Marked Distributional Difference of α-Melanocyte-Stimulating Hormone (α-MSH)-like Immunoreactivity in the Brain between Two Elasmobranchs (Scyliorhinus torazame and Etmopterus brachyurus): An Immunohistochemical Study

Abstract

Application of α-MSH immunohistochemistry to the brain of elasmobranchs (Scyliorhinus torazame and Etmopterus brachyurus) demonstrated a marked species difference concerning the distribution of the α-MSH-like molecule in the brain. In S. torazame, α-MSH-like immunoreactive cells were present in the hypothalamus, mainly in the tuberculum posterius and the nucleus lateralis tuberis, and also in the distal and neurointermediate lobes of the hypophysis. Labeled varicose fibers were densely distributed in the hypothalamus, but they were sparse or absent in other portions of the brain. In striking contrast to the results for S. torazame, the immunoreactivity in the E. brachyurus brain was associated exclusively with the glial system, represented by astrocytes and tanycytes, throughout the central nervous system; no immunoreactivity was found in the neuronal elements. In the E. brachyurus hypophysis, the labeled cells were present in the distal and intermediate lobes, similarly to their presence in S. torazame, but in the intermediate lobe the immunoreactivity was confined to the peripheral cell cord closely adjacent to the neural lobe. The present findings are the first as regards the occurrence of α-MSH-like immunoreactivity in the glial system of the central nervous system of vertebrates and suggest diversity of expression and/or processing of proopiomelanocortin in the brain of elasmobranchs.