Received for publication June 13, 2006.
Revised November 8, 2006.
Accepted for publication November 9, 2006.

Nefiracetam potentiates NMDA receptor function via PKC activation and reduces magnesium block of NMDA receptor

Abstract

Nicotinic acetylcholine receptors and NMDA receptors are known to be down-regulated in the brain of the Alzheimer's disease patients. We have previously demonstrated that the nootropic drug nefiracetam potentiates the activity of both nicotinic acetylcholine and NMDA receptors, and that nefiracetam modulates the glycine binding site of the NMDA receptor. Since the NMDA receptor is also modulated by Mg²⁺ and protein kinases, we studied their roles in nefiracetam action on the NMDA receptor by the whole-cell patch clamp technique and immnoblotting analysis using rat cortical neurons in primary culture. The nefiracetam potentiation of NMDA currents was inhibited by the PKC inhibitor chelerythrine but not by the PKA inhibitor H- 89. In immnoblotting analysis, nefiracetam treatment increased the PKC activity with a bell-shaped dose-response relationship peaking at 10 nM, thereby increasing phosphorylation of PKC substrate and NMDA receptor. Such an increase in PKC-mediated phosphorylation was prevented by chelerythine. Nefiracetam treatment did not affect the PKA activity. Analysis of the current-voltage relationships revealed that nefiracetam at 10 nM largely eliminated voltage- dependent Mg²⁺ block. It was concluded that nefiracetam potentiated NMDA currents via interactions with PKC, and that it attenuated voltage-dependent Mg²⁺ block through allosteric interactions with the Mg²⁺ binding site.

Key words: Glutamate, Protein Kinase C, Immunocytochemistry