Copper Modulation of NMDA Responses in Mouse and Rat Cultured Hippocampal Neurons

Abstract

The effect of Cu²⁺ on NMDA receptors was studied in cultured mouse and rat hippocampal neurons using whole‐cell patch‐clamp and a fast perfusion system. Analysis of the Cu²⁺ concentration‐response curve for inhibition of NMDA‐induced currents suggests that free Cu²⁺ directly inhibits NMDA receptors with an IC₅₀ of 0.27 μM. Cu²⁺ was ineffective in blocking NMDA receptor activity when complexed with NMDA or glycine; NMDA‐Cu²⁺ and glycine‐Cu²⁺ complexes acted as agonists of similar potency to the free amino acids. The inhibition by Cu²⁺ (10–100 μM) of responses to 10 μM NMDA was essentially voltage‐independent. The onset of inhibition by 100 μM Cu²⁺ of responses to 2 FM glutamate acting at NMDA receptors was significantly faster than NMDA receptor deactivation evoked by a sudden decrease in the concentration of glycine or glutamate, or of both agonists. This suggests that CU²⁺ acts as a non‐competitive antagonist, and does not directly interfere with the binding of glutamate or glycine to their recognition sites on the NMDA receptor complex. In the absence of NMDA the apparent association rate constant for binding of Cu²⁺ to NMDA receptors, calculated from the rate of onset of block by Cu²⁺ of test responses to NMDA, was 19 times slower than in the presence of 30 μM NMDA, suggesting that Cu^z+ interacts preferentially with agonist‐bound receptors. Our results show that Cu²⁺ is a potent inhibitor of NMDA receptor‐mediated responses.