The role of kisspeptin in the regulation of seasonal reproduction in Siberian hamsters (Phodopus sungorus)

Most non-tropical animals exhibit seasonal changes in reproduction, and photoperiod (i.e., day length) acts as the main environmental cue regulating the timing of breeding. The integration of photoperiodic cues ensures offspring are born during favorable environmental conditions. The precise mechanisms by which photoperiodic information are integrated to directly regulate the reproductive neuroendocrine axis has, however, remained less well specified. The neuropeptide kisspeptin has recently been identified as a potent positive regulator of reproductive function and is associated with the onset of puberty in laboratory mammals and humans. I investigated the potential role of kisspeptin in regulating seasonal changes in reproduction in the photoperiodic Siberian hamsters (Phodopus sungorus). Specifically, I examined photoperiodic changes in kisspeptin content in the hypothalamus of hamsters held on "winter-like" short- or "summer-like" long-day photoperiods. Next, to examine whether observed differences in kisspeptin immunoreactivity are driven by photoperiod directly or indirectly via changes in gonadal steroids, castration and sex steroid replacement studies were employed in hamsters held on either short or long days. Lastly, functional studies were conducted to assess the effects of exogenous kisspeptin on the activity of the reproductive hormonal axis (e.g., luteinizing hormone, sex steroids) and gonadal morphology in reproductive and non-reproductive hamsters. My findings demonstrate significant differences in the kisspeptin system in hamsters held in long or short days; these changes are regulated by both gonadal-dependent and independent (i.e., photoperiodic) mechanisms. Further, kisspeptin is able to stimulate the HPG axis in both reproductive and non-reproductive males; using identical methods, non-reproductive females display decreased HPG axis v i activity. Collectively, these findings provide evidence for an important role of kisspeptin in coordinating and relaying environmentally relevant information to the reproductive axis, as well as potential sex differences, and suggest a key role for this peptide in regulating appropriate changes in reproductive status.