Heroin DependenceNarcoticsAnalgesics, OpioidOpioid-Related DisordersMorphine DerivativesMethadoneMorphineNaloxoneBuprenorphineReceptors, OpioidCodeineEnkephalin, MethionineNarcotic AntagonistsNaltrexoneOpioid PeptidesSelf AdministrationPainOxycodoneEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Drug OverdoseFentanylReceptors, Opioid, kappaEnkephalinsStreet DrugsHydromorphoneReceptors, Opioid, deltaSubstance Abuse DetectionDrug and Narcotic ControlEndorphinsPrescription DrugsCocaine-Related DisordersAnalgesicsSubstance Withdrawal SyndromeSubstance-Related DisordersOpiate Substitution TreatmentBehavior, AddictiveSubstance Abuse, IntravenousPain MeasurementEnkephalin, D-Penicillamine (2,5)-Chronic PainPrescription Drug MisuseAnalgesiaDrug ToleranceAlfentanilOxymorphoneCocaineDose-Response Relationship, DrugPain, IntractableDynorphinsOpiate AlkaloidsEnkephalin, Leucinebeta-EndorphinDrug UsersSufentanilEuphoriaReceptors, Opioid, muConditioning, OperantHydrocodoneBenzeneacetamidesNucleus AccumbensCrimeInjections, SpinalTramadolDrug InteractionsReinforcement (Psychology)HeroinSubstance Abuse Treatment CentersEnkephalin, Leucine-2-AlanineBehavior, AnimalMorphine DependenceMeperidineRats, Sprague-DawleyEtorphineAnalgesics, Non-NarcoticDrug PrescriptionsMethadyl AcetateGas Chromatography-Mass SpectrometryPyrrolidinesInjections, IntravenousArkansasReinforcement SchedulePain ManagementNociceptive PainHyperalgesiaPeriaqueductal GrayPain, PostoperativePain ThresholdChronic DiseasePiperidinesDiprenorphineAnalgesia, EpiduralLaw EnforcementTime FactorsHypnotics and SedativesDrug-Seeking BehaviorNeonatal Abstinence SyndromeExtinction, PsychologicalDrug Administration RoutesRats, Long-EvansLevorphanol