic studies indicate that there are a large number of mutations that are associated with schizophrenia. Some of these mutations are very rare, only occurring in fewer than 1% of patients with schizophrenia, but they have a strong effect. That is, if you have this mutation, you are more likely to have the disease. Conversely, there are a large number of genetic variants that have a weak effect. That is, they only slightly increase your chance of developing the disease. Taken together, we believe that it is a combination of environmental and genetic factors, both rare and common variants, that combine to underlie schizophrenia. Slide 8: So, how do we go about testing this theory that mutations in genes associated with schizophrenia can result in altered synaptic structure or function and, therefore, impact brain wiring? Well, the two most commonly used approaches are to either use animal models – where the gene of interest has been knocked out or mutated – or to use primary neuronal cell cultures – where cells are grown in a dish and we, again, manipulate the expression of genes to try and understand the role that the protein may play in controlling synaptic structure or function. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 2. Both of these experimental approaches have their benefits as well as their caveats. For example, using an animal model, we can not only look at the overall morphology of the cell, but we can also examine how altering the expression of specific genes may impact the behaviour of an animal. But you may also argue, that how can you model the behaviour of an animal with schizophrenia? Whereas, on the other hand, looking at primary neuronal cell cultures, it is a very easy way to manipulate gene expression and also allows you to examine dendritic spines in quite a bit of detail. Slide 9: Let's take the approach of growing neurons in a dish and explore how we can use this experimental approach to examine or model synaptic deficits in schizophrenia. What we can do in this approach is to grow neurons on a glass coverslip. And, then, using some clever molecular biology, we manipulate the expression of our target gene. After this, we can use a microscope to image the morphology of the neuron and then to perform detailed analysis of the dendritic spines. Slide 10: Let's take a real-life example of this approach. In this experiment, we have chosen to target the DISC1 or 'disrupted in schizophrenia one' gene. This gene encodes for a protein that is found in dendritic spines and is involved in a number of processes at the synapse. The mutation in this gene has been linked with a range of psychiatric disorders, including schizophrenia, autism spectrum disorders, depression as well as a number of other disorders. It was first identified in a Scottish family where a number of individuals were found to have mutations in this gene and to also have schizophrenia or bipolar disorder. Mutations of the DISC1 gene often seem to result in a reduction in the expression of the protein or in a dominant negative effect. So, in order to test whether this one protein is important for regulating dendritic spine number – and, therefore, wiring within the brain – we decided to try and reduce the expression of DISC1 in cultured neuronal cells – or cells grown in a dish – and to compare them with a control cell. Hopefully, you can see here a control cell and, if we zoom in on the dendrite, you can see the dendritic spines that are shown here with the red arrows. However, in cells where there's a reduction in the levels of DISC1, you can see that there are fewer dendritic spines – as shown with the red arrows. These data are consistent with a number of previous studies that have shown the same effect using a wide range of different approaches. And, simply put, this experiment allows us to say that by reducing DISC1 levels, we can negatively impact the number of dendritic spines. This allows us, therefore, to suggest that DISC1 plays an important role in, at least, the maintenance of dendritic spines. And, therefore, alterations in the expression of DISC1 protein, as seen in patients with various psychiatric disorders, may impact the synaptic connectivity within their brain. Slide 11: In this lecture, we have covered what the basic function of dendritic spines are, what their overall structure is and what they contain. We have explored how dendritic spines are the site for where the majority of excitatory synapses occur and discussed the model whereby dendritic spines can form new synaptic connections. We then went on to investigate in more depth, the idea that dendritic spine structure is strongly linked with synaptic function and that changing these two parameters are coordinated in response to different stimuli. We then went on to examine the evidence that dysfunction in dendritic spine function – and, therefore, altered synaptic connectivity – was linked with a number of neurodevelopmental and psychiatric disorders. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 3. In particular, we focused on evidence coming from genetic studies that implicate these structures in the pathogenesis of disease. Finally, we touched on some of the approaches whereby we can test the hypothesis that dendritic spine dysfunction may contribute to a complex disorder, like schizophrenia. Moreover, I have tried to show you that we could test the idea that altering the expression of proteins associated with disease, allows us to see how these proteins may contribute to the pathophysiology of disorders, such as schizophrenia. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 4. Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 3 The effects of activity, experience and deprivation on the nervous system - Part 1 of 5 Dr Sam Cooke Lecturer in Neurobiology of Neurodevelopmental Disorders, Basic and Clinical Neuroscience, King’s College London Slide 3: Hello, my name is Sam Cooke. I'm a lecturer, here, at King's College London and I work on learning and memory, the processes by which sensory experience and deprivation modify the brain to store information which can later be retrieved in the appropriate context. This work is important not just for increasing our understanding of how the brain works, but as you'll see at the end of this lecture, it's also highly relevant to understanding what goes wrong in disorders of the nervous system and for identifying potential treatments. In this lecture, we will delve into how neural activity and sensory experience and deprivation can shape brain function. We will start with a quick refresher on synaptic plasticity, focusing on Hebbian synaptic plasticity which you will have already covered in a lecture by Professor Peter Geza. We will then apply this knowledge to start thinking about how the selective responses of neurons in the brain to neural activity or to sensory input, which – as we will see – are not necessarily the same thing, can be shaped through Hebbian synaptic plasticity to both segregate and integrate inputs. For the purposes of this lecture, we will focus mostly on the visual system. As this is an important sensory modality for humans, it's intuitive to understand and it is probably the sensory modality that we have the deepest understanding of. However, it's also important to note that most of the concepts that we described are relevant to the postnatal development of other sensory modalities, such as auditory or somatosensory systems and to higher order functions, such as the development of language faculties or executive function. Slide 4: Okay, so let's briefly revisit Hebbian plasticity in order to make sure that we have a command of the key characteristics which will then enable understanding of how activity and experience can shape functional properties of the nervous system. