MorphineNaloxoneEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Morphine DependenceEnkephalin, MethionineEnkephalin, LeucineAnalgesics, OpioidReceptors, Opioid, muNarcoticsDrug ToleranceMorphine DerivativesEnkephalin, D-Penicillamine (2,5)-Enkephalin, Leucine-2-AlanineNarcotic AntagonistsReceptors, OpioidReceptors, Opioid, deltaSubstance Withdrawal SyndromeEndorphinsNaltrexoneLocus CoeruleusPeriaqueductal GrayLevallorphanReceptors, Neurokinin-3Rats, Sprague-DawleyAnalgesiaPain MeasurementDose-Response Relationship, DrugEthylketocyclazocineOpioid PeptidesNeurokinin BPainCyclazocineReceptors, Opioid, kappaTachykininsReceptors, TachykininInjections, SpinalNucleus AccumbensAnalgesicsD-Ala(2),MePhe(4),Met(0)-ol-enkephalinBehavior, AnimalSubstance PHyperalgesiaTime FactorsNeurokinin ASpinal CordDynorphinsCodeineReceptors, Neurokinin-2BrainPain, PostoperativeOpioid-Related DisordersNeuronsMorphinansAdenylate CyclaseDrug Administration ScheduleBenzomorphansMotor ActivityRats, WistarArrestinsbeta-EndorphinCyclic AMPMice, Inbred C57BLG Protein-Coupled Inwardly-Rectifying Potassium ChannelsCyclic AMP-Dependent Protein KinasesFentanylGuinea PigsEtorphineRNA, Transfer, GlyBenzeneacetamidesDiprenorphineNalorphine