MorphineEnkephalinsMorphine DependenceDrug ToleranceEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Analgesics, OpioidEnkephalin, MethionineReceptors, Opioid, muEnkephalin, LeucineNarcoticsNaloxoneSubstance Withdrawal SyndromeNarcotic AntagonistsEnkephalin, D-Penicillamine (2,5)-Enkephalin, Leucine-2-AlanineNaltrexoneReceptors, OpioidReceptors, Opioid, deltaMorphine DerivativesEndorphinsRats, Sprague-DawleyPain MeasurementBehavior, AnimalDose-Response Relationship, DrugLocus CoeruleusReceptors, Neurokinin-3LevallorphanPeriaqueductal GrayNeurokinin BEthylketocyclazocineCyclazocineOpioid PeptidesTachykininsAnalgesiaReceptors, TachykininReceptors, Opioid, kappaD-Ala(2),MePhe(4),Met(0)-ol-enkephalinNucleus AccumbensPainSubstance PNeurokinin AReceptors, Neurokinin-2HyperalgesiaDynorphinsSpinal CordAdenylate CyclaseBrainOpioid-Related DisordersTime FactorsNeuronsAnalgesicsInjections, SpinalArrestinsRNA, Transfer, GlyG Protein-Coupled Inwardly-Rectifying Potassium ChannelsBenzomorphansDrug Administration ScheduleCyclic AMP-Dependent Protein KinasesCyclic AMPbeta-EndorphinMotor ActivityMice, Inbred C57BLBenzeneacetamidesCarboxypeptidase HMorphinansGuinea PigsGTP-Binding Protein beta SubunitsRats, WistarDiprenorphineEtorphineRNA, MessengerExcitatory Postsynaptic PotentialsReceptors, N-Methyl-D-AspartateSynaptic TransmissionNeurokinin-1 Receptor AntagonistsCodeineInhibitory Postsynaptic PotentialsMice, KnockoutPain, PostoperativeOligopeptidesExcitatory Amino Acid AntagonistsGlycineFentanylAnalysis of VarianceReceptors, Neurokinin-1ThiorphanPiperidinesAmino Acid SequenceProtein PrecursorsSignal TransductionPhosphorylationInjections, Intraventricular