The influence of social environment on endocrine, cardiovascular and tissue responses in the rabbit.

Previous work from our lab demonstrated that social environment influences the progression of atherosclerosis in genetically hyperlipidemic rabbits. The purpose of the current study was to examine behavioral and physiological responses associated with these distinct chronic social conditions. Normolipidemic rabbits were exposed to one of three social environments for 4 hours/day over 20 weeks: 1) an Unstable Group in which animals were paired weekly with a different unfamiliar rabbit, 2) a Stable Group in which rabbits were paired with the same littermate for the entire study, and 3) an Individually Caged Group in which animals were socially isolated. It was found that the Unstable Group, characterized by increased agonistic behavior and relatively less affiliative behavior, exhibited physiological responses indicative of chronic stress (increased urinary norepinephrine, plasma cortisol, splenic weight, and decreased visceral fat and body weight compared to the other groups). These animals also had increased acute plasma oxytocin responses relative to the other groups 10 minutes into the social pairing. In contrast, the Stable Group exhibited more affiliative behavior and less stressful physiological and tissue responses. The Individually Caged Group had elevated urinary norepinephrine relative to the Stable Group, and they exhibited higher heart rates at the end of the study compared to the other groups, suggesting that this social environment is also associated with chronic sympathetic arousal. It was concluded that distinct social contexts lead to different patterns of behavioral and physiological responses, and these responses are relevant to the pathophysiology of atherosclerosis and cardiovascular disease.