Prolactin (PRL), also known as lactotrope, is a protein that in humans is encoded by the PRL gene.
Prolactin is a peptide hormone discovered by Oscar Riddle and important later work was done by Henry Friesen. Although it is perhaps best known for its role in lactation, prolactin already existed in the oldest known vertebrates—fish—where its most important functions were 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. Prolactin has important cell cycle related functions as a growth-, differentiating- and anti-apoptotic factor. As a growth factor binding to cytokine like receptors it has also profound influence on hematopoiesis, angiogenesis and is involved in the regulation of blood clotting through several pathways. In summary, “more than 300 separate actions of PRL have been reported in various vertebrates, including effects on water and salt balance, growth and development, endocrinology and metabolism, brain and behavior, reproduction, and immune regulation and protection”. Prolactin 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 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.[not verified in body]
Vasoactive intestinal peptide and peptide histidine isoleucine help to regulate prolactin secretion in humans, but the functions of these hormones in birds can be quite different.
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.
Several variants and forms are known per species. Many fishes have variants prolactin A and prolactin B. Most vertebrates including humans also have the closely related somatolactin. In humans 3 smaller (4, 16, and 22 kDa) and several larger (so called big and big-big) variants exist.[not verified in body]