Groups of adult female Syrian hamsters (Mesocricetus auratus) were injected daily at 17:00 hr with 2.5, 15, or 25 μg 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‐μg group of Cl‐Mel. Only uterine weights of noncycling animals treated with either 25 or 15 μg of Mel or Cl‐Mel were statistically significantly depressed versus controls. Cl‐Mel (25 μg) significantly suppressed the total number and size of antral follicles (P> 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‐μg 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 μg of Cl‐Mel are more effective than 25 μg 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.
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