Neuromuscular Junction &Synapses

7/23/2019 Neuromuscular Junction &Synapses

1/26

CHAPTER

8

'.::__.rifi'.1,

a.,:t:..

'.

,:..--

):.

t

t

-';;t't'.

Synaptic

Transmission

and

the Neuromuscular

f

unction

Edward

G. Moczydlowski

The

ionic

gradients

hat cells maintatl

lcLoss hrn mcmhranes rovide

a

lbrm of stored

electrochemical nergy

cells can use lor electrical

signalling.

The

combination

of a

resting membrane

porential

of

-60

to

g0

mV and

a

diverse array

of voltage

gated on

channelsallorvsexcitable

cells to gener

ate

action

potentials

that propagate

over long clistances long

the surlace

membrane

o[ a singlenerve axon or musc]e iber. However,

another

classol

mechanisms s

necessary

o transmit

such electrical nfomation

from cell to

cell

throughout

the

myriad of neuronal

netu'orks that link

the brain lrrrh

sensoryand effector organs. Electrical signals must pass across he special

. \ . . ^

, h 1 ^ 5 : * p

. . 1 1 6 n 6 1 1 2 r e ,

r

. r r -

,

, l l c . l

a

" l

l b

sy-napse.The

process underifing this

cell to-cell transfer of electr.ical

ig-

nals is tenned

synaptic transmission.

Communication betr,veen

ells at a

synapse can

be erLherelecuical or chemical.

Electrical synapscspror.icle

direct electrical

continuity beti,r,eencells

by means of gap

lunctions,

u,hereaschemicaL

slnapses link two cells

iogether by a chemical neuro-

transmitter thaL

s released

rom

or.re eLland diffuses

o another.

ln this chapter

tve discr-rsshe generalpropertjes

ol synaptic transnissioli

and Lhen focus mainly on s1'napttc ransmissionbenveen a notor neuron

and

a skeletal muscle fiber. This interface

betg'een the motor neuron

ancl

the muscLe

ell is called Lheneuromuscular

uncljon.

In Chaprer12,

Lhe

locus is on

synaplic t ransmission between neulons in

the central nen'ous

s)'stem

CNS).

s3@

w

MEC}IANISMS

OF

SYNAPTICTRANSMISSION

ElectricalContinuity BetweenCells s Established ither by

DirectFIow

of

CurrentThrough

Gap

unct ion

Channels

t

an Electrical

Synapse

r by Diffusion

of a Neurotransmitter

acrossa Chemical

Synapse

Once the cor.rcept

l bioelectrlcitl' hacl taken l.iold among physiologists

of

l h e

J I L

e - r .

r r . b e , a m c ,

, r ' h . r r e

q u c < l . o n

" l

h o u

c l . " r r ' . a l

i g r a .


2/26

TABLE

A-I

CHEMI(AL

ELICTRICAL l onotropi( M etabotropic

Agonist

Membrane

proteln

speed

Effect

ACh,

acetylcholine;

-, membrane

potential.

A simiLar

calculation based

on the geometry and cable

propeflies

of a typical nenre-muscle ).napse uggests

hat

an

action

polential arriving al a

nerve terminal could

depolarize

he

posts)'naptic

membrane by

only 1

pV

after

crossing

he s).naptic

gap-an attenuation of 105. ClearLy,

Lhe evolution

of complex

multicellular organisms

equired

the

development

of specialsplaptic mechanisms

or

elec-

trical

signalling

to seNe as a

workable means of interceL-

Lular

communication.

lwo

.ompel inghyoothesesnerged

-

the loh cen-

tury to

explain

how closely apposed cells could commu-

nicate

electrically. One

schooLof thought proposed that

ce

15 are

dirpcll) l i nled b; micro,coDic cornecL 8

bridges

thar

enable electrical signals to

flow directly.

Other

pioneering

physiologists

used

pharmacologicobser-

vations to infer that cell -lo-ceLl ransmissionwas chemicaL

in nature.

Ultimate resolution of this

question awaited

both the

developmentof

electron microscopic echniques,

which

permitted

yisualization

of

the intimate contact re

gion

between cells,

and further studies n neurochemistry,

which

identifred

the small, organic molecules that are

responsible

or neurotransmission.

By 1960,

accumulated

evidence

ed to the

general recognition that cells use

borh

direct electricaland indirect chemical modes of transmis-

sion

lo

communicate

with

one another.

The

essential tructural

element o[ interceiLuLarommu-

nicarion,

the

slrrapse, is a speciaiized

point

of

contact

between

the

membranesof two different, but connected,

cells.

Electrical

and chemical

rynapses

have unique mor-

phologies,

distinguishable by electron

microscopy.

One

major

distinction is the dismnceof

separationbetween he

two apposingcell membranes.At electrical s)mapses, he

acljacent

cell

membranes are

separaled

by about 3 nm

and

appear

to be nearly sealed together by a

plate-1ike

structure

lhat is a lraction of a micrometer

in diameter.

Freeze-fracture

mages of the intramembraneplane

in

this

region

reveal a cLusterof closely packed

ntramembranous

particles

that

represent a gap

junction.

As described n

Svnaptic

ransmissionnd the

NeuromuscLrlar

unction

8

is

as large as 50

nm

at lhe

vertebratenewe-muscle syn-

apse.An additional

characteristicof a chemtcal

$mapse

s

the

presenceof numerous s)'naptic

vesicles

on

the side

of

the slnapse that

initiates the signal transmission,

ermed

the

pres).napticside. These vesicles

are

sealed,

sphedcal

membranebound structures that mnge in diameter tiom

40

to 200 nm and contain a

high concentrationof chemi-

cal

lreurotransmitter.

The contrasting

morphologiesof electricaland chemical

s1-napsesnderline

the contrasting

mechanisrnsby which

they

function

(Table

8 l). Electrical

slnapses pass volt

age

changesdirectly from one

cell to another across he

low-resistance continuity that

is provided by the con

nexon channels. On the other hand, chemical syrapses

link

two ceLlsby the diffusion

of a chemica l transmitter

across he large

gap

separating

hem. The neurotransmlt

ter rhat

is

stored

in the

ry.naptic

vesicles

s

released nto

the synaptlc

space,diffusesacross

he cleft of the slrrapse,

and

activates he

posts)'naptic

ell by blnding

to a specifrc

receptor

protein

on the

posts).naptic ell membrane.

Direct evidence

or the existenceof chemical transrnis-

sion

predated the experimental confirmalion of

electrical

slnapses.

The foundations of

s),Traptic hysioLogycan be

traced back io

early studies of the aulonomic

newous

system.

Early in the 1900s,

researchers oted that adrenal

gland extracts,which contain

epinephrine, elicited

physio

Logicaleffects

(e.g.,

an

increase n heart rate) that were

similar to those

elicired by stimulation of sl.mpathetic

nerve

hbers. ln 1904, ElLiot

proposed

that

sympathelic

nenes

might release a subsmnce

thal is analogous to

epinephrine that would functlon in chemicaL ransmission

beLueen

ner reand rs

r , i rget

"gar . ' r 'a r

< lLd.es

ug-

gested that the vagus nerve,

which is parasyrpathetic,

produces re ld ted ub5lan\e

hat

L re 'oons ib 'e r de-

nrp


3/26

8

/

Synaptic ransmissionnd

he Neuromuscular

unction

/

I

(eleclrotonic

urrent)

Ce

l-cel l

gap

juncl ion

FICURE

8

1 An eleclr ical

ryrpse

All e lecrr ic. l l

\

n ipsc

consinsol

cules.

Electrical nd ChemicalSynapsesBoth

Convey

Signals romOne

Cell o Another,

but

Differ Greatly n the Particulars

ELECTRI(AI"

YNAPSES.

hereas

overwhelnT

ng

sullporl

lo" . l r ,

-n . . l l

. y r .

l . .

Jn . n s r to t r

ac \unu la

eJ

in h \

lirst hall ol the

20th

century. Lhe irst direct evidence or

p l c . l . . , .

r ' : ] nm r - - . o

r d n

r u . h

l : r . r f r o r

c l . . r r o . l l ) ' l

ologic ecorclingsf a crayfish ervepreparation.n 1959,

t u , - h 1 " r

r d P o e r u . e J r r r . '

D ' r i . o '

. t

n

. r

r g : n C

recording

electrodeso shor,vhat

depolarization f a pre-

- ) n J p , r .

n r , e

f b e r '

t l t

' . l )

r - , o J o r ' r

- . - \ r

r r -

sulted

n excjtation l a

posts),naptic

erve

ce]]

(the

mo-

tor

nene to fie tail

nruscle)

wlth virtually no

time clelay.

In contrast,

chemrcal

s1'napses xhibiL a characLerjstic

e-

J \ o

df f ro \ lnd l r l .

r r -

r

L l r .

- . r ,

-1n" rur .o l r :g ,

gn. r l [ te" er ,

t - .

r

o l

1r . pre ,

n . rp t r ' " L T l re

c le . ror

stration

of an e lectrical s1-napse

elrveen L\\,onervc mem-

branes

highlighted an Lmportant unctional

differetlcebe

r \ ^ c c n

l ( l r

r r l

r d . \ e r ' . 1 1

y r

- c -

r r c ' r J . -

r g

.

propagation

electricaL)

ersus

rieilv clelayeclommunlca

t ion

(chemrcal)

hrough he

lunction.

An electrical

synapse rs a true

strucLrLral onnecLion

Linearll,rvith the translunctional voltage

(i.e.,

the

y,,,

dif-

Ierence

between

he tu'o cells). However, the crayfish

syn

apse

described b1. Furshpan

and Potter allorvs depolariz

. l

E . L r

n l

t o

. r , ,

r e a d

' o n l ) - o n , d r r

r t o

f f o | . h e

'

. . , - - i .

r

I . h . n ^ - r . .

- i .

c e l . ) L . n . e L l - d l

synapsesare called rectifying synapses to inclicate thar

the underl,vir'rg

ur.ictional

conducLance s voltage

depen-

d . r r

- r

, - ^

. 1 , . . r

' , 1

" " r r r q , . q 1 l

. T n . r r n .

