Prolactin is secreted by the lactotroph cells of the anterior pituitary. The control of its secretion, like that of other anterior pituitary hormones, is regulated by the hypothalamus. Unlike the other anterior pituitary hormones, however, the hypothalamic influence is of tonic inhibition (Fig. 4.1).
The hypothalamus secretes two hypothalamic factors to control prolactin secretion: a prolactin-releasing factor (PRF) and a prolactin release-inhibiting factor (PIF). The latter is almost certainly the catecholamine dopamine, although the possibility of the existence of noncatecholamine PIFs cannot be excluded. Additionally, g-aminobutyric acid (GABA) may play a role as an inhibitor, and there may well also be one or more PIF peptides. The nature of PRF is unclear, although it is known that thyrotrophin-releasing hormone (TRH) can act as a PRF. Other candidates that may be involved in the release of prolactin include vasoactive intestinal peptide (VlP) and PHM-27, the latter being a peptide with structural homology to VIP.
