Divalent cations modulate N-methyl-D-aspartate receptor function at the glycine site

The modulation of the N-methyl-D-aspartate (NMDA) receptor (NMDAR) by divalent cations was examined using (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten 5,10-imine maleate ([H-3]MK-801) binding as a functional indicator of NMDAR function. Ca2+ and Mg2+ produce a biphasic effect on the binding of [H-3]MK-801 to the NMDAR channel in extensively washed adult rat brain membranes. Concentrations of Ca2+ and Mg2+ between I and 600 mu M potentiate binding, but higher concentrations inhibit binding. The potentiating effect of Ca2+ and Mg2+ on [H-3]MK-801 binding is due to an increase in the maximal number of binding sites (B-max) with no effect on binding affinity (K-d). Ca2+- and Mg2+-induced potentiation is the result of an apparent increase in the affinity of the NMDAR for glycine. The ontogeny of NMDAR potentiation by Ca2+ and Mg2+ was also investigated. The number of [H-3]MK-801 binding sites associated with divalent cation potentiation ave present at low levels shortly after birth, and increase to peak level at 17 days of age before declining to adult levels. The potency of Ca2+ and Mg2+ to stimulate [H-3]MK-801 binding did not change as a function of age. Lead (Pb2+) and zinc (Zn2+), potent inhibitors of the NMDAR, antagonize NMDAR potentiation by Ca2+ and Mg2+. These findings indicate that divalent cations differentially regulate NMDAR function by modulation of the glycine site. The NMDAR glycine site may be Important in the regulation of glutamatergic neurotransmission by physiologically and toxicologically relevant cations.