Intracellular Signals in Pituitary Hormone Secretion

Abstract

The regulation of the secretion of hormones from the anterior pituitary gland depends, to a large degree, on the release of substances from the hypothalamus into the hypophysial portal vessels of the median eminence. These neural substances act in conjunction with other blood-borne substances from the periphery to regulate the function of the various anterior pituitary cells. Thus far, the only hypothalamic substances that have been unequivocally demonstrated are the peptides; thyrotropin-releasing hormone (TRH), luteinizing hormone-releasing hormone (LHRH) and somatostatin (or GHRIH). In addition, most evidence favours the supposition that dopamine is the major prolactin inhibitory factor (PIF), although there have been sporadic reports of a separate PIF (notably GABA). Dopamine acts directly on pituitary cells to inhibit prolactin secretion, it is found in the portal blood in concentrations sufficient to affect prolactin release, and its release from the hypothalamus appears to be influenced by effectors of prolactin secretion in vivo. There is also evidence for the existence of corticotropin-releasing factor (CRF), growth hormone releasing factor (GRF), and a prolactin releasing factor (PRF). This latter activity may be distinct from TRH which also stimulates prolactin secretion.