MorphineHeroin DependenceAnalgesics, OpioidReceptors, Opioid, muReceptors, OpioidMorphine DerivativesReceptors, Opioid, deltaNarcoticsMorphine DependenceReceptors, Opioid, kappaNaloxoneOpioid PeptidesNarcotic AntagonistsDrug ToleranceNaltrexoneOpioid-Related DisordersMethadoneBuprenorphineSubstance Withdrawal SyndromeCodeineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Pain MeasurementAnalgesiaPainAnalgesicsEndorphinsEnkephalin, D-Penicillamine (2,5)-Injections, SpinalDose-Response Relationship, DrugHydromorphoneSelf AdministrationFentanylDrug OverdosePain, PostoperativeDynorphinsMorphinansEnkephalin, LeucineHeroinRats, Sprague-DawleySubstance Abuse DetectionEnkephalin, Leucine-2-AlanineOpiate Substitution TreatmentStreet DrugsDiprenorphinebeta-EndorphinBehavior, AnimalPeriaqueductal GrayCocaine-Related DisordersBehavior, AddictiveAnalgesia, Patient-ControlledCocaineDrug InteractionsSubstance-Related DisordersPain ThresholdNalbuphineConditioning, OperantOxymorphoneSubstance Abuse, IntravenousBenzomorphansPentazocineBenzeneacetamidesDrug and Narcotic ControlNalorphineNucleus AccumbensLevorphanolMeperidineAnalgesics, Non-NarcoticHyperalgesiaInjections, IntraventricularPyrrolidinesPain, IntractableTime FactorsInjections, IntravenousSpinal CordOxycodoneBenzylidene CompoundsButorphanolTramadolReinforcement (Psychology)OpiumOpiate AlkaloidsAnalgesia, EpiduralDrug UsersDouble-Blind MethodPrescription Drug MisuseMethadyl AcetateHydrocodoneInjections, SubcutaneousThebaineGas Chromatography-Mass SpectrometryPrescription DrugsBrainEthylketocyclazocineCyclazocineNociceptorsRewardLocus CoeruleusMotor ActivityRats, Wistar