Experience-induced neurogenesis in the senescent dentate gyrus

Abstract

We demonstrate here that under physiological conditions neurogenesis continues to occur in the dentate gyrus of senescent mice and can be stimulated by living in an enriched environment. Neurogenesis was investigated by confocal microscopy of three-channel immunofluorescent staining for the proliferation marker bromodeoxyuridine (BrdU) and neuronal and glial markers. Quantification was performed with unbiased stereological counting techniques. Neurogenesis decreased with increasing age. Stimulation of adult and aged mice by switching from standard housing to an enriched environment with opportunities for social interaction, exploration, and physical activity for 68 d resulted in an increased survival of labeled cells. Phenotypic analysis revealed that, in enriched living animals, relatively more cells differentiated into neurons, resulting in a threefold net increase of BrdU-labeled neurons in 20-month-old mice (105 vs 32 cells) and a more than twofold increase in 8-month-old mice (684 vs 285 cells) compared with littermates living under standard laboratory conditions. Corresponding absolute numbers of BrdU-positive astrocytes and BrdU-positive cells that did not show colabeling for neuronal or glial markers were not influenced. The effect on the relative distribution of phenotypes can be interpreted as a survival-promoting effect that is selective for neurons. Proliferation of progenitor cells appeared unaffected by environmental stimulation.

Fig. 1.

A, One of the cages for an enriched environment; B, One standard cage at the same scale (the trays for food and water are removed). Enrichment consisted of social interaction (13 mice in the large cage vs 3 in the standard cage), stimulation of exploratory behavior with objects such as toys and a rearrangeable set of tunnels, and C, physical activity in a running wheel. In addition to standard food and water ad libitum, enriched mice received occasional treats, including cheese, crackers, and fruits.

Fig. 2.

Experimental design applied to each of the two age groups (6 or 18 months at the beginning of the experiment). The 12 d period of daily BrdU injections is symbolized by syringes.WMZ, Behavioral testing on the water maze task.

Fig. 3.

A, Total number of BrdU-positive cells per dentate gyrus 1 d after the last injection of BrdU to estimate ongoing proliferation (hatched bars) and 4 weeks later to assess survival of BrdU-positive cells (open bars). B, C, Percentage of the surviving BrdU-positive cells (compare with B) that differentiated into either neuronal (filled bars) or glial (hatched bars) phenotype or showed neither differentiation (open bars). D, the total number of BrdU-labeled neurons generated over the 12 d of BrdU injections (values derived from the phenotypic ratio times the number of BrdU-positive cells at 4 weeks). Note that because the number of BrdU-positive cells is influenced by cell cycle parameters, no absolute statements about the size of the population of proliferating neuronal progenitor cells are possible. *p < 0.05; **p < 0.005. For details on statistical analyses see Materials and Methods.

Fig. 4.

Confocal microscopic analysis of immunofluorescent triple-labeling of BrdU-positive cells (red) 1 d (A) and 4 weeks (B–D) after the last injection of BrdU. A, Overview of the distribution of BrdU-positive cells along the border between the arrowhead-shaped granule cell layer of the dentate gyrus and the hilar area CA4 (compare with hatched columns in Fig.3A). In addition there are some proliferating cells in the hilus itself and in the molecular layer. No qualitative difference among the four groups could be found. Four weeks after injection the phenotypes of BrdU-labeled cells were examined (B–D). Markers were NeuN (green) for neurons and S100β (blue) for astrocytes. B, Two BrdU-labeled neurons (orange = red + green) and one BrdU-positive cell that is neither NeuN- nor S100β-positive in an Enr-6 animal. C, BrdU-labeled neuron with the typical chromatin pattern of a granule cell in an Enr-18 animal.D, One BrdU-labeled astrocyte (pink = red + blue, left) and one BrdU-labeled neuron (orange, right). Scale bar (in A): A, 200 μm; B, C, 12 μm;D, 20 μm.

Fig. 5.

Data from the water maze test for 18-month-old mice. Performance curves for 6-month-old animals looked similar. Overall analysis of time to reach the platform revealed significant differences between controls and enriched living mice. The curve for swim path paralleled A in appearance but in overall analysis no significant difference between the groups could be found. Although experience induced functional improvements, a causal relation to neurogenesis remains to be established.