Postsynaptic localization of ?-aminobutyric acid transporters and receptors in the outer plexiform layer of the goldfish retina: An ultrastructural study

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Abstract
The γ‐aminobutyric acid (GABA)‐ergic system in the outer plexiform layer (OPL) of the goldfish retina was studied via light and electron immunohistochemistry. The subcellular distributions of immunoreactivity (‐IR) of plasma membrane GABA transporters GAT2 and GAT3, the α1 and α3 subunits of the ionotropic GABAA receptor, and the ρ1 subunit of the ionotropic GABAC receptor were determined. The localization of the GAT2‐IR and GAT3‐IR to horizontal cell dendrites at the base of the cone synaptic complex was the main characteristic at the ultrastructural level. Very rarely, GAT2‐IR and GAT3‐IR were found in horizontal cell dendrites innervating rod spherules. α1‐IR and α3‐IR were seen in wide bands in the OPL, whereas ρ1‐IR appeared as a narrow band in the OPL. Most α1‐IR was intracellular in rod and cone terminals. Membrane‐associated α1‐IR was observed in cone pedicles but not in rod spherules; postsynaptic elements were also labeled. α3‐IR was concentrated in the lateral elements of horizontal cell dendrites in cone pedicles. In contrast, ρ1‐IR was found mainly on the spinules of the horizontal cell dendrites in cone pedicles. In addition, in another type of cone pedicle, ρ1‐IR was found at the position of OFF‐bipolar cell dendrites. α3‐IR and ρ1‐IR were rarely found in horizontal cell dendrites innervating rods. We suggest that two GABAergic pathways exist in the outer retina— first, a GABAergic positive loop with GABA receptors mainly on the horizontal cell dendrites and spinules and, second, a GABAergic feedback pathway involving GABA receptors on cone pedicles and GABA transporters on horizontal cells and that this pathway presumably modulates feedback strength from horizontal cells to cones. J. Comp. Neurol. 474:58–74, 2004.