Conditioning properties of social subordination in rats: behavioral and biochemical correlates of anxiety

To develop a socially based model of anxiety, the contextual fear conditioning properties of social defeat were examined in rats. Social threat consisted of exposing intruders to aggressive residents in resident home cage, separated by a partition. During 3 daily encounters, intruders were either defeated or threatened by residents, providing the defeated-threatened (DT) and threatened-threatened (TT) groups respectively. On Day 4, both DT and TT animals were subjected to a social threat only. Additional animals received a 4-day exposure to a novel empty cage (EC group). Further DT, TT, and EC rats were confronted to a different context on Day 4. DT rats exhibited a robust and context-specific anxiety-like response, characterized by significant behavioral and biochemical alterations. DT rats showed increased risk assessment and decreased exploration compared to TT and EC rats that in turn were not different towards each other. DT and TT rats exhibited increased ACTH levels, while only DT rats showed enhanced corticosterone and decreased testosterone levels compared to EC. These differences were context-specific since they were absent confronting animals to a different context and since they were not long lasting. Overall, these data demonstrate the induction of an anxiety-like state in rats through a context conditioning process based upon social factors. The social basis of this paradigm offers good face validity with anxiety disorders, which in humans are mainly related to social factors and associated with HPA axis deregulations. The present procedure may provide a useful experimental model to further investigate the neurobiological mechanisms underlying anxiety-related disorders.     

The hormonal and behavioral response to group formation, seasonal changes, and restraint stress in the highly social Malayan Flying Fox (Pteropus vampyrus) and the less social Little Golden-mantled Flying Fox (Pteropus pumilus) (Chiroptera: Pteropodidae)

This study examined behavioral and physiological responses (changes in inter-animal spacing, total glucocorticoids, testosterone, and body mass) to the formation of breeding and same-sex groups in two bat species, the socially gregarious Malayan Flying Fox (Pteropus vampyrus) and the less social Little Golden-mantled Flying Fox (Pteropus pumilus). We hypothesized that social instability, especially in the breeding groups and especially in P. vampyrus, would result in elevated glucocorticoids and that social facilitation of breeding and/or male-male competition would result in persistently higher levels of testosterone in breeding males. Seasonal rhythms in all measures were also predicted, and the glucocorticoid stress response was expected to vary by sex, season, and group type. Nearly all animals responded to group formation with elevated glucocorticoids, but, for breeding animals (especially aggressive male P. vampyrus), these responses persisted over time. In both species, breeding group formation resulted in elevated testosterone in males. Glucocorticoids, testosterone, testes volume, and body mass generally peaked in the breeding season in males (late summer and early autumn), but the seasonal glucocorticoid peak in females occurred in late winter and early spring. All animals responded to restraint stress with elevations in glucocorticoids that largely did not differ by sex, time of year, reproductive condition, group type, or, in lactating females, the presence of her pup. Changes in both behavior and physiology were more evident in P. vampyrus than in P. pumilus, and we believe that their underlying social differences influenced their responses to group formation and to the changing seasonal environment.