MorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Morphine DependenceEnkephalin, MethionineReceptors, Opioid, muEnkephalin, LeucineAnalgesics, OpioidNarcoticsDrug ToleranceMorphine DerivativesNaloxoneEnkephalin, D-Penicillamine (2,5)-Enkephalin, Leucine-2-AlanineNarcotic AntagonistsReceptors, Opioid, deltaReceptors, OpioidSubstance Withdrawal SyndromeEndorphinsNaltrexoneLocus CoeruleusPeriaqueductal GrayLevallorphanRats, Sprague-DawleyReceptors, Neurokinin-3Pain MeasurementAnalgesiaOpioid PeptidesReceptors, Opioid, kappaEthylketocyclazocineDose-Response Relationship, DrugPainNeurokinin BCyclazocineAnalgesicsTachykininsInjections, SpinalNucleus AccumbensReceptors, TachykininBehavior, AnimalD-Ala(2),MePhe(4),Met(0)-ol-enkephalinHyperalgesiaSpinal CordSubstance PDynorphinsNeurokinin ANeuronsDiprenorphineCodeineEtorphineBenzomorphansBenzeneacetamidesTime FactorsBrainPain, PostoperativeOpioid-Related DisordersReceptors, Neurokinin-2Adenylate CyclaseMorphinansG Protein-Coupled Inwardly-Rectifying Potassium ChannelsArrestinsbeta-EndorphinDrug Administration ScheduleFentanylCyclic AMPMotor ActivityRats, WistarMice, Inbred C57BLGuinea PigsOligopeptidesRNA, Transfer, GlyCyclic AMP-Dependent Protein KinasesAnalgesia, Patient-ControlledNalorphinePyrrolidinesExcitatory Postsynaptic PotentialsInjections, IntraventricularSynaptic TransmissionPentazocineReceptors, N-Methyl-D-AspartateHeroinCarboxypeptidase HHydromorphoneGTP-Binding Protein beta SubunitsSomatostatinPain ThresholdRNA, MessengerMice, KnockoutDrug InteractionsAnalysis of VarianceOxycodoneExcitatory Amino Acid AntagonistsOxymorphoneNalbuphineGuanosine 5'-O-(3-Thiotriphosphate)Neurokinin-1 Receptor AntagonistsInhibitory Postsynaptic PotentialsMicroinjectionsNociceptorsMethadone