Download Synaptic Transmission 1

SynapticTransmission
Neuroscience201A
PeterSargent
1.
2.
3.
4.
Electricalvs.ChemicalSynapticTransmission
CharacteristicsofLigand-GatedIonChannels
Transmitter Release
TransmitterReleaseStatistics
“Non-traditional”methodsofmeasuringtransmitterrelease(voltage
sensitivedyes,voltammetry,capacitance)– notcovered
IntegrativeMechanisms– KevinBender
SynapticPlasticity– RogerNicoll
NeurotransmitterRelease/Uptake– RobEdwards
SynapticModulation(transmittersactingviametabotropicreceptors)–
JenniferWhistler
Whatisasynapse?
• Asynapseisasite ofcloseapposition betweenaneuronandatarget
cell,whereanelectricalsignalinaneuronleadstoachangeinthe
probabilitythatitstargetcellwillgiveanactionpotential.
– iftheprobabilityincreases,thesynapseisexcitatory
– iftheprobabilitydecreases,thesynapseisinhibitory
Here,wearelookingatasectionofasynaptic
boutonwithtwosynapticcontacts,ortwo
synapses.
tripartite synapse
(pre,post,glia)
tetrapartite synapse
(pre,post,glia,ECM)
(ECM=extracellular
matrix)
DityatevA,RusakovD
(2011)Curr.Opin.
Neurobiol.21:353-359.
Electricalsynapses
“gapjunction” =
collectionofconnexons
Electricalsynapsespermitdirectcurrentflowfrom
onecelltotheother
Smallmoleculescanreadilypassbetweencells
connectedbygapjunctions(connexons)
• Moleculesofupto~2kDcan
passfromacelltoits
electricallycoupled
neighbors
• Fluorescentmoleculessuch
asluciferyellowwillpass
fromcelltocoupledcell–
suchcellsaresaidtobe
“dye-coupled.”
PatricioO'Donnell&AnthonyA.Grace,AlbanyMedical
College,Albany,NYandUniversityofPittsburgh,
Pittsburgh,PA
Whyhavechemicalsynapses?
Advantages
Amplification
Unlesstheterminalandthetargetcellarecomparableinsize,
electricalsynapsesarenotlikelytobeeffective(impedance
mismatch)
Polarity(+,-)
Modifiability/Plasticity
Disadvantages
Lossofreliability
Speed(losetimeduetosynapticdelay)??
Howdoyoudistinguishanelectricalfroma
chemicalsynapse?
• Anatomy
– dyecoupling?
1
• Physiology
– synapticdelay?
– quantalfluctuations?
– reversalpotential?
2
Sequenceofevents(chemicalsynapses)
1.
2.
3.
4.
5.
6.
7.
Anactionpotentialarrivesintheterminal,
which
activatesvoltage-dependentcalcium
channels.
Calciumenterstheterminalandpromotes
fusionofsynapticvesicleswiththeplasma
membrane.
Transmitterisreleasedintothesynaptic
cleft,and
bindstoreceptorsonthepostsynaptic
membrane.
Thechannelsassociatedwiththese
receptorsopen,whichallowionstoflow
downtheirelectrochemicalgradient,
excitingorinhibitingthecell.
Transmitterisremovedfromthecleft,and
thesynapseis“reset.”
OUTLINE
(wearegoingbackwards)
1.
2.
3.
4.
Electricalvs.ChemicalSynapticTransmission
CharacteristicsofLigand-GatedIonChannels
TransmitterRelease
TransmitterReleaseStatistics
Simplestatefunctionfor
anionotropicreceptor
(skeletalmuscleacetylcholinereceptor– AChR)
A+R « AR +A« A2R « A2R* « A2R
closed
Inside-out
patch
closed
closed open desensitized
(closed)
bursting:movement
betweenopen(A2R*)and
closed(A2R)states
A+R« AR+A« A2R« A2R*« A2R
closedopendesensitized
(closed)
b
A2RA2R*
a
closedopen
forthemuscleAChR,a =700s-1andb =30,000s-1
Intheabsenceoftransmitter,therateofclosingwillbe
governedbya.Thetimeconstantdescribingthefallin
currentwillbe1/a,whichis1-2ms,whichoutlaststhe
waveformof[ACh]whenacetylcholinesteraseispresent.The
decaytimeofthecurrentthusreflectsthebehaviorofthe
channels(notofthetransmitter,whichhasbeendestroyed).
