The structure and hormonal activity of the thyroid gland and its capacity to bind (3)H-melatonin were studied in young adult Chinchilla rabbits with pineal gland hypofunction induced by 2-month exposure to constant illumination. After 2 months of constant exposure to light, the experimental animals exhibited signs of the so-called "latent hypothyroidism" and more intense binding of (3)H-melatonin by the thyroid gland. This fact indicated intactness of its receptor system underlying the possibility of restoring activity of the thyroid gland under conditions of melatonin replacement therapy in hypothyroidism induced by chronic melatonin insufficiency.
The pituitary-thyroid axis of young sexually mature rabbits kept under a 24-hour daylight photoperiod was shown to undergo phase-modulated variations of hormonal activity with its initial increase (during the first month) and subsequent progressive decrease (within 2-5 months after the onset of exposure to light). These changes correlated with the time-dependent fall in the blood T3, T4, and TSH levels. Simultaneously, the animals developed pathological changes in the histological structure of the thyroid gland similar to those in patients with secondary or tertiary hypothyroidism. It is concluded that hormonal and structural changes in the thyroid gland during long-term hypopinealism should be regarded as an experimental model of hypothyroidism of neuroendocrine origin.
The authors of this review generalize and analyze the published data, as well as their own findings, related to melatonin-mediated interaction of the pineal gland (the brain epiphysis) with the hypothalamo-hypophysealthyroid axis (HHTA). The review deals with (i) historical aspect of development of the concepts on neuroendocrine mechanisms underlying the control of functions of the thyroid gland, (ii) data on circadian and seasonal rhythms of the functioning of the epiphysis and HHTA, (iii) basic facts about direct and feedback connections between the brain epiphysis and HHTA, and (iv) overview of the experimental data on age-related changes in the epiphysis-thyroid axis.
In the literature of recent years it was widely discussed the issue of violation of the light mode as a risk factor for the formation of obesity — one of the leading symptoms of metabolic syndrome and type 2 diabetes. However, until the real time, the effect of the long round-the-clock illumination of different duration on histological and leptinproducing activity of adipose tissue remains unclear. Studies of the dynamics of weight indicators, histological characteristics and the level of leptin in the blood plasma of rats subjected to long round-the-clock illumination, for 3, 5 and 10 months, led to the conclusion that, originally, animals subjected to long round-the-clock illumination observed an activation of hyperplastic processes in adipose tissue and increasing of its leptinsynthesizing activity. Long round-the-clock illumination as an inducer of expressed hypopinealizm is the cause hypoplasia of adipose tissue, a sharp drop of its cytokineproducing function and the development of its nonspecific inflammation.
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