MIP-like substance in the brain of Perinereis nuntia Savigny, 1818 (Polychaeta, Annelida) detected by immunocytochemistry

Abstract

Molluscan insulin-like peptides (MIPs) were first identified from the cerebral ganglion of a freshwater pulmonate, the snail Lymnaea stagnalis. Recently, MIPs received considerable attention due to their molecular similarity to insulins of higher taxa including humans. It has been suggested that MIPs play an important role in the growth and cell differentiation of peripheral organs of mollusks. The striking similarity between the embryology of mollusks and polychaetes has long been recognized and supports the view that polychaetes and mollusks are evolutionarily related. In this study, we used polyclonal antibodies against the MIP1C-chain of L. stagnalis to detect the MIP immunoreactive cells in the brain of the locally common polychaete Perinereis nuntia. MIP immunoreactive cells were detected in the P. nuntia brain by confocal and epifluorescent microscopy. These cells were located in the periphery and posterior regions of the brain but not in the anterior region. The cells varied from 10 to 20 µm in diameter. Electron microscopic studies of ultra-thin sections of the P. nuntia brain, labeled by an immuno-gold secondary antibody, confirmed the presence of MIP-immunoreactive neurons in the brain. MIP-immunoreactive secretory granules were found in axon terminals, which co-existed with synaptic vesicles in the neuropile of the brain. The potential functions of MIP-like substances in polychaetes are discussed.