- a v e

shown that the voltagedependence f electrical

ynapses

arises

rom uniqrie gating properties

o[ dillerenLconnexjn

)u

or rs

.o

n. . -o lo , r

-

r t r lLage

eper rJ rnr

.

, r ,

others are

loltage

independent. lntdr-rsic -ectilicaLion

an

also be altereclbl Lhe brmation of a gap

uncLion

Lhat s

c o n 1 ' o . e d 1 1 1 n" p i . l ; - - c l - . . l c h m a l eu 1 ' t r . l e r

' '

\ l ,

L

L _ i d

c " r n e r t n s a 1 L - J - L o

heterotyprcchannels.

CHEMICAL YNAPSES.y

thejr

\,ery nalure,

chemical

synapses

re inherently rectif,ving

or

polarized.

They prop-

J 3 , e , - l

e n l

i n o n r d r "

l t o n

" r '

l r e

f - c . ) n , r l r l i .

" l l

Lhat

releases

he transnirter ro the posrs).naptic

ell that

. r

' i n .

l - r -

. r r ^ . ,

h r ' r .

. o - i

e

o T J

. t n .

t

.

L d 1 5

_ l r ' '

Fo r ' r . r h r r ' - -e

'

a \ \ .

i o t

, , r

' a t l l -

t l l ch .m- -

. " 1

- v , f

i .

r n - r r

. "

n

h p l

. t h p p o - - b i l

r 1

r l - a . h e

p . - t : 1 n L p , , . l l

, . r n

r r r f r e r . e

r ) n ,

p . .

. . , r J o n o r

transmittel

release

b1' the preslnaptrc cell.

Studies ol s1n

apse clevelopment ancl regulation hale sholltr

that postsy

naptic

ceLls also p1a1

an actrve role in s,r'napse ormation

n rh r fNS no< t *n rnn r r c l l< m,

. l \ ' l lSO p roc lU(e r ' o

grade signall ing molecules,

such as

nitdc

oxide

(NO),

that

d1ffuse back into the presvnaptic Lerminal and modulate

l c

'

,

e l

o h ,

. ) n . r t .

c o

- e c r i o

r f . 1 2 2 .

F r r f , c r -

- , . ' " i l .

"

hanh . . - ,

. , - , , .

. r l

) n m e

) n d l

e

. O T

rains receptor-s hat lnay eiLher inhibiL

or

facilltate

the

le [ - :e

o

l r

n , r r i . ie

b] b .o , -emi . np. h . r r

>m' .

T

u , .

. h ' c ; l

- r

; - e ,

" u l , l

h e

,

^

, 1 .

,

J , u n r d . r e . L i u r r

pathu.a). or signal propagation

that can be modulated

by

bidirectional chemical commur-rication

etween two inler-

actingcells.

The process o[ chemical transmissic]n

an be summa-

rizeclby

the l,rl lowing

eries f steps

Frg.

8 2):

\ (e

,

.

\ c r . to t rdn,n

le

- ro lecJ

e- ._ ' r 'e cL. rgcd r to

\ )

d p t .

\ ( . t

h -

' l r , ( i 5 .

- d

l o f l l f , . e i n .

I n

I n c

r \ < t -

cle membrane use the energl of an H gradient

Lo

energizeuptake o[ lhe neurotransniLLern the vesicle.

\ lcp

2 . A r

, .

on LrJr (

"

r r l r i .h

in ro

rc ,

ro l t "ge

B. r r '

N r

. r

d

(

c h , - n e l ,

. )

1 8 2 1 .r r , . . - , r

r h c

p - c s y - a p

Ltc

ne nre Le mtnal.

Step

3r Depoladzation opens voltage-gated

Ca2t channels,

* h . . l - - l lo ' r

\

J '

lo

c

le .

t

.e prP)yr l . , rpL

lermin l l .

SLep4;

The increase

n intracellular

Cart concentration

([Ca2-1,)

riggers he fusionof s] 'napric esicles ith

Lhe


4/26

Synaptic ransmissionnd he Neuromuscular

unction

8

Extfacellular

space

i(electrotonic

current)

Presynaptic

nerve erminal

of he nerve

cell

\

str.ucrion

f t l.re ransmitler

e.g..

hydro\,siso[ ACh b,v

acetyLcholinesterase

AChEl),

(2)

uptake of transniuer

lnLo the

presynaptic erve

erminal or

into

other celLs

by

Nr*-dependent ranspor-t) 'stems, r

(3)

diffusionoI

r l - , . r : n < m , r r n r m . l . . r r l . < t r \ \ , \ 1 r

O l L n ( \ n r P \

FICUREB

2. A chcmic al )nrpsc SlnapLic mnsmission1 r rhcmicalstnapsc an

be rh)rghtu[ . ]soccLrnnsrrs| r . . t .Lr.

A neurotransmitter

reaksdown,

is takenup by the presynaptic

terminal or oiher cel1s, i

diffuses

away ftom the syr'upse.

h

.

t

u . : .

, .

: r - r rd 'd l

r ln ^b

l )

|

,

. , '

, l

i

' . \B 1 .

e l y

.

. d F .

. . p r ,

-

' r . h . - . n J u r ' p l

a J e r

\ c o h . ' l r n ,

The

Transmitter at a ChemicalSynapseCan

Activate Either an lonotropic Receptor hat

ls

. , ^ 2 +vorrage gareoLa

channels oDen.

Postsynapticell


5/26

IONOTROPIC ECEPTOR

. , :

Axol

t\;

Electr ical

I l ; : l : i : l

srmulus

I |

'1 ,

\, ' ,,/ \ . N"./

. l l

";

-'--"

@ ,

. .

. . . . . .

:

,

i l , ,

: . '

. , , ,

|

.

. . : . . , . . . . . , , .

. : .

. . . , . : t . .

. : . .

^

' ' .

- r i

. . . _ ) , i . , 1 t : 1 , . , , . , .

, / t

"

2Oa 8 Sylapt iL

-tdn 'Ti. . 'on

a1d

t l-" \eL-omJ)culat

,LtL

01

B METABOTROPICBECEPTOR

Acetylcholine

Skeletalmuscle

fibermembrane

mate.

Ellutamete

receptors

thaL are ton

chilt-tr-rels rc

knorvnas

ionotropic

receptors,

and

gluLanate

ecepLors

cor,4t1ed

o C proLcinsare

calleclmetabotropic

receptors.

fhis nonenclature is berng increasingly

used ro

clescribe

thc Lwo

malor l)rpes

)1

rcccptols of

Lfansmitters

ther

llun

llutamate.

lonotroprc

ancl metabotropic

eceptors

determine he

ult imate

unctionll response

o tftnsmiter re]ease.

ctiva-

Atrialmuscle

cellmembrane

FICURE 3. id lotropic

xnd n.rr

botrof ic . rc. t \ ( l r {r lmr

fe.eprors A,

This e\mplc i l lusrf :urs

nicor ir l ic '

acel) lchol inc cccptor

shrch is

r

tLg.Lml-gaLcdhannel

on Lhe

possr'-

naptic nembr.rnc In

r sktleral mLrs

cle,

Ihe fnd

result

s mrLsclc

rr

Lf lcLion B, This

eramptc l luslratcs

I rruscrf jr r ic

; l .er) lchol inc

crcp

1of.uhrrh is couplcd o e Lcrcrour

mcric

a;

t rotern.

ln I crr . l i rc rrLrs-

clc. Lhccml Lcsul t s. lecreis. . l

rcart

rr te. Noie

thrt rhc

frcr\nair t rc

r. ,

l .1se l r \ ( lh is \ . r \

s irr i l r r l1ef .

. rnd LLr - \Ch.

arcr\ l .hohnri

( ,1P.

3LLrnonne

f

phosphxre

a a

95). B) lreir YeD nalure, onotropic

receptors tecliaLe

fast ionic slnaptic responses hat occur elt it nillisccond

t r c

- . . ' 1 , .

. l t . r , . - , c . '

u -

\ .

r . . . p r

r -

,

.

/ r . . , \ \ .

biochenricalll '

edirted

sy'napticesponses

n rhe range

of

seconds

ominuteS.

Flgurc B-3 compares

Lhe basic processes

mecliare.lb) '

t\vo prototyPicACh receprors

AChRs): 1)

rhe ACh acLi

vatecl

on

channcl lt the neuromuscLLlar

unction

of skele

Nico

ch

I

l

,

L

: inicACh rece

annelactivatir

lr

IM".b"r*

lepolarization

lr-

"tb"

p"t""tb

exctaiion

ptor

ln

IVIusc

contraction

[,4uscarinicCh eceptor

I

acrvar n

]

lr

R"b"r"

"l

"TP

-

P./

from he

helerotrimeric

protein

-

]r

Activation

oi

lnward

I

rectifier

K- channelby

py

----r-

t-r.,r"-.t

*.

l

I

nyperporararon

]

t-

Decrease n

nean TaIe


6/26

tinic)

receptor, opening

of the AChR channel

results n

a

transient

increase

n permeability

to Na* and K*, which

directly

produces

a brief depolarization

hat activates

he

muscle

fiber. In

the caseof the

metabotropic

(muscarinic)

receptor,

activation

of the G protein-coupLed

recepior

opens an inward rectiFer K+ channel, or GIRK (p. I97),

via

p7

subunits released rom

an activatedheterotrimedc

G

protein. Enhanced

opening of

these GIRKs produces

membrane hyperpoLarization

and leads to inhibiiion

of

cardiac excitarion

(p.

488). These

wo funcLionallydistinct

mechanisms

are the

molecular basis for

the seemingly

conflictrng

observations

of early physiologisrs

rhar ACh

(VagusstofJ)

ctivates skeletal

muscle bur inhibirs

hearr

muscle.

