Prolactin

Clinic

• PRL also known as lactotropin, is a protein best known for its role in enabling mammals to produce milk.
• It is influential in over 300 separate processes in various vertebrates, including humans.
• PRL 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.
• It plays an essential role in metabolism, regulation of the immune system and pancreatic development.
• It is associated with milk production
• It also acts in regulating of the immune system.
• It has important cell cycle-related functions as a growth, differentiating and anti-apoptotic factor.
• As a growth factor, binding to cytokine-like receptors, it influences hematopoiesis and angiogenesis and is involved in the regulation of blood clotting through several pathways.

• Pituitary prolactin secretion is regulated by endocrine neurons in the hypothalamus.
• The most important of these are the neurosecretory tuberoinfundibulum (TIDA) neurons of the arcuate nucleus that secrete dopamine (a.k.a. Prolactin Inhibitory Hormone) to act on the D₂ receptors of lactotrophs, causing inhibition of prolactin secretion.
• TRH has a stimulatory effect on prolactin release, although prolactin is the only anterior pituitary hormone whose principal control is inhibitory.

Several variants and forms are known per species. Many fish have variants prolactin A and prolactin B. Most vertebrates, including humans, also have the closely related somatolactin. In humans, three smaller (4, 16, and 23 kDa) and several larger (so-called big and big-big) variants exist.^{[not verified in body]}

Function

• 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.

• 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 GnRH secretion.
• 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 CNS
• 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.