Direct action of serotonin on prolactin, growth hormone, corticotropin andluteinizing hormone release in cocultures of anterior and posterior pituitary lobes: Autocrine and/or paracrine action of vasoactive intestinal peptide

There is extensive evidence that serotonin (5-HT) is implicated in the neur oendocrine control regulating the secretion of several anterior pituitary h ormones. It has also been reported that the posterior pituitary is necessar y for prolactin (PRL) response to 5-HT as well as to suckling, in which 5-H T implication has been demonstrated. As we have previously shown that vasoa ctive intestinal peptide (VIP) mediates through an autocrine or paracrine a ction the PRL release induced by insulin-like growth factor I, thyrotropin- releasing hormone (TRK) and dopamine withdrawal, the aim of the present wor k was to determine whether 5-HT has a direct action on pituitary secretion and to study the possible role of pituitary VIP in this situation. Cells fr om the anterior pituitary lobe (AP) were cultured either alone or together with cells from the posterior pituitary lobe (PP). As melanotropes from PP express glucocorticoid receptors in vitro, both AP cultures and cocultures of AP/PP cells were incubated in the presence or absence of corticosterone (0.1 mu g/ml), thus designing four experimental conditions. Then both AP an d mixed cultures were incubated with 5-HT (100 nhl) for 20, 45 and 180. The release of PRL, growth hormone (GH), corticotropin (ACTH) and luteinizing hormone (LH) was stimulated by 5-HT, but only in cocultures of AP/PP cells preincubated with corticosterone, whereas follicle-stimulating hormone and thyroid-stimulating hormone release was not modified. As AP cultures did no t show any response to 5-HT, both in the presence or absence of corticoster one, and as melanotropes are the main cellular type present in the PP cultu res, we studied the response of a-melanocyte-stimulating hormone (aMSH) to 5-HT in PP cells cultured with or without corticosterone. Serotonin did not modify aMSH release either in the absence or the presence of corticosteron e. VIP release was also stimulated by 5-HT in the cocultures, and the time response profile was only similar to that of PRL. In order to study whether pituitary VIP is implicated in 5-HT action, cocultures preincubated with c orticosterone were incubated in the presence of 5-HT, a VIP-receptor antago nist (VIP-At) or simultaneously with 5-HT plus VIP-At. PRL response to 5-HT was abolished by the simultaneous presence of VIP-At, whereas GH, ACTH and LII response remained unchanged. These data demonstrate that: (1) 5-HT sti mulates the secretion of PRL, GH, ACTH, LH and VIP acting directly at pitui tary level on PP, probably by releasing an unidentified mediator from melan otropes; (2) glucocorticoids make the response of AP cells to 5-HT possible due to the presence of PP cells in the coculture; (3) PRL response to 5-HT is mediated through an autocrine and/or paracrine action of VIP.