ESTROGEN-RECEPTORS ARE EXPRESSED IN A SUBSET OF TYROSINE HYDROXYLASE-POSITIVE NEURONS OF THE ANTERIOR PREOPTIC REGION IN THE RAINBOW-TROUT

A double immunocytochemical procedure, with two different chromogens, was used to compare the respective distribution of estrogen receptor-i mmunereactive cells and tyrosine hydroxylase-immunoreactive neurons on the same sections of the preoptic region of adult female rainbow trou t (Oncorhynchus mykiss). Estrogen receptor-immunoreactive cells were o bserved in the anterior preoptic region surrounding the preoptic reces s and its large lateral extensions. Tyrosine hydroxylase-immunoreactiv e cells were consistently detected in the ventral and ventrolateral wa lls of the preoptic recess, in an area that was named nucleus preoptic us pars anteroventralis. Dopamine immunohistochemistry and Dil retrogr ade transport studies indicated that part of these catecholaminergic n eurons are dopaminergic and could project to the pituitary. Double sta ining studies showed consistently that most estrogen receptor-positive cells located ventral to the large extensions of the preoptic recess are also tyrosine hydroxylase-positive, indicating that this region is a major target for estradiol feedback. The results are discussed in r elation to the role of the nucleus preopticus pars anteroventralis in mediating the negative feedback actions of estradiol on the secretion of gonadotrophin (GTH2) secretion. A hypothesis is drawn in order to e xplain the synchronizing role of estradiol at the time of ovulation in rainbow trout.