Analgesics, OpioidReceptors, OpioidReceptors, Opioid, muReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesNarcotic AntagonistsOpioid-Related DisordersNaloxoneNarcoticsMorphineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-NaltrexoneEnkephalinsPainEndorphinsEnkephalin, D-Penicillamine (2,5)-FentanylDynorphinsBuprenorphineEnkephalin, LeucineAnalgesicsHydromorphonePain Measurementbeta-EndorphinEnkephalin, Leucine-2-AlanineEnkephalin, MethionineOxycodoneDrug ToleranceBenzeneacetamidesChronic PainAnalgesiaMethadoneDiprenorphinePrescription DrugsOxymorphonePrescription Drug MisusePain, IntractableAlfentanilMorphinansPyrrolidinesHeroinInjections, SpinalBenzomorphansSufentanilHydrocodoneCodeineMorphine DerivativesDose-Response Relationship, DrugOpiate Substitution TreatmentSubstance Withdrawal SyndromeTramadolLevorphanolOpiate AlkaloidsMorphine DependencePeriaqueductal GrayRats, Sprague-DawleyMeperidinePain ThresholdButorphanolAnalgesics, Non-NarcoticDrug OverdosePain ManagementHyperalgesiaPain, PostoperativeD-Ala(2),MePhe(4),Met(0)-ol-enkephalinHeroin DependenceNalbuphineDrug InteractionsDrug PrescriptionsSubstance-Related DisordersMethadyl AcetateNociceptive PainEthylketocyclazocineEuphoriaPiperidinesArkansasCyclazocineLoperamideSpinal CordDrug and Narcotic ControlAcute PainChronic DiseaseAnalgesia, EpiduralOligopeptidesNociceptorsBenzylidene CompoundsSubstance Abuse DetectionEtorphineAnalgesia, Patient-ControlledDiterpenes, ClerodaneGuanosine 5'-O-(3-Thiotriphosphate)SomatostatinLocus CoeruleusNeuralgiaDouble-Blind MethodBehavior, AnimalPain ClinicsAnesthetics, LocalStreet Drugs