AnalgesicsAnalgesics, OpioidReceptors, Opioid, muDexmedetomidineAnalgesiaAnalgesics, Non-NarcoticPainReceptors, OpioidPain MeasurementMorphineNaloxoneReceptors, Opioid, deltaHypnotics and SedativesPain ThresholdReceptors, Opioid, kappaHyperalgesiaOpioid PeptidesKetamineDose-Response Relationship, DrugPlant ExtractsRats, Sprague-DawleyNarcotic AntagonistsBehavior, AnimalOpioid-Related DisordersNaltrexonePain, PostoperativeNarcoticsTramadolEnkephalinsDrug ToleranceEnkephalin, Ala(2)-MePhe(4)-Gly(5)-BuprenorphineEndorphinsFentanylEnkephalin, D-Penicillamine (2,5)-AcetaminophenDynorphinsPhenacetinHydromorphoneMorphinansEnkephalin, LeucineCodeineDextropropoxypheneInjections, SpinalAnti-Inflammatory Agents, Non-SteroidalDouble-Blind MethodEnkephalin, Leucine-2-Alaninebeta-EndorphinMeperidineMethadonePentazocinePain ManagementBenzeneacetamidesPain, IntractableAnesthetics, LocalDiprenorphineButorphanolHydrocodoneChronic PainOxymorphoneMorphine DerivativesAnalgesia, EpiduralDipyroneAnalgesia, Patient-ControlledLevorphanolBupivacaineBenzomorphansTime FactorsOxycodoneDrug Administration ScheduleAdministration, OralAlfentanilSubstance Withdrawal SyndromeNalbuphineNociceptorsPyrrolidinesCyclazocineNociceptionDrug InteractionsHeroinPrescription DrugsCarrageenanKetoprofenTolmetinSufentanilNeuralgiaTreatment OutcomeKetorolacMeptazinolAcupuncture AnalgesiaMorphine DependenceLethal Dose 50Drug OverdoseCyclohexanecarboxylic AcidsNalorphineIbuprofenSpinal CordInjections, IntravenousKidney Papillary NecrosisRats, Wistar