Prolactin (PRL) or Luteotropic hormone (LTH) is a
peptide hormoneprimarily associated with lactation. In breastfeeding, the act of an infantsuckling the nipplestimulates the production of prolactin, which fills the breastwith milkvia a process called lactogenesis, in preparation for the next feed. Oxytocin, another hormone, is also released, which triggers milk let-down.
Production and regulation
Prolactin or luteotropic hormone is synthesised and secreted by
lactotropecells in the adenohypophysis(anterior pituitary gland). It is also produced in other tissues including the breast, the decidua, parts of the central nervous systemand the immune system. citation | author = Mancini, T.
year = 2008 | title = Hyperprolactinemia and Prolactinomas | journal = Endocrinology & Metabolism Clinics of North America | volume = 37 | pages = 67 | doi = 10.1016/j.ecl.2007.10.013
pmid = 18226731 ] The gene encoding prolactin in humans is located on
Pituitary prolactin secretion is regulated by neuroendocrine neurons in the
hypothalamus, the most important ones being the neurosecretory tuberoinfundibulum (TIDA) neurons of the arcuate nucleus, which secrete dopamineto act on the dopamine-2 receptors (D2-R) of lactotrophs, causing inhibition of prolactin secretion. Thyrotropin-releasing factorhas a stimulatory effect on prolactin release. Vasoactive intestinal peptideand peptide histidine isoleucinehelp to regulate prolactin secretion in humans, but the functions of these hormones in birds can be quite different. [cite journal |author=Kulick R, Chaiseha Y, Kang S, Rozenboim I, El Halawani M |title=The relative importance of vasoactive intestinal peptide and peptide histidine isoleucine as physiological regulators of prolactin in the domestic turkey |journal=Gen Comp Endocrinol |volume=142 |issue=3 |pages=267–73 |year=2005 |pmid=15935152 |doi=10.1016/j.ygcen.2004.12.024]
Prolactin has many effects including regulating
lactation, orgasms, and stimulating proliferation of oligodendrocyte precursor cells.
It stimulates the
mammary glands to produce milk ( lactation): Increased serum concentrations of prolactin during pregnancycause enlargement of the mammary glands of the breasts and increases the production of milk. However, the high levels of progesteroneduring pregnancy act directly on the breasts to stop ejection of milk. It is only when the levels of this hormone fall after childbirth that milk ejection is possible. Sometimes, newborn babies (males as well as females) secrete a milky substance from their nipples. This substance is commonly known as Witch's milk. This is caused by the fetus being affected by prolactin circulating in the mother just before birth, and usually stops soon after birth.
Prolactin provides the body with sexual gratification after sexual acts: The
hormonecounteracts the effect of dopamine, which is responsible for sexual arousal. This is thought to cause the sexual refractory period. [ [http://www.newscientist.com/channel/sex/mg18925405.900.html New Scientist article on prolactin function relating to sex] - University of Paisleyand the ETH Zürich] 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).Prolactin also stimulates proliferation of oligodendrocyte precursor cells. These cells differentiate into oligodendrocytes, the cells responsible for the formation of myelincoatings on axons in the central nervous system. [cite journal |author=Gregg C, Shikar V, Larsen P, "et al" |title=White matter plasticity and enhanced remyelination in the maternal CNS |journal=J. Neurosci. |volume=27 |issue=8 |pages=1812–23 |year=2007 |pmid=17314279 |doi=10.1523/JNEUROSCI.4441-06.2007 |url=]
Prolactin also has a number of other effects including contributing to
surfactantsynthesis of the fetal lungs at the end of the pregnancy and immune toleranceof the fetus by the maternal organism during pregnancy;it also decreases normal levels of sex hormones — estrogenin women and testosteronein men." [ [http://www.mayoclinic.com/health/prolactinoma/DS00532 Prolactinoma] - Mayo Clinic] . Prolactin delays hair regrowth in mice [cite journal |author=Craven AJ, Nixon AJ, Ashby MG, "et al" |title=Prolactin delays hair regrowth in mice |journal=J. Endocrinol. |volume=191 |issue=2 |pages=415–25 |year=2006 |month=November |pmid=17088411 |doi=10.1677/joe.1.06685 |url=http://joe.endocrinology-journals.org/cgi/content/abstract/191/2/415] .
Variance in levels
pregnancy, high circulating concentrations of estrogenpromote prolactin production. The resulting high levels of prolactin secretion cause further maturation of the mammary glands, preparing them for lactation.
childbirth, prolactin levels fall as the internal stimulus for them is removed. Sucking by the baby on the nipple then promotes further prolactin release, maintaining the ability to lactate. The sucking activates mechanoreceptors in and around the nipple. These signals are carried by nerve fibers through the spinal cord to the hypothalamus, where changes in the electrical activity of neurons that regulate the pituitary gland cause increased prolactin secretion. The suckling stimulus also triggers the release of oxytocinfrom the posterior pituitary gland, which triggers milk let-down: Prolactin controls milk production (lactogenesis) but not the milk-ejection reflex; the rise in prolactin fills the breast with milk in preparation for the next feed.