SYNAPTIC

RANSMISSION

T

THE NEUROMUSCULAR

UNCTTON

Neuromuscular

unctions

re Specialized

Synapses

ith Active

Zones

of Synaptic

Vesicles n PresynapticNeuronal)

Membranes

nd Highly

Amplified

unctional

Folds n

Postsynaptic

Muscle)

Membrane

The chemical

ry.napse

etween

peripheral nerve

terminals

and skeletal muscle fibers is

the most intensely

studied

synaptic connection in Lhe neryous

system. Even rhough

the detailed

morphology

and the

specific molecular com-

ponents

(e.g.,

neurotransmitters

and receptors)

differ con-

siderably among different iypes of slnapses, ihe basic

electrophysiologic

principles

of the neuromuscular

unc-

tion

are applicable o many

other

qpes

of chemical slrr-

apses,

including neuronal

sl.naptic connections

in the

brain, to

which

we wiLl retum

in Chaprer 12. In

this

chapter, we focus on the neuromuscular

unction

in

dis-

cu5s inSheba>rc r in . ip les f

: lnapLic rdnsmi5s ,on.

Motor

neurons with

cell bodies in rhe

spinal cord have

long axons that branch extensive lynear the point of con-

tact

wirh the target muscle

(Fig.

B-4). These axon proc-

esses

each inneruate a

separate iber of skeletal muscle.

The whole assembly of muscle

fibers innervated

by the

axon from one

motor

neuron is

called a motor unit.

T1pically, an axon makes

a single

point

of synapdc

contact

with a skeletal

muscle hber,

midway along

the

lengt\

oi rhe muscle 6oer.

thrs


7/26

210 8

/

Synaptic

ransmissionnd

the Neuromuscular

unction

Spinacord

Nerve ellbody

Axon/

Muscle ell

or

ber

Postjunctional

folds

Synaptic

vestcles

'o

I

o

I

Acetylchollne

receprors

Postjunctional

folds

Postsynaptic

membrane

FICURE -4.

Ihc

\efrcbr: r te

b o u 1 o n s ' i 1 s s ' c L l l s t h c s p c c i a l i : . r L i o l s o | l 1 r c p o s l s , v n a p l L

.onL:rLLrirg

he

tionrl loltls).

Depol^iz.rLiorl

Act ve zone

Acetylcholine

(re

eased rom

vesrcles

Presynaptic

memorane

,t

laT.rna

@k'

\@


8/26

FICURI 8-5.

End pla ie poterhals

el ic iredar rhc lrog

nuro-

muscular

lunclion

by sLmulaturg the mot,:lr neuron. The mag-

r i r ' ' " o f t l . . . c r . ' r

\

I o . w

1 i .

r . l I D P

.

s , . . . .

near thc end

plxre

and deca)s farther awry

(De e

fonr

Fati I,

Kelz B: An anal)sis of the end-p1ate otenLial

ecorddwirh an

inLracel lu larlectrode. l ,hysio l l5 :120

370, 195i. )

cle cell) of

lhe neuromuscular

unction.

Normally,

nen'e

stimulatron

rvouid

drir.e the V,,, of the muscle

above

threshold

and elici t an action

potentlal

p.

172). How-

ever,

Falt and Katz were interestecl

not in seeing

the

Synaptic ransmission

nd he Neuromuscular

unction

/

8

0

Excitatorv

1o

postsynaptic

(or

end

plate)

0

potenlial

(mv)

1 0

0

1 0

0

1 0

0

r h , m r n m . ' ' r r . r n v , . " h o p . i n \

' . ' ' ' ' , /

spoto[ themuscle

ell.

\ V h e n

J l

" i

d h a t -

e l e c t r L " l l r \ ( i r e d h , ' m o r o r

neNe axon, they' obsen'ed a transient

depolarizaLion n

211

Stimulusf

motor

erye

1 0

Voltageecordiqg

\\ llJ-

MorotneNe

The muscle s treatedwith

curare o limit

ACh

receptorachvahon

o

subthreshold esponses.

The delay in

response is a

function of

acetylcholi11e

release,

diffusion, and

activation ol

Pos6ynaPtlc

aecepto$,

The delay

in

response

tlme

increases s

a

function

of the

distance

hom the

end

plate.

1 .0

mm)


9/26

212

8

/

Synaptic

ransmissionnd the Neuromusculaf

unction

A EXPERII\,lENTAL

REPARATION

END-PLATE

URRENTSOBTAINED

I

VARIOUS

OLDINGOTENTIALS

400

200

End-plate

current 0

(nA)

-200

-400

0 1 2 3 4 5 6 7 8

Time

msec)

C I-V

F]ELATIONSHIP

ORPEAKEND.PLATE URRENT

Clamped

membrane

potential

(mv)

FICURE

8 6.

End-plate currents obtained at dillerenr

membrane

poten

tials in a vohage-clamp expenmen . A, Two electrode vokage cLamp s

used to

measure

he end-plate cunent in a frog

muscle 6ber. The tips of

rhe rwo

microelectrods

are in the muscle fiber.

B, The

six

records

represenr

end-plaLe

currents that \'r'reobiaind while the motor

nerve

rvas stimulated

and the

poslslnaptic membrane was clamped to

V,,,

values

of

-I20,

91, 68, 37,

+24, +38 mV Not ice that ihe

peak

cunem

reveres

from inward to out\rard as the

holding polential shifts

from

-37

to

+14 mV.

C,

The rvelsaL

potential

is near 0 mV because

atory

postslrnaptic potential. lt

is produced

by

the tran-

s ienr

open inB l

AChR

thannel . .

nht ,h

are >e rL ' \e l )

permeable

o monovalent

calionssuch as Na*and

K-.

The

increase n Na* conductance

drives V* to a more

p o c r

e v d l L e n l \ e

\ r c r n I )

o ' t h e e - d - p J a L e

e B o n .

n

rhis expenment, curare blockade al1ows only a small

number

o[ AChR channels o open,

so that the EPP does

nor reach the

threshold to produce an action

polential. If

the experiment

s repeatedby inserting

the microelectrode

at

various distances

rom the end

plate,

the amplitude of

the

potential change

is successivelydiminished

and its

peak

is increasingly deLayed.This decrement

with dis-

lance

o. .ur ( becausehe EPPorg

rate . a t

the

ond-p l " te

region and spreads away from this site according to the

^ , . < ' . p

r h p

n . o n e r e q

n

' C 0 r

o [ L h e

r . u ' . e f b e r .

Thus,

the EPP n Figure B 5

is an example o[ a

propa-

gared.

g raded

responce.

ouerer . wrhout t l ' e curd re

blockade,

more AChR channels would open and

a larger

EPP rvould

ensue,which would drive

V,,,

above

threshold

and consequently

trigger a regenerating

action potentiai

(p.

172).

What

ions pass through the AChR channels

during

generation

of rhe EPP?This question can be answeredby

using

the same

voltage-clamp technique that was

also

. sed

ro , t r d1 the

bac iso f l he dc l

o ' po ler t l

r 5ee

F.8.

7 5B).

Figure 8-64 illustrates he

experimenlal prepara

tion

for a two-electrode voltage-clamp

expedment

in

which the motor

nerve is stimulated

while

the

muscle

fiber in the

region

of

i|s end plate is voltage-clamped

o a

chosen

V-. The recorded current, which

is proportional

to the conductance change at the muscLeend plate, is

called

the end-plate

current

(EPC).

The

EPC has a char-

acrerisric

rime course that rises to

a peak within 2 ms

after

stimulation of the motor

newe and falls exponen-

L i a l l ;

a c r

o

e r e

q f ' g

B - o B ' .

l h e l i m e c o u - " e f

, h e

EPC

corresponds o lhe opening

and closing of a

popula-

tion

of

AChR

channels,

governed by the

rapid

binding

and d i

,ppearan,e [ ACh as r t d i l lu 'e .

o

the

po. tsynap-

tic membrane and is hydrolyzed by AChE.

Aegn -11. .he , :gon. 'L-

binding site 1or ACh is in

the extracelLularN-terminal

domain o1 the a subunit. For

each of the subunits,

the

M2 transmembrane egment ines

the aqueouspore

of the


11/26

214

8

/

Synaptic

ransmissionnd the Neuromuscur

unction

A SINGLE-CHANNEL

URRENTS

Single-channel

currents

PA)

Embryonic

ozJltS

0

Single-channel

^

currents

pA)

-z

0 20 40 60 B0 100

120 140

Time msec)

B

t-v RELATTONSHIPS

2

-100

Embryonic

crz0t6

\naut,

0rP.6

have

clilferent

functional properties. The uniLary conduct

ence of

nonjunctionaL

receptors 1s approximately 50%

larger

and

the single channel

iletime is longer in duraLion

rhan thar

of junciionaL receptors.The basis for this phe-

nomenon

is

a difference n subunit composilion.

The non-

y t r ' r ,: . r r tc i

o

cn b r r

onrc l

ecPFlo15

c r Den l . r r - rc r ' .o ln

plex with

a subunit composition of

arp76 1n mammals,

just

as

in rhe eLectric

organ of the TolTedo ay. For the

.luncriondl

AChR

in adult skeletal muscle, subsiitution

ol

an

e subunit

for the leta]

7

subunit

results n a compLex

ivlth tl.re

composition

arPe6.

T e l u n . ' r o n " lr o p e r t e s l t . e t u o t ; p . o ' e e p t o - s

l.ravebeen

studied by coexpressrng

he cloned subunits

r.t

) , , ' . ,pu ,

oocvre ,

rgLrc

B-84

-Los

pach c lamo eco .d

ings of

single

ACh-activatedchannels n oocytes hat

had

been

injected

u'ith nRNA encoding either

a,

B. 7,

6

or

a,

B,

e, 6.

Measurements f currents at different

vohages

yreLded single-cl.rannel

-V cun'es

(Fig.

