Analgesics, OpioidOpioid-Related DisordersReceptors, Opioid, muMorphineChronic PainReceptors, OpioidReceptors, Opioid, deltaArkansasPainReceptors, Opioid, kappaDrug IncompatibilityOxycodoneDrug ToleranceEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Morphine DependenceNarcoticsNaloxoneOpioid PeptidesEnkephalinsMethadoneChronic DiseasePain MeasurementSubstance Withdrawal SyndromeNarcotic AntagonistsNaltrexoneDose-Response Relationship, DrugEnkephalin, D-Penicillamine (2,5)-EndorphinsDynorphinsBuprenorphineEnkephalin, LeucineEnkephalin, Leucine-2-AlanineDiprenorphinebeta-EndorphinBenzeneacetamidesAnalgesicsMorphinansFentanylHydromorphoneBenzomorphansPyrrolidinesAnalgesiaOpiate Substitution TreatmentHeroinPrescription Drug MisuseLevorphanolButorphanolPrescription DrugsDiterpenes, ClerodaneHeroin DependenceOxymorphoneEthylketocyclazocineMorphine DerivativesInjections, SpinalRats, Sprague-DawleyPain, IntractableOligopeptidesBenzylidene CompoundsDrug OverdosePain ThresholdCyclazocineAnalgesics, Non-NarcoticPeriaqueductal GrayD-Ala(2),MePhe(4),Met(0)-ol-enkephalinHyperalgesiaAlfentanilNalbuphineHydrocodoneTramadolGuanosine 5'-O-(3-Thiotriphosphate)Drug InteractionsRadioligand AssaySufentanil