Biology for Engineers: Cellular and Systems Neurophysiology Christopher Fiorillo BiS 521, Fall , Part 4: Synaptic Transmission Reading: Bear, Connors, and Paradiso Chapter 5

A Single Neuron with Synapses in Yellow

Types of Synaptic Contacts –Axodendritic: Axon to dendrite –Axosomatic: Axon to cell body –Axoaxonic: Axon to axon –Dendrodendritic: Dendrite to dendrite Synapses Are Physical Contacts between Neurons that Enable Fast Transmission of Information

Two Types of Synaptic Transmission Chemical Transmission –1921- Otto Loewi Electrical Transmission –1959- Furshpan and Potter There was a long-lasting debate about whether transmission was chemical or electrical. Both occur, but chemical transmission is much more common.

Direction of Information Flow Information usually flows in one direction –First neuron = Presynaptic neuron –Target cell = Postsynaptic neuron Postsynaptic neuron Presynaptic neuron

Gap junction are large channels –Large enough (1-2 nm) to allow all ions plus other small molecules to pass –A Connexon spans the membrane - formed by six connexin proteins Cells are said to be “electrically coupled” –Flow of ions from cytoplasm to cytoplasm Electrical Synapses Are Composed of Gap Junctions

Electrical Synapses Very fast transmission –Chemical transmission has a delay Postsynaptic potentials (PSPs) have the same form as the presynaptic potential, but are smaller Most electrical synapses are bidirectional, but some are unidirectional

A Chemical Synapse The synaptic cleft is a nm gap between the presynaptic terminal and the postsynpatic membrane Neurotransmitter is released into the cleft and activates postsynaptic receptors

Electron Micrograph of a Chemical Synapse Synaptic Vesicles –Made of phospholipid membrane –50 nm in diameter –Filled with molecules of neurotransmitter Dense-core Vesicles –Contains peptide neurotransmitters Vesicles release neurotransmitter when they fuse with the presynaptic membrane

CNS Synapses (Examples) –Gray’s Type I: Asymmetrical, usually excitatory –Gray’s Type II: Symmetrical, usually inhibitory Two Synaptic Morphologies

Larger synapses allow the presynaptic neuron to have a larger and more reliable effect on the postsynaptic neuron Synapses Vary in Size and Strength

Neurotransmitter Synthesis and Storage –Small neurotransmitters (amines, amino acids) Synthesized in vesicles within terminal –Peptides Synthesized within soma and transported to terminal

Recording Methods: Current and Voltage Clamp “Current Clamp” –Measures voltage across cell’s membrane –A constant current is injected through the electrode The current can be manually adjusted by the experimenter “Voltage Clamp” –Measures current passing through the cell’s membrane –Clamps voltage across the cell’s membrane A feedback circuit calculates, and injects through the electrode, the amount of current that is necessary to keep the voltage constant (This slide and the next one should have been presented in the last section on “membrane voltage.”)

Current versus Voltage Clamp: which is appropriate? Advantages of current clamp –More physiological. Voltage is never clamped under physiological conditions –Fast and accurate voltage clamp can be difficult to achieve Especially in the dendrites of a large neuron Advantages of voltage clamp –Greater experimental control: it eliminates voltage as a variable Keeps driving force constant Better for studying voltage-gated channels Better temporal resolution for fast channels, because it removes the effect of membrane capacitance In general, voltage-clamp is better for studying the properties of ion channels. Current clamp is better for studying the properties of neurons.

Basic Steps of Chemical Synaptic Transmission –Action potential invades synaptic terminal –Depolarization-activated Ca2+ channels open –Ca2+ triggers vesicles to fuse into membrane of presynaptic terminal (exocytosis) –Neurotransmitter spills into synaptic cleft –Binds to postsynaptic receptors –Biochemical/Electrical response elicited in postsynaptic cell –Removal of neurotransmitter from synaptic cleft –New vesicles formed by endocytosis –Vesicles are filled with neurotransmitter and prepared for release

Removal of neurotransmitter is important in order to limit the duration of postsynaptic stimulation. This enables high frequencies of information transmission Three Mechanisms –Diffusion –Reuptake: Transporters bind neurotransmitter and transport it to inside of presynaptic terminal This is the most important mechanism for removing neurotransmitters Cocaine and Prozac (fluoxetine) block reuptake of dopamine and serotonin –Enzymatic destruction in synaptic cleft Acetylcholineesterase eliminates acetylcholine. It is the only example of this method. Removal of Neurotransmitter from the Synaptic Cleft

A action potential causes the release of a discrete number of vesicles (or quanta) –Neuromuscular junction: About 200 synaptic vesicles, EPSP of 40mV or more –CNS synapse: Single vesicle, EPSP of few tenths of a millivolt Each vesicle contains about the same amount of neurotransmitter –Quantal content (the amount of transmitter per vesicle) is not a physiologically important variable Spontaneous release of a single vesicle causes a miniature postsynaptic potential (current) –Often called a “mini” Neurotransmitter Release is Quantal

Studies of NMJ established principles of synaptic transmission Synapses between neurons are very similar to NMJ The Neuromuscular Junction

Miniature Postsynaptic Currents Are Caused by Release of a Single Vesicle “Minis” (mEPSCs and mIPSCs) are caused by spontaneous release of a single vesicle in the absence of a presynaptic action potential Minis can be calcium-dependent or independent Time course of mPSCs are identical to PSCs ~3 ms for EPSC ~30 ms for IPSC Amplitude of mPSC depends on postysynaptic receptors vesicles all contain the same amount of transmitter, which can saturate postsynaptic receptors Frequency of mPSCs depends on presynaptic factors At most synapses, < 0.01 mPSC / second At some synapses, > 0.1 mPSC / second Glutamate EPSC

