Leptin acts on metabolism in a photoperiod-dependent manner, but has no effect on reproductive function in the seasonally breeding Siberian hamster (Phodopus sungorus)

Leptin may play a role in appetite regulation and metabolism, but its repro ductive role is less clear. In photoperiodic Siberian hamsters, seasonal ch anges in fatness, leptin gene expression, and metabolism occur synchronousl y with activation or suppression of reproduction, analogous to puberty. Her e, we test the hypothesis that seasonal changes in leptin secretion mediate the photoperiodic regulation of reproduction. Mature male and ovariectomiz ed estrogen-treated female Siberian hamsters were kept in long (LD; 16 h of light, 8 h of darkness) or short days (SD: 8 h of light, 16 h of darkness) for 8 weeks, and recombinant murine leptin (15 mug/day) was infused for 2 weeks via osmotic minipumps. SD hamsters exhibited significant weight and f at losses, reduced serum leptin and food intake, and suppressed pituitary L II concentration. Leptin did not suppress food intake over the a-week treat ment on either photoperiod, but significantly reduced fat reserves in SD ha msters. Leptin had no significant effect on pituitary LH concentrations in either sex or photoperiod or on testicular size and testosterone concentrat ions in males. These results suggest hamsters are more responsive to leptin on SD than on LD and that effects on food intake and fat loss can be disso ciated in this species. Our data suggest that leptin does not mediate photo periodic reproductive changes.