Synaptic Transmission and Plasticity (BIOL2052) PDF

Synaptic integration and synaptic plasticity Lectures Dr. Mariana Vargas-Caballero  Methods used to record neuronal activity Extracellular Intracellular Voltage recording or Current clamp Current recording or Voltage Let’s picture some electrophysiological equi...

Synaptic integration and synaptic plasticity Lectures Dr. Mariana Vargas-Caballero  Methods used to record neuronal activity Extracellular Intracellular Voltage recording or Current clamp Current recording or Voltage Let’s picture some electrophysiological equipment  Shared property: Neurons generate and transmit electrical impulses. This enables functions such as sensory perception, motor control, and information processing within the nervous system. Frog nerve Rat neurons in culture Squid giant https://elifesciences.org/articles/ axon Human 32–35 ˚C mV 41714.pdf https://www.researchgate.net/publication/283682257_Biophysics_of_Extracellular_Spikes ms Learning outcomes Lecture 1 Describe passive and active electrical properties of neurons. Compare and evaluate different methods used to measure voltage and current changes in neurons. Describe the mechanisms underlying postsynaptic responses. Lecture 2 Explain how neurons integrate excitatory and inhibitory inputs. Explain synaptic plasticity and some of its key mechanisms.  Neurons have both passive and active electrical properties  Passive electrical properties of cells Can be mimicked with capacitors/resistors (electrical components) Faraday cage Bath Pipette holder Capacitor and resistor in parallel  Ohms law The electrical circuit will consist of three related electrical variables called: Voltage, ( V, in volts ), Current, ( I, in amperes ) and Resistance, (R, in ohms Ω ) or conductance (G, in siemens = 1/R). V = IR V Voltage step from 0 to 100 pA R = V/I R I 250 MΩ Voltage response reaches steady state at 25 mV  Passive and active electrical properties of neurons Voltage steps from zero to: -150, -100, 50, 0, 50, and 100 pA Passive Membrane Properties time constant – τ (tau) length constant – λ (lambda) Time The voltage-effect of a membrane current takes time due to membrane capacitance (time cosntant) The voltage effect of a membrane current decreases with distance from the injection site (length constant)) The voltage change as a response to current injection depends on the cells resistance R (input resistance) = V/I Purves et al  How do these properties impact actual neuronal function? Example 1. Dentate gyrus: old vs new neurons https://www.biorender.com/template/adult-neurogenesis-in-the-dentate-gyrus Example 1. Dentate gyrus: old vs new neurons New neuron Small High input resistance Low Capacitance Old neuron Low input resistance High Capacitance Passive and active electrical properties of neurons Passive properties + active properties: Capacitor and resistor in parallel: passive properties To understand more, we can go inside a simulator https://www.youtube.com/watch?v=YgJ5ZEn67tk  How can we dissect the contribution of different currents to neuronal activity? Voltage clamp Injects the necessary current to keep cell at desired potential https://nerve.bsd.uchicago.edu/nervejs/ MAP1.html By smonsays - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php? curid=76499842  How can we dissect the contribution of different currents to neuronal activity? Ion channel blockers/ inhibitors Allow separation of currents pharmacologically 4-AP and TEA block K+ TTX blocks Na+ currents, currents, allow isolation of allows isolation of K+ delay-rectifier Na + current current  Sir John Carew Eccles, Alan Lloyd Hodgkin and Andrew Fielding Huxley 1963 Nobel Prize in Physiology or Medicine "for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane" Ionic basis of action potential determined using VOLTAGE CLAMP The membrane current can be separated into its components These individual currents can be used to calculate how the membrane current or potential change in response to a stimulus Seminal Paper: A Quantitative Description of Membrane Current and its Applicaton to Conduction and Excitation in Nerve, J. Physiol. (I952) 117, 500- 544 Passive and active electrical properties of neurons Passive properties + active properties: Capacitor and resistor in parallel: passive properties  Active membrane properties Slow spiking neuron Active Slow spiking neuron + Kv3.4 Guo et al 2018 DOI: 10.1016/j.isci.2018.10.014 Delayed rectifier Kv1 There are multiple types of potassium channels wit h very different properties (check link) Kv3.4 Inactivating e know which ion channels are present in a neuron? siolgy, mRNA amplification (single cell) or both (see Baranauskas et al 2003 https://www.nature.com/article https://learning.edx.org/course/course-v1:EPFLx+SimNeuroX+3T2021/ Neuronal subtypes are characterized by specific gene expression patterns. The expression of those genes into proteins (+ interaction with their microenvironment) gives them their  Propagation of signals Passive and active electrical properties of neurons https://www.youtube.com/watch?v=2_MIjvwWsrg Passive propagation of signals is also known as: Cable properties or Electrotonic propagation of signals.  