Displaced starburst amacrine cells of the rabbit retina contain the 67-kDa isoform, but not the 65-kDa isoform, of glutamate decarboxylase.

The cholinergic identity of retinal starburst amacrine neurons is well established, but recent evidence suggests that these cells are GABAergic as well. Confirmation of this dual transmitter function requires the demonstration of glutamate decarboxylase (GAD), the biosynthetic enzyme for GABA, within starburst cells. The current work was undertaken to determine whether rabbit retinal starburst amacrine neurons contain either of the two known isoforms of GAD. To do this, we have examined the localization of the following: (1) the 65-kDa isoform of GAD; (2) the 67-kDa isoform of GAD; (3) choline acetyltransferase; and (4) the fluorescent dye DAPI, a marker for cholinergic amacrine cells. In addition, we labeled displaced starburst neurons directly, by injecting them with Lucifer Yellow in vitro. Four strata within the inner plexiform layer contained immunoreactive GAD65. A non-GAD65-immunoreactive zone separated the two innermost strata (G3 and G4); this zone contained (1) the dendrites of individual Lucifer Yellow-injected, displaced starburst amacrine cells; (2) dendrites immunoreactive for choline acetyltransferase; and (3) processes of DAPI-labeled amacrine cells. Immunoreactive GAD67 appeared in the same strata that contained GAD65, and in at least two additional strata, one of which lay at precisely the same depth as the proximal cholinergic stratum. In addition, the somas of displaced starburst cells were strongly immunoreactive for GAD67, but not for GAD65. These results demonstrate (1) that displaced starburst amacrine cells contain the 67-kDa isoform of GAD, but not the 65-kDa isoform; and (2) that the dendrites of starburst (67-kDa GAD) amacrines, and the dendrites of 65-kDa-GAD-containing amacrines, occupy different strata within the inner plexiform layer. Thus, displaced starburst cells do contain GAD, and can, presumably, manufacture GABA. The reasons for their preferential use of the 67-kDa GAD isoform remain to be elucidated.