ESTRADIOL-17-BETA AND THYROTROPIN-RELEASING-HORMONE STIMULATE PROLACTIN-RELEASE FROM THE PITUITARY-GLAND OF A TELEOST FISH INVITRO

The effects of estradiol-17.beta. (E2) and thyrotropin-releasing hormone (TRH) on prolactin (PRL) release were investigated using the organ-cultured rostral pars distalis (RPD) of the tilapia, Oreochromis mossambicus. Spontaneous PRL release into hyperosmotic medium increased in a dose-related manner following E2 pretreatment in vitro. In addition, TRH stimulated a dose-related increase in PRL release from E2-preincubated RPD''s, but had no effect on tissues not previously exposed to E2. The maximal PRL response, nearly three times control levels, occurred at 50 nM TRH. Higher doses of TRH were less effective in stimulating PRL release. These findings indicate that TRH may be an important hypothalamic prolactin-releasing factor in the tilapia. Furthermore, the marked potentiation of the action of TRH on PRL release following exposure to E2 suggests that there may be a shift in the control of PRL secretion with changes in the reproductive state of the tilapia.