Short-day aggression is independent of changes in cortisol or glucocorticoid receptors in male Siberian hamsters (Phodopus sungorus).

Testosterone mediates aggression in many vertebrates. In some species, aggression remains high during the non-breeding season (e.g., winter), when testosterone levels are low. In Siberian hamsters (Phodopus sungorus), we have demonstrated photoperiodic changes in aggression with hamsters housed in short, "winter-like" days displaying significantly more territorial aggression than long-day animals, despite low levels of testosterone. The mechanisms by which photoperiod regulates aggression, however, remain largely unknown. Adrenocortical hormones (e.g., glucocorticoids) have been implicated in mediating seasonal aggression; circulating concentrations of these hormones have been correlated with aggression in some species. The goal of this study was to examine the role of cortisol and glucocorticoid receptors in mediating photoperiodic changes in aggression in male Siberian hamsters. Males were housed in long or short days and treated with either exogenous cortisol or vehicle. Circulating levels of cortisol, adrenal cortisol content, and aggression were quantified. Lastly, photoperiodic effects on glucocorticoid receptor (GR) protein levels were quantified in limbic brain regions associated with aggression, including medial prefrontal cortex, amygdala, and hippocampus. Short-day hamsters were more aggressive than long-day hamsters, however cortisol treatment did not affect aggression. Photoperiod had no effect on serum or adrenal cortisol or GR levels in the brain regions examined. Taken together, these data suggest that increases in cortisol levels do not cause increases associated with short-day aggression, and further that GR protein levels are not associated with photoperiodic changes in aggression. The results of this study contribute to our understanding of the role of adrenocortical steroids in mediating seasonal aggression.