Atothersynapsesdecayofcurrentmaybeinfluencedby
transmittersurvival.
[transmitter],mM
Whathappenswhentransmitterisavailablefora
briefperiodoftime?
6
5
4
3
2
1
0
0
0.5
1.0
Time,ms
1.5
2.0
Principlesofrapid/focalsynaptictransmissionvia
ionotropicreceptors
• Theaffinityoftransmitterforreceptorsisusuallylow(10-100µM)
• Releaseleadstoahighconcentrationoftransmitter(1-10mM)fora
briefperiodoftimeinasmallvolume
• Receptoroccupancycanbesubstantial,despitelowaffinityof
receptorsfortransmitter(arereceptorssaturated?)
• Diffusionanduptakeremovetransmitterfromthecleftquickly,so
thattransmittertypicallyhasonly“onechance” tobindtoreceptor
• Thesynapseiswelldesignedforrepeateduseathighfrequency(up
to»1kHz)
IonicBasisofExcitation/Inhibition
• Excitation
– Cation-selectivechannels
(Na+ isthemajorchargecarrier)
• ligand-gated:nicotinic,5-HT3,AMPA,NMDA,…
• TRPchannels(transientreceptorpotential)
• Inhibition
– Anion-selectivechannels
(Cl- isthemajorchargecarrier,HCO3- isalsoinvolved)
• ligand-gated:glycine,GABAA,GABAC
• Q:Canactivationofchloride-selectivechannelsbeexcitatory?
– K+ selectivechannels
Measuringtherelativepermeabilitiesofionotropic
receptorchannels
• Reversalpotential:thepotentialatwhichthecurrentis“reversed” in
polarity,ornulled:thepotentialatwhichthereisnonetcurrent
producedbyopeningthechannels
• Whatisthereversalpotentialofalloftheleakchannelsofthecell,
actingtogether?
• Q: Howtomeasurethepermeabilityofachannel
(orasetofchannels,actingtogether)?
• A: Measureeffectofchangingioniccompositionuponreversal
potential
Thenicotinicacetylcholinereceptorchannelisa
cationchannel
replacemostofextracellular
sodiumwithglucosamine
(impermeant)
AdamsDJ,DwyerTM,Hille B(1980)Thepermeabilityof
endplatechannelstomonovalentanddivalentmetal
cations.JGenPhysiol 75:493-510.
Thenicotinicacetylcholinereceptorchannelisa
cation channel(cont.)
E1andE2representreversalpotentialsforNaandforthe
ionX,respectively
Einthisequationcandescribeonechannelorafamilyof
channels(e.g.,allleakchannels).
DoesAChopenonechannelortwo?
FattandKatz(1951)andTakeuchi
andTakeuchi(1960)established
thatactivationofAChRsdrivesthe
membranepotentialtoavalue
near0mV.
ThiscouldbeexplainedifACh
openedonechannelclasspermeable
tobothsodiumandpotassium,buta
secondpossibilityisthatAChopens
twodifferentchannels,oneforNa+
andoneforK+.
TheGABAA channelisananionchannel
here,inwardcurrent
(negativeinsign,by
convention)means
movementofCl- in
whichdirection?
MouginotD,KombianSB,PittmanQJ(1998)ActivationofpresynapticGABAB receptorsinhibitsevoked
IPSCsinratmagnocellularneuronsinvitro.JNeurophysiol79:1508-1517.
