MorphineReceptors, Opioid, muAnalgesics, OpioidEnkephalinsReceptors, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, D-Penicillamine (2,5)-Enkephalin, LeucineEnkephalin, MethionineNarcotic AntagonistsNaloxoneNaltrexoneEnkephalin, Leucine-2-AlanineBenzeneacetamidesOpioid PeptidesEndorphinsDynorphinsPyrrolidinesEthylketocyclazocineNarcoticsDiterpenes, ClerodaneD-Ala(2),MePhe(4),Met(0)-ol-enkephalinPain MeasurementBenzomorphansDose-Response Relationship, DrugMorphinansAnalgesicsCyclazocineDrug ToleranceMorphine DerivativesMorphine DependenceOligopeptidesRats, Sprague-DawleyDiprenorphineBenzylidene CompoundsFentanylButorphanolReceptors, Neurokinin-3PainInjections, SpinalGuinea PigsNeurokinin-1 Receptor Antagonistsbeta-EndorphinEtorphineSpiro CompoundsNeurokinin BSelf StimulationRadioligand AssayTachykininsDrug InteractionsPiperidinesPurinergic P1 Receptor AgonistsLoperamideReceptors, TachykininHyperalgesiaVas DeferensBehavior, AnimalRats, WistarSpinal CordSerotonin Receptor AgonistsAnalgesiaSubstance PDopamine AgonistsPiperazinesBenzamidesIleumSerotonin 5-HT1 Receptor AgonistsElectric StimulationNeurokinin ASubstance Withdrawal SyndromeSerotonin 5-HT2 Receptor AgonistsReceptors, Neurokinin-2Opioid-Related DisordersMotor ActivityAdenosine A1 Receptor AgonistsNeuronsCricetinaeGuanosine 5'-O-(3-Thiotriphosphate)Structure-Activity RelationshipPeriaqueductal GrayCricetulusLevorphanolInjections, IntraventricularBuprenorphineHeroinFormaldehydeLigandsGABA AgonistsSomatostatinBinding, CompetitiveBrainCannabinoid Receptor AgonistsAdenosine A2 Receptor AgonistsAdrenergic alpha-2 Receptor AgonistsGABA-B Receptor AgonistsGABA-A Receptor AgonistsSerotonin 5-HT4 Receptor AgonistsPain Threshold