神經(jīng)生物學(xué) 5、遞質(zhì)系統(tǒng)（臨床醫(yī)學(xué)5年制）課件

NeurotransmitterSystemsJianhongLuo,Ph.D.DepartmentofNeurobiologyZhejiangUniversitySchoolofMedicineMainReference:NeuroscienceExploringtheBrain,3rdEd.ByM.F.Bear,B.W.Connors,andM.A.ParadisoIntroductionStudyingNeurotransmitterSystemsLocalizationoftransmittersandsynthesizingenzymesStudyingtransmitterrelease/synapticmimicry/receptorsNeurotransmitterChemistryCholinergic/Catecholaminergic/Serotonergic/minoacidergicneuronsOtherneurotransmittercandidatesandintercellularmessengersTransmitter-gatedChannelsThebasicstructureoftransmitter-gatedchannelsAminiacid-gatedchannelsG-Protein-CoupledReceptorsAndEffectorsThebasicstructureofG-protein-coupledreceptorsTheubiquitousG-proteinG-protein-coupledeffectorsystemsDivergenceandConvergenceinNeurotransmitterSystemsNeurotransmitters(aminoacids,amines,andpeptides)PlusThe

molecularmachinery(fortransmittersynthesis,vesicularpackaging,reuptakeanddegradation,andtransmitteraction)Acetylcholine(ACh),thefirstNT,identifiedinthe1920s.TheneuronsproducingandreleasingAchgiventhetermcholinergicbyBritishpharmacologistHenryDale(sharedthe1936NobelPrizewithLoewi).Thesuffix–ergic:noradrenergic,glutamatergic,GABAergic,peptidergic,andsoon,forthevarioussynapses,neuronsandneurotransmittersystems.IntroductionIntroductionElementsofneurotransmittersystemsStudyingNeurotransmitterSystemsCriteriatoidentifyaneurotransmitter:Themoleculemustbesynthesizedandstoredinthepresynapticneuron.Themoleculemustbereleasedbythepresynapticaxonterminaluponstimulation.Themolecule,whenexperimentallyapplied,mustproducearesponseinthepostsynapticcellthatmimicstheresponseproducedbythereleaseofneurotransmitterfromthepresynapticneuron.LocalizationofTransmittersandtheirSynthesisEnzymesWhatevertheinspiration,thefirststepinconfirmingthehypothesisonanewneurotransmitteristoshowthatthemoleculeis,infact,localizedin,andsynthesizedby,particularneurons.Manymethodshavebeenusedtosatisfythiscriterionfordifferentneurotransmitters.Twoofthemostimportanttechniquesusedtodayareimmunocytochemistry（免疫細(xì)胞化學(xué)）andinsituhybridization（原位雜交）.StudyingNeurotransmitterSystemsImmunocytochemistry.Thismethoduseslabeledantibodiestoidentifythelocationofmoleculeswithincells.StudyingNeurotransmitterSystemsImmunocytochemistrycanbeusedtolocalizeanymoleculeforwhichaspecificantibodycanbegenerated,includingtheneurotransmittersthemselvesandthesynthesizingenzymesfortransmittercandidates.ImmunocytochemicallocalizationofapeptideneurotransmitterinneuronsStudyingNeurotransmitterSystemsStrandsofmRNAconsistofnucleotidesarrangedinaspecificsequence.Eachnucleotidewillsticktooneothercomplementarynucleotide.Inthemethodofinsituhybridization,asyntheticprobeisconstructedcontainingasequenceofcomplementarynucleotidesthatwillallowittosticktothemRNA.Iftheprobeislabeled,thelocationofcellscontainingthemRNAwillberevealed.InsituhybridizationStudyingNeurotransmitterSystemsInsituhybridizationofthemRNAforaneuropeptidetransmitterinhippocampusofmice(A.wildtypeandB.thepeptideKnockout).OnlyneuronswiththepropermRNAarelabeled,withclustersofwhitedots.StudyingNeurotransmitterSystemsStudyingTransmitterReleaseToshowthataneurotransmittercandidateisactuallyreleaseduponstimulation.

