Mechanism of Action of Galantamine onN-Methyl-d-Aspartate Receptors in Rat Cortical Neurons

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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, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate currents were not affected by galantamine. The maximum potentiation of NMDA currents to approximately 130% of the control was obtained at 1 microM 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 microM 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, Gi/Go proteins, and Gs 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.