Multiple sites of divalent cation modulation of delta opioid receptor binding

Having established differential ontogeny for rat brain mu, delta, and kappa opioid receptors (Spain et al. 1985) we have examined the effect of divalent cations on delta receptors during postnatal development. The pronounced inhibition of the binding of 1 nM [3H]D-Ala2-D-Leu5-enkephalin (DADLE) by 100 mM Na+ was reversed by Mn++ to the levels seen in the absence of ions both in adults and in 10-day-old neonates. Mg++ and Ca++ on the other hand were much less potent in adults, and ineffective in neonates. These results suggest that, in the presence of Na+, regulation of delta binding with Mg++ or Ca++ emerges subsequent to Mn++ modulation in postnatal development. Dose-dependency studies in adults performed with each ion in the presence of fixed concentrations of either of the other ions revealed that while Mg++ or Ca++ antagonized stimulation of delta binding by Mn++ and vice versa, Mg++ and Ca++ displayed no antagonism toward each other. Our data suggest the existence of at least two divalent cation regulatory sites for delta opioid receptors, one of which appears to be specific for Mn++ and can readily be detected in the presence of Na+.