Differential ontogeny of divalent cation effects on rat brain delta-, mu-, and kappa-opioid receptor binding

The effect of the divalent cations, Mn2+, Mg2+ and Ca2+ on rat forebrain delta-, mu- and kappa-receptor binding was examined during postnatal development. It was found that delta-receptor binding, assessed with 3H]D-Ala2-D-Leu5-enkephalin (3H]DADLE) (+ 10 nM D-Ala2- MePhe4-Gly-ol5-enkephalin (DAMGE)), was stimulated by the 3 cations in a dose- and developmental time-dependent manner. delta-Binding was most sensitive to the cations during the first week postnatal, prior to the appearance of high-affinity delta-binding. In contrast, inhibition of mu-receptor binding (3H]DAMGE) by divalent cations appeared early in development and remained constant throughout the postnatal period. Divalent cation inhibition of kappa-binding (3H]ethylketocyclazocine (3H]EKC) + 100 nM DAMGE and 100 nM DADLE) appeared after the second week postnatal. These results demonstrate that the characteristics and postnatal development of divalent cation modulation of mu-, delta- and kappa-binding is distinctly different. Thus, the neonate may be a good model system to examine the binding properties and functions of delta- and kappa-receptor subtypes.