8-BB) showing

FICURE

8 8. Properties ol embrlonic and

adrlt acetylcholine

(ACh)

receptors

fron skeletal muscle A, T he resulLsol

pelch

cLamp

expe

-

rnenrs,

sith

lhe

p:rlch pipeues in th o utside out configutation

end the

parch exposed o 0.5

pNl

ACh, are sumnarizcd ln thc rpper

pdncl, rhe

ir estigatoE

expresscciLhc

emb$onic AChR. $'hich has the subuni

compositjon

d:87.5, in Xdrril]rrls oc]tes. ln thc ldv.f

ldnul,

thc

invesli

garors expresscd hc

.ldrlt,A.Chlt, $,hich has the subunii co]nposlion

arB5.

No(ice thlr rhe

meen open times ar

great,rr

urr he errbfyonic

forn,

s'hereas Lhc unitary currens are

greater lor the adult lornl. B,

Ihe lrfo

lires summarize d:ia that are simihr

to those obtarned rn A.

The single channel conductance

of the aduLt lorm

(i9

pS) is hiilher

rhan

rhaLol Lhe

eLnbry'tnlicorm

(40

pS)

(Data

{rori \,lishina M, Takai

T,

Imoro K. ei a]: lvloleclrlardisrincrion

between feLaland aduLt forns

ol

muscle ceLl lchol ine

eccpLofNaLure 21:406-411, 1986.)

" h l "

. . f l D r , l i D i r . n D . i r l i ? a ; r ^ 1 . < r r ' ) ' l *PL

Lrdnrm

>ro l r

ier>

-

cle\eofrl.c.l i

a d

,1nrp-e

9r1n"11"n

MolecuLar

cloning of

genes

hat

encode AChR subuniLs

of the TorTedo ay electric organ and mammalian skeletal

r

. .1 , r

cd ro rh " rd r t . f ' o t .on o [

. . r r rge

-umbcr

o [

relaLed

genes for AChR channel

proteins. For example,

mammals

have a family ol at

least eight genes hat encode

homologous

a subunits of

nicotrnic ACh activatecl

ecep-

L o

.

, l | e

o

s ,b r , n . r

f

t h ,

- L r l . t "

n u r c e

e c e p l o - , -

h p

procluct

of a gene called al. Seven

additional genes des-

iElnatecl

2

through

aB encode a subur-rilsLl-rat

re ex-

pressed n neuronal Lissues.Only the proLein producLsol

genes

n1, a7, and aB bind the

snake venom

protein

called

a bungarotoxin.

In

addiLion,

at least lour

B

sub-

units

exist. Besides he

B

subunit

of the skeletal muscle

AChR

which

is

calLed

B]-there

are three

neuronaL

homologs

(82.

83. P+).

Heteromeric associatron

f differ

ent combinalions

oI these subunrts

could potentially

pro


12/26

(5-HT-

receptor), lycine

GlyR),

and GABA

(GABA,

re -

, e f . u i . A q m n t . n c d p c \

o u r ) .

\ r h l

r n d

5

H l r e

ceptor channels are boLh

permeable

o cations and

thus

produce

exciLaLory urrents, rl''hereas lycine

acti\,ated

ancl

GAB{,

chrnnels are permeable

o anions such as Cl and

produce rnhibitory crirrenls. Fillure

8-9 shorvs exanples

o - r r J \ r o r . n p c

. r r d

L r r r i t a r

l

.

r

c n L -

n e d

a L e q

_

5 1 ,

- e

r .

r v r t e d

d u { B A . . h . r

. l -

C l " n e d

e , c >

e n -

'

od i -g

.u lun t

-

o

hc- .

a t to r ' r p lu r . rnn , r

r .oJe

t ) t r ' . c i r -

h . r '

1 , o n o l o o . u . .

q r

h R > L r b - '

.

f h . r

primary amino acid sequences harc a common arrangc-

ment of ML, N {2. M3, and M4 transmembraneegmenLs,

as

described arlier or the mcotinicACl.rR

see

Fjg. 8-7).

T L

- e

p r o L e L h . s l b . l o n g

r o

- J E .

d c r e

r . r r y r L J l

is knoi,vn as the

ligand-gated

ion channel

superfamily

f p

L 0 \ .

. , o r . ,

. e r l r l , . i .

o l l - c . c

I

n c -

. u t L , . r - . L J l

' h p .

' ^ ' "

. , I n , a , r c e - l o

h , b

- j .

r o r

. r . L r T

\ c

- .

r r r i , ,

. ' e . r ' \

\

a D D c r o c - i d e

' l '

1

rvjthin

the tr42 segmenr. 4utatlon

of only three resLdues

ri.ithin Lhe

M2

segment of a cation-selective subunit of

a neuronal

nicotinic

AChR is sufFcLent o conven it

lo an

anion-selectrve hannel actlr.ated y ACl.r.

AcetylcholineReceptorChannels

Cannot

Open

Until Two Acetylcholine

Synaptic

ransmission

nd he NeuaomLrscular

unction

8

0

1

2

3

Equation 8- I

ln the case of an cgoni-st ctivateclchannel, such as the

AChR channel, ar least

one additional state must be

p r ee n l b ( . l J - \

t h ,

. o ' c d

. r ; n n . c :

'

e i t h e r

' r n d

l B

nisl or not:

Equation 8-2

a - la . r.-]

Closed hannel Closed hannel

Open

channel

No aElonist Agonist lound Aplomstbound

I t h i . t r ' . . p - h , ' c

L h c l o : c d L , . L c

, I

o t . ' ec . r o

nel must brnd one molecule of the agonist ACh

Lo lorm

J r d s ^ r i - l " u n C \ n n " l

L . r r

-

. l o , " d

l r

' - 1 e

. ;

r

r ,

. , . - -

, l _ , n " , I r l - ,

(AO).

Llon'evcr. even this scheme s oi'er1y

simplistic be-

cause

u.e knolr,

that each of the two cv subunits of the

AChR channel must bir.rd ACh srmultaneously for the

cl-rannel o open:

215

A GLYCINE

Macro-

scopic

current

(nA)

2

0

S ngle-

"

hannel

'

current

-4

(pA) -6

-8

0

_,1

0

_,1

B GABA

2 3 4 5 6

Tinre

(sec)

2 3 4 5 6

Tirne

sec)

Macro-

scopic

cutrent

(nA)

5

ngre-

-

channel u

curreni

-2

(pA)

4

1000 1500

Tme

msec)

FICURE B

9.

(:ltfrcnrs

acttrrred by glycrne and 1aminobut).ric Acid

(cAB,\)

,A These e\pernrcnts Nere performed on culLurcd mousc splnaLcord

neurofs usjng

palch

clamp lechniques The lcli pnncl

sho*,s

rhe

macroscopic Cl clrrrcnl,

*hich

s rneasured n the rvhole ce11

on[iguration and

c a r r i c d b 1 ' g l 1 c L n e | e c c P l o ] ( G \ ' R ) c h a n n c l s t v h e n e \ p o s e d 1 o g ] l c i l 1 ' T h e l l g h t p d n . l s h o r i ' s s l g l c . c h a n n c l

ouL

patch

conngufallof

In

both scenariLrs, he hol.ting porenlial \\'rs

-70

m\r n, The L/t pancl shoNs rhe mrcros.opic Cl currenr

ihai

is

carried bv

a;ABA,

re.eptor chan ncls when exposed

to CABA. The lighl

pdncl

shors single-chamreL

urrents

(Det:r

lroln Bofmann.l,

Hamill OP. Salinann B:

\ ' lechxnjsnofaniLrnpernreet iL lL, rLhloughchanrre

Equation

8- 3

2o


13/26

216 8

/

Synapticransmission

nd heNeuromoscular

unction

charLnel,

ncluding some local

anesthetics, ct

by enteing

rhe lumen

of rhe channel and

blocking the flow of ionic

current.

Figure B-l0A shorvs he results

of a patch-clamp

experiment

in

which

a single AChR

channel opened and

closed n

response o its

agonist, ACh. After adding

QX-

222, an analogof the loca1anestheticagent idocaine (p.

189), to

the extracellular slde,

the channel exhibits a

rapidly flickering behavior. This flickering represents

a

series of

brief interuptions of the

open state by numer-

ous

closures

Fig.

B-

10B).

This tlpe of flickering

biock is

caused

by rapid binding and unbinding of the

anesthetic

drug to a site

in the mouth

of the open channel. When

the drug binds,

it block the channel

to the flow of ioru

(ArB).

Conversely,when the drug dissociates,

he channeL

becomes

unblocked

(ArO):

Equaron 8-4

-

d / O - A r B

Blocked

Channel

blockers have proved

to be effective tools to

study the mechanism of ion pemeation. For example,

QX-222

helped ln locating

amino acid residues on the

M2 transmembrane

egment hat form part

of the blocker

binding

site, thus identifiring residues hat line

the aque-

ous

pore.

Miniature

End-Plate otentials

eveal

he

Quantal

Natureof Transmitter

Releaserom

the Presynapticerminals

Under

physiological conditions,

an action

potential

in a

presynaptic

motor newe axon produces

a depolarizing

pastsynaptic

PP rhat

peaks

at approximately

40 mV more

positive than the

resting

V.. This large signal results

rom

the releaseof

ACh from

only about 200 sp.Laptic esicles,

each containing

6000 to 10,000

molecules of ACh. The

neuromuscular

unction

is clearly designed or

excessca-

pacity inasmuch as a single end plate is composed of

numerous

s)'napric contacts

(-1000

at the frog muscle

end

plate), each with an active zone that is lined with

dozenr

oI

malure

Synr'ptrc

esicles.

hus, a large

-rer

tory

of ready

vesicles

)l0a),

together with the

ability to

s)'nthesize

ACh and package t

into new vesicles,allows

the

neuromuscular

unction

to maintain

a high rate of

successful ransmlssion

without

significant oss of function

as a result of presy.rapticdepletionof vesiclesor ACh.

The originaL

notion

of a vesicular mode

of transmitter

delivery

is

based

on

classic observationsof EPPs

under

conditions

of reduced ACh release.

1n 1950, Fait

and

Katz observed

an interesling kind

of electrophysioLogic

"noise"

in their continuous, high-resolution

recordings of

V- with a microelectrode nserted at the

end-plate region

normal EPP, they were named miniature

end-plate po-

tentials

(also

known

as

"MEPPs"

or "minis").

