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Prolactin is a hormone produced in the pituitary that
stimulates the mammary glands to produce milk. Prolactin has other
functions including an essential role in the maintenance of immune system
functions. Prolactin levels rise during the latter part of pregnancy but
the effects on lactation are suppressed by high levels of progesterone.
Once progesterone drops at childbirth, milk secretion begins.
The second study below is interesting because it suggests
that copper from a copper IUD can stimulate lactation. Whether it
increases prolactin is unclear from the study but the possibility is there that
copper causes an increase in prolactin production.
Prog Food Nutr Sci 1990;14(1):1-43
A review of the hormone prolactin during lactation.
Department of Nutritional Sciences, University of Connecticut, Storrs.
The principal lactogenic hormone, prolactin, secreted by the anterior
pituitary is critical to the establishment of lactation, milk macronutrient
content and milk production. The concentration of circulating prolactin
increases during pregnancy so that by the end of gestation, levels are 10 to
20 times over normal amounts. However, prolactin is prevented from exerting
its effect on milk secretion by elevated levels of progesterone. Following
clearance of progesterone and estrogen at parturition, copious milk
secretion begins. The minimal hormonal requirements for normal lactation to
occur are prolactin, insulin and hydrocortisone. Prolactin stabilizes and
promotes transcription of casein mRNA; may stimulate synthesis of alpha-lactalbumin,
the regulatory protein of the lactose synthetase enzyme system; and
increases lipoprotein lipase activity in the mammary gland. Prolactin levels
decrease as lactation is established but nursing stimulates prolactin
release from the pituitary which promotes continued milk production.
Prolactin is secreted into milk at levels representative of the average
circulating concentration. The physiological significance of milk prolactin
to the infant is uncertain. Prolactin exists in three heterogenic forms
which possess varying biological activity. The monomer with a molecular
weight of 23 kDa is found in greatest quantity and is the principal
biologically active form. The pattern of heterogeneity changes during
pregnancy to favor even more monomer in proportion to the dimer. However,
during lactation, the proportion of the monomer in circulation decreases in
response to selective uptake of the monomer by the mammary gland. Over 90
percent of the prolactin in milk is present as the monomer. Prolactin may
exert some of its biological effect by a shift in the ratio of active to
less active forms of the molecule.
- Normoprolactinemic galactorrhea in a fertile woman with a copper
intra-uterine device (copper IUD).
- Giampietro O; Ramacciotti C; Moggi G
- Acta Obstet Gynecol Scand, 63(1):23-5 1984
We report a case of galactorrhea in a normoprolactinemic fertile woman (30
years old) wearing a copper intra-uterine device (Gravigard). The
Gravigard was first inserted in July 1977. In February 1979 our patient noted
spontaneous galactorrhea, mainly on the left, but it was also present on the
right, after breast pressure. X-ray film of the sella turcica, visual-field
examination, thyroid function and basal prolactin levels were all within normal
limits. In May 1979 the Gravigard was withdrawn and milk loss stopped finally in
December 1979. In March 1980 the IUD was replaced; after only 3 days, mild
spontaneous lactation again ensued, on the right side. The patient never took
drugs which might have occasioned a prolactin rise. Possible explanations for
this unusual phenomenon are discussed.
|J Exp Zool 2000 May;286(6):625-631
Direct influence of melatonin on the thyroid and
comparison with prolactin.
Wright ML, Cuthbert KL, Donohue MJ, Solano SD, Proctor KL
Biology Department, College of Our Lady of the Elms, Chicopee, Massachusetts
[Record supplied by publisher]
Melatonin administered in vivo had previously been shown to inhibit thyroid
cell proliferation and subsequent in vitro thyroxine (T(4)) secretion in
anuran tadpoles. Melatonin in vitro also directly reduced the sensitivity of
the thyroid to thyrotropin (TSH). The present work sought to determine
whether melatonin directly affected baseline, unstimulated T(4) secretion,
and to compare its effect with that of prolactin (PRL). Thyroids from larval
Rana catesbeiana or adult Rana pipiens were incubated in control or
melatonin (0.01 to 100 mug/ml) media. Melatonin directly inhibited T(4)
secretion by thyroids from both tadpoles and frogs at all concentrations of
melatonin used and at both prometamorphic and climax tadpole stages. PRL,
used in vitro at 10 mug/ml, did not influence the response of the thyroid to
TSH (0.2 mug/ml) in young tadpoles, or the baseline secretion of T(4) by
thyroids at any stage of larval life except climax, when T(4) secretion was
significantly decreased by the third day of culture. Thus although both
melatonin and PRL have been shown to antagonize the action of T(4) in vitro,
and to decrease metamorphic rate, melatonin is a much more effective thyroid
gland inhibitor than PRL. J. Exp. Zool. 286:625-631, 2000. Copyright 2000
|Physiol Behav 2000 Jun 1-15;69(4-5):391-7
Hypothyroidism increases prolactin secretion and
decreases the intromission threshold for induction of pseudopregnancy in
adult female rats.
Tohei A, Taya K, Watanabe G, Voogt JL
Laboratory of Veterinary Physiology, Tokyo University of Agriculture and
Technology, Fuchu, 183-8509, Tokyo, Japan.
[Medline record in process]
In order to understand the mechanism by which thyroid hormones alter
prolactin (PRL) secretion, we investigated the role of tuberoinfundibular
dopamine (TIDA) neurons and pituitary and hypothalamus vasoactive intestinal
peptide (VIP) in thiouracil- (0. 03% in drinking water for 16 days)
induced-hypothyroid adult female rats. The intromission threshold for
induction of pseudopregnancy also was examined to evaluate the PRL response
to coital stimulation in hypothyroid rats. Hypothyroidism in adult female
rats did not affect TIDA neuronal activity as measured by tyrosine
hydroxylase activity (DOPA accumulation 30 min after administration of m-hydroxybenzylhydrazine
dihydrochloride, 100 mg/kg, i.p.) in the stalk-median eminence compared with
that in euthyroid rats, whereas pituitary concentration of VIP was
dramatically increased. Plasma concentration of PRL was higher at 1100 h of
proestrus and estrus in hypothyroid rats as compared with that of euthyroid
rats. The proportion of female rats exhibiting pseudopregnancy was higher in
hypothyroid animals (100%) receiving seven intromissions than in euthyroid
animals (43%). Administration of L-thyroxine in hypothyroid rats decreased
the proportion of pseudopregnancy (40%) to the level of euthyroid animals.
These results indicate that the increased level of pituitary VIP probably
affects PRL secretion in a paracrine or autocrine manner and account for the
hyperprolactinemia induced in hypothyroid female rats. No role for TIDA
neurons in PRL elevation can be ascribed. A decrease in the intromission
threshold for induction of pseudopregnancy might be due to increased levels
of PRL in hypothyroid female rats.