The aim of the present study was to elucidate the putative interactions between the serotoninergic and the GABAergic systems in the lower vertebrate retinas. Experiments were performed on 12-14 µm frozen frog and turtle retinal sections, processed with more than 10 different antibodies directed against serotonin, GABA and their membrane receptors by means of the indirect immunocytochemical method. The results showed that some of the GABAergic retinal neurons possess the serotonin receptors 5-HT2A and 5-HT3A on their cell membrane, and vice versa: certain serotoninergic neurons possess the ionotropic GABA A and GABA C receptors. A simple neuronal chart is proposed to explain how the activation of serotonin receptors on a subgroup of GABAergic amacrine cells can decrease the GABA C receptor mediated inhibition on the retinal bipolar cells, the main generators of ERG. As the retina is a natural biological model of CNS, the relationships between both neurotransmitter systems described here may elucidate their participation in certain brain functions and to explain the possible mechanisms of action of some antidepressant drugs.

Abstract. Serotonin (5-HT) is one of the major monoamines in the brain. It is also found in the retina, a part of the brain, where a subgroup of amacrine cells are considered to be the sole retinal source of 5-HT. The data, obtained so far, about the serotonin receptors' function are contradictory. Therefore, our aim was to study the distribution of the 5-HT synthesizing neurons and the serotonin receptors from 5-HT1A to 5-HT7 in frog and turtle retinas, using the indirect immunofluorescent method. We found strong Tryptophan hydroxylase immunoreactivity in many amacrine and bipolar cells. Some horizontal, ganglion and glial cells were also stained, as well as a bundle of parallel axons in the proximal retina. All 5-HT receptors were also very well expressed. Our data show that the serotonin retinal sources are more numerous than was previously thought. The great variety of receptors expressed implies that serotonin may fulfill different functions, serving both as neuro-and glio-transmitter/modulator.