Figure 3

Cd²⁺ binding to the activated NMDA receptor channel with a higher affinity than to the closed channel.

(a) NMDA currents are elicited by essentially the same protocols as in Fig. 1c but with different concentrations of Cd²⁺ applied during the NMDA pulse. Cd²⁺ shows a dose-dependent inhibition of the peak as well as the sustained NMDA currents during the NMDA pulse. (b) The amplitude of the sustained current in Cd²⁺ is normalized to that in control to obtain the relative current, and is plotted against Cd²⁺ concentration (n = 7). The data points are fitted with a Hill equation (equation (5) in Methods) with Kd and n of ~1.6 μM and 1, respectively. The groups of Kd values obtained from fits of each individual data sets in Fig. 2b (7.6 ± 1.2 μΜ), 2d (3.4 ± 0.66 μΜ) and 3b (1.4 ± 0.16 μΜ) are also compared with one-way ANOVA followed by the Bonferroni-Holm test. There is significant difference between Figs 2b and 3b (P = 0.0042), and between Figs 2d and 3b (p = 0.028), but not between Fig. 2b and d (p = 0.065). (c) The activation curve is shifted by 3 μM Cd²⁺ (n = 4). P = 0.0087, 1.0 * 10⁻⁵, 7.8 * 10⁻¹⁰, 4.6 * 10⁻¹⁰, and 9.9 * 10⁻⁷ compared with control for NMDA concentrations of 1 to 100 μM, respectively. The curves are fits to the data points in control with equation (1) with a fixed apparent dissociation constant of NMDA (K_N) of 47 μM (the dotted curve) or 40 μM (the solid curve, see Methods for more details), or to the data points in 3 μM Cd²⁺ with equation (2) with a K_Cd2+,o value of 2.5 μM and a r value of 8. (d) The desensitization curve is shifted by 3 μM Cd²⁺ (n = 4). P = 0.19, 0.00059, 0.0018, 1.7 * 10⁻⁵, and 0.00035 compared with control for NMDA concentrations of 1 to 100 μM, respectively. The lines are fits to the data points in control with equation (3) with a fixed Kn value of 35 μM (the dotted curve) or 40 μM (the solid curve, see Methods for more details), or to the data points in 3 μM Cd²⁺ with equation (4) with the same K_Cd2+,o and r values given in part c.