Olfactory signals mediate social buffering of conditioned fear responses in male rats.

In social animals, the presence of an affiliative conspecific alleviates acute stress responses, and this is called social buffering. We previously reported that social buffering mitigates the fear responses of male rats to auditory conditioned stimuli that had been paired with foot shocks. Subsequent studies revealed that signals that are perceived by the main olfactory epithelium are important for social buffering. Because olfactory signals are the signal perceived by the main olfactory epithelium, we hypothesized that we could induce the social buffering of conditioned fear responses by presenting olfactory signals that were derived from a conspecific. In order to test this hypothesis, we exposed fear-conditioned subjects to a conditioned stimulus either in a clean test box or in a test box that was odorized by keeping a conspecific in it as an odor donor beforehand. When the subjects were tested in the clean test box, they showed behavioral fear responses and enhanced Fos expression in the paraventricular nucleus. In contrast, the presence of conspecific olfactory signals blocked these fear responses and Fos expression. These results suggested that olfactory signals suppress conditioned fear responses. Fos expression in the posteromedial region of the olfactory peduncle and amygdala suggested that this suppression involves the same neural mechanisms as those of social buffering. Taken together, we concluded that olfactory signals mediate the social buffering of conditioned fear responses.