Maternal aggression as a model of acute social stress in the rat: a behavioral-electrocardiographic study.

Maternal aggression of lactating rats was used to induce acute social stress in a resident-intruder paradigm. Behavioral and electrocardiographic responses of male intruders were simultaneously recorded. Cardiac electrical activity was measured by means of a telemetry system which ensured preservation of intruder's behavioral repertoire. Behavioral response to maternal attack was evaluated in terms of percent time spent in passive/submissive (p/s) and active/non-submissive (a/ns) patterns. This allowed us to classify the intruders as submissive, non-submissive, and intermediates. During the test, a significant decrease of average R-R interval (R-R) compared to baseline conditions and the occurrence of arrythmias of different nature were found. Ventricular arrhythmias, including premature ventricular beats and ventricular tachycardias, were present in all the intruders while R-R fluctuations and II degree atrioventricular (A-V) blocks only appeared in submissive and intermediate rats. Most ventricular arrhythmias occurred within the upper range of heart rate and were preceded by periods of low R-R variability. These electrocardiographic events were temporally associated with a/ns behavioral patterns and were related to an increased prevalence of sympathetic activity. On the other hand, R-R fluctuations and II degree A-V blocks preceded by high R-R variability were synchronized with p/s behavior and were ascribed to a predominant inhibitory effect of vagal activation which becomes more pronounced as the level of sympathetic activity increases.