The effect of injection of exogenous melatonin on formation of the nocturnal peak of the melatonin concentration and intrapineal hormonogenesis was studied on mature male rats. It was shown that 3H-melatonin is selectively uptaken by pinealocytes, and the uptake intensity depends on the degree of saturation of an organism with exogenous melatonin. Exogenous melatonin exerts an inhibitory influence on the hormone self-production, at the same time apparently stimulating synthesis of epiphyseal peptides. The results can be considered an indication of the existence of an ultra-short connection between the mechanisms of production of epiphyseal indoles and peptides, which apparently plays an important role in autocrine regulation of the epiphyseal functions.

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The study explores the effect of pineal peptide hormones (epitalamin) on the pituitarythyroid system in cold exposure. It is shown that pineal hormones block hyperreactivity of the thyroid to acute cold stress, while in chronic cooling they modulate the adaptive reaction of the thyroid gland to cold. Thus, pineal peptides can modulate the thyroid reaction to cooling. Epitalamin modulates the intensity of peripheral deiodination of thyroxine.Key Words: epitalamin; pineal peptide hormones; thyroid gland; thyroid hormones; thyroid stimulating hormone; cold stress Since the thyroid gland (TG) is involved in thermogenesis, the hypothalamus-pituitary-thyroid axis readily responses to cooling. A good illustration is presented in the report of E. Tang et al. [5], who observed elevation of the serum triiodothyronine (1"3), thyroxine (T4) , and thyroid stimulating hormone (TSH) in albino rats exposed to 4~ for 1 and 5 hours. Other investigators observed a 2-fold activation of T 4-deiodinase in rat pineal gland after 4-h cooling at 4~ attesting to enhanced utilization of thyroid hormones by pinealocytes in hypothermia [4].On the other hand, our previous studies on clinical (intraoperation biopsy specimens of human TG) and experimental material demonstrated the involvement not only of pineal indoles but also of pineal peptide hormones into regulation of thyroid function [1][2][3]. These data were obtained under normal temperature, while the role of pineal hormones in the regulation of adaptation of the hypothalamus-pituitary-thyroid system to cold stress has not been evaluated. In this study we attempted to tackle this problem. U]u-ainian Institute of Pharmacology and Endocrinological Diseases, Kharkov MATERIALS AND METHODSExperiments were carried out on 78 adult male Wistar rats. The animals were maintained at normal temperature, exposed to acute cold stress (4~ for 30 and 180 min), or chronic cooling (10 days at 10~ receiving standard food and water supply.Epitalamin (EP), a peptide preparation from cattle pineal glands, kindly provided by Dr. V. A.Khavinson (Tsistomedin, St. Petersburg) was injected intraperitoneally in a dose of 0.25 mg/kg independently or immediately before cold exposure.Serum concentrations of total T3, T4, and TSH were measured by enzyme-linked immunosorbent assay using commercial kits (Immunotekh, Russia); concentrations of free T 3 and T 4 were measured by radioimmunoassay using Amersham kits. RESULTSAs seen from Table 1, 30-min cooling (4~ leads to a 2.5-fold rise of serum T 3 against the background of a 40% decrease in the T 4 level. This indicates substantial peripheral conversion of T 4. The same pattern was noted for free hormones. The enhanced production of total and free T 3 in cold stress is apparently a compensatory reaction. Judging from