<b>THE INFLUENCE OF LARGE DOSES OF MELATONIN ADMINISTERED IN THE AFTERNOON ON THE NEUROENDOCRINEREPRODUCTIVE SYSTEM OF FEMALE SYRIAN HAMSTERS MAINTAINED EITHER IN A LONG OR SHORT </b><b>PHOTOPERIOD </b>

Abstract

Adult female hamsters were maintained on either a long (l4L: 10D; lights on 0600) or short (l0L: 14D; lights on 0600) photoperiod. Within each photoperiodic regimen the animals were injected daily with either vehicle, 25 pg 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 pg mel injections for ll 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 pg of mel while no effect on these hormones 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 pg 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 pg 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.When Syrian hamsters are exposed to short photoperiodic conditions (<12.5h light/day), collapse of the reproductive system ensues (23, 11). Surgical extirpation of the pineal gland prevents the inhibitory effects of reduced photoperiod on the reproductive capacity of these animals (21). The pineal methoxyindole, melatonin (mel), is known to mimic the antigonadotrophic effects of short photoperiod in hamsters when 'low' doses (25 pg or less) are injected daily. However, mel administration must be carried out late in the light phase to inhibit reproduction (36, 37). In light of the suppressive actions of mel, it seems inconsistent that the same indole can also thwart the antigonadal influence of a short photoperiod (14, 30) and even more interestingly, its own antigonadotrophic effects (29, 37) when administered as a subcutaneous implant. Recently, Chen er a1., (7) and Richardson er al. (31) have demon-

Splenic hypertrophy and extramedullary hematopoiesis induced in male Syrian hamsters by short photoperiod or melatonin injections and reversed by melatonin pellets or pinealectomy

“…Essentially, this is what was observed in the present set of experiments. Melatonin injections inhibit estrous cyclicity and fecundity in female hamsters; the large, brown ovaries of these acyclic females are full of interstitial tissue (Richardson et al, 1982) but still produce some progesterone (Little et al, 1986). The observed depression in relative spleen weights associated with small uteri conforms to the paradigm of hypogonadism also in that sex.…”