AnalgesicsAnalgesics, OpioidReceptors, Opioid, muDexmedetomidineAnalgesiaAnalgesics, Non-NarcoticPainReceptors, OpioidPain MeasurementMorphineNaloxoneReceptors, Opioid, deltaHypnotics and SedativesReceptors, Opioid, kappaPain ThresholdHyperalgesiaPharmacologyPharmacology, ClinicalOpioid PeptidesKetamineDose-Response Relationship, DrugRats, Sprague-DawleyPlant ExtractsNarcotic AntagonistsBehavior, AnimalOpioid-Related DisordersNaltrexoneNarcoticsPain, PostoperativeEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-TramadolDrug ToleranceBuprenorphineEndorphinsFentanylEnkephalin, D-Penicillamine (2,5)-DynorphinsPhenacetinAcetaminophenHydromorphoneInjections, SpinalEnkephalin, LeucineMorphinansDextropropoxypheneCodeineAnti-Inflammatory Agents, Non-SteroidalEnkephalin, Leucine-2-AlanineMethadonebeta-EndorphinMeperidinePain ManagementPentazocineAnesthetics, LocalBenzeneacetamidesPain, IntractableDiprenorphineButorphanolDouble-Blind MethodHydrocodoneChronic PainOxymorphoneMorphine DerivativesDipyroneAnalgesia, EpiduralLevorphanolAnalgesia, Patient-ControlledBenzomorphansSubstance Withdrawal SyndromeBupivacainePyrrolidinesOxycodoneDrug InteractionsNociceptorsAlfentanilRadioligand AssayCyclazocineNociceptionNalbuphinePrescription DrugsHeroinPiperidinesLigandsNeuralgiaTolmetinDrug OverdoseCarrageenanKetoprofenSufentanilMeptazinolKetorolacAcupuncture AnalgesiaTime FactorsSpinal CordMorphine DependenceNalorphineKidney Papillary NecrosisNociceptive PainAcute PainPeriaqueductal Gray