REGIONAL DIFFERENCES IN GABA AND GAD IMMUNOREACTIVITY IN RABBIT HORIZONTAL CELLS

Mammalian horizontal cells are believed to be GABAergic because, in mo st species, they contain both GABA and glutamic acid decarboxylase (GA D), and their terminals are presynaptic to GABA receptors. In adult ra bbit, however, GABA and GAD immunoreactivity have not been consistentl y demonstrated in horizontal cells, casting doubts on the assumption t hat they too are GABAergic. Here we demonstrate that all rabbit horizo ntal cell terminals-dendritic terminals of type A, and both dendritic and axonal terminals of type B-immunostain for one isoform of GAD, GAD (67). In addition, we show that type A horizontal cell somas and prima ry dendrites in the visual streak (identified by their immunoreactivit y to calbindin) are immunoreactive for the other GAD isoform, GAD(65). Double-labeling experiments for GAD(65) and GABA reveal that every ce ll that stains for GAD(65) also stains for GABA. The presence of GAD(6 7) in horizontal cell terminals suggests that rabbit horizontal cells are GABAergic. The segregation of the two GAD isoforms to different ce ll compartments suggests that GABA is released at different sites, pos sibly by two different mechanisms.