The influence of large doses of melatonin administered in the afternoon on the neuroendocrine-reproductive system of female Syrian hamsters maintained either in a long or short photoperiod

Adult female hamsters were maintained on either a long (14L:10D; lights on 0600) or short (10L:14D; lights on 0600) photoperiod. Within each photoperiodic regimen the animals were injected daily with vehicle, 25 μg of melatonin (mel) or 2.5 mg of mel at 1600. Short photoperiod resulted in an inhibitory influence on vaginal cyclicity, uterine weight, plasma follicle-stimulating hormone (FSH) and pituitary prolactin (PRL). One hundred percent of both 14:10 and 10:14 LD female hamsters receiving 25 μg mel injections for 11 weeks became acyclic along with significant decreases in uterine and anterior pituitary weights. Plasma luteinizing hormone (LH), FSH and PRL titers were significantly depressed in 14:10 LD animals injected with 25 μg of mel while no effect on these homones was observed in 10:14 LD animals. Pituitary concentrations of LH and FSH were increased in both short day and long day exposed hamsters injected with 25 μg mel; pituitary PRL was depressed in both groups. Hypothalamic luteinizing hormone releasing hormone (LH-RH) was increased in the 10:14 LD animals receiving 25 μg of mel. Injections of 2.5 mg of mel failed to alter any of the parameters measured in 14:10 LD animals with the exception of plasma FSH which was depressed. However, in 10:14 LD hamsters the 2.5 mg injections appeared to prevent most of the antigonadotrophic effects of short photoperiod. These results demonstrated that a large bolus of mel given in the afternoon when smaller doses of the indole are known to have inhibitory actions on reproduction, either failed to have any effect or counteracted the antigonadotrophic influence of a short photoperiod.