MorphineDrug ToleranceEnkephalinsAnalgesics, OpioidEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, muReceptors, OpioidEnkephalin, LeucineReceptors, Opioid, deltaAnalgesicsEnkephalin, MethionineEnkephalin, D-Penicillamine (2,5)-NarcoticsMorphine DependenceNarcotic AntagonistsNaloxoneEnkephalin, Leucine-2-AlanineReceptors, Opioid, kappaAnalgesiaPain MeasurementPainEndorphinsPain ThresholdNaltrexoneMorphine DerivativesOpioid PeptidesTranscutaneous Electric Nerve StimulationDose-Response Relationship, DrugRats, Sprague-DawleySubstance Withdrawal SyndromeHyperalgesiaBenzeneacetamidesEtorphineOpioid-Related DisordersLocus CoeruleusPeriaqueductal GrayEthylketocyclazocineSpinal CordBuprenorphineAnalgesics, Non-NarcoticPyrrolidinesDynorphinsReceptors, Neurokinin-3BenzomorphansFentanylD-Ala(2),MePhe(4),Met(0)-ol-enkephalinMorphinansCyclazocineLevorphanolNalbuphineDiprenorphineHydromorphoneImmune ToleranceMice, Inbred C57BLReceptors, N-Methyl-D-AspartateMethadoneNeurokinin BInjections, SpinalTachykininsOligopeptidesOxycodoneReceptors, Tachykininbeta-EndorphinBehavior, AnimalGuinea PigsMethadyl AcetateDrug InteractionsGuanosine 5'-O-(3-Thiotriphosphate)Pain, PostoperativeSubstance PNeuronsTime FactorsPentazocineNeurokinin-1 Receptor AntagonistsNeurokinin ABrainHeroinNucleus AccumbensDiterpenes, ClerodaneAzocinesButorphanolReceptors, Neurokinin-2Rats, WistarCodeineAdenylate CyclaseGlucose Tolerance TestTegmentum MesencephaliDrug ImplantsTramadolG Protein-Coupled Inwardly-Rectifying Potassium ChannelsNociceptorsInjections, IntraventricularVas DeferensBenzamidesCyclic AMPDrug Administration ScheduleArrestinsElectric StimulationVentral Tegmental AreaIleum