Analgesics, OpioidNarcoticsOpioid-Related DisordersMorphineNaloxoneNarcotic AntagonistsEnkephalin, MethionineReceptors, OpioidOpioid PeptidesOxycodonePainEnkephalin, Ala(2)-MePhe(4)-Gly(5)-FentanylReceptors, Opioid, kappaEnkephalinsEndorphinsAnalgesicsHydromorphoneReceptors, Opioid, deltaPain MeasurementChronic PainNaltrexoneEnkephalin, D-Penicillamine (2,5)-Prescription DrugsAlfentanilOxymorphoneAnalgesiaPain, IntractableBuprenorphineDynorphinsDrug ToleranceEnkephalin, Leucinebeta-EndorphinPrescription Drug MisuseCodeineSufentanilInjections, SpinalHydrocodoneBenzeneacetamidesMethadoneDose-Response Relationship, DrugTramadolEnkephalin, Leucine-2-AlanineRats, Sprague-DawleyMorphine DerivativesOpiate AlkaloidsMeperidineEtorphineHeroinPyrrolidinesArkansasPain ManagementNociceptive PainEuphoriaHyperalgesiaPeriaqueductal GrayPain, PostoperativeDrug PrescriptionsDiprenorphineAnalgesia, EpiduralPain ThresholdDrug InteractionsSubstance-Related DisordersHypnotics and SedativesChronic DiseaseLevorphanolPiperidinesAcute PainAnalgesics, Non-NarcoticDrug and Narcotic ControlNeuralgiaDrug OverdoseOpiate Substitution TreatmentMethadyl AcetateNalbuphineD-Ala(2),MePhe(4),Met(0)-ol-enkephalinBreakthrough PainTime FactorsPain ClinicsSpinal CordSubstance Withdrawal SyndromeBenzomorphansSubstance Abuse DetectionDisease Models, AnimalMorphinansAnalgesia, Patient-ControlledPalliative CareMorphine DependenceNociceptorsBehavior, AnimalAnesthetics, LocalNeuronsDouble-Blind MethodDrug UtilizationButorphanolLoperamidePruritusBenzodiazepinesRats, WistarPrescription Drug Diversion