Enkephalin, D-Penicillamine (2,5)-Receptors, OpioidEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, muReceptors, Opioid, deltaReceptors, Opioid, kappaNarcotic AntagonistsBenzeneacetamidesAnalgesics, OpioidNaltrexoneEnkephalinsOpioid PeptidesNaloxoneDynorphinsEnkephalin, Leucine-2-AlaninePyrrolidinesNarcoticsMorphineBuprenorphineEtorphineOpioid-Related DisordersLevorphanolBenzomorphansEndorphinsFentanylNalbuphineMethadoneMethadyl AcetateDrug ToleranceHydromorphoneEnkephalin, LeucineDiprenorphineDose-Response Relationship, DrugAnalgesicsMorphinansEthylketocyclazocineDiterpenes, ClerodaneAzocinesPain MeasurementEnkephalin, MethioninePainMorphine DerivativesOxycodoneD-Ala(2),MePhe(4),Met(0)-ol-enkephalinRats, Sprague-DawleyPeriaqueductal GrayDopamine AgonistsOligopeptidesButorphanolbeta-EndorphinOpiate Substitution TreatmentHeroinSubstance Withdrawal SyndromeGuanosine 5'-O-(3-Thiotriphosphate)Drug InteractionsBenzamidesPiperazinesNeurokinin-1 Receptor AntagonistsRadioligand AssaySelf StimulationVas DeferensMorphine DependenceGABA AgonistsAnalgesics, Non-NarcoticHyperalgesiaInjections, SpinalPain ThresholdCinnamatesBehavior, AnimalLigandsSpiro CompoundsGuinea PigsPentazocineSpinal CordIleumNociceptorsPurinergic P1 Receptor AgonistsBinding, CompetitiveRats, WistarMuscarinic AgonistsCHO CellsAdrenergic alpha-2 Receptor AgonistsNicotinic AgonistsGTP-Binding ProteinsSelf AdministrationCocaineSerotonin 5-HT2 Receptor AgonistsAdrenergic AgonistsInjections, IntraventricularPertussis ToxinConditioning, OperantCricetinaeSomatostatinSerotonin 5-HT1 Receptor AgonistsG Protein-Coupled Inwardly-Rectifying Potassium ChannelsSaimiriNeuronsLocus CoeruleusCricetulusCyclic AMP