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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,DalyandBrunshaverecentlyproposedthatcaffeineaffectsbehaviorbycounteringtheactivityin(5)thehumanbrainof
2、anaturallyoccurringchemicalcalledadenosine.Adenosinenormallydepressesneuronfiringinmanyareasofthebrain.Itapparentlydoesthisbyinhibitingthereleaseofneurotransmitters,chemicalsthatcarrynerveimpulsesfromoneneurontothenext.(10)Likemanyotheragentsthataffectneuronfiring,adenosinemustfirst
3、bindtospecificreceptorsonneuronalmembranes.Thereareatleasttwoclassesofthesereceptors,whichhavebeendesignatedA1andA2.Snyderetalproposethatcaffeine,whichisstruc-(15)turallysimilartoadenosine,isabletobindtobothtypesofreceptors,whichpreventsadenosinefromattachingthereandallowstheneurons
4、tofiremorereadilythantheyotherwisewould.Formanyyears,caffeine’seffectshavebeenattri-(20)butedtoitsinhibitionoftheproductionofphosphodi-esterase,anenzymethatbreaksdownthechemicalcalledcyclicAMP.AnumberofneurotransmittersexerttheireffectsbyfirstincreasingcyclicAMPconcentra-tionsintarg
5、etneurons.Therefore,prolongedperiodsat(25)theelevatedconcentrations,asmightbebroughtaboutbyaphosphodiesteraseinhibitor,couldleadtoagreater34新东方在线[www.koolearn.com]网络课堂电子教材系列GMAT阅读讲义amountofneuronfiringand,consequently,tobehav-ioralstimulation.ButSnyderetalpointoutthatthecaffeineconc
6、entrationsneededtoinhibittheproduction(30)ofphosphodiesteraseinthebrainaremuchhigherthanthosethatproducestimulation.Moreover,othercom-poundsthatblockphosphodiesterase’sactivityarenotstimulants.Tobuttresstheircasethatcaffeineactsinsteadbypre-(35)ventingadenosinebinding,Snyderetalcomp
7、aredthestimulatoryeffectsofaseriesofcaffeinederivativeswiththeirabilitytodislodgeadenosinefromitsreceptorsinthebrainsofmice.“Ingeneral,”theyreported,“theabilityofthecompoundstocompeteatthereceptors(40)correlateswiththeirabilitytostimulatelocomotioninthemouse;i.e.,thehighertheircapac
8、itytobindattherecep
