The Role of Calcium-Dependent Desensitization in the Potentiation by Gne-9278 of NMDA Receptor Currents in Rat Cortical Neurons in vitro.

Positive allosteric modulators (PAMs) are of great interest to compensate for the lack of NMDA receptor function in the CNS in dementia. Known PAMs increase the amplitude of whole-cell ionic currents carried by NMDA receptors, but do not affect their calcium-calmodulin dependent desensitization. We studied the possibility of modulating NMDA receptor desensitization by the recently synthesized PAM GNE-9278, which has a unique binding site on the transmembrane domain. Experiments were performed on native NMDA receptors expressed in rat cortical neurons in primary tissue culture. Using the method of "patch-clamp" recording of transmembrane currents, a comparative study of the effect on NMDA receptor desensitization of three substances potentiating NMDA receptor currents was carried out: GNE-9278 (10 µM), dithiothreitol (1 mM) and copper ions (5 µM). These substances increased the amplitude of currents evoked by 100 µM NMDA, but only GNE-9278 reduced the difference between equilibrium and peak current amplitudes by 15%. In addition, GNE-9278 doubled the time constant of the decline from peak to equilibrium state thus attenuating NMDA receptor desensitization. Because GNE-9278 did not alter the effective concentration of extracellular calcium to induce desensitization, its effect probably does not interfere with receptor-calmodulin interaction. Analysis of the shape of the currents within the kinetic model showed that GNE-9278 reduced two kinetic parameters: the channel closure rate, which determines the open state time, and the rates of receptor entry into and exit from the desensitized state, which determines the probability of the open state of the channel. Modulation of calcium-dependent NMDA receptor desensitization places GNE-9278 apart from other known PAMs, which is likely determined by the GNE-9278 binding site in the pre-M1 segment of the GluN1 subunit. [ABSTRACT FROM AUTHOR]