MorphineMorphine DependenceMorphine DerivativesAnalgesics, OpioidDrug ToleranceNarcoticsNaloxoneReceptors, Opioid, muNarcotic AntagonistsAnalgesiaInjections, SpinalPain MeasurementCodeineSubstance Withdrawal SyndromePain, PostoperativeAnalgesicsPainReceptors, OpioidNaltrexoneRats, Sprague-DawleyAnalgesia, Patient-ControlledNalorphineFentanylHeroinHydromorphoneDose-Response Relationship, DrugEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Periaqueductal GrayOxycodoneDrug Administration ScheduleMorphinansPentazocineDouble-Blind MethodEnkephalinsNalbuphineTime FactorsReceptors, Opioid, deltaInjections, IntravenousDrug InteractionsMeperidineInjections, SubcutaneousBehavior, AnimalPain ThresholdAdministration, OralThebaineAnalgesia, EpiduralOxymorphoneLethal Dose 50Radiation DosageOpiumMethadoneEndorphinsInjections, IntraventricularReceptors, Opioid, kappaSpinal CordInfusions, IntravenousTramadolBupivacaineLevallorphanLocus CoeruleusAnalgesics, Non-NarcoticPain, IntractableDrug SynergismPapaverClonidineMotor ActivityRats, WistarBuprenorphineEtorphineRadiometryHyperalgesiaHalf-LifeArea Under CurveAnesthesia, SpinalOpioid-Related DisordersLevorphanolInjections, IntraperitonealTreatment OutcomeEnkephalin, D-Penicillamine (2,5)-PruritusAnesthetics, LocalKetorolacNociceptors