Elsevier

Brain Research

Volume 1727, 15 January 2020, 146571
Brain Research

Research report
NMDA receptors containing GluN2C and GluN2D subunits have opposing roles in modulating neuronal oscillations; potential mechanism for bidirectional feedback

https://doi.org/10.1016/j.brainres.2019.146571Get rights and content

Highlights

  • NMDA receptor channel blockers induced stronger neuronal oscillations in GluN2C knockout mice.

  • NMDA receptor channel blockers induced weaker neuronal oscillations in GluN2D knockout mice.

  • NVP-AAM077 and Ro 25-6981induced oscillations appear to be unchanged in GluN2C and GluN2D knockouts.

  • GluN2C/D-containing NMDARs may be modulating oscillations through bidirectional feedback.

Abstract

NMDA receptor (NMDAR) antagonists such as ketamine, can reproduce many of the symptoms of schizophrenia. A reliable indicator of NMDAR channel blocker action in vivo is the augmentation of neuronal oscillation power. Since the coordinated and rhythmic activation of neuronal assemblies (oscillations) is necessary for perception, cognition and working memory, their disruption (inappropriate augmentation or inhibition of oscillatory power or inter-regional coherence) both in psychiatric conditions and with NMDAR antagonists may reflect the underlying defects causing schizophrenia symptoms. NMDAR antagonists and knockout (KO) mice were used to evaluate the role of GluN2C and GluN2D NMDAR subunits in generating NMDAR antagonist-induced oscillations. We find that basal oscillatory power was elevated in GluN2C-KO mice, especially in the low gamma frequencies while there was no statistically significant difference in basal oscillations between WT and GluN2D-KO mice. Compared to wildtype (WT) mice, NMDAR channel blockers caused a greater increase in oscillatory power in GluN2C-KO mice and were relatively ineffective in inducing oscillations in GluN2D-KO mice. In contrast, preferential blockade of GluN2A- and GluN2B-containing receptors induced oscillations that did not appear to be changed in either KO animal. We propose a model wherein NMDARs containing GluN2C in astrocytes and GluN2D in interneurons serve to detect local cortical excitatory synaptic activity and provide excitatory and inhibitory feedback, respectively, to local populations of postsynaptic excitatory neurons and thereby bidirectionally modulate oscillatory power.

Introduction

The rhythmic and coordinated activation of groups of neurons results in neuronal oscillations which are thought to be necessary for cognition, perception and working memory (Uhlhaas et al., 2008, Uhlhaas and Singer, 2012). Thus, it may be related to disease symptoms that in patients with schizophrenia, most studies (but not all) have found an increase in basal oscillatory power (Kikuchi et al., 2011, Spencer, 2011) and a decrease in task-related gamma oscillation power (Cho et al., 2006, Spencer et al., 2008). These electrophysiological changes are thought to underlie the reduced functional connectivity between brain regions observed in patients with schizophrenia using functional magnetic resonance imaging (fMRI) (Uhlhaas and Singer, 2015). Of note, there are also alterations in the task-related synchrony of oscillations between regions which may additionally contribute to symptoms (Block et al., 2007, Spencer et al., 2003, Spencer et al., 2009, Uhlhaas and Singer, 2015).

The ability of the NMDAR antagonists to mimic the spectrum of symptoms seen in schizophrenia (SZ) initiated the NMDAR hypofunction hypothesis of schizophrenia. This hypothesis is now supported by a variety of pharmacological and genetic studies in both humans and rodents, for reviews see (Kantrowitz and Javitt, 2010, Lisman et al., 2008). Consistent with this hypothesis, NMDAR channel blockers, or genetic deletion, has been associated with a robust augmentation of local neuronal oscillations and a deficit in interregional coherence (Dzirasa et al., 2009, Hakami et al., 2009, Korotkova et al., 2010, Pinault, 2008). Such augmentation of oscillations may be due to the preferential inhibition by NMDAR channel blockers of NMDARs on the fast-spiking parvalbumin-containing interneurons (Homayoun and Moghaddam, 2007). Their reduced excitation results in reduced GABAergic inhibition of pyramidal cells and thus increased excitatory activity. In short, NMDAR blockade may be disrupting cognition and perception by decreasing the signal to noise ratio of gamma oscillations, and/or altering oscillatory coherence between brain regions. Presently, it is not clear how different NMDAR subtypes participate in the oscillatory generators.