In usual circumstances, in the absence of
galactorrhea, lactation will cease within one or two weeks of the end of demand breastfeeding.
High prolactin levels also tend to suppress the ovulatory cycle by inhibiting the secretion of both follicle-stimulating hormone (
FSH) and gonadotropic-releasing hormone ( GnRH).
Prolactin levels peak during REM sleep, and in the early morning. Levels can rise after exercise, meals, sexual intercourse, or minor surgical procedures. [cite book |author=Melmed S, Jameson JL |chapter=333 Disorders of the Anterior Pituitary and Hypothalamus |editor=Jameson JN, Kasper DL, Harrison TR, Braunwald E, Fauci AS, Hauser SL, Longo DL. |title=Harrison's principles of internal medicine |publisher=McGraw-Hill Medical Publishing Division |location=New York |year=2005 |edition=16th ed. |pages= |isbn=0-07-140235-7 |oclc= |doi= |accessdate= |url=http://highered.mcgraw-hill.com/sites/0071402357/information_center_view0/]
Prolactin is a single-chain
polypeptideof 199 amino acids with a molecular weight of about 24,000 daltons. Its structure is similar to that of growth hormoneand placental lactogen. The molecule is folded due to the activity of three disulfide bonds. Significant heterogeneity of the molecule has been described, thus bioassays and immunoassays can give different results due to differing glycosylation, phosphorylation, sulfation, as well as degradation. The non-glycosylated form of prolactin is the dominant form of prolactin that is secreted by the pituitary gland.
Little prolactin is apparently the result of removal of some amino acids, whereas big prolactin can be the product of interaction of several prolactin molecules.
Pit-1 is a
transcription factorthat binds to the prolactin gene at several sites to allow for the production of prolactin in the pituitary gland. A key regulator of prolactin production is estrogens that enhance growth of prolactin-producing cells and stimulate prolactin production directly, as well as suppressing dopamine.
Human prolactin receptors are insensitive to mouse prolactin [cite journal |author=Utama FE, LeBaron MJ, Neilson LM, "et al" |title=Human prolactin receptors are insensitive to mouse prolactin: implications for xenotransplant modeling of human breast cancer in mice |journal=J. Endocrinol. |volume=188 |issue=3 |pages=589–601 |year=2006 |pmid=16522738 |doi=10.1677/joe.1.06560 |url=http://joe.endocrinology-journals.org/cgi/content/abstract/188/3/589] .
Prolactin receptors are present in the mamillary glands, ovaries, pituitary glands, heart, lung, thymus, spleen, liver, pancreas, kidney, adrenal gland, uterus, skeletal muscle, skin and areas of the central nervous system. When prolactin binds to the receptor it causes it to dimerize with another prolactin receptor. This results in the activation of
Janus kinase 2a tyrosine kinase which initiates the JAK-STAT pathway. The activation of the prolactin receptor also results in the activation of mitogen-activated protein kinasesand Src kinase.
Prolactin levels may be checked as part of a sex hormone workup, as elevated prolactin secretion can suppress the secretion of FSH and GnRH, leading to
hypogonadism, and sometimes causing erectile dysfunctionin men.
Prolactin levels may be of some use in distinguishing epileptic seizures from
psychogenic non-epileptic seizures. The serum prolactin level usually rises following an epileptic seizure. [cite journal |author=Banerjee S, Paul P, Talib V |title=Serum prolactin in seizure disorders |journal=Indian Pediatr |volume=41 |issue=8 |pages=827–31 |year=2004 |pmid=15347871]
Conditions causing elevated prolactin secretion
Hyperprolactinaemiais the term given to having too-high levels of prolactin in the blood.
thyrotropin-releasing hormone(TRH), usually in primary hypothyroidism
* Emotional stress
Conditions causing decreased prolactin
Use of breastfeeding as contraceptive
World Health Organizationstates that demand breastfeeding is more than 98% effective as a contraceptivein the first six months postpartum. This effect is said to be responsible for the natural spacing of children seen in countries where contraception is not widely available, and is thought to be an evolutionary means of ensuring adequate care is provided to each newborn. The 98% effectiveness applies only if three criteria are met:
# The mother has had no menstrual periods at all (
# The baby is exclusively breast-fed
# It is six months or less since birth.
If one or more of these conditions are broken, lactational amenorrhea is no longer a reliable form of birth control. This contraceptive method is highly effective as long as the three conditions stated above are fulfilled. Further, the WHO suggests that a woman that is still amenorrheic has a less-than-5% chance of getting pregnant in the first year of her baby's life, as long as she is still breastfeeding on demand.
Psychogenic non-epileptic seizures
* [http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/hypopit/prolactin.html Pathophysiology of Endocrine System - Prolactin at colostate.edu]
* - "Hyperprolactinemia"
* - "Prolactin Deficiency"
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