MorphineMorphine DependenceMorphine DerivativesAnalgesics, OpioidDrug ToleranceNarcoticsNaloxoneReceptors, Opioid, muNarcotic AntagonistsAnalgesiaInjections, SpinalPain MeasurementCodeineSubstance Withdrawal SyndromePain, PostoperativeAnalgesicsPainReceptors, OpioidNaltrexoneAnalgesia, Patient-ControlledNalorphineDose-Response Relationship, DrugHeroinHydromorphoneFentanylEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Periaqueductal GrayRats, Sprague-DawleyOxycodonePentazocineMorphinansEnkephalinsNalbuphineReceptors, Opioid, deltaMeperidineThebainePain ThresholdOxymorphoneAnalgesia, EpiduralBehavior, AnimalOpiumEndorphinsMethadoneReceptors, Opioid, kappaInjections, SubcutaneousSpinal CordDrug InteractionsInjections, IntraventricularPapaverDouble-Blind MethodLevallorphanLocus CoeruleusHyperalgesiaPain, IntractableEtorphineTramadolBupivacaineInjections, IntravenousPruritusClonidineBuprenorphine