Modulation of Burst Firing of Neurons in Nucleus Reticularis of the Thalamus by GluN2C-Containing NMDA Receptors

Abstract

The GluN2C subunit of the NMDA receptor is enriched in the neurons in the nucleus reticularis of the thalamus (nRT), but its role in regulating their function is not well understood. We found that deletion of GluN2C subunit did not affect spike frequency in response to depolarizing current injection or hyperpolarizationinduced rebound burst firing of nRT neurons. D-Cycloserine or CIQ (GluN2C/GluN2D positive allosteric modulator) did not affect the depolarization-induced spike frequency in nRT neurons. A newly identified highly potent and efficacious coagonist of GluN1/GluN2C NMDA receptors, AICP ((R)-2-amino-3-(4-(2-ethylphenyl)-1H-indole-2-carboxamido)propanoic acid), was found to reduce the spike frequency and burst firing of nRT neurons in wild type but not GluN2C knockout. This effect was potentially due to facilitation of GluN2C-containing receptors, because inhibition of NMDA receptors by AP5 did not affect spike frequency in nRT neurons. We evaluated the effect of intracerebroventricular injection of AICP. AICP did not affect basal locomotion or prepulse inhibition but facilitated MK-801induced hyperlocomotion. This effect was observed in wildtype but not in GluN2C knockout mice, demonstrating that AICP produces GluN2C-selective effects in vivo. Using a chemogenetic approach, we examined the role of nRT in this behavioral effect. G q-or G i-coupled DREADDs were selectively expressed in nRT neurons using Cre-dependent viral vectors and PV-Cre mouse line. We found that similar to AICP effect, activation of G q-but not G i-coupled DREADD facilitated MK-801-induced hyperlocomotion. Together, these results identify a unique role of GluN2C-containing receptors in the regulation of nRT neurons and suggest GluN2C-selective in vivo targeting of NMDA receptors by AICP.

Facilitation of GluN2C-containing NMDA receptors in the external globus pallidus increases firing of fast spiking neurons and improves motor function in a hemiparkinsonian mouse model

“…6F – I ). In addition, we have previously shown that intracerebroventricular injection of AICP does not affect basal locomotor activity in normal mice ( Liu et al, 2019 ). Together, electrophysiology and behavioral results demonstrate that DCS and AICP improves motor function in a mouse model of PD by facilitating the function of GluN2C-containing NMDA receptors in the GPe.…”

“…In contrast, facilitation of GluN2C-containing receptors increases the firing of GPe fast spiking neurons. We have recently shown that in the nRT neurons, GluN2C ablation similarly does not affect basal excitability and burst firing, but facilitation of GluN2C-containing receptors by AICP modulates these properties ( Liu et al, 2019 ). Thus, it appears that increasing ongoing activity of GluN2C-containing receptors can modulate neuronal function.…”

“…For electrophysiology and biochemistry studies wildtype or Grin2C tm1(EGFP/cre/ERT2)Wtsi mouse line on pure C57BL/6 N background were used ( Ravikrishnan et al, 2018 ; Shelkar et al, 2019 ; Liu et al, 2019 ). Conditional deletion of GluN2D from PV neurons was achieved by crossing PV-Cre mouse line (Jax stock number 017320) with GluN2D flox/flox mice.…”

“…Immunohistochemistry was performed as previously described ( Ravikrishnan et al, 2018 ; Liu et al, 2019 ). Briefly, mice were transcardially perfused, brains were collected and cryoprotected and stored at − 80 °C.…”

Synaptic and intrinsic mechanisms impair reticular thalamus and thalamocortical neuron function in a Dravet syndrome mouse model

“…For example, a recent study indicated that the GluN2A positive allosteric modulator GNE-0723 reduces low-frequency oscillations and epileptiform activity in a DS mouse model, providing evidence that NMDA receptor modulation could be a viable therapeutic option (Hanson et al, 2020). The GluN2C and GluN2D subunits of NMDA receptors have more restricted expression patterns, including relatively high expression in the thalamus, and recently developed GluN2C/2D-selective modulators could offer a means to tune thalamic function with more limited adverse effects (Acker et al, 2011; Khatri et al, 2014; Liu et al, 2019; Mullasseril et al, 2010; Swanger et al, 2018; Yi et al, 2020). Thus, the data presented here lay the foundation to explore glutamatergic synapse modulation as a possible therapeutic strategy to correct thalamic dysfunction in DS.…”