Mechanisms of Action Underlying Conductance-modifying Positive Allosteric Modulators of the NMDA Receptor.

Abstract

NMDA receptors (NMDARs) are ionotropic glutamate receptors that mediate a slow, Ca2+-permeable component of fast excitatory neurotransmission. Modulation of NMDAR function has the potential for disease modification as NMDAR dysfunction has been implicated in neurodevelopment, neuropsychiatric, neurological, and neurodegenerative disorders. We recently described the thieno[2,3-d]pyrimidin-4-one (EU1622) class of positive allosteric modulators, including several potent and efficacious analogs. Here we have used electrophysiological recordings from Xenopus oocytes, HEK cells, and cultured cerebellar and cortical neurons to determine the mechanisms of action of a representative member of this class of modulator. EU1622-240 enhances current response to saturating agonist (doubling response amplitude at 0.2-0.5 µM), slows the deactivation time course following rapid removal of glutamate, increases open probability, enhances co-agonist potency, and reduces single channel conductance. We also show that EU1622-240 can transform NMDARs so that they can be opened when only glutamate or glycine is bound. EU1622-240-bound NMDARs channels activated by a single agonist (glutamate or glycine) open to a unique conductance level with different pore properties and Mg2+ sensitivity, in contrast to channels arising from activation of NMDARs with both co-agonists bound. These data demonstrate that previously hypothesized distinct gating steps can be controlled by glutamate and glycine binding and shows that the 1622-series modulators enable glutamate- or glycine-bound NMDARs to generate open conformations with different pore properties. The properties of this class of allosteric modulators present intriguing therapeutic opportunities for the modulation of circuit function. Significance Statement NMDA receptors are expressed throughout the CNS and are permeable to calcium. EU1622-240 increases open probability and agonist potency, while reducing single channel conductance and prolonging the deactivation time course. EU1622-240 allows NMDA receptor activation by the binding of one co-agonist (glycine or glutamate), which produces channels with distinct properties. Evaluation of this modulator provides insight into gating mechanisms and may lead to the development of new therapeutic strategies.