MorphineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, muMorphine DerivativesMorphine DependenceAnalgesics, OpioidNaloxoneDrug ToleranceEnkephalinsNarcoticsNarcotic AntagonistsEnkephalin, D-Penicillamine (2,5)-Receptors, Opioid, deltaReceptors, OpioidNaltrexoneAnalgesicsReceptors, Opioid, kappaPain MeasurementInjections, SpinalAnalgesiaOpioid PeptidesPeriaqueductal GrayRats, Sprague-DawleyCodeineSubstance Withdrawal SyndromePainPain, PostoperativeDose-Response Relationship, DrugEtorphineDiprenorphineFentanylBenzeneacetamidesAnalgesia, Patient-ControlledNalorphineSpinal CordHeroinGuanosine 5'-O-(3-Thiotriphosphate)HydromorphonePyrrolidinesLocus CoeruleusEndorphinsDrug InteractionsPain ThresholdMicroinjectionsSomatostatinEnkephalin, Leucine-2-AlanineOxycodoneBehavior, AnimalEnkephalin, LeucinePentazocineMorphinansBenzomorphansInjections, IntraventricularNociceptorsNalbuphineOligopeptidesHyperalgesiaMethadoneNucleus AccumbensMeperidineThebaineOxymorphoneAnalgesics, Non-NarcoticAnalgesia, EpiduralDynorphinsNeuronsbeta-EndorphinEthylketocyclazocineOpiumSubstantia GelatinosaElectric StimulationInjections, SubcutaneousEnkephalin, MethionineMedulla OblongataRats, WistarTime FactorsMiniature Postsynaptic PotentialsBaclofenPapaverDouble-Blind MethodLevallorphanSulfur RadioisotopesG Protein-Coupled Inwardly-Rectifying Potassium ChannelsPain, IntractableInjectionsTramadolVentral Tegmental AreaBupivacaineInjections, IntravenousAnalysis of VarianceGTP-Binding Proteins