MorphineMorphine DependenceMorphine DerivativesAnalgesics, OpioidDrug ToleranceNarcoticsNaloxoneReceptors, Opioid, muNarcotic AntagonistsAnalgesiaPain MeasurementInjections, SpinalCodeineSubstance Withdrawal SyndromePain, PostoperativeAnalgesicsDose-Response Relationship, DrugPainReceptors, OpioidNaltrexoneMolecular StructureAnalgesia, Patient-ControlledNalorphineStructure-Activity RelationshipRats, Sprague-DawleyHeroinHydromorphoneFentanylMorphinansEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Periaqueductal GrayOxycodonePentazocineEnkephalinsReceptors, Opioid, deltaNalbuphineThebaineStereoisomerismMeperidineMagnetic Resonance SpectroscopyPain ThresholdOxymorphoneBehavior, AnimalAnalgesia, EpiduralEndorphinsDrug InteractionsOpiumInjections, SubcutaneousReceptors, Opioid, kappaTime FactorsSpinal CordChromatography, High Pressure LiquidMethadoneGas Chromatography-Mass SpectrometryIndicators and ReagentsKineticsInjections, IntraventricularHematoporphyrin DerivativeDouble-Blind MethodInjections, IntravenousPapaverLevallorphanLocus CoeruleusHyperalgesiaMolecular Sequence DataPain, IntractableEtorphineTramadolDrug SynergismBupivacaineClonidineRats, WistarPruritusBuprenorphineSpectrophotometry, UltravioletAnalgesics, Non-NarcoticMotor Activity