Briefly describe the functions and the regulation of secretion of prolactin.

Outline:

·        Synthesis and release

·        Regulators of secretion

·        Actions on:

- breast development

- milk production

- gonadotropin production

Essay:

            Prolactin is a single-chain protein of molecular weight 23,000. In humans, prolactin is a protein hormone concerned with breast development and milk production. In addition, it exerts an influence on reproductive function and immune responses.

            Prolactin secretion is tonically inhibited by the hypothalamus. Dopamine is the prolactin inhibiting factor which inhibits prolactin release. A dopaminergic tract runs from the hypothalamus to the median eminence. The other hypothalamic inhibitor is somatostatin. Prolactin inhibits its own secretion via a short-loop feedback. It does so directly by increasing the synthesis and release of dopamine.

            Prolactin secretion increases steadily during pregnancy. This increase is mediated by the large increase in estrogen, which stimulates hyperplasia of prolactin-producing cells and synthesis of the hormone by inducing transcription of the gene. Prolactin secretion from mammotrophs is also stimulated in paracrine fashion by the presence of neighboring gonadotrophs, which are themselves responding to GnRH. These gonadotrophs, which release angiotensin II, increase prolactin release by nearby mammortrophs.

            Prolactin binds to a receptor belonging to a superfamily that includes the growth hormone receptor and receptor for many cytokines and hematopoietic growth factors. Prolactin participates in stimulating the original development of breast tissue and its further hyperplasia during pregnancy. It is the principal hormone responsible for lactogenesis. Before and after puberty, prolactin, together with estrogens, progesterone, cortisol and growth hormone, stimulates the proliferation and branching of ducts in the female breast. During pregnancy, prolactin, along with estrogen and progesterone, causes the development of lobules of alveoli within which milk is produced. Finally, after parturition, prolactin, together with insulin and cortisol, stimulates milk synthesis and secretion. Prolactin induces the transcription of genes for the milk proteins casein, lactalbumin and b-lactoglobulin, and also stabilizes their mRNAs.

            After parturition, prolactin suppresses reproductive function by inhibiting GnRH secretion and its action on the pituitary, antagonizing the action of gonadotropins on the ovaries. Ovulation is inhibited, and the ovaries are inactive, so estrogen and progesterone output falls to low levels.