SEROTONIN-IMMUNOREACTIVE NEURONS AND ENDOGENOUS SEROTONIN IN THE OPISTHOSOMAL VENTRAL NERVE CORD OF THE HORSESHOE-CRAB, LIMULUS-POLYPHEMUS

It has been suggested that serotonin serves as a neurotransmitter in t he horseshoe crab, Limulus polyphemus. While some studies of identifie d groups of central neurons have been conducted, little is known conce rning the neuronal organization in Limulus central ganglia. This study was undertaken to determine the localization of serotoninergic neuron s in the opisthosomal ventral nerve cord of Limulus and to construct a basis for further comparative biochemical and pharmacological studies of the specific function of these neurons. Endogenous serotonin was d etected in the ventral nerve cord (chain of abdominal ganglia) by high performance liquid chromatography and electrochemical detection (HPLC- EC). Endogenous serotonin was quantified in the 9th through 13th gangl ia, anterior (hemal) nerves, posterior (branchial) nerves, and connect ives. The serotonin content in the abdominal ganglia was significantly reduced when the ganglia were incubated for 24 hours in Leibovitz's ( L-15) medium containing reserpine or 5,7-dihydroxytryptamine (5,7-DHT) , neurotoxins that block the uptake of serotonin into storage vesicles . The distribution of serotonin-immunoreactive neurons in the ventral nerve cord was determined by indirect immunocytochemistry. Treatment o f the chain of ganglia with an anti-serotonin antiserum followed by tr eatment with a fluorescent-labeled antiserum raised against the primar y antibody demonstrated specific staining in each ganglion, the gangli onic roots, and connectives. Clusters of serotonin-immunoreactive neur ons were observed anteriolaterally and posteriorly in each ganglion. P rocesses from dense fiber bundles extended from these clusters of neur ons to the central region of each ganglion. These results demonstrate that serotonin-immunoreactive neurons are present in the opisthosomal ventral nerve cord of the horseshoe crab and that serotonin may functi on as a neurotransmitter. (C) 1994 Wiley-Liss, Inc.