Different regulation of adult hippocampal neurogenesis in Western house mice (Mus musculus domesticus) and C57BL/6 mice

Abstract

Adult hippocampal neurogenesis (AHN) of laboratory rodents is enhanced by physical exercise in a running wheel. However, little is known about modulation of AHN in wild-living rodent species. The finding that AHN cannot be modulated by voluntary exercise in wild wood mice suggests that AHN may be regulated differently under natural conditions than in laboratory adapted animals. In order to minimize genetic influences, we aimed to investigate the genetically closest wild-living relatives of laboratory mice. Here, C57BL/6 mice and F1 offspring of wild house mice (Mus musculus domesticus) were tested in two different running paradigms: voluntary running and running-for-food--a condition in which mice had to run for their daily allowance of food. In house mice, we found a non-significant trend towards increased numbers of proliferating cells and doublecortin-positive immature neurons in both voluntary runners and runners-for-food. Voluntary running in C57BL/6 mice resulted in a 30% increase in cell proliferation and a pronounced 70% increase in doublecortin-positive cells. C57BL/6 runners-for-food ran as much as voluntary runners, but they showed no enhancement of cell proliferation, a small increase in the number of doublecortin-positive cells and more pyknotic cells compared to controls. Taken together, these findings suggest that motivational aspects of running are critical determinants of the increased cell proliferation in C57BL/6 mice. In contrast, running has smaller and context-independent effects in house mice. The findings imply a difference in the regulation of AHN in C57BL/6 mice and their wild-derived conspecifics.

Fig. 1. Visualization of proliferating cells, DCX-positive and pyknotic cells

(a) Immunohistochemical staining of proliferating cells with Ki67 antibody. Scale bar: 10 μm; hi: hilus, gcl: granule cell layer). (b) DCX-positive cells of neuronal lineage, counterstained with haematoxylin; scale bar: 10 μm. (c) Condensed chromatin of a Giemsa-stained pyknotic cell, forming two distinct nuclear bodies; scale bar: 10 μm.

Fig. 2. Performance in the running wheel does not differ between voluntary and running-for-food mice

In C57BL/6− and house mice total performance does not differ between voluntary runners and runners-for-food. No significant difference between C57BL/6− and house mice is found. Bars represent mean standard deviation (SD).

Fig. 3. Cell numbers in C57BL/6

(a) Cell proliferation is significantly increased in voluntary running C57BL/6 mice but not in runners-for-food. (b) The number of DCX-positive cells is significantly increased in voluntary runners and runners-for-food, but the increase is smaller in runners-for-food. (c) C57BL/6 runners-for-food display an enhanced number of pyknotic cells compared to controls. *: p<0.05, **: p<0.01. Bars = SD.

Fig. 4. Cell numbers in house mice

In house mice, (a) we find no significant running-induced increase in proliferation and (b) in the number of DCX-positive cells, although there is a consistent 30% trend of an increased number of proliferating cells and cells of the neuronal lineage. (c) No group differences are found in the number of pyknotic cells. Note also that proliferation and neuronal differentiation in house mice is overall lower than in C57BL/6. Bars = SD.

Fig. 5. Equal distribution of cell numbers in female and male house mice

(a) No gender differences are found in the numbers of proliferating cells, (b) DCX-positive cells and (c) pyknotic cells in house mice. The distribution of female data points within the groups suggests that there would be no gender differences even with a larger number of females.

Fig. 6. Missing correlation between cell counts and performance

No correlations between individual running performance and (a) proliferation, (b) neuronal differentiation or (c) pyknosis are found in either C57BL/6 or house mice. Larger inter-individual variability is observed in the number of proliferating and DCX-positive cells in C57BL/6 mice, whereas house mice show small variability on an overall lower level. However, house mice show larger variability in performance than C57BL/6. Data points represent total values (estimated cell numbers and wheel revolutions in the two weeks of the experiment) of the individual mice.