These ob-

servations

suggested

hat even in the absence

of

nerve

stimulation,

there is

a certain low probability

of transmit-

ter release at the presl'naptic

terminal, resulting in rhe

opening ol a small number of AChRs n the postslnaptic

membrane.

An

examination of the size

of

individual

MEPPs suggested hat they

occur

in

discrete muLtiplesof

a unitary amplitude. This findlng led to the

notion that

ACh release s quantized,

with the

quantum

event corre-

sponding

to ACh release rom

one slnaptic vesicle.

Another way of studying

the

quantal

releaseof ACh is

lo stimulate the preslnaptlc motor neuron

and monitor

V-

at the end plate

under conditions when the probabil-

ity of ACh release s greatly

decreased.How can we de-

crea5p

he

probability

o' ACh release? he a-nplitude

ol

the EPP hat is evoked n response

o nerve stimulation is

de.reased 1 lowering

[Ca2

1.

ard

-creasinS

[tt4g'

. A

low

[,-;2 l.

decreases

a

'

errD

nro

Lhe

pre'ynapLic

re r r

a l

rF ig .

B

2.

s t ry 3 ) . A h igh

IVg, ] .

pan ia l ly

blocks

the pres)-naptic

Ca'z* channels and thus also

de-

creases

Ca2+ entry. Therefore,

the consequenceof either

decreasedCa'z*1" r increased Mgz*J. is a falL n [Ca'zt],

in the

presFaptic

terminal, which reduces transmitter

releaseand thus the amplirude

of the

EPP

(Fig.

I 11).

Del e"-tr l lo

and Kat-

explorted hrs suppressior

f trans

miter

reLease

nder conditiors of 1ow

[Ca,*].

and high

fMg'?*1"

o obsewe the

V- changescausedby the quantal

releaseof transmitter. Figure

B-12A shows seven super-

imposed

records

of MEPPs hat were recorded

rom a frog

muscle hber during seven repetitive trials of newe stirnu-

lation under conditions of reduced

[Ca'z*].

and elevated

l \ , 4 o r I T h e r c , a r d < z r c r l i o n p r l r r r h p n n < r i n n n f r h e

nerye

stimulus

artifact. The amplitudes of the peak re-

sponses

occur in

dlscrete multiples of approximately

0.4

mV. Among the

seyen records were one

"nonfe-

sponse,"

two responses

of approximately 0.4 mV, three

response) l approximatell0.8 rV. ard

o-e re5ponse [

appro \ lma le l )

1 .2 mV.

One o [ the record ing< l .o

-e -

vealed a spontaneousMEPP with a quantal amplitude of

approxlmately 0.4

mV

that appeared ater in the

trace.

Del Castillo and Katz proposed

that the macroscopicEPP

is the sum of many unitary

events,each having a magni-

tude of approximately 0.4 mV. Microscopic

observationo[

numerous vesicles n the synaplic terminal naturally led

to the

supposition

that a single vesicle eleases relarively

fixed amount of ACh and therebv

Droduces a unitary

MFDP. c.o rd ing o tn jsv iew. he qu in r izedVEPP< nus

cor

espond o lhe f l..or of

d'screte

urrbers

of svnapti.

*

ves iL .e>

, l , 2 .

l ,

and so on .

@

For elucidating the mechanismof

spraptic transmission

at the

neuromuscular

unction,

Bemard Kaiz

shared the

.

1970

Nobel Prize

n Physiologyor Medicine.

@


14/26

Synaptic ransmissionnd he Neuromusculaf

unction

8 217

CONTROL

FIGURE

8-10. The ef fectof a local

anestheric n

rhe acerylcholLLle

eceplor channeL

AChR)

A, Sin

gle-charnel

recording ol nicotinic ACh receplor

ex

pressd n a

.Xenopfts ocyre The patch

rvas n rh

ou[side

out conllguralion, and the holding potentia]

rvas

-150

mV. The continuous presence

of

L

pM

A r

' . u * d

b

"

\ , r r e o p r g B . l l - ,

e ' l

i

ment is similar

to

lhai

in A,

excepr hat in addition

Lo rhe ACh,

the lidocaine

analog

QX

222

(20

/rM)

was

preseni

at

the

exrracellularsur{acof th recp-

1 . . -

- ,

| , c l

\ o

p

' r r

e

t r "

r r " l

o p n . r g . - d ,

^ r p , n r

o b .

r . T , .

l i

r " r

" g

a . : d L , r n a n o r c f

channel

closures

The

dme scale of th lower

panel

is expanded

10 fold.

(Data

rom Leonard RJ, r-abarca

CG. Charnet

P, et al: Il,rdence that

lhe M2

mem

brane'spanning

region lines rLle on

channel

pore

ol

rhe nicot in ic

recepror.

Science

242:1578-1581,

l9BB.)

500

750

Time

msec)

B LIDOCAINE

NALOG

close to the

pres).naptic

erminal membmne o[ a leech

neuron lhat uses serolonin as its only neurotransmitler.

The carbon liber is an electrochemica]detector of sero-

ronin

(see

Flg. B-13A), the currenl measured

by this

electrode

corresponds

o

four

eLectrons

er

serotonin

mol

ecule

oxidized at the

tip. Stimulating the

leech neuron

to

produce an action potential aLsoelicits an

oxidation cur-

rent, as measuredby the carbon hber, that corresponds

o

rhe releaseo[ serotonln. At a

[Ca'?*].

of 5 mM, rhe cur-

enr

J arge nd

r

omposed 'man, 'mal l


15/26

218

8

/

Synaptic

ransmission

nd he NeLrromuscular

unction

A MINIATURE

ND-PLATE OTENTIALS

MEPPS)

0.9

(rnv)

0.3

molecules.

Thus, the

amount of serotonin releasedby the

small

slnaptic

vesicles of the leech

neuron is about half

l.JLrfiber of

q',jania

the str.englh

f a particular

slnapse and thereby

give

rise

ro

dn l l t c rar .on bchauor . hree

ype-

o

5) r r , : rpr

moo

-0.3

22

20

1 8

1 4

1 2

1 0

B

6

4

2

0

0 5 1 0 1 t

Time

msec)

DISTRIBUTION

F

\,1EPPN4PLITUDES

Number f

observations

0.8 1 .2 1 .6 2 .0 2 .4

Amplitudef EPP

mV)

Number

of

quanta

released

x)

FICURE 8-12. Evoked and sponlaneousminiature end-plate potentiaLs MEPPS)A, The rnvstrgators ecorded y,,, in trog skeleral nuscle ibcrs LhaL

rvere exposed o

extracellular

solulions having a

tca'z'l

of 0.5 mM and a

IMg,'l

of 5

mN4.These alues mininrize

lransrnifter

release,

nd

(herefore

t

rvas possible o

resolve he srnallesrpossible

MEPP, whrch correspondsLo

the

reLease f a single synaptic vesicle

(i

e

,

1 quantum) The invenjgators

stimulatd

Lhe molor

neuron

seven consecutive imes and recordd the

evohed MEPPS

ln

one irial, ihe stimulus eloked no

response

0

quanta).

In

rwo lrials,

the

peak MEPP was

ebout 0.4 mV

(1

quantum). In LhreeoLhers,

he

peek responsewas

abour 0.8 mV

(2

quanla)

Finally, in one, rhe

peak

was about

l.2mV

(l

quanta)

ln once

case,e MEPP ol Lhe smallest magnirude appeardspontaneously.B, The histogram summarizes

claLa

rom 198

trials

on a ca

neuromuscular

unctioLl

in

Lhe

presencc

of 12 5-mM extracellularMgz . The data are in bins $'r h a width of

0.1

mV The

disiribution

has ight

peaks The first represenls

limuli that evoked no responses. he

olher seven

epresentslimuli that eYokecL EPPS haLwere

rcLlghlyLntcgral

muhiples

of the smallesr

MEPP

Thc cur.,'eoverlying each clusrer ol bins

rs a

gaussianor'normal" luncuon

and facjliralescalculaiion of the average

MEPP for each cluster

of

bins

The peak

r,rlues of these

gaussians

ollow

a PojssoLl

istribution.

(Data

trom Magleby KL: Neuromuscular

ransmission.

In Engel AG, Franzini-Armsuong C (eds):Myolo$/, Basicand Clinical, 2nd ed r.welv orL. Mccraw-Hil1, pp 442 ,163,1994.)

3.2

.4

Gaussian

urve

for5

quanta


16/26

A EXPERIMENTALPREPARATION

Synaptic ransmissionnd the Neuromuscur

Junction

8 219

SEROTONIN

ELEASE

(mv)

Current

orcatoon

fiber

pA )

80

./Serotonin

Postsynaptic

membrane

/iled

and

pronour]ced

ncrease n transmitter release

hal

, . . - r ' :

I

. t r

i ' , , -

1 -

' d o l r i g l rf c . . u c r r c )

\ ' .

i r - r - u -

larion. This ellect can lasL br minuLcsafter the conclition

lng stimulus.

Petentiatior-l av

be causecl y' a

ll.rtoc,

ol

1 r ,

r e

e

\ L

F i r e . ^ , h I . - L a . e .

(

"

i n

L c

I ' r . - . -

naptic erminaL

nd thus rncreases

he

probability

of exo-

cytosis.

Sl-naplic

depression Ls a lr.l sicnt decrease n the

elfr-

L c r ' . )

o L

J . . r ' r . 1 . . -e l . J

- r r , l .

o r . n , l .

n l

) .

J . , o r \

It ' luoo

L i

"

10msec

Sn-a clea.,a., .a'n,""*ne.h ra,+'v,'*.y-.*.-^

+

vesces

\ l

r.^,,*"*-.-**"*****"*.4

Curreri

\

I

o ' carbon

'

\ /

fioer

pAr

[*.' t

",*"*,,."*.