UnderwhatconditionswouldmovementofCl- beexcitatory?
Inhibitionoccursvia twomechanisms
• Hyperpolarization
– Whenthemembraneishyperpolarized,itismoredistantfrom
thresholdand,generally,morecurrentisrequiredtoreach
threshold
• Shunting(short-circuiting)
– Evenifthemembraneisnothyperpolarizedmuchbytheactionof
aninhibitorytransmitter,theeffectisstillinhibitory,sinceCl- (or
K+)movementwillopposetheconsequencesofinwardcurrent
producedbytheactionofexcitatorytransmitters.
– Shuntingisthepredominantformofinhibitioninthenervous
system!
OUTLINE
1.
2.
3.
4.
Electricalvs.ChemicalSynapticTransmission
CharacteristicsofLigand-GatedIonChannels
TransmitterRelease
TransmitterReleaseStatistics
Calciumchannelsopeninresponseto
depolarization
Voltage-clamp
•
CalyxofHeldsynapse
Isolatingcalciumcurrents
pharmacologically
•
Sourcesofsynapticdelay
YangY-M,WangL-Y(2006)Amplitudeandkineticsofactionpotential-evokedCa2+ currentandItsefficacyintriggering
transmitterreleaseatthedevelopingcalyxofHeldsynapse.J.Neurosci.26:5698-5708.
Calciumisnecessaryforrelease
• ConsequencesofinjectingBAPTA,afast-actingCa2+ buffer
BucurencluI,KulikA,SchwallerB,JonasP(2008)NanodomaincouplingbetweenCa2+channelsandCa2+
sensorspromotesfastandefficienttransmitterreleaseatacorticalGABAergicsynapse.Neuron57:536-545.
Calciumissufficientforrelease
• Consequencesofelevating[Ca2+]i artificially.
Squidgiantsynapse
DelaneyK,ZuckerRS(1990)CalciumreleasedbyphotolysisofDM-nitrophenstimulatestransmitterreleaseat
squidgiantsynapse.JPhysiol426:473-498.
Releaseisdependentonthe≅4th powerof[Ca2+]i
CalyxofHeld(auditorybrainstem)
SchneggenburgerR,NeherE(2000)Intracellularcalciumdependenceoftransmitterreleaseratesatafast
centralsynapse.Nature406:889-893.
Fateofreleasedtransmitter
• Diffusion(Dis~0.5µm2/msec fortransmitters)
– Diffusiontimeisdependentonthesquareofdistance(t≈x2/D)
• Bindingtoreceptors
• Removal[Uptake,Hydrolysis(ACh)]
• Whathappensifyouinterferewithremovaloftransmitter?
Distance (µm)
Time (ms)
(approx.)
0.1
0.01
1
1
10
100
100
10,000
1000
1,000,000
1 meter
32 years
Directandindirectmethods
ofmeasuringtransmitterrelease
• Indirect
– PostsynapticResponse
– PresynapticResponse
• Dyedestaining(e.g.,FM1-43)
• Capacitancechanges
• Direct
– Voltammetry(amperometry)
OUTLINE
1.
2.
3.
4.
Electricalvs.ChemicalSynapticTransmission
CharacteristicsofLigand-GatedIonChannels
TransmitterRelease
TransmitterReleaseStatistics
Spontaneousrelease(“minis”)
(followingfromtheworkofBernardKatzinthe1950s)
1.
occurrandomly
2.
occurintheabsenceof
electricalactivity
3.
causedbytherelease
ofmulti-molecular
packetsoftransmitter
Evokedreleaseinlow[Ca2+]o shows“failures”
(followingfromtheworkofBernardKatzinthe1950s)
SaharaY,TakahashiT
(2001)JPhysiol
536:189-197.
Note“jitter”
(asynchrony)inrelease
process!
Note
fluctuationin
responsesize.
