Receptors, Opioid, muReceptors, OpioidAnalgesics, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesNarcotic AntagonistsNaloxoneOpioid-Related DisordersNaltrexoneEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-MorphineNarcoticsEnkephalin, D-Penicillamine (2,5)-EndorphinsDynorphinsBuprenorphineEnkephalin, LeucineEnkephalin, Leucine-2-AlaninePainDiprenorphinebeta-EndorphinAnalgesicsBenzeneacetamidesDrug ToleranceMorphinansFentanylMethadonePain MeasurementHydromorphoneBenzomorphansPyrrolidinesSubstance Withdrawal SyndromeAnalgesiaOpiate Substitution TreatmentHeroinChronic PainDose-Response Relationship, DrugPrescription Drug MisuseLevorphanolMorphine DependenceButorphanolPrescription DrugsDiterpenes, ClerodaneHeroin DependenceOxymorphoneRats, Sprague-DawleyEthylketocyclazocineMorphine DerivativesInjections, SpinalOligopeptidesPain, IntractableDrug OverdoseBenzylidene CompoundsPain ThresholdAnalgesics, Non-NarcoticCyclazocinePeriaqueductal GrayHyperalgesiaD-Ala(2),MePhe(4),Met(0)-ol-enkephalinAlfentanilNalbuphineHydrocodoneTramadolGuanosine 5'-O-(3-Thiotriphosphate)Drug InteractionsRadioligand AssayLoperamideSufentanilPain, PostoperativeOpiate AlkaloidsPain ManagementSpinal CordPiperidinesMethadyl AcetateCodeineAzocinesNociceptorsLigandsPentazocineBehavior, AnimalSomatostatinTime FactorsBrainMeperidineTreatment OutcomeSubstance-Related DisordersMeptazinolGuinea PigsDihydromorphineChronic DiseaseDrug PrescriptionsInjections, IntraventricularDouble-Blind MethodRats, WistarCHO CellsVas DeferensBinding, CompetitiveSelf Administration