****,

I

l-*'

{ywdl"airp.n****r,r,,r+,,r,.r*r.y,

' /

: : i : " " "1" ,J: :

. ' ' * * , ' t *u ' t , i . . , t t t - ; , , , - .n^* t /

I

--_1

u

oo

.

10msec

Stimulus

artilact

F | c U R E 8 ] 3 ' D c t e c 1 i o ] r 0 [ s c r o L o n l n d r a L L s l c l c a s c d | r o n s Y n t 1 p t i . v e s i c l e s 1 , . I h c s c r o ( D L n

n c u r n . l n b c d e t e c t e d e l e c t r o c h e n l i c e l l 1 ' u s L n g a c l t b o n | r b e r n r l c r o c L c c L r o c l c I h e c u [ e n | c l r r ] e d b 1 l h e

rccordccl

ront

( b l g h l e \ ' e ] o | 5 c r o t o n i n r e L e a s c ) a n d a [ ( - ^ : - ] . , ' o |

rd illLlsrLiLes

h.rt the Ielease

\ ' e s i C 1 c s l n d l a L g c d c n s c c o l e R s i c I e s ' b o t h o | w h i c h c 1 n l l . l l 1 ) s e r \ c d

.-ensmilrcf

fele.rse rom single

s)'napric

csiclcs

N.{urc 377:62 65. l9g5

)

' C . - . '

r , . h . '

- r r .

f

r r , . d o J - rt

l n ,

- . a n d . n g

o , r r ,

dir iclual

nene

ternrinalsmay

learn"

p.

318)

Synaptic

Vesicles

Package,Store, and Deliver

Neurotransmitters

f n e D h . . o . " , o l - \ n

r . . r

r . r . i . r ' r o nr

h . - r r ,

. r l

t h , n c ' r - . b )

. n J o .

, n , .


17/26

22O 8

/

Synaptic

ransmission

nd he Neuromuscular

unction

A B IOGENES]S

Endoplasmic

reticulum

,/+s.

N,4yelin

heath

W)

. Y Y

cts. trans

Golgi

Golgi

---.-.

Endosomes

(veslcles)

ucleus

CELLBODY

Nerve

Terminal

B EXOCYTOSIS

FIGURE 14. Slnihesisrnd ,eclcJmg

f sy'naprLr:

resrcLes

nd hcif

onLcnL

. . ,

q

o r .

p , r L . \ , .

L .

.

i J . r [ ' e J . r

o

.

f r ,

. l L ,

,

LhaL are

homologous

Lo those associatecl \ritlt

s).naptic

vesiclesol higl-tervertebrates.Thus, the processesuncler-

I ) n -

. l u . i o n | c t c t

1 L ,

r J r

' , a

lular exocl,Losis nd encLocytosis.

Synaptic

r.esrcles

are sphelical

organelles

wtrh

a cliame

Ler of

'10

to 200 nm. ,\s

shou,n rn Flgure

8 14A, syrap,

r ic

vesicles are procluced n

rhe neuronal

cel1 boc\, by a

process similar ro Lhc secretory pathway

(p.

36).

- lhus,

port

(p.

26) mediated

1.the microtubule

system, 'hich

alsocarrLes itochondria

o the ten-ninal.

VesiclesdestinedLo c ontain pepti,le eurotransmjtters

travcl

down

the axon with Lhe presynthesizecl eprLdes

r

pcplide

precursors

already .rside. On arrival

at the ner\.c

terninal

(Fig.

8-148). the vesicles-now

calledsynaprlc

vcsicles Lhat carry pepticle

-reurotransmrtters

ecome aL

tached o the acrinbasecL

yLoskeleLaLet\\'orh.

Other ves

. . 1 , . . ' c .

' l c J

, r i h

r o n f . . J n ,

u o d r - n

r ,

r -

c . g . .

Vesicleand peptide

neurotransmitter

recursors

nd

enzymes

rc synlhesizedn ihe

cell and are released

rom Colgi.

Vesicles

ravel through the

axon

on

rnicrotubr e tracks

via

fast axonal ransport.

Peptidc

ncurotransmitters

are

already n somevesicles.

Nonpeptide

neurotransmitters

rc

sy1'Ithesized

nd transported nto

vcsicles he n nerue ern]inal.

A nonpeptide

neurotransmitters

svnihesizedn the nerve erminal

and transported

nto a

vesicle.

**:;oii

a. ) - f l


18/26

OUTSIDE YNAPTIC

ESICLE

INSIDE YNAPTIC ESICLE

Synapticransmission

nd heNeuromuscular

unction

8 221

cles

p..12)

recolers

membrane ompenents

nd recycles

them to an endosorne

omparLmentn Lhc erminal.

Syn-

ap1lc

vesicles

may then be reformed

within rh e rerminal

for

reuse in

neurotransmission,

or the)' ma)' be

trans-

ported

back ro Lhe ccll bod).for tu].nover

nd degrada

Lton

,

i r .

r

r L .

b . u r . ,

d . .

. L u

,

n , n l l , " I ' e n o u .

) s -

tem zrnd heir- elatively'

niform sizeand

molecular om

posit ion,

ynaptic esjcles

an be obrained n largequantr-

t ies

trom

ya

ous sources

uch

as

the rat bra in and

the

eleciricorganol the loipf, lo ray. The puril ication

of

syn,

aptrc

veslcleshas

n.rade t possible

to anal)'zeLheir com,

position, r,vhichhas

hciLitaLedgene cloning

ancl the mo-

lecularcharacterizatjonl nra[y proteins hat are ntrinsic

to

s)'napticvesicle

function. Figure

8 15 sunmarizes

a number ol the n-rajor

lasses f s)'naptic esiciepro-

te ins .

l

.

, L e

,

n l ,

l t . J (

t . , l o 1 a '

r .

1 e

- .

r , ,

u r . .

phshedb1.the combinaLion

l a vacuolar

ype

H' ATPase

' t

l

t .

r

' r r o

, I

r L

t . . n

- o t .

r l r o . c t n .

p

\ a c u o l a r .

type

H+ pump

is a large, n.rulLjsultur.rir

omplex

thar

cat;r1,vzeshe inr'varclmovemenl of H- into

the vesicle,

coupled Lo the hvdrol l,sis

[ cyrosolicATP

to adenosine

diphosphrte

ADP)

and inorgar.riclosphaLc

p.

64). The

resulting pH

ancl

voltage gradients

across

tlte vesicle

' ' _

"

,

. . , . t o l r

) n t L e r J i n L J

the \esicle by a unique fami\'

ol neurotransmitter

trans-

por t pro te ln< l

\ . h"

I

Pur^ l r .n-

r ' l '

-

In

|

.

c )

tosol

for Hf in

the r.esicLe.hls family'

of transporrers

includes members

specilic br ACh, monoanines

(e.g..

serotonin).catccholaminese.g., norepineplrrine), lr,rta

male, i1nd

GABA,/glycine

Another cl onccl sYnaltt ic esicle protein

named

SV2

(Ior

synaptic vesiclcproteln2)

structurally esembles

transpor-t

rotein:

hou'er.er,

transpor-t ubsLrateor

SV2

l.urs

not

been icLentillccl ncl is function

is unknor,r'n.

Synaptobrevin is an lg-kDa

s).naptjc vesicle

prorcin

contalning one transnembrane scgntent.

SlmapLobrevin,

which is a v-SNARE p. 39), ]s essentialor transmitter

ielease.As

discussecln the next section,

slnapLobrei' in

nn

tL.

, t

i , n r

, r ' ,

-

I

' - r ' .

. - r

. r '1p | .1

\ '1

. \ \

. -s

' p r T

o n

r , ,

. . . I 1 ; .

, .

r 1 L r .

, n C

" ,

p - d t \ .

. c .

.

,

l . r .

n l f . , c . ' .

1

r , 1

1 t

1 , " 1 , 1 i . 1 , 1 1 . '

) : r .

B , D , l , . n J

{ .

are

encLoproteinases

hrt digestsynaptobrevin

nd a re po-

, . n t r n h h r n ' - n l - . n , ' r r i . . - ' . 1 ,

' | \ o

) ' o

Rab3 is a mcmber

ol a

large anrily

of lou-molecular-

rveightGTP

binding

proreins

har appears o be unl\,er-

sally invoLvecl n cellular membrane

rafllcking

(p.

39)

via

the blnding and hydroLysisf

GTP.Synaptotagmin s he

slnaptic

:esicle

az' recepLor, proLein

'ith two external

repetiti\'e

domains tl.iaL re homologous

o the C2 clonuin

of

protein l


19/26

@

222

8

/

Synapticransmissionnd heNeuromuscular

unction

s).naptic

Yeslcle

proteins that are phosphorylatedby both

cyclic adenosine

monophosphate

(cAMPldependent

and

calmoduLin-dependent

protein kinases. Interactions

of

sl,napsins

wlth

cytoskeletalproteins

and their

inhibition

by

phosphorylation

have led to the notion thar s)'napsins

normally mediate the attachment o[ synaptic vesicles o

lhc ac

n cytos l ,e le to l

W' t l ^ r , rL -edsen

[ (

a

I

; r rd

subsequent

phosphorylation, the synapsin detaches and

permi ts

e . lc

e ' to

ro \e

io a t . i re ' i tes a r ,he ' y .nao- i ,

membrane.

Neurotransmitter

ReleaseOccursby

Exocytosis f SynapticVesicles

Al houg.

rhe

necha-

sn by

wn.L

s1-ap t ic

es ' .

e ,

fu ,e

with the plasma

membrane and release

heir

contents is

far from

fuLLy nderstood, ve have working models

(Fig.

8-16)

for the function of various ke)' co-Oon"rlrr und

steps

involved in s)'naptic

vesicle

release.These models

are based

on a

variety

of

in vitro

experimenls.

The

use of

speciflc

toxins

that acr at nerve

slnapses

and elegant

functional studies of genetic mutanls in Drosphiha, C.

elegans,

nd

gene knockout mice have provided important

information

on the roLeo[ various components.

