Target-Cell-Specific Left-Right Asymmetry of NMDA Receptor Content in Schaffer Collateral Synapses in ϵ1/NR2A Knock-Out Mice

Abstract

Input-dependent left-right asymmetry of NMDA receptor ⑀2 (NR2B) subunit allocation was discovered in hippocampal Schaffer collateral (Sch) and commissural fiber pyramidal cell synapses (Kawakami et al., 2003). To investigate whether this asymmetrical ⑀2 allocation is also related to the types of the postsynaptic cells, we compared postembedding immunogold labeling for ⑀2 in left and right Sch synapses on pyramidal cells and interneurons. To facilitate the detection of ⑀2 density difference, we used ⑀1 (NR2A) knock-out (KO) mice, which have a simplified NMDA receptor subunit composition.The labeling density for ⑀2 but not 1 (NR1) and subtype 2/3 glutamate receptor (GluR2/3) in Sch-CA1 pyramidal cell synapses was significantly different between the left and right hippocampus with opposite directions in strata oriens and radiatum; the left to right ratio of ⑀2 labeling density was 1:1.50 in stratum oriens and 1.44:1 in stratum radiatum. No significant difference, however, was detected in CA1 stratum radiatum between the left and right Sch-GluR4-positive (mostly parvalbumin-positive) and Sch-GluR4-negative interneuron synapses. Consistent with the anatomical asymmetry, the amplitude ratio of NMDA EPSCs to non-NMDA EPSCs in pyramidal cells was approximately two times larger in right than left stratum radiatum and vice versa in stratum oriens of ⑀1 KO mice. Moreover, the amplitude of long-term potentiation in the Sch-CA1 synapses of left stratum radiatum was significantly larger than that in the right corresponding synapses. These results indicate that the asymmetry of ⑀2 distribution is target cell specific, resulting in the left-right difference in NMDA receptor content and plasticity in Sch-CA1 pyramidal cell synapses in ⑀1 KO mice.

Left-right asymmetry of the hippocampal synapses with differential subunit allocation of glutamate receptors

“…2A). In VHCT mice, CA1 pyramidal cells would have exclusively ipsilateral projections from CA3 5 days after surgery (3,13), as the VHC conveyed commissural fibers. Because the great majority of glutamatergic synapses in the SR is found on pyramidal cell spines, contamination from synapses on interneurons is virtually negligible.…”

“…Note that NR2B belongs to this group. Because there is no significant difference in the total number of synapses or in the number of Schaffer collateral synapses between the left and right CA1 areas (13) (Fig. S6), net expression of NR2B is actually symmetrical between the left and right hemispheres, as well as between the GluR1 dense synapses and the NR2B dense synapses.…”

NMDA Receptor GluN2B (GluRε2/NR2B) Subunit Is Crucial for Channel Function, Postsynaptic Macromolecular Organization, and Actin Cytoskeleton at Hippocampal CA3 Synapses

“…Although the reason for this discrepancy is uncertain, the two studies differ in that Fukushima et al (2009) analyzed basal dendrites, whereas we analyzed apical dendrites. Considering that GluN2B subunit is differentially allocated between apical and basal dendrites of single pyramidal cells (Kawakami et al, 2003;Wu et al, 2005;Shinohara et al, 2008), the role of GluN2B subunit for dendritic spine formation or maintenance could be different according to its asymmetrical allocation.…”

“…Although the two subunits confer distinct properties in pharmacological profiles, synaptic plasticity, receptor trafficking, and left-right asymmetrical synaptic delivery (Ito et al, 1997;Quinlan et al, 1999Quinlan et al, , 2004Barria and Malinow, 2002;Kawakami et al, 2003;L. Liu et al, 2004;Wu et al, 2005;Shinohara et al, 2008), it remains unknown how they play roles in synaptic expression and function of NMDARs in the brain, mainly because of the lack of viable GluN2B-KO animal models.…”

Glutamate and GABA receptors and transporters in the basal ganglia: What does their subsynaptic localization reveal about their function?

“…The freeze-substitution technique, nevertheless, has been instrumental for the localization of ionotropic GABA and glutamate receptors in various brain regions, including the basal ganglia (e.g. Nusser et al, 1994;Baude et al, 1995;Nusser et al, 1995a;Matsubara et al, 1996;Bernard et al, 1997;Ottersen and Landsend, 1997;Fujiyama et al, 2002;Wu et al, 2005).…”