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 1. Slide 5: Donald Hebb was a Canadian psychologist who used his knowledge of animal learning to identify some important theoretical criteria for the biological mechanisms that must support this critical faculty. In his famous text, The Organization of Behavior, Hebb describes many theories that remain influential to this day. Perhaps his most famous theory describes the process of Hebbian synaptic plasticity: ‘When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that the efficiency of A, as one of the cells firing B, is increased’. This somewhat wordy postulate is depicted pictorially here. The key concept to get hold of here is that existing chemical synapses on any one neuron arise from many different sources and are independently modifiable in strength based on the pattern of activity between the two connected cells. This fact would allow the synapse to serve as the major unit of information storage in the brain and reflect the history of activity at that synapse. Although Hebb never discussed bidirectional modification, it is also important – for the purposes of this lecture – to appreciate that synapses can be strengthened or weakened, depending on whether pre- and post-synaptic cells are correlated in their activity or uncorrelated in activity respectively. Many of you may have come across the phrase, 'fire together, wire together', which was coined as a mnemonic device to understand and remember the key aspects of Hebbian plasticity. I would like to point out that, while you may find it useful in some way, this slogan misses the mark and does not really describe Hebbian plasticity, because it implies that this plasticity is occurring between cells that are not already connected. A critical component of Hebbian theory is that synaptic plasticity allows experience to shape connections that already exist by increasing or decreasing their efficacy. There are certain examples of rewiring that occur in the nervous system that may actually be critical for recovery of function after brain damage, or perhaps even aspects of learning and memory, but they are very different phenomenon from Hebbian plasticity. Slide 6: These processes of Hebbian plasticity are commonly studied experimentally using high frequency trains of electrical stimulation, known as a 'tetanus', which is applied to axonal pathways that are afferent to a population of neurons, whose activity can be recorded using methods known as electrophysiology. This stimulation allows experimenters to guarantee electrical activation of pre-synaptic terminals at the same time as producing activation of post-synaptic neurons. The precise conditions that Hebb described as being necessary for the strengthening of synapses. Some experimental preparations also allow for the isolation of separate axonal inputs to the same cell, allowing experimenters to test the Hebbian theory that synaptic plasticity can occur at one synapse without affecting its neighbour – an important property, known as 'input specificity'. Slide 7: The most commonly studied form of Hebbian plasticity, known as 'long-term potentiation' or 'LTP', relies upon these electrophysiological stimulations and recording techniques. This very well studied phenomenon was originally discovered and characterised by British neuroscientist, Tim Bliss, and his Norwegian colleague, Terje Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 2. Lømo, in the hippocampus of anaesthetised rabbits. LTP is now commonly studied in surgically excised tissue, which helps greatly with positioning, stimulating and recording electrodes and for washing drugs on and off to determine underlying mechanisms. Here is a slice of human hippocampus, which has been removed from a patient with otherwise intractable epilepsy as an emergency treatment. The neurons in this transverse hippocampus slice can be kept alive by maintaining it at the correct temperature in carefully oxygenated solutions that contain all the required ionic concentrations and metabolites. Visualisation of the slice, under a microscope, allows precise positioning of recording electrodes by the cell bodies of hippocampal neurons and, in this case, the granule cells of the dentate gyrus – where LTP was first recorded by Bliss and Lømo in the early 70s. Two stimulating electrodes are positioned on either side of these cells to stimulate different afferent pathways, each of which evokes a response in the postsynaptic cells, demonstrating independent synaptic inputs. On the right is a graph showing the strength of the synaptic response to electrical pulses delivered to each of these pathways at a test frequency, which is delivered at one pulse every minute, and it does not induce plasticity. After a half hour baseline, to ensure stability, a high frequency tetanus of 100 Hz is delivered to just one of these pathways – which is depicted with black circles – while the other pathway continues to receive the very low frequency test pulses. As you can see, the tetanised pathway undergoes potentiation which then lasts for at least an hour without the control pathway being affected. This is the famous phenomenon of LTP which is an input-specific, long-lasting Hebbian form of synaptic plasticity. Slide 8: Since LTP was discovered, many considered it a theoretical imperative that the reverse phenomenon, 'longterm depression' or 'LTD', must exist at synapses since activity-dependent potentiation would quickly saturate synaptic strength and lead to hyperexcitability in the nervous system. After many years of trying it, it was discovered that low frequency tetanus of 1 Hz, still much higher in frequency than the test pulses that don't induce plasticity at all, would produce the reverse effect of LTD – in contrast to the 100 Hz tetanus that induces LTP. Importantly, both forms of plasticity could be observed longitudinally at the same synapses. On the right is a modification curve, which is a graph mapping the effects of different stimulus frequencies on the strength of synapses. As you can see, a range of low frequency stimuli will induce LTD while higher frequencies induce LTP. There's also a frequency of around 10 Hz that induces no change at all, which is known as the modification threshold. Much work has been conducted to show that the frequencies which result in LTP do so by ensuring strongly correlated pre- and post-synaptic activity, just as Hebb had originally described, while the lower frequency stimuli that induce LTD do so by ensuring explicitly uncorrelated activity between pre- and post-synaptic cells. As we shall see later in the lecture, this bidirectional plasticity, the direction of which reflects the recent history of activity at the synapse, is a perfect system to shape the functional response of neurons in the brain to activity and to sensory input. