INTERACTION OF DAILY INJECTIONS AND SUBCUTANEOUS RESERVOIRS OF MELATONIN ON THE REPRODUCTIVE PHYSIOLOGY OF FEMALE SYRIAN HAMSTERS

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