Axonstimulation→testbiologicalactivity→chemicalanalysis(asLoewiandDaledidinidentificationofAChasatransmitteratmanyperipheralsynapses)AdiversemixturesynapsesinCNSmakesitimpossibletostimulateasinglepopulationofsynapses.Researchershavetocollectandmeasureallthechemicalmixture(e.g.usingbrainslicesinasolutioncontainingahighK+concentration).AlsohavetoshowCa2+dependencyofrelease,andfromthepresynapticaxonterminaluponStudyingNeurotransmitterSystemsStudyingSynapticMimicryTomeetthethirdcriterion,microionophoresis

(離子微電泳)

isoftenusetoassessthepostsynapticactionsofatransmittercandidate.Thecandidatesinsolutionsinaglasspipetteisejectedoninverysmallamountsbypassingelectricalcurrentthroughthesurfaceofneurons,andthemembranepotentialcanbemeasured.Ifitmimicstheeffectsoftransmitterreleasedatthesynapse,andiftheothercriteriaoflocalization,synthesis,andreleasehavebeenmet,thenthemoleculeandthetransmitterusuallyareconsideredtobethesamechemical.StudyingNeurotransmitterSystemsMicroionophoresisStudyingNeurotransmitterSystemsStudyingReceptorsAsarule,notwoneurotransmittersbindtothesamereceptor;however,oneneurotransmittercanbindtomanydifferentreceptors,receptorsubtype.e.g.twodifferentcholinergicreceptorsubtypes.Threeapproachestostudythedifferentreceptorsubtypes:NeuropharmacologicalAnalysis.Forinstance,cholinergicreceptorsubtypesresponddifferentlytovariousdrugs.Nicotine(煙堿),areceptoragonistinskeletalmuscle,nicotinicAChreceptors(channels).Curare(筒劍毒)isits

selectiveantagonist.Muscarine(毒蕈堿),areceptoragonistintheheart,muscarinicreceptors(GPCR).AtropineisitsselectiveantagonistNicotinicandmuscarinicreceptorsalsoexistinthebrain.StudyingNeurotransmitterSystemsThreesubtypesofglutamatereceptorsatthesynapticexcitationintheCNS:AMPAreceptors,NMDAreceptors,andkainatereceptors,eachnamedforadifferentchemicalagonist.Theneurotransmitterglutamateactivatesallthesubtypes,butAMPAactsonlyattheAMPAreceptor…..TwosubtypesthesofNEreceptors,αandβ,andofGABAreceptors,GABAAandGABAB.Thus,selectivedrugshavebeenextremelyusefulforcategorizingreceptorsubclasses.Inaddition,neuropharmacologicalanalysishasbeeninvaluableforassessingthecontributionsofneurotransmittersystemstobrainfunction.StudyingNeurotransmitterSystemsTheneuropharmacologyofcholinergicsynaptictransmission.Sitesontransmitterreceptorscanbindeitherthetransmitteritself(ACh),anagonistthatmimicsthetransmitter,oranantagonistthatblockstheeffectsofthetransmitterandagonists.StudyingNeurotransmitterSystemsTheneuropharmacologyofglutamatergicsynaptictransmission

Threesubtypesofglutamatereceptors,eachofwhichbindsglutamate,andeachofwhichisactivatedselectivelybyadifferentagonist.

