The study shows that in hamsters, the hormone melatonin controls a brain signal called kisspeptin, which in turn drives the reproductive system. When day length is short, melatonin lowers kisspeptin levels and suppresses fertility, but giving extra kisspeptin can jump‑start testicular activity even in those conditions. This reveals a link between light cycles, melatonin, and reproductive hormones, but it’s based on animal experiments and doesn’t give direct guidance for human use.

In seasonal species, photoperiod (i.e. daylength) tightly regulates reproduction to ensure that birth occurs at the most favorable time of year. In mammals, a distinct photoneuroendocrine circuit controls this process via the pineal hormone melatonin. This hormone is responsible for the seasonal regulation of reproduction, but the anatomical substrate and the cellular mechanism through which melatonin modulates sexual activity is far from understood. The Syrian hamster is widely used to explore the photoneuroendocrine system, because it is a seasonal model in which sexual activity is promoted by long summer days (LD) and inhibited by short winter days (SD). Recent evidences indicate that the products of the KiSS-1 gene, kisspeptins, and their specific receptor GPR54, represent potent stimulators of the sexual axis. We have shown that melatonin impacts on KiSS-1 expression to control reproduction in the Syrian hamster. In this species, KiSS-1 is expressed in the antero-ventral-periventricular and arcuate nuclei of the hypothalamus at significantly higher levels in hamsters kept in LD as compared to SD. In the arcuate nucleus, the downregulation of KiSS-1 expression in SD appears to be mediated by melatonin and not by secondary changes in gonadal hormones. Remarkably, a chronic administration of kisspeptin restores testicular activity in SD hamsters, despite persisting photoinhibitory conditions. Overall, these findings are consistent with a role of KiSS-1/GPR54 in the seasonal control of reproduction. We propose that the photoperiod, via melatonin, modulates KiSS-1 neurons to drive the reproductive axis.