Table 2.

Subunit-specific voltage-independent Zn²⁺ inhibition

	1a/2A	1b/2A	1a/2B	1b/2B	1a/2C
IC50(nm)	∼10	70	490	2500	14,000
	(see Fig. 3)	(n = 4)	(see Fig.3)	(n = 5)	(n = 4)

Recombinant NMDA receptors were expressed in Xenopusoocytes. The IC₅₀ values were determined as described in Figure 3B for NR1b–NR2A receptors and Figure 3Cfor NR1b–N2B and NR1a–NR2C receptors. As for NR1a–NR2A receptors, the voltage-independent Zn²⁺ inhibition of NR1b–NR2A receptors is partial, saturating at ∼70% of the maximal current. In contrast, as for NR1a–NR2B receptors, NR1b–NR2B receptors and NR1a–NR2C receptors are fully inhibited by Zn²⁺, mainly via a voltage-independent mechanism. The external Ba²⁺ concentration was 0.3 mm, and the external pH was 7.3.