Opioid PeptidesReceptors, Opioid, muReceptors, OpioidNaloxoneEndorphinsNarcotic AntagonistsAnalgesics, OpioidReceptors, Opioid, deltabeta-EndorphinReceptors, Opioid, kappaNaltrexoneEnkephalin, MethionineEnkephalinsDynorphinsEnkephalin, LeucineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-MorphineNarcoticsAnalgesiaPain MeasurementPainMorphinansPain ThresholdAnalgesicsEnkephalin, D-Penicillamine (2,5)-BenzeneacetamidesD-Ala(2),MePhe(4),Met(0)-ol-enkephalinAzocines2-HydroxyphenethylaminePeripheral NervesOpioid-Related DisordersAnalgesics, Non-NarcoticFentanylPlacebo EffectAcute PainDrug ToleranceSomatostatinSubstance Withdrawal SyndromeRats, Sprague-DawleyInjections, SpinalPro-OpiomelanocortinOligopeptidesDose-Response Relationship, DrugPyrrolidinesThiorphanInjections, IntraventricularLocus CoeruleusBrainHyperalgesiaPeriaqueductal GrayPeripheral Nervous SystemLuteinizing HormoneNociceptorsPeripheral Nervous System DiseasesProtein PrecursorsAnalysis of VarianceBuprenorphineRewardEthanolBehavior, AnimalPeripheral Vascular DiseasesEnkephalin, Leucine-2-AlanineRadioligand AssayProlactinNucleus AccumbensSpinal CordHypothalamo-Hypophyseal SystemConditioning, OperantDouble-Blind MethodTime FactorsRadioimmunoassayDiprenorphineHypothalamusElectric StimulationAdrenocorticotropic HormoneLeukocytes, MononuclearStress, PhysiologicalRats, WistarMotor ActivityPositron-Emission TomographyPeripheral Nerve InjuriesPeripheral Arterial DiseaseNeuronsMice, KnockoutMice, Inbred C57BLHydrocortisoneMethadoneRats, Inbred StrainsRNA, MessengerSheepHydromorphoneBenzomorphansLymphoma, T-Cell, PeripheralProgesteroneLymphocytesPregnancySciatic NerveOpiate Substitution TreatmentCells, CulturedT-Lymphocytes