Mapping of serotonin-like immunoreactivity in the lobster nervous system.

Serotonin exerts a wide range of physiological actions on many different lobster tissues. To begin the examination of the role of serotonin in lobsters at a cellular level, we have used immunohistochemical methods to search for presumptive serotonergic neurons, their central and peripheral projections, and their terminal fields of arborization. Whole mount preparations of the ventral nerve cord and various peripheral nerve structures have been used for these studies. With these tissues, more than 100 cell bodies have been found that show serotonin-like immunoreactivity. Although a few of the cell bodies are located peripherally (near the pericardial organs, a well known crustacean neurohemal organ), the vast majority are located in central ganglia. Every ganglion in the ventral nerve cord contains at least one immunoreactive cell body. The projections of many of the neurons have been traced, and we have constructed a map of the system of serotonin-immunoreactive cell bodies, fibers, and nerve endings. In addition, a dense plexus of nerve endings showing serotonin-like immunoreactivity surrounds each of the thoracic second roots in the vicinity of groups of peripheral neurosecretory neurons. These peripheral nerve plexuses originate from central neurons of the ventral nerve cord. In some cases we have been able to trace processes from particular central cell bodies directly to the peripheral nerve root plexuses; in other cases we have traced ganglionic neuropil regions to these peripheral endings.