Receptors, Opioid, muAnalgesics, OpioidReceptors, OpioidPainReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesChronic PainPain ManagementPain ThresholdPain MeasurementNarcotic AntagonistsPain, PostoperativeLow Back PainBack PainPain, IntractableNaloxoneOpioid-Related DisordersAbdominal PainNeck PainNaltrexoneMorphinePain PerceptionPelvic PainAcute PainEnkephalinsAnalgesicsFacial PainEnkephalin, Ala(2)-MePhe(4)-Gly(5)-NarcoticsPain, ReferredEnkephalin, D-Penicillamine (2,5)-EndorphinsBuprenorphineDynorphinsShoulder PainEnkephalin, LeucineMusculoskeletal PainAnalgesiaEnkephalin, Leucine-2-AlanineFentanylbeta-EndorphinDiprenorphineNociceptive PainHyperalgesiaNeuralgiaDrug ToleranceBenzeneacetamidesHydromorphoneMethadoneMorphinansChronic DiseaseNociceptorsInjections, SpinalAnalgesics, Non-NarcoticBenzomorphansPyrrolidinesSubstance Withdrawal SyndromeOpiate Substitution TreatmentDose-Response Relationship, DrugMyofascial Pain SyndromesRats, Sprague-DawleyPrescription Drug MisuseComplex Regional Pain SyndromesTreatment OutcomeVisceral PainHeroinTramadolButorphanolArthralgiaLevorphanolAnesthetics, LocalLabor PainPrescription DrugsSpinal CordDouble-Blind MethodMorphine DependenceOxymorphoneTime FactorsMorphine DerivativesDiterpenes, ClerodaneQuestionnairesPeriaqueductal GrayHeroin DependenceNociceptionEthylketocyclazocineHydrocodoneOligopeptidesDrug OverdoseFibromyalgiaCatastrophizationDisability EvaluationBehavior, AnimalAlfentanilBenzylidene CompoundsProspective StudiesAnalgesia, Patient-ControlledCodeineNalbuphineCyclazocine