MorphineDrug ToleranceEnkephalinsAnalgesics, OpioidEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, muNaloxoneEnkephalin, LeucineMorphine DependenceEnkephalin, MethionineNarcoticsReceptors, OpioidReceptors, Opioid, deltaNarcotic AntagonistsEnkephalin, Leucine-2-AlanineEnkephalin, D-Penicillamine (2,5)-Receptors, Opioid, kappaSubstance Withdrawal SyndromeOpioid-Related DisordersEndorphinsMorphine DerivativesOpioid PeptidesPain MeasurementLocus CoeruleusNaltrexonePainInjections, SpinalRats, Sprague-DawleyReceptors, Neurokinin-3Periaqueductal GrayDose-Response Relationship, DrugAnalgesicsPain ThresholdEthylketocyclazocineImmune ToleranceHyperalgesiaSpinal CordCyclazocineNeurokinin BTachykininsPiperidinesReceptors, TachykininDynorphinsD-Ala(2),MePhe(4),Met(0)-ol-enkephalinReceptors, N-Methyl-D-AspartateEtorphineSubstance PNeurokinin Abeta-EndorphinNucleus AccumbensAnalgesiaGuinea PigsBenzeneacetamidesBenzomorphansDiprenorphineReceptors, Neurokinin-2MorphinansGlucose Tolerance TestBehavior, AnimalTegmentum MesencephaliDrug ImplantsNeuronsOligopeptidesBrainVentral Tegmental AreaTime FactorsFentanylMice, Inbred C57BLGuanosine 5'-O-(3-Thiotriphosphate)ArrestinsRNA, Transfer, GlyDrug InteractionsDrug Administration ScheduleAdenylate CyclaseTransplantation ToleranceCarboxypeptidase HBuprenorphinePyrrolidinesCodeineRats, Inbred StrainsPain, PostoperativeHydromorphoneMotor ActivityTyrosine 3-MonooxygenaseRaphe NucleiG Protein-Coupled Inwardly-Rectifying Potassium ChannelsRats, WistarCyclic AMPNeurokinin-1 Receptor AntagonistsLimbic SystemMethadoneHeroinElectric StimulationExcitatory Amino Acid AntagonistsAmino Acid SequenceRNA, MessengerMolecular Sequence DataInjections, IntraventricularMice, KnockoutProtein Precursors