Analgesics, OpioidReceptors, Opioid, muReceptors, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaNarcotic AntagonistsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, D-Penicillamine (2,5)-Opioid PeptidesNaltrexoneBenzeneacetamidesNarcoticsEnkephalinsNaloxoneMorphineBuprenorphineEtorphinePyrrolidinesOpioid-Related DisordersEnkephalin, Leucine-2-AlanineLevorphanolBenzomorphansEndorphinsFentanylNalbuphineDynorphinsMethadoneMethadyl 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 Administration