Immunocytochemical localization of octopamine in the central nervous system of Limulus polyphemus: a light and electron microscopic study.

We have determined the distribution and localization of the monoamine octopamine in the prosomal central nervous system of the horseshoe crab, Limulus polyphemus, by light and electron microscopic immunocytochemistry. Sixteen discrete clusters of octopamine-like immunoreactive neurons are situated bilaterally in the tritocerebrum and circumesophageal ring of fused thoracic ganglia. Two pairs of anterior clusters are located laterally in the cheliceral and first pedal ganglia; the remaining six pairs of clusters are located ventromedially in the second through fifth pedal ganglia, chilarial ganglia, and opercular ganglia. The immunoreactive somata range from about 40 to 100 microns in diameter and occur in clusters of 12-24 cells. There is extensive distribution of octopamine-immunoreactive nerve fibers in Limulus; dense fiber tracts course anteroposteriorly through the central nervous system, and most neuropil regions are innervated by immunoreactive processes and terminals. This wide distribution of octopamine-like immunoreactivity provides an anatomical basis for the several effects of octopamine in Limulus. We determined the subcellular localization of octopamine by postembedding immunoelectron microscopy. The immunogold-labelled terminals are morphologically unique; they contain large, distinctively shaped dense-core granules, typically cylindrical with a prominent indentation in one end. These large granules are 100-150 nm in diameter and range from 150-400 nm in length. The dense labelling of these unusual granules with immunogold particles indicates that octopamine is sequestered in or associated with the granules.