SYNAPTIC CONTACTS OF SEROTONIN-LIKE IMMUNOREACTIVE AND 5,7-DIHYDROXYTRYPTAMINE-ACCUMULATING NEURONS IN THE ANURAN RETINA

The synapses of serotonin-like immunoreactive retinal neurons were stu died in Bufo marinus and Xenopus laevis and those of 5,7-dihydroxytryp tamine-labelled cells in Xenopus. Immunoreactivity to serotonin was mo stly confined to amacrine cells. Synapses formed by profiles of labell ed cells were almost uniformly distributed in the inner plexiform laye r in both species. Interamacrine synapses were the most frequent, and in some cases two labelled amacrine cell profiles made a gap junction. Some of the labelled amacrine cells synapsed on to presumed ganglion cell dendrites and onto bipolar cell terminals. Labelled bipolar cell terminals synapsed on to non-labelled amacrine cell dendrites and rece ived inputs both from labelled and non-labelled amacrine cells. Labell ed bipolar cell profiles were not observed in the outer plexiform laye r. After preloading and photoconversion of 5,7-dihydroxytryptamine in the Xenopus retina, labelled bipolar cell dendrites in the outer plexi form layer were observed to be postsynaptic to cone pedicles and less frequently to rods and horizontal cells. In the inner plexiform layer, synapse types formed by labelled bipolar cells were similar to those with serotonin immunoreactivity. The frequency of synapses formed by 5 ,7-dihydroxytryptamine-labelled amacrine cells increased, compared wit h serotonin immunocytochemistry. Labelled amacrine cells synapsed most ly with non-labelled amacrine cells, although the ratio of contacts fo rmed by two labelled profiles increased. Synapses from labelled amacri ne cell dendrites to non-labelled bipolar cell terminals and from non- labelled bipolar cell terminals to labelled amacrine cell profiles inc reased in number, while those from labelled amacrine cells to presumed ganglion cell dendrites decreased. The quantitative data obtained by the two approaches enabled us to propose different neuronal circuits f or serotonin-synthesizing and -accumulating neurons of the Xenopus ret ina.