Lecture6 BMS2011 2023-24 Lecture slides-1 (1).pptx

Excitatory Synaptic function Lecture 6 - BMS2011  Learning Objectives 1) Understand the structure and function of ionotropic glutamate receptors* 2) Understand the structure and function of metabotropic glutamate receptors* 3) Understand the mechanisms of NMDAR- dependent L...

Excitatory Synaptic function Lecture 6 - BMS2011  Learning Objectives 1) Understand the structure and function of ionotropic glutamate receptors* 2) Understand the structure and function of metabotropic glutamate receptors* 3) Understand the mechanisms of NMDAR- dependent LTP* 4) Other forms of synaptic plasticity (guided self- directed study*) *Note – see Canvas: additional slides (baby blue Fluorescent labeling of the presynaptic terminals (red) and the postsynaptic cell (green) reveals 1000s of synapses onto a  Glutamate The Neurotransmitter  Glutamate Principle excitatory neurotransmitter in the vertebrate nervous system Activation of postsynaptic ionotropic glutamate receptors, causes a transient opening of ion channels allowing net influx of cations, generating an excitatory current Metabotropic glutamate receptors play a modulatory role in synaptic transmission Glutamate plays a role in learning, memory etc. as well as various disorders (e.g. epilepsy, schizophrenia, brain damage etc.) Glutamate Glutamatergic synapses  Basic Synapse Presynaptic Axon Mitochondrion Synaptic Vesicle Synaptic Cleft Neurotransmitter Neurotransmitter Receptor Postsynaptic Cell Glutamatergic synapses - location 1. When the postsynaptic neuron is excitatory, the glutamatergic synapse is usually found on spine or dendritic shaft of the excitatory cell (e.g. hippocampal pyramidal cell). 2. When the postsynaptic neurons is inhibitory , the glutamatergic synapse is usually found on the soma or dendritic shaft of the inhibitory cell (e.g. parvalbumin-positive interneuron). Dendritic Spines * Dendrites of excitatory neurons often possess spines*, which typically receive synaptic inputs from presynaptic axons. Spines are dynamic, plastic, changeable. Glutamate receptors  Glutamate Receptors 1. Ionotropic receptors (iGluR) = Fast transmission, Ions flow in/out of neuron, Millisecond responses. botropic receptors (mGluR) = Slow synaptic transmission, Activation messenger cascades, Seconds for responses to arise 1. 2. Ionotropic Glutamate Receptors iGluRs  A M P A R K A I N A T E R N M D A R Protein Gene Old Nomenclature  Localisation of ionotropic GluRs (iGluRs) AMPARs and NMDARs are most of the time co-localised at glutamatergic synapses where they mediate ‘fast’ chemical synaptic transmission NMDARs, AMPARs and KainateRs can be both synaptic and extrasynaptic NMDARs, AMPARs and KainateRs can be both pre-synaptic (autoreceptors) and post- synaptic  AMPA receptors a-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptors  AMPA receptors AMPARs open and close quickly, and are thus responsible for most of the fast excitatory synaptic transmission in the central nervous system AMPARs are permeable to sodium, potassium, and some of them to calcium They are formed of a combination of 4 subunits, GluA1,GluA2, GluA3 and GluA4 AMPAR activity can be regulated by second messenger cascades of PKA, PKC, CaMKII and other kinases NMDA receptors N-methyl D-aspartate receptors  NMDA receptors NMDA (N-methyl D-aspartate) receptors are named after the selective specific agonist (NMDA) NMDARs are tetramers and assembled from a choice of 3 types of subunits NMDARs have slow activation and deactivation kinetics Activation requires binding of glutamate and the co- agonist glycine NMDAR allows the entry of calcium into the neuron, as well as sodium and potassium NMDARs are ligand & voltage sensitive receptors  NMDARs are ligand & voltage sensitive receptors At resting membrane potential NMDARs carry little current, as magnesium ions block permeability to cations The magnesium block is relieved by depolarisation - reason for slow activation and depolarisation Coincident Detectors To function, NMDARs require: Glutamate, Glycine, and a depolarised membrane potential - NMDARs are active upon sustained Act to reinforce a glutamate response stimulation - Calcium flux through NMDARs is thought to play a critical role in synaptic plasticity - the cellular mechanism for learning and memory  Kinetics of NMDA & AMPA Receptors Non-NMDA receptors (AMPA and measure of synaptic current Kainate receptors) generate the Normal large and early component of synaptic EPSCs, whereas NMDA receptors contribute to the