Mapping brain response to social stress in rodents with c-fos expression: a review.

Social defeat is an important event in the life of many animals, and forms part of the process of social control. Adapting to social defeat is thus an intrinsic part of social "homeostasis", and mal-adaptation may have pathological sequelae. Experimental models of social defeat (e.g. inter-male aggression) have existed for many years. However, very few studies have investigated the changes in brain activity in male animals exposed to the social stress of being defeated by another conspecific male, and in all these studies the expression of the immediate-early gene c-fos has been used as the marker of neuronal activity. In general, the results obtained inform that many areas of the brain, especially those involved in the general stress response, increase their activity when animals are exposed to an acute defeat. However, when animals are defeated repeatedly over many consecutive days, the level of activation of the brain shows different patterns of adaptation depending on the brain areas (varying from complete habituation to persistent activation). Discrepancies between studies may be due to differences in the experimental procedure. On the other hand, further research has to be conducted in order to understand what these changes in the brain activity mean in relation to the other stress responses to social defeat. Furthermore, knowing that the corresponding protein products of many immediate-early genes are transcription factors that can promote or inhibit the expression of target genes, research following this approach is also necessary.

Effect of predatory stress on sucrose intake and behavior on the plus-maze in male mice.

In this study, the effect of the exposure of male mice to sensory stimuli from rats was assessed on both sucrose intake and the elevated plus-maze tests. CDl male mice were trained in the sucrose intake task (the prestress phase) and, subsequently, distributed into two groups. The stressed group was accommodated in the same room as rats and the control group with mice (the stress phase). After being transferred, animals were tested on sucrose intake and the plus-maze (acute tests) and retested three times a week for sucrose intake and once on plus-maze on the last day (chronic tests). After acute exposure to the predator, the only difference between stressed and control animals was a higher number of fecal boli left on the plus-maze by the former. During the chronic phase, stressed animals showed a lower level of sucrose intake and higher level of anxiety than controls. In conclusion, this study shows that chronic exposure of male mice to stimuli from rats reduces the sensitivity to the rewarding properties of sucrose and prevents the habituation to the plus-maze observed in controls. Thus, this study suggests that exposure of mice to sensory stimuli from rats may provide an animal model of stress, and that these species should not be routinely housed together.