Immunohistochemical and quantitative analysis of 5-hydroxytryptamine in the retina of some vertebrates

5-Hydroxytryptamine immunoreactive neurons were found in retinae from chicken, pigeon, frog and goldfish. They were localized among the amacrine cells with a distribution of cell bodies and nerve fibres that varied with the species. In chicken and pigeon, bipolar-like cell bodies were also found in the middle of the inner nuclear layer, sending processes inwards to the inner plexiform layer and outwards to the horizontal cells. The signalling direction of these cells is doubtful. No 5-hydroxytryptamine immunoreactivity was found in retinae from cow, pig, cat, rabbit, guinea-pig, rat or mouse. Quantitative analyses were performed with HPLC on extracts from chicken, pigeon, frog and goldfish retinae. High concentrations were found in goldfish and frog whereas less, about 100 ng/g, was observed in chicken and pigeon. The results suggest that 5-hydroxytryptamine is the transmitter of a set of amacrine cells in cold-blooded vertebrates and perhaps also in birds. The transmitter of the indoleamine accumulating neurons of mammals remains to be further elucidated.