Analgesics, OpioidReceptors, Opioid, muReceptors, OpioidOpioid-Related DisordersMorphineNarcotic AntagonistsReceptors, Opioid, deltaNaloxoneNarcoticsBuprenorphineReceptors, Opioid, kappaDrug ToleranceOpioid PeptidesPainNaltrexoneEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, D-Penicillamine (2,5)-EndorphinsDynorphinsEnkephalin, LeucineEnkephalin, Leucine-2-AlanineDiprenorphinebeta-EndorphinBenzeneacetamidesAnalgesicsMorphinansFentanylMethadonePain MeasurementHydromorphoneBenzomorphansPyrrolidinesAnalgesiaSubstance Withdrawal SyndromeOpiate Substitution TreatmentHeroinChronic PainPrescription 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 AssaySufentanilLoperamideOpiate AlkaloidsPain, PostoperativePain ManagementMethadyl AcetateCodeineAzocinesNociceptorsSpinal CordPiperidinesLigandsPentazocineSomatostatinBehavior, AnimalMeperidineMeptazinolDihydromorphineBrainGuinea PigsSubstance-Related DisordersInjections, IntraventricularDrug Prescriptions