Vesicular γ‐aminobutyric acid transporter expression in amacrine and horizontal cells

Abstract

The vesicular gamma-aminobutyric acid (GABA) transporter (VGAT), which transports the inhibitory amino acid transmitters GABA and glycine, is localized to synaptic vesicles in axon terminals. The localization of VGAT immunoreactivity to mouse and rat retina was evaluated with light and electron microscopy by using well-characterized VGAT antibodies. Specific VGAT immunoreactivity was localized to numerous varicose processes in all laminae of the inner plexiform layer (IPL) and to the outer plexiform layer (OPL). Amacrine cell somata characterized by weak VGAT immunoreactivity in the cytoplasm were located in the ganglion cell layer and proximal inner nuclear layer (INL) adjacent to the IPL. In rat retina, VGAT-immunoreactive cell bodies also contained GABA, glycine, or parvalbumin (PV) immunoreactivity, suggesting vesicular uptake of GABA or glycine by these cells. A few varicose VGAT-immunoreactive processes entered the OPL from the IPL. VGAT immunoreactivity in the OPL was predominantly localized to horizontal cell processes. VGAT and calcium binding protein-28K immunoreactivities (CaBP; a marker for horizontal cells) were colocalized in processes and terminals distributed to the OPL. Furthermore, VGAT immunoreactivity overlapped or was immediately adjacent to postsynaptic density-95 (PSD-95) immunoreactivity, which is prominent in photoreceptor terminals. Preembedding immunoelectron microscopy of mouse and rat retinae showed that VGAT immunoreactivity was localized to horizontal cell processes and their terminals. Immunoreactivity was distributed throughout the cytoplasm of the horizontal cell processes. Taken together, these findings demonstrate VGAT immunoreactivity in both amacrine and horizontal cell processes, suggesting these cells contain vesicles that accumulate GABA and glycine, possibly for vesicular release.

Cellular distribution and subcellular localization of molecular components of vesicular transmitter release in horizontal cells of rabbit retina

“…VGAT is strongly expressed in mammalian horizontal cells Cueva et al, 2002;Jellali et al, 2002), consistent with earlier findings of GAD expression in horizontal cells and a transmitter role for GABA. The highest level of VGAT immunostaining is in horizontal cell processes underneath the photoreceptor terminals, and in the dendritic and axonal endings within the synaptic triad Cueva et al, 2002;Jellali et al, 2002).…”

“…VGAT is strongly expressed in mammalian horizontal cells Cueva et al, 2002;Jellali et al, 2002), consistent with earlier findings of GAD expression in horizontal cells and a transmitter role for GABA. The highest level of VGAT immunostaining is in horizontal cell processes underneath the photoreceptor terminals, and in the dendritic and axonal endings within the synaptic triad Cueva et al, 2002;Jellali et al, 2002). Furthermore, the presence of small, clear-core vesicles in the horizontal cell processes and endings at photoreceptor synapses (Dowling et al, 1966;Fisher and Boycott, 1974;Kolb, 1977;Brandstä tter et al, 1996) indicates that the morphological substrate for vesicular release is also present in these cell processes.…”

“…Indeed, the highest level of VGAT immunoreactivity occurs in horizontal cell processes underneath the photoreceptor terminals and in the dendritic and axonal endings within the synaptic triad Cueva et al, 2002). The localization of synaptic proteins involved in regulated transmitter release, such as complexin I/II, syntaxin-1, and synapsin I, to rabbit horizontal cells provides further molecular support for a vesicular release mechanism.…”

“…The supposition that rabbit horizontal cells use a vesicular mode of transmitter release was prompted by the immunolocalization of VGAT, a synaptic vesicle protein necessary for loading inhibitory transmitter into synaptic vesicles (McIntire et al, 1997) to these cells Cueva et al, 2002;Jellali et al, 2002). Indeed, the highest level of VGAT immunoreactivity occurs in horizontal cell processes underneath the photoreceptor terminals and in the dendritic and axonal endings within the synaptic triad Cueva et al, 2002).…”

Neurotransmission plays contrasting roles in the maturation of inhibitory synapses on axons and dendrites of retinal bipolar cells

“…VIAAT is expressed by both mouse horizontal cells and amacrine cells (21). To assess the importance of GABA and glycine release on the formation and maintenance of inhibitory synapses on BCs, we perturbed vesicular release of inhibitory neurotransmitters specifically in the retina by abolishing VIAAT expression using αPax6-Cre (22) mice (VIAAT KO).…”