Affinity of naloxone and its quaternary analogue for avian central delta and mu opioid receptors.

Quaternary narcotic antagonists that are assumed not to penetrate the blood-brain barrier following systemic administration are commonly used to distinguish between peripheral and central actions of opiates. In mammals, these antagonists have a lower affinity for opioid receptors than their tertiary parent compounds. The relative affinity of quaternary vs. tertiary antagonists either for opioid receptors in non-mammalian species or for specific receptor subtypes has, however, not been determined. Using brain tissues from a passerine songbird (Junco hyemalis), we found the affinity of the quaternary antagonist, naloxone methiodide (Nal MI), for brain opioid receptors to be less than 10% that of Nal HCl. Further, Nal MI affinity for mu and delta receptors is 8.7% and 3.7%, respectively, that of Nal HCl. These results confirm that tertiary narcotic antagonist quaternization substantially reduces the affinity of these derivatives for central opioid receptors. They show that this reduction is receptor-type selective, and they extend previous reports demonstrating functional similarities between mammalian and non-mammalian central opioid receptors.