Prolactin

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Prolactin
Identifiers
Symbol PRL
Entrez 5617
HUGO 9445
OMIM 176760
RefSeq NM_000948
UniProt P01236
Other data
Locus Chr. 6 p22.2-p21.3

Prolactin (PRL) is a peptide hormone primarily associated with lactation. In breastfeeding, the act of an infant suckling the nipple stimulates the production of prolactin, which fills the breast with milk via a process called lactogenesis, in preparation for the next feed. Oxytocin, another hormone, is also released, which triggers milk let-down.

Contents

[edit] Production and regulation

Prolactin is synthesised and secreted by lactotrope cells in the adenohypophysis (anterior pituitary gland). It is also produced in other tissues including the breast and the decidua.

Pituitary prolactin secretion is regulated by neuroendocrine neurons in the hypothalamus, the most important ones being the neurosecretory dopamine neurons of the arcuate nucleus, which inhibit prolactin secretion. Thyrotropin-releasing factor has a stimulatory effect on prolactin release.

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.[1]

[edit] Effects

Prolactin has many effects:

  • To stimulate the mammary glands to produce milk (lactation): Increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and increases the production of milk. However, the high levels of progesterone during 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.
  • To contribute to 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[5].
  • In the rat, Dopamine inhibits plasma Prolactin. Production of prolactin is transiently inhibited by L-dopa. Also plasma prolactin levels are decreased (in 30 mins) and restored (in 3 hours) by L-dopa by a peripheral processes (i.e., tissues outside pituitary). [6]

[edit] Variance in levels

There is a diurnal as well as an ovulatory cycle in prolactin secretion.

During pregnancy, high circulating concentrations of estrogen promote prolactin production. The resulting high levels of prolactin secretion cause further maturation of the mammary glands, preparing them for lactation.

After 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 oxytocin from 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.[7]

[edit] Structure

Prolactin is a single-chain polypeptide of 199 amino acids with a molecular weight of about 24,000 daltons. Its structure is similar to that of growth hormone and 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 factor that 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[8].

[edit] Prolactin receptor

See prolactin receptor

[edit] Diagnostic use

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 dysfunction in 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.[9]

[edit] Conditions causing elevated prolactin secretion

Hyperprolactinaemia is the term given to having too-high levels of prolactin in the blood.

[edit] Conditions causing decreased prolactin

[edit] Use of breastfeeding as contraceptive

The World Health Organization states that demand breastfeeding is more than 98% effective as a contraceptive in 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:

  1. The mother has had no menstrual periods at all (amenorrhea)
  2. The baby is exclusively breast-fed
  3. 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.

[edit] See also

[edit] References

  1. ^ Kulick R, Chaiseha Y, Kang S, Rozenboim I, El Halawani M (2005). "The relative importance of vasoactive intestinal peptide and peptide histidine isoleucine as physiological regulators of prolactin in the domestic turkey". Gen Comp Endocrinol 142 (3): 267-73. doi:10.1016/j.ygcen.2004.12.024. PMID 15935152. 
  2. ^ New Scientist article on prolactin function relating to sex - University of Paisley and the ETH Zürich
  3. ^ Gregg, C. et al. (2007). White Matter Plasticity and Enhanced Remyelination in the Maternal CNS. Journal of Neuroscience 27(8): 1812-1823.
  4. ^ Prolactinoma - Mayo Clinic
  5. ^ Prolactin delays hair regrowth in mice - Craven et al. 191 (2): 415 - Journal of Endocrinology
  6. ^ Prolactin inhibition test with L-dopa: decrease and restoration of plasma prolactin levels in the rat by a peripheral process - van der Gugten et al. 68 (3): 369 - Journal of Endocrinology
  7. ^ Melmed S, Jameson JL. In: Harrison’s Principles of Internal Medicine. 16th ed. New York, NY:McGraw-Hill Professional; 2004
  8. ^ Human prolactin receptors are insensitive to mouse prolactin: implications for xenotransplant modeling of human breast cancer in mice - Utama et al. 188 (3): 589 - Journal of Endocrinology
  9. ^ Banerjee S, Paul P, Talib V (2004). "Serum prolactin in seizure disorders". Indian Pediatr 41 (8): 827-31. PMID 15347871. 

[edit] External links

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Endocrine system: hormones/endocrine glands (Peptide hormones, Steroid hormones)
Hypothalamic-pituitary
Adrenal axis
Thyroid axis
Gonadal axis
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Non-end. glands
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