Effect of GABA and its antagonists, bicuculline and picrotoxin, on nerve cell discharges of the photosensory pineal organ of the frog, Rana esculenta.

The effect of gamma-aminobutyric acid (GABA) and its antagonists, bicuculline and picrotoxin, was studied on pineal neurons of the frog, Rana esculenta. The drugs were applied by microiontophoresis while monitoring the spontaneous activity and light-evoked responses of electrophysiologically identified achromatic (luminance) neurons of the pineal organ. Almost all neurons investigated were sensitive to GABA. The inhibitory action was characterized by its rapid onset and its reversibility. The GABA antagonists, bicuculline and picrotoxin, were able to antagonize the inhibitory action of the amino acid. The light-evoked inhibition of the maintained ganglion cell activity interfered with the GABA-induced inhibition, i.e. light reduced the strength of inhibition and shortened the effect of GABA. The investigation suggests a major role of GABAergic mechanisms in the ganglion cell output of pineal neurons.