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Leptin Effects on Immune Function and Energy Balance Are Photoperiod Dependent in Siberian Hamsters (Phodopus sungorus)¹

Departments of Psychology (D.L.D., R.J.N.), Neuroscience (R.J.N.), and Biochemistry and Molecular Biology (R.J.N.), Division of Reproductive Biology, The Johns Hopkins University, Baltimore, Maryland 21218-2686; and Center for Behavioral Neuroscience, Department of Biology, Georgia State University (G.E.D.), Atlanta, Georgia 30303

Address all correspondence and requests for reprints to: Randy Nelson, Department of Psychology, 09 Townshend Hall, Ohio State University, Columbus, Ohio 43210. E-mail: rnelson+{at}osu.edu

Many adaptations have evolved in small mammals to maximize survival during winter. One such coping tactic in many species is an alteration of immune function in advance of the stressful conditions of winter. Leptin is a hormone produced by adipose tissue, and in addition to its central role in energy metabolism, leptin mediates the interactions among energy allocation, immune function, and reproduction. To examine this interaction further, exogenous leptin was administered for 2 weeks via osmotic minipumps to Siberian hamsters (Phodopus sungorus) housed in long or short days for a total of 12 weeks. Short-day hamsters displayed the expected reductions in humoral immune function, body mass, fat mass, and food intake. In Exp 1, exogenous leptin counteracted the reduction in food intake and the suppression of immune function in short days. In Exp 2, when the leptin-induced increase in food intake in short-day hamsters was prevented, leptin did not enhance immune function. In most of the measured fat pads and body mass, leptin had no effect in long days. In sum, leptin administered to short-day animals caused them to respond, in many cases, like long-day animals. Taken together, these data suggest that leptin acts indirectly to mediate energy allocation to humoral immune function. Additionally, leptin appears to act differentially, according to photoperiod, to regulate both immune and energetic parameters.