MorphineReceptors, Opioid, muAnalgesics, OpioidEnkephalinsReceptors, OpioidReceptors, Opioid, deltaEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, kappaEnkephalin, D-Penicillamine (2,5)-NaloxoneNarcotic AntagonistsEnkephalin, LeucineEnkephalin, MethionineNaltrexoneMorphine DependenceNarcoticsEnkephalin, Leucine-2-AlanineDrug ToleranceOpioid PeptidesBenzeneacetamidesEndorphinsDynorphinsPain MeasurementPyrrolidinesDose-Response Relationship, DrugEthylketocyclazocineMorphine DerivativesDiterpenes, ClerodaneD-Ala(2),MePhe(4),Met(0)-ol-enkephalinBenzomorphansSubstance Withdrawal SyndromeMorphinansRats, Sprague-DawleyAnalgesicsCyclazocinePainOligopeptidesDiprenorphineBenzylidene CompoundsFentanylPeriaqueductal GrayLocus CoeruleusButorphanolReceptors, Neurokinin-3HyperalgesiaInjections, SpinalAnalgesiaBehavior, AnimalLevallorphanGuinea PigsSpinal CordRats, WistarNeurokinin-1 Receptor Antagonistsbeta-EndorphinOpioid-Related DisordersEtorphineSpiro CompoundsNeurokinin BSelf StimulationRadioligand AssayTachykininsDrug InteractionsPiperidinesPurinergic P1 Receptor AgonistsNucleus AccumbensNeuronsMotor ActivityLoperamideReceptors, TachykininVas DeferensSerotonin Receptor AgonistsSubstance PDopamine AgonistsPiperazinesBenzamidesIleumBrainAdenylate CyclaseCyclic AMPTime FactorsSerotonin 5-HT1 Receptor AgonistsElectric StimulationNeurokinin ASerotonin 5-HT2 Receptor AgonistsReceptors, Neurokinin-2ArrestinsAdenosine A1 Receptor AgonistsCricetinaeGuanosine 5'-O-(3-Thiotriphosphate)Mice, Inbred C57BLStructure-Activity RelationshipCricetulusLevorphanolInjections, IntraventricularBuprenorphineG Protein-Coupled Inwardly-Rectifying Potassium ChannelsHeroinFormaldehydeCyclic AMP-Dependent Protein KinasesLigands