late component of response to the EPSCs glutamate AMPA Receptors NMDA Receptors Fast Kinetics Slow Kinetics application Glutamate + NMDAR antagonist (e.g. D-AP5) = fast 1. Na+ 2. Na+ synaptic NMDARs have a slower transmissio AMPARs open first opening and closing time n + AMPAR 1. antagonist (e.g. EPSC 2. SYM2206) = slow synaptic transmission Time (ms)  Metabotropic Glutamate receptors mGluRs  Metabotropic Glutamate Receptors Metabotropic glutamate receptors (mGluRs) are single polypeptide chain proteins that bind glutamate They have 7 transmembrane domains, with an intracellular C- terminus and extracellular N- terminus. mGluRs link glutamate binding to the activation of G-proteins mediated signalling cascades There are 8 subtypes of mGluRs that embody 3 distinct functional groups Group I mGluR pathways1) Glutamate binds to the group I receptor, which is coupled to PLC 2) Cleaves PIP2 into IP3 and DAG 3) IP3 diffuses to the cytoplasm and binds to ER which releases Ca2+ PLC = phospholipase C PIP2 = phosphatidylinositol-4,5- Increase synaptic biphosphate function IP3 = inositol-1,4,5-triphosphate Decrease synaptic DAG = diacylglycerol function Effects of activating Group I mGluRs Calcium release from stores = Calcium is implicated in synaptic plasticity. E.g. calcium activates calmodulin causing synapse remodelling and synaptic vesicles mobilization. PKC increase = phosphorylates a huge range of synaptic proteins, and enhances NMDAR activity. Homer protein released = Important for trafficking mGluRs into and out of synapses and functionally connect mGluRs to iGluRs Inhibition of K+ conductances = Increasing the excitability of neurons Less negative membrane potential = Increases the probablilty of action potentials arising in a neuron  Group II & III mGluRs Group II/III = Mainly located on presynaptic terminals where they act as autoreceptors They reduce the amount of glutamate release from terminal Linked to Gi/o proteins Decrease synaptic function  mGluR group II/III properties Excitotoxicity refers to the ability of glutamate and related compounds to destroy neurons by prolonged excitatory synaptic transmission (e.g. trauma or hypoxia-ischaemia). If abnormally high concentrations of glutamate accumulate in the cleft, the excessive activation of neuronal glutamate receptors can literally excite neurons to death. It is generally suggested that activation of groups II and III mGluRs have neuroprotective action by reducing presynaptic activity. iGluRs vs mGluRs summary 1) mGluR activation has a much slower onset and is longer lasting than iGluR responses 2) iGluRs operate as on/off switches at discrete regions in the neurons, whereas mGluR effects are seen throughout neurons, activating a range of channels and 2nd messenger systems 3) mGluRs modulate synaptic activity, by both increasing and decreasing channel opening in presynaptic neurons  Structure AMPAR NMDAR AMPA-R NMDA-R  Function Role in Synaptic Function and Plasticity The Hebbian Synapse A B From The Organization of Behavior by Donald Hebb, 1949: “When an axon of cell A is near enough to excite 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 A's efficiency, as one of the cells firing B, is increased.” I.e. as Carla Shatz famously coined: “neurons that fire together, wire together.” Hebb postulated that this behaviour of synapses in neuronal networks would permit the networks to store memories. The synapse is strengthened by repeated activity (e.g., experience, learning or electrical stimulation). The activity may be adaptive (i.e. improved function) or maladaptive (i.e. impaired function) A Hebbian synapse is a “coincidence detector” NMDA receptors, back-propagating action potentials, and summation of EPSPs appear to be the components that confer “Hebbian” behaviour on the synapse.  PLASTICITY is the ability to change and occurs on many interdependent levels Plasticity. Behavioural plasticity is the product of network plasticity; networks are comprised of circuits. Plasticity of circuits is caused by neuronal plasticity including neurogenesis, dendritic branching, and dendritic spine growth. At the centre of it all is synaptic plasticity, or the change in strength of connection between axon terminal and dendritic spine. Synaptic plasticity directly underlies macroscopic changes and triggers signalling molecules to alter mRNA and protein production. Modified from: Brown et al. https://www.neuromodulationjournal.org/article/S1094- 7159(21)06173-0/fulltext  Plasticity Nervous System is plastic – learning occurs – Structural changes at synapses Changes in synaptic efficiency, e.g. – Long-term potentiation (LTP) – Long-term depression (LTD) – Other forms LTP & LTD throughout brain – Many different mechanisms ~ Hippocampus The Hippocampus - a Key Region for Memory and Learning Long-Term Potentiation Occurs in the Hippocampus CA3 - CA1 LTP NMDA-Dependent Long-Term Potentiation in the Hippocampus Record Stim.  