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Prolactin (PRL), also known as luteotropic hormone or luteotropin, is a protein that in humans is best known for its role in enabling mammals, usually females, to produce milk; however, it is influential over a large number of functions with over 300 separate actions of PRL having been reported in various vertebrates.〔 Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation, and nursing. Prolactin is secreted in a pulsatile fashion in between these events. Prolactin also plays an essential role in metabolism, regulation of the immune system, and pancreatic development. Discovered in non-human animals around 1930 by Oscar Riddle 〔 〕 at Cold Spring Harbor Laboratory on Long Island, New York, and confirmed in humans in 1970 by Henry Friesen 〔 〕 prolactin is a peptide hormone, encoded by the ''PRL'' gene. 〔 〕 Although often associated with human milk production, prolactin plays a wide range of other roles in both humans and other vertebrates. (For example, in fish—the oldest known vertebrates—an important function is probably related to control of water and salt balance.) Prolactin also acts in a cytokine-like manner and as an important regulator 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 also has profound influence on hematopoiesis, angiogenesis and is involved in the regulation of blood clotting through several pathways. The hormone acts in endocrine, autocrine, and paracrine manner through the prolactin receptor and a large number of cytokine receptors. Pituitary prolactin secretion is regulated by endocrine neurons in the hypothalamus, the most important ones being the neurosecretory tuberoinfundibulum (TIDA) neurons of the arcuate nucleus, which secrete dopamine (aka Prolactin Inhibitory Hormone) to act on the D2 receptors of lactotrophs, causing inhibition of prolactin secretion. Thyrotropin-releasing factor (thyrotropin-releasing hormone) has a stimulatory effect on prolactin release, however Prl is the only adenohypophyseal 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 22 kDa) and several larger (so called big and big-big) variants exist. == Effects == Prolactin has a wide range of effects. It stimulates the mammary glands to produce milk (lactation): increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and prepare for the production of milk. Milk production normally starts when the levels of progesterone fall by the end of pregnancy and a suckling stimulus is present. Sometimes, newborn babies (males as well as females) secrete a milky substance from their nipples known as witch's milk. This is in part caused by maternal prolactin and other hormones. Prolactin also has been found to play an important role in maternal behavior. Prolactin provides the body with sexual gratification after sexual acts: The hormone counteracts the effect of dopamine, which is responsible for sexual arousal. This is thought to cause the sexual refractory period. The amount of prolactin can be an indicator for the amount of sexual satisfaction and relaxation. Unusually high amounts are suspected to be responsible for impotence and loss of libido (see hyperprolactinemia symptoms). Highly elevated levels of prolactin decrease the levels of sex hormones — estrogen in women and testosterone in men.〔(Prolactinoma )—Mayo Clinic〕 The effects of mildly elevated levels of prolactin are much more variable, in women both substantial increase or decrease of estrogen levels may result. 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. The exact mechanism by which it inhibits GnRH is poorly understood although expression of prolactin receptors (PRL-R) have been demonstrated in rat's hypothalamus, the same has not been observed in GnRH 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. Prolactin also has a number of other effects including 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 delays hair regrowth in mice. Prolactin promotes neurogenesis in maternal and fetal brains. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「prolactin」の詳細全文を読む スポンサード リンク
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