GLYCINE-DEPENDENT AND CALCIUM-DEPENDENT EFFECTS OF LEAD ON N-METHYL-D-ASPARTATE RECEPTOR FUNCTION IN RAT HIPPOCAMPAL-NEURONS

The effects of lead (Pb++) on N-methyl-D-aspartate (NMDA) receptor fun ction of rat hippocampal neurons in culture were studied by use of the whole-cell patch-clamp technique. Currents activated by NMDA (100 mu M) in the presence of nonsaturating concentrations of glycine (0.01-0. 05 mu M) were potentiated in a voltage-independent manner by Pb++ (1-1 0 mu M), and the potentiation was antagonized by 50 mu M kynurenic aci d. Increasing extracellular Ca++ from 1 to 10 mM similarly potentiated the NMDA-activated currents in the presence of a nonsaturating concen tration of glycine (0.2 mu M). The potentiation of NMDA-activated curr ents by low micromolar concentrations of Pb++ may be mediated by this cation's ability to increase the affinity of the NMDA receptor for gly cine in the presence of 10 mu M glycine and 2 mM Ca++, Pb++ reduced th e peak amplitudes of currents activated by NMDA (100 mu M) in a voltag e-independent manner (IC50 = 5.9 mu M Pb++, Hill coefficient (n(H)) = 1.2). Also, steady-state currents activated by NMDA (50 mu M) were inh ibited by rapid application of Pb++ (IC50 = 3.2 mu M, n(H) = 0.7). Inc reasing extracellular Ca++, in the presence of 10 mu M glycine, reduce d the NMDA-activated currents and shifted the Pb++ concentration-respo nse curves to the right: at 0.2, 2 and 20 mM Ca++, the IC50 values of Pb++ were 3.0, 5.9 and 12.5 mu M and the n(H) values were 0.9, 1.2 and 1.1, respectively. The finding that external Ca++ antagonized the inh ibitory effect of Pb++ suggests that the noncompetitive inhibitory act ion of Pb++ with respect to glycine and NMDA may be mediated by Pb++ c ompetition with Ca++ for a site on the NMDA receptor.