Hypothyroidism increases prolactin secretion and decreases the intromission threshold for induction of pseudopregnancy in adult female rats.
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.
Laboratory of Veterinary Physiology, Tokyo University of Agriculture and Technology, Fuchu, 183-8509, Tokyo, Japan. tohei@ps.toyaku.ac.jp
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Tohei
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Taya
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Watanabe
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Voogt
D000818:Animals; D013956:Antithyroid Agents / Q000494:pharmacology; D004295:Dihydroxyphenylalanine / Q000378:metabolism; D004971:Estrus / Q000097:blood; D005260:Female; D007037:Hypothyroidism / Q000139:chemically induced / Q000378:metabolism*; D008297:Male; D008473:Median Eminence / Q000378:metabolism; D010060:Ovulation / Q000187:drug effects; D010286:Paraventricular Hypothalamic Nucleus / Q000378:metabolism; D010903:Pituitary Gland, Anterior / Q000378:metabolism; D011388:Prolactin / Q000097:blood / Q000378:metabolism* / Q000494:pharmacology; D011555:Pseudopregnancy / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012726:Sexual Behavior, Animal / Q000187:drug effects; D013889:Thiouracil / Q000494:pharmacology; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism; D014660:Vasoactive Intestinal Peptide / Q000378:metabolism
D013956:Antithyroid Agents; D014284:Triiodothyronine; D014660:Vasoactive Intestinal Peptide; D013889:Thiouracil; D004295:Dihydroxyphenylalanine; D011388:Prolactin; D013972:Thyrotropin; D014446:Tyrosine 3-Monooxygenase; D013974:Thyroxine