Sex-dependent effects of chronic exercise on cognitive flexibility in aging mice

Abstract

Cognitive impairments associated with advanced age are a growing concern in our aging society. Such impairments are associated with alterations in brain structure and function, especially in the hippocampus, which changes to experience throughout life. It is well-known that regular exercise can maintain hippocampus volume. The hippocampus is critical for cognitive flexibility involved with extinction and reinstatement of conditioned fear. Therefore, we asked whether voluntary chronic exercise in middle-aged mice can improve extinction and/or reinstatement of conditioned fear compared to standard housing. Eight-month-old male and female C57Bl/6J mice had access to a running wheel or remained in standard housing until 11 months of age. Alongside control standard-housed young adult (3-month-old) mice, they received tone-footshock pairings, which were subsequently extinguished with tone-alone presentations the next day. Half of the mice then received a reminder treatment in the form of a single footshock. Both male and female 11-month-old mice housed in standard conditions exhibited impaired reinstatement compared to young adult mice. However, for males that had access to a running wheel from 8 months of age, the reminder treatment rescued reinstatement ability. This was not observed in females. Additionally, exercise during middle age in both sexes increased expression of Bdnf mRNA in the hippocampus, specifically exon 4 mRNA. These results show that, at least for males, physical exercise is beneficial for reducing age-related decline in cognitive abilities. Despite not rescuing their impaired reinstatement, exercise also increased Bdnf gene expression in the female hippocampus, which could potentially benefit other forms of hippocampal-dependent cognition.