DIFFERENTIAL MELANIN-CONCENTRATING HORMONE GENE-EXPRESSION IN 2 HYPOTHALAMIC NUCLEI OF THE TELEOST TILAPIA IN RESPONSE TO ENVIRONMENTAL-CHANGES

For some teleosts, a role has been established for melanin-concentrati ng hormone (MCH) background adaptation and stress response. in teleost fishes, prepro-MCH (ppMCH) mRNA is expressed in the hypothalamus, pre dominantly in neurons of the nucleus lateralis tuberis (NLT) and in sc attered cells of the nucleus recessus lateralis (NRL). The response of mature tilapia to different environmental challenges was studied by a ssessing ppMCH mRNA levels in these two hypothalamic nuclei by quantit ative dot blot analysis. Changes in background colour induced pronounc ed differences in ppMCH mRNA expression in the NLT, but not in the NRL . The NLT of tilapia adapted to a white background contained 2.5 to 3 times more ppMCH mRNA than the NLT of black-adapted fish. The NLT of f ish kept on neutral background contained intermediate levels of ppMCH mRNA, which were significantly lower than the levels in white-adapted fish. Oral administration of dexamethasone lowered plasma cortisol con centrations, but had no effect on ppMCH mRNA levels in white- and blac k-adapted fish. In tilapia exposed to strongly acidified water (pH 3.5 ), plasma cortisol and ACTH concentrations were highly elevated, and p lasma chloride concentrations considerably lower than in controls. The se fish responded with a 70% rise in ppMCH mRNA levels in the NLT, whi ch is most probably associated with a stress response evoked by inadeq uate osmoregulation. After exposure to a milder acidification (pH 4.0) or to seawater no significant changes in ppMCH mRNA levels occurred i n either the NLT or the NRL, nor in plasma chloride, cortisol and ACTH levels. A specific increase of ppMCH mRNA levels in the NRL was obser ved in repeatedly disturbed tilapia. We conclude that MCH neurons in t he NLT and NRL of this teleost differentially respond to background co lour, acidification and disturbance stress, and that this response is not strictly associated with changes in plasma ions and activity of th e pituitary-interrenal axis.