Multihormonal regulation of the pituitary gland binding and secretory responses to hypothalamic neuropeptides in rat GH pituitary strains in culture.

The parenchyma of the anterior pituitary gland is constituted of distinct specialized glandular cell types, some of them being able to secrete two, or more, genetically related hormones. The synthesis and the release of each hormone is a discontinuous process controlled by a variety of chemical signals of hypothalamic and peripheral origin. These signals, namely neuropeptides, may cross-react at the level of one cell type and each signal may govern the activity of more than one cell type. Such integrated systems hamper the analysis of binding and secretion coupling of regulatory neuropeptides at the cellular and molecular levels. Prolactin (PRL) and growth hormone (GH) rat clonal pituitary tumor-derived cell lines (GH cells) by contrast offer a reductionist model suitable for such investigations on TRH and somatostatin hypophysiotropic neuropeptides and on their hormonal regulation, as reviewed in this paper. The tetradecapeptide somatostatin inhibits similarly PRL and GH release by GH cells. Using (125)I-Tyr-somatostatin, specific binding sites were characterized; their occupation presents dose dependence and stereospecific requirements similar to the induction of the inhibition of hormone release. In cells cultured in serum-supplemented media, TRH regulates in a biphasic manner somatostatin binding sites: TRH acutely increases and chronically decreases their number without altering their affinity. In cells cultured in serum-free media, TRH competes with (125)I-Tyr-somatostatin and displace it with a similar affinity and nearly identical efficiency as unlabelled somatostatin. These effects of TRH are totally prevented by preculturing the cells in a medium supplemented with tri-iodotyronine, which in addition increases somatostatin binding capacity. The tripeptide TRH stimulates similarly the acute release of PRL and GH by GH cells. By contrast, long-term exposure to TRH inversely regulates the production of both hormones, PRL being augmented and GH inhibited. This results from an opposite regulation of the transcription of the rPRL and rGH genes and of the half-life of their respective encoded mRNA, the stimulation of the rPRL gene being rapid and transient. These effects are mediated by the occupation of specific binding sites, extensively studied using (3)H-TRH, displaying the same dose- and time-dependence, as well as stereospecific requirements, as the regulation of hormone secretion. Factor regulating per se the PRL and GH secretion also alter TRH interaction with GH cells. TRH and tri-iodotyronine both down-regulate the number of TRH binding sites while estrogens or glucocorticoids augment them. TRH binding and secretion coupling are not always regulated in parallel. Evidences for plasma membrane located events were reported for TRH, but also for its internalization. Thus the subcellular localization of the initial signal(s) triggering acute hormone release on the one hand and alteration of specific genes transcription on the other hand is still debated.