Vesicular inhibitory amino acid transporter
dendrite
Cellular localization of the vesicular inhibitory amino acid transporter in the mouse and human retina.
Horizontal cells are classically thought to mediate lateral inhibition by gamma-aminobutyric acid (GABA)-transporter mediated release. In the mammalian retina, however, GABA uptake and cloned GABA transporter were not detected in horizontal cells. Furthermore, the vesicular inhibitory amino acid transporter (VIAAT or VGAT) that loads GABA and glycine into synaptic vesicles was reported recently to be expressed in horizontal cells. To further assess synaptic transmission in mammalian horizontal cells, we examined the subcellular distribution of VIAAT in mouse and human retina by confocal microscopy with specific cell markers. VIAAT was observed in the mouse outer plexiform layer as punctate structures that localized in calbindin-positive horizontal cells. These structures were in close apposition with synaptophysin-, PSD-95-, dystrophin-, and bassoon-immunopositive photoreceptor terminals, suggesting that VIAAT is localized in horizontal cell tips at photoreceptor terminals. VIAAT-positive puncta were also in apposition to lectin-labeled cone terminals or dendrites of PKCalpha-immunopositive rod bipolar cells, indicating that VIAAT is expressed in horizontal cell tips at both rod and cone terminals. By contrast, only a very few puncta were observed in the human outer plexiform layer, whereas the inner plexiform layer remained labeled as in the mouse retina. When using adult human retinal cells in culture, horizontal cells identified by parvalbumin immunostaining were found to contain VIAAT, either at their terminals or throughout the entire cell similarly as in syntaxin-immunopositive cells. These differences between human retinal tissue and cultured cells were attributed to VIAAT degradation in postmortem retinal tissue. VIAAT localization in mouse and human horizontal cells further support the role of inhibitory transmitters in lateral inhibition at the photoreceptor terminals.