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1、新东方在线[www.koolearn.com]网络课堂电子教材系列GMAT阅读讲义GMAT阅读讲义主讲：王鹏欢迎使用新东方在线电子教材GMAT核心阅读Passage1Caffeine,thestimulantincoffee,hasbeencalled“themostwidelyusedpsychoactivesubstanceonEarth.”Synder,DalyandBrunshaverecentlyproposedthatcaffeineaffectsbehaviorbycounteringtheact
2、ivityin(5)thehumanbrainofanaturallyoccurringchemicalcalledadenosine.Adenosinenormallydepressesneuronfiringinmanyareasofthebrain.Itapparentlydoesthisbyinhibitingthereleaseofneurotransmitters,chemicalsthatcarrynerveimpulsesfromoneneurontothenext.(10)Likemanyot
3、heragentsthataffectneuronfiring,adenosinemustfirstbindtospecificreceptorsonneuronalmembranes.Thereareatleasttwoclassesofthesereceptors,whichhavebeendesignatedA1andA2.Snyderetalproposethatcaffeine,whichisstruc-(15)turallysimilartoadenosine,isabletobindtobotht
4、ypesofreceptors,whichpreventsadenosinefromattachingthereandallowstheneuronstofiremorereadilythantheyotherwisewould.Formanyyears,caffeine’seffectshavebeenattri-(20)butedtoitsinhibitionoftheproductionofphosphodi-esterase,anenzymethatbreaksdownthechemicalcalled
5、cyclicAMP.AnumberofneurotransmittersexerttheireffectsbyfirstincreasingcyclicAMPconcentra-tionsintargetneurons.Therefore,prolongedperiodsat(25)theelevatedconcentrations,asmightbebroughtaboutbyaphosphodiesteraseinhibitor,couldleadtoagreater34新东方在线[www.koolearn
6、.com]网络课堂电子教材系列GMAT阅读讲义amountofneuronfiringand,consequently,tobehav-ioralstimulation.ButSnyderetalpointoutthatthecaffeineconcentrationsneededtoinhibittheproduction(30)ofphosphodiesteraseinthebrainaremuchhigherthanthosethatproducestimulation.Moreover,othercom
7、-poundsthatblockphosphodiesterase’sactivityarenotstimulants.Tobuttresstheircasethatcaffeineactsinsteadbypre-(35)ventingadenosinebinding,Snyderetalcomparedthestimulatoryeffectsofaseriesofcaffeinederivativeswiththeirabilitytodislodgeadenosinefromitsreceptorsin
8、thebrainsofmice.“Ingeneral,”theyreported,“theabilityofthecompoundstocompeteatthereceptors(40)correlateswiththeirabilitytostimulatelocomotioninthemouse;i.e.,thehighertheircapacitytobindattherecep
