hypothalamic–pituitary–prolactin axis hypothalamic–pituitary–prolactin axis the hypothalamic–pituitary–prolactin axis (hpp axis), also known as the hypothalamic–pituitary–mammary axis or hypothalamic–pituitary–breast axis, is a hypothalamic–pituitary axis which includes the secretion of prolactin (prl; luteotropin) from the lactotrophs of the pituitary gland into the circulation and the subsequent action of prolactin on tissues such as, particularly, the mammary glands or breasts. it is involved in lobuloalveolar maturation of the mammary glands during pregnancy and the induction and maintenance of lactation following parturition. hormones that control the secretion of prolactin from the pituitary gland include dopamine ("prolactin-inhibiting factor", or "pif"), estradiol, progesterone, thyrotropin-releasing hormone (trh), and vasoactive intestinal peptide (vip). prolactin, a major hormone of the hpp axis.