Prolactin (PRL), also known as lactotropin, is a protein best known for its role in enabling mammals to produce milk. Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation, and nursing. It is secreted heavily in pulses in between these events. Prolactin plays an essential role in metabolism, regulation of the immune system and pancreatic development.
Prolactin has a wide variety of effects. It stimulates the mammary glands to produce milk (lactation): increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands and prepare for milk production, which normally starts when levels of progesterone fall by the end of pregnancy and a suckling stimulus is present. Prolactin plays an important role in maternal behavior.
It has been shown in rats and sheep that prolactin affects lipid synthesis differentially in mammary and adipose cells. Prolactin deficiency induced by bromocriptine increased lipogenesis and insulin responsiveness in adipocytes while decreasing them in the mammary gland.
In general, dopamine inhibits prolactin but this process has feedback mechanisms.
Elevated levels of prolactin decrease the levels of sex hormones—estrogen in women and testosterone in men. The effects of mildly elevated levels of prolactin are much more variable, in women, substantially increasing or decreasing estrogen levels.
Prolactin is sometimes classified as a gonadotropin although in humans it has only a weak luteotropic effect while the effect of suppressing classical gonadotropic hormones is more important. Prolactin within the normal reference ranges can act as a weak gonadotropin, but at the same time suppresses gonadotropin-releasing hormone secretion. The exact mechanism by which it inhibits gonadotropin-releasing hormone is poorly understood. Although expression of prolactin receptors have been demonstrated in rat hypothalamus, the same has not been observed in gonadotropin-releasing hormone neurons. Physiologic levels of prolactin in males enhance luteinizing hormone-receptors in Leydig cells, resulting in testosterone secretion, which leads to spermatogenesis.
Prolactin also stimulates proliferation of oligodendrocyte precursor cells. These cells differentiate into oligodendrocytes, the cells responsible for the formation of myelin coatings on axons in the central nervous system.
Other actions include contributing to pulmonary surfactant synthesis of the fetal lungs at the end of the pregnancy and immune tolerance of the fetus by the maternal organism during pregnancy. Prolactin promotes neurogenesis in maternal and fetal brains.