'Prolactin' ('PRL') is a
peptide hormone primarily associated with
lactation. In breastfeeding, the
infant suckling the
teat stimulates the production of prolactin, which fills the
breast with
milk (
lactogenesis) in preparation for the next feed.
Oxytocin, a similar hormone, is also released, which
triggers milk let-down.
Production and regulation
It 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, most importantly by 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]
Effects
"The major effect of increased prolactin is a decrease in normal levels of sex hormones — estrogen in women and testosterone in men."
[1]
Prolactin has many effects:
★ The most important of which is 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.
★ Another effect, recently discovered by the
University of Paisley and
the ETH Zürich, is to provide the body with
sexual gratification after
sexual acts. The
hormone represses the effect of
dopamine, which is responsible for
sexual arousal, thus causing the sexual
refractory period.
[2] 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 has been found to stimulate 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.
[3]
★ Other possible functions of prolactin include the
surfactant synthesis of the fetal
lungs at the end of the pregnancy and
immune tolerance of the fetus by the maternal organism during pregnancy.
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 fibres 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.
Usually, 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
FSH and
GnRH.
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, while '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 are
estrogens that enhances growth of prolactin producing cells and stimulates prolactin production directly as well as suppressing
dopamine.
Prolactin receptor
:''See
prolactin receptor''
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.
[4]
Conditions causing elevated prolactin secretion
Hyperprolactinaemia is the term given to having too-high levels of prolactin in the blood.
★
Prolactinoma;
★ Excess
thyrotropin-releasing hormone (TRH), usually in primary
hypothyroidism.
★ A side effect of many
anti-psychotic medications
Conditions causing decreased prolactin
★
Bulimia;
★ Excess of
dopamine.
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 only applies if three criteria are met:
# The mother has had no menstrual periods at all (
amenorrhea);
# 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 above are fulfilled. Further, the WHO suggests that a woman who 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.
See also
★
weaning
★
breast feeding
★
baby food
★
male lactation
★
hormones
References
1. The relative importance of vasoactive intestinal peptide and peptide histidine isoleucine as physiological regulators of prolactin in the domestic turkey, Kulick R, Chaiseha Y, Kang S, Rozenboim I, El Halawani M, , , Gen Comp Endocrinol, 2005
2. New Scientist article on prolactin function relating to sex
3. Gregg, C. et al. (2007). White Matter Plasticity and Enhanced Remyelination in the Maternal CNS. Journal of Neuroscience 27(8): 1812-1823.
4. Serum prolactin in seizure disorders, Banerjee S, Paul P, Talib V, , , Indian Pediatr, 2004
External links
★
Pathophysiology of Endocrine System - Prolactin at colostate.edu
★
★ - "Hyperprolactinemia"
★ - "Prolactin Deficiency"
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