Prolactin

Prolactin (PRL) is secreted by lactotrophs of the anterior lobe. The PRL is a glycoprotein with a alike amino acid series to growth hormone. It is one of numerous hormones which stimulate the growth of the mammary glands during pregnancy. The PRL concentrations are maximum during lactation and pregnancy. Suckling generates a reflex secretion of prolactin that stimulates the synthesis and secretion of milk. Similar to growth hormone, exercise and stress stimulate prolactin discharge.

PRL synthesis and secretion are inhibited by dopamine discharged into the portal system by dopaminergic neurons in the arcuate nucleus. Lactotrophs of the anterior pituitary express D2 dopamine receptors. These are GPCRs at which dopamine generates a fall in cAMP concentration, thereby decreasing transcription of the prolactin gene.PRL discharge is stimulated physiologically by:

- Vasoactive intestinal peptide (VIP) secreted by hypothalamic neurons into the portal system. This is the major short-term PRL discharging factor.

- Estrogens (generate a mid-cycle peak in PRL secretion); significant in late pregnancy.

- Oxytocin from the posterior pituitary that gains access to the anterior lobe through tiny blood vessels known as short portal vessels. This is significant in lactating women.

With common suckling (every 2–3 hours) the amount of prolactin discharged is adequate to block ovulation by suppressing LH and GnRH secretion, therefore lactating women are somewhat infertile.