GAMMA-AMINOBUTYRIC-ACID ENHANCES THE LIGHT RESPONSE OF GANGLION-CELLSIN THE TROUT PINEAL ORGAN

Electrical recordings from achromatic second order neurons of intact s uperfused pineal organs of the rainbow trout were used to investigate the role of gamma-aminobutyric acid (GABA) in the transmission of phot oreceptor signals onto centrally projecting ganglion cells. Bath-appli ed GABA decreased the spike discharge rate of 98% of achromatic gangli on cells in a dose-dependent manner. GABA was also active if applied d uring synaptic blockade, demonstrating the presence of GABAergic recep tors at the ganglion cell level. Responsiveness of ganglion cells to l ight was reversibly enhanced by GABA. The light response curve of gang lion cells, which was obtained by plotting spike rate versus light int ensity, was significantly shifted to lower frequencies by GABA, indica ting that GABA is an important inhibitory modulator of ganglion cell a ctivity in the trout pineal organ.