Analgesics, OpioidAnalgesicsReceptors, Opioid, muAnalgesiaReceptors, OpioidDexmedetomidinePainAnalgesics, Non-NarcoticMorphineNaloxonePain MeasurementReceptors, Opioid, deltaReceptors, Opioid, kappaPain ThresholdHypnotics and SedativesHyperalgesiaOpioid PeptidesNarcoticsKetamineOpioid-Related DisordersNarcotic AntagonistsOpiate AlkaloidsDose-Response Relationship, DrugRats, Sprague-DawleyPlant ExtractsEnkephalinsNaltrexoneBehavior, AnimalEndorphinsBuprenorphineCodeineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-MethadoneDrug ToleranceHeroinMorphine DerivativesEnkephalin, Leucine-2-AlanineEnkephalin, LeucinePain, PostoperativeTramadolFentanylEnkephalin, D-Penicillamine (2,5)-MorphinansDynorphinsSubstance Withdrawal SyndromeCyclazocineHydromorphoneDiprenorphineMorphine DependenceHeroin Dependencebeta-EndorphinOpiumPentazocineOxymorphoneDextropropoxypheneBenzomorphansHydrocodonePhenacetinDihydromorphineAcetaminophenEthylketocyclazocineEtorphineMeperidineSubstance Abuse DetectionOxycodoneInjections, SpinalPain ManagementAnti-Inflammatory Agents, Non-SteroidalButorphanolPain, IntractableNalorphineDrug OverdoseEnkephalin, MethionineBenzeneacetamidesAnesthetics, LocalDouble-Blind MethodChronic PainPyrrolidinesMethadyl AcetateMeptazinolAnalgesia, EpiduralAlfentanilNociceptorsDipyroneBupivacaineLevorphanolAnalgesia, Patient-ControlledPrescription DrugsLevallorphanSufentanilLocus CoeruleusNociceptionSpinal CordNalbuphineNeuralgia