MorphineDrug ToleranceEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Analgesics, OpioidEnkephalin, LeucineAnalgesicsEnkephalin, MethionineReceptors, Opioid, muMorphine DependenceAnalgesiaNarcoticsReceptors, Opioid, deltaEnkephalin, Leucine-2-AlanineReceptors, OpioidPain MeasurementEnkephalin, D-Penicillamine (2,5)-NaloxonePain ThresholdPainTranscutaneous Electric Nerve StimulationNarcotic AntagonistsMorphine DerivativesEndorphinsHyperalgesiaSubstance Withdrawal SyndromeRats, Sprague-DawleyLocus CoeruleusDose-Response Relationship, DrugSpinal CordReceptors, Neurokinin-3NaltrexonePeriaqueductal GrayAnalgesics, Non-NarcoticImmune ToleranceCyclazocineReceptors, N-Methyl-D-AspartateEthylketocyclazocineNeurokinin BMice, Inbred C57BLTachykininsReceptors, Opioid, kappaReceptors, TachykininOpioid PeptidesEtorphineD-Ala(2),MePhe(4),Met(0)-ol-enkephalinOpioid-Related DisordersPain, PostoperativeSubstance PInjections, SpinalNeurokinin AReceptors, Neurokinin-2Behavior, AnimalDynorphinsGuinea PigsNucleus AccumbensGlucose Tolerance TestTegmentum MesencephaliCodeineDrug ImplantsNeuronsTime FactorsBrainMorphinansBenzomorphansTramadolFentanylVentral Tegmental AreaOligopeptidesbeta-EndorphinDrug Administration ScheduleRNA, Transfer, GlyDrug InteractionsArrestinsBenzeneacetamidesPentazocineTransplantation ToleranceGuanosine 5'-O-(3-Thiotriphosphate)Adenylate CyclaseCarboxypeptidase HOxycodoneRats, Inbred StrainsAnalgesia, Patient-ControlledDiprenorphineMotor ActivityMeperidineRats, WistarRaphe NucleiTyrosine 3-MonooxygenaseG Protein-Coupled Inwardly-Rectifying Potassium ChannelsNalorphineAcetaminophenHydromorphonePhenacetinNeurokinin-1 Receptor AntagonistsLimbic SystemCyclic AMPPiperidinesExcitatory Amino Acid AntagonistsElectric Stimulation