Neurogenesis Inhibition Prevents Enriched Environment to Prolong and Strengthen Social Recognition Memory, But Not to Increase BDNF Expression

Abstract

Hippocampus-dependent memories, such as social recognition (SRM), are modulated by neurogenesis. However, the precise role of newborn neurons in social memory processing is still unknown. We showed previously that 1 week of enriched environment (EE) is sufficient to increase neurogenesis in the hippocampus (HIP) and the olfactory bulb (OB) of mice. Here, we tested the hypothesis that 1 week of EE would enhance SRM persistence and strength. In addition, as brain-derived neurotrophic factor (BDNF) may mediate some of the neurogenesis effects on memory, we also tested if 1 week of EE would increase BDNF expression in the HIP and OB. We also predicted that neurogenesis inhibition would block the gain of function caused by EE on both SRM and BDNF expression. We found that EE increased BDNF expression in the HIP and OB of mice; at the same time, it allowed SRM to last longer. In addition, mice on EE had their SRM unaffected by memory consolidation interferences. As we predicted, treatment with the anti-mitotic drug AraC blocked EE effects on SRM. Surprisingly, neurogenesis inhibition did not affect the BDNF expression, increased by EE. Together, our results suggest that newborn neurons improve SRM persistence through a BDNF-independent mechanism. Interestingly, this study on social memory uncovered an unexpected dissociation between the effect of adult neurogenesis and BDNF expression on memory persistence, reassuring the idea that not all neurogenesis effects on memory are BDNF-dependent.

Social isolation impairs the persistence of social recognition memory by disturbing the glutamatergic tonus and the olfactory bulb-dorsal hippocampus coupling

“…To assess social short-term memory (STM), 1:30 h after TR, mice were habituated exactly as describe above and the social investigation was scored. The social long term-memory (LTM) was measured 24 h after TR 40 . None of the juvenile mice were used more than three times.…”

Pro-neurogenic effect of fluoxetine in the olfactory bulb is concomitant to improvements in social memory and depressive-like behavior of socially isolated mice

“…Bromodeoxyuridine (BrdU; Sigma) was dissolved in 0.9% NaCl and administered intraperitoneally at 75 mg/kg, once a day for a total of 7 days [38][39][40] .…”

“…After, slices were washed in PBS and incubated for 90 min at room temperature with Alexa Fluor 488 (1:400, Invitrogen) and Alexa Fluor 568 (1:400, Invitrogen), washed in PBS, and mounted. Slices were fixed in Vectashield (Vector Laboratories) 39,40 .…”

“…The experimenter counted the cells blind to the condition. The exposure time for each filter was determined through the pixel saturation histogram, and the maximum possible number of pixels below the saturation limit was always used 39,40 .…”

“…Interestingly, enriched environment blunted the effect of SI on memory 36 in a neurogenesis-dependent manner 38 . In fact, the formation of social recognition memory seems reliant on neurogenesis 39,40 .…”