Release Probability Not every action potential evokes vesicle release Release probability (P r ) given action potential Some synapses release multiple vesicles, but most release just 0 or 1 vesicle P r depends primarily on calcium concentration in terminal’s cytosol, which depends on: –Action potential –Recent history of action potentials –Activation of neurotransmitter receptors on synaptic terminal P r varies from one synapse to another, as shown in the histogram at right. A typical value is 0.3

Paired-Pulse Depression and Facilitation PPD and PPF at 3 synapses. 10 stimuli at 50 Hz (20 ms intervals) Presynaptic [Ca2+] at PF synapse PPD and PPF are universal features of synapses. Some synapses show PPD, some show PPF, and some show both –All synapses may have multiple mechanisms mediating both depression and facilitation PPD and PPF are caused primarily by a decrease or increase, respectively, in vesicle release probability Electrical stimuli (each lasting about 0.2 ms) are applied to a brain slice maintained in vitro. This evokes postsynaptic potentials (or currents, if measured in voltage clamp). –Excitatory Postsynaptic Potential (Current): EPSP (EPSC) –Inhibitory Postsynaptic Potential (Current): IPSP (IPSC) Each stimulus evokes action potentials in many axons, and it therefore causes vesicle release from many terminals –A PSP (PSC) is caused by release of multiple vesicles (quanta) But if a low stimulation current is used, it is possible to stimulate only a single axon, and that axon may have only one release site. In this case, some stimuli may not release any vesicles. The amplitude of a PSP (PSC) depends on the release probability at stimulated synapses

Causes of Synaptic Depression and Facilitation PPD and PPF at 3 synapses. 10 stimuli at 50 Hz (20 ms intervals) Presynaptic [Ca2+] at PF synapse The most common cause of facilitation is an increased calcium concentration –This is due primarily to the fact that calcium is cleared slowly after an action potential The most common cause of depression is a loss of “docked” (releasable) vesicles –Most vesicles in the terminal are “undocked,” meaning that they are not close to the membrane and bound to the vesicle-release machinery –There may be just one docked vesicle. Once it is released, it takes time for another vesicle to be docked and ready to release. –The rate of recovery from depression (docing of vesicles) is increased by calcium There are many ways in which release probability might be modified –Changes in membrane voltage –Changes in the properties of ion channels, particularly calcium channels, that are activated during the action potential –Modification of proteins involved in vesicle release There are probably multiple depressing and facilitating processes happening simultaneously at each synapse.

Modulation of Release Probability by Presynaptic Neurotransmitter Receptors Neurotransmitter receptors on presynaptic terminals act to augment or suppress release probability –These receptors therefore alter PPD or PPF Many receptors suppress vesicle release, including “autoreceptors” –Suppression often occurs through inhibition of Ca2+ channels and activation of K+ channels Presynaptic [Ca2+] at PF synapse is suppressed by cannabinoid receptor activation Suppression of glutamate EPSCs by adenosine receptors

How can we know whether a change in amplitude of a synaptic potential is pre- or postsynaptic? Two Easy Tests: –Paired-pulse ratio (PPF or PPD) A change suggests a presynaptic effect No change suggests a postsynaptic effect –Minis A change in frequency suggests a presynaptic effect A change in amplitude suggests a postsynaptic effect These tests are not definitive; there are exceptions to these rules Suppression of glutamate EPSCs by adenosine receptors

Analogies between Presynaptic Terminals and Somatodendritic Compartment 1.Output: quantal vesicle release, usually 0 or 1 2.Integration medium: [Ca2+] 3.Imaginary quantity: “Release Probability” 4.Spontaneous release 5.Inputs: action potential, synaptic neurotransmission, voltage- regulated ion channels 1.Output: All-or-none action potential 2.Integration medium: membrane potential 3.Imaginary quantity: “Instantaneous Firing Rate” 4.Spontaneous action potentials 5.Inputs: synaptic neurotransmission, voltage-regulated ion channels Synaptic Terminal Somatodendritic Compartment

Synaptic Integration Synaptic Integration: The process by which multiple synaptic potentials sum together within one postsynaptic neuron This occurs in the dendrites and soma The “decision” point in most neurons is the axon hillock, where the neuron “decides” whether to emit an action potential

Inhibition –Takes membrane potential away from action potential threshold –Excitatory vs. inhibitory synapses: Bind different neurotransmitters, allow different ions to pass through channels Most synaptic inhibition is mediated by GABA-gated Cl- channels E Cl- is -65 mV If membrane potential is less negative than -65mV, GABA mediates hyperpolarizing IPSP. Two Mechanisms of Inhibition: –Hyperpolarization –Shunting Inhibition: Inhibiting current flow from dendrites and soma to axon hillock Synaptic Inhibition

Shunting Inhibition: Inhibiting current flow to axon hillock Increasing membrane conductance will decrease membrane space constant Therefore, opening any channel will cause an EPSP to decay over a shorter distance This is called “shunting” inhibition. It prevents depolarizing current from reaching the axon hillock and eliciting an action potential. By opening Cl- channels, GABA can cause a shunting inhibition even if it causes a depolarization towards E Cl.

A Single Neuron with Synapses in Yellow

Synaptic Plasticity The strength of a synapse can change; it is “plastic” –A neuron can therefore select its own synapses Synaptic plasticity is thought to be the main mechanism of learning and memory Synaptic plasticity has probably been the most popular topic in neuroscience for the last years Synaptic plasticity plays a critical and necessary role in many computational neuroscience models and in all artificial neural networks We will cover synaptic plasticity from both computational and mechanistic perspectives in future lectures