Chronic PainAnalgesicsAnalgesics, OpioidPainReceptors, Opioid, muReceptors, OpioidPain MeasurementPain ManagementReceptors, Opioid, deltaSleep, REMReceptors, Opioid, kappaPain ThresholdPain, IntractableOpioid PeptidesAnalgesics, Non-NarcoticPain, PostoperativeAnalgesiaChronic DiseaseMorphineNarcotic AntagonistsOpioid-Related DisordersAcute PainPain PerceptionNaloxoneBack PainNeuralgiaHyperalgesiaLow Back PainNeck PainPain ClinicsSleep DeprivationAbdominal PainFacial PainNaltrexoneElectroencephalographySleepTramadolNarcoticsSleep StagesDrug ToleranceWakefulnessBuprenorphinePelvic PainMusculoskeletal PainNociceptorsEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Injections, SpinalPharmacovigilanceFentanylNociceptive PainEndorphinsHydromorphoneRats, Sprague-DawleyOxycodoneFibromyalgiaAnesthetics, LocalShoulder PainEnkephalin, D-Penicillamine (2,5)-NociceptionDynorphinsAcetaminophenAnti-Inflammatory Agents, Non-SteroidalPain, ReferredCodeineElectromyographyTime FactorsSpinal CordDose-Response Relationship, DrugMethadonePhenacetinQuestionnairesDouble-Blind MethodBrain Wavesbeta-EndorphinHydrocodoneArousalTreatment OutcomeMorphinansElectrooculographyEnkephalin, LeucineDextropropoxypheneComplex Regional Pain SyndromesBupivacaineMeperidineAnalysis of VariancePrescription Drug MisuseArthralgiaEnkephalin, Leucine-2-AlanineOxymorphoneCyclohexanecarboxylic AcidsRats, WistarPentazocineDisease Models, AnimalAcupuncture AnalgesiaPrescription DrugsNerve BlockCarrageenanPosterior Horn CellsAnalgesia, Patient-Controlled