Chronic social comparison elicits depression- and anxiety-like behaviors and alterations in brain-derived neurotrophic factor expression in male rats.

Social comparison is a fundamental human characteristic; however, long-term social comparison may induce psychological stress and can lead to depression and anxiety. Recent studies have shown that nonhuman primates compare themselves with others; however, no studies have investigated whether social comparisons exist among rodents. In the present study, we established a rat model of social comparison. This model was subsequently used to examine the effects of the differential environment of a partner on depression- and anxiety-like behaviors in male rats, as well as to assess the changes in serum, medial prefrontal cortex (mPFC), and dorsal hippocampus brain-derived neurotrophic factor (BDNF) levels induced by long-term social comparison. Compared to rats whose partners were exposed to the same environment, rats whose partners were exposed to two combined enriched environmental stimuli for 14 days showed significantly decreased social novelty preference and sucrose consumption. No anxiety-like behaviors were observed. Rats whose partners were exposed to one enriched environment for 31 days showed significantly increased immobility time in the forced swimming test, and significantly decreased time spent in the center area in the open-field test. Further, rats whose partners were exposed to one enriched environment for 31 days showed lower BDNF levels in the mPFC and dorsal hippocampus, but not following partner exposure for 14 days. These results suggest that social comparisons exist in rats and can induce psychosocial stress and other negative affect. This model will not only provide the possibility to reveal the neurobiological basis of the emotional impact of social comparison, but could also be used to confirm the conservative evolutionary characteristics of social comparison as a behavioral attribute.

Distinct mechanisms mediate the reinstatement and spontaneous recovery of extinguished fear in rats.

It is controversy whether the summation of new contextual conditioning with residual conditioned response contributes to the reinstatement phenomenon. Here, we examined the summation hypothesis in the auditory conditioned fear paradigm by comparing the freezing levels during pre-tone and tone presentation periods. Unexpectedly, we found that the onset of tone relieves fear during the reinstatement test, which is not predicted by the summation hypothesis; whereas the onset of tone enhances fear during the spontaneous recovery test and early extinction session. These results implicated that return of fear in two conventional assays after extinction, i.e., reinstatement assay and spontaneous recovery assay, are mediated by different mechanisms.