Modulation of NMDA Receptor Properties and Synaptic Transmission by the NR3A Subunit in Mouse Hippocampal and Cerebrocortical Neurons

Abstract

Expression of the NR3A subunit with NR1/NR2 in Xenopus oocytes or mammalian cell lines leads to a reduction in N-methyl-d-aspartate (NMDA)-induced currents and decreased Mg(2+) sensitivity and Ca(2+) permeability compared with NR1/NR2 receptors. Consistent with these findings, neurons from NR3A knockout (KO) mice exhibit enhanced NMDA-induced currents. Recombinant NR3A can also form excitatory glycine receptors with NR1 in the absence of NR2. However, the effects of NR3A on channel properties in neurons and synaptic transmission have not been fully elucidated. To study physiological roles of NR3A subunits, we generated NR3A transgenic (Tg) mice. Cultured NR3A Tg neurons exhibited two populations of NMDA receptor (NMDAR) channels, reduced Mg(2+) sensitivity, and decreased Ca(2+) permeability in response to NMDA/glycine, but glycine alone did not elicit excitatory currents. In addition, NMDAR-mediated excitatory postsynaptic currents (EPSCs) in NR3A Tg hippocampal slices showed reduced Mg(2+) sensitivity, consistent with the notion that NR3A subunits incorporated into synaptic NMDARs. To study the function of endogenous NR3A subunits, we compared NMDAR-mediated EPSCs in NR3A KO and WT control mice. In NR3A KO mice, the ratio of the amplitudes of the NMDAR-mediated component to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated component of the EPSC was significantly larger than that seen in WT littermates. This result suggests that NR3A subunits contributed to the NMDAR-mediated component of the EPSC in WT mice. Taken together, these results show that NR3A subunits contribute to NMDAR responses from both synaptic and extrasynaptic receptors, likely composed of NR1, NR2, and NR3 subunits.

Supralinear potentiation of NR1/NR3A excitatory glycine receptors by Zn 2+ and NR1 antagonist

“…The in vivo functions of NR1/NR3 receptors are still enigmatic, because, so far, only limited evidence for native glycine-activated NR1/NR3 receptors has been reported (12, see also ref. 10). The results presented here indicate that for efficient channel activation, at least two ligands have to bind to NR1/NR3A receptors.…”

“…Coexpression of the glycinebinding NR3A or NR3B subunit with NR1 and NR2 subunits generates heteromeric receptors composed of NR1, NR2, and NR3 subunits, which also require glutamate and glycine for gating but exhibit altered biophysical properties (7)(8)(9)(10). In contrast, recombinant NMDA receptors assembled from two NR1 and two NR3 subunits (11) do not respond to glutamate but are exclusively activated by glycine (''excitatory glycine receptors''; ref.…”

New Insights into the Not-So-New NR3 Subunits of N-Methyl-d-aspartate Receptor: Localization, Structure, and Function

“…In HEK293 cells, however, the NR1/ NR3A or NR1/NR3B receptor combination does not express well on the surface. A combination of NR1/NR3A/NR3B instead was found to be necessary in HEK293 cells to form functional glycine-activated receptors (Pina-Crespo and Heinemann, 2004;Smothers and Woodward, 2007 Ká radóttir et al, 2005;Tong et al, 2008). Apart from these studies that have been conducted under heterologous conditions, others have embarked on studies in attempts to characterize these different types of NR3-containing receptors in native neurons.…”

Stoichiometry ofN-Methyl-d-Aspartate Receptors Within the Suprachiasmatic Nucleus

“…Due to the lack of nonspecific ligands and restricted developmental expression, the functional presence of these channels has been difficult to demonstrate in native systems. Although there is some evidence to support glycineactivated NMDAR in neurons (Chatterton et al 2002), recent advances show that when NR3A expression is induced with tetracycline in transgenic neurons, glycine-activated currents were not recorded (Tong et al 2008). Rather NR3 subunits appear to form triheteromers with NR1/NR2 subunits that retain NMDA and glycine affinity but decreased [Mg] (Tong et al 2008).…”

“…Although there is some evidence to support glycineactivated NMDAR in neurons (Chatterton et al 2002), recent advances show that when NR3A expression is induced with tetracycline in transgenic neurons, glycine-activated currents were not recorded (Tong et al 2008). Rather NR3 subunits appear to form triheteromers with NR1/NR2 subunits that retain NMDA and glycine affinity but decreased [Mg] (Tong et al 2008). Therefore although NR3 can form functional diheteromeric channels in heterologous expression systems, the triheteromeric NR3 channels appear to be the predominant form in native neurons.…”