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 3. Slide 9: The phenomena of LTP and LTD have been observed at most synapses throughout the nervous system. This slide shows work in ex vivo slices taken from rat hippocampus, rat visual cortex and cat visual cortex. All showing very similar degrees of LTP and LTD when assessed with electrophysiology. This fact will be highly relevant to most of the remainder of this topic, which will focus on activity-dependent plasticity in the primary visual cortex. Slide 10: Critical mechanisms, at the heart of many forms of LTP and LTD, are the AMPA and NMDA subclasses of ionotropic glutamate receptors. One of these receptors, the AMPA receptor, is opened by glutamate and is an ion channel that allows the flow of positively charged ions, mostly sodium ions, into a neuron. This receptor carries the majority of synaptic current and is responsible for much excitatory fast synaptic transmission. Changes in the properties or number of AMPA receptors are a major expression mechanism of both LTP and LTD. The NMDA receptor is also an ion channel that allows positively charged ions to flow into neurons. However, it's more complex than the AMPA receptor because, as well as glutamate-binding, it is also voltagedependent. Meaning that the channel will only open when glutamate is bound and the post-synaptic neuron is also depolarised, or active. This property arises from a magnesium ion that blocks the channel pore unless the post-synaptic membrane is depolarised. The NMDA receptor, therefore, has the ideal properties to serve as a critical coincidence detector for the Hebbian criterion of pre- and post-synaptic coactivity. And the key ions that flow through the NMDA receptor, and indicates that Hebbian conditions have been met, are calcium ions. Slide 11: This slide shows the original experimental evidence that AP5 (or APV), the specific NMDA receptor antagonist, blocks the induction of both LTP and LTD. This was ground-breaking work as it demonstrated how biology serves Hebb's theory. This experiment also demonstrates an invaluable experimental advantage of the ex vivo slice – which not only allows drugs to be washed on at the appropriate time but also washed off to demonstrate the synapses are not irreparably altered by drug delivery and the LTP can still be induced subsequent to washout. Slide 12: How can it be that the same receptor serves opposing directions of synaptic change? The answer is in the conduction of calcium ions through the NMDA receptor. Because of the different dynamics of post-synaptic activation, produced by high- and low-frequency stimulation, the concentration of post-synaptic calcium is very different – as it summates to high concentrations for high-frequency stimulation while remaining elevated, but considerably lower in concentration, as a result of pulsatile, non-summating increases in calcium ion concentration. This result in activation of different types of calcium-sensing enzymes, some kinases that will phosphorylate targets – such as AMPA receptors – to change their properties and some phosphatases which dephosphorylate and have the reverse effect. We will not go into the details of the particular signalling systems that play, as Professor Peter Geza has already discussed this to some degree previously, but it is important to understand how LTP and LTD can coexist at the same synapses and share many key mechanisms. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 4. Slide 13: So, to summarise this brief overview of Hebbian plasticity, Hebbian plasticity is an activity-dependent strengthening of synapses between coactive neurons or a weakening of synapses between neurons with uncorrelated activity. Hebbian plasticity is modelled experimentally in vitro and in vivo through electrical stimulation to produce longterm potentiation, LTP or long-term depression, LTD which respectively strengthen or weaken synapses. The frequency of stimulation is a major determinant of the direction of change – high for LTP and low for LTD. LTP and LTD occur at most synapses in the nervous system. Hebbian plasticity is input-specific, as it occurs only at synapses that have undergone activity and does not occur at neighbouring inactive synapses on the same neuron. It is also long-lasting. The NMDA subclass of glutamate receptor is often a key mechanism in the induction of LTP as it is an ion channel that conveys calcium ions only when two coincident events occur: glutamate-binding and post-synaptic depolarisation. Thus, it serves as the detector of the defining events in Hebbian LTP: correlated pre- and postsynaptic activity. It's also a key mechanism for many forms of Hebbian LTD. Hebbian plasticity is not accurately described by the statement 'fire together, wire together'. Hebbian plasticity can only change existing synapses. It does not involve the formation of new synapses. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 5. Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 3 The effects of activity, experience and deprivation on the nervous system - Part 2 of 5 Dr Sam Cooke Lecturer in Neurobiology of Neurodevelopmental Disorders, Basic and Clinical Neuroscience, King’s College London Slide 3: Now, let's move on to thinking about how Hebbian plasticity could explain the effects of activity on the nervous system during postnatal development. Slide 4: The neocortex is a six-layered structure that exists only in mammals. It's highly plastic and is known to be critical for long-term memory. The primary sensory regions of neocortex are by far the best studied and understood regions of the neocortex because, first, they receive relatively unprocessed sensory information, which is relayed from the relevant sensory operators – usually via very few intermediary structures. Second, they potentially provide a general model of neocortical function because they contain all the key circuit and molecular elements that are found in higher-order regions, such as prefrontal cortex. Third, their structure and function are relatively well understood and often exhibits visible specialisations that reflect its topographical organisation as a spatial recapitulation of the sensory world. For instance, in this slide, you can see a stained surface mount of three primary sensory areas in mouse neocortex. The primary visual cortex, known as 'V1', the primary auditory cortex, known as 'A1', and the primary somatosensory cortex, known as 'S1'. This view reveals particularly striking anatomical specialisations in somatosensory cortex, known as 'whisker barrels', which are columnar organisations that are each dedicated to input from a single whisker. Thus, it's possible to constrain sensory stimulation to a very specific region of interest and study the plasticity that results. Slide 5: For this topic, we're going to focus on the visual system and the similar specialisations, known as 'ocular dominance columns', that exist in the primary visual cortex V1 of most mammals – including primates, such as ourselves, and other well-studied species – notably including carnivore species, such as cats and ferrets. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 1. The organisation of ocular dominance layers in the thalamus and columns in the neocortex have been tracked experimentally using radioactive transsynaptic tracers injected into one eye of an animal. This enabled autoradiographic tracing of this functional segregation, which is seen here in a cat brain. On the left, is a section through the primary visual relay nucleus of the thalamus, the 'lateral geniculate nucleus' or 'LGN', revealing layers that are dedicated to the contralateral eye – which is not injected and, therefore, without tracer present in the thalamus – and the ipsilateral eye – which is injected and, therefore, with tracer in the thalamus. On the right are two views of primary visual cortex. First, a view from above of visual cortex showing the interdigitated zones dedicated to one eye across the visible layer 4. And, second, a transverse section revealing white matter projections up to V1, showing the restriction of these labelled ocular dominance zones to layer 4 of cortex. Slide 6: Here's a schematic of the segregation of ocular inputs and the maintenance of this segregation through the cat visual system, up to primary visual cortex. Spatially segregated zones are dedicated to processing visual information provided through the contralateral eye – in blue – or the ipsilateral eye – in yellow. The segregation is maintained in the optic nerve and the lateral