RELEASE OF PITUITARY GONADOTROPINS GTH-I AND GTH-II IN THE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) - MODULATION BY ESTRADIOL AND CATECHOLAMINES

The present study focused on the role of catecholaminergic neurons and estrogens on the release of gonadotropins I and II in immature and ea rly vitellogenic female rainbow trout. The ovariectomy-induced increas e of GtH I blood levels (from about 10 to 15 ng/ml) was prevented in v itellogenic fish by E2 supplementation. E2 implantation of immature fi sh decreased blood GtH I levels (from about 6 to 1 ng/ml). Blood level s of GtH II were low (about 0.5 ng/ml) and not altered by ovariectomy and E2 treatment. These data demonstrate that estrogens exert a negati ve feedback on the release of GtH I in trout. A treatment with alpha-m ethyl-p-tyrosine (MPT), an inhibitor of catecholamine synthesis, incre ased blood GtH II levels of sham-operated vitellogenic fish and ovarie ctomized fish implanted with E2, but had no effects in ovariectomized fish. MPT did not modify blood GtH I levels in any experimental group. A treatment of E2-implanted immature or vitellogenic fish with the do pamine antagonist pimozide also increased blood GtH II levels, but did not significantly change blood GtH I levels. These data demonstrate t hat release of GtH II. but not of GtH I, depends on an E2-activated DA inhibitory tone. (C) 1998 Academic Press.