Narcotic AntagonistsNaloxoneOpioid PeptidesReceptors, OpioidNaltrexoneEndorphinsReceptors, Opioid, mubeta-EndorphinEnkephalin, MethionineAnalgesics, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaDynorphinsEnkephalinsNarcoticsEnkephalin, LeucineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-MorphineAnalgesiaD-Ala(2),MePhe(4),Met(0)-ol-enkephalinPainAnalgesicsPain MeasurementLevallorphanDose-Response Relationship, DrugOpioid-Related DisordersDrug ToleranceRats, Sprague-DawleyPain ThresholdEnkephalin, D-Penicillamine (2,5)-DiprenorphineEnkephalin, Leucine-2-AlanineBenzeneacetamidesMorphinansFentanylAzocinesBuprenorphineBenzomorphansPyrrolidinesOxymorphoneSubstance Withdrawal SyndromeAnalysis of VarianceMorphine DependenceLuteinizing HormoneHeroinLevorphanolPiperidinesEtorphineInjections, SpinalBenzylidene CompoundsAnalgesics, Non-NarcoticDrug InteractionsPruritusInjections, IntraventricularDiterpenes, ClerodanePeriaqueductal GrayOligopeptidesMeptazinolAntipruriticsDrug Inverse AgonismHormone AntagonistsBehavior, AnimalRats, WistarTetrahydroisoquinolinesNociceptorsHydromorphoneExcitatory Amino Acid AntagonistsHyperalgesiaGuinea PigsNeurokinin-1 Receptor AntagonistsDopamine AntagonistsSpinal CordTime FactorsOxycodoneConditioning, OperantSomatostatinMotor ActivityChronic PainEthanolInjections, SubcutaneousHallucinogensMethadoneHistamine H2 AntagonistsElectric StimulationSelf AdministrationStereoisomerismPrescription DrugsInterleukin 1 Receptor Antagonist ProteinMice, Inbred ICRGABA AntagonistsMuscarinic AntagonistsBrainCapsaicinPain, IntractablePrescription Drug MisuseLigandsAlfentanilNeuronsRats, Inbred StrainsHistamine H1 Antagonists