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 catecholam inergic neurons on the release of gonadotropin II (GtH(2)) in female r ainbow trout at different stages of the reproductive cycle. Estradiol (E(2)) implants in sexually immature female increased blood E(2) level s (from 0.55 to about 9 ng/ml) and pituitary GtH(2) contents (from 15 to about 8500 ng/pit), but did not modify blood GtH(2) levels or pitui tary dopamine (DA) and norepinephrine contents. Subsequent treatment b y alpha-methyl-p-tyrosine (MPT) increased blood GtH(2) levels (from 0. 5 to about 3 ng/ml) and decreased pituitary DA levels, only in E(2)-im planted fish. MPT also increased blood GtH(2) levels of sexually recru descent (about threefold) and sexually mature (about twofold) fish. In the periovulatory period, there was a highly significant positive cor relation between blood E(2) levels-which fall up until ovulation-and a bility of MPT to increase GtH(2) release. GtH(2) release in rainbow tr out is concluded to be inhibited by a brain-pituitary DA tone and ther e are relationships between this inhibitory tone and the level of estr adiol. (C) 1995 Academic Press, Inc.