StudyingNeurotransmitterSystemsStudyingNeurotransmitterSystemsLigand-BindingMethods.Selectivedrugsprovideanopportunitytoanalyzereceptorsdirectly,evenbeforetheneurotransmitteritselfhadbeenidentified.StoryofdiscoveryofOpiatereceptorsSolomonSnyder(1938-)atJohnsHopkinsUniversityOpiateseffectsonthebrainHypothesis:opiatesareagonistsforspecificreceptorsinCNS.Radioactivelylabeledopiatecompoundslabeledspecificsitesonsomeneuronsinthebrain.Ledtothediscoveryofopiatereceptors,andidentificationofendogenousopiates,orendorphins(內(nèi)啡肽),e.g.enkephalinOpiateneurotransmittersystemseventuallyproved.StudyingNeurotransmitterSystemsOpiatereceptorbindingtoasliceofratbrain.Specialfilmwasexposedtoabrainsectionthathadradioactiveopiatereceptorligandsboundtoit.Thedarkregionscontainmorereceptors.StudyingNeurotransmitterSystemsMolecularAnalysisEnableustodividetheneurotransmitterreceptorproteinsintotwogroups:transmitter-gatedionchannelsandG-protein-coupled(metabotropic)receptorsThestructureofreceptorsubunitsbymolecularanalysispresentedabroadextentofthediversityinsubunitcompositione.g.EachGABAreceptorchannelrequiresfivesubunits,fromfivemajorclasses,α,β,γ,δandρ,α1-6isoforms,β1-4,γ1-4.Theoretically,thereare151,887possiblecombinationsandarrangementsofsubunits.Whatthismeans?StudyingNeurotransmitterSystemsEvolutionisconservativeandopportunistic,anditoftenputscommonandfamiliarthingstonewuses.Aminoacidsareessentialtolife.Mostoftheknownneurotransmittermoleculesareeither(1)aminoacids,(2)aminesderivedfromaminoacids,or(3)peptidesconstructedfromaminoacids.AChisanexception;butitisderivedfromacetylCoA,aubiquitousproductofcellularrespirationinmitochondria,andcholine,importantforfatmetabolism.Aminoacidandaminetransmittersaregenerallyeachstoredinandreleasedbyseparatesetsofneurons,so-calledDale’sprinciple.However,manypeptide-containingneuronsviolateDale’sprinciple.Co-transmitters:peptide+aminoacidorpeptide+amineNeurotransmitterChemistryCholinergicNeuronsAcetylcholine(ACh)TheneurotransmitteratNMJ,synthesizedbyallthemotorneuronsinthespinalcordandbrainstem.OthercholinergiccellscontributetothefunctionsofspecificcircuitsinthePNSandCNS.AChsynthesisneedsanenzyme,cholineacetyltransferase(ChAT).OnlycholinergicneuronscontainChAT,sothisenzymeisagoodmarker,e.g.antibody-ICC.ChATtransfersanacetylgroupfromacetylCoAtocholine.Transportofcholineintotheneuronistherate-limitingstepinAChsynthesis.Acetylcholinesterase(AChE)secretedfromCholinergicandnoncholinergicneuronstodegradesACh.InhibitionofAChEdisruptstransmissionatcholinergicsynapsesonskeletalmuscleandheartmuscle.NeurotransmitterChemistryNeurotransmitterChemistryThelifecycleofAChlowmicromolarconcentrationsrate-limitingstepwiththefastestcatalyticrate.thetargetofnervegasesandinsecticidesNeurotransmitterChemistryAcetylcholine.(a)AChsynthesis.(b)AChdegradation.NeurotransmitterChemistryCatecholaminergicNeuronsTheaminoacidtyrosineistheprecursorforthreedifferentamineneurotransmittersthatcontainachemicalstructurecatechol,collectivelycalledcatecholamines(兒茶酚胺).Includedopamine(DA),norepinephrine(NE),andepinephrine.Catecholaminergicneuronsarefoundinregionsofthenervoussystemforregulationofmovement,mood,attention,andvisceralfunction.CatecholgroupDopaminenorepinephrineepinephrine(nonadrenaline)adrenalineNeurotransmitterChemistryAllsuchneuronscontaintyrosinehydroxylase(TH),whichcatalyzesthefirststepincatecholaminesynthesis,theconversionoftyrosinetoacompoundcalleddopa(L-dihydroxyphenylalanine二羥苯基丙氨酸).TheactivityofTHisratelimitingforcatecholaminesynthesis,regulatedbyvarioussignalsinthecytosoloftheaxonterminal(end-productinhibition,increasewhen[Ca2+]ielevatedbyahighraterelease).Dopaisconvertedintotheneurotransmitterdopaminebytheenzymedopadecarboxylase.Parkinson’sdiseaseanddopasupplementtherapy.NeurotransmitterChemistry酪氨酸二羥苯基丙氨酸（多巴）多巴胺去甲腎上腺素腎上腺素酪氨酸羥化酶多巴脫羧基酶多巴胺β羥化酶苯基乙醇胺-N-甲基轉(zhuǎn)移酶ratelimitingenzymelocatedwithinthesynapticvesicleslocatedinthecytosolNeurotransmitterChemistryNeuronsthatuseNEasaneurotransmittercontain,inadditiontoTHanddopadecarboxylase,theenzymedopamineβ-hydroxylase(DBH),whichconvertsdopaminetonorepinephrine.DAistransportedfromthecytosoltothesynapticvesicles,andthereitismadeintoNE.Thelastinthelineofcatecholamineneurotransmittersisepinephrine(adrenaline).AdrenergicneuronscontaintheenzymephentolamineNmethyltransferase(PNMT),whichconvertsNEtoepinephrine.Inadditiontoservingasaneurotransmitterinthebrain,epinephrineisreleasedbytheadrenalglandintothebloodstream.Circulatingepinephrineactsatreceptorsthroughoutthebodytocoordinatevisceralresponse.NeurotransmitterChemistryTheactionsofcatecholaminesinthesynapticcleftareterminatedbyselectiveuptakeoftheneurotransmittersbackintotheaxonterminalviaNa+-dependenttransporters.Thisstepissensitivetoanumberofdifferentdrugs.Forexample,amphetamineandcocaineblockcatecholamineuptake.Onceinside,theaxonterminal,thecatecholaminesmaybereloadedintosynapticvesiclesforreuse,ortheymaybeenzymaticallydestroyedbytheactionofmonoamineoxidase(MAO),anenzymefoundontheoutermembraneofmitochondria.NeurotransmitterChemistrySerotonergicNeuronsTheamineneurotransmitterserotonin,alsocalled5-hydroxytryptamineandabbreviated5-HT,isderivedfromtheaminoacidtryptophan.Serotonergic

