Animal age and enriched environment affect the “retirement” of adult‐generated dentate granule cells.

Adult neurogenesis occurs in the subgranular zone of the dentate gyrus, resulting in the addition of functionally integrated neurons. These new cells go through a period of increased excitability around 4-6 weeks after cell division, but then attain more mature cell properties. The idea that mature granule cells may eventually become difficult to activate and essentially “retire” was a recent hypothesis. However, this hypothesis remains controversial. We addressed this controversy using a within-animal protocol for birth-dating different cohorts of adult-born granule cells in 10-month-old Sprague-Dawley rats. The thymidine analogs chloro-deoxyuridine (CldU) and iodo-deoxyuridine (IdU) were injected s.c. or i.p. to label cells born 4, 6, 12, or 35 weeks prior to death. Activation of adult-born cells was assessed post-mortem by quantifying co-expression of the immediate early gene Zif268, using double-label immunofluorescence techniques. The results produced a U-shaped curve with the co-expression of Zif+ in XdU+ (positive for either CldU or IdU) in the 4, 6, 12 and 35-week-old cells. However, it was not clear whether the significantly increased activity in the 35-week-old cells was because they came out of “retirement” due to their age, or because they were born at a young adult age (2 months) and never really “retired”. To test whether cell age was a factor, we birth-dated 12 and 35-week-old granule cells that were born at a mature adult age (7 and 12 months) and then studied when rats were 15 months old. In this 15-month group, the XdU-labelled cells were only active at percentages of 0.8±0.1% and 0.6±0.2% for the 12 and 35-week-old cells, respectively. The 15-month group had a significantly lower percentage of active cells compared to the 10-month-old group, with respect to the 35-week-old cells (p=0.0005). This suggested that depending on the age of the animal at which the cells are born, populations of cells with equivalent ages can have significantly different levels of activity. Therefore, “retirement” of adult-born granule cells is affected by the age of the animal at which the granule cells are born. We then attempted to wake the less active cells by exposing the animals to an enriched environment (EE) for 10 nights prior to death in 10-month-old animals. The results showed an increased Zif+ in XdU+ percentages of 6.6±1.2%, 4.4±1.9%, 3.1±0.5%, and 5.5±3.1% for 4, 6, 12 and 35-week-old cells, respectively (p=0.03 comparing home cage vs EE). This suggested that cells that are less active or “retired” could be woken up, which indicates that these cells were not truly “retired”. However, the U-shaped curve remained representative of the activity of adult-born cells, despite being exposed to EE. Thus, the pattern of the effect of cell age for 10-month-old animals appeared to be relatively unaltered by exposure to EE. These results suggested that the adult-born dentate granule cells (DGCs) do not truly retire but the animal age at which the DGCs are born affects their phenotypic excitability as they age.