MorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, LeucineEnkephalin, MethionineMorphine DependenceReceptors, Opioid, muEnkephalin, Leucine-2-AlanineNarcoticsDrug ToleranceAnalgesics, OpioidEnkephalin, D-Penicillamine (2,5)-NaloxoneNarcotic AntagonistsMorphine DerivativesReceptors, OpioidReceptors, Opioid, deltaEndorphinsSubstance Withdrawal SyndromeLocus CoeruleusReceptors, Neurokinin-3NaltrexonePeriaqueductal GrayNeurokinin BEthylketocyclazocineCyclazocineTachykininsRats, Sprague-DawleyReceptors, TachykininReceptors, Opioid, kappaPain MeasurementD-Ala(2),MePhe(4),Met(0)-ol-enkephalinOpioid PeptidesEtorphineSubstance PDose-Response Relationship, DrugOpioid-Related DisordersNeurokinin AReceptors, Neurokinin-2Nucleus AccumbensTegmentum MesencephaliDynorphinsGuinea PigsDrug ImplantsAnalgesicsPainBehavior, AnimalNeuronsVentral Tegmental AreaInjections, SpinalSpinal CordBrainRNA, Transfer, GlyBenzomorphansAnalgesiaOligopeptidesArrestinsbeta-EndorphinPain ThresholdBenzeneacetamidesCarboxypeptidase HAdenylate CyclaseMorphinansDrug Administration ScheduleGuanosine 5'-O-(3-Thiotriphosphate)Time FactorsDiprenorphineDrug InteractionsTyrosine 3-MonooxygenaseRaphe NucleiRats, Inbred StrainsG Protein-Coupled Inwardly-Rectifying Potassium ChannelsHyperalgesiaMice, Inbred C57BLLimbic SystemReceptors, N-Methyl-D-AspartateNeurokinin-1 Receptor AntagonistsCodeineCyclic AMPMotor ActivityPain, PostoperativeExcitatory Amino Acid AntagonistsExcitatory Postsynaptic PotentialsElectric StimulationGlycineRats, WistarSynaptic TransmissionFentanylCyclic AMP-Dependent Protein KinasesColforsinReceptors, Neurokinin-1RNA, MessengerThiorphanNorepinephrinePiperidinesAmino Acid SequenceProtein PrecursorsDopamineInjections, IntraventricularMolecular Sequence Data