In light of the receptor density to size relationship demonstrated in hippocampal synapses, we propose that glutamate receptor subunits can be classified into three groups based on their expression patterns as regards to PSD area (Figure ) – Group 1 the receptor density is inversely related to the PSD area (e.g., NR2B). Group 2 the receptor density is constant and hence the receptor expression can be estimated by a linear function of PSD area; Group 3 the receptor density increases with the PSD area (e.g., GluR1).
Figure 3 Schematic drawings illustrating different ways in which receptor density (ρ) varies with postsynaptic spine area (A). In Group 1, receptor expression remains constant regardless of PSD area. In Group 2, receptor expression grows linearly with (more ...)
Among the three major NMDA receptor subunits, NR2B is the only subunit that belonged to Group 1. This view is in agreement with a previous study that showed NR2B subunits are enriched in smaller synapses (Sobczyk et al., 2005
). Interestingly, metabotropic glutamate receptor 5 (mGluR5), the major postsynaptic mGluR in CA3–CA1 synapses, shows a similar laterality-dependent expression pattern as NR2B (Figure ) and hence is classified as Group
1. Four major subunits – GluR2, GluR3, NR1, and NR2A are categorized as members of Group 2. As the number of these subunits is linearly proportional to the PSD area, increase of the receptor subunits is a passive reflection of the volume change of the synapses. As far as we investigated, GluR1 is the only glutamate receptor subunit that shows a supralinear relationship to PSD area in the CA1 area. As the PSD area is linearly related to the spine volume (Harris and Stevens, 1989
), the grouping holds valid for receptor expression vs. spine volume classification.
Figure 4 mGluR5 shows a similar left–right distribution to NR2B in ventral hippocampal commissure transected mice. Immunoreactive bands were observed at approximately 130 and 260kD, which represent monomeric and dimeric forms of mGluR5, respectively. (more ...)
As relatively constant amounts of Group 1 molecules are expressed in a synapse regardless of the PSD area, it is tempting to regard them as “unbiased-sensor” glutamate receptors for synaptic plasticity. Indeed, NR2B expression declines only mildly during the developmental process (Liu et al., 2004
; Nase et al., 1999
), and the profound increase in the NR2A/NR2B ratio during development is mainly attributed to upregulation of NR2A subunits (Liu et al., 2004
; Nase et al., 1999
; Yashiro and Philpot, 2008
). Therefore, in young animals where plastic changes of synapses are constantly taking place, activation of NR2B is thought to be critical to allow calcium influx into synapses in order to modify their efficacy. Likewise, activation of mGluR5 is essential in some forms of LTP (Lu et al., 1997
; Rodrigues et al., 2002
). Interestingly, both NR2B and mGluR5 are also known to be extrasynaptic receptor subunits (Lujan et al., 1996
; Scimemi et al., 2004
; Stocca and Vicini, 1998
; Tovar and Westbrook, 1999
As NR1 is an essential subunit of NMDAR, the question arises as to why NR1 and NR2B belong to different groups in our analysis. A parsimonious explanation is that the majority of NR2 subunits are NR2A in mature animals (Sans et al., 2000
; Stocca and Vicini, 1998
) and therefore the relationship between NR1 expression and synapse area is largely reflected by NR2A behavior.
What makes GluR1 special over other receptor subunits? In late-phase LTP, AMPARs increase in number while NMDARs elicit little change (Malinow and Malenka, 2002
). Electron microscopic studies also demonstrate that NMDAR immunoreactivity at a spine correlates little with the synapse area (Racca et al., 2000
; Takumi et al., 1999
). At the same time, LTP-inducing stimulations result in a morphological enlargement of the spine (Matsuzaki et al., 2004
). Because of the supralinear receptor expression vs. synapse area relationship, GluR1 is likely to be actively involved in the expression of synaptic potentiation, in clear contrast to the subunits that belong to Group 2.
Overwhelming increase of GluR1 over GluR2 in large spines essentially results in an increase of non-GluR2-containing AMPARs, in particular homotetrameric GluR1 receptors (Wenthold et al., 1996
). As the presence of GluR2 in AMPAR determines Ca2+
impermeability, AMPAR mediated Ca2+
influx should be present in large and GluR1-dense synapses. Indeed, a recent study demonstrated transient incorporation of GluR2-lacking AMPAR into stimulated synapses during LTP as well as the existence of pools of GluR2-lacking AMPARs in hippocampal slice culture and acute slice, as well as in the cerebral cortex in vivo
(Bagal et al., 2005
; Clem and Barth, 2006
; Plant et al., 2006
). Although the Ca2+
permeability and functionality of these GluR2-lacking AMPA receptors remain controversial (Adesnik and Nicoll, 2007
), a close look at electron micrographs of SDS-digested synaptic membrane surface for GluR1 shows that multiple 5-nm immunogold particles are often found around a single intramembrane particle cluster. Perhaps the densely distributed immunogold particles observed in CA1 stratum radiatum synapses are indicative of GluR1 homomeric receptors (Figure , for an example). It is worth noting, however, that such large synapses represent only a small fraction of hippocampal CA1 synapses. In large spines, GluR1/GluR1 homotetramer AMPARs potentially serve as a source of Ca2+
entry, in addition to NMDR and voltage gated calcium channels, to keep the synapse potentiated. In smaller excitatory synapses, stimulation by ligands leads to a large elevation of Ca2+
concentration via relatively dense NR2B-containing NMDARs.
Figure 5 Electronmicrographs showing SDS-FRL labeling of the GluR1 and NR1 subunits of a small synapse (A) and a large synapse (B) in CA1stratum radiatum. PSD areas are demarcated by dotted lines. GluR1 is labeled by 5nm immunogold particles (more ...)