geniculate nucleus all the way up into V1 where ocular dominance columns are maintained in layer 4, which is the first layer of neocortex to receive the thalamic input. If we now look at the laminar organisation in V1, we can see that intracortical connections integrate these two separated inputs in layers 2, 3, and 5 into binocular representations – which are shown in green. In this topic, we will address both how segregation in ocular dominance columns of layer 4 can be initiated by Hebbian plasticity and how integration into binocular representations can occur in layer 2/3, also through Hebbian plasticity. Let's start with how ocular dominance columns may arise from activity within the nervous system. Slide 7: A key question for neuroscientists has been whether this segregation of function arises from genetic programming, that determines the organisation of the developing visual system, or whether the activity of neurons plays a critical role in the development of ocular dominance territories in the brain, as had been hypothesised by British neuroscientist, David Willshaw. An important observation came from Lamberto Maffei's laboratory, in Italy, that retinal neurons produce spontaneous activity. And this was followed up by Carla Shatz's laboratory, in the US, who used calcium imaging to show that there were, in fact, waves of activity that passed across the retina. This slide shows an example of such a wave which can be recorded in a dish with a calcium imaging dye and reveals the time course of the progression of a wave of activity across the retina, here taken in time snaps over seconds. These retinal waves have subsequently been a major area of investigation. The retinal waves were found to occur long before the eyes of many species open, an event that usually happens a considerable time after birth. Slide 8: A first question to address was, what would be the consequence of inactivating the retina and preventing the spontaneous activity from occurring during postnatal development prior to eye opening? To accomplish this, experimenters turned to toxins that are extracted from the animal world. In this case, 'Epibatidine', extracted from the skin of a species of Ecuadorian frog known as 'Anthony's poison arrow frog', and to 'Tetrodotoxin' or 'TTX', which is taken from the puffer fish and is the active ingredient that numbs the lips of those that eat puffer Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 2. fish, a delicacy in Japan. These nerve toxins target different molecular mechanisms. Epibatidine is an antagonist of many different acetylcholine receptors and TTX blocks voltage-gated sodium channels but both block neural activity and are commonly used, now, as experimental tools to assess the importance of activity in specific neural populations. Slide 9: It had previously been shown by Carla Shatz and Michael Striker, in the US, that application of TTX in the prenatal cat embryo, prevented normal segregation of ocular dominance zones, indicating that spontaneous neural activity must play a major role. However, blockade of activity in the retina of postnatal ferret pups prior to eye opening, thereby preventing retinal waves, also had a striking effect on the segregation of ocular dominance zones. In the work shown here, from Andrew Huberman and colleagues in the US, you can see that the boundaries between the zones dedicated to ipsilateral and contralateral eyes in the LGN – which are, here, marked with different coloured trans-neuronal dyes delivered to each eye – are significantly blurred if activity is blocked in the retina. Slide 10: Similarly, when the retinae are inactivated during postnatal development prior to eye opening, ocular dominance columns in V1 do not segregate at all. Slide 11: How does this lack of synchrony between the activity of the two eyes contribute to segregation of ocular dominance layers in the thalamus and ocular dominance columns in layer 4 of the neocortex? The key factor to note, here, is that while the retinae and other parts of the nervous system are exhibiting a high degree of spontaneous activity at this stage of development, that activity is in no way correlated since the retinae are not receiving shared sensory input. That lack of correlation plays a major role in the ability of Hebbian plasticity to segregate zones of the visual system that are dedicated to input from one eye or the other. Slide 12: Remember that Hebbian synaptic weakening, as modelled by LTD, occurs when there is a lack of correlation between activity in the presynaptic neuron and the postsynaptic neuron. Thus, where post-synaptic neurons are having their activity driven by one eye slightly more powerfully than by the other eye, because these two inputs are out of synchrony, the slightly weaker input will be further weakened until it eventually is unable to elicit any activity in the post-synaptic neuron. Thus, the post-synaptic neuron can be said to have a monocular receptive field, more or less dedicated to processing information from one of the two eyes only. Subsequently, inputs from this favoured eye will be strengthened even further through Hebbian potentiation, as modelled by LTP, given the increasingly reliable coincidence between pre-synaptic activity and post-synaptic response. This overall scenario is depicted in the schematic to explain how highly segregated zones or 'ocular dominance columns' – in blue and yellow – could arise from a population of neurons that initially had a largely binocular response – shown in green. All that would be required for this to occur is the prior existence of a very slight bias in one direction or another. This bias may arise through chance or it could be that some genetic mechanisms that are not activitydependent do create some very rough bias before this is hugely refined by activity. This latter scenario would Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 3. explain why, for instance, there's some vestige of zones dedicated to input from one eye or the other when the retinas are silenced postnatally. Slide 13: A major question posed by this hypothesis is whether segregation requires the NMDA receptor in cortical neurons. Remember, that this ionotropic glutamate receptor acts as a detector of coincidence between preand post-synaptic elements and is critical for many forms of Hebbian LTP and LTD. The mouse is the only mammalian species in which genetic engineering can easily be used to ablate, or 'knock out', a gene and, thereby, the expression of the protein encoded by that gene. Although modern technology is changing that – soon we will be able to knock out or manipulate genes in almost any species. This knock out approach is critical to determine whether Hebbian plasticity is required for functional segregation based on spontaneous activity. This slide shows data from a mouse in which the NMDA receptor has been functionally ablated from glutamatergic neurons of the cortex. Mice don't exhibit ocular dominance columns, unlike most other mammalian species, but they do have analogous functional segregation in primary somatosensory cortex, known as 'whisker barrels' – as we discussed at the beginning of this section. On the right, you can see anatomical markers and stains of neural activity that reveal severely ill-defined whisker barrels in the primary somatosensory cortex of mice that do not express NMDA receptors in the neocortex. This requirement, for NMDA receptors to achieve functional segregation in primary sensory areas, is further evidence for a key role played by Hebbian plasticity. Slide 14: In summary, 'ocular dominance columns' are zones of cortex that only respond to input through one or another eye. They are present in the primary visual cortex of many species – for example, cats and humans. And functional segregation also exists in the visual thalamus. In many species, the eyes open sometime after birth, but ocular dominance columns still emerge during this period. Spontaneous neural activity can be recorded in the retina prior to eye opening and these are known as 'retinal waves'. Similar spontaneous activity can be detected in the visual thalamus. Retinal waves are not correlated between the two eyes. Inactivation of the retinae to prevent retinal waves prevents the formation of discrete ocular dominance columns. Evidence suggests that blockade of NMDA receptors also prevents segregation of ocular dominance columns and whisker barrels in the somatosensory cortex. Hebbian synaptic plasticity is hypothesised to progressively sharpen the boundaries between ocular dominance columns by weakening connections between neurons that are uncorrelated in activity – ie responsive to opposite eyes – and strengthen connections between neurons that are correlated– ie those that are responsive to waves in the same retina. