Does 5HT play a neurotransmitter role in mammalian retina? Studies on uptake and potassium-stimulated release of 14C-5HT and 14C-GABA from bovine and rabbit retina slices.

Slices of the bovine and rabbit retina actively took up 14C-5HT from the incubation medium; the process was temperature- and Na+-dependent. Concentration-dependent inhibition of 14C-5HT uptake into retinal slices by fluoxetine and citalopram, as well as relative ineffectiveness of nomifensine and nisoxetine, indicate that the uptake process is specific. Exposition of the retina slices of both rabbit and cow to high K+ concentrations (25-70 mM) did not result in any change in the release of radioactivity (compared to basal outflow) when studied in a standard superfusion system. Under the same experimental conditions, 50 mM K+ stimulation of the bovine retina slices preloaded with 14C-GABA clearly enhanced outflow of 14C-radioactivity. Inability of K+ stimulation to increase the release of 14C-5HT from the bovine and rabbit retina slices does not support the idea that 5HT acts as a retinal neurotransmitter in the studied mammals. A possibility is discussed that in mammals 5HT, being actively and selectively taken up by some retinal elements, may serve as a precursor for a functionally active compounds, e.g. melatonin. In contrast to 5HT, K+ evoked 14C-GABA release from the bovine retina slices supports the contention that GABA is a neurotransmitter in some retinal cells.