Higher ambient synaptic glutamate at inhibitory versus excitatory neurons differentially impacts NMDA receptor activity

Abstract

Selective disruption of synaptic drive to inhibitory neurons could contribute to the pathophysiology of various brain disorders. We have previously identified a GluN2A-selective positive allosteric modulator, GNE-8324, that selectively enhances N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic responses in inhibitory but not excitatory neurons. Here, we demonstrate that differences in NMDAR subunit composition do not underlie this selective potentiation. Rather, a higher ambient glutamate level in the synaptic cleft of excitatory synapses on inhibitory neurons is a key factor. We show that increasing expression of glutamate transporter 1 (GLT-1) eliminates GNE-8324 potentiation in inhibitory neurons, while decreasing GLT-1 activity enables potentiation in excitatory neurons. Our results reveal an unsuspected difference between excitatory synapses onto different neuronal types, and a more prominent activation of synaptic NMDARs by ambient glutamate in inhibitory than excitatory neurons. This difference has implications for tonic NMDAR activity/signaling and the selective modulation of inhibitory neuron activity to treat brain disorders.

Increasing endogenous activity of NMDARs on GABAergic neurons increases inhibition, alters sensory processing and prevents noise-induced tinnitus

“…It is possible that lateral movement of NMDAR could be altered by M-8324, although we think this possibility is remote. Since GNE-8324 potentiates both synaptic and extrasynaptic NMDAR responses 10 , it is unlikely that the impact of NMDAR-PAMs on the NMDAR trafficking plays a major role since it is unlikely that the same drug enhances NMDAR trafficking to both synaptic and extrasynaptic sites.

Figure 1 M-8324 enhances spontaneous spiking of GABAergic neurons in vitro.

…”

“…There was also no effect of M-8324 on AMPAR-mediated EPSPs in the GABAergic neurons. Whether M-8324 affects only GluN2A-NMDARs, as is the case for GNE-8324 9 , 10 , was not tested. In addition to neurons in AI, we have also found enhanced NMDAR-EPSCs/EPSPs and spiking in GABAergic neurons in mouse prefrontal cortex and hippocampus, suggesting that this enhancement is likely brain-wide.…”

“…The pressure changes for every startle stimulus were recorded. The gap was presented 50 ms between six different frequencies (5,7,10,14,20,28 kHz) at 75 dB and the startle stimulus (white noise, 105 dB, 20 ms). After a 3 min acclimation period, 150 gap trials and 150 no-gap trials under six frequencies were performed randomly for each mouse.…”

“…Endogenous behavior likely requires coordinated activation of many subtypes of GABAergic neurons 14 , and modulation of diverse GABAergic neurons may be needed or required to achieve therapeutic benefit since various subtypes are altered in diseases 22 , 23 ; (3) these approaches are not ready for clinical use yet. By using small molecule NMDAR- PAMs, we circumvent the above limitations, since PAMs enhance endogenous activity rather than imposing activity artificially, and likely affect the majority of inhibitory neurons 10 .…”

“…One of the PAMs, GNE-8324, is of particular interest since it selectively enhances activity of synaptic NMDARs on inhibitory but not excitatory neurons 9,10 . This novel selectivity is mediated by higher ambient glutamate concentration in the synaptic cleft during resting (non-stimulating) conditions in inhibitory neurons 10 . However, whether this selective enhancement translates to a selective increase in inhibitory neuronal activity in vivo is unknown.…”