• Reductionof[Ca2+]oandorelevationof[Mg2+]oreducesEPSC
sizeandleadsto“failures”
ThesmallestEPSCsareequaltothesizeofthe
spontaneouslyoccurringevents
Note“jitter” (asynchrony)in
releaseprocess!
Notethatquantalresponses
arenotuniform!
IsaacsonJS,Walmsley B(1995)Countingquanta:directmeasurementsoftransmitterreleaseatacentral
synapse.Neuron15:875-884.
The“quantumhypothesis”
(delCastilloandKatz,1954)
• Transmitterisreleasedinmultimolecularpackets,or
quanta.Thesepacketsarereleasedspontaneouslyatlow
frequency.Thearrivalofanactionpotentialinthenerve
terminalgreatlyincreasesthefrequencyofrelease.(from
1persecto~100permsec inthecaseofthefrognervemusclejunction)
• Whatisthesignaldirectlyresponsibleforincreasingthe
frequencyofrelease?
TheKatzformalism
• “Supposewehave,ateachnerve-musclejunction,a
populationofN units capableofrespondingtoanerve
impulse.Suppose,further,thattheaverageprobabilityof
respondingisP,…,thenthemeannumberofunits
respondingtooneimpulseism=N*P.”
(delCastilloandKatz,J.Physiol.124:560-573(1954)
• Whatisthephysicalidentityoftheunits?
• foreachmemberofthepopulationN,theremayormay
notareleaseeventinresponsetothearrivalofanaction
potential
• transmitterreleaseisaprobabilistic,orstochastic,event.It
canbestudiedusingstatistics.
Howtotestthe“quantumhypothesis?”
(doestransmitterreleasereallyoperatethisway?)
• Trytopredicttheaveragenumberofquantathatare
released,assumingthatthehypothesisiscorrect,and
comparethistoadirectmeasurement ofthe“quantum
content.”
• Thedirectmeasurementofquantalcontentisthenumber
ofquanta.Usually,however,youcannotcountquanta.
Instead,youdividetheaverageamplitudeoftheevoked
response(epp)bytheaverageamplitudeofthemepp
(assuminglinearsummation).
– IsitOKtoassumelinearsummation?
Usethebinomialdistributiontopredictthequantal
content
• Thebinomialtheoremdescribesdistributionsofoutcomes
acrossasetoftrialswherethereareonlytwopossible
outcomes(heads/tails,success/failure,release/norelease)per
trialpermemberofthepopulationN.
• Ifp=probabilityofsuccesspertrial,thentheprobabilityP
(caps!)ofgettingksuccessesinntrialsis
ö
ænö æ
n!
ænö k
÷÷
çç ÷÷ = çç
P(k ; n, p) = çç ÷÷ p (1 - p) ( n -k )
,where è k ø è k!(n - k )!ø
k
è ø
• Theaveragenumberofsuccessesism=N*p
• Problem!– whatifyoudon’tknowNorp?
“nchoosek”
UsingthePoissondistributiontopredict
quantalcontent
(see“binomial”handout)
• Poissonisaspecialcaseofthebinomial,wherep<<1and
n® ¥
m k -m
P(k ; n, p) =
k!
e
Youdon’thavetoknowp!!!
• Probabilityofgetting0successes(afailure)
m0 -m -m
P(0; n, p) =
e =e
0!
T
P(0; n, p) = 0 ,whereTisthenumberoftrialsandT0
isthenumberoftrials
T
resultinginafailure
æT ö
m = ln çç ÷÷ the“failuresmethod”
è T0 ø
(later,theCVmethod(m=1/CV2)
SynapsesobeyPoissonstatisticsatlowprobabilities
ofrelease
mbythe“failuresmethod”
mbythe
“direct
method”
IsaacsonJS,WalmsleyB(1995)Countingquanta:
directmeasurementsoftransmitterreleaseata
centralsynapse.Neuron15:875-884.
• Measurem(directmethod)andcomparetomcalculated
fromthefailuresmethod
Thequantumhypothesisisverified.
