Localization of aspartate-like immunoreactivity in the retina of the turtle (Pseudemys scripta)

Aspartate has been reported to be a putative excitatory neurotransmitter in the retina, but little detailed information is available concerning its anatomical distribution. We used an antiserum directed against an aspartate-albumin conjugate to analyze the anatomy, dendritic stratification, and regional distribution of cell types with aspartate-like immunoreactivity in the turtle retina. The results showed dramatic differences in immunoreactivity in the peripheral versus the central retina. Strong aspartate-like immunoreactivity was shown in the peripheral retina, with many well-labeled processes in the inner plexiform layer. Many bipolar, horizontal, amacrine, and ganglion cells, some photoreceptors, and some unidentified cells were strongly immunoreactive in the peripheral retina. In contrast, although the central retina showed well-labeled horizontal cells, there was only light labeling in the inner plexiform layer with weakly immunoreactive amacrine and ganglion cells and no labeled bipolar cells. There were several strongly immunoreactive efferent nerve fibers which left the optic nerve head and arborized extensively in the retina. At the electron microscopic level, electron-dense reaction product was associated with synaptic vesicles at bipolar and amacrine cell synapses in the inner plexiform layer. These results suggest that aspartate may be involved in many diverse synaptic interactions in both the outer plexiform layer and the inner plexiform layer of the turtle retina.