Prepubertal increases in the levels of two salmon gonadotropin-releasing hormone mRNAs in the ventral telencephalon and preoptic area of masu salmon

Ontogenic changes in the expression levels of two salmon gonadotropin-releasing hormone genes (sGnRH-I and -II) were examined in the forebrain region including the ventral telencephalon and preoptic area of masu salmon by competitive reverse transcription-polymerase chain reaction (RT-PCR). Two genes showed similar expression patterns throughout the lifetime in both sexes, although the levels of sGnRH-II mRNA were about 20 times higher than those of sGnRH-I mRNA. In males, the levels of sGnRH mRNAs increased at the beginning of the second year and reached their maximum in the autumn. The levels decreased gradually until the autumn of the third year when fish sexually matured. In females, the levels reached their maximum in the first autumn and fluctuated considerably along with the seasons in the third year. These results suggest that, in the salmon brain, sGnRH genes are activated long before the sexual maturation under sexually different control mechanisms. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.