A familiar conspecific is more effective than an unfamiliar conspecific for 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 in male rats that the presence of a conspecific mitigates conditioned fear responses to auditory conditioned stimulus paired with foot shocks. Subsequent studies revealed that we could observe this social buffering when rats were tested in a box odorized by a conspecific. Because we previously used an unfamiliar conspecific, the effects of familiarity with a conspecific on the intensity of social buffering remain unclear. Here, we examine this question by preparing a familiar conspecific that had been housed with a subject for 3 weeks in the same cage. We exposed fear-conditioned subjects to a conditioned stimulus in either a clean control box or a box odorized beforehand by either an unfamiliar or a familiar conspecific. When the subjects were tested in the control box, they showed freezing and Fos expression in the paraventricular nucleus. These responses were suppressed when we placed rats in the box odorized by a conspecific. However, the suppression was greater when the box was odorized by a familiar conspecific rather than by an unfamiliar conspecific. Fos expression in the lateral amygdala was also suppressed in the same manner. These results suggest that a familiar conspecific is more effective for social buffering of conditioned fear responses.