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 4. Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 3 The effects of activity, experience and deprivation on the nervous system - Part 3 of 5 Dr Sam Cooke Lecturer in Neurobiology of Neurodevelopmental Disorders, Basic and Clinical Neuroscience, King’s College London Slide 3: Now, let's consider a very different event in the postnatal development of the nervous system, that nevertheless requires Hebbian synaptic plasticity. This is the integration of inputs on to shared post-synaptic targets, to create more complex receptive fields. For this purpose, we're going to focus on the postnatal development of binocular vision. As we shall see, this process requires visual experience. Slide 4: The important starting point for this section is to take into consideration that after the eyes open, visual stimuli will start to evoke activity in the retinae. This activity is very different from the spontaneous activity occurring during retinal waves for the simple reason that it is highly correlated across the two eyes. Thus, activation of neurons in the visual system will start to reflect the statistics of the environment, and this activity will be shared across the segregated zones. Slide 5: We'll now focus on one aspect of the very famous work developed by Canadian neuroscientist, David Hubel and Swedish neuroscientist, Torsten Wiesel, who won the Nobel Prize for their ground-breaking work together at Harvard, in the US, on the development of receptive fields in the visual system. One aspect of the work that they conducted was in coming to understand how neurons in the brain could take on binocular representations that are required for such important faculties as depth perception. Slide 6: They worked in carnivore and primate species, which all have excellent binocular vision, unlike some prey species such as mice and deer. This is accounted for by their front-facing eyes, which allows both eyes to serve a match of the same extent of the visual field. One of the species of choice was the cat – the visual system of which is, again, depicted here. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 1. As we discussed before, ocular dominance columns are maintained as separate for each eye within layer 4 of visual cortex – shown here in yellow or blue – but cells from each of these columns then make common contact with other neurons within the cortex, particularly in layers 2 and 3. These neurons then take on a binocular representation – shown in green – as inputs from each eye drive activity in the same binocular neuronal population. But, how does this integration occur? Slide 7: A very well used, experimental strategy to study the plasticity of ocular dominance and binocularity was to reversibly close one eye, preventing visual input. This was usually achieved by carefully suturing the eyelids closed. This may sound like a cruel procedure but, when performed with surgical precision, it allows for careful reopening of the eye after several days or weeks without any compromise to the function of the eye. And, assessment of response of the brain to input through that eye compared to the other eye that had remained open. Thus, ocular dominance plasticity could be measured. The first thing to note is that the ocular dominance columns in cortical layer 4 undergo dramatic reorganisation when the contralateral eye of kittens is sutured for several weeks. Here you can see, using the same radioactive tracer technique described earlier to track ocular dominance columns, that the territory dedicated to the open, ipsilateral eye has dramatically expanded into the columns previously dedicated to the contralateral eye. Slide 8: Another important technique was developed by Hubel and Wiesel to record the electrical activity of neurons in visual cortex of anaesthetised cats with tungsten microelectrodes. This revealed an additional striking functional effect of ocular dominance. As we've described in previous slides, neurons in layers 2 and 3 and further intercortical networks of primary visual cortex exhibit binocularity. Using Hubel and Wiesel's electrophysiology approach and masking visual input through one eye or the other, it could be observed that most neurons in layer 2/3 were either completely or partially binocular in their response to visual inputs, as shown on the right. Just around 10 to 20% of neurons were monocular in their response within this layer in kittens, undergoing normal visual experience. Slide 9: However, as Hubel and Wiesel showed, as well as others, such as Colin Blakemore, here in the UK. Monocular deprivation through, lid suture of the contralateral eye, led to a profound shift in these binocular responses so that neurons in layer 2/3 became almost exclusively responsive to the open ipsilateral eye – here depicted in shades of yellow – even after the eye had been reopened. This effect is known as ocular dominance plasticity as a result of monocular deprivation, and it has been a deeply studied phenomenon – as it likely provides broad insight into how experience and deprivation shape the nervous system. Slide 10: An alternative experimental approach was to create an artificial strabismus in kittens, in which the eyes were forced to view different parts of the visual field. This was achieved by surgically cutting one of the muscles around the eyeball. Like monocular deprivation, the strabismus treatment more or less eradicated binocular receptive fields from layer 2/3 neurons. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 2. However, in contrast to monocular deprivation, strabismus led to equal responsiveness through the two eyes as each eye was delivering equal amounts of activity. That activity was just not correlated between the two eyes. Slide 11: Importantly, this process of deprivation-dependent plasticity appeared to rely upon Hebbian mechanisms, as work from Wolf Singer's lab showed – in which they blocked NMDA receptors in primary visual cortex with a selective receptor antagonist and prevented the ocular dominant shift resulting from monocular deprivation in kittens. Slide 12: Another dramatic observation was that the ocular dominance shift did not occur if both eyes were sutured for the same period as the previous monocular deprivation experiments. Thus, more deprivation did not result in more plasticity or, indeed, much plasticity at all. This was a really important observation because it showed the ocular dominance plasticity is a competitive process that requires not just deprivation of input through one eye but, also, experience through the other. Slide 13: So, in summary for section 3, binocular vision