Voluntary exercise followed by chronic stress strikingly increases mature adult-born hippocampal neurons and prevents stress-induced deficits in ‘what–when–where’ memory

Highlights

• Voluntary exercise, chronic stress or its combination preceded a What–when–where task.

• The amount of voluntary exercise performed was notably reduced by stress.

• However, voluntary exercise improved Www memory and prevented the stress deficits.

• Memory correlated with hippocampal c-Fos and treatments modulated functional networks.

• Exercise followed by stress enhanced the new cells with mature neuron phenotype.

Abstract

We investigated whether voluntary exercise prevents the deleterious effects of chronic stress on episodic-like memory and adult hippocampal neurogenesis. After bromodeoxyuridine (BrdU) administration, mice were assigned to receive standard housing, chronic intermittent restraint stress, voluntary exercise or a combination of both (stress starting on the seventh day of exercise). Twenty-four days later, mice were tested in a ‘what–when–where’ object recognition memory task. Adult hippocampal neurogenesis (proliferation, differentiation, survival and apoptosis) and c-Fos expression in the hippocampus and extra-hippocampal areas (medial prefrontal cortex, amygdala, paraventricular hypothalamic nucleus, accumbens and perirhinal cortex) were assessed after behavior. Chronic intermittent restraint stress impaired neurogenesis and the ‘when’ memory, while exercise promoted neurogenesis and improved the ‘where’ memory. The ‘when’ and ‘where’ memories correlated with c-Fos expression in CA1 and the dentate gyrus, respectively. Furthermore, analysis suggested that each treatment induced a distinct pattern of functional connectivity among the areas analyzed for c-Fos. In the animals in which stress and exercise were combined, stress notably reduced the amount of voluntary exercise performed. Nevertheless, exercise still improved memory and counteracted the stress induced-deficits in neurogenesis and behavior. Interestingly, compared with the other three treatments, the stressed exercising animals showed a larger increase in cell survival, the maturation of new neurons and apoptosis in the dentate gyrus, with a considerable increase in the number of 24-day-old BrdU + cells that differentiated into mature neurons. The interaction between exercise and stress in enhancing the number of adult-born hippocampal neurons supports a role of exercise-induced neurogenesis in stressful conditions.

Introduction

New cells are constantly generated in the adult hippocampus. The cells mature into neurons that are functionally integrated into the circuits of the dentate gyrus and have a potential role in behavior (Castilla-Ortega et al., 2011, Deng et al., 2010, Jessberger and Kempermann, 2003). These adult-born hippocampal neurons show an enhanced susceptibility to modulation by experience (Castilla-Ortega et al., 2011, Deng et al., 2010), and are highly sensitive to the effects of chronic stress and voluntary exercise. Therefore, while chronic stress downregulates the proliferation, differentiation and survival of new neurons and impairs many forms of hippocampal-dependent memory, voluntary exercise has the opposite effect, potentiating both hippocampal neurogenesis and cognition (Conrad, 2010, Klaus and Amrein, 2012, Schoenfeld and Gould, 2012). Furthermore, voluntary exercise has been shown to be a critical factor in the improvement of neurogenesis and memory that occurs following the broadly used environmental enrichment protocols that combine novel stimulation with free access to a running wheel (Mustroph et al., 2012). Despite the reported ability of voluntary exercise to prevent or enable recovery from stress-induced deficits in hippocampal plasticity and behavior (Head et al., 2012, Zheng et al., 2006), few studies that combine these two treatments have focused on adult hippocampal neurogenesis (Kiuchi et al., 2012, Nakajima et al., 2010, Yau et al., 2011b, Yau et al., 2012). Those studies revealed that exercise counteracts the suppressive effects of chronic stress or corticosterone administration on hippocampal cell proliferation (Kiuchi et al., 2012, Nakajima et al., 2010, Yau et al., 2011b, Yau et al., 2012), but other aspects of neurogenesis such as cell survival have been less frequently assessed and may not benefit from this protective effect (Nakajima et al., 2010). On the other hand, the assessment of adult hippocampal neurogenesis under conditions of stress and voluntary exercise is of great interest because the adult-born neurons may be part of the mechanism by which exercise counteracts the deleterious effects of stress (Snyder et al., 2011, Yau et al., 2011a, Yau et al., 2011b).

Here, we studied the impact of chronic intermittent restraint stress and voluntary exercise, both separately and in combination, on adult hippocampal neurogenesis. In addition, hippocampal dependent memory was evaluated in the three-trial ‘what–when–where’ object recognition task (Www-Task) developed by Dere, Huston, and De Souza Silva (2005). This task requires mice to remember both the temporal order in which a familiar