NMDAR complexes are composed of subunits from seven genes - GluN1, GluN2A-GluN2D, and GluN3A-GluN3B (Masu et al., 1993, Mishina et al., 1993, Monyer et al., 1994, Traynelis et al., 2010). These subunits assemble into hetero-tetrameric complexes in various combinations resulting in functionally distinct NMDARs. Many NMDARs are thought to be composed of two GluN1 subunits and two GluN2 subunits. The different alternatively-spliced GluN1 isoforms have largely similar pharmacological and physiological properties whereas the GluN2 subunits confer distinct physiological, biochemical, and pharmacological properties to the NMDAR complex (Buller et al., 1994, Hollmann et al., 1993, Ikeda et al., 1992, Monyer et al., 1994, Sugihara et al., 1992, Vicini et al., 1998). For example, NMDAR containing GluN2C or GluN2D subunits display a reduced voltage-dependency due to a weaker Mg++ block, do not desensitize, have relatively high affinity for L-glutamate, and have slow-decaying current responses (Glasgow et al., 2015, Paoletti et al., 2013). These properties, combined with their varied developmental profiles and anatomical distributions (Alsaad et al., 2019, Watanabe et al., 1992, Watanabe et al., 1993a), imply that NMDARs containing these subunits may have a function in the CNS that is markedly distinct from the major GluN2A- and GluN2B-containing receptors.

In recent studies, we have found that the potent non-selective NMDAR antagonist MK-801 augmented gamma oscillations in GluN2C-KO mice more than in wildtype mice (Gupta et al., 2016). In contrast, in the GluN2D-KO mouse, ketamine-induced oscillations were greatly diminished (Sapkota et al., 2016). Since ketamine and MK-801 each have additional off-target activities (Briggs and McKenna, 1996, Clarke and Reuben, 1995, Sleigh et al., 2014), we sought in the present study to directly compare multiple NMDAR antagonists in both GluN2C-KO and GluN2D-KO mice to determine if the NMDAR antagonists effects on GluN2C-KO and GluN2D-KO mice are a general feature of NMDAR antagonists. Furthermore, since memantine has a potency, selectivity, and mechanism of action very similar to ketamine, but has less association with psychotomimetic effects, we were also interested to determine if memantine displays different effects on neuronal oscillations than other general NMDAR antagonists.

We find that NMDAR channel blockers, in general, have a larger effect on neuronal oscillations in the GluN2C-KO mouse than in wildtype (WT) mice and have very little effect in GluN2D-KO mice. In contrast to the results with channel blockers, preferential blockade of GluN2A- and GluN2B-containing receptors induced oscillations that did not appear to be changed in either KO animal.

Section snippets

Baseline power in GluN2C-KO and GluN2D-KO mice

Electroencephalographic (ECoG) recordings of awake WT, GluN2C-KO and GluN2D-KO mice displayed typical ECoG traces before and after drug administration. Relative to WT mice, GluN2C-KO mice, displayed higher average basal oscillatory power at all frequencies analyzed (30 – 200 Hz, p < 0.001, Fig. 1A; see Section 4.5 for statistical analysis). A plot of the GluN2C/WT power ratio at different frequencies (Fig. 1B) suggests that basal power was especially elevated within the beta and low gamma range

NMDAR channel blockers induce stronger oscillations in GluN2C-KO mice and weaker oscillations in GluN2D-KO mice

The synchronous, rhythmic activation of neuronal assemblies is necessary for perception, cognition, and working memory, and their disruption may reflect the underlying defects causing symptoms in schizophrenia and other neuropsychiatric disorders (Haenschel et al., 2009, Spencer et al., 2003, Uhlhaas et al., 2008, Uhlhaas and Singer, 2012). Blockade of NMDA receptor activity, or their genetic deletion, strongly modulates neuronal oscillations and impairs inter-regional coherence (Hakami et al.,

Surgery

All procedures were approved by the University of Nebraska Medical Center’s Institutional Animal Care and Use Committee (IACUC) in compliance with the National Institutes of Health guidelines. 12–16 weeks old WT, GluN2C-KO and GluN2D-KO mice were surgically implanted with tripolar electrodes (MS333/2; Plastics One, Roanoke, VA) under xylazine/ketamine/ acepromazine anesthesia as required by IACUC regulations. Two holes were made in the skull 3 mm posterior to bregma at 1 mm and 2.5 mm lateral.

CRediT authorship contribution statement

Zhihao Mao: Conceptualization, Investigation, Methodology, Data curation, Formal analysis, Visualization, Writing - original draft. Shengxi He: Investigation. Christopher Mesnard: Investigation. Paul Synowicki: Methodology, Investigation. Yuning Zhang: Investigation. Lucy Chung: Investigation. Alex I. Wiesman: Formal analysis, Writing - review & editing. Tony W. Wilson: Formal analysis, Writing - review & editing. Daniel T. Monaghan: Conceptualization, Methodology, Visualization, Writing -

Acknowledgements

This work was supported by the National Institutes of Mental Health [grants MH60252 and GM110768]. The authors thank Dr. Andres Buonanno and colleagues for having provided the GluN2C-KO mouse line and Dr. Masayoshi Mishina and colleagues for the GluN2D-KO mouse. We also thank Robin Taylor for expert graphical assistance.

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