MorphineDrug ToleranceEnkephalinsAnalgesics, OpioidEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, muNaloxoneEnkephalin, LeucineEnkephalin, MethionineReceptors, OpioidReceptors, Opioid, deltaMorphine DependenceEnkephalin, D-Penicillamine (2,5)-Enkephalin, Leucine-2-AlanineNarcoticsReceptors, Opioid, kappaNarcotic AntagonistsEndorphinsOpioid-Related DisordersMorphine DerivativesOpioid PeptidesSubstance Withdrawal SyndromePain MeasurementInjections, SpinalReceptors, Neurokinin-3PainAnalgesicsNaltrexoneEthylketocyclazocineImmune ToleranceDose-Response Relationship, DrugCyclazocineNeurokinin BPain ThresholdTachykininsPiperidinesReceptors, TachykininDynorphinsD-Ala(2),MePhe(4),Met(0)-ol-enkephalinHyperalgesiaRats, Sprague-DawleySpinal CordSubstance PNeurokinin Abeta-EndorphinAnalgesiaBenzeneacetamidesBenzomorphansDiprenorphineReceptors, Neurokinin-2Receptors, N-Methyl-D-AspartateMorphinansGlucose Tolerance TestFentanylRNA, Transfer, GlyGuinea PigsTransplantation ToleranceOligopeptidesCarboxypeptidase HBuprenorphinePyrrolidinesCodeinePeriaqueductal GrayEtorphinePain, PostoperativeHydromorphoneNeurokinin-1 Receptor AntagonistsMethadoneHeroinAmino Acid SequenceMolecular Sequence DataInjections, IntraventricularBrainProtein PrecursorsOxymorphoneGlycinePentazocineBehavior, AnimalReceptors, Neurokinin-1Locus CoeruleusDrug InteractionsThiorphanIleumTime FactorsPeptide FragmentsSomatostatinAnalgesia, Patient-ControlledBenzylidene CompoundsKineticsNalbuphineVas DeferensSelf ToleranceNeuronsNalorphineRats, Inbred StrainsMice, Inbred C57BLLevorphanolNeprilysinMeperidineBinding, Competitive