MorphineNarcoticsEnkephalinsAnalgesics, OpioidOpioid-Related DisordersEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, MethionineEnkephalin, LeucineMorphine DependenceReceptors, Opioid, muNaloxoneDrug ToleranceEnkephalin, Leucine-2-AlanineEnkephalin, D-Penicillamine (2,5)-Receptors, OpioidNarcotic AntagonistsMorphine DerivativesReceptors, Opioid, deltaSubstance Withdrawal SyndromeEndorphinsNaltrexoneLocus CoeruleusLevallorphanReceptors, Neurokinin-3EthylketocyclazocineCyclazocinePeriaqueductal GrayRats, Sprague-DawleyAnalgesiaOpioid PeptidesNeurokinin BPain MeasurementOpiate AlkaloidsTachykininsReceptors, Opioid, kappaDose-Response Relationship, DrugReceptors, TachykininNucleus AccumbensPainD-Ala(2),MePhe(4),Met(0)-ol-enkephalinSubstance PCodeineDynorphinsBehavior, AnimalNeurokinin AEtorphineReceptors, Neurokinin-2AnalgesicsBenzomorphansHyperalgesiaBrainSpinal CordAdenylate CyclaseHeroinTime FactorsNeuronsDiprenorphineMorphinansbeta-EndorphinInjections, SpinalArrestinsMethadoneDrug Administration ScheduleRNA, Transfer, GlyG Protein-Coupled Inwardly-Rectifying Potassium ChannelsCyclic AMPCyclic AMP-Dependent Protein KinasesMotor ActivityMice, Inbred C57BLGuinea PigsDihydromorphineBenzeneacetamidesRats, WistarOpiumCarboxypeptidase HGTP-Binding Protein beta SubunitsFentanylRNA, MessengerExcitatory Postsynaptic PotentialsReceptors, N-Methyl-D-AspartateSynaptic TransmissionMice, KnockoutSubstance Abuse DetectionHeroin DependencePain, PostoperativeOligopeptidesOxymorphonePentazocineNeurokinin-1 Receptor AntagonistsBuprenorphineInhibitory Postsynaptic PotentialsNalorphineExcitatory Amino Acid AntagonistsHydromorphoneOxycodoneAnalysis of Variance