Analgesics, OpioidOpioid-Related DisordersBuprenorphineReceptors, Opioid, muOpiate AlkaloidsMethadoneNaloxoneReceptors, OpioidNarcotic AntagonistsSubstance Abuse Treatment CentersMorphineDrug and Narcotic ControlOpiate Substitution TreatmentReceptors, Opioid, deltaReceptors, Opioid, kappaPainChronic PainOpioid PeptidesPain MeasurementUnited StatesNaltrexoneEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-NarcoticsEnkephalin, D-Penicillamine (2,5)-EndorphinsDynorphinsEnkephalin, LeucineEnkephalin, Leucine-2-AlanineDiprenorphinebeta-EndorphinBenzeneacetamidesAnalgesicsDrug ToleranceMorphinansFentanylHydromorphoneBenzomorphansPyrrolidinesAnalgesiaSubstance Withdrawal SyndromeHeroinPrescription Drug MisuseLevorphanolDose-Response Relationship, DrugMorphine DependenceButorphanolPrescription 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 AssaySufentanilLoperamidePain, PostoperativePain ManagementMethadyl AcetateCodeineAzocines