The proper administration of melatonin has well been documented to induce testicular regression in seasonal breeding animals. The subcutaneous injections of melatonin in the afternoon, not in the morning, consistently occurred testicular involution in the male Syrian (golden) hamsters whose reproductive activity is regulated by the photoperiod. But the effects of daily melatonin via gavage have not been estimated. Golden hamsters housed in long photoperiod (LP) were divided into 5 groups: the control animals housed in LP or in short photoperiod (SP) and animals treated daily with low (15 μg), middle (150 μg), and high dosages (1,500 μg) of pure melatonin by using gavage in the evening for 8 weeks. As results, LP control animals had large testes and SP controls displayed small and entirely regressed testes. The animals treated with various dosages of melatonin showed collectively degenerating effects on the weights of testes, epididymides, and seminal vesicles in the middle and high dosage groups, with the individual differences as well. The high dosages induced testicular regression in more proportion than the middle dosages did. The low dosage had large testes like the LP control animals. The small and inactive testes shown in some animals of both middle and high groups presented the complete regression as those of the animals maintained in SP. These results strongly suggest that the administrations of melatonin lead to testicular involution in the male golden hamsters when it is administered through gavage.

The Syrian (golden) hamsters are seasonal breeders whose reproductive functions are active in summer and inactive in winter. In experimental facility mimicking winter climate, short photoperiod (SP) induces gonadal regression. The blood-testis barrier (BTB) of the sexually involuted animals have been reported to be permeable, allowing developing germ cells to be engulfed or sloughed off the epithelium of the seminiferous tubules. The expressions of genes related to the tight junction composing of BTB were investigated in the reproductive active and inactive testes. Claudin-11, occludin, and junctional adhesion molecule (JAM) were definitely expressed in the active testes but not discernably detected in the inactive testes. And spermatozoa (sperm) were observed in the whole lengths of epididymides in the active testes. They were witnessed in only cauda region of the epididymides but not in caput and corpus regions in animals with the inactive testes. The results imply that the disorganization of BTB is associated with the testicular regression. The developing germ cells are swallowed into the Sertoli cells or travel into the lumen, as supported by the presence of the sperm delayed in the last region of the epididymis. These outcomes suggest that both apoptosis and desquamation are the processes that eliminate the germ cells during the regressing stage in the Syrian hamsters.