We

have already

ntroduced

the key proteins located n

the s)'naptic

vesicle. Of these,

we

now focus on the

v

SNARE

yrap tobrer -

and the

La sen)o- )mdPlo lag

min.

ln addition,

several other protelns-Located in the

rarget area

o[ the presynaptic membrane of the nerve

terminal-play

an

important role

in the fusion

process.

S).ntaxin

is anchored n lhe preslnaptic membrane by a

single

membrane-spanningsegment. SNAP-25

(slnapto-

come-eqsocir ' led

rote1

-21

kDa)

i. tethe-ed o tle

pr e

- ) r - ldpr r

memhran(

ia

pc r

to )

,de cLa in ,

BoLh

yr-

taxin

and SNAP-25 are I-SNARES

(p.

39).

Borulinunr

ro^ ; rsA ar rd

t . r r \ch a -c e dop loLea 'e ' .

' pecf i , r1 ly

cleave

SNAP-25,

whereas another endoproteinase,botuli-

num toxin

C],

specifically

cleavessyntaxin.

These

toxins

b lock he us ion l synapt rcestc1c. .

According

to the

model

shown in Figure 8 16, dock

ing of

the vesicle

o the pres)-naptic

membrane occurs as

n Secl

dissoclates

rom slntaxin. The free

ends of s1'nap-

tobrevin,

s)'ntaxin,

and SNAP-25

begin

to coil around

each other.

The result is a ternary compLex,an extraordi-

narily

stable

rod-shaped

structure o[ a helices. As the

energeticaliy

avorablecoiling of the three

SNARES

ontin-

ues, th vesicle membrane is pulled ever closer to the

pres),'naptic

membrane.

Car+

enters

through

voltage-gated

Ca2*

channels

ocated in register with the active zone

of

Lhe

pre .w. rp t ic

membrane.

'o . : l

in . e rse n

[Cr ]

triggers

he

final event, usion and exocytosis.The synap-

ric vesicle

protein synaptotagmin is believed to be the

actual

sensor

of increased

[Ca2+],

because

knockout n-rice

whereas he , l ,n ra^ inand SNAP

5

on lhe p res)Tapl ic

membrane are availabLe

or

the

next round

of

vesicle

fusion.

The

modeL

ust

presented eavesunanswered

some

im-

portant queslions. For example, whal is the struclure of

the fusion pore detectedby electrophysiologicalmeasure-

menLsas a

primary

event

in membrane fusion? Also,

the

model

does not fuily explain the basis for the rapid catal-

ysis of lusion by Ca'z*.Neuroscientists re very interested

in the

deLailso[ synaptic vesicle usion because his exo-

cytotic

process might be a target lbr controlling synapllc

srrengrh

and may thus play a role in the synaptic plasric-

ity that is responsible or changes n animal behavior.

Re-Uptake r Cleavageof the

NeurotransmitterTerminates

Its Action

Eflective transmission across chemical syeapses equires

not only

releaseof the neurotransmitterand

aciivaiion of

the

receptor on the postsynapticmembranebut aLso apid

and e lh . ien tmechdn i ,msor removing he ran .m.L te r t

synapses

where ACh is released, hls removal is

accom-

plished by enzymatic destruction of the neurotransmitter.

However, the more

general mechanism in

lhe nervous

- r . re r

r ro l res

-e- r . t rLe

n f

the

neJroL-dnsr ' t

Ler

ned. -

ated by

specific, high-affinity transporl systems ocated n

the

presynaptic plasma membrane and surrounding glial

cel

.

fl-ese secordaryaclive r-d->po-L ys .emsu5e lhe

normal

ionic gradients

of

Na+, K*, H*, or

Cl to

achieve

concentralive

uptake of transmilter. Vertebrateshave two

ots nct

[a r ' res

o f

neL-oL_a

n

t re r

I rdn5porL roLe >

The first

family is

characterized y

a

common

motif

of

12

membrane-spanning

segments and incLudes

ransporte$

wi ln

>pec

c i t l fo cr rec l -o lan ines.

e roron ,n .

ABA.

1-

c ine .

and ,ho l ine . rnergy coup l ingo I l ranspo l in lh ic

class

o[ 'v,rems is generally asedon tor"ansporto[ the

substrate

with Na* and

Cl

. The second farnily is repre-

sentedby transporters or the excitatory amino acids glu

lamaLe

nd asparLaLe.n

i l -e 'e

a t te '

.1 te ' r ' .

sub ' t -a te

t r rnspor t

Bener r l l )

o

rp l " ,

o

co l ra rcLdrunct ,o r . ALhF acl i l ) can be de-

tected throughout

the nervous system.The enz).meoccurs

in a

variety of physical forms. The globular or G forms

exist

as monomers, din-rers,or tetramers of a common,

approximately

72 kDa glycoprotein catalytic subunit.

These molecules can be lound either

in

soLuble

orm

or

bound

to cer m.nb-a-e .v ia a CPI Ln lage

p.

t5 ) in


20/26

E

Vesicles

with svnaptotagrrinand

synaptobrevin

a

V-SNARE)

move

to

the nerve

erminal membrane,

h.hich contains

yntaxinand

SNAP-25

both

SNAREs).

Synaptic

ransmlssionnd he Neuromuscular

unction

8

INITIALSTATE

FORI\,1ATION

F

TEBNARY

COIVIPLEXF SNARES

q-SNAP

and the ATPaseNSI bind

to

the ternary SNAREcomplexand

use

the energyof

ATP

hydrolysis

o

disassemblehe SNARES.

TIGHTENING F TERNARY

SNARECOI\,4PLEX

The entry

of Ca2*and ts binding

to synaptoiagmin

riggers usion.

Synaplotagmin

Synaptic

I

vesrcle_

|

.

,,,.,,,,

,,,

l(

. - t

. ,F

"

' ' a " /

' '

. . t . .

- \ l

.

- : -

)

.::

:.

. .,.. ::

::.:

-

;hravin

-

'

n-Sec-1

l*

?

n'Sec

1

memorane

RECYCLING

FSNARES

0-SNAP\

Zt

n sec 1 dissociatesrom syntaxin,

allowing

the

syntaxin

ard

SNAP-25

o

Iorm

a

complex.The distal end of synaptobrevin

begins o

l\rind

around

the syntaxin/SNAP-

25 complex, onning a temary conplex.

It

The three SNARE5 synaptobrevin,

syntaxinand SNAP-25-continue

to form a tight bundle of o helices,

draliring

he vesicleand presynaptic

membranes nto closeapposition.

Syntaxin

DISASSEI\,lBLY F TERNARY

SNARECOMPLEX

FUSIONNDEXOCYTOSIS

fr

,-il

]

With

the endocytosis f the

vesicle,

he synaptobrevin

is

effectivel)'

ecycled.The

syntaxinand SNAP-25are

now ftee

for an additional

cycleof

vesicle usion.

F | c U R E 8 - 1 6 ' N l o d e ] o | s ] n a ] r r i c ' e s i c ] e l u s i o n a n c | e r o c \ ) S i s , { D P .

scnsiri|r

lacLori sN.{l'-25.

slnaptosone-associared

rorein

2i

l{Da: d

S\AP. solublc

NSF rt(dchlnenr

pforern:

SN-\RF. SNAP

r.LtPio

o-SNAP

D

&

NSF

I

I

. )


21/26

8

/

Synaptic ransmission

nd

the Neuromuscular

unction

DISEASES

FTHE

HUMANACETYLCHOLINE

ECEPTOR:

MYASTHENIA

RAVIS

ND A

CONGENITAL

YASTHENIC

YNDROME

The term "myasthenia"means

muscle

weakness

from

the Creek

mys

and asthenia)

nd is

used clinically

o

usually

mean

weaknessn

the absence

f

primary

muscle

disease, europaLhy,

r central ervous

yitem

d'isorder.

Myasthenia

gravls,

one specific

ype of myasthenia

and the mostcommon

adult orm,

afflicts 5

to 125 of

every1 mill ion

people.

t

can occur

at any age

but hasa

bimodaldistribution, ith peak

ncidences

ccurring

among

people

n th eir 20s

and

60s.Those lf l icted

t an

early age tend to be women

with

hyperplasia

f the

thymus, whereas hosewho are older are more

likely o

be

men wilh coexisting

ancer

t the

thymus

gland:

Th e

cellsof the thymus possess

icotinic

acetylcholine

ecep-

tors

(AChRt,

and the

disease

rises sa result

f antibod-

iesdirected gainst

hese eceptors.

he

antibodieshen

lead o skeletalmuscle

weakness

ausedn

Dart

bv com-

petit ive

nLagonism

f AChRs.

ymptomsnclude'fatigue

and weakness f skeletal

muscle.

Two maior

forms of the

disease re recognized:

ne hat

involves eakness

f

only the extraocular uscles nd another hat resultsn

generalized eakness

f

all skeletal

muscles.n

either

(ase,

myastheniaravis

s

ypilied

by

flucLuating

ymp-

toms,

with weaknessrealesl

oward

he end ot the

day

or after exertion. n severe

ases,paralysis

f the respira-

tory muscles an lead to

death. Treatment

directed

at

enhancing holinergicransmission,

loneor combined

with thymectomy r immunosuppression,

s highly

effec-

tive

in most

patients.

Progressowardachieving n understandingf the

causeof myasthenia

gravis

was

first made

when electro-

physiologic

analysis

f involved muscle

evealed

hat the

amplitude f the miniature

nd-plate otential

was de-

creased,although the

frequency

of

quantal

events

was

normal.This inding

suggested

ither

a defect n the

postsynaptic

esponse

o ACh

or a reduced

concentration

of ACh n the synaptic

esicles.

major

breakthrough

occurred n 1973,when

Patrick

nd Lindstrom

ound hat

symptoms imilar o th oseof humanswith myasthenia

developed

n rabbits

mmunized

with

AChR

piotein puri-

l ied rom the eleclric

el.This indinq

was

shortlv ol-

lowed by the demon stration

f anti-AchR

ntibodiesn

human

patients ith

myastheniaravis

nd a severe

e-

duction

n the

surface ensity

f AChR

n the

iunctional

folds.

