Drug ToleranceMorphineReflex, RightingEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, LeucineEnkephalin, MethionineMorphine DependenceAnalgesics, OpioidReceptors, Opioid, muNarcoticsMorphine DerivativesNaloxoneEnkephalin, Leucine-2-AlanineEnkephalin, D-Penicillamine (2,5)-Narcotic AntagonistsReceptors, OpioidReceptors, Opioid, deltaEndorphinsSubstance Withdrawal SyndromeLocus CoeruleusNaltrexonePeriaqueductal GrayReceptors, Neurokinin-3Pain MeasurementRats, Sprague-DawleyImmune ToleranceEthylketocyclazocineNeurokinin BCyclazocineDose-Response Relationship, DrugReceptors, Opioid, kappaTachykininsReceptors, TachykininInjections, SpinalOpioid PeptidesAnalgesiaEtorphineAnalgesicsPainOpioid-Related DisordersD-Ala(2),MePhe(4),Met(0)-ol-enkephalinSubstance PBehavior, AnimalNeurokinin ANucleus AccumbensCodeineDynorphinsSpinal CordGuinea PigsDrug ImplantsTegmentum MesencephaliPain, PostoperativeReceptors, Neurokinin-2Pain ThresholdGlucose Tolerance TestMorphinansNeuronsBrainTime FactorsBenzomorphansVentral Tegmental AreaDrug InteractionsMice, Inbred C57BLDrug Administration Schedulebeta-EndorphinFentanylOligopeptidesArrestinsHyperalgesiaRNA, Transfer, GlyNalorphineBenzeneacetamidesAnalgesia, Patient-ControlledDiprenorphineGuanosine 5'-O-(3-Thiotriphosphate)Transplantation ToleranceAdenylate CyclaseMotor ActivityPentazocineRats, Inbred StrainsHeroinReceptors, N-Methyl-D-AspartateHydromorphoneCarboxypeptidase HTyrosine 3-MonooxygenaseRaphe NucleiOxycodoneRats, WistarInjections, IntraventricularG Protein-Coupled Inwardly-Rectifying Potassium ChannelsLimbic SystemExcitatory Amino Acid AntagonistsCyclic AMPNalbuphineOxymorphoneNeurokinin-1 Receptor AntagonistsElectric StimulationRNA, MessengerMice, Knockout