Colocalization of enkephalin-, glucagon-, and corticotropin-releasing factor-like immunoreactivity in GABAergic amacrine cells in turtle retina.

The large number of amacrine cells which contain gamma-aminobutyric acid (GABA) in the turtle retina makes it difficult to examine specific GABAergic cell types. In order to selectively label subpopulations of GABAergic neurons, we have used fluorescent double-labeling immunocytochemical techniques to examine the localization of GABA-like immunoreactivity (LI) in amacrine cells which contain antigens resembling the neuropeptides glucagon (GLUC), corticotropin-releasing factor (CRF) or enkephalin (ENK). GABA-LI was found in 41% of the cells with GLUC-, 100% of the cells with CRF-, and 69% of the cells with ENK-LI. There were regional differences in the presence of GABA-LI in amacrine cell populations with ENK-LI. GABA-LI was present in about 80% of the cells with ENK-LI outside of the visual streak, while only 37% of the cells within the streak had GABA-LI. Based on the distinct morphologies and regional distributions of these peptidergic amacrine cells, we conclude that they represent different subpopulations of GABAergic amacrine cells in the turtle retina. Future studies can now utilize existing information regarding the synaptic connectivity of these peptidergic amacrine cells to help delineate the functions of GABAergic amacrine cells in the turtle retina.