MorphineMorphine DependenceMorphine DerivativesAnalgesics, OpioidDrug ToleranceNarcoticsNaloxoneReceptors, Opioid, muNarcotic AntagonistsAnalgesiaPain MeasurementInjections, SpinalPain, PostoperativeCodeineSubstance Withdrawal SyndromeAnalgesicsPainReceptors, OpioidRats, Sprague-DawleyNaltrexoneRescue WorkAnalgesia, Patient-ControlledDose-Response Relationship, DrugNalorphineFentanylHeroinHydromorphoneDrug Administration ScheduleDouble-Blind MethodTime FactorsOxycodoneEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Periaqueductal GrayMorphinansPentazocineEnkephalinsNalbuphineInjections, IntravenousBehavior, AnimalReceptors, Opioid, deltaDrug InteractionsInjections, SubcutaneousMeperidineAdministration, OralPain ThresholdAnalgesia, EpiduralThebaineOxymorphoneLethal Dose 50Radiation DosageSpinal CordOpiumInjections, IntraventricularMethadoneEndorphinsReceptors, Opioid, kappaInfusions, IntravenousBupivacaineTreatment OutcomeTramadolDisease Models, AnimalAnalgesics, Non-NarcoticMotor ActivityDrug SynergismLevallorphanRats, WistarLocus CoeruleusPain, IntractablePapaverPostoperative Nausea and VomitingClonidineAnesthesia, SpinalBuprenorphineEtorphineHalf-LifeMutationRadiometryHyperalgesiaAnesthetics, LocalInjections, IntraperitonealArea Under CurveKetorolacMice, KnockoutBrainOpioid-Related DisordersDrug Therapy, CombinationAnalysis of VarianceLevorphanolNeuronsEnkephalin, D-Penicillamine (2,5)-PruritusNociceptors