Effects of repeated stimuli on prolactin release in vitro from normal and adenomatous rat lactotrophs

Abstract

It is now well known that dopamine (DA) plays a major role in the inhibitory control of prolactin (PRL); however, the mechanisms that are physiologically involved in the stimulation of PRL release are still under investigation. Indeed, although suppression of DA inhibitory tonus, administration of thyrotropin-releasing hormone (TRH) or vasoactive intestinal peptide (VIP) are all known PRL releasers, it is not clear whether they interact during physiological periods of PRL release such as suckling and estrus. No clear indications exist, furthermore, on whether they all act upon a same pituitary pool that may become depleted following repeated exposure to stimuli. Refractoriness to a single or a repeated stimulus has been reported to occur in prolactinoma-bearing or normal humans, respectively, the mechanism of which is still matter for discussion. Our present studies performed by perifusing normal or adenomatous rat lactotrophs attached to Cytodex I microcarrier beads was undertaken to try and answer some of these questions. The experimental period consisted in perifusing the cells for l h with Dulbecco's modified Eagle's medium (DMEM) containing DA HT −5 M, then for 2 h with either DMEM, DMEM and TRH 10 −8 M, DMEM and VIP 10 −7 M, then again with DA in DMEM for 1 h, and finally with DMEM, DMEM and TRH, or DMEM and VIP. Three experiments of various combinations were performed. Lower PRL levels were observed under DA, while two periods (first and second) of PRL release followed the suppression of DA infusion with or without the addition of either one of the two peptides. With both normal and adenomatous cells, more PRL was released during the first period with TRH or VIP addition than with DMEM alone. A previous period of DA suppression (perifusion with DMEM alone) did not modify the releasing potency of TRH or VIP when the peptides were perifused during the second period. However, a previous period of either TRH or VIP infusion (combined with suppression of DA inhibition) partially or totally suppressed the releasing effects of a second period of DA suppression even when combined with TRH or VIP, particularly when using tumoral cells. The latter, as a whole, released less PRL than normal cells during the various experimental conditions. No significant differences were found between the releasing effects of TRH and VIP when infused in various combinations. These results confirm previously published data indicating that TRH acts upon a pool of PRL previously transformed by an interruption of DA inhibition and thus rendered releasable by the peptide. They show that VIP acts