We investigated inbred SWR/J and AKR/J mice, two established models for different susceptibility to diet-induced obesity (DIO), to scrutinize the contribution of physical activity and energy assimilation to the etiology of developing obesity. Body mass gain and body composition of mice fed a high-energy (HE) or a low caloric control diet were monitored. In parallel, assimilated energy, locomotor activity and thermoregulatory behaviour were measured. Activity was continuously registered by radio telemetry and, in addition, Open Field (OF) behaviour was used as a quick screening tool for spontaneous activity before and after the feeding trial. Energy assimilation was increased in both strains on HE (AKR/J: +60.7% and SWR/J: +42.8%) but only in AKR/J, body mass (+8.1%) and fat mass (+40.7%) were significantly elevated. As a trend, total home cage activity was increased and was more scattered in SWR/J. Interestingly, HE stimulated OF activity only in SWR/J in the second trial at the end of the feeding experiment. The spatial pattern of OF activity also differed between strains with obese mice avoiding the core area. Under housing conditions, nest building behaviour was more pronounced in AKR/J. To further evaluate OF behaviour as a marker for spontaneous activity an obese mouse line was investigated. Mice lacking the leptin receptor (db/db) showed already before the onset of obesity lowest activity levels in OF. Adjustment of energy intake, higher activity levels and energy consuming thermoregulatory behaviour are mechanisms employed by SWR/J mice to dissipate excess energy as a defence against the onset of obesity. Therefore our results deciphering mechanisms of DIO-sensitivity in mice contribute to the understanding of inter-individual differences in body weight development in an adipogenic environment.
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