1 Bi / CNS 150 Lecture 11 Synaptic inhibition; cable properties of neurons Wednesday, October 15, 2013 Bruce Cohen Chapter 2 (p ); Chapter 10 ( )  -Aminobutyric acid (GABA)

2 Crystal structure of GABA A Receptor (Miller et al, 2014) Homomer of 5 GABA A β3 subunits Overall structure similar to nicotinic receptor Positive charges in vestibule contribute to anion selectivity

The pentameric GABA A and glycine receptors resemble ACh receptors; but they are permeable to anions (mostly Cl -, of course) 1.  -amino-butyric acid (GABA) is the principal inhibitory transmitter in the brain. 2. Glycine is the dominant inhibitory transmitter in the spinal cord & hindbrain. GABA A receptors are more variable than glycine receptors in subunit composition and therefore in kinetic behavior... Cation channels become anion channels with only one amino acid change per subunit, in this approximate location

A Synapse “pushes” the Membrane Potential Toward the Reversal Potential (E rev ) for the synaptic Channels 4 At E rev, the current through open receptors is zero. Positive to E rev, current flows outward Negative to E rev, current flows inward ACh and glutamate receptors flux Na + and K +, (and in some cases Ca 2+ ), and E rev ~ 0 mV Membrane potential Resting potential E K E Na At GABA A and glycine receptors, E rev is near E Cl ~ -70 mV Like Figure 10-11

5 Benzodiazepines (= BZ below): Valium (diazepam), (Ambien, Lunesta are derivatives) Pharmacology of GABA A Receptors: activators phenobarbital site is unceratin The natural ligand binds at subunit interfeces (like ACh at ACh receptors)

(Miller et al, 2014 Nature) 6 The GABA agonist benzamidine also makes cation-  interactions with its binding site

7 GABA A and Glycine blockers bind either at the agonist site or in the channel Agonist site Picrotoxin (GABA A & glycine receptors) Strychnine (glycine receptor) Bicuculline (GABA A receptor

8 Like Figure 2-1 (rotated) Parts of two generalized CNS neurons synaptic cleft direction of information flow apical dendrites Excitatory terminal cell body (soma) nucleus axon presynaptic terminal postsynaptic dendrite Inhibitory terminal presynaptic terminal axon hillock neuron Presynaptic neuron Postsynaptic basal dendrites initial segment node of Ranvier myelin (apex) (base) little hill

9 Types of synapses Synapses can form on dendrites, axons, soma, and even the presynaptic teriminals Two types of dendritic synapses are shown, one on spines, the other on the shaft Type 1 synapses tend to be excitatory Type 2 tend to be inhibitory Figure 10-3

10 Concentration of acetylcholine at NMJ (because of acetylcholinesterase, turnover time ~ 100 μs) Number of open channels ms 0 high closed open State 1State 2 k 21 all molecules begin here at t= 0 units: s -1 Duration of postsynptic current

11 How are glutamate & GABA cleared from the synapse? Na + -coupled transporters for glutamate & GABA are present at densities of > 1000 / μm 2 near each synapse This density is probably high enough to sequester each transmitter molecule as it leaves a receptor At the nerve-muscle synapse, acetylcholinesterase is present at densities of > 1000 / μm 2 near each synapse This density is high enough to destroy each transmitter molecule as it leaves a receptor

12 1. Temporal 2. Spatial 3. Excitatory-inhibitory Types of synaptic integration

13 Membrane capacitance integrates the current that goes across it This integration slows down voltage changes across the membrane Figure 9-6 Capacitive current is proportional to the derivative of voltage with respect to time Integrating both sides and solving for voltage we get,

14 1B. Temporal Summation 2. Spatial summation Recording Synaptic Current Synaptic Potential Long time constant (100 ms) Short time constant (20 ms) Axon Synaptic Current Synaptic Potential Long length constant (1 mm) Short length constant (0.33 mm) VmVm VmVm 2 mV 25 ms Figure ~ 100 pA

15 1. Passive dendrites act like leaky cables Gulledge & Stuart (2005) J. Neurobiol 64:75, V EPSP measured in soma V EPSP measured in dendrite Excitatory synapses

16 Temporal and spatial integration of the EPSP from two synapses Gulledge & Stuart (2005) J. Neurobiol 64:75, Δt = 0 Simultaneous, colocalized EPSPs (two individual trials) V Nearly simultaneous, colocalized EPSPs (two individual trials) V Δt = 5 ms Simultaneous, Spatially distinct EPSPs V Δt = 0 Prolonged rising phase Inspect the simulation, and run the movie, at

17 Active propagation in dendrites Patch-clamp recordings from dendrites in brain slices show that dendrites can generate action potentials A neuron was filled with a fluorescent dye Voltage was recorded at two sites in current clamp mode, one in soma, one in dendrite The occurrence of action potentials at the dendritic site shows that action potentials can actively propagate in dendrites Gulledge & Stuart (2005) J. Neurobiol 64:75,

18 immunocytochemistry 25 μm Whitaker, Brain Res, 2001 Magee & Johnston, J Physiol (1995) Now break the patch, to fill the cell with dye: Averaged traces Voltage-gated Na + channels in a dendrite Voltage-gated Na+ channels were recorded from a patch of membrane on the apical dendrite of a pyramidal neuron (left traces) Step depolarization of the membrane patch opened the channels The traces at the bottom left show the ensemble currents evoked by repeated voltage steps Image in the middle shows the location of the recording site in a cell filled with fluorescent dye

19 Back propagation of the action potential The existence of voltage-gated Na + and Ca 2+ channels in dendrites means that action potentials initiated at the axon hillock can “back propagate” to the dendrites The back propagation of action potentials into the dendrites can have an important effect on synaptic signaling at excitatory synapses because NMDA receptors act as co-incidence detectors. Gulledge & Stuart (2005) J. Neurobiol 64:75, brain slice

20 3.Excitatory-inhibitory integration: The “veto principle” of inhibitory transmission Inhibitory synapses work best when they are “near“ the excitatory event they will inhibit. “Near” means < one cable length. A. Inhibitory synapses on dendrites do a good job of inhibiting EPSPs on nearby spines B. Inhibitory synapses on cell bodies and initial segments do a good job of inhibiting spikes

21 End of Lecture 11