neuronsarerelativelyfewinnumber,buttheyappeartoplayanimportantroleinthebrainsystemsthatregulatemood,emotionalbehavior,andsleep.Serotoninsynthesisappearstobelimitedbytheavailabilityoftryptophanintheextracellularfluidbathingneurons.Thesourceofbraintryptophanistheblood,andthesourceofbloodtryptophanisthediet.5-HTisremovedfromthesynapticcleftbytheactionofaspecifictransporter,whichissensitivetoanumberofdifferentdrugs.e.g.antidepressantdrugslikefluoxetine.Oncebackinthecytosol,5-HTiseitherreloadedtoSVsordegradedbyMAO.NeurotransmitterChemistryThesynthesisofserotoninfromtryptophan.色氨酸5-羥色氨酸5-羥色胺色氨酸羥化酶5-羥基色氨酸脫羧酶NeurotransmitterChemistryAminoAcidergicNeurons

glutamate(Glu)Glycine(Gly)GABAAminoacidneurotransmittersGlu,Gly,andGABAserveasneurotransmittersatmostCNSsynapsesGlutamateandglycinearesynthesizedfromglucoseandotherprecursorsbyenzymesexistinginallcells.Differencesamongneuronsarequantitativeratherthanqualitative.e.g.theglutamatergicterminalshaveabout20mMGlu,only2-3timeshigherthannonglutamatergiccells.Importantly,inglutamatergicterminalss,butnotinother’s,theglutamatetransporterconcentratesGluinSVstoreachabout50mM.NeurotransmitterChemistry

