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

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.

Pinealectomy increases oxidant damage in kidney and testis caused by hyperthyroidism in rats

Thyroid hormones regulate energy metabolism and act on mitochondria which are an important source of free radicals in the cell. The pineal gland activates antioxidant systems via melatonin secretion and thus has a protective function in body tissues. The present study was conducted to determine the oxidative damage caused by hyperthyroidism in kidney and testis tissues of pinealectomized rats. Experimental animals were allocated to three groups: 1, control group; 2, sham pinealectomy-hyperthyroidic group; and 3, pinealectomy-hyperthyroidic group. Hyperthyroidism was induced by A 3-week intraperitoneal administration of thyroxin after sham pinealectomy or pinealectomy. Malondialdehyde (MDA) and glutathione (GSH) levels were determined in kidney and testis tissues. MDA levels of the kidney and testis tissue in the pinealectomy and hyperthyroidic groups were significantly higher than those in the sham pinealectomy-hyperthyroidic group and the control group (p < 0.001). GSH levels of both kidney and testis tissues were significantly higher in the sham-pinealectomy-hyperthyroidic group when compared to the other two groups (p < 0.001). This increase in GSH levels was more evident in the pinealectomy-hyperthyroidic group than in the control group (p < 0.001). The results of our study demonstrate that MDA and GSH levels in kidney and testis tissues increased due to hyperthyroidism and that pinealectomy made the increase in MDA levels more apparent, while decreasing GSH levels.

Opposite effects of zinc and melatonin on thyroid hormones in rats

The present study was conducted to investigate how thyroid function in rats is affected by administration of 3 mg per kg per day of zinc and/or melatonin. The study was conducted with 40 Sprague-Dawley adult male rats equally divided into four groups: 1 (controls), 2 (zinc-only), 3 (melatonin-only) and 4 (zinc- and melatonin-supplemented). The supplementation was continued for 4 weeks after which the animals were sacrificed and plasma samples were obtained for determination of zinc, melatonin, free- and total triiodothyronine (T3), thyroxine (T4) and thyroid-stimulating hormone (TSH) levels. The free T3, T4 and TSH levels were lower in the melatonin group than in all other groups (P<0.01), while free- and total T3 levels were higher in the zinc group (P<0.01). The group that received zinc and melatonin combined had free thyroid hormone levels higher than the only melatonin group. These results show that melatonin has a thyroid function suppressing action, just the opposite to the actions of zinc. When zinc is administered along with melatonin, its thyroid function suppression is diminished.

Pinealectomy and zinc deficiency have opposite effects on thyroid hormones in rats

The present study was conducted to investigate how pinealectomy and zinc deficiency separately or in combination affected thyroid hormones in rats. The study was carried out on 40 Sprague-Dawley male rats. The rats were equally allocated to four groups: Group 1 (control group), Group 2 (zinc-deficient group), Group 3 (pinealectomized group) and Group 4 (pinealectomized and zinc-deficient group). At the end of a 4-week study period, the rats were decapitated and blood samples were taken. The samples were examined in terms of plasma zinc, melatonin, free and total T3, T4, and TSH. It was found that free T3 and T4 levels in the pinealectomized group (Group 3) were higher than all others (p < 0.01) while free T3, T4, and TSH levels in the zinc-deficient group (Group 2) were lower than all others (p < 0.01). Free T3 and T4 levels in the pinealectomized zinc-deficient group (Group 4) were lower than those in Groups 1 and 3 and higher than those in Group 2 (p < 0.01). The findings obtained at the end of the study period show that pinealectomy has a stimulating and zinc deficiency has a suppressing effect on thyroid hormones and that the suppressing effect caused by zinc deficiency is partially balanced by pinealectomy.

Differential effects of melatonin on plasma levels of thyroxine and triiodothyronine levels in the air-breathing fish, Clarias gariepinus, during breeding and quiescent periods

Effects of morning and evening injections of three doses of melatonin were studied on the plasma levels of thyroxine (T(4)) and triiodothyronine (T(3)) in an air-breathing fish Clarias gariepinus during its quiescent phase and breeding phase. Depending on seasons and time of administration, melatonin increased, decreased or had no effects on the plasma levels of T(4) and T(3). Present findings strongly suggest that melatonin is involved in the regulation of thyroid hormones in C. gariepinus, and differentially influences T(4) and T(3) level.

Effect of melatonin on the response of the thyroid to thyrotropin stimulation in vitro

Thyroidal-melatonin interactions are of particular importance to amphibian development since the thyroid controls the progress of metamorphosis while melatonin may coordinate its rate with prevailing environmental conditions. Melatonin antagonized thyroxine (T4) action at the tissue level and directly inhibited baseline T4 secretion in culture, so the present work sought to determine if it antagonized the response of the thyroid to thyroid stimulating hormone (TSH) as well. A preliminary experiment showed that, in Rana pipiens, the concentration of TSH (0.2 microg/ml) used in the culture of tadpole thyroids stimulated T4 secretion as much as frog pituitaries, but more than late premetamorphic tadpole pituitaries. There was no significant effect of 1 to 15 microg/ml melatonin in TSH-containing thyroid cultures of various Rana species of tadpoles and frogs in experiments with media collected once every 24 or 48 hr, although 15 microg/ml melatonin tended to depress T4 secretion. In a final experiment, a higher melatonin concentration was used as well as more frequent media collections. Thyroids from prometamorphic Rana catesbeiana tadpoles were cultured in L-15 media with periodic stimulation by 0.2 microg/ml TSH, or TSH and 10 or 100 microg/ml melatonin. Media were collected at the end of two 3-hr TSH pulses, and every 8 hr thereafter for the next 3 days. Melatonin was administered until the end of Day 2 while TSH was given only on Day 2 in addition to the original 3-hr pulses. The secretion of T4 was inhibited significantly by 10 microg/ml melatonin at only two of the early media collections. In contrast, 100 micro;g/ml melatonin significantly depressed T4 secretion in response to TSH at all but one interval and completely inhibited the thyroidal response to TSH reintroduced into the media on Day 2. The findings suggest that a high concentration of melatonin is inhibitory to the thyroidal response to TSH, but that lower concentrations do not significantly overcome the TSH stimulus.

Acclimation of a non-indigenous sub-Arctic population: seasonal variation in thyroid function in interior Alaska

Total, as well as free, T4 and T3 levels were obtained over four seasons for young male infantry soldiers assigned to interior Alaska. Significant seasonal variations were found in both T3 and T4. Total T4 and T3 levels were highest in winter, while free T4 and T3 levels were highest in early spring. Correlations with melatonin levels from a concurrent study showed an association between late day (17.00) mean spot melatonin levels during the preceding summer and T3 levels in winter and spring. Differences in seasonal T4 and T3 levels between indigenous and newly arrived people in the sub-Arctic may be related not only to cold acclimation but also to light.