Receptors, OpioidReceptors, Opioid, muNaloxoneDihydromorphineEnkephalin, Leucine-2-AlanineEnkephalinsEndorphinsAnalgesics, OpioidEnkephalin, LeucineReceptors, Opioid, deltaEtorphineMorphineDiprenorphineReceptors, Opioid, kappaEthylketocyclazocineNarcoticsEnkephalin, MethionineCyclazocineNaltrexoneNarcotic AntagonistsOpioid PeptidesLevorphanolBenzomorphansEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Opioid-Related DisordersDynorphinsMorphine DerivativesMorphinansFentanylEnkephalin, D-Penicillamine (2,5)-Opiate Alkaloidsbeta-EndorphinPhenazocineAnalgesicsDextrorphanAnalgesiaLevallorphanHybrid CellsBrainLoperamideBinding, CompetitiveBuprenorphineDrug ToleranceMethadonePainHeroinNeuroblastomaBrain ChemistryGuanylyl ImidodiphosphatePyrrolidinesRats, Inbred StrainsCodeineSubstance Withdrawal SyndromeIleumDose-Response Relationship, DrugHeroin DependenceMorphine DependenceGuinea PigsGliomaKineticsKetamineOpiumReceptors, DrugSubstance Abuse DetectionClonidineAdenylate CyclaseHydromorphoneOligopeptidesRats, Sprague-DawleySpinal CordStereoisomerismReceptors, AdrenergicBrain StemReceptors, Adrenergic, alphaOxymorphonePain MeasurementThalamusCorpus StriatumLigandsAutoradiographyMethadyl AcetateNeuronsStructure-Activity RelationshipBenzeneacetamidesCell MembraneHydrocodoneDrug OverdoseCatsGuanosine TriphosphateTime FactorsCell LineLocus CoeruleusCerebral CortexPentazocineStreet DrugsButorphanolTomography, Emission-ComputedMolecular Sequence DataAmino Acid SequenceMuscle Contraction