REEVALUATION OF GONADOTROPIN-RELEASING-HORMONE (GNRH) ACTION ON PITUITARY CELL-DIFFERENTIATION WITH SPECIAL REGARD TO ITS EFFECT ON LH AND TSH CELL-TYPES

We have previously demonstrated that TRH induces several hypophyseal c ell type differentiation. In the present study, we reevaluated the eff ect of GnRH on gonadotroph differentiation, and we determined whether gonadotropin is implicated in the appearance of other pituitary cell t ypes. These effects were compared to the influence of TRH on LH and TS H cell differentiation. Pituitary primordia of 11 day-old rat embryos were cultured in a synthetic medium until the equivalent of term, and the LH and TSH cell differentiation was visualized using an immunohist ochemical technique. The effect of GnRH on pituitary hormone cells was evaluated by measuring the percentage of immunoreactive area per sect ion. GnRH, when added on the first day of culture, induced LH and TSH cell differentiation in a dose-dependent manner. GnRH was more effecti ve than TRH in inducting gonadotropic phenotype expression. This sugge sts that GnRH, which evokes LH release, is the most effective factor i nducing gonadotroph differentiation in early fetal life. However, GnRH was as effective as TRH in thyrotroph differentiation. Lactotrophs we re detected in primordia cultured with GnRH. As lactotroph phenotype d ifferentiation is controlled by the alpha-subunit of glycoproteic horm one, further studies are required to determine whether or not GnRH eff ect on lactotroph is direct. GnRH did not affect the corticotroph and somatotroph differentiation. These results confirm that hypothalamic f actors such as TRH or GnRH are capable of inducing more than one pitui tary cell phenotypes.