Defensive aggregation to predatory threat in the laboratory rat: behavioural, neural, pharmacological and epigenetic correlates

Abstract

Abstract: Defensive aggregation is the tight clumping together of conspecifics observed in response to predatory threat across many species. While much field research has explored this social response to threat, in particular the important survival advantages it affords, it has received little examination in the laboratory. Chapter 2 of this thesis presents the first laboratory rodent model of defensive aggregation, demonstrating that it can be readily elicited in groups of four rats presented with an unconditioned stressor (cat fur or bright light). This provides a novel opportunity to explore the more subtle benefits accrued from defensive aggregation as well as its underlying neurobiology and pharmacology. Chapter 3 illustrates that defensive aggregation has a hitherto unknown social buffering effect that reduces neural and behavioural stress responsivity and facilitates reengagement in important non-threat-related behaviours. It also demonstrates that stable active and passive stress coping rats exist amongst populations that are group exposed to predator threat. Chapter 4 demonstrates that the neuropeptide oxytocin acts at vasopressin V1ARs to selectively promote social responding to threat without increasing anxiety-like behaviour. This suggests that developing novel pharmacotherapies that target V1ARs may prove useful for the treatment of chronic social withdrawal in the face of stress, which occurs in numerous psychiatric disorders. Finally, Chapter 5 provides the first report of striking epigenetic differences in the medial amygdala AVP system between active and passive coping rats, providing a potential mechanism through which the proactive response style seen in some animals confronted with threat might be maintained. It is hoped that the work presented in this thesis has served as a foundation for the future investigation of the neurobiological mechanisms driving, and adaptive benefits underlying, the social response to threat and an active stress coping strategy.