Sowhat?
• Whatisthesignificanceofthefactthatthepoisson
distributionsuccessfullypredictsm?
• Biology
– thatevokedresponsesareconstructedfromthesame“units”
thatoccurspontaneously(thisiscontroversial– e.g.,Kavalali’s
work)
– thatthese“units” (releasablequanta)arereleasedindependently
• Methodology
– youmightbeabletoestimatewhetherachangeinthesizeofan
EPSCiscausedbyachangeinquantalcontent
“Synchronous” vs.asynchronousrelease
Releaseisnevertrulysynchronous,
SaharaY,TakahashiT
(2001)JPhysiol 536:189197.
Note“jitter”
(asynchrony)in
releaseprocess!
but itcanbeclosetosynchronous(esp.atphysiologictemperature).
SargentPB,Saviane C,NielsenTA,DiGregorio DA,SilverRA(2005)
J.Neurosci.25:8173-8187.
Asynchronous(Delayed)Release
AtluriPP,RegehrWG(1998)Delayedreleaseofneurotransmitterfromcerebellargranulecells.JNeurosci18:8214-8227.
IntroducingaCa2+ bufferintotheterminalreduces
delayedrelease
• parallelfiber– Purkinjecell
synapseincerebellum
• EGTA-AMintothebath
EGTA-AM
iCa
PSC
PSC
esterase
PSC
EGTA
ChenC,RegeherWG(1999)Contributionsofresidualcalciumtofastsynaptictransmission.JNeurosci19:6257-6266.
Synapticconnectionsshowwidediversityinthe
importanceofasynchronousrelease
HefftS,JonasP(2005)AsynchronousGABAreleasegenerateslong-lastinginhibitionatahippocampalinterneuron–principal
neuronsynapse.NatureNeurosci.8:1319-1328.
StructureoftheNerveTerminal
• synapticvesicles
• “docked” vesicles
• activezones(AZs)
• postsynapticdensities
(PSDs)
fromHeuserlabwebsite
Thevesiclehypothesis
Aquantumoftransmitteristhatamountstoredinasynapticvesicle.
Releaseoccursvia exocytosis.
•
Stimulationproducesexocytoticevents,hereseenaspitsontheCfaceofthe
plasmamembraneinfreezefracture.
HeuserJE,ReeseTS,DennisMJ,JanL,JanYN,EvansL(1979)Synapticvesicleexocytosiscapturedbyquick
freezingandcorrelatedwithquantaltransmitterrelease.J.CellBiol,81:275-300.
Thevesiclehypothesis
Aquantumoftransmitteristhatamountstoredinasynapticvesicle.
Releaseoccursvia exocytosis.
• Capacitancemeasurementsshowthatreleaseofaquantumof
transmitterisaccompaniedbyanincreaseinthesurfaceareaofthe
terminalbythesurfaceareaequivalenttoa~50nmsphericalvesicle.
160,776
SunJ-Y,WuX-S,WuJ-G(2002)Singleandmultiplevesiclefusioninducedifferent
ratesofendocytosisatacentralsynapse.Nature417:555-559.
320,541
Nerveterminalscontaindistinctpoolsofvesicles
•
•
•
WhatisthephysicalcorrelateofN,thenumberof
readilyreleasablevesicles?
N<<#vesicles
N@ numberofanatomicallydockedvesicles(very
approximately!)
readilyreleasablepool- RRP
RizzoliSO,BetzWJ(2005)Synapticvesicle
pools.NatRevNeurosci6:57-69.
SchneggenburgerR,SakabaT,NeherE(2002)TrendsNeurosci
25:206-212.
Remainingcontroversies(partiallist!)
•
Isexocytosisaccompaniedby“fullcollapsefusion” orthe
vesicle,orbya“kissandrun” mechanism?
–
–
•
•
Capacitancemeasurementssuggestthat“kissandrun”
maynotbeprevalent.(Whatkindofdatawouldsupport
“kissandrun?”)
