Effects of afternoon injections of O-acetyl-5-methoxytryptophol, melatonin or 5-methoxytryptophol in female syrian hamsters

Female Syrian hamsters were maintained in a 14:10 light: dark photoperiod (lights on 6 a.m.) and injected each afternoon between 4 p.m. and 6 p.m. with diluent or with 25 μg melatonin, 0-acetyl-5-methoxytryptophol (aML) or 5-methoxytryptophol (ML). After 7 or 12 weeks of treatment, only the melatonin-treated hamsters were acyclic. Gross inspection of their ovaries revealed that they were smooth, brown, and devoid of corpora lutea and large follicles. Progesterone levels of the acyclic animals were midway between the high levels observed during estrus and postestrus and the low levels observed on cycle days 2 and 3. Additionally, the melatonin-treated hamsters had significantly reduced uterine weights and pituitary and plasma levels of prolactin. Hamsters treated with aML had significantly lower pituitary prolactin and plasma and pituitary levels of LH only at the 7 week timepoint. ML-treated hamsters had lower pituitary prolactin levels also at 7 weeks. The endocrine changes wrought by injections of aML and ML, however, were insufficient to ultimately affect vaginal cyclicity. In conclusion, it would appear that at the dose used and the time administered melatonin is the more active of the indoles tested in the female hamster, at least in terms of alterations in reproductive physiology.