The distribution of melanin-concentrating hormone in the lamprey brain

In addition to its novel, colour-regulating hormonal role in teleosts, the melanin-concentrating hormone (MCH) serves as a neuromodulatory peptide in all vertebrate brains. In gnathostome vertebrates, it is produced in severa l neuronal cell groups in the hypothalamus. The present work examines the o rganisation of the MCH system in the brain of lampreys, which separated fro m gnathostome vertebrates at an early stage in evolution. In all three lamp rey genera examined-Petromyzon, Lampetra, and Geotria spp.-MCH perikarya we re found in one major anatomical site, the periventricular dorsal hypothala mic nucleus of the posterior hypothalamus, Axons from these cell bodies pro jected medially into the ventricular cavity, and laterally to the neuropile of the lateral hypothalamus. From here, they extended anteriorly and poste riorly to the fore- and hindbrain. Other fibres extended dorsomedially to t he habenular nucleus. In Lampetra, but not in Petromyzon, MCH fibres were s een in the pituitary neurohypophysis, most prominantly above the proximal p ars distalis. The hypothalamic region in which the MCH perikarya are found forms part of the paraventricular organ (PVO), which is rich in monoamines and other neuropeptides. The association of MCH neurones with the PVO, whic h occurs also in many other nonmammalian vertebrates, may reflect the prima ry location of the MCH system. These MCH neurones were present in ammocoete s, postmetamorphic juveniles, and adults. They were more heavily granulated in adults than in young lampreys but showed no marked change in secretory appearance associated with metamorphosis or experimental osmotic challenge to indicate a role in feeding or osmoregulation. In sexually maturing Lampe tra fluviatilis, however, a second group of small MCH neurones became detec table in the telencephalon, suggesting a potential role in reproduction and /or behaviour. (C) 2001 Academic Press.