MorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Morphine DependenceEnkephalin, MethionineEnkephalin, LeucineAnalgesics, OpioidReceptors, Opioid, muNarcoticsDrug ToleranceMorphine DerivativesNaloxoneEnkephalin, D-Penicillamine (2,5)-Enkephalin, Leucine-2-AlanineNarcotic AntagonistsReceptors, OpioidReceptors, Opioid, deltaSubstance Withdrawal SyndromeEndorphinsNaltrexoneLocus CoeruleusPeriaqueductal GrayLevallorphanReceptors, Neurokinin-3AnalgesiaPain MeasurementRats, Sprague-DawleyEthylketocyclazocineDose-Response Relationship, DrugOpioid PeptidesNeurokinin BCyclazocinePainReceptors, Opioid, kappaTachykininsReceptors, TachykininInjections, SpinalNucleus AccumbensD-Ala(2),MePhe(4),Met(0)-ol-enkephalinAnalgesicsBehavior, AnimalSubstance PHyperalgesiaNeurokinin ASpinal CordCodeineDynorphinsPain, PostoperativeOpioid-Related DisordersReceptors, Neurokinin-2Time FactorsBrainNeuronsMorphinansAdenylate CyclaseBenzomorphansDrug Administration ScheduleArrestinsMotor Activitybeta-EndorphinG Protein-Coupled Inwardly-Rectifying Potassium ChannelsFentanylEtorphineRNA, Transfer, GlyCyclic AMPCyclic AMP-Dependent Protein KinasesMice, Inbred C57BLRats, WistarDiprenorphineAnalgesia, Patient-ControlledNalorphineBenzeneacetamidesGuinea PigsPentazocineReceptors, N-Methyl-D-AspartateHeroinCarboxypeptidase HHydromorphoneGTP-Binding Protein beta SubunitsRNA, MessengerExcitatory Postsynaptic PotentialsOxycodoneSynaptic TransmissionMice, KnockoutInjections, IntraventricularExcitatory Amino Acid AntagonistsPain ThresholdOxymorphoneNalbuphineNeurokinin-1 Receptor AntagonistsOligopeptidesAnalysis of VarianceInhibitory Postsynaptic PotentialsMeperidinePiperidinesThebaineDrug InteractionsGlycineReceptors, Neurokinin-1Signal Transduction