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 interestto compensate for the lack of NMDA receptor function in the CNSin dementia. Known PAMs increase the amplitude of whole-cell ionic currentscarried by NMDA receptors, but do not affect their calcium-calmodulindependent desensitization. We studied the possibility of modulatingNMDA receptor desensitization by the recently synthesized PAM GNE-9278,which has a unique binding site on the transmembrane domain. Experimentswere performed on native NMDA receptors expressed in rat corticalneurons in primary tissue culture. Using the method of “patch-clamp”recording of transmembrane currents, a comparative study of theeffect on NMDA receptor desensitization of three substances potentiatingNMDA receptor currents was carried out: GNE-9278 (10 µM), dithiothreitol(1 mM) and copper ions (5 µM). These substances increased the amplitudeof currents evoked by 100 µM NMDA, but only GNE-9278 reduced thedifference between equilibrium and peak current amplitudes by 15%.In addition, GNE-9278 doubled the time constant of the decline frompeak to equilibrium state thus attenuating NMDA receptor desensitization.Because GNE-9278 did not alter the effective concentration of extracellular calciumto induce desensitization, its effect probably does not interferewith receptor-calmodulin interaction. Analysis of the shape of thecurrents within the kinetic model showed that GNE-9278 reduced twokinetic parameters: the channel closure rate, which determines theopen state time, and the rates of receptor entry into and exit fromthe desensitized state, which determines the probability of the openstate of the channel. Modulation of calcium-dependent NMDA receptordesensitization places GNE-9278 apart from other known PAMs, whichis likely determined by the GNE-9278 binding site in the pre-M1segment of the GluN1 subunit.