MorphineMorphine DependenceAnalgesics, OpioidDrug ToleranceNarcoticsReceptors, Opioid, muMorphine DerivativesSubstance Withdrawal SyndromeNaloxoneNarcotic AntagonistsLevallorphanEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Locus CoeruleusReceptors, Opioid, deltaPeriaqueductal GrayNaltrexoneEnkephalinsReceptors, OpioidRats, Sprague-DawleyAnalgesiaPain MeasurementNucleus AccumbensPainDose-Response Relationship, DrugEnkephalin, MethionineOpioid PeptidesHyperalgesiaBehavior, AnimalOpioid-Related DisordersAdenylate CyclaseDrug Administration ScheduleTime FactorsArrestinsSpinal CordG Protein-Coupled Inwardly-Rectifying Potassium ChannelsCyclic AMP-Dependent Protein KinasesNeuronsGTP-Binding Protein beta SubunitsMotor ActivityMice, Inbred C57BLCyclic AMPInjections, SpinalBrainReceptors, N-Methyl-D-AspartateExcitatory Postsynaptic PotentialsCodeineInhibitory Postsynaptic PotentialsRats, WistarPain, PostoperativeExcitatory Amino Acid AntagonistsMice, KnockoutRNA, MessengerSynaptic TransmissionAnalgesicsAnalysis of VarianceAnalgesia, Patient-ControlledNalorphineSignal TransductionPhosphorylationUp-RegulationHeroinHydromorphoneBlotting, WesternFentanylGene Expression RegulationOxycodonePentazocineMorphinansNalbuphine