Action and localization of gamma-aminobutyric acid in the cat retina.

The effects of iontophoretically applied GABA (gamma-aminobutyric acid) and bicuculline on retinal ganglion cells were studied in the optically intact eye of the anaesthetized cat. GABA suppressed both the spontaneous activity and light-evoked discharge of all retinal ganglion cells, regardless of their type and regardless of the visual stimulus used. Bicuculline antagonized the action of iontophoretically applied GABA. Bicuculline enhanced the spontaneous activity of on-centre cells, but suppressed the spontaneous activity of most off-centre cells. The light-evoked response of on-centre cells was increased by bicuculline. A more complicated picture emerged for off-centre cells. Weak light responses were suppressed by bicuculline, but during strong light responses the initial transient phase of the response was dramatically enhanced. Amacrine cells of the inner nuclear layer and displaced amacrine cells of the ganglion cell layer were labelled, using glutamic acid decarboxylase (GAD) immunohistochemistry and [3H]muscimol uptake. GAD-positive dendrites were found throughout the inner plexiform layer and no sign of dendritic stratification was detected.