NaloxoneNarcotic AntagonistsAnalgesics, OpioidMorphineReceptors, OpioidNarcoticsEnkephalin, MethionineEndorphinsOpioid-Related DisordersOpioid PeptidesBuprenorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-OxycodoneReceptors, Opioid, kappaPainAnalgesicsFentanylbeta-EndorphinEnkephalin, LeucineNaltrexoneReceptors, Opioid, deltaReceptors, Opioid, muMorphine DependenceEnkephalin, D-Penicillamine (2,5)-Pain MeasurementDrug ToleranceDynorphinsDrug OverdoseHydromorphoneSubstance Withdrawal SyndromeEnkephalin, Leucine-2-AlanineInjections, SpinalDose-Response Relationship, DrugOxymorphoneMorphinansAlfentanilLevorphanolMeperidineBenzeneacetamidesHeroinCodeineEtorphineMethadoneChronic PainOpiate AlkaloidsMorphine DerivativesPrescription DrugsLoperamideDiprenorphinePyrrolidinesCyclazocineSufentanilAnalgesiaPeriaqueductal GrayPentazocinePain, IntractableLevallorphanRats, Sprague-DawleyHeroin DependenceD-Ala(2),MePhe(4),Met(0)-ol-enkephalinPain ThresholdInjections, IntraventricularPrescription Drug MisuseNociceptorsNalorphineDrug InteractionsOpiate Substitution TreatmentHyperalgesiaAnalgesics, Non-NarcoticHydrocodoneSpinal CordAdministration, SublingualMeptazinolMethadyl AcetateTramadolClonidinePiperidinesEthylketocyclazocineNalbuphinePain ManagementElectric StimulationPruritusSubstance-Related DisordersPain, PostoperativeBehavior, AnimalHypnotics and SedativesTime FactorsElectroacupunctureGuinea Pigs