Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis

Abstract

Exposure to an enriched environment and physical activity, such as voluntary running, increases neurogenesis of granule cells in the dentate gyrus of adult mice. These stimuli are also known to improve performance in hippocampus-dependent learning tasks, but it is unclear whether their effects on neurogenesis are exclusive to the hippocampal formation. In this study, we housed adult mice under three conditions (enriched environment, voluntary wheel running and standard housing), and analysed proliferation in the lateral ventricle wall and granule cell neurogenesis in the olfactory bulb in comparison to the dentate gyrus. Using bromodeoxyuridine to label dividing cells, we could not detect any difference in the number of newly generated cells in the ventricle wall. When giving the new cells time to migrate and differentiate in the olfactory bulb, we observed no changes in the number of adult-generated olfactory granule cells; however, voluntary running and enrichment produced a doubling in the amount of new hippocampal granule cells. The discrepancy between the olfactory bulb and the dentate gyrus suggests that these living conditions trigger locally through an as yet unidentified mechanism specific to neurogenic signals in the dentate gyrus.

Environmental enrichment increases progenitor cell survival in the dentate gyrus following lateral fluid percussion injury

“…This finding is in general agreement with several previous studies that have demonstrated that EE can increase neural progenitors in the hippocampus [6,7,16,22,54,79,81]. However, this finding was also novel and somewhat unexpected in comparison to previous studies because 1) in intact animals, EE produced bilateral increases in neural progenitors in the granule cell layer, and no increases in the hilus [6,54,81], and 2) following an ischemic injury, two weeks of EE did not have an additive effect on the number of progenitor cells in the dentate gyrus [5]. There are several possible explanations for the differences between our findings and those of previous studies.…”

“…Unlike the BrdU data, which demonstrated increased survival of cells that were proliferating at the time of injury, the ki67 labeling did not differ between groups or sides of the brain. These findings are in agreement with previous studies where EE was demonstrated to increase cell survival, but not ongoing proliferation in control animals [6,9,43,54,73,81]. Interestingly, running increases both cell proliferation and survival of progenitors in the dentate gyrus [16], which underscores the importance of clarifying how specific components of these behavioral interventions can differentially modulate progenitor cells.…”

“…For example, EE has been reported to improve behavior following FP injury in male rats, but not in female rats [77]. Besides gender [54,76], there are numerous other factors, including species, [6], age [81], the onset and duration of the EE [20,54], and the type of injury [5] that may also explain the differential effects of EE among various studies. Furthermore, differences in the number, timing, and dose of BrdU injections [8,54] may also contribute to different study outcomes.…”

Olfactory system modulation of hippocampal cell death

“…These results demonstrate that input from ipsilateral olfactory structures plays a role in hippocampal dentate cell death, in addition to the roles previously demonstrated for exploration and learning [9,25], exercise [4,6,28] and hormones and stress [20]. The fact that the effects were limited to the ipsilateral hemisphere in unilateral OBX animals argues against a generalized or widespread system degeneration.…”

“…Two regions uniformly agreed to display continual neurogenesis through life are the olfactory bulb and the hippocampal dentate gyrus [1,2,15,18]. In both structures, survival of adult generated neurons is dependent on their being incorporated into an active circuit [4,9,18,23]. For example, exposure to specific odors enhances the survival of adult-born granule cells within spatial regions of the olfactory bulb activated by those odors [23].…”