Localization of putative GABAergic neurons in the larval tiger salamander retina by immunocytochemical and autoradiographic methods.

Putative GABAergic neurons in the larval tiger salamander retina were localized by a comparative analysis of glutamate decarboxylase immunoreactivity (GAD-IR), GABA-like immunoreactivity (GABA-IR), and high-affinity 3H-GABA uptake at the light microscopical level. Preliminary data showed that all GAD-IR neurons were double labeled for GABA-IR. However, because the weak somatic labeling with GAD-IR, we could not determine if the converse were true. Neurons commonly labeled with GABA-IR and 3H-GABA uptake include horizontal cells, type I (outer) and type II (inner) bipolar cells, type I (inner) and type II (outer) amacrine cells, and cell bodies in the ganglion cell layer (GCL). In addition, interplexiform cells were identified with GABA-IR. The presence of GABA-IR ganglion cells was indicated by GABA-IR fibers in the optic fiber layer and optic nerve as well as by a GABA-IR cell in the GCL that included a labeled axon. The percentage of labeled somas in the inner nuclear layer (INL) compared to all cells in each layer was similar for the two methods: 30% in INL 1 (outer layer of somas), 15% in INL 2 (middle layer), 43-52% in INL 3 (inner layer), and about 21-26% in the GCL. Labeled processes were found in three bands in the inner plexiform layer, with the densest band located in the most proximal part. Postembedding labeling of 1-micron Durcupan resin sections for GABA-IR showed the same general pattern as obtained with 10-microns cryostat sections, with additional staining, however, of type II (inner) bipolar cell Landolt's clubs. Extensive colocalization of labeling was indicated, and we conclude that GABA-IR can serve as a valid and reliable marker for GABA-containing neurons in this retina and suggest that GABA serves as a transmitter for horizontal cells, several types of amacrine cell, a type of interplexiform cell, and perhaps a small percentage of type I and type II bipolar cells and ganglion cells.