Fig. 3.

The same three pairs of cysteine residues involved in potentiation of NMDA-evoked currents by DTT also mediate potentiation by tricine and voltage-independent Zn²⁺inhibition. A, Current tracings showing potentiation of NMDA-evoked (200 μm NMDA plus 100 μmglycine) currents by tricine (10 mm) in oocytes expressing wild-type NR1–1a/NR2A receptors (top trace), and absence of potentiation by tricine in oocytes expressing NR1–1a(C79S, C308S, C744A, C798A)/NR2A(C87A, C320A) mutant receptors (bottom trace). Unlike potentiation by DTT (Fig.2A), there was no slow component of potentiation by tricine (10 mm). B, Potentiation of NMDA-evoked currents by tricine in the various cysteine mutant combinations expressed as a percentage of the response to NMDA alone (mean ± SEM; n = 6–21 for each group). Thedashed line represents the absence of potentiation of NMDA-evoked currents. The amplitude of potentiation was measured 15 sec after the onset of tricine and NMDA coapplication (similar to DTT potentiation). Tricine potentiation for all mutants was significantly different from that of wild-type NR1–1a/NR2A receptors, and potentiation of NR1–1a(C79S, C308S, C744A, C798A)/NR2A(C87A, C320A) receptors was significantly different from that of the other mutants (p < 0.01, ANOVA followed by Fisher's PLSD test). C, NMDA-evoked (200 μm NMDA plus 100 μm glycine) currents expressed as a percentage of the current recorded in the presence of the Zn²⁺chelator tricine (10 mm). Currents were recorded at a holding potential of −40 mV and pH 7.3 for the following subunit compositions: NR1–1a/NR2A (closed circles), NR1–1a(C744A, C798A)/NR2A (open circles), NR1–1a(C79S, C308S)/NR2A (open squares), NR1–1a/NR2A(C87A, C320A) (closed squares), and NR1–1a(C79S, C308S, C744A, C798A)/NR2A(C87A, C320A) (closed triangles). Each point represents the mean ± SEM of the responses obtained from 4–10 oocytes. For IC₅₀ values and Hill coefficients, see Table2.