Follicular growth and intraovarian and extraovarian oocyte release after daily injections of melatonin and 6‐chloromelatonin in the Syrian hamster

Abstract

Groups of adult female Syrian hamsters (Mesocricetus auratus) were injected daily at 17:00 hr with 2.5, 15, or 25 micrograms of melatonin (Mel) or 6-chloro-melatonin (Cl-Mel) for 12 weeks. An ovary from each animal was completely serially sectioned for light microscopic investigation. Judging from the presence of corpora lutea, there were some animals in each group that continued to cycle, although the postestrous, white mucous discharge had disappeared. Noncycling animals were most often found in the 25-micrograms group of Cl-Mel. Only uterine weights of noncycling animals treated with either 25 or 15 micrograms of Mel or Cl-Mel were statistically significantly depressed versus controls. Cl-Mel (25 micrograms) significantly suppressed the total number and size of antral follicles (P less than 0.05). Follicular ruptures with incomplete or complete release of the oocyte out of the follicular compartment were observed. The oocyte release occurred either into the ovary ("intraovarian oocyte release: IOR") or outside of the ovary ("extraovarian oocyte release: EOR"). Compared with controls, the total number of IOR was increased in all experimental groups with the exception of the 2.5-micrograms group of Cl-Mel. IOR appeared in both preantral and antral follicles, and often IOR was complete. In controls, only preantral follicles were involved in IOR; these were primarily incomplete ones. IOR was seen in cycling and noncycling animals. By contrast, EOR was exclusively observed in noncycling hamsters. It is concluded that the cessation of postestrous, white mucous discharge is not necessarily an index for a halt in cyclic ovarian function. Injections of 25 micrograms of Cl-Mel are more effective than 25 micrograms of Mel in suppressing ovarian function. Both Mel and Cl-Mel increase the frequency of IOR. Finally, noncycling hamsters show EOR that is regarded as an abnormal ovulation.

Melatonin has dose-dependent effects on folliculogenesis, oocyte maturation capacity and steroidogenesis

“…There is almost no information in literature on the influence of melatonin upon ovarian follicle differentiation. In rats, night time administration of melatonin (10 g/ml drinking water) delayed puberty and reproductive senescence without affecting the rate that primordial follicles are lost (Meredith et al, 2000) and it was reported that 6-chloro-melatonin significantly reduced the number and size of antral follicles in the Syrian hamster (Spanel-Borowski et al, 1983).…”