Mammalian horizontal cells are believed to be GABAergic because, in most species, they contain both GABA and glutamic acid decarboxylase (GAD), and their terminals are presynaptic to GABA receptors. In adult rabbit, however, GABA and GAD immunoreactivity have not been consistently demonstrated in horizontal cells, casting doubts on the assumption that they too are GABAergic. Here we demonstrate that all rabbit horizontal cell terminals—dendritic terminals of type A, and both dendritic and axonal terminals of type B—immunostain for one isoform of GAD, GAD67. In addition, we show that type A horizontal cell somas and primary dendrites in the visual streak (identified by their immunoreactivity to calbindin) are immunoreactive for the other GAD isoform, GAD65. Double-labeling experiments for GAD65 and GABA reveal that every cell that stains for GAD65 also stains for GABA. The presence of GAD67 in horizontal cell terminals suggests that rabbit horizontal cells are GABAergic. The segregation of the two GAD isoforms to different cell compartments suggests that GABA is released at different sites, possibly by two different mechanisms.
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