EnkephalinsReceptors, OpioidReceptors, Opioid, muReceptors, Opioid, deltaEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, kappaEnkephalin, D-Penicillamine (2,5)-Enkephalin, LeucineEnkephalin, MethionineAnalgesics, OpioidEnkephalin, Leucine-2-AlanineNaloxoneNarcotic AntagonistsOpioid PeptidesNaltrexoneBenzeneacetamidesDynorphinsEndorphinsMorphinePyrrolidinesDiterpenes, ClerodaneBenzomorphansMorphinansEthylketocyclazocineD-Ala(2),MePhe(4),Met(0)-ol-enkephalinNarcoticsAnalgesicsCyclazocineOligopeptidesDiprenorphineBenzylidene CompoundsRats, Sprague-DawleyDrug ToleranceDose-Response Relationship, DrugButorphanolFentanylPain MeasurementPainSpiro Compoundsbeta-EndorphinEtorphineInjections, SpinalPurinergic P1 Receptor AgonistsGuinea PigsRadioligand AssaySelf StimulationLoperamideDrug InteractionsVas DeferensSerotonin Receptor AgonistsRats, WistarHyperalgesiaDopamine AgonistsPiperazinesNeurokinin-1 Receptor AntagonistsBehavior, AnimalSpinal CordIleumPiperidinesSerotonin 5-HT1 Receptor AgonistsSerotonin 5-HT2 Receptor AgonistsBenzamidesElectric StimulationAdenosine A1 Receptor AgonistsOpioid-Related DisordersNeuronsMotor ActivityCricetulusCricetinaeGuanosine 5'-O-(3-Thiotriphosphate)Structure-Activity RelationshipGABA AgonistsAnalgesiaLevorphanolLigandsMorphine DerivativesAdenosine A2 Receptor AgonistsFormaldehydeCannabinoid Receptor AgonistsGABA-A Receptor AgonistsGABA-B Receptor AgonistsBuprenorphineSomatostatinSerotonin 5-HT4 Receptor AgonistsAdrenergic alpha-2 Receptor AgonistsInjections, IntraventricularBinding, CompetitiveBrainCHO CellsPeriaqueductal GrayPurinergic P2 Receptor AgonistsSubstance Withdrawal SyndromeMorphine DependenceCyclic AMPReceptors, Dopamine D2HeroinCarboxypeptidase HAdenosine A3 Receptor AgonistsHistamine AgonistsMicroinjections