INJECTIONS AND/OR IMPLANTS OF 6-CHLOROMELATONIN AND MELATONIN: EFFECTS ON PLASMA THYROID HORMONES IN MALE AND FEMALE SYRIAN HAMSTERS

Plasma thyroid hormone concentrations were evaluated in three experiments after treatment of male and female Syrian hamsters with two indoles, 6-chloromelatonin and melatonin. In experiment 1, female hamsters given daily afternoon subcutaneous injections of 2.5, 15 or 25 μg of either 6-chloromelatonin or melatonin had the same doserelated depression of circulating levels of thyroxine (T4) and free T4 index compared to the saline-injected control animals. No effect of either indole was observed on T3 levels or on the free T3 index. Daily afternoon injections of melatonin reduced T4 titers and the free T4 index in male hamsters; these effects were prevented if the melatonin-injected hamsters were concurrently receiving a biweekly subcutaneous implant of 1 mg of 6-chloromelatonin or 1 mg of melatonin (experiment 2). Injections of 6-chloromelatonin caused a significant reduction in free T4 index but not in plasma T4 titers. The effect on free T4 index in these 6-chloromelation-treated animals was reversed by biweekly implantation of a 1 mg melatonin pellet (experiment 3). In conclusion, daily afternoon injections of 6-chloromelatonin are as effective as melatonin in depressing circulating concentrations of T4 and the free T4 melatonin in female hamsters; in male animals, melatonin injections were somewhat more effective than 6-chloromelatonin in this regard. However, either melatonin or 6-chloromelatonin implants are capable of reversing the T4 suppression induced by melatonin injections. Thus, 6-chloromelatonin acts as a melatonin agonist and may be of further use in determining the role of melatonin in future physiological studies.