Receptors, Opioid, kappaNaltrexoneNarcotic AntagonistsDynorphinsBenzeneacetamidesDiterpenes, ClerodaneReceptors, OpioidReceptors, Opioid, muNaloxoneAnalgesics, OpioidReceptors, Opioid, deltaOpioid PeptidesEndorphinsMorphineNarcoticsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-EnkephalinsDose-Response Relationship, DrugMorphinansEnkephalin, D-Penicillamine (2,5)-Enkephalin, LeucineDiprenorphineAnalgesicsPyrrolidinesRats, Sprague-DawleyEnkephalin, Leucine-2-AlanineEnkephalin, MethionineAzocinesBenzomorphansOpioid-Related Disordersbeta-EndorphinPiperidinesHormone AntagonistsSubstance Withdrawal SyndromePain MeasurementPainDopamine AntagonistsNeurokinin-1 Receptor AntagonistsBuprenorphineExcitatory Amino Acid Antagonists