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Bi / CNS 150 Lecture 11

Synaptic inhibition; cable properties of neurons

Wednesday, October 15, 2013

Bruce Cohen

Chapter 2 (p. 28-35); Chapter 10 (227-232)

-Aminobutyric acid (GABA)

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Crystal structure of GABAA Receptor (Miller et al, 2014)

• Homomer of 5 GABAA β3 subunits

• Overall structure similar to nicotinic receptor

• Positive charges in vestibule contribute to anion selectivity

The pentameric GABAA 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.

GABAA 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 (Erev) for the synaptic Channels

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At Erev , the current through open receptors is zero.

Positive to Erev, current flows

outward

Negative to Erev, current flows inward

ACh and glutamate receptors flux Na+ and K+,

(and in some cases Ca2+),and Erev ~ 0 mV.

-20

-50

-80

-100

-5

+20

+40

+60

+80Membrane potential

Resting potential

EK

ENa

At GABAA and glycine receptors,

Erev is near ECl ~ -70 mV

Like Figure 10-11

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Benzodiazepines (= BZ below):

Valium (diazepam), (Ambien, Lunesta are derivatives)

Pharmacology of GABAA 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-p interactions with its binding site

GABAA and Glycine blockers bind either at the agonist site or in the channel

Agonist site

Picrotoxin

(GABAA & glycine

receptors)

Strychnine

(glycine receptor)

Bicuculline

(GABAA receptor

8Like Figure 2-1

(rotated)

Parts of two generalized CNS neurons

synaptic cleft

direction of information flow

apicaldendrites

Excitatoryterminal

cell body

(soma)

nucleus

axon

presynaptic terminal postsynaptic

dendrite

Inhibitoryterminal presynaptic

terminal

axon hillock

neuronPresynaptic

neuronPostsynaptic

basaldendrites

initialsegment

node of Ranvier

myelin

(apex)

(base)

little hill

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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

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Concentration of acetylcholine at

NMJ(because of

acetylcholinesterase,turnover time

~ 100 μs)

Number of open channels

ms

0

high closed open

State 1 State 2

k21

all molecules begin here at

t= 0

units: s-1

Duration of postsynptic current

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How are glutamate & GABA cleared from the

synapse?

• Na+ -coupled transporters

for glutamate & GABA

are present at densities of >

1000 / μm2 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 / μm2 near each synapse

• This density is high enough to destroy each

transmitter molecule as it leaves a receptor

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1. Temporal

2. Spatial

3. Excitatory-inhibitory

Types of synaptic integration

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Membrane capacitance integrates the current that goes across it

This integration slows down voltage changes across the membrane

C

C

dVI C dT

IV

C

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,

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1B. Temporal Summation 2. Spatial summation

Recording Recording

SynapticCurrent

SynapticPotential

Long time constant(100 ms)

Short time constant(20 ms)

Axon Axon

SynapticCurrent

SynapticPotential

Long length constant(1 mm)

Short length constant(0.33 mm)

Vm

Vm

2 mV25 ms

Figure 10-14

~ 100 pA

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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

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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

http://www.neuron.yale.edu/neuron/static/about/stylmn.html

Inspect the simulation, and run the movie, at

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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,

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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

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Back propagation of the action

potential

• The existence of voltage-gated Na+ and

Ca2+ 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

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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

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End of Lecture 11