Receptors, Opioid, muNarcotic AntagonistsReceptors, OpioidAnalgesics, OpioidReceptors, Opioid, deltaNaltrexoneReceptors, Opioid, kappaNaloxoneOpioid PeptidesEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, D-Penicillamine (2,5)-NarcoticsMorphineEnkephalinsBenzeneacetamidesEndorphinsOpioid-Related DisordersBuprenorphinePyrrolidinesEnkephalin, Leucine-2-AlanineDynorphinsEnkephalin, LeucineDiprenorphineEtorphineBenzomorphansMorphinansDose-Response Relationship, DrugAnalgesicsAzocinesEnkephalin, MethionineLevorphanolFentanylDrug TolerancePain Measurementbeta-EndorphinPainRats, Sprague-DawleyDiterpenes, ClerodaneSubstance Withdrawal SyndromeMethadoneHeroinMorphine DependenceNalbuphineHydromorphoneOligopeptidesPeriaqueductal GrayMethadyl AcetateDrug InteractionsEthylketocyclazocineAnalgesics, Non-NarcoticInjections, SpinalNeurokinin-1 Receptor AntagonistsOxymorphonePiperidinesBenzylidene CompoundsMorphine DerivativesInjections, IntraventricularBehavior, AnimalAnalgesiaRats, WistarD-Ala(2),MePhe(4),Met(0)-ol-enkephalinGuinea PigsNociceptorsPain ThresholdHyperalgesiaSpinal CordButorphanolOpiate Substitution TreatmentDopamine AgonistsRadioligand AssayLigandsOxycodoneConditioning, OperantDrug Inverse AgonismMeptazinolSelf AdministrationSomatostatinGuanosine 5'-O-(3-Thiotriphosphate)PiperazinesPruritusTime FactorsElectric StimulationStereoisomerismAntipruriticsTetrahydroisoquinolinesDopamine AntagonistsCapsaicinHormone AntagonistsBinding, CompetitiveBenzamidesNeuronsBrainExcitatory Amino Acid AntagonistsVas DeferensAnalysis of VarianceMotor ActivitySpiro CompoundsGABA AgonistsCHO CellsMacaca mulatta