“Kissandrun” and“fullcollapsefusion” maybothoccur
naturallyatdifferenttimes,dependingonthestateofthe
terminal.
Arespontaneouslyoccurringevents(“mEPSCs”)identical
tothebuildingblocksofEPSCs?
Howisendocytosislinkedtoexocytosis?
Poissonandbinomialdistributions
• Isthepoissondistributionrelevantwhensynapses
operateatphysiologicalcalcium?
• NO!!!
– Pisnolongersmall!
– Atmanysynapses,Nisnotlarge,andsothePoissonwouldnot
apply,strictly,evenif[Ca2+]o werereduced.
– Note,however,thatinpractice,itdoeswork,aslongaspissmall.
• FormostCNSsynapsesatphysiological[Ca2+]o,the
binomialdistribution,notthepoissonone,shouldbeused
toanalyzetransmitterrelease.
• Topredictthedistributionofoutcomesusingthe
binomial,weneedtoknowmorethansimplym,alas.
Additionalchallengestostudyingthestatisticsof
transmitterreleaseintheCNS
1.
Inmanycells,youcannotrecordsynaptic
responsesfaithfullyfromthecellbody(space
clamperrors).
2.
Mostcentralneuronsaremultiply
innervated.Youmaybeabletorecord
evokedresponsesfromoneinput,butyou
cannoteasilystudyspontaneousreleaseonly
fromthatinput.(convergence)
3.
SimplebinomialmodelsassumethatPis
uniformacrossthepopulationN,butit’snot.
(non-uniformityofP)
4.
SimplebinomialmodelsassumethatQis
uniform,butit’snot.(non-uniformityofQ)
Measuringquantalparameterswithoutknowingabout
mEPSCs:variance-meananalysis
• Measurethemeanandthevarianceofsetsofresponses
tostimulationofasingleinputunderdifferent
probabilitiesofrelease.
• Youdonotneedtomeasurespontaneousevents,
• Fromthebinomialdistribution,thevarianceandmeanare
relatedaccordingto:
s2 =QI– I2/N
where
I=thecurrent,
Q=quantalsize,meaningthe
responsetoonequantum,and
N=thenumberofreleasablequanta
Fromthebinomial
1. Mean=NPorNPQ
2. s2 =NP(1-P)orNPQ2(1-P)
SubstituteforP
EstimatingN,P,andQ(cont.)
σ2 =QI– I2/N
y=Ax– Bx2
ClementsJD,SilverRA(2000)Unveilingsynapticplasticity:anewgraphicaland
analyticalapproach.TrendsNeurosci 23:105-113.
Note:thisisthesimplebinomial versionoftheexpression,butthis
analysiscanreadilybemorphedintoonethataccountsfornonuniformityinPandinQ(themultinomial).
IsQindependentofP?
• Binomial-basedanalysesoftransmitterreleaseassume
thatthequantalresponse,Q,isindependentofP
– i.e.,thattheresponsetoeachquantumisthesame,regardlessof
howmanyquantaarereleased
• Isthisagoodassumption?Aresynapses“linear?” (i.e.,
doesthepostsynapticcellprovidealinearreadoutofthe
numberofquantareleased?)
Aresynapses“linear?” (cont.)
(IsQindependentofP?)
• Thesuitabilityofthepoisson statisticssuggeststhat
releaseoccursindependentlyformembersofthe
populationN(presynaptic).
• Butwhatabouttheconsequences ofrelease?Isthe
responsetoreleaseofonequantumindependentofthe
releaseofanother?(postsynaptic)
1.
2.
–
Whatisthespatialrelationshipbetweenseparatereleasesites
thatconstituteasynapticconnection?
Canmorethanonequantumoftransmitterbereleasedper
activezone?
Ifreleasesitesarewellseparated,andifnomorethanone
quantumcanbereleasedperactionpotentialpersite,thenthe
postsynapticcellshouldbeabletosumtheresponsesto
individualquantalinearly.
