MorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, LeucineEnkephalin, MethionineMorphine DependenceReceptors, Opioid, muEnkephalin, Leucine-2-AlanineNarcoticsDrug ToleranceAnalgesics, OpioidEnkephalin, D-Penicillamine (2,5)-NaloxoneNarcotic AntagonistsMorphine DerivativesReceptors, OpioidReceptors, Opioid, deltaEndorphinsSubstance Withdrawal SyndromeLocus CoeruleusReceptors, Neurokinin-3NaltrexonePeriaqueductal GrayNeurokinin BEthylketocyclazocineRats, Sprague-DawleyCyclazocineTachykininsReceptors, TachykininPain MeasurementReceptors, Opioid, kappaD-Ala(2),MePhe(4),Met(0)-ol-enkephalinOpioid PeptidesEtorphineSubstance POpioid-Related DisordersDose-Response Relationship, DrugNeurokinin AReceptors, Neurokinin-2Nucleus AccumbensGuinea PigsTegmentum MesencephaliDynorphinsDrug ImplantsAnalgesicsBehavior, AnimalPainNeuronsInjections, SpinalDrug Administration ScheduleSpinal CordVentral Tegmental AreaBrainBenzomorphansRNA, Transfer, GlyTime FactorsAnalgesiaOligopeptidesArrestinsbeta-EndorphinPain ThresholdDrug InteractionsBenzeneacetamidesMorphinansCarboxypeptidase HAdenylate CyclaseGuanosine 5'-O-(3-Thiotriphosphate)DiprenorphineRats, Inbred StrainsTyrosine 3-MonooxygenaseRaphe NucleiG Protein-Coupled Inwardly-Rectifying Potassium ChannelsHyperalgesiaMice, Inbred C57BLLimbic SystemReceptors, N-Methyl-D-AspartateCodeineNeurokinin-1 Receptor AntagonistsMotor ActivityCyclic AMPRats, WistarPain, PostoperativeExcitatory Amino Acid AntagonistsElectric StimulationFentanylGlycineExcitatory Postsynaptic PotentialsSynaptic TransmissionRNA, MessengerPiperidinesCyclic AMP-Dependent Protein KinasesColforsinReceptors, Neurokinin-1NorepinephrineInjections, IntraventricularDopamineThiorphanAmino Acid SequenceProtein PrecursorsAnalysis of Variance