Daily melatonin injections reduce reproductive and thyroid hormones in male Syrian hamsters. The interrelationship between the decline in these hormones is not known. To explore this relationship, male Syrian hamsters were divided into four groups: castrated, implanted with testosterone (5-mm silastic implants), both treatments, or neither treatment. One-half of each group of hamsters (n = 7 or 8) were injected with melatonin (25 rog) daily at 1730 h. The other half of each group received daily vehicle injections. Ten weeks later, the hamsters were anesthetized and decapitated. Testes weights, serum testosterone, and serum thyroxine levels were measured. As expected, testes and serum testosterone levels were uniformly low in all of the melatonin-treated hamsters. All of the melatonin-treated groups also had lower than normal thyroxine values irrespective of gonadal treatment. Interestingly, in the non-melatonin-treated hamsters, serum thyroxine values were decreased in the castrated group and increased in the testosterone-implanted group. These results suggest that castration can reduce serum thyroxine levels in male Syrian hamsters and that replacement of testosterone restores these levels to normal. Notably, the declines in thyroxine levels produced by daily melatonin injections were not restored by testosterone implants in castrated or intact hamsters. Therefore, melatonin-induced reductions in thyroxine are not mediated by concurrent reductions in serum testosterone levels. It appears that melatonin-induced reductions in serum thyroxine levels do not use the same mechanism as castration-induced reductions.
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