Distribution of monoamines in the central nervous system of the nudibranch gastropod, Hermissenda crassicornis

The distribution of monoamines in the central ganglia of the nudibranch gastropod Hermissenda crassicornis was examined through the histological localization of both glyoxylic acid-induced fluorescence and serotonin-like immunoreactivity. Glyoxylic acid histochemistry revealed several clusters of catecholamine-containing cells which were located principally in the cerebropleural ganglia. One large unpaired catecholamine-containing cell was also located in the right pedal ganglion. Glyoxylic acid histochemistry and immunohistochemistry together revealed several serotonin-containing cells. The most prominent of these was a bilateral pair of cells (the metacerebral giants or MCG's) with somata located in the anterior lobes of the cerebropleural ganglia and each with a single large axon running through the ipsilateral cerebrobuccal connective to the buccal ganglia. Apart from the MCG's and a few smaller ones in the cerebropleural ganglia, most other serotonergic cells were located in the pedal ganglia. Among the serotonergic cells identified in the pedal ganglia was a single unpaired giant cell (LP1) located only on the left side. The neurites of LP1 projected through the cerebropleural ganglia to the contralateral pedal ganglion. Similarities in the distribution of monoamines in different gastropod species are discussed.