Tonic Activation of GluN2C/GluN2D-Containing NMDA Receptors by Ambient Glutamate Facilitates Cortical Interneuron Maturation

Abstract

Developing cortical GABAergic interneurons rely on genetic programs, neuronal activity, and environmental cues to construct inhibitory circuits during early postnatal development. Disruption of these events can cause long-term changes in cortical inhibition and may be involved in neurological disorders associated with inhibitory circuit dysfunction. We hypothesized that tonic glutamate signaling in the neonatal cortex contributes to, and is necessary for, the maturation of cortical interneurons. To test this hypothesis, we used mice of both sexes to quantify extracellular glutamate concentrations in the cortex during development, measure ambient glutamate-mediated activation of developing cortical interneurons, and manipulate tonic glutamate signaling using subtype-specific NMDA receptor antagonists in vitro and in vivo. We report that ambient glutamate levels are high (Ϸ100 nM) in the neonatal cortex and decrease (to Ϸ50 nM) during the first weeks of life, coincident with increases in astrocytic glutamate uptake. Consistent with elevated ambient glutamate, putative parvalbumin-positive interneurons in the cortex (identified using G42:GAD1-eGFP reporter mice) exhibit a transient, tonic NMDA current at the end of the first postnatal week. GluN2C/GluN2D-containing NMDA receptors mediate the majority of this current and contribute to the resting membrane potential and intrinsic properties of developing putative parvalbumin interneurons. Pharmacological blockade of GluN2C/GluN2D-containing NMDA receptors in vivo during the period of tonic interneuron activation, but not later, leads to lasting decreases in interneuron morphological complexity and causes deficits in cortical inhibition later in life. These results demonstrate that dynamic ambient glutamate signaling contributes to cortical interneuron maturation via tonic activation of GluN2C/ GluN2D-containing NMDA receptors. Inhibitory GABAergic interneurons make up 20% of cortical neurons and are critical to controlling cortical network activity. Dysfunction of cortical inhibition is associated with multiple neurological disorders, including epilepsy. Establishing inhibitory cortical networks requires in utero proliferation, differentiation, and migration of immature GABAergic interneurons, and subsequent postnatal morphological maturation and circuit integration. Here, we demonstrate that ambient glutamate provides tonic activation of immature, putative parvalbumin-positive GABAergic interneurons in the neonatal cortex via high-affinity NMDA receptors. When this activation is blocked, GABAergic interneuron maturation is disrupted, and cortical networks exhibit lasting abnormal hyperexcitability. We conclude that temporally precise activation of developing cortical interneurons by ambient glutamate is critically important for establishing normal cortical inhibition.

GluN2D-mediated excitatory drive onto medial prefrontal cortical PV+ fast-spiking inhibitory interneurons

“…Our findings demonstrate that both GluN2C/D-mediated potentiated excitatory drive (EPSCs) and enhanced intrinsic excitability act synergistically to increase the output of this neuronal functional class. The addition of CIQ (+) depolarized the resting membrane potential of FSINs, suggesting GluN2C/D-containing receptors are active at rest, as has been reported in interneurons of the developing cortex [37]. This unique feature of GluN2C/D-containing NMDARs can be attributed to basal glutamatergic tone acting with higher potency (~0.5 μM) at GluN2D-containing NMDARs, which are active at rest due to weak Mg 2+ -induced rectification [20,38].…”

GluN2B but Not GluN2A for Basal Dendritic Growth of Cortical Pyramidal Neurons

“…A recent study in mouse cortex has revealed that parvalbuminergic interneurons mature via tonically active GluN2C/2D-containing receptors which transform the depolarizing signaling of ambient glutamate into an increase of dendritic complexity. Interestingly, the action is temporally limited to an early postnatal time window: a pharmacological block of GluN2C/2D receptors between day 7 and 9 yields at day 21 interneurons with less dendritic branch points and reduced inhibitory activity (Hanson et al, 2019).…”

NMDAR-mediated transcriptional control of gene expression during the development of medial ganglionic eminence-derived interneurons

“…Unlike mature interneurons where iGluRs differentially contribute towards synaptic transmission, immature and migrating interneurons express different glutamate receptor subunits including the NMDA-type iGluR (NMDAR) and AMPA/Kainate-type iGluR (AMPAR/KAR) [13–15] prior to the expression of any functional synapses. This becomes particularly important as the developing brain contains higher ambient glutamate levels than the adult brain [16]. Collectively, higher ambient glutamate, developmental expression of iGluRs and recruitment of glutamatergic signaling is considered to be trophic [8, 17, 18] and thought to engage mechanisms to regulate various aspects of interneuron development including morphological and electrical maturation to promote appropriate circuit integration [9, 11, 14, 16, 19–22].…”