EXPRESSION OF TILAPIA PREPRO-MELANIN-CONCENTRATING HORMONE MESSENGER-RNA IN HYPOTHALAMIC AND NEUROHYPOPHYSEAL CELLS

Melanin-concentrating hormone (MCH) is a neuropeptide involved in back ground adaptation in teleost fish, and in multiple regulatory function s in mammals and fish. To study the expression of the MCH preprohormon e (ppMCH) in teleosts, eve first cloned a hypothalamic cDNA encoding t he complete ppMCH of tilapia (Oreochromis mossambicus), and a cRNA pro be derived from a 270 bp ppMCH cDNA fragment was used for the expressi on studies. The level of ppMCH mRNA expression in tilapia hypothalamus , measured by dot blot analysis, was significantly higher in fish adap ted to a white background than in black-adapted animals, which is in a ccordance with the reported MCH plasma and tissue concentrations in fi sh. Northern blot analysis not only revealed a strong ppMCH mRNA signa l in the hypothalamus, but also the presence of ppMCH mRNA in the neur ointermediate lobe (NIL) of the pituitary. In situ hybridization and i mmunocytochemistry showed that ppMCH mRNA as well as MCH immunoreactiv ity are located in perikarya of two hypothalamic regions, namely in th e nucleus lateralis tuberis (NLT) and the nucleus recessus lateralis ( NRL). Quantitative analysis by dot blot hybridization revealed about e ight times more ppMCH mRNA in the NLT than in the NRL and NIL of matur e tilapias. ppMCH mRNA in the NIL could be localized to cell bodies of the neurohypophysis, which were also MCH immunoreactive.