EnkephalinsReceptors, OpioidEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, muEnkephalin, LeucineEnkephalin, MethionineReceptors, Opioid, deltaEnkephalin, D-Penicillamine (2,5)-Receptors, Opioid, kappaEnkephalin, Leucine-2-AlanineAnalgesics, OpioidNarcotic AntagonistsNaloxoneEndorphinsNaltrexoneOpioid PeptidesMorphineDynorphinsDiprenorphineEthylketocyclazocineBenzeneacetamidesCyclazocineReceptors, Neurokinin-3NarcoticsBenzomorphansMorphinansAnalgesicsPyrrolidinesbeta-EndorphinDose-Response Relationship, DrugNeurokinin BD-Ala(2),MePhe(4),Met(0)-ol-enkephalinEtorphineTachykininsPurinergic P1 Receptor AgonistsReceptors, TachykininRats, Sprague-DawleySerotonin Receptor AgonistsDopamine AgonistsSubstance PBenzylidene CompoundsOligopeptidesSerotonin 5-HT1 Receptor AgonistsDrug ToleranceNeurokinin ADiterpenes, ClerodaneGuinea PigsReceptors, Neurokinin-2Serotonin 5-HT2 Receptor AgonistsAdenosine A1 Receptor AgonistsPainPiperidinesRadioligand AssayPain MeasurementFentanylOpioid-Related DisordersGuanosine 5'-O-(3-Thiotriphosphate)LigandsGABA AgonistsAnalgesiaLevorphanolMorphine DerivativesAdenosine A2 Receptor AgonistsRats, WistarCannabinoid Receptor AgonistsBinding, CompetitiveGABA-A Receptor AgonistsInjections, SpinalSomatostatinGABA-B Receptor AgonistsBuprenorphineSerotonin 5-HT4 Receptor AgonistsAdrenergic alpha-2 Receptor AgonistsInjections, IntraventricularDrug InteractionsNeurokinin-1 Receptor AntagonistsBehavior, AnimalCHO CellsBrainPeriaqueductal GrayAmino Acid SequencePurinergic P2 Receptor AgonistsSubstance Withdrawal SyndromeNeuronsRNA, Transfer, GlyCyclic AMPSpinal CordMorphine DependenceMolecular Sequence DataCricetinaeStructure-Activity RelationshipButorphanolIleumReceptors, Neurokinin-1Vas DeferensKineticsReceptors, Dopamine D2HeroinProtein PrecursorsCarboxypeptidase H