Differential distribution of serotoninergic inputs on the goldfish Mauthner cell

The morphology and distribution of the serotoninergic (5-HT) input to the Mauthner cell (M cell) of a teleost, Carassius auratus, were analyzed at the light microscopic level. Immunohistochemical methods revealed that 1) most fibers innervating the M cell originate from the ventral and lateroventral regions of the rhombencephalon; 2) two groups of fibers contribute to this innervation, thick ones (type I, 0.4-0.7 microns in diameter) with terminal endings and thin ones (type II, less than 0.2 microns) that issue numerous beaded varicosities 4-10 microns from the target cell and only occasional side endings contacting it; 3) the density of immunoreactive profiles is uneven over the whole cell and predominates on the ventral dendrite; and 4) the two sets of axons, although overlapping, do not have the same distribution. Specifically, both classes are present on the ventral dendrite, whereas type II fibers are the only ones observed on the soma, in the region of the initial segment of the axon, and in the vicinity of the lateral dendrite. Functionally identified inputs on the M cell also have a regionalized distribution, depending, for example, on whether they belong to excitatory or inhibitory networks. Thus we propose that 5-HT inputs have specific influences that are a function of their respective localization.