Odor Enrichment Increases Hippocampal Neuron Numbers in Mouse

Abstract

The hippocampus and olfactory bulb incorporate new neurons migrating from neurogenic regions in the brain. Hippocampal atrophy is evident in numerous neurodegenerative disorders, and altered hippocampal neurogenesis is an early pathological event in Alzheimer's disease. We hypothesized that hippocampal neurogenesis is affected by olfactory stimuli through the neural pathway of olfaction-related memory. In this study, we exposed mice to novel pleasant odors for three weeks and then assessed the number of neurons, non-neuronal cells (mainly glia) and proliferating cells in the hippocampus and olfactory bulb, using the isotropic fractionator method. We found that the odor enrichment significantly increased the neuronal cell numbers in the hippocampus, and promoted cell proliferation and neurogenesis in the olfactory bulb. In contrast, the glial cell numbers remained unchanged in both of the regions. Our results suggest that exposure to novel odor stimuli promotes hippocampal neurogenesis and support the idea that enriched environments may delay the onset or slow down the progression of neurodegenerative disorders.

Keywords: Alzheimer's disease; hippocampus; neurodegeneration; neurogenesis; odor stimuli; olfactory bulb.

Fig. 1. Cage setting for standard home cage and for odor stimulus pre-tests. (A) Mice were housed in standard home cage; red arrows indicate the location of mice. (B) Mice were tested to determine their “interest” in the odor provided; red arrow indicates odour stimulant bottle and blue arrows indicate empty vector bottles. (C) A typical enriched cage; red arrows indicate the location of mice.

Fig. 2. Pictorial presentation of the experimental scheme.

Fig. 3. The effect of odor exposure on the hippocampal cell constitution. (A) Odor exposure reforms the hippocampus by increasing the number of neurons and reducing the non-neuronal to neuronal cell ratio (indicated as “cellular constitution”). ^*p<0.05, ^**p<0.01. (B) Representative immunofluorescence images captured from the dentate gyrus of treated and control mice showing similar distribution and density of DCX-positive (red) and Ki-67-positive (green) cells. Arrowheads indicate Ki-67-positive cells. The scale bar in the last image applies to all other images.

Fig. 4. An analysis of the effect of enriched environment on hippocampal cell constitution. ^*p<0.05, ^***p<0.001.

Fig. 5. The effect of odor exposure on the olfactory bulb. (A) The odor exposure increases the number of total cells, neurons, and proliferating cells, and reduces the ratio of non-neuronal to neuronal cell numbers (indicated as “cellular constitution”). ^*p<0.05, ^***p<0.001. (B) Representative immunofluorescence images captured from the rostral migration stream of treated and control mice show the distribution and density of DCX-positive (red) and Ki-67-positive (green) cells. Arrowheads indicate Ki-67-positive cells. The scale bar in the last image applies to all other images.