Regulation of body mass and adiposity in the field vole, Microtus agrestis: a model of leptin resistance.

Adult mammals are typically highly resistant to perturbations in their energy balance. In obese humans, however, this control appears to be lost. Apart from a few exceptional cases, this loss of control occurs despite appropriate levels of circulating leptin -- suggesting that elevated adiposity may be a consequence of failure to respond to the leptin signal: leptin resistance. When cold-acclimated male field voles (Microtus agrestis) are transferred from short (SD, 8 h light) to long (LD, 16 h light) photoperiods, they increase dramatically in body mass and fatness for about 4 weeks. After this period, their mass stabilizes at a new plateau about 25% higher than animals maintained in SD. The increase in adiposity is not caused by significant increases in food intake, but reflects an increase in digestive efficiency. Measures of circulating leptin reveal that the increased adiposity is matched by increased circulating leptin. By infusing voles with exogenous leptin, we have demonstrated that SD voles are leptin sensitive (reducing both body mass and food intake), whereas LD animals are leptin resistant. Voles may therefore be a useful model for understanding the process of leptin resistance. The change in leptin sensitivity in voles was not associated with changes in the levels of gene expression of the orexogenic or anorexogenic neuropeptides, such as neuropeptide Y, agouti-related peptide, POMC and cocaine- and amphetamine-regulated transcript, measured in the hypothalamic arcuate nucleus (ARC). During the phase that body mass was increasing, however, there was a transient increase in the ARC expression of suppressor of cytokine signalling-3 (SOCS3). These data suggest that the changes in the expression of SOCS3 in the ARC may be involved in leptin resistance. However, the mechanism by which these changes may be linked to alterations in digestive efficiency that underpin the changes in adiposity, or how the differences are signalled by changes in photoperiod, remains unclear.