Neurogenesis decreases with age in the canine hippocampus and correlates with cognitive function

Abstract

New neurons are continually produced in the adult mammalian brain from progenitor cells located in specific brain regions, including the subgranular zone (SGZ) of the dentate gyrus of the hippocampus. We hypothesized that neurogenesis occurs in the canine brain and is reduced with age. We examined neurogenesis in the hippocampus of five young and five aged animals using doublecortin (DCX) and bromodeoxyuridine (BrdU) immunostaining. The total unilateral number of new neurons in the canine SGZ and granule cell layer (GCL) was estimated using stereological techniques based upon unbiased principles of systematic uniformly random sampling. Animals received 25mg/kg of BrdU once a day for 5 days and were euthanized 9 days after the last injection. We found evidence of neurogenesis in the canine brain and that cell genesis and neurogenesis are greatly reduced in the SGZ/GCL of aged animals compared to young. We further tested the hypothesis that an antioxidant fortified food or behavioral enrichment would improve neurogenesis in the aged canine brain and neurogenesis may correlate with cognitive function. Aged animals were treated for 2.8 years and tissue was available for six that received the antioxidant food, five that received the enrichment and six receiving both treatments. There were no significant differences in the absolute number of DCX or DCX-BrdU neurons or BrdU nuclei between the treatment groups compared to control animals. The number of DCX-positive neurons and double-labeled DCX-BrdU-positive neurons, but not BrdU-positive nuclei alone, significantly correlated with performance on several cognitive tasks including spatial memory and discrimination learning. These results suggest that new neurons in the aged canine dentate gyrus may participate in modulating cognitive functions.

Figure 1

DCX and BrdU immunohistochemical staining in the SGZ and GCL of the hippocampus in a young (A,C) and aged (B,D) Beagle. Only a subset of DCX-positive neurons colabeled with BrdU (arrow). Scale in A and B is 100 μm and scale in C and D is 20 μm. Confocal image showing colocalization. Scale in E, F and G is 5 μm.

Figure 2

Graphs plotting the numbers of (A) BrdU-positive nuclei, (B), DCX-positive new neurons, (C) double labeled DCX-BrdU-positive neurons, and (D) the proportion of BrdU cells expressing DCX in young, aged and treated-aged canines. Young animals had significantly higher number of cells and proportions compared to aged animals (p < .05). Only the proportion of BrdU cells expressing DCX (D) was higher in the treatment groups compared to age-matched controls even though this did not achieve statistical significance. Control = aged untreated animals; antox = aged animals receiving the antioxidant fortified food; enriched = aged animals receiving behavioral enrichment; antenr = aged animals receiving both the antioxidant food and behavioral enrichment. Errors bars = SEM.

Figure 3

Higher numbers of new neurons in the SGZ and GCL of the aged canine hippocampus correlated with lower error scores on the black-white discrimination reversal learning task (A, B) and the spatial memory task (C).