Hormetic effects by exercise on hippocampal neurogenesis with glucocorticoid signaling

Abstract

Exercise enhances adult hippocampal neurogenesis (AHN), although the exact nature of how this happens remains controversial. The beneficial effects of exercise vary depending upon the exercise condition, especially intensity. Most animal studies, however, have used wheel running, which only evaluates running distance (exercise volume) and does not consider intensity. In our rat model, we have found that exercise-induced neurogenesis varies depending on the intensity of the exercise and have found that exercise-enhanced neurogenesis is more pronounced with mild exercise than with moderate and/or intense exercise. This may be due, at least in part, to increased glucocorticoid (CORT) secretion. To test this hypothesis, we used our special exercise model in mice, with and without a stress response, based on the lactate threshold (LT) in which moderate exercise above the LT increases lactate and adrenocorticotropic hormone (ACTH) release, while mild exercise does not. Adult male C57BL/6J mice were subjected to two weeks of exercise training and AHN was measured with a 5-Bromo-2-deoxyuridine (BrdU) pre-injection and immunohistochemistry. The role of glucocorticoid signaling was examined using intrapertioneal injections of antagonists for the glucocorticoid receptor (GR), mifepristone, and the mineralocorticoid receptor (MR), spironolactone. We found that, while mild exercise increased AHN without elevating CORT blood levels, both MR and GR antagonists abolished mild-exercise-induced AHN, but did not affect AHN under intense exercise. This suggests a facilitative, permissive role of glucocorticoid and mineralocorticoid receptors in AHN during mild exercise (234/250).

Keywords: Glucocorticoid receptor; hippocampus; lactate threshold; mild exercise; mineralocorticoid receptor; neurogenesis.

Fig.1

A typical lactate threshold (LT) profile for a mouse during a gradual increase of running speed protocol (0–35 m/min) (A) To collect blood during treadmill running, a silicone catheter was inserted through the jugular vein to the right atrium three days before the graded exercise test. (B) Typical data for LT of one mouse (No. 4) is shown. The LT (around 20 m/min) was determined from the non-linear increase in blood lactate levels vs running speed using a modified regression analysis. Inset is average LT for eight mice.

Fig.2

Determination of blood lactate, glucose, and plasma corticosterone after different intensities of exercise (A) Protocols for different intensities of exercise, (B) blood lactate levels (F _(3,24)  = 23.47, p <  0.0001), (C) plasma corticosterone levels (F _(3,24)  = 6.31, p = 0.0026), and (D) blood glucose levels (F _(3,24)  = 1.17, p = 0.34). Data represent the mean ± SEM (n = 7 mice per group). ^**, p <  0.01, ^***, p <  0.001 in comparison with control and ^# # # , p <  0.001 in comparison to sub-LT (one-way ANOVA Tukey post hoc tests).

Fig.3

Effects of different exercise intensities on adult neurogenesis of male mice (A) Timeline of experiment, (B) A representative confocal micrograph of triple labeling of BrdU (red), the immature neuron marker DCX (green), and the mature neuron marker NeuN (blue) in the hippocampal dentate area. Scale bars, 50μm. (C) Total number of BrdU⁺ cells (F _(2,21)  = 3.65, p = 0.044). (D) Number of BrdU⁺/DCX⁺ cells (F _(2,21)  = 10.69, p = 0.0006). (E) Number of BrdU⁺/NeuN⁺ cells in the dentate gyrus (F _(2,21)  = 4.69, p = 0.02). Data represent the mean ± SEM (n = 8 mice per group). ^*, p <  0.05, ^***, p <  0.001 in comparison to control mice and ^# # , p <  0.01 in comparison with supra-LT (one-way ANOVA Tukey post hoc tests).

Fig.4

Effects of glucocorticoid receptor antagonists on exercise-induced adult hippocampal neurogenesis The role of glucocorticoid signaling was evaluated with the injection of the mineralocorticoid receptor (MR) antagonist spironolactone, and the glucocorticoid receptor (GR) antagonist mifepristone. (A) Total number of BrdU+ cells. There was a significant main effect of treadmill exercise (F _(2,49)  = 4.15, p <  0.05), but not glucocorticoid receptor antagonist (F _(2,49)  = 3.07, p = 0.06), and a significant glucocorticoid receptor antagonist×treadmill exercise interaction (F _(4,49)  = 4.7, p <  0.01). (B) Number of BrdU+/DCX+ cells. There was a significant main effect of treadmill exercise (F _(2,49)  = 5.59, p <  0.01), but not glucocorticoid receptor antagonist (F _(2,49)  = 3.72, p = 0.69), and a significant glucocorticoid receptor antagonist×treadmill exercise interaction (F _(4,49)  = 4.86, p <  0.01). (C) Number of BrdU+/NeuN+ cells in the dentate gyrus. There was a significant main effect of treadmill exercise (F _(2,47)  = 3.23, p <  0.05), but not glucocorticoid receptor antagonist (F _(2,47)  = 0.61, p = 0.55), and no significant glucocorticoid receptor antagonist×treadmill exercise interaction (F _(4,47)  = 1.87, p = 0.14). Data represent the mean ± SEM (n = 6 -7 mice). ^*, p <  0.05, ^**, p <  0.01, ^***, p <  0.001 in comparison with respective control mice and #, p <  0.05, ##, p <  0.01 in comparison with supra-LT (two-way ANOVA and Bonferroni post hoc tests).