MorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, LeucineEnkephalin, MethionineMorphine DependenceReceptors, Opioid, muEnkephalin, Leucine-2-AlanineNarcoticsDrug ToleranceAnalgesics, OpioidEnkephalin, D-Penicillamine (2,5)-NaloxoneNarcotic AntagonistsReceptors, OpioidMorphine DerivativesReceptors, Opioid, deltaEndorphinsSubstance Withdrawal SyndromeLocus CoeruleusReceptors, Neurokinin-3NaltrexonePeriaqueductal GrayNeurokinin BEthylketocyclazocineCyclazocineRats, Sprague-DawleyTachykininsReceptors, TachykininReceptors, Opioid, kappaPain MeasurementOpioid PeptidesD-Ala(2),MePhe(4),Met(0)-ol-enkephalinEtorphineDose-Response Relationship, DrugSubstance POpioid-Related DisordersNeurokinin AReceptors, Neurokinin-2RNA, Transfer, PheNucleus AccumbensTegmentum MesencephaliDynorphinsGuinea PigsDrug ImplantsAnalgesicsPainNeuronsBehavior, AnimalVentral Tegmental AreaInjections, SpinalSpinal CordBrainRNA, Transfer, GlyBenzomorphansOligopeptidesAnalgesiaArrestinsbeta-EndorphinPain ThresholdBenzeneacetamidesCarboxypeptidase HAdenylate CyclaseMorphinansGuanosine 5'-O-(3-Thiotriphosphate)Drug Administration ScheduleTime FactorsDiprenorphineDrug InteractionsTyrosine 3-MonooxygenaseRats, Inbred StrainsRaphe NucleiG Protein-Coupled Inwardly-Rectifying Potassium ChannelsMice, Inbred C57BLHyperalgesiaAmino Acid SequenceLimbic SystemPhenylalanineReceptors, N-Methyl-D-AspartateNeurokinin-1 Receptor AntagonistsCyclic AMPMolecular Sequence DataCodeineMotor ActivityPain, PostoperativeGlycineExcitatory Amino Acid AntagonistsExcitatory Postsynaptic PotentialsElectric StimulationRats, WistarSynaptic TransmissionFentanylRNA, MessengerReceptors, Neurokinin-1ColforsinThiorphanProtein PrecursorsKineticsNorepinephrinePiperidines