Immunoreactivity in Limulus . II. Studies of serotoninlike immunoreactivity, endogenous serotonin, and serotonin synthesis in the brain and lateral eye

Abstract Serotoninlike immunoreactivity was examined by the fluorescein‐iso‐thiocyanate‐labeled secondary antibody technique in the lateral eye and brain of Limulus . Endogenous serotonin was measured with high‐perfor‐mance liquid chromatography and electrochemical detection. The synthesis of [ 3 H]serotonin from [ 3 H]tryptophan was measured in the presence and absence of reserpine. Fibers with serotoninlike immunoreactivity were found in the proximal stalks of the corpora pedunculata, in the neuropil of the central body, in the neuropils of the visual centers (lamina, medulla, and ocellar gnaglion), in the optic tract that connects the ocellar ganglion with the posterior medial medulla, and in the central neuropil of the brain. Immunoreactive somata were found in four groups in the brain. Up to 50 somata were scattered through each side of the dorsal medial group that lies centered on the dorsal surface within the curve of the centra body. These neurons innervate the central body neuropil and send processes into the central neuropil. Three of four reactive somata formed the ventral pole of each medullar group. These may provide the innervation of the proximal stalk of the corpora pedunculata. Five to ten reactive neurons were observed anteriorly in the ventral posterior lateral group #2 on each side that send processes into the central neuropil. Ten to 15 reactive somata were found on either side of the midline in the dorsal anterior part of the ventral medial group that contribute processes to the central neuropil. The remainder of the brain was not immunoreactive. No immunoreactive fibers or somata were found in the lateral eye or in the lateral optic nerve. Serotoninlike and substance P‐like immunoreactivities were not found to be colocalized anywhere in the brain. Significant amounts of endogenous serotonin were detected in the lamina and medulla whose neuropils are rich in immunoreactive fibers and in the central body and dorsal medial group that are also rich in immunoreactive somata and fibers. No endogenous serotonin was detected in either the lateral eye of the lateral optic nerve. The lamina, medulla, and central body and dorsal medial group also synthesized and stored [ 3 H]serotonin from [ 3 H]tryptophan. It is likely that serotonin is a neurotransmitter in the brain, but not in the lateral eye of the horseshoe crab. In particular, it appears that serotoninergic neurons may play a role in central visual processing.