This study investigated the ability of continuously available melatonin (from sc melatonin-beeswax pellets that were implanted bi-weekly) in overcoming the antigonadotrophic action of single daily melatonin (25 μg) injections given late in the light period to female Syrian hamsters kept in light:dark cycles of 14:10. The melatonin-beeswax pellets contained either 1, 50, 500 μg, 1 or 10 mg melatonin and 25 mg beeswax. Daily melatonin injections into hamsters implanted with beeswax only cause vaginal acyclicity in 60% of the animals within 6 weeks. After 10 weeks of treatment, uterine weights and pituitary levels of immunoreactive prolactin were still greatly depressed. Beeswax pellets containing either 500 μg, 1 or 10 mg melatonin overcame the inhibitory effects of daily melatonin injections. Thus, the vaginal cycles and pituitary hormone levels of these animals were indistinguishable from those of untreated control animals. Beeswax pellets containing either 1 to 50 μg melatonin were incapable of counteracting the antigonadotrophic effects of daily injections of melatonin. This is the first report that continuously available melatonin can negate the effects of daily injections of melatonin in female hamsters. When measured after 10 weeks, none of the treatment procedures significantly influenced circulating levels of luteinizing hormone or prolactin.
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