Hippocampal cytogenesis and spatial learning in senile rats exposed to chronic variable stress: effects of previous early life exposure to mild stress

Abstract

In this study, we exposed adult rats to chronic variable stress (CVS) and tested the hypothesis that previous early-life exposure to stress changes the manner in which older subjects respond to aversive conditions. To this end, we analyzed the cytogenic changes in the hippocampus and hippocampal-dependent spatial learning performance. The experiments were performed on 18-month-old male rats divided into four groups as follows: Control (old rats under standard laboratory conditions), Early-life stress (ELS; old rats who were exposed to environmental noise from postnatal days, PNDs 21-35), CVS + ELS (old rats exposed to a chronic stress protocol who were previously exposed to the early-life noise stress) and CVS (old rats who were exposed only to the chronic stress protocol). The Morris Water Maze (MWM) was employed to evaluate the spatial learning abilities of the rats at the end of the experiment. Immunohistochemistry against 5'Bromodeoxyuridine (BrdU) and glial fibrillar acidic protein (GFAP) was also conducted in the DG, CA1, CA2 and CA3 regions of the hippocampus. We confocally analyzed the cytogenic (BrdU-labeled cells) and astrogenic (BrdU + GFAP-labeled cells) changes produced by these conditions. Using this procedure, we found that stress diminished the total number of BrdU+ cells over the main proliferative area of the hippocampus (i.e., the dentate gyrus, DG) but increased the astrocyte phenotypes (GFAP + BrdU). The depleted BrdU+ cells were restored when the senile rats also experienced stress at the early stages of life. The MWM assessment demonstrated that stress also impairs the ability of the rats to learn the task. This impairment was not present when the stressful experience was preceded by the early-life exposure. Thus, our results support the idea that previous exposure to mild stressing agents may have beneficial effects on aged subjects.

Keywords: aging; astrocytes; dentate gyrus; glia; memory; noise.

Figure 1

General procedure: Illustrates the general procedure followed in our experiment for the (A) Control group, (B) Early-life stress (ELS) group, (C) Early-life stress + Chronic variable stress (ELS + CVS) group, and (D) Chronic variable stress (CVS) group. The experimental procedures are chronologically depicted above and below the line. The postnatal age is written below the line.

Figure 2

Morris water maze (MWM) assessment. Figure shows the results of the escape latencies across trials (means ± SEM). Significant interactions were found for the escape latency for trials 1, 3 and 4 (*p < 0.05), where the ELS-exposed animals showed increased latencies. A significant interaction was found for trial 2, where the ELS + CVS-exposed animals decreased the latency to find the hidden platform (*p < 0.05). The ELS-exposed rats were no different than the control rats.

Figure 3

Hippocampal cytogenesis in senile male rats. The figure contains graphics illustrating the means ± SEM of the BrdU-positive cells in the (A) dentate gyrus (DG), (B) cornus ammonis (CA3), (C) CA2, and (D) CA1 subregions. The ELS + CVS-exposed rats exhibited an increased number of BrdU+ cells in the DG (***p < 0.001), CA3 (***p < 0.001), CA2 (***p < 0.001), and CA1 (***p < 0.001). The CVS-exposed rats exhibited a decreased number of 5′Bromodeoxyuridine (BrdU+) cells in the DG (*p < 0.05). The ELS-exposed rats exhibited an increased number of BrdU+ cells in the CA1 region (**p < 0.01).

Figure 4

Hippocampal gliogenesis in senile male rats. The right panel contains graphics illustrating the means ± SEM percentage of glial fibrillar acidic protein (GFAP) + BrdU-positive cells in the (A) dentate gyrus, (B) CA3, (C) CA2, and (D) CA1 subregions. The ELS + CVS-exposed rats exhibited an increased number of GFAP + BrdU+ cells in the DG (*p < 0.05). The CVS-exposed rats exhibited an increased percentage of GFAP + BrdU-positive cells in the CA2 region (*p < 0.05). The ELS-exposed rats exhibited an increase in the numbers of GFAP + BrdU-positive cells in the CA3 region (*p < 0.05). The left panel illustrates co-labeling in the two statistically relevant regions: (A′) DG, and (B′) CA2 (slices 730 μm × 730 μm). The white boxes in (C′) and (D′) illustrate co-labeling; the round green cells are BrdU+, and the star-shaped red cells are GFAP+.