Is full physical contact necessary for buffering effects of pair housing on social stress in rats?

Our previous study showed that pair housing with a familiar male prevented an increase in anxiety caused by social defeat in male rats. The present study attempted to identify the aspects of social interactions with a familiar male that are needed for the emergence of such a pair-housing effect. In Experiment 1, the subject was repeatedly exposed to the cage and bedding used by a familiar pairmate, after two instances of social defeat. Mere exposure to the soiled cage and bedding did not prevent an increase in anxiety in the elevated plus-maze test performed two weeks after social defeat. In Experiment 2, the subject was separated from a familiar pairmate with a wire mesh partition, which allowed visual, auditory, and limited physical contact, in addition to olfactory contact with the pairmate. The separation with a wire mesh partition abolished the buffering effect of pair housing on anxiety. These results indicate that visual, auditory, and olfactory contact with a familiar male was not sufficient in reducing the anxiogenic effect of social defeat in male rats. It was suggested that full physical contact is necessary for the emergence of the buffering effect of pair housing on social stress.