Size and Receptor Density of Glutamatergic Synapses: A Viewpoint from Left-Right Asymmetry of CA3-CA1 Connections

Abstract

Synaptic plasticity is considered to be the main mechanism for learning and memory. Excitatory synapses in the cerebral cortex and hippocampus undergo plastic changes during development and in response to electric stimulation. It is widely accepted that this process is mediated by insertion and elimination of various glutamate receptors. In a series of recent investigations on left-right asymmetry of hippocampal CA3-CA1 synapses, glutamate receptor subunits have been found to have distinctive expression patterns that depend on the postsynaptic density (PSD) area. Particularly notable are the GluR1 AMPA receptor subunit and NR2B NMDA receptor subunit, where receptor density has either a supralinear (GluR1 AMPA) or inverse (NR2B NMDAR) relationship to the PSD area. We review current understanding of structural and physiological synaptic plasticity and propose a scheme to classify receptor subtypes by their expression pattern with respect to PSD area.

Keywords: AMPAR; NMDAR; PSD; glutamate; mGluR5; spines.

Figure 1

Right–left (R/L) ratios of ipsilateral synaptic glutamate receptor proteins in hippocampal CA1 stratum radiatum. (A) Original densitometry R/L ratio (data taken from Kawakami et al., ; Shinohara et al., 2008). Among major ionotropic glutamate receptors, NR2B and GluR1 ratios show significant left–right differences (**p < 0.01). (B) Schematic figure of CA3–CA1 hippocampal projections (left). In investigating only ipsilateral projections, contralaterally projecting axons were excluded from analysis by ventral hippocampal commissure transection. Average PSD areas of ipsilateral CA3–CA1 projections (right). Ipsilateral synapses in left side are generally smaller (**p < 0.01). (C) R/L ratios were scaled by PSD area to estimate the relative receptor amounts per synapse. Most of the subunits showed significant left–right differences (*p < 0.05, **p < 0.01). L and R represent left and right sides, respectively.

Figure 2

Glutamate receptor subunits have distinct expression patterns according to PSD size. Examples of paired SDS-digested freeze-fracture replica labeling (SDS-FRL) for NR2B (E-face) vs. NR2A (P-face) (A), and for GluR1 (E-face) vs. NR2B (P-face) (B) in CA1 stratum radiatum synapses. Upper rows are examples of small synapses, whereas lower rows are that of large synapses. Large black dots present in all E-faces (10 nm gold) in (A, B) are NR1 labeling, which is used for a marker of synapses. Boundaries of intramembrane particles (IMP) shown in dotted lines in E-faces correspond to exposed PSD areas (ePSD). Scale bars: 100 nm. Relationship between the ePSD and density of synaptic NR2B (C), NR2A (D), or GluR1 (E) labeling, as assessed by SDS-FRL method. The data set published in Shinohara et al. (2008) is used in the analysis. Hyperbolic fit matches well for NR2B, whereas positive correlation is seen for GluR1. No significant correlation was observed for NR2A. For detailed descriptions of SDS-FRL, see Masugi-Tokita and Shigemoto (2007).

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 PSD area, and the receptor density is constant. In Group 3, receptor expression grows supralinearly with PSD area, as the receptor density is an incremental function of synapse area.

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 260 kD, which represent monomeric and dimeric forms of mGluR5, respectively. Left side PSD preparation from ipsilateral CA3–CA1 projection contains more mGluR5 than that of right side. Molecular weights are shown in left in kilodalton.

Figure 5

Electronmicrographs showing SDS-FRL labeling of the GluR1 and NR1 subunits of a small synapse (A) and a large synapse (B) in CA1 stratum radiatum. PSD areas are demarcated by dotted lines. GluR1 is labeled by 5 nm immunogold particles and NR1 is labeled by 10 nm immunogold particles. GluR1 is more densely distributed in the large synapse. (C) GluR1 labeling in (B) is labeled by yellow or red dots. Gold particles with inter-particle distance <10 nm is labeled in red. Scale bar: 100 nm.