AnalgesicsAnalgesics, OpioidReceptors, Opioid, muDexmedetomidineAnalgesiaAnalgesics, Non-NarcoticPainReceptors, OpioidPain MeasurementMorphineNaloxoneReceptors, Opioid, deltaPain ThresholdHypnotics and SedativesReceptors, Opioid, kappaHyperalgesiaCentral Nervous SystemOpioid PeptidesKetamineNervous SystemDose-Response Relationship, DrugRats, Sprague-DawleyPlant ExtractsNarcotic AntagonistsBehavior, AnimalOpioid-Related DisordersNaltrexonePeripheral Nervous SystemCentral Nervous System DiseasesPain, PostoperativeEnkephalinsNarcoticsTramadolEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Drug ToleranceEndorphinsBuprenorphineFentanylEnkephalin, D-Penicillamine (2,5)-Enteric Nervous SystemCentral Nervous System NeoplasmsBrainDynorphinsPhenacetinAcetaminophenAutonomic Nervous SystemInjections, SpinalSpinal CordEnkephalin, LeucineHydromorphoneSympathetic Nervous SystemMorphinansCodeineNeuronsDextropropoxypheneNervous System Physiological PhenomenaEnkephalin, Leucine-2-Alaninebeta-EndorphinAnti-Inflammatory Agents, Non-SteroidalPain ManagementMeperidinePentazocineMethadoneCentral Nervous System InfectionsBenzeneacetamidesPain, IntractableNervous System DiseasesNervous System NeoplasmsAnesthetics, LocalDiprenorphineChronic PainButorphanolHydrocodoneDouble-Blind MethodMorphine DerivativesOxymorphoneDipyroneNociceptorsAnalgesia, EpiduralLevorphanolBupivacaineTime FactorsAnalgesia, Patient-ControlledBenzomorphansNociceptionNeuralgiaOxycodoneSubstance Withdrawal SyndromeAlfentanilPyrrolidinesCyclazocineNalbuphinePrescription DrugsCentral Nervous System Viral DiseasesCarrageenanInjections, IntraventricularHeroinCentral Nervous System AgentsTolmetinAcupuncture Analgesia