P. Deviche, AFFINITY OF NALOXONE AND ITS QUATERNARY ANALOG FOR AVIAN CENTRAL DELTA-OPIOID AND MU-OPIOID RECEPTORS, Brain research, 757(2), 1997, pp. 276-279
Quaternary narcotic antagonists that are assumed not to penetrate the
blood-brain barrier following systemic administration are commonly use
d 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 quater
nary vs. tertiary antagonists either for opioid receptors in non-mamma
lian species or for specific receptor subtypes has, however, not been
determined. Using brain tissues from a passerine songbird (Junco hyema
lis), we found the affinity of the quaternary antagonist, naloxone met
hiodide (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 previo
us reports demonstrating functional similarities between mammalian and
non-mammalian central opioid receptors.