MODULATION BY MAGNESIUM OF THE AFFINITY OF NMDA RECEPTORS FOR GLYCINEIN MURINE HIPPOCAMPAL-NEURONS

1. The effects of the divalent cation Mg2+ on NMDA currents recorded f rom cultured fetal mouse and acutely isolated neonatal rat hippocampal neurones were studied using the whole-cell patch-clamp technique. 2. Current-voltage relations were measured in the presence or absence of applied Mg2+ and added glycine. NMDA-evoked currents were studied in t he absence or in a low concentration (0.2 mM) of applied Ca2+ in order to minimize Ca2+-dependent inactivation of the responses. Mg2+ unexpe ctedly enhanced NMDA-activated currents at positive membrane potential s. At negative membrane potentials Mg2+ caused a previously characteri zed voltage-dependent block of inward NMDA-activated currents. 3. The potentiation by Mg2+ of outward currents activated by NMDA was concent ration dependent (EC(50), similar to 3 mM; Hill coefficient, similar t o 2). Mg2+ also reduced the desensitization of the NMDA receptor. The maximal enhancement of steady-state NMDA-activated currents was 2.7-fo ld and at 6 mM the time constant of desensitization was doubled. 4. Co mparisons of concentration-response curves for glycine and 7-chloro-ky nurenic acid demonstrated that Mg2+ significantly increased the affini ty of the NMDA receptor for glycine. The EC(50) for glycine was 380 nM in the absence of Mg2+ and 163 nM in 3 mM Mg2+. Mg2+ had little effec t on the forward rate of the glycine response but halved the off-rate (2.34 to 1.15 s(-1)) and thus similarly reduced the apparent dissociat ion constant. 5. There was a good correlation between the concentratio n of extracellular Ca2+ and a reduction in the time constant of the gl ycine-sensitive component of NMDA receptor desensitization. Ca2+ could enhance these NMDA-activated currents briefly following exposure to h igh concentrations of Ca2+. These results are consistent with a Ca2+-d ependent enhancement of the affinity of the NMDA receptor for glycine. 6. Mg2+ can enhance NMDA-mediated currents and reduce desensitization of this receptor by allosterically interacting with the glycine bindi ng site. This interaction may be a key physiological mechanism through which modulation of the NMDA receptor is achieved.