Dark exposure inhibits the mitotic activity of thyroid follicular cells in male mice with intact pineal

The relation between reduced serum melatonin levels and zinc in rats with induced hypothyroidism

The objective of the study was to explore the changes in melatonin and zinc levels in rats with induced hypothyroidism. Thirty adult male rats used in the study were allocated to three groups with equal numbers. Group 1: General control group which was not subjected to any procedure. Group 2: Sham-hypothyroidism group to which was administered 10 mg kg(-1) intraperitoneal (i.p.) physiologic saline (0.09% NaCl) for 4 weeks. Group 3: Hypothyroidism group which was supplemented with intraperitoneal 10 mg kg(-1) propylthiouracil (PTU) for 4 weeks. Blood samples collected from all animals at the end of the study by decapitation were analysed for serum Total T4 (TT4), Total T3 (TT3), Free T4 (FT4), Free T3 (FT3) (ELISA) as well as for melatonin (RIA) hormones and zinc levels (atomic emission). Comparison of the study groups in terms of thyroid hormones, melatonin and zinc levels showed that TT4, TT3, FT4, FT3, melatonin and zinc levels in group 3 were lower than those in groups 1 and 2 (p < 0.01). These parameters were not different in groups 1 and 2. The results of the study demonstrate that PTU supplementation for 4 weeks results in a significant inhibition in both melatonin and zinc levels. Inhibited melatonin levels may result from the decrease in zinc levels.

Inhibitory influence of late afternoon melatonin injections and the counter-inhibitory action of melatonin-containing pellets on thyroid growth process in male Wistar rats: comparison with effects of other indole substances

Previous studies performed in our laboratory indicated that melatonin, when administered in late afternoon (1600-1800) as s.c. injections for 10 days, exerted an inhibitory effect on thyroid growth in mice and Sprague-Dawley rats. The goal of the present study was to examine the effects of melatonin and other indole substances (5-methoxytryptophol, N-acetylserotonin, 6-methoxytryptamine), administered for 10 days, for either 4, 8, or 10 weeks, on thyroid growth processes in adult male Wistar rats. We have also compared the action of melatonin administered in late afternoon as s.c. injections with effects of the melatonin-released chronically from s.c. implanted beeswax pellets containing 1 mg of the hormone. Additionally, the effects of melatonin injections in animals with the pineal gland removed and in thyroid stimulating hormone (TSH)-injected rats have been studied. We examined the mean mitotic activity rates (MMARs) of thyroid follicular cells (TFC) and the wet thyroid weights. We concluded that melatonin, of the examined substances, is the most potent inhibitor of thyroid growth; the hormone reduced the MMAR of TFC and the thyroid weight at all time-points. 5-Methoxytryptophol, like melatonin, exerted the inhibitory influence on the mitotic activity; however, it did not affect thyroid weight. The influence of other indole substances was much less pronounced. It was shown that melatonin-pellets prevented the inhibitory effect of late afternoon melatonin injections on thyroid growth processes. This observation is consistent with results of other authors concerning the counter-antithyroid effect of melatonin on thyroid hormone secretion. Pinealectomy revealed the stimulatory effect on thyroid growth processes, while melatonin treatment reversed the effect of the surgery.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of melatonin on [3H]-thymidine incorporation into DNA of rat thyroid lobes in vitro

The effects of 4-h incubation in the presence of melatonin and N-acetylserotonin on the incorporation of [3H]-thymidine into DNA of the rat thyroid lobes were investigated. Additionally, thyroid lobes were incubated in the exposure to melatonin with thyrotropin jointly. Melatonin, when applied in the lowest concentration examined (10(-9)M), inhibited [3H]-thymidine incorporation, the effect of other concentrations (10(-6)M-10(-8)M) being not significant. N-acetylserotonin (10(-6)M) did not affect the rate of incorporation of that labelled nucleoside. As expected, thyrotropin enhanced the [3H]-thymidine incorporation into DNA of thyroid lobes. Melatonin (10(-7)M) suppressed that stimulatory effect of thyrotropin. The participation of melatonin in the regulation of thyroid growth processes is considered.

Studies on the effects of the pineal hormone melatonin on an androgen-insensitive rat prostatic adenocarcinoma, the Dunning R 3327 HIF tumor

The effects of the pineal indole melatonin on a transplantable androgen-insensitive fast-growing rat prostatic adenocarcinoma, the Dunning R 3327 HIF tumor, were examined. Afternoon injections of melatonin given to intact male rats enhanced tumor growth, while leading to a reduction in the weights of gonads and dorsal and ventral prostates. In anosmic hosts, this treatment had no effect on tumor growth or reproductive organ size. In castrated anosmic hosts, melatonin injections led to reductions in the growth of tumors, when compared with those in castrated hosts. Continuously available melatonin, in beeswax pellets, had no effect on growth of these tumors. These results are discussed in relation to the anticancer effects of melatonin and the effects of tumors on endogenous melatonin rhythms.

Melatonin inhibits the basal and TSH-stimulated mitotic activity of thyroid follicular cells in vivo and in organ culture

The aim of the present study has been to examine the effect of melatonin, administered to mice as daily subcutaneous injections for a total of 10 days, on the mitotic activity of thyroid follicular cells (TFC). The colchicine metaphase-arrest technique was employed in the experiment. We found that melatonin (10 micrograms and/or 100 micrograms injection) decreased significantly the mean mitotic activity rate (MMAR) of TFC in both male and female mice. Moreover, melatonin totally suppressed the stimulatory effect of TSH on the MMAR of TFC in both sexes of mice. Furthermore, the effect of melatonin (5 X 10(-7) M) on the proliferation of TFC in the organ-cultured rat and mouse thyroid explants was investigated. It was found that melatonin almost totally suppressed the MMAR of TFC in organ culture. Moreover, melatonin blocked the stimulatory effect of TSH on the MMAR of TFC in both rat and mouse thyroid explants. N-acetylserotonin (NAc-5HT, 10(-6) M) also decreased the MMAR of cultured thyroid explants, but its effect was less expressed when compared to melatonin inhibition. The present data indicate that melatonin can exert its inhibitory effect on the proliferation of TFC directly at the thyroid level, since this pineal indoleamine has been shown to suppress not only basal but also TSH-stimulated mitotic activity. The results are in agreement with the hypothesis of a pineal-thyroid negative feedback, assuming the direct inhibitory effect of melatonin on the thyroid growth.

Possible mechanisms of TSH--independent thyroid growth

Considerable evidence has been accumulated which indicates the participation of the autonomic nervous system in the growth of adrenals, ovaries, testes and thyroid lobes. Results have been gathered indicating that the pituitary is not required for the growth of the thyroid, adrenals and ovaries; interest is currently focused on the involvement of the pineal gland in the control of growth not only of the gonads, but also of the thyroid. This paper summarizes the data currently available on the concepts of thyroid hypertrophic and hyperplastic mechanisms, which occur independently of thyrotropin (TSH), and which suggest the existence of a reciprocal relationship between the pineal and the thyroid.