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

Abstract

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‐μm Durcupan resin sections for GABA‐IR showed the same general pattern as obtained with 10‐μm 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.