Direct influence of melatonin on the thyroid and comparison with prolactin

Abstract

Melatonin administered in vivo had previously been shown to inhibit thyroid cell proliferation and subsequent in vitro thyroxine (T4) 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 T4 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 μg/ml) media. Melatonin directly inhibited T4 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 μg/ml, did not influence the response of the thyroid to TSH (0.2 μg/ml) in young tadpoles, or the baseline secretion of T4 by thyroids at any stage of larval life except climax, when T4 secretion was significantly decreased by the third day of culture. Thus although both melatonin and PRL have been shown to antagonize the action of T4 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. © 2000 Wiley‐Liss, Inc.

Melatonin Receptor (Mt1 and Mt2) Proteins Expression in Thyroid Gland and Level of Thyroxin (T4), Thyrotropin (Tsh) Hormone During Progression of Age in Male Swiss Albino Mice

“…Moreover, aging also causes the declining of the melatonin hormone which is well known for maintenance of immune function, contributing to the onset of the immunosenescence and age-related diseases 46 . Melatonin also plays an important role in intra-thyroid hormone production 27,47 . Melatonin exerts many physiological actions through its two high affinity membrane bound receptors, MT1 and MT2 in mammals.…”

“…Melatonin may regulate the thyroid gland activity through stimulatory effect on hypothalamus-pituitary axis, playing a modulatory action on HPT axis and performing as a servo-mechanism that diminishes or increases responses when stimuli are respectively too strong or too low 25 . Besides, melatonin has a direct inhibitory effect on T 4 secretion and also, suppresses the response of the thyroid to TSH 26,27 . Furthermore, melatonin has also play a protective role against oxidative stress in the rat thyroid gland [28][29][30] .…”

Role of Melatonin in the Mechanisms Underlying Neuroendocrine Control of the Hypothalamo-Hypophyseal-Thyroid Axis

“…According to the existing concepts, biologically active substances of epiphyseal nature are also directly involved, together with TRH and TTH, in the mechanisms underlying central neuroendocrine regulation of morphofunctional activity of the thyroid gland [9,32,[34][35][36][37][38][39].…”

Circadian rhythms of In Vitro incorporation of 3H-melatonin into organs of the hypothalamic-pituitary-thyroid system in rats

“…A peculiar feature of the pineal gland hormone melatonin (MT) is that normally its inhibitory effect is observed at all levels of HPTS functioning: injection of MT inhibits the formation of thyrotropinreleasing hormone in the hypothalamus, of thyrotropin in the pituitary, and thyroid hormones in the thyroid gland, which suggests the central (mediated) effect of MT on thyroid activity. However, in vivo and in vitro experiments demonstrate principal possibility of direct (also inhibitory) effect of MT on intrathyroid hormone production [2,8, 15]. Melatonin modulates virtually all stages of thyroid hormone biosynthesis and metabolism, including iodine capture and incorporation in thyrocytes, its organification, synthesis and secretion of thyroxine and its metabolic transformations; the inhibitory effect of MT on triiodothyronine secretion is less pronounced.…”