Seasonal changes in expression of neurohypophysial hormone genes in the preoptic nucleus of immature female masu salmon

In relevance to osmoregulatory and reproductive functions, activity of the hypothalamic magnocellular neurosecretory system may vary seasonally in tel eosts. The changes in the expression of vasotocin (VT) and isotocin (IT) ge nes were thus studied by an in situ hybridization technique and an immunohi stochemical avidin-biotin complex method in immature female masu salmon (On corhynchus masou). The plasma levels of testosterone and estradiol were als o measured by enzyme immunoassay. Fish were sampled in March, May, August, and November 1994 and January 1995. The intensity of autoradiographic hybri dization signals and immunoreactivity were determined in individual neurose cretory cells (NSC) in the rostroventral, middle, and dorsocaudal regions o f the magnocellular part of the preoptic nucleus (PM). The VT hybridization signals and immunoreactivity were high in November, along with the elevati on of plasma levels of testosterone and estradiol. These results suggest th at sex steroid hormones are involved in seasonal regulation of VT gene expr ession. The hybridization signals for IT mRNA were increased in May and dec reased in November, whereas IT immunoreactivity was low in March and high i n November. NSCs thus showed seasonal variations in the intensity of hybrid ization signals for VT and IT mRNAs and immunoreactivity of VT and IT, alth ough the patterns of changes were different between VT and IT. VT and IT ge nes may be seasonally expressed under different regulatory mechanisms. (C) 1999 Academic Press.