Mechanism of action of galantamine on N-methyl-D-aspartate receptors in rat cortical neurons

Galantamine, a new Alzheimer's drug approved in the United States, is known to inhibit acetylcholinesterase and potentiate acetylcholine-induced currents in brain neurons. However, because both cholinergic and N-methyl-D-aspartate (NMDA) systems are down-regulated in the brain of Alzheimer's patients, we studied the effects of galantamine on NMDA receptors. NMDA-induced whole-cell currents were recorded from the rat multipolar cortical neurons in primary culture. NMDA currents recorded in Mg²⁺-free media without addition of glycine were reversibly potentiated by bath and U-tube applications of galantamine at 10 to 10,000 nM, showing a bell-shaped dose-response relationship. However, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate currents were not affected by galantamine. The maximum potentiation of NMDA currents to ∼130% of the control was obtained at 1 μM galantamine. The potentiation was due to a shift of the NMDA dose-response curve in the direction of lower NMDA concentrations. Glycine at 1 to 3000 nM enhanced NMDA currents, and potentiation by 1 μM galantamine and 1 to 300 nM glycine was additive. The glycine site antagonist 7-chlorokynurenic acid did not prevent the galantamine action. These results suggested that galantamine did not interact with the glycine binding site. Experiments with various concentrations of Mg²⁺ indicated that galantamine did not affect the Mg²⁺ blocking site of the NMDA receptor. PKC was involved in galantamine potentiation of NMDA currents, but protein kinase A, G _i/G_o proteins, and G_s proteins were not involved. Potentiation of the activity of NMDA receptors is deemed partially responsible for the improvement of cognition, learning, and memory in Alzheimer's patients.