Endothelin-like immunoreactivity in the nervous system of invertebrates and fish

There have been no reports on the distribution of immunoreactive endothelin (ir-ET) in lower vertebrates and invertebrates, except for our previous studies on the nereid Neanthes diversicolor and the earthworm Eisenia foetida. In the present study, we found ET-like immunoreactivity in five species of invertebrates and two species of fish with antiserum against synthetic endothelin-1 (ET-1). Immunoreactive perikarya and nerve fibers were observed in the central nervous system of the slug Limax marginatus, the freshwater snail Indoplanorbis exustus, and the mussel Mytilus edulis in mollusks, the field cricket Gryllus bimaculatus in insects, and the tube tunicate Ciona intestinalis in protochordates. In the medaka, Oryzias latipes, ir-ET was found in the hypothalamoneurohypophysial system, the caudal neurosecretory system, the gill, and the kidney. Immunoreactive cells were also found in the mucous gland of the slug and in the adenohypophysis of the lamprey, Lampetra japonica. The wide distribution of ET-like substances in invertebrates and fish provides evidence for the case that ET found in mammals has a long evolutionary history.