MorphineMorphine DependenceMorphine DerivativesAnalgesics, OpioidIsomerismDrug ToleranceNarcoticsNaloxoneStereoisomerismReceptors, Opioid, muNarcotic AntagonistsAnalgesiaInjections, SpinalPain MeasurementCodeineSubstance Withdrawal SyndromePain, PostoperativeAnalgesicsPainReceptors, OpioidDose-Response Relationship, DrugNaltrexoneAnalgesia, Patient-ControlledNalorphineRats, Sprague-DawleyHeroinHydromorphoneFentanylMorphinansEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Periaqueductal GrayOxycodonePentazocineLinoleic Acids, ConjugatedEnkephalinsNalbuphineReceptors, Opioid, deltaChromatography, High Pressure LiquidMeperidineThebainePain ThresholdOxymorphoneBehavior, AnimalAnalgesia, EpiduralOpiumEndorphinsReceptors, Opioid, kappaGas Chromatography-Mass SpectrometryMethadoneDrug InteractionsSpinal CordInjections, SubcutaneousDouble-Blind MethodBupivacaineLevorphanolInjections, IntraventricularTime FactorsPapaverLevallorphanHyperalgesiaLocus CoeruleusInjections, IntravenousPain, IntractableEtorphineTramadolPruritusClonidineKetamineBuprenorphine