Heroin DependenceReceptors, Opioid, muAnalgesics, OpioidReceptors, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaNarcoticsOpioid PeptidesNaloxoneOpioid-Related DisordersNarcotic AntagonistsMethadoneNaltrexoneMorphine DerivativesBuprenorphineMorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Self AdministrationDrug OverdoseEnkephalin, D-Penicillamine (2,5)-Substance Withdrawal SyndromeEndorphinsCodeineDynorphinsOpiate Substitution TreatmentEnkephalin, LeucineStreet DrugsCocaine-Related DisordersEnkephalin, Leucine-2-AlanineBehavior, AddictiveSubstance Abuse DetectionPainSubstance Abuse, IntravenousDiprenorphineDrug and Narcotic ControlHydromorphoneDrug Tolerancebeta-EndorphinAnalgesicsSubstance-Related DisordersBenzeneacetamidesDose-Response Relationship, DrugFentanylPain MeasurementCocaineMorphinansMorphine DependenceDrug UsersConditioning, OperantPrescription Drug MisuseAnalgesiaPrescription DrugsBenzomorphansNucleus AccumbensPyrrolidinesSubstance Abuse Treatment CentersBenzylidene CompoundsHeroinReinforcement (Psychology)CrimeBehavior, AnimalRats, Sprague-DawleyDrug InteractionsOpiate AlkaloidsChronic PainLevorphanolAnalgesics, Non-NarcoticAzocinesButorphanolReinforcement ScheduleDiterpenes, ClerodaneOxymorphonePain ThresholdEthylketocyclazocineOligopeptidesExtinction, PsychologicalMethadyl AcetateGas Chromatography-Mass SpectrometryInjections, SpinalHyperalgesiaPain, IntractableTime FactorsLaw EnforcementRewardInjections, IntravenousEuphoriaCyclazocinePupilInjections, IntraventricularRats, Long-EvansGuanosine 5'-O-(3-Thiotriphosphate)PiperidinesDrug-Seeking BehaviorPeriaqueductal GrayDrug ContaminationBrainD-Ala(2),MePhe(4),Met(0)-ol-enkephalinFlupenthixolAnalysis of Variance