Action potential threshold Image modified from: https://commons.wikimedia.org/wiki/File:Anatomy_of_a_Neuron_with_Synapse.png So far: Passive electrical properties of neurons: electrical circuits, shape, size. The active properties of neurons depend on the type of ion channels they express and the specific localisation of these within the cell. We can measure membrane voltage or current with different methods: voltage clamp (and pharmacology) allows dissection of currents that contribute to activity. Let’s now discuss: Many postsynaptic receptors are ion channels and their activation can directly impact membrane potential. Studying the function and activation of Ion channels 10.3389/fchem.2016.00040  Single electrode voltage clamp 10 MW GW 100 MW - GW 400pA 12.5MW V = RI  Patch clamp methods Clean glass Air pressure Membrane High resistance seal Gigaohm Voltage recording or Current clamp Current recording or Voltage https://www.leica-microsystems.com/science-lab/life-science/the-patch- Clamp what? … Patch clamp A technique to make a high resistance seal between glass and bilipid membrane. Can be used to measure voltage or current. None of these: To measure individual currents we need to clamp the voltage. We can record voltage or current from a cell with patch clamp Using voltage clamp with amplifier allows measurement of currents (how much current do I need to inject to keep this cell at the desired voltage). Y axis in pA To measure the membrane voltage one uses a different mode in the amplifier some times called bridge mode or current clamp (I-clamp). Y axis in mV  Ion current plotting convention inward Entry of positive current into the cell eg. AMPA or  depolarises the cell (Vm more (glutamate positive) ) receptors opening at Exit of negative current from the cell rest -70mV outward Entry of negative current into the cell eg. GABAA or  hyperpolarises the cell (Vm more receptors negative) opening at -50mV  Recall the concept of equilibrium potential to discuss how ligand-gated ion channels encode postsynaptic electric signals. Permeability is dictated by ion channel molecule: Glutamate-activated ion channels (iGLuRs) are selective cation channels. Similar concentration 0 mV of cations in/out the cell. Reversal potential for cation channels in neurons is approx… When they open, the cell depolarises. By bringing the cell close to threshold they have an Driving force  Reversal potential Driving force Ecations Vm – Ecations ECl Vm – ECl The current carried by an ion is determined by its conductance multiplied by the driving force for that ion. The calculations of current carried by each ion are key terms of Hodgkin and Huxley equations Remember the membrane potential is always changing: neurons are dynamic.  Synaptic integration and synaptic plasticity  Integration of excitatory and inhibitory inputs. Neurons received multiple synaptic inputs The resulting currents are summed by the cell, integrating the input information. The net effect of these inputs modifies the output of the neuron  Dendritic spines Synapse observed with transmission electron microscopy Image Vargas-Caballero lab, obtained by Pippa Richardson and Anton Page Image modified from: https://commons.wikimedia.org/wiki/File:Anatomy_of_a_Neuron_with_Synapse.png  Many neurotransmitter receptors are ion channels AMPA receptors: NMDA receptors: Glu Glu These recordings were obtained in a solution with 0 Mg To observe the behaviour of NMDA receptors at -60 mV NOT physiological https://www.jneurosci.org/content/20/6/2073 https://doi.org/10.1016/S0006-3495(91)8  We can distinguish the single NMDA receptors ion channels in these synaptic currents Mg2+-free 10 uM Mg2+ Robinson and Kawai 1991 1mM (1000 uM) Mg2+ ese recordings were obtained in a solution with 0 Mg2+ observe the behaviour of NMDA receptors at -60 mV NOT physiological The inhibitory transmitter GABA also activates ion channels (GABAA receptors, selective for Cl-) https://www.jneurosci.org/content/19/8/2960 NMDAR act as coincidence detectors 41 Gly Glu Glu Glu Gly Glu Glu Glu Mg2+ Mg2+ Mg2+ Na + Ca2 + K NMDA R AMPA They only open when there is glutamate R release and postsynaptic depolarisation Mg2+ block makes NMDA receptors voltage dependent. NMDAR are Ca2+ permeable https://www.jneurosci.org/content/17/1/204  Henry Markram et al. Science 1997;275:213-215 Published by AAAS Repetitive activation of AMPA and NMDA receptors results in synaptic plasticity Gly Glu Glu Glu Mg2+ Mg2+ Calcium flux through NMDARs initiates a series of events that results in more AMPA receptors at the synaptic membrane. This type of plasticity is observed in CA3- CA1 hippocampal synapses and in many cortical synapses. 