HORIZONTAL CELLS IN CAT AND MONKEY RETINA EXPRESS DIFFERENT ISOFORMS OF GLUTAMIC-ACID DECARBOXYLASE

The neurotransmitter used by horizontal cells in mammals has not been identified. GABA has been the leading candidate, but doubt has remained because of failure to clearly demonstrate the GABA synthetic enzyme, glutamic acid decarboxylase (GAD) in these cells. Because GAD was recently shown to exist as two isoforms, 65 kDa and 67 kDa, we considered whether there might be a mismatch between the forms of GAD expressed in horizontal cells and the probes used to detect it. Accordingly, we stained sections of mammalian retina with antibodies specific for each isoform. Cat horizontal cells of both types (A and B) were immunoreactive for GAD(67) but negative for GAD(65); monkey horizontal cells of both types (H-I and H-II) were positive for GAD(65) and negative for GAD(67),. The findings reconcile previous, apparently conflicting, observations and strengthen considerably the hypothesis that mammalian horizontal cells are GABAergic.