In mixed-sex rat groups housed in semi-natural conditions, a clear hierarchical organization is usually present, with one dominant male, socially active subdominant males and subordinate animals. The present paper reports preliminary data on the relationship between social rank in wild-type rats housed in colony conditions and cardiac sympatho/vagal balance in baseline and challenging conditions. Social structure was determined using advanced image analysis techniques (EthoVision, Noldus Information Technology b.v.) and cardiac electrical activity recorded by means of a radiotelemetry system (transmitters manufactured by Data Sciences International and receivers by the Department of Electronic Engineering, Biological Center, University of Groningen).

Four colonies were studied, each composed of 4 males and 4 females housed in large cages (120x150x200 cm). Experimental males (n=16) were provided with a colored perspex cap on top of the skull for automated imaging recording and with an intraperitoneal transmitter for ECG recording. After 2 weeks of individual housing following surgery, animals were grouped and kept in colony conditions for 2 weeks. 15-min baseline ECGs were recorded both in individual housing conditions and after grouping. At the end of the grouping period, each male was also exposed to a final 15-min defeat test obtained using a standard resident-intruder paradigm, immediately followed by additional 15-min psychosocial challenge. One day after, animals were sacrificed and the brain, heart, adrenals, thymus and seminal vesicles removed for weight determination and/or morphological analysis. Additional males (n=6) paired with a female were used as controls. Quantification of inter-male behavioral interactions in the colony were mainly based on the parameters speed moving towards and speed moving from, obtained converting X-Y positions of each individual male (an option provided by the EthoVision system). These parameters describe the spatial movements of each individual in pairwise interactions with other simultaneously tracked animals. At the end of data analysis, a net relative movement of each male can be obtained for each of its three pairwise interactions with the other male members of the colony and a dominance score established by averaging these three values. A negative sign of this parameter indicates that the animal is moving relatively more away from the others, a positive one indicates more approaching-like behaviors. Quantification of sympatho/vagal balance was obtained by means of simple time-domain parameters such as average R-R interval (RR), standard deviation of RR (SD), coefficient of variance (SD/RR), root-mean-square of successive RR differences (r-MSSD), together with the incidence of different cardiac arrhythmias such as ventricular and supraventricular extrasystoles and II degree atrio-ventricular blocks.

Male rats housed in the colony had higher organ/body weight ratios as compared to controls, significantly for the right and left ventricles of the heart (p<0.05) and tendentially for the adrenals (p=0.06). Moreover, the weight of the right ventricle was positively correlated with the dominance score (p<0.01). The body weight of the animals were positively correlated with the dominance scores, both when before grouping and after grouping body weight values were considered (p<0.01). We also found a tendency toward a negative correlation between dominance scores and SD and SD/RR values during the final defeat test: the higher the social position, the lower the variability of RR (p=0.07). Finally, a significant positive correlation was found between the overall number of ventricular extrasystoles observed in each animal and its dominance score (p<0.01).

In conclusion, high social rank in rat colonies (as estimated by means of an automated video tracking system) was found to be correlated with larger body mass, signs of cardiac hypertrophy, sympathetic dominance during an acute social stress test and higher vulnerability to ventricular arrhythmias.