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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.

Ostrom KM

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 01013.

[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 Wiley-Liss, Inc.
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.