These nti-AChR

ntibodies

re

directed

gainst

one or

moresubunits

f

the receptor, here

hey

bind

and activate ompl ement

nd

accelerate

esLruction

f

the receptors,The most common target of theseantibod-

ies s a reg ionof the AChR

d subunit

calledMIR

main

immunogenic egion).

Myasthenia

ravis

s now

recognized

o be

an acquired

autoimmune isordern which

he

spontaneous

roduc-

Lionol anti-AchR ntibodies

esulGn

proqressive

oss f

muscleAChRs nd degeneration

f poatju;ctional

olds.

patient's erum).Somepatients ith mya sthenia ravis

havea thymoma

a

tumor of

the thymus

gland)

hat is

often readily eenon routine

hest

adiographs.n

these

patients,

emoval f the

thymoma eads

o clinical

m-

provement

n

nearly75o/o

f the cases.Enhancement

f

cholinergic ctivity

s achieved

hrough he

useof AChE

inhibitors,

ith

pyridostigmine

eing he

mostwidely

usedagent.The dosage

f thesedrugs

must be

carefully

monitored

to

prevent

oyerexposure

f

the remaining

AChRs o ACh.

Overexposure

an lead o overstimuiation

of the

postsynaptic

eceptors,prolonged

depolarization

f

the

postsynaptic

embrane,nactivation

f

neighboring

Nat

channels, nd thus

synaptic

lockade.

Another

condition characterized

y progressive

muscle

weakness

nd fatigue s

the Lambert-Eaton

syndrome

(see

box on

p.

193).Lambert-Eaton

yndromes

caused

by antibodieshat attack

he

presynaptic

a2"

channel

and can be distinguished

rom myasthenia ravis

n sev-

eral

ways.First,

t

primarily

ttackshe limb

muscles,

ot

the ocularand bulbarmuscles. econd, epetit ive

timula,

t ion of a

particular

muscle eads

o enhanied

amplitude

of the

postsynaptic

ction

potential,

whereas n

patients

with myasthenia, epetitive

stimulation

eads

o

progres-

sive essening f

the action potential.

Thus, repeated

muscle t imulationeads

o increasinq

ontractile

trenqth

in

patients

ith Lambert-Eaton

yndr-omend

to decreis-

ing strength n

patients

with

myasthenia.

The term congenital

myasthenic

syndromes

(CMS)

refers o a variety of inheriteddisorders,presentat birth,

lhal aflectneuromuscular

ransmissionn

a

varietv

f

ways.Because

pecif ic ases

an nvolve cetylcholinester-

asedeficienc, abnormal presynaptic

elease

f ACh,

or

defective

AChR

unction

(without

the

presence

f antire-

ceptorantibodies),he signs

and symptoms

an alsovary

widely. n 1995,an

unusual xample

f a CMS

d;sorder

was raced o a mutation

n the

o

subunit

of the human

AChR.Single-channel

ecordings

rom

biopsiedmuscle

i-

bers of a young myasthenicpatient revealeda profound

alterationn AChR

inetics. he

burstduration

of AChR

openingswas

greatly

prolonged

n

comparison

ith that

of normalhumanAChR

hannels. he

molecular

efect s

a

point

mutation

of Thr to Pro

at

posit ion

64

in the

adult subunit

of the AChR. his

bminoacid

esidue

correspondso an

evolutionarily

onserved

osit ion

n the

M2 membrane-spanning

egment,which

is nvolved

n

Jormation

f the channel ore.

Thus,a human

mutation

in the pore egionof the AChRprotein esultsn failure f

the channe l o

closenormall,

thereby ausjng

xcessive

depolarizationnd pathologic

onsequencest

the mus-

cle end

plate.

The aforementioned

utation

s only one

of at least

3

mutationsn

55 different

inshipshat have

been denti-

f ied n the AChR.

Someof

the othe r mutations

esult n


22/26

FICURE

B 17. Pharma.oLogr of

rhe rcrlebrate

neuronuscular

jun.

lion N1:uy

of thc pnneins rhal are

jnvohed in slrapl ic lransmissron

at

rhe mammalian

neuromuscLrlar

lLrncrion

a.c

thf 1.rr8ers

l

narLrnll)

occurnng or

slnLLleLi.

drugs

The

anugonEts rfe

sbown

:rs

'

signs

highlithtd

jn

fcd

Thc agonisLs re

shown

as

+'

signs hiilhlighted in

gr..n

. e r n \ d , r c

" h , .

I

\

d \ e . a L e

l f o u p

r

. o . - r e n L

)

. . r .

. l r l , o ' - r i n e u r n r r n n n , h n n n ; . , n f h F - F ^ . d - ' ,

is the

hydrolysLs and release

ol this acetate.

as

\\,eLl

- - r h e t c p

p n / . n .

\ r h

" h - . f F .

r r \

- . o

n , , i

uplake

syslem, he nerve lerminal recovers he chollne

formed in Lhrs eactionand uses t lor the svnthesis f

ACh.

TOXINS

AND DRUGS FFECTING

SYNAPTIC

RANSMISSION

Much ol our

knou'ledge

of the s)'naptic

physiology

of

the

neuromuscularunction and the identltres f its various

molecular

components have

been derived from experi

, r l f . . F , m , n l n o r . o p n , , . , 1 , \ n - L - i

l l e - r

f r .

i " n ; l

d i

- '

o r o h ,

)

p - r

- g . e 8 . 7

illustrates he reLative yl.Iap lic ocation and corresponding

pharmacology

of AChE, as well as several on channeis

irnd

pfoteins

involved 1n

exocytosis.

Synaptic

ransmissionnd the Neuromuscular

onction

8

entire

process.

s cliscussecln Chapter7, LhedepoLarlz-

ing

phase

ol LheacLion

otential s mecliated

y voltage

dependent

Na'

channels hat are specif ically Jocked ,v

nanomolar oncentratiolrsl the small

guanidiniun

neu

rotorins tetrodotoxin

(TTX)

and saxitoxin

(STX)

(see

F ig.7 5C) .

The mamba snaLe to)iin dendrotoxin

(p.

19.1)has

an

effccL haLs precisely pposi Le hatol TTX: it facilitates

tl ' le

release

f

ACh

that

is

evoked by

nerve

stimulation.

1pr ' .

^ r^ 1 \ / ' / ' |

" f

,

) o \ rnr te \ , r . l

e r . tLr -

proteinsuJiih

hreedisulf idebonds that block certain

so

f . m . , f , k . h ' n n l - h . n r n

, r r - . . ] I ' r l i , r

.

. n . h P r . . , J . , t r n

" t \

L 6

. l [

-

'

" h . . .

n r

-

- e

r

-

- . -

I n

- c l .

. , 1

. .

. l ' a . e ,

in terminating he process [ transmltter elease. lockadc

ol

preslnapticK

clrannels l dendrotoxin

nhibiLs epo

l a r z , t n o l

i \ ,

1 - , - 1

, f l . r r ' n ' - h , r c L

; 1 r " l " r r g

I n g l . J r ' i o n o ,

c . , t o

p t (

r .

, r d f u . r t - t . g

L l r c

releasc o[ transmittel in responsc to the entry of extra

Caz-

nLo

he nen'e e rmLnaL.

Acetylcholine

Nicotine

E

d-Tubocurarine

E.r-Bungarotox

unction


23/26

226

8

/

SynapticTransmission nd the NeuromLrsculaf

TABLE

A_2

TARGET

"

h r n ,

r

i n l p , i n n

L r

h e c e

l o q 1 q 1 , c d n l e d d t o

death

because he toxins that they slnthesize are potent

inhibitors of neurotransmitter release.This inhibition oc-

cum

becauseboth tetanus and botulinum toxin proteins

harez ' - t -depedenL ndopro re ina.e- r i \ i l y

lab le

8 2)

These toxins enter nene terminals and specificallycleave

three

different proteins required for slnaptic

vesicle exo-

cytosis.

Tetanus

toxin and botulinum toxils B, D, F,

and G

cleave s1-naptobrer.in, n inlegral membrane pro-

tein of the s)'naptic vesicle membrane. Botulinum toxins

( 1

" - A

a r r

. " . - . . , , , . " t . . . , , . c y r t d y r n

n J S \ A l 2 5 .

two

proteins associated

with the

preslrraptic

membrane.

These

neurotoxins can have

uselul

medical

applications.

For example, botuLinum toxin is used Lo treat cenain

disorders characterizedby muscle spasms. njection of a

smal1

amount of botulinum toxin into the eye muscles

of

a patient

with

strabismus

a

condition in which both eyes

cannot

focus

on the same object because of abnormal

hyperactivity of

particular

eye

muscLes)s

able to suppress

aberrant

muscLe pasmsand restorenormal vision-

Both Agonistsand Antagonistsof the

Nicotinic

Acetylcholine

ReceptorCan Prevent

Synaptic

Transmission

The ionotropic

(nicotinic)

AChR channeL ocated in the

posts)'napticmuscle membrane

(see

Fig. 8- 17) aLsohas a

rich and diverse

pharmacology

har can be expLoi ted or

clinical applications,as

weLl

as

for

elucidating many func-

to-a l a .pecr . f t he neuro. rus ,u lar1 . - rn ,.o . . F igu reB-

lB shows the chemical structures of two classes f agents

that

act on the nicotinic AChR. Theseagentsare

cLassifred

as

agonistsor antagonistsaccording to whether they acti-

\ a le

open ingo1 the .hannel or prevent rc ac l iva l ion.

Many agonists have a structure similar to that of the

' , lu rJ l

neuroLrdn) r r . tLereh. In

genera l .

uch agonis t '

activate

the

opening of

AChR

channels

with

the same

u r i t a r y

o n d u , t a n c e.

t h o . e

a . n v a t e d1 A ( h . b u r

^ h

d. f ferenL e l ' c , o l chan ne ope-

rg

a-d

co, ng rhe

synthe

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