Social buffering reduces male rats' behavioral and corticosterone responses to a conditioned stimulus

In social species, the presence of an affiliative same-sex conspecific ameliorates acute stress responses in threatening conditions. We previously found that the presence of an unfamiliar male rat separated by a wire mesh barrier blocks the behavioral responses and Fos expression in the paraventricular nucleus of the hypothalamus (PVN) in a male subject rat that had previously been exposed to an auditory conditioned stimulus (CS) paired with foot shocks. Based on the Fos expression in the PVN, we hypothesized that the presence of a conspecific ameliorated the hypothalamic–pituitary–adrenal (HPA) axis activation and induced social buffering of conditioned fear responses. The direct evidence for this hypothesis, however, is still lacking. To clarify this point, we exposed fear-conditioned and non-conditioned subjects to the CS either alone or with a conspecific separated by a wire mesh barrier. When the fear-conditioned subject alone was re-exposed to the CS, it exhibited increased freezing, decreased sniffing, and elevated corticosterone levels. In contrast, the presence of the conspecific suppressed these behavioral and HPA axis responses to a level similar to those observed in the non-conditioned subjects. These results suggest that the presence of a conspecific suppressed the behavioral responses and HPA axis activation to the CS. The present results provide direct evidence for the existence of social buffering of conditioned fear responses in male rats.