Transmitter release from mammalian horizontal cells

Horizontal cells mediate lateral inhibition in the outer retina participating the formation of the center-surround antagonism of the bipolar cells and the ganglion cells. The mechanism of the transmitter release from mammalian horizontal cells has remained unclear. Gamma-aminobutyric acid (GABA) is likely expressed in horizontal cells of all species, although conflicting physiological findings has led to considerable controversy regarding its role as a transmitter in the outer retina.;This study has evaluated key components of the GABA system in the outer retina of mouse and guinea pig. The presence of GABA, its rate-limiting synthetic enzyme, glutamic acid decarboxylase (GAD65 and GAD67 isoforms), the plasma membrane GABA transporters (GAT-1 and GAT-3) and the vesicular GABA transporter (VGAT) were evaluated using immunohistochemistry. The presence of GAD65 and GAD67 mRNAs was also evaluated using laser capture microdissection and RT-PCR or degenerate RTPCR.;Specific GARA, GAD65 and VGAT immunostaining was localized to horizontal cell bodies, their processes and tips in the outer plexiform layer of the adult guinea pig retina. In the mouse retina, however, strong GARA and GAD67 immunoreactivities were observed in horizontal cell bodies, processes and tips at around postnatal day 5 (P5). GABA immunoreactivity was weakly detected in the adult mouse horizontal cells, while GAD67 immunoreactivity was no longer detectable in these cells. The mRNA of GAD67, but not GAD65, was detected in the adult mouse horizontal cells, In contrast, GAT-1 or GAT-3 immunoreactivity was not detected in horizontal cells, but their immunoreactivities were distributed to Muller cell processes in the outer retina.;In addition, a retinal neuronal culture system was developed using a sandwich culture method using guinea pig retinas. Activity dependent dye loading experiments indicated vesicle exo-/endocytosis activities in the processes of the cultured horizontal cells.;These studies demonstrate the presence of GARA, GAD and VGAT in mammalian horizontal cells, provided preliminary evidence for vesicle recycle at the horizontal cell processes, and support the idea that GABA is synthesized from GAD, taken up into synaptic vesicles by VGAT, and likely released by a vesicular mechanism from horizontal cells.