Receptors, OpioidReceptors, Opioid, muReceptors, Opioid, deltaReceptors, Opioid, kappaAnalgesics, OpioidOpioid PeptidesNarcotic AntagonistsNaltrexoneNaloxoneEnkephalin, Ala(2)-MePhe(4)-Gly(5)-EnkephalinsEnkephalin, D-Penicillamine (2,5)-MorphineDiprenorphineDynorphinsEnkephalin, LeucineEnkephalin, Leucine-2-AlanineNarcoticsBenzeneacetamidesOpioid-Related DisordersMorphinansEndorphinsBenzomorphansAnalgesicsbeta-EndorphinPyrrolidinesBuprenorphinePainDiterpenes, ClerodaneDrug ToleranceEtorphineFentanylEthylketocyclazocinePain MeasurementLevorphanolCyclazocineBenzylidene CompoundsAnalgesiaOligopeptidesDose-Response Relationship, DrugButorphanolMorphine DerivativesRats, Sprague-DawleyHeroinMethadoneSubstance Withdrawal SyndromeEnkephalin, MethionineGuanosine 5'-O-(3-Thiotriphosphate)LigandsMorphine DependenceInjections, SpinalHydromorphoneRadioligand AssayPeriaqueductal GrayProtein BindingMolecular Sequence DataCHO CellsBinding, CompetitiveLoperamideHyperalgesiaPain ThresholdSomatostatinAnalgesics, Non-NarcoticAmino Acid SequenceNociceptorsCricetinaeBinding SitesOpiate AlkaloidsSpinal CordD-Ala(2),MePhe(4),Met(0)-ol-enkephalinHeroin DependenceSalviaInjections, IntraventricularAzocinesChronic PainCricetulusDrug InteractionsBrainGuinea PigsOpiate Substitution TreatmentBehavior, AnimalSpiro CompoundsCell LineNalbuphineOxymorphoneTramadolDihydromorphinePiperidinesRats, WistarG-Protein-Coupled Receptor Kinase 3ArrestinsPrescription Drug MisuseVas DeferensMicroinjectionsHydrocodoneGTP-Binding ProteinsNociceptionNucleus AccumbensStructure-Activity RelationshipKinetics