is critical for depth perception and survival. Once the eyes open, activity switches from being uncorrelated between the two eyes to being correlated, due to shared visual input from the outside world over much of the visual field. The visual system integrates inputs from the two eyes through experience to form binocular representations – ie neurons that respond to shared visual inputs from both eyes. In carnivores and primates, intra-cortical synapses originating from segregated ocular dominance columns in layer 4 converge on neurons in layers 2/3 and 5 of primary visual cortex to form binocular receptive fields. Ocular dominance plasticity, which results when vision through one eye is deprived or altered, provides insight into the mechanisms that support binocular integration. Closure of one eye in kittens or monkeys shifts the response of neurons in layer 2/3 of visual cortex away from the closed eye and towards the open eye. This shift remains even after the eye is opened. Strabismus, in which muscles are cut to prevent the eyes from focusing on the same part of the visual field, has a different effect of forcing neurons in layer 2/3 to become responsive to just one eye or the other. Hebbian plasticity mediates formation of binocularity. Blockade of the NMDA receptor prevents ocular dominance plasticity. If both eyes are closed, no plasticity occurs, showing the competition between inputs is critical for ocular dominance plasticity. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 3. Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 3 The effects of activity, experience and deprivation on the nervous system - Part 4 of 5 Dr Sam Cooke Lecturer in Neurobiology of Neurodevelopmental Disorders, Basic and Clinical Neuroscience, King’s College London Slide 3: Now, let's turn our attention to a related developmental process that introduces a major permissive factor to this deprivation-induced plasticity – the critical period. Slide 4: Another Nobel laureate, who really crystallised the concept of the critical period, was the Austrian ethologist, Conrad Lorenz. Lorenz conducted many fascinating experiments on the phenomenon of imprinting, in which he became the major parental figure to numerous different bird species. If he served as the primary provider and carer for chicks, goslings or cygnets during a critical period of postnatal development, they formed a powerful, unbreakable attachment to him that could not be superseded by a member of their own species. Importantly, this attachment persisted if he took on this role during and beyond the close of the defined period, which was termed a critical period. This concept of the critical period – a relatively brief window during which defining plasticity was permitted – has become influential throughout education, psychology, psychiatry and neuroscience and it's highly relevant to the effects of visual experience in deprivation on the neocortex of mammals. Slide 5: This is illustrated by a series of experiments in kittens and cats of various ages. The first key observation is that the effects of monocular deprivation are highly reversible if the deprivation occurred during an early critical period. In five-week-old kittens, monocular deprivation would not only result in the ocular dominance shift in the response of layer 2/3 neurons, as we've discussed already, but after un-suturing the deprived eye, a reverse suture of the opposite eye would result in an equivalent shift in the opposite direction. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 1. Slide 6: If this experiment were carried out in the same way with monocular deprivation during the critical period of 5 weeks of age, but then un-suturing and reverse suturing occurred much later, at 14 weeks of age, then not only did the reversal of ocular dominance in Layer 2/3 not happen, but recovery from the initial shift did not occur. Slide 7: Similarly, the ocular dominance plasticity does not occur at all if eyelid suture occurs in the adult animal, demonstrating very clearly that the capacity of the cortex for plasticity is lost after the critical period. Slide 8: A key question, of course, is whether this permanent shift not only compromises response in layer 2/3 of visual cortical neurons but also actually impairs vision itself. In humans, we would test vision with a Snellen chart, which many of you may be familiar with. The Snellen chart is a test of visual acuity, and it asks you to resolve lines that are different distances apart and this is known as varying spatial frequency. At some point, a threshold can be found beyond which you cannot differentiate the letters ‘M’, ‘W’, ‘E’ and the number ‘3’, which is the determinant of your visual acuity. 20/20 vision just means that your vision at 20 feet matches normal vision at 20 feet. Slide 9: One classic test of vision teaches a cat to associate a specific orientation of lines with a reward – say vertical stripes but not horizontal stripes. Once this association is formed then one can just assess vision by changing the spatial frequency and determining how often the cat chooses to jump to the rewarding orientation. If one eye or other is covered during this test, then vision can be tested independently through each eye. Here you can see work from Canadian vision scientists Donald Mitchell and Kevin Duffy, in which monocularly deprived kittens show normal binocular vision a week or so after the eye is open post-critical period. However, vision limited to the deprived eye never recovers and the animals remain functionally blind through this eye even though the eye, itself, is fully operational. Thus, if visual experience does not return to normal until after closure of the critical period, then there is no functional recovery. Slide 10: The visual cortical critical period varies in time and longevity from one species to another. Rather conveniently, this roughly lines up in weeks for cats, months for monkeys and years for humans, as shown in this graph, with closure of the critical period occurring around eight to nine weeks, months or years depending on the species. Slide 11: Much work has now been done by several laboratories, notably including those of Mark Bear and Takao Hensch in the US, to demonstrate that a key determinant of both the opening and the closing of the critical period is the degree of cortical inhibition. Inhibition develops late in the cortex, relative to excitation circuits, and we now know that the critical period really represents a sweet spot between too little and too much inhibition. Slide 12: So, how can inhibition be a key determinant in whether Hebbian plasticity occurs or does not? Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 2. Here, we can see a schematic of how a simple feed-forward circuit in the visual cortex looks, with tick marks representing action potentials. As the schematic shows, cortical inhibition after the eyes open, but before the critical period opens, is too low to really impact the activity of cortical circuits resulting from visual input. This means that Hebbian plasticity cannot operate to integrate signals because there's too much noise in the system. After inhibition has started to develop, during the critical period, conditions are optimised so that only the strongest visual inputs will drive enough cortical activation to modify synaptic strength through Hebbian plasticity. It is during this period that the cortex is primed to be modified by visual experience and deprivation. The closure of the critical period appears to arise once inhibition is so powerful that it suppresses the propagation of activity through cortical circuits for all but the very strongest sensory input. Slide 13: Thus, the critical period is closed once inhibition in the cortex is matured. However, it is important to note that the capacity for change still exists in cortical circuits if inhibition can be modified. The opening of the critical period can be advanced by positively modulating GABA receptors with benzodiazepines. The critical period can be reopened with treatments that reduce inhibition, such as a genetic knockdown of the key enzyme for synthesising GABA or, interestingly, by grafting immature inhibitory neurons into the visual cortex of mature mice. In the next section, we will consider some of the therapeutic implications that this work gives rise to. Slide 14: So, in summary for this section on critical periods: Critical periods define the time window during which the effects of sensory experience or deprivation on the nervous system are most pronounced, usually occurring quite early in post-natal development. Critical periods vary for brain regions and sensory modalities, for example, the critical period for plasticity in somatosensory cortex opens and closes earlier than for visual cortex. Higher order regions of cortex, such as prefrontal cortex, have even later critical periods. Critical periods vary from species to species, for example, the critical period for ocular dominance plasticity closes much earlier for mice than cats, and earlier for cats than primates. Several lines of evidence indicate that inhibitory neurons play a key role in critical period duration, with development of inhibition opening the critical period of maturation and maturation of cortical inhibition closing it. Increasing inhibition can prematurely open the critical period and reducing inhibition can re-open the critical period after it has closed. Inhibition is believed to serve as a permissive factor for Hebbian plasticity by reducing overall activity at the opening of the critical period, thereby reducing 'noise' and allowing differentiation of correlated and uncorrelated activity. However, too much inhibition can prevent enough post-synaptic activity to allow Hebbian plasticity to occur, thereby closing the critical period. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 3. Module: Biological Foundations of Mental Health Week 4 Biological basis of learning, memory and cognition Topic 3 The effects of activity, experience and deprivation on the nervous system - Part 5 of 5 Dr Sam Cooke Lecturer in Neurobiology of Neurodevelopmental Disorders, Basic and Clinical Neuroscience, King’s College London Slide 3: Now, let's complete our topic by considering the therapeutic possibilities that exist, because of all this fundamental neuroscience work. How might we treat sensory deprivation, and are there further reaching implications for psychiatric disorders? A major focus in this regard is asking whether we could re-open the critical period in mature patients in order to recover developmental disruptions that may have arisen from deprivation during childhood. Slide 4: As well as the various invasive treatments that we discussed for reopening the critical period – including genetic modifications in mouse to reduce GABA synthesis and the grafting of inhibitory neurons, precursors into visual cortex – considerable work has been done to develop non-invasive means to influence inhibition and, thereby, extend or re-open the critical period. These non-invasive approaches would be much more palatable as potential treatments in humans than genetic modifications or surgical grafts – although nothing can be discounted if the condition is severe enough and the patients are willing. Among treatments tested in rodents, that show promise in returning the cortex of adults to critical period levels of plasticity, include environmental enrichment, dark exposure, caloric restriction, physical exercise and perceptual training. In addition, certain drugs that are already available for use in humans, such as Selective Serotonin Reuptake Inhibitors – or 'SSRIs' – which are used as antidepressants, appear to influence cortical inhibition and return it to a critical period-like state. Slide 5: Focusing on one of the most promising of these treatments in the visual domain, we can look at some work revealing that dark exposure for several days in rodents alters inhibition within the visual cortex and, as a result, alters the modification threshold for synaptic plasticity. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 1. On the left are direct measurements of cortical inhibition, using intracellular electrophysiological recordings, from slices of visual cortex, taken from rodents that have either been raised under normal lighting conditions, or raised in this manner but, then, briefly exposed to extended dark over several days. The top panel shows that dark exposure has no effect on cortical inhibition if it occurs during the critical period and is compared to critical period mice on a normal light cycle. However, if the same experiment is conducted in adult animals, in which the critical period is closed and inhibition is fully matured, the dark exposure substantially reduces the amplitude of inhibitory, post-synaptic currents – 'IPSCs' – relative to controls. This effect reveals the capacity of dark exposure to recover cortex to critical period levels of inhibition. In the right panels, we can see that if animals are raised in the dark, the direction of Hebbian synaptic plasticity can be altered in primary visual cortex, reflecting altered inhibition and a shifted modification threshold. Low frequency stimulation produces less LTD in dark-reared animals than their littermate controls, raised under normal lighting, and higher frequency stimulation, of around 40 Hz, induces more LTP. Thus, the expectation would be that dark exposure could either reduce the impact of monocular deprivation – remembering the important fact that binocular deprivation does not induce a shift in ocular dominance in the brain – or, more dramatically, recover lost visual function in adult animals after extended monocular deprivation. Slide 6: If we return to cats and the behavioural measure of their visual acuity carried out by Donald Mitchell's laboratory, we can see a stunning experimental result that is highly relevant to the treatment of human disorder. Here you can see the kittens that underwent monocular deprivation through the critical period – around one month of age – retain major visual deficits long after that eye is open. These deficits, in visual acuity, are akin to almost complete blindness through the deprived eye for months after the eye has been opened and in contrast to the open eye, which exhibits normal visual acuity. The amazing thing is that exposing the animals to 10 days in the dark, at three months of age, leads to a complete recovery of function through the deprived eye over just a few days of further visual experience. The weight of evidence, therefore, points towards dark exposure as being a strong candidate for recovery of function in the visual system by modifying inhibition. Slide 7: Monocular deprivation in animals is, essentially, a model of a not uncommon human condition, known as 'amblyopia' – in which monocular deprivation occurs during childhood as a result of several possible ocular conditions. Sometimes, this deprivation is not detected early enough during childhood and it extends beyond the critical period, to ages eight and upwards. Meaning, that the visual cortex is slowly dedicated to responding to the fully functional eye and cannot be recovered for binocularity even with good treatment of the eye in adulthood. The condition of amblyopia is colloquially described as 'lazy eye'. Amblyopia reduces visual acuity to varying extents resulting in almost no depth perception, this affects around 