MorphineAnesthesia, RectalAnalgesics, OpioidPain, PostoperativeInfusions, IntralesionalAnalgesia, Patient-ControlledPain MeasurementReceptors, Opioid, muReceptors, OpioidReceptors, Opioid, deltaMorphine DependenceMorphine DerivativesReceptors, Opioid, kappaOpioid PeptidesNaloxoneNarcotic AntagonistsNarcoticsDrug ToleranceNaltrexoneOpioid-Related DisordersEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-AnalgesiaPainAnalgesicsSubstance Withdrawal SyndromeEndorphinsInjections, SpinalEnkephalin, D-Penicillamine (2,5)-BuprenorphineFentanylCodeineHydromorphoneMorphinansDynorphinsEnkephalin, LeucineHeroinDose-Response Relationship, DrugEnkephalin, Leucine-2-AlanineMethadoneDiprenorphinebeta-EndorphinRats, Sprague-DawleyPeriaqueductal GrayNalbuphineOxymorphoneBenzomorphansPentazocineBenzeneacetamidesPain ThresholdNalorphineLevorphanolMeperidinePain, IntractablePyrrolidinesBehavior, AnimalDrug InteractionsHyperalgesiaSpinal CordOxycodoneButorphanolTramadolOpiumAnalgesics, Non-NarcoticAnalgesia, EpiduralHeroin DependenceHydrocodoneInjections, IntraventricularThebaineEthylketocyclazocineCyclazocineNociceptorsDouble-Blind MethodLocus CoeruleusOpiate Substitution TreatmentChronic PainInjections, SubcutaneousSufentanilAlfentanil