Hippocampus lesions induced deficits in social and spatial recognition in Octodon degus

Previous studies of rodents reported that the hippocampus plays an important role in social behavior as well as spatial behavior. However, there are inconsistencies between reports of the effects of hippocampal lesions on social behavior. The present study sought to clarify the aspects of social behavior in which the hippocampus plays a role in the degu, Octodon degus, a social rodent. We examined the effects of hippocampal lesions on social behavior in the degu using familiar and novel partners. When placed in a familiar environment with a familiar partner after surgery, sham operation control (S.Cont) degus exhibited affinitive behavior longer compared with hippocampal lesioned (HPC) degus. In a novel environment, S.Cont degus exhibited longer aggressive behavior toward novel partners, and longer affinitive behavior with familiar partners compared with HPC degus. HPC degus did not show evidence of differentiation in social behavior, regardless of partner's novelty. The results of an anxiety test confirmed that these findings could not be attributed to changes in emotional state. We conducted an object-recognition test with the same subjects. HPC degus showed an impairment in spatial recognition but not object recognition. Taken together, these results suggest that the degu hippocampus plays an important role not only in spatial recognition but also social recognition. The changes in social behavior resulting from hippocampal lesions were interpreted as due to an impairment of social recognition rather than an impairment in novelty detection.