Receptors, Opioid, muNarcotic AntagonistsReceptors, OpioidAnalgesics, OpioidReceptors, Opioid, deltaNaltrexoneReceptors, Opioid, kappaNaloxoneOpioid PeptidesEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, D-Penicillamine (2,5)-NarcoticsMorphineEnkephalinsBenzeneacetamidesDose-Response Relationship, DrugEndorphinsOpioid-Related DisordersPyrrolidinesBuprenorphineDynorphinsEnkephalin, Leucine-2-AlanineEnkephalin, LeucineDiprenorphineEtorphineBenzomorphansMorphinansRats, Sprague-DawleyAnalgesicsAzocinesEnkephalin, MethionineLevorphanolFentanylPurinergic P2 Receptor AntagonistsDrug TolerancePain MeasurementPainOligopeptidesbeta-EndorphinBinding, CompetitivePiperidinesStructure-Activity RelationshipRadioligand AssaySubstance Withdrawal SyndromeDiterpenes, ClerodaneGuinea PigsNeurokinin-1 Receptor AntagonistsNicotinic AntagonistsMethadoneHeroinMorphine DependenceLigandsDrug InteractionsNalbuphineHydromorphonePeriaqueductal GrayStereoisomerismMethadyl AcetateHormone AntagonistsEthylketocyclazocineCHO CellsDopamine AntagonistsRats, WistarAnalgesics, Non-NarcoticInjections, SpinalExcitatory Amino Acid AntagonistsBehavior, AnimalConotoxinsInjections, IntraventricularCricetinaeOxymorphoneBenzylidene CompoundsReceptors, NicotinicMorphine DerivativesAnalgesiaHistamine H2 AntagonistsHyperalgesiaBicyclo Compounds, HeterocyclicCell LineReceptors, SerotoninNociceptorsDopamine AgonistsPain ThresholdMuscarinic AntagonistsSpinal CordKineticsD-Ala(2),MePhe(4),Met(0)-ol-enkephalinPeptides, CyclicMuscle ContractionPiperazinesIndolesViper VenomsInterleukin 1 Receptor Antagonist ProteinGABA AntagonistsSulfonamidesConditioning, OperantSignal TransductionTime FactorsCyclic AMPButorphanol