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Leptin Acts on Metabolism in a Photoperiod-Dependent Manner, But Has No Effect on Reproductive Function in the Seasonally Breeding Siberian Hamster (Phodopus sungorus)¹

School of Biological Sciences (Z.A., F.R.A.C., J.A.S., I.D.M., A.S.I.L.), University of Manchester, Manchester, United Kingdom M13 9PT; AstraZeneca CTL (A.N.B.), Cheshire, United Kingdom SK10 4TJ; School of Biomedical Sciences (F.J.P.E.), University of Nottingham, United Kingdom NG7 2UH; Fachbereich Biologie/Zoologie (M.K.), Philipps Universtat Marburg, D-35043 Marburg, Germany

Address all correspondence and requests for reprints to: Dr. Andrew S. I. Loudon, School of Biological Sciences, 3.614 Stopford Building, Oxford Road, University of Manchester, Manchester, United Kingdom M13 9PT. E-mail: andrew.loudon{at}man.ac.uk

Leptin may play a role in appetite regulation and metabolism, but its reproductive role is less clear. In photoperiodic Siberian hamsters, seasonal changes in fatness, leptin gene expression, and metabolism occur synchronously with activation or suppression of reproduction, analogous to puberty. Here, we test the hypothesis that seasonal changes in leptin secretion mediate the photoperiodic regulation of reproduction. Mature male and ovariectomized 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 µg/day) was infused for 2 weeks via osmotic minipumps. SD hamsters exhibited significant weight and fat losses, reduced serum leptin and food intake, and suppressed pituitary LH concentration. Leptin did not suppress food intake over the 2-week treatment on either photoperiod, but significantly reduced fat reserves in SD hamsters. Leptin had no significant effect on pituitary LH concentrations in either sex or photoperiod or on testicular size and testosterone concentrations 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 dissociated in this species. Our data suggest that leptin does not mediate photoperiodic reproductive changes.

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