5-hydroxytryptamine: its facilitative action on [3H]dopamine release from the retina

Retinal neurotransmitter interaction as reflected in horizontal cell spatial behaviour

The effects of 5-hydroxytryptamine (5-HT) and its precursors 5-hydroxytryptophan (5-HTP) and L-tryptophan (L-Tryp) on the spatial properties of horizontal cells were studied in the isolated and perfused retina of the teleost Eugerres plumieri. All three compounds produce a contraction of the receptive field, evaluated by the ratio of responses evoked by local and distant light stimuli. This is the result of cell uncoupling, revealed by the hindrance to diffusion of intracellularly injected Lucifer yellow. Similar effects are produced by dopamine (DA) and the effectiveness is DA much greater than 5-HT greater than 5-HTP greater than L-Tryp. All these effects are blocked by Haloperidol. HPLC studies of endogenous DA release reveal that it occurs when isolated retinas are incubated with 50 mM potassium, 10 microM 5-HT or 5-HTP, but is not found with up to 1 mM L-Tryp. The results indicate that indolaminergic cells induce the release of DA from interplexiform cells, which in turn uncouples horizontal cells in the fish retina.

Putative serotonergic bipolar and amacrine cells in the chicken retina

Four populations of putative serotonergic cells could be detected in the chicken retina by histofluorescence and immunohistochemistry. Numerous (10,000/mm2) small (6 micron diameter) bipolar cells were located towards the middle of the inner nuclear layer, as were sparser (1000/mm2) larger (12 micron diameter) amacrine cells. Very sparse large (greater than 30 micron diameter) and more numerous small (12 micron diameter) ganglion cells were also detected. Prominent fibre plexuses were detected in the inner plexiform layer, close to the inner nuclear and ganglion cell layers, and appeared to be formed by the processes of the bipolar cells, amacrine cells and at least the large ganglion cells. Exogenous serotonin (5-HT) was detected in the chicken retina. From the effects of neurotoxins on 5-HT levels and 5-HT-like immunoreactivity (5-HTLI), most of this appeared to be associated with the amacrine cells. 5-HTLI bipolar cells were selectively destroyed by intravitreal injections of 5-10 nmol of kainic acid, while 5-HTLI amacrine cells were destroyed by N-methyl-D,L-aspartic acid and 5,7-dihydroxytryptamine. The sensitivity of the bipolar cells to kainic acid indicates that they are OFF-cells.