44  Synaptic plasticity often accompanied by structural remodeling and actual spine growth. Ras, a small GTP-binding protein, is an important component of signal transduction pathways (eg. downstream of Ca2+ signalling).  Our cerebral cortex is responsible for the integration Cortical integration of most sensory signals, mediates cognitive processes, and is likely the substrate of our conscious experience.  Signal integration an example. Back-propagating action potential activated Ca2+ spike firing (BAC firing) Larkum and Sakmann 1999 https://www.nature.com/articles/18686  Temporal and spatial summation An example that puts it all together: Action potentials can back propagate into dendritic arbours of cortical neurons. This enhances the integrative properties of neurons and offers the possibility of coincidence integration and plasticity Larkum et al 1999 events. https://www.nature.com/articles/18686 GABAA receptors are selective for Cl- Actions of GABAA in neurons Early in development, chloride concentrations are higher inside neurons (as neurons mature KCC2 symporter is expressed andExtracellular lower Cl concentration 110 intracellular [Cl] is achieved) mM Intracellular E Cl Cl- concentration ECl ? Young 30 mM Old 5 mM Postnatal brain: GABAA actions … More mature brain: GABAA …  In vivo recording of a In vitro recording cortical neuron in a mouse cortex (whisker sensory input) DOI: 10.1016/j.neuron.2007.09.017 https://www.sciencedirect.com/science/article/pii/  Integration of synaptic inputs in cortical neurons Larkum and Sakmann 1999 https://www.nature.com/articles/18686  Signal integration in cortical neurons A single back-propagating sodium action potential generated in the axon facilitates the initiation of calcium action potentials when it coincides with distal dendritic input within a time window of several milliseconds. Calcium spikes in the dendrite (downstream signaling - plasticity) Bursts of axonal action potentials (strengthen output) Inhibitory dendritic input can selectively block the initiation of dendritic calcium action potentials, preventing bursts of axonal action potentials. Mechanism by which the main cortical output neurons can 1999 doi: 10.1038/18686 associate inputs arriving at different cortical layers. A new cellular mechanism for coupling inputs arriving at different cortical layer Larkum and Sakmann Additional slides provided for your own study Use the Nernst equation intuitively. Can you fill this table without a using a calculator? ION INSIDE OUTSIDE Equilibrium potential K+ 100 mM 10 mM Na+ 10 mM 100 mM Cl- 10 mM 100 mM All positive 110 mM 110 mM ions Glossary Membrane potential is the electrical potential difference across the plasma membrane, which results from the separation of charges (ions) on either side of the membrane. This potential arises because of differences in ion concentrations and the selective permeability of the membrane to different ions. Energy dependent mechanisms required to keep these ion concentration differences. Recall: Na/K pump, KCC2 (Cl-), NCX3 (Na+/Ca2+) transporter). When allowed to move between compartments, the flow of electrical ion charges down their electrochemical gradient produces an electrical current (In neurons synaptic currents are in the order of pA). Capacitance the ability of a system to store an electric charge (phospholipid membranes act as capacitors with a relatively constant capacitance per area) (1mF/cm2).  Ions are atom or molecules with a net electric charge (Physiological: K+, Na+, Ca2+, Mg2+,Cl-,HCO3- , experimental: Cs+, F+, Ba2+). A resistance is the restriction to ion flow expressed in Ohms (impermeable plasma membrane or physical obstacles at intracellular and extracellular spaces). The counterpart of resistance is conductance (G) expressed in Siemens (e.g. ion channel opening, the size of the pore in relation to permeant ions will determine the conductance (eg. NMDAR channels ~50pS). I V = I/G I = VG V G  Example 1. Dentate gyrus: old vs new neurons New neuron Small High resistance Low Capacitance Old neuron Low resistance High Capacitance 1 synaptic receptor AMPA type 10pS conductance 10 pA: 14 synaptic receptors 200 pA: 286 synaptic receptors The workings are mostly correct but there is an error in the answer by ChatGTP can you spot it? Continues in next page… Starts in previous page… The workings are mostly correct but there is an error in the answer by ChatGTP can you spot it?  Can you solve now for 144 megaohm resistance i.e. old granule cell?: Question: What change in voltage would occur in a cell with 144 X 106 W resistance if at -70 mV a 10 pS conductance with reversal potential of 0 mV opens. How many AMPA receptors could need to be active to depolarise the dendrite by 20 mV?