GABAisnotoneofthe20aminoacidsusedtoconstructproteins,itissynthesizedinlargequantitiesonlybytheneuronsthatuseitasaneurotransmitter.TheprecursorforGABAisglutamate.Thekeysynthesizingenzymeisglutamicaciddecarboxylase(GAD),agoodmarkerforGABAergicneurons.Onechemicalsteptoconvertthemajorexcitatoryintothemajorinhibitoryneurotransmitterinthebrain!ThesynapticactionsofGABAareterminatedbyselectiveuptakeintotheterminalsandgliaviaspecificNa+-dependenttransporters.Insidethecytosol,GABAismetabolizedbytheenzymeGABAtransaminase.GADOtherNeurotransmitterCandidatesandIntercellularMessengersATPisconcentratedinvesiclesatmanysynapsesintheCNSandPNS,releasedintothecleftbypresynapticspikesinaCa2+-dependentmanner.ATPisoftenpackagedinvesiclesalongwithanotherclassictransmitter(e.g.catecholamine)whichmeanstheyareprobablyco-transmitters.ATPdirectlyexcitessomeneuronsbygatingacationchannel.ATPbindstopurinergicreceptors,bothtransmitter-gatedionchannelsandalargeclassofG-proteincoupledpurinergicreceptors.NeurotransmitterChemistryTheinterestingdiscoveryinthepastfewyearsisthatsmalllipidmolecules,endocannabinoids(大麻酚,endogenouscannabinoids),canbereleasedfrompostsynapticneuronsandactonpresynapticterminals,calledretrogradesignaling;thus,endocannabinoidsareretrogrademessengers,akindoffeedbackregulation.Vigorousfiringinthepostsynapticneuron→Ca2+influxthroughvoltage-gatedcalciumchannelsofpostsynapticneurons→[Ca2+]iincrease→stimulatesthesynthesisofendocannabinoidmoleculesfrommembranelipids.NeurotransmitterChemistryRetrogradesignalingwithendocannabinoidsThereareseveralunusualqualitiesaboutendocannabinoids:Theyarenotpackagedinvesicleslikeotherneurotransmitters;instead,theyareproducedrapidlyandon-demand.Theyaresmallandmembranepermeable;oncesynthesized,theycandiffuserapidlyacrossthemembranetocontactneighboringcells.TheybindselectivelytotheCB1typeofcannabinoidreceptor,mainlylocatedoncertainpresynapticterminals.NeurotransmitterChemistry

CannabisMarijuana大麻THC(Δ9-tetrahydrocannabinol)Short-termeffectsonbrain?NeurotransmitterChemistryCB1receptorsareGPCRs,andtheirmaineffectisoftentoreducetheopeningofpresynapticcalciumchannels,andinhibitreleaseofitsneurotransmitter.

Gaseousmolecule,nitricoxide(NO).Carbonmonoxide(CO)hasalsobeensuggestedasamessenger,beingextensivelystudiedandhotlydebated.Manyofthechemicalswecallneurotransmittersmayalsobepresentandfunctioninnon-neuralpartsofthebody.(Aminoacids,ATP,Nitricoxide,Ach,serotonin)Transmitter-gatedChannelsThetransmitter-gatedionchannelsaremagnificenttinymachines.Asinglechannelcanbeasensitivedetectorofchemicalsandvoltage,itcanregulatetheflowofsurprisinglylargecurrentswithgreatprecision,itcansiftandselectbetweenverysimilarions,anditcanberegulatedbyotherreceptorsystems.Yeteachchannelisonlyabout11nmlong,justbarelyvisiblewiththebestcomputer-enhancedelectronmicroscopicmethods.Transmitter-gatedChannelsTheBasicStructureofTransmitter-GatedChannelsThemostthoroughlystudiedtransmitter-gatedionchannelisthenicotinicAChreceptoratNMJ.Fivesubunitsarrangelikeabarreltoformasinglepore.Fourdifferentsubunitsα,β,γ,δareused.ThereisoneAChbindingsiteoneachoftheαsubunits.ThenicotinicAChreceptoronneuronsisalsoapentamer,but,unlikethemusclereceptor,mostofthemarecomprisedofαandβsubunitsonly.ThesubunitsofGABA-andGlycine-gatedchannelshaveasimilarprimarystructuretothenicotinicAChreceptor,withfourhydrophobicsegmentstospanthemembrane,alsothoughttobepentamericcomplexes.Transmitter-gatedChannelsThesubunitarrangementofthenicotinicAChreceptorSideviewshowinghowthefourα-helicesofeachsubunitpackedtogether.TopviewshowingthelocationofthetwoAChbindingsites.Transmitter-gatedChannelsSimilaritiesinsubunitstructurefordifferenttransmitter-gatedionchannels(a)TheyhaveincommonthefourregionscalledM1–M4,whicharesegmentswherethepolypeptideswillcoilintoalphahelicestospanthemembrane.Kainatereceptorsaresubtypesofglutamatereceptors.(b)M1–M4regionsoftheAChsubunit,astheyarebelievedtobethreadedthroughthemembrane.?Transmitter-gatedChannelsTheglutamate-gatedchannelsaremostlikelytetramersstructure.TheM2regiondoesnotspanthemembrane,butinsteadformsahairpinthatbothentersandexitsfromtheinsideofthemembrane,resemblingpotassiumchannels.Thepurinergic(ATP)receptorsalsohaveanunusualstructure.Eachsubunithasonlytwomembrane-spanningsegments,andthenumberofsubunitsofacompletereceptorisnotknown.Differenttransmitterbindingsitesletonechannelrespondtodistincttransmitters;certainaminoacidsaroundthenarrowionporeallowonlyNa+andK+toflowthroughsomechannels,Ca2+throughothers,andonlyCl-throughyetothers.Transmitter-gatedChannelsAminoAcid-GatedChannelsAminoacid-gatedchannelsmediatemostofthefastsynaptictransmissionintheCNS.Severalpropertiesofthesechannelsdistinguishthemfromoneanotheranddefinetheirfunctionswithinthebrain.Allthesepropertiesareadirectresultofthemolecularstructureofthechannels.

