MorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, LeucineEnkephalin, MethionineMorphine DependenceNarcoticsReceptors, Opioid, muEnkephalin, Leucine-2-AlanineAnalgesics, OpioidNaloxoneDrug ToleranceEnkephalin, D-Penicillamine (2,5)-Receptors, OpioidNarcotic AntagonistsMorphine DerivativesEndorphinsReceptors, Opioid, deltaSubstance Withdrawal SyndromeLocus CoeruleusNaltrexoneReceptors, Neurokinin-3Opioid-Related DisordersEthylketocyclazocinePeriaqueductal GrayCyclazocineNeurokinin BEtorphineRats, Sprague-DawleyOpiate AlkaloidsTachykininsReceptors, Opioid, kappaReceptors, TachykininOpioid PeptidesPain MeasurementD-Ala(2),MePhe(4),Met(0)-ol-enkephalinDose-Response Relationship, DrugSubstance PCodeineDynorphinsNucleus AccumbensNeurokinin ATegmentum MesencephaliReceptors, Neurokinin-2AnalgesicsGuinea PigsBenzomorphansPainDrug ImplantsAnalgesiaBehavior, AnimalHeroinDiprenorphineBrainNeuronsMorphinansbeta-EndorphinVentral Tegmental AreaSpinal CordInjections, SpinalMethadoneRNA, Transfer, GlyOligopeptidesPain ThresholdArrestinsAdenylate CyclaseTime FactorsDihydromorphineDrug InteractionsDrug Administration ScheduleBenzeneacetamidesGuanosine 5'-O-(3-Thiotriphosphate)Rats, Inbred StrainsOpiumCarboxypeptidase HTyrosine 3-MonooxygenaseLimbic SystemFentanylRaphe NucleiHyperalgesiaMice, Inbred C57BLReceptors, N-Methyl-D-AspartateG Protein-Coupled Inwardly-Rectifying Potassium ChannelsMotor ActivityCyclic AMPSubstance Abuse DetectionHeroin DependencePain, PostoperativeOxymorphonePentazocineNeurokinin-1 Receptor AntagonistsBuprenorphineNalorphineExcitatory Amino Acid AntagonistsElectric StimulationHydromorphoneOxycodoneExcitatory Postsynaptic PotentialsRats, WistarSynaptic Transmission