Estradiol-17β and thyrotropin-releasing hormone stimulate prolactin release from the pituitary gland of a teleost fish in vitro

Abstract

The effects of estradiol-17β (E 2) 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 E 2 pretreatment in vitro. In addition, TRH stimulated a dose-related increase in PRL release from E 2-preincubated RPD's, but had no effect on tissues not previously exposed to E 2. The maximal PRL response, nearly three times control levels, occurred at 50 n M 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 E 2 suggests that there may be a shift in the control of PRL secretion with changes in the reproductive state of the tilapia.