Individual differences in cardiovascular response to social challenge.

An important determinant of cardiovascular stress reactivity and morbidity is the individual behavioral strategy of coping with social challenge. This review summarizes the results of a number of studies that we performed in rats, aimed at investigating the relationship between aggression and cardiovascular responsivity under social stress conditions. We show that rats belonging to the 'aggressive tail' of a population are characterized by a higher sympathetic-adrenomedullary activation during social and non-social stress episodes. Wild-type rats are characterized by a larger sympathetic dominance and a higher susceptibility to cardiac arrhythmias during defeat as compared to Wistars. Cardiovascular habituation takes place when social challenge is an intermittent victory experience, whereas no habituation is observed across repeated defeat episodes. Dominant rats whose social dominance is challenged by the aggression of another subject display long-term alterations of heart rate circadian rhythmicity. Such changes are linked to individual proness to defend social dominance: the more the animal counterattacks the aggressor, the smaller the subsequent rhythm disturbance. These data underline how important it is to carefully consider individual differences in aggression and the context in which aggression is expressed, when studying cardiovascular effects of social interactions.