In humans 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. In
music psychology, it is conjectured that prolactin may play a role in the pleasurable perception of sad music, as the levels of the hormone increase when a person feels sad, producing a consoling psychological effect.
In other vertebrates The primary function of prolactin in
fish is
osmoregulation, i.e., controlling the movement of water and salts between the tissues of the fish and the surrounding water. Like mammals, however, prolactin in fish also has reproductive functions, including promoting sexual maturation and inducing breeding cycles, as well as brooding and parental care. In the South American
discus, prolactin may also regulate the production of a skin secretion that provides food for larval
fry. An increase in brooding behaviour caused by prolactin has been reported in
hens. Prolactin and
its receptor are expressed in the skin, specifically in the hair follicles, where they regulate hair growth and moulting in an
autocrine fashion. Elevated levels of prolactin can inhibit hair growth, and knock-out
mutations in the prolactin gene cause increased hair length in cattle and mice. Additionally, prolactin delays hair regrowth in mice. Analogous to its effects on hair growth and shedding in mammals, prolactin in birds controls the
moulting of feathers, as well as the age at onset of feathering in both turkeys and chickens.
Pigeons,
flamingos and male
emperor penguins feed their young a cheese-like secretion from the upper digestive tract called
crop milk, whose production is regulated by prolactin. In rodents,
pseudopregnancy can occur when a female is mated with a sterile male. This mating can cause bi-daily surges of prolactin which would normally occur in rodent pregnancy. Prolactin surges initiate the secretion of progesterone which maintains pregnancy and hence can initiate pseudopregnancy. The false maintenance of pregnancy exhibits the outward physical symptoms of pregnancy, in the absence of a foetus. Prolactin receptor activation is essential for normal
mammary gland development during
puberty in mice. Adult virgin female prolactin receptor
knockout mice have much smaller and less developed mammary glands than their
wild-type counterparts. Prolactin and prolactin receptor signaling are also essential for maturation of the mammary glands during pregnancy in mice. == Regulation ==