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MELATONIN

 

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.