pharmacology

kinetics

selectivity

conductanceTransmitter-gatedChannelsGlutamate-GatedChannels.Threeglutamatereceptorsubtypes:AMPA,NMDA,andkainate.TheAMPA-andNMDA-gatedchannelsmediatethebulkoffastexcitatorysynaptictransmissioninthebrain.AMPA-gatedchannelsarepermeabletobothNa+andK+,andmostofthemarenotpermeabletoCa2+.TheneteffectistoadmitNa+ionsintothecell,causingarapidandlargedepolarization.NMDA-gatedchannelsdifferfromAMPAreceptorsintwoveryimportantways:(1)NMDA-gatedchannelsarepermeabletoCa2+,and(2)inwardioniccurrentthroughNMDA-gatedchannelsisvoltagedependent.Transmitter-gatedChannelsThecoexistenceofNMDAandAMPAreceptorsinthepostsynapticmembraneofaCNSsynapse.(a)Animpulsearrivinginthepresynapticterminalcausesthereleaseofglutamate.(b)GlutamatebindstoAMPA-gatedandNMDA-gatedchannelsinthepostsynapticmembrane.(c)TheentryofNathroughtheAMPAchannels,andNaandCa2throughtheNMDAchannels,causesanEPSP.Transmitter-gatedChannelsInwardioniccurrentthroughtheNMDA-gatedchannel.(a)Glutamatealonecausesthechanneltoopen,butattherestingmembranepotential,theporebecomesblockedbyMg2+ions.(b)DepolarizationofthemembranerelievestheMg2+blockandallowsNa+andCa2+toenter.Transmitter-gatedChannelsItishardtooverstatetheimportanceofintracellularCa2+tocellfunctions.presynapticalandpostsynaptical;physiologicalandpathological.Thus,activationofNMDAreceptorscan,inprinciple,causewidespreadandlastingchangesinthepostsynapticneuron.ThemagnitudeofthisinwardCa2+andNa+throughNMDA-gatedchannelsdependsonthepostsynapticmembranepotentialinanunusualway,foranunusualreason.“magnesiumblock”,voltagedependentrelease.Bothglutamateanddepolarizationmustcoincidebeforethechannelwillpasscurrent.ThispropertyhasasignificantimpactonsynapticintegrationatmanylocationsintheCNS.Transmitter-gatedChannelsGABA-GatedandGlycine-GatedChannels.GABAmediatesmostsynapticinhibitionintheCNS,andglycinemediatesmostoftherest.Botharechloridechannels.Synapticinhibitionmustbetightlyregulatedinthebrain.Toomuchcausesalossofconsciousnessandcoma;toolittleleadstoaseizure.ItiswhytheGABAAreceptorhasseveralothersiteswherechemicalscandramaticallymodulateitsfunction.e.g.Benzodiazepines（苯二氮卓）andbarbiturates（巴比妥）.WhenGABAispresent,increasethefrequency,orthedurationofchannelopenings,respectively,thus,moreCl-current,strongerIPSPs,enhancedbehaviorinhibition.AndselectiveforGABAAreceptor,noeffectonglycinereceptor.Transmitter-gatedChannels

