Analgesics, OpioidHydromorphonePainChronic PainReceptors, Opioid, muReceptors, OpioidPain MeasurementReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesNarcotic AntagonistsNaloxoneOpioid-Related DisordersNaltrexoneEnkephalinsMorphineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-NarcoticsEnkephalin, D-Penicillamine (2,5)-EndorphinsDynorphinsBuprenorphineEnkephalin, LeucineEnkephalin, Leucine-2-AlanineDiprenorphineAnalgesicsDrug Tolerancebeta-EndorphinBenzeneacetamidesFentanylMorphinansMethadoneBenzomorphansPyrrolidinesSubstance Withdrawal SyndromeAnalgesiaRats, Sprague-DawleyOpiate Substitution TreatmentDrug Administration ScheduleHeroinRadiation DosageLethal Dose 50Time FactorsAdministration, OralLevorphanolPrescription Drug MisuseMorphine DependenceDouble-Blind MethodDrug InteractionsDose-Response Relationship, DrugInjections, SpinalButorphanolPrescription DrugsHeroin DependenceOxymorphoneDiterpenes, ClerodaneAnalgesics, Non-NarcoticMorphine DerivativesOligopeptidesEthylketocyclazocineTreatment OutcomePain, IntractableRadiometry