Short-term social isolation induces depressive-like behaviour and reinstates the retrieval of an aversive task: mood-congruent memory in male mice?

Background: Although mood-congruent memory (MCM), or the tendency to recall information consistent with one's mood, is a robust phenomenon in human depression, to our knowledge, it has never been demonstrated in animals.

Methods: Mice were subjected to social isolation (SI) or crowding for 12 hours and had their depressive-like behaviour (evaluated by the forced swim, tail suspension, sucrose preference and splash tests) or their serum corticosterone concentrations evaluated. In addition, we determined the temporal forgetting curve of the plus-maze discriminative avoidance task (PM-DAT) and examined the effects of SI or crowding on memory retrieval in the PM-DAT. Finally, we verified the effects of metyrapone pretreatment on reinstatement of memory retrieval or on the increase of corticosterone levels induced by SI.

Results: Twelve hours of SI produced depressive-like behaviour, enhanced corticosterone concentration and reinstated retrieval of a forgotten discriminative aversive (i.e., negatively valenced) task. Depressive-like behaviour was critical for this facilitative effect of SI because 12 hours of crowding neither induced depressive-like behaviour nor enhanced retrieval, although it increased corticosterone levels at the same magnitude as SI. However, corticosterone increase was a necessary condition for MCM in mice, in that the corticosterone synthesis inhibitor metyrapone abolished SI-induced retrieval reinstatement.

Limitations: Our study did not investigate the effects of the social manipulations proposed here in a positively valenced task.

Conclusion: To our knowledge, the present paper provides the first evidence of MCM in animal models.