Ethanol,anotherpopulardrug,stronglyenhancesGABAAreceptorfunctioninawayofdependenceonthereceptorspecificstructure.EthanolhasalsocomplexeffectsonNMDA,glycine,nicotinicACh,andserotoninreceptors.Whatistheendogenousligandsforthesedrugbindingsites?Theymayserveasregulatorsofinhibition.Substantialevidenceindicatesthatnaturalbenzodiazepine-likeligandsexist.OthergoodcandidatesasnaturalmodulatorsofGABAAreceptorsaretheneurosteroids(神經(jīng)甾體),naturalmetabolitesofsteroidhormones,butalsoinglialcellsofthebrain.Someneurosteroidsenhanceinhibitoryfunctionwhileotherssuppressit,andtheyseemtodosobybindingtotheirownsiteontheGABAAreceptor.Transmitter-gatedChannelsThebindingofdrugstotheGABAAreceptorThedrugsbythemselvesdonotopenthechannel,butchangetheeffectwhenGABAbindstothechannelatthesametimeasthedrug.G-ProteinCoupledReceptorsandEffectorsTransmissionatGPCRsinvolvesthreesteps:(1)bindingoftheneurotransmittertothereceptorprotein,(2)activationofG-proteins,and(3)activationofeffectorsystems.TheBasicStructureofGPCRsAfamilymembers(about100),asinglepolypeptide,sevenmembrane-spanningα-helices,twoextracellularloops(bindingsites).Twointracellularloops(bindingtoandactivatingG-proteins)StructuralvariationsatthesetwositesdeterminewhichagonistbindingandwhichG-proteinsandeffectorsystemsactivatedinresponsetotransmitterbinding.Transmitter-gatedChannelsTransmitter-gatedChannelsThebasicstructureofaG-protein-coupledreceptor.Mostmetabotropicreceptorshavesevenmembrane-spanningα-helices,atransmitterbindingsiteontheextracellularside,andaG-proteinbindingsiteontheintracellularside.G-ProteinCoupledReceptorsandEffectorsTheUbiquitousG-ProteinsG-proteinsarethecommonlinksignalingpathways;GTPbindingprotein,about20familymembers;Lessthantransmitterreceptors.Thesamebasicmodeofoperation:Eachhasthreesubunits,α,β,andγ.Intherestingstate,GDPisboundtotheGα.IfthisG-proteinhitstheproperreceptorwithatransmitterbound,thenreleasesitsGDPandexchangesitforaGTP.TheactivatedG-proteinsplitsinto2parts:theGαplusGTP,andtheGβγcomplex.Bothcaninfluencevariouseffectors.TheGαisitselfanenzymethateventuallybreaksdownGTPintoGDP,andterminatesitsownactivity.TheGαandGβγsubunitscomebacktogether,allowingthecycletobeginagain.ThebasicmodeofoperationofG-proteins

(a)Initsinactivestate,theαsubunitoftheG-proteinbindsGDP.(b)WhenactivatedbyaG-protein-coupledreceptor,theGDPisexchangedforGTP.(c)TheactivatedG-proteinsplits,andboththeGα(GTP)subunitandtheGβγsubunitbecomeavailabletoactivateeffectorproteins.(d)TheGsubunitslowlyremovesphosphate(PO4)fromGTP,convertingGTPtoGDPandterminatingitsownactivity.G-ProteinCoupledReceptorsandEffectorsG-ProteinCoupledReceptorsandEffectorsG-Protein-CoupledEffectorSystemsActivatedG-proteinsexerttheireffectsbybindingtoeitherof:G-protein-gatedionchannelsandG-protein-activatedenzymes.TheShortcutPathway:Avarietyofneurotransmittersusetheshortcutpathway,fromreceptortoG-proteintoionchannel.e.g.①themuscarinicreceptorsinthehearttoexplainwhyAChslowstheheartrate.②neuronalGABABreceptors.ThefastestoftheG-protein-coupledsystems(30–100msec)sincenointermediarybetweenreceptorandchannel.Andalsoverylocalizedsinceitiswithinthemembraneandcannotmoveveryfar.Theshortcutpathway.(a)G-proteinsinheartmuscleareactivatedbyAChbindingtomuscarinicreceptors.(b)TheactivatedGsubunitdirectlygatesapotassiumchannel.G-ProteinCoupledReceptorsandEffectorsSecondMessengerCascades.:G-proteinscanalsodirectlyactivatecertainenzymesandthelaterstriggeranelaborateseriesofbiochemicalreactions,acascadethatoftenendsintheactivationofother“downstream”