M. Marchioro et al., GLYCINE-DEPENDENT AND CALCIUM-DEPENDENT EFFECTS OF LEAD ON N-METHYL-D-ASPARTATE RECEPTOR FUNCTION IN RAT HIPPOCAMPAL-NEURONS, The Journal of pharmacology and experimental therapeutics, 279(1), 1996, pp. 143-153
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.