2% of the UK population and, even in the subtlest of cases, prevents those people from entering certain professions that require depth perception, such as being a pilot or a fireman or a firewoman. In more extreme cases, it results in complete cortical blindness through that eye and would prevent you from driving. It would also reduce your quality of life in many ways and potentially also lead onto some mental health issues. In the developing world, such as countries in Asia and Africa, the problem is more prevalent – as easily treated ocular problems, such as cataracts, are often not Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 2. tended to at all or until it is too late. In these countries, amblyopia also has a more dramatic effect on one's ability to earn a living and can, therefore, be a catastrophic condition. Slide 8: Of the causes of amblyopia, some are very easy to detect, such as cataracts or strabismus, due to an obvious physical manifestation. These conditions would likely be remedied early in life, in the UK, and a child can go on to have perfectly normal vision. In the developing world, these dysfunctions may not be attended to, due to a lack of money or facilities. In countries like the UK, amblyopia can occur due to less noticeable conditions, such as anisometropia – in which the two lenses are of different refractory indices, and one provides a clearer view of the world than the other. It is obviously critical to have good tests of visual function when children are young, to give them the best chance of recovery prior to closure of the critical period. Slide 9: The current best clinical practice is to use surgery to return the 'bad' eye back to normal before dealing with residual ocular dominant shift during the critical period. This recovery can be accelerated by performing the equivalent of a reverse suture experiment, either by patching the good eye or by using eye drops of belladonna extracts – or atropine – which prevent muscles in the good eye from working properly. This punishment of vision through the good eye is not ideal, given that the visual system is still developing in numerous other ways. Development of novel treatments for amblyopia, especially in adults – in which function cannot currently be recovered, would have a major societal impact. The work on dark exposure and related, non-invasive treatments is, therefore, extremely important. The therapeutic implications of the fundamental neuroscience that we have discussed extends way beyond the visual system, however. The work on the effects of visual deprivation on the visual cortex provides deep insight into the likely consequences for deprivation in other sensory systems and in higher order systems. Slide 10: Insight into the development of inhibitory systems in the neocortex and how that can influence the effects of experience and deprivation on the nervous system is likely relevant to a slew of conditions, including neurodevelopmental psychiatric disorders: such as epilepsy, intellectual disability, autism spectrum disorders and schizophrenia – where dysfunctions in the postnatal development of balanced excitation and inhibition is heavily implicated. This so-called 'E-I balance' has been studied in the context of these neurodevelopmental disorders. Highly penetrant genetic causes of these conditions often target synaptic proteins, such as 'neurexins' and 'neuroligins' – which are transsynaptic signalling molecules that are critical for either normal inhibition of excitatory neurons or normal excitation of excitatory neurons. Mutations in the genes that encode these proteins often result in E-I imbalance and intellectual disability, autism spectrum disorders or schizophrenia. Critical receptors for Hebbian plasticity, such as the NMDA receptors or associated signalling systems, appear to be risk factors for schizophrenia and there is ample evidence, in this condition, that inhibitory neurons in the cortex are reduced in number and in the production of GABA – indicative of E-I imbalance. Another major risk factor of neurodevelopmental disorder is Fragile X Mental Retardation Protein – 'FMRP' – Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 3. which regulates activity-induced protein synthesis required for many synaptic processes, notably including lasting Hebbian synaptic plasticity. Disruption of FMRP function, as its name suggests, results in one of the more common forms of intellectual disability – 'Fragile X Syndrome' – which is often comorbid with epilepsy and autism spectrum disorders and exhibits a clear E-I imbalance at various stages of development. Other risk genes encode transcription factors, such as MeCP2 – which is the protein that is mutated to cause the debilitating neurodevelopmental disorder known as 'Rhett's syndrome' and which appears to play a critical role in the production of enzymes necessary for GABA production in inhibitory neurons, thereby resulting in major E-I imbalance. Slide 11: As we have seen, critical periods reflect the normal development of E-I balance in the cortex, but the timecourse of critical periods is very different from region to region of the cortex, reflecting the different functions of these regions. While the sensory critical periods that affect plasticity in primary sensory areas occur early, around and after birth, similar developmental windows are extended much later into life, for language development or socialisation. It's possible that these later critical periods are affected in autism spectrum disorders. Executive function or context/rule-dependent behavioural control, which arises from higher order cortical regions in the frontal lobe may not be fully developed until late into adolescence. Disrupted development of these faculties may contribute to numerous psychiatric disorders, including schizophrenia. It is a relatively new concept that lost, delayed or exaggerated critical period plasticity – or that deprivation or aberrant experience that occurs during the relevant critical period – may be causal factors in a range of neurodevelopmental disorders. Much further work is now required in this domain. Slide 12: To summarise this section, non-invasion means to manipulate inhibition may re-open the critical period, returning the brain to peak plasticity and maximising the therapeutic effects of sensory experience. Promising methods include environmental enrichment, sensory deprivation, dietary restriction and exercise. Placing animals in the dark for an extended period greatly reduces the level of inhibition in the visual cortex. Mature cats, that have previously undergone monocular deprivation as kittens and have severe loss of vision through the previously deprived eye, can show dramatic visual recovery after being placed in the dark for 10 days. This approach holds promise for a debilitating condition, known as 'amblyopia', which results in a visual cortical deficit due to childhood deprivation, that persists even after the eye is rendered fully functional through surgery later in life. Amblyopia affects around 1 to 2% of people in the UK, but many more in the developing world – where treatment of fixable ocular conditions is less likely to occur, in a timely fashion, and where poor vision carries more severe consequences. Work on the visual system also provides general insight into how cortical function is shaped by deprivation and experience and how altered critical period plasticity may contribute to a wealth of neurodevelopmental disorders, including intellectual disability, autism spectrum disorders and schizophrenia. Please note that this is a transcript. It is not a learning object. Please refer to topics for visuals and full lecture content. Week 4 @ King’s College London 4.