AnalgesicsOpioid PeptidesNaloxoneAnalgesics, OpioidReceptors, Opioid, muNarcotic AntagonistsEndorphinsReceptors, Opioidbeta-EndorphinEnkephalin, MethionineAnalgesiaPainNaltrexoneReceptors, Opioid, deltaMorphineReceptors, Opioid, kappaDynorphinsEnkephalinsPain MeasurementDexmedetomidineAnalgesics, Non-NarcoticNarcoticsPain ThresholdEnkephalin, LeucineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-D-Ala(2),MePhe(4),Met(0)-ol-enkephalinHypnotics and SedativesHyperalgesiaLevallorphanRats, Sprague-DawleyDose-Response Relationship, DrugDrug ToleranceKetamineOpioid-Related DisordersPlant ExtractsBehavior, AnimalFentanylBuprenorphineTramadolEnkephalin, D-Penicillamine (2,5)-Pain, PostoperativeOxycodoneAnalysis of VarianceHydromorphoneLuteinizing HormoneCodeineChronic PainPain, IntractableInjections, SpinalOxymorphoneEnkephalin, Leucine-2-AlanineMethadoneBenzeneacetamidesPrescription DrugsAlfentanilMeperidineHydrocodoneMorphinansPain ManagementAcetaminophenDextropropoxyphenePhenacetinMorphine DerivativesDiprenorphineSufentanilPrescription Drug MisuseAnalgesia, EpiduralLevorphanolHeroinButorphanolPyrrolidinesAnesthetics, LocalPentazocineAnalgesia, Patient-ControlledBenzomorphansAnti-Inflammatory Agents, Non-SteroidalDouble-Blind MethodNalbuphineNociceptive PainNociceptorsSubstance Withdrawal SyndromePeriaqueductal GrayNeuralgiaBupivacaineCyclazocineDipyroneAcute PainDrug OverdoseOpiate AlkaloidsMorphine DependenceOpiate Substitution TreatmentDrug InteractionsNociceptionSpinal CordDrug PrescriptionsTolmetinKetorolacPiperidinesDrug and Narcotic ControlChronic Disease