Abstract
NMDA receptors are ionotropic glutamate receptors that mediate excitatory neurotransmission in the central nervous system where they play critical roles in normal and pathological brain functions and neurodevelopment. Glutamate/glycine-activated GluN2-containing NMDA receptors (GluN1/GluN2) have been extensively studied, whereas the physiological roles and pharmacology of glycine-activated GluN3-containing receptors (GluN1/GluN3) remain less understood. Although GluN1/GluN3 receptors exhibit unique functional properties and play distinct roles in neuronal development and synapse maturation, studies of their precise roles in neurophysiology and circuit function are impeded by limited availability of GluN3-selective pharmacological tools. This study explores UCM-A86, a novel GluN3-selective positive allosteric modulator, with EC(50) values of 21 μM and 19 μM at GluN1/GluN3A and GluN1/GluN3B receptors, respectively. UCM-A86 selectively potentiates GluN1/GluN3A and GluN1/GluN3B receptors by 436% and 174%, respectively, relative to activation by glycine, with no activity at GluN1/GluN2A-D receptors. Furthermore, UCM-A86 selectively potentiates responses from native GluN1/GluN3A receptors expressed in somatostatin-expressing interneurons of the somatosensory cortex with no modulation of hippocampal AMPA receptor-mediated and GluN1/2 NMDA receptor-mediated excitatory postsynaptic currents. Mechanistic studies suggest that UCM-A86 modulation is facilitated by agonist binding (or channel gating) and that UCM-A86 primarily potentiates GluN1/GluN3A by increasing open probability with no effect on mean channel conductance. These findings advance the synthetic pharmacology of GluN1/GluN3 receptors and provide a novel tool for modulation of native GluN3-containing NMDA receptors. Significance Statement: This study introduces UCM-A86 as the first positive allosteric modulator selective for GluN3-containing NMDA receptors, addressing a critical gap in the pharmacological toolbox for investigating these understudied receptor subtypes. Using electrophysiological approaches in both recombinant and native systems, UCM-A86 demonstrates specific modulation of GluN3-containing NMDA receptors without affecting GluN2-containing NMDA receptors or AMPA receptors. UCM-A86 therefore provides new avenues to investigate the physiological roles of GluN3 subunits in normal and pathological brain function.