Enriched environment increases neurogenesis and improves social memory persistence in socially isolated adult mice

Social memory consists of the information necessary to identify and recognize cospecifics and is essential to many forms of social interaction. Social memory persistence is strongly modulated by the animal's experiences. We have shown in previous studies that social isolation (SI) in adulthood impairs social memory persistence and that an enriched environment (EE) prevents this impairment. However, the mechanisms involved in the effects of SI and EE on social memory persistence remain unknown. We hypothesized that the mechanism by which SI and EE affect social memory persistence is through their modulation of neurogenesis. To investigate this hypothesis, adult mice were submitted to 7 days of one of the following conditions: group-housing in a standard (GH) or enriched environment (GH+EE); social isolation in standard (SI) or enriched environment (SI+EE). We observed an increase in the number of newborn neurons in the dentate gyrus of the hippocampus (DG) and glomerular layer of the olfactory bulb (OB) in both GH+EE and SI+EE mice. However, this increase of newborn neurons in the granule cell layer of the OB was restricted to the GH+EE group. Furthermore, both SI and SI+EE groups showed less neurogenesis in the mitral layer of the OB. Interestingly, the performance of the SI mice in the buried food-finding task was inferior to that of the GH mice. To further analyze whether increased neurogenesis is in fact the mechanism by which the EE improves social memory persistence in SI mice, we administered the mitotic inhibitor AraC or saline directly into the lateral ventricles of the SI+EE mice. We found that the AraC treatment decreased cell proliferation in both the DG and OB, and impaired social memory persistence in the SI+EE mice. Taken together, our results strongly suggest that neurogenesis is what supports social memory persistence in socially isolated mice.