Cholinergic and GABAergic neurons occur in both the distal and proximal turtle retina.

Turtle retinas were processed immunocytochemically and histochemically to detect the presence of choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and glutamate decarboxylase (GAD). We observed cholinergic and gamma-aminobutyric acid (GABA)ergic neurons in the proximal retina, as expected, and in the distal retina as well. ChAT immunoreactivity in the distal retina was observed within the axons and pedicles of numerous cone photoreceptors, suggesting that a population of turtle cone photoreceptors uses ACh as a neurotransmitter. Type L2 horizontal cells were immunoreactive for GAD, and their dendrites invaginated into cone pedicles. AChE histochemistry revealed processes within the outer plexiform layer which formed a loosely organized lattice. In the proximal retina, labeling for ChAT and GAD was similar to that reported by previous investigators. Processes from ChAT-labeled amacrine cells in the inner nuclear layer formed a stratum within the distal inner plexiform layer (IPL) (at 16-21% relative IPL depth), and processes from ChAT-labeled amacrines in the ganglion cell layer formed a proximal ChAT stratum (at 55-58% relative IPL depth). In addition, six AChE-labeled bands and five GAD-labeled bands were observed within the IPL of stained retinas. Therefore, we determined that the two broadest AChE-labeled bands and the two broadest GAD-labeled bands overlapped the two labeled ChAT strata. The evidence for cholinergic and GABAergic processes in both the inner plexiform layer and the outer plexiform layer, combined with electrophysiological evidence from other investigators, raises the possibility that distal retinal neurons may be involved in the encoding of directional information.