DIFFERENTIAL REGULATION OF THE ESTROGEN-RECEPTOR MESSENGER-RNA BY ESTRADIOL IN THE TROUT HYPOTHALAMUS AND PITUITARY

In an attempt to understand the molecular mechanisms by which steroids can modulate brain functions in fish, we first localized the cells wh ich produce estrogen receptor mRNA in the rainbow trout forebrain (Sal bert et al., 1991). We now report how estradiol itself can alter the e strogen receptor mRNA content of these cells in a sterile strain of fe male rainbow trout. We also examined liver and pituitary levels of the estrogen receptor mRNA under the same estrogenic treatment. As reveal ed by slot blot and in situ hybridisations, a single injection (1.5 mg /kg) of estradiol can induce a strong increase (about five-fold) in th e estrogen receptor mRNA levels in the liver, as well as a moderate in crease (about two-fold) in two nuclei of the hypothalamus/preoptic are a: the nucleus lateralis tuberis and the nucleus preopticus periventri cularis. Conversely, no modifications of these levels were observed in the pars intermedia and the proximal pars distalis of the pituitary. Moreover, a comparison between estrogen receptor mRNA levels in the br ain of sexually active female trout and in the brain of these sterile animals revealed that, in these latter, estrogen receptor mRNA levels are lower but can be increased by a single estradiol injection and rea ched the levels observed in mature females.