Diversity of glycine receptors in the mouse retina: Localization of the α2 subunit

Abstract

Gamma-aminobutyric acid (GABA) and glycine are the major inhibitory neurotransmitters in the retina, glycine being produced in approximately half of all amacrine cells. Whereas retinal cell types expressing the glycine receptor (GlyR) alpha1 and alpha3 subunits have been mapped, the role of the alpha2 subunit in retinal circuitry remains unclear. By using immunocytochemistry, we localized the alpha2 subunit in the inner plexiform layer (IPL) in brightly fluorescent puncta, which represent postsynaptically clustered GlyRs. This was shown by doubly labeling sections for GlyR alpha2 and bassoon (a presynaptic marker) or gephyrin (a postsynaptic marker). Synapses containing GlyR alpha2 were rarely found on ganglion cell dendrites but were observed on bipolar cell axon terminals and on amacrine cell processes. Recently, an amacrine cell type has been described that is immunopositive for glycine and for the vesicular glutamate transporter vGluT3. The processes of this cell type were presynaptic to GlyR alpha2 puncta, suggesting that vGluT3 amacrine cells release glycine. Double labeling of sections for GlyR alpha1 and GlyR alpha2 subunits showed that they are clustered at different synapses. In sections doubly labeled for GlyR alpha2 and GlyR alpha3, approximately one-third of the puncta were colocalized. The most abundant GlyR subtype in retina contains alpha3 subunits, followed by those containing GlyR alpha2 and GlyR alpha1 subunits.

Glycinergic transmission in the mammalian retina

“…In the inner IPL (stratum S3–S5) smaller GlyR α1 immunoreactive puncta can be observed representing synapses onto ganglion cell dendrites and rod bipolar cell axons (Ivanova et al, 2006; Majumdar et al, 2007). The GlyR α2 subunit is more uniformly distributed across stratum 1–4 (Figure 3D), and GlyR α2 immunoreactive puncta occur at the highest density amongst the four α subunits (Haverkamp et al, 2004). The GlyR α3 subunit (Figure 3E) shows four bands of higher density of puncta (Haverkamp et al, 2003).…”

“…In the adult two copies of the α subunit and three copies of the β subunit form the pentameric receptor protein (Grudzinska et al, 2005). Subunit selective antibodies have recently become available that recognize the four GlyR α subtypes (Harvey et al, 2004; Haverkamp et al, 2003, 2004; Heinze et al, 2007). When these antibodies were applied to lightly fixed sections of the mammalian retina, they each produce a distinct punctate immunofluorescence pattern (Figure 3).…”

“…However, when retinal sections were double labelled for the GlyR α1 subunit and the other three GlyR α subunits, no statistically significant coincidence rate of immunoreactive puncta was observed (Figures 4A–C). When retinal sections were double labelled for the GlyR α2 and α3 subunits a coincidence rate of 26.7% was found (Haverkamp et al, 2004). In retinal sections double labelled for the GlyR α3 and α4 subunits no significant coincidence rate was found (Heinze et al, 2007).…”

“…All four α subunits of the GlyR have been localized to specific synapses within the mammalian retina (Haverkamp et al, 2003, 2004; Heinze et al, 2007; Sassoè-Pognetto et al, 1994). To date, selective agonists or antagonists that distinguish different isoforms of synaptic GlyRs have not been identified (Betz and Laube, 2006; Harvey and Betz, 2000; Legendre, 2001; Lynch, 2004).…”

Glycine receptor mutants of the mouse: what are possible routes of inhibitory compensation?

“…Within the variety of glycinergic amacrine cells, all α-subunits have been identified in specific synapses (Sassoe-Pognetto et al, 1994; Haverkamp et al, 2003, 2004; Heinze et al, 2007). Immunoreactivity of the GlyRα2 subunit was found to be evenly distributed across the inner plexiform layer representing the most frequent type of GlyRs in this area (Haverkamp et al, 2004). …”