Involvement of estradiol in a catecholamine inhibitory tone of gonadotropin release in the rainbow trout (Oncorhynchus mykiss).

The present study aimed to examine the inhibitory action of catecholaminergic neurons on the release of gonadotropin II (GtH2) in female rainbow trout at different stages of the reproductive cycle. Estradiol (E2) implants in sexually immature female increased blood E2 levels (from 0.55 to about 9 ng/ml) and pituitary GtH2 contents (from 15 to about 8500 ng/pit), but did not modify blood GtH2 levels or pituitary dopamine (DA) and norepinephrine contents. Subsequent treatment by alpha-methyl-p-tyrosine (MPT) increased blood GtH2 levels (from 0.5 to about 3 ng/ml) and decreased pituitary DA levels, only in E2-implanted fish. MPT also increased blood GtH2 levels of sexually recrudescent (about threefold) and sexually mature (about twofold) fish. In the periovulatory period, there was a highly significant positive correlation between blood E2 levels--which fall up until ovulation--and ability of MPT to increase GtH2 release. GtH2 release in rainbow trout is concluded to be inhibited by a brain-pituitary DA tone and there are relationships between this inhibitory tone and the level of estradiol.