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Bi / CNS 150 Lecture 8

Synaptic inhibition; cable properties of neurons;electrical integration in cerebellum

Wednesday, October 15, 2013

Henry Lester

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

Binding region

Membrane region

Cytosolicregion

Colored by secondary

structure

Colored by subunit(chain)

~ 2200 amino acids in 5 chains

(“subunits”),

MW ~ 2.5 x 106

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Nicotinic ACh, GABAA , and glycine receptors look alike at this resolution (prev. lecture)

The pentameric GABAA and glycine receptors look like 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

Like a previous lecture

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)

The AChBP interfacial “aromatic box” occupied by nicotine (prev. lecture)

Y198C2

Y190C1

Y93A

W149B

non-W55D

(Muscle Nicotinic numbering)6

. . . GABA and glycine also make cation- interactions

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

How does the receptor transduce binding into channel gating? (prev. lecture)

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OPENCLOSED

Twist?

Corringer, J

Physiol 2010

Swivel?

Miyazawa, Nature

2003

. . . Both ideas are also in play for

GABA or glycine receptors

We have Completed our Survey of Synaptic Receptors

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A. ACh, Serotonin 5-HT3, GABA, (invert. GluCl, dopamine, tyrosine)

receptor-channels

Most

^

Figure 10-7

10Like 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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Molecular layer

Purkinje cell layer

Ganule cell layer

Whitematter

Figure 42-4

10% of the neurons in the CNS are

cerebellar granule cells

The cerebellum: a famous circuit in neuroscience.In today’s lecture,

it exemplifies pre- and postsynaptic structures.

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A plurality of synapses in the CNS (> 1013 ) occur between parallel fiber axons and Purkinje cell dendritic spines

500 nm

Molecular layer

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Types of synapses

(Don’t mind the Type I, Type II stuff)

Figure 10-3

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1. TemporalA. Molecular lifetimesB. Capacitive filtering

2. Spatial

3. Excitatory-inhibitory

Types of synaptic integration

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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

Synaptic integration 1A.Molecular lifetimes

Previous lecture

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What causes the ~ δ-function of glutamate & GABA at CNS synapses?

Na+ -coupled transporters for glutamate & GABA

are present at densities of > 1000 / μm2 near each synapse,

probably high enough to sequester each transmitter molecule

as it leaves a receptor

(more in a few slides).

At the nerve-muscle synapse, acetylcholinesterase is present

at densities of > 1000 / μm2 near each synapse,

high enough to destroy each transmitter molecule

as it leaves a receptor

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Synaptic Integration 1B. Capacitive filtering

CC ICVdTdVCI ;

Figure 9-6

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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

Improved from Figure 10-14

~ 100 pA

CC ICVdTdVCI ;

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1. If dendrites were passive, they would 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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. . . and passively integrate inputs . . .

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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. . . but two-photon microscopes allow

researchers to visualize patch-clamped dendrites in

living animals . . .

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 * = axon hillock

. . . dendrites are not passive. They have Na channels

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. . . voltage-gated Na+ and Ca2+ channels

in dendrites

lead to

partial “backpropagation”

of

action potentials,

implying

that parts of cells

can process signals

semi-independently.

Stay tuned!

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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“Veto” inhibition at the axon initial segment:Schematic of a GABAergic “chandelier cell” in

human cerebral cortex

Ch terminals

from Felipe et al, Brain (1999) 122, 1807

Ch. axon

InhibitoryChandelier

Cell

Ch terminals

PyramidalCells

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Molecular layer

Purkinje cell layer

Ganule cell layer

Whitematter

Now we localize the inhibitory “vetos” of cerebellar Purkinje cells

by “pinceaux” (paintbrushes) of basket cells

Figure 42-4

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NH2

A fusion protein: GABA transporter (GAT1)-GFP

How to localize and quantify inhibitory synapses

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cerebellum

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Molecular layer (basket cells stain)

Purkinje cell layer“pinceux” (paintbrushes)stain heavily

Granule cell layer

<ImmunocytochemistryFor GABA transporter

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Molecular layer (basket cells stain)

Purkinje cell layer“pinceaux” stain heavily, showing soma-hillock “veto”

Granule cell layer

mGAT1 GFP knock-in fluorescence >

<ImmunocytochemistryFor GABA transporter

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GAT1-GFP expression in cerebellum: basket cell terminals in molecular layer,Showing dendritic “veto”GABA transporter density is ~1000/(μm2)

50 m

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End of Lecture 8