Leptin, but not immune function, is linked to reproductive responsiveness to photoperiod

Energetic demands are high while energy availability is minimum during wint er. To cope with this energetic bottleneck, animals exhibit numerous energy -conserving adaptations during winter, including changes in immune and repr oductive functions. A majority of individual rodents within a population in hibits reproductive function (responders) as winter approaches. A substanti al proportion of small rodents within a species, however, fails to inhibit reproduction (nonresponders) during winter in the field or in the laborator y when maintained in winter-simulated day lengths. In contrast, immune func tion is bolstered by short day lengths in some species. The specific mechan isms that link reproductive and immune functions remain unspecified. Leptin is a hormone produced by adipose tissue, and several studies suggest that leptin modulates reproductive and immune functions. The present study sough t to determine if photoperiodic alterations in reproductive function and le ptin concentrations are linked to photoperiod-modulated changes in immune f unction. Siberian hamsters (Phodopus sungorus) were housed in either long ( LD 16:8) or short (LD 8:16) day lengths for 9 wk. After 9 wk, blood samples were collected during the middle of the light and dark phase to assess lep tin concentrations. One week later, animals were injected with keyhole limp et hemocyanin to evaluate humoral immunity. Body mass, body fat content, an d serum leptin concentrations were correlated with reproductive responsiven ess to photoperiod; short-day animals with regressed gonads exhibited a red uction in these measures, whereas short-day nonresponders resembled long-da y animals. In contrast, immune function was influenced by photoperiod but n ot reproductive status. Taken together, these data suggest that humoral imm une function in Siberian hamsters is independent of photoperiod-mediated ch anges in leptin concentrations.