Synaptic Plasticity Can Be Measured in the Hippocampal Circuits LTP occurs at several sites in the hippocampal formation The CA1 region has two main kinds of ionotropic glutamate receptors: NMDA receptors (after its selective ligand, N-methyl-D-aspartate) AMPA receptors (after its selective ligand, AMPA) Glutamate first activates AMPARs. NMDARs do not respond until enough AMPARs are stimulated, and the neuron is partially depolarized. − When depolarization reaches a threshold, Mg++ moves − Ca++ can then move through the NMDARs Roles of the NMDA and AMPA Receptors in the Induction of LTP in the CA1 Region  Ca2+-mediated Effects Activation of protein kinases: – Protein Kinase C (PKC) – Ca2+/calmodulin-dependent protein kinase (CaMKaII) – Kinases’ Targets: AMPAR & other signaling proteins CaMKaII important role: – Block CaMKaII  No LTP – Self-phosphorylation  LTP duration CaMKaII is activated by acutely high levels of Ca2+/CaM (Calmodulin) complexes due to the NMDAR activation and opening. Phosphorylation by CaMaKII causes conformational changes in AMPARs. The new conformation opens the pore to let more Na + in. This triggers a series of complex reactions leading to delivery and insertion of AMPA receptors into the synapse. Steps in the Neurochemical Cascade during the Induction of LTP LTP will lead to new synaptic contacts THER FORMS OF SYNAPTIC PLASTICIT Guided self-directed study Distinct Molecular Bases of LTP at Three Synapses in Hippocampus  LTP AT SCHAFFER COLLATERAL (CA3)-CA1 SYNAPSES IS DUE TO POSTSYNAPTIC CALCIUM INFLUX AND CaM KINASE ACTIVITY P at CA3-CA1 synapse is blocked by MDAR antagonist APV and by inhibitors CaM kinase  LTP AT MOSSY FIBER (DG)--CA3 SYNAPSES IS DUE TO PRESYNAPTIC CALCIUM INFLUX AND cAMP/PKA PATHWAY Bidirectional synaptic plasticity in the hippocampus  Two important types of synaptic plasticity ： Long-term potentiation (LTP ) (Bliss & Lomo, 1973); Long-term depression (LTD ) (Dudeck & Bear, 1992) They are two potential mechanisms that underlie learning and memory  Two cellular processes underlie the major changes during LTP and LTD LTP – AMPARs insertion into the postsynaptic membrane or LTD - AMPARs removal from the postsynaptic membrane Growth or shrinkage of the spine via reshaping of the actin cytoskeleton. Changes in synaptic structure (e.g. PSD size)  Models of Learning Neural basis of memory: Learning and memory can result from modifications of synaptic transmission – Synaptic modifications can be triggered by conversion of neural activity into intracellular second messengers – Memories can result from alterations in existing synaptic proteins – Synaptic Plasticity in the Hippocampus (LTP; LTD) Spatial learning is impaired by block of NMDA receptors (Morris, 1989) platform Morris water maze rat  Examples of Forms of Long Term Synaptic Plasticity Guided self-directed study I. Frequency-dependent Long-term Potentiation (LTP) II. Frequency-dependent Long-term Depression (LTD) III. Spike-timing dependent synaptic plasticity (STDP)  Examples of Forms of Short Term Synaptic Plasticity Guided self-directed study I. Paired pulse facilitation II. Short-term Synaptic Depression III. Post-tetanic Potentiation IV. Spike-timing dependent synaptic plasticity (STDP) Presynaptic vs. Postsynaptic Mechanisms Guided self-directed study I. The size of synaptic potentials can be modulated: A. by regulating the number of vesicles (quanta) released (Presyn.) B. by regulating the size of the current generated by a released quantum at the postsynaptic membrane. II. Short term modulation (Duration: ms - min) A. The mechanisms of these forms of modulation are almost always presynaptic. B. Paired-pulse facilitation (~10 to 100 ms) C. Synaptic depression (50 ms to min) D. Post-tetanic potentiation (min) III. Long-term plasticity (Duration: min - hr) A. The mechanisms of these forms of modulation are usually both pre- and postsynaptic B. LTP (30 min to hr) C. LTD (min to hr) General references: Bear et al. Neuroscience: Exploring the Brain 4th Ed. Kandel et al. Principles of Neural Science 5th Ed. Squire et al. Fundamental Neuroscience 4th Ed. Equivalent Chapters in New Editions