UnivesicularRelease
SilverRA,LubkeJ,Sakmann
B,FeldmeyerD(2003)
Science302:1981-1984.
• MeasureN(variance-meananalysis)and
measure#boutons.
• N=#boutons.
• Releaseisunivesicular,andsincerelease
sitesarewidelydistributed,theresponse
shouldscalelinearlywiththenumberof
quantareleased
Somaanddendritesofpresynapticcellareinred
Axonofpresynapticcellisinblue
Somaanddendritesofpostsynapticcellareinblack
Axonofpostsynapticcellisingreen,
MultivesicularRelease
BiroAA,HolderithNB,NusserZ
(2006)JNeurosci.26:1248712496.
• MeasureN
(variance-mean
analysis)and
measure#boutons.
• N>#boutons.
Furtherevidenceformultivesicularrelease
Climbingfiber-Purkinjecell(CF-PC)
• AsPincreases,mwillincrease
synapseincerebellum
• Whathappenstopeak[glu]?
• Lookateffectsofslow-offandfast-offantagonists
NBQX isahighaffinity
competitiveAMPAR
antagonist.Itsefficacy
willnotdependupon
[glu].
g-DGG isalowaffinity,
competitiveAMPAR
antagonist.Itsefficacy
willdependupon[glu].
Showing
EPSC
before
andafter
g-DGG
WadicheJI,JahrCE(2001)Multivesicularreleaseatclimbingfiber-Purkinjecellsynapses.
Neuron32:301-313.
Furtherevidenceformultivesicularrelease
(cont.)
ClimbingfiberPurkinjecell
synapsein
cerebellum
Pairedpulsedepression
PPRisdifferentincontrol
andinthepresenceofgDGG.It’snotdifferentin
NBQX.
PPR(control)=0.45
PPR(g-DGG)=0.25
What’sgoingon?
Depressioniscausedin
partbydepletionof
vesicles.Fewerquantaare
releasedinresponsetothe
secondstimulus.Thedata
suggestthatpeak[glu]is
largerinresponsetothe
firstpulse(g-DGGisless
effective).Thisagainis
evidenceofmultivesicular
release.
PPR(control)=0.50
PPR(NBQX)=0.50
WadicheJI,JahrCE
(2001)Multivesicular
releaseatclimbing
fiber-Purkinjecell
synapses.Neuron
32:301-313.
Q(quantalsize)isnotuniform
CV is0.2-0.6 (CV=S.D./mean)
• Intrasite/intersitevariability
• Sourcesofvariability
– intrasite
IsaacsonandWalmsley(1995)
• amountofglutamateinvesicle?
• isalloftheglutamateinthevesiclereleased,
andreleasedrapidly?(areallexocytotic eventssimilar?)
• probabilisticnatureofevents(Popen forreceptor’schannel)
– intersite
• isthedensity/numberofglutamatereceptors(acrosssites)similar?
• isthedensity/numberofglutamatetransporters(acrosssites)similar?
• isthegeometryofeachsynapticcontactsimilar?
OUTLINE
1.
2.
3.
4.
5.
Electricalvs.ChemicalSynapticTransmission
CharacteristicsofLigand-GatedIonChannels
Transmitter Release
TransmitterReleaseStatistics
SynapticPlasticity(iftime)
ICING!!!
SynapticStrength
I=m*Q=N*P*Q
I=response,measuredtypicallyincurrent(e.g.,peakofEPSC)
m=avg.numberofquantareleased(=N*P)
N=numberofreleasablequanta[readilyreleasablepool(RRP)of
vesicles]
P=averageprobabilityofrelease,and
Q=quantalresponse:thesizeoftheresponseofthepostsynaptic
celltoonequantumoftransmitter
Note:thisassumesthatthesystemislinear!
Synapticstrengthisdependentuponuse(upon
history)
• Facilitation
neuromuscular
junction(lowCa2+)
• Paired-pulse
facilitation
4 pA
mossyfiber– granule
synapseincerebellum
2 msec
Ispaired-pulsefacilitation(PPF)
presynapticorpostsynaptic?
• Synapticstrength:I=N*P*Q
– Nisgenerallyassociated withpresynapticfactors
– Pisgenerallyassociated withpresynapticfactors
N*P=m(numberofquanta)
– Qisgenerallyassociated withpostsynapticfactors
• Howtoresolvethequestion?
– measurem,fromthefailuresmethod
m=ln(T/T0)
IsaacsonJS,WalmsleyB
(1995)Countingquanta:
directmeasurementsof
transmitterreleaseata
centralsynapse.Neuron
15:875-884.
Whatcausesanincreaseinm forthesecondoftwo
stimuli?
• Largeractionpotentials?
– butaren’tactionpotentialsallornone?
• Morecalciumenteringduringtheactionpotential?
– iCa canbelargerforthesecondoftwoAPs
– however,facilitationcanoccureveniCa isunchanged.
• Residualcalciumhypothesis: Nerveterminalsfacilitatebecausesome
ofthecalciumthatenterstheterminalduringthefirstaction
potentialisstillpresentwhenthesecondimpulsearrives.This
“residualcalcium” isresponsibleforgreaterreleaseafterthesecond
impulse.
– Thisisunlikelytobetheentireexplanation,sincetheamountof“residualcalcium”
measurableisnotsufficienttoexplainthedegreeoffacilitation.
• Calciumbuffersaturationhypothesis (relatedtotheresidualcalcium
hypothesis).Calciumbuffersarepartiallysaturatedasaresultofthe
calciumthatenterstheterminalinresponsetothefirstimpulse,and
lesscalciumissequesteredfollowingthesecondimpulse.
Interplaybetweenfacilitationanddepression
1.25 mM Ca2+, 3.00 mM Mg2+
4 pA
2 msec
3.00 mM Ca2+, 0.75 mM Mg2+
20 pA
granule
cell
mossy
fiber
2 msec
SargentPB,NielsenT,DiGregorioDA,SilverRA(unpublished)
Theprobabilityofreleaseisdependent
upontwofactors
• Ifthereareanatomicallydefinablesitesfromwhich
transmitterisreleased,thenPwilldependbothon
whetherthesiteisoccupiedwithavesicleandwhether,if
occupied,thevesiclefuseswiththeplasmamembrane
• P=Poccupied *Pfusion=Po *Pf
• Ifthesiteisnotoccupiedwitha“release-ready” vesicle,
thentherewillbenoreleaseeventthere.
Whydoesasynapsethatfacilitatesatlow[Ca2+]o
depressathigh[Ca2+]o?
• Depression(someforms) iscausedbythefailureofthe
nerveterminaltoreplacevesiclesreleasedduringthefirst
response.Thisprocesstakestime– generallytensofms.
• Recallthatm=N*P=N*Po *Pf
– “residualcalcium” willincreasePf
– depletionofvesicleswilldecreasePo
– ifPf increasesmorethanPo decreases,wewillhavefacilitation.
Butwhen[Ca2+]o ishigh,therewillbemuchdepletioninresponse
tothefirstpulse,andPo willdecreasebymorethanPf increases,
resultingindepression.
Synapsesvaryintheirresponse
totrainsofstimuli
Climbingfiber–
Purkinjecell
(cerebellum)
Phigh
depressionduringtrain
Parallelfiber–
Purkinjecell
(cerebellum)
Plow
facilitationduringtrain
DittmanJS,KreitzerAC,RegehrWG(2000)Interplaybetween
facilitation,depression,andresidualcalciumatthreepresynaptic
terminals.JNeurosci20:1374-1385.
Longer-lastingformsofplasticity
Facilitation
Augmentation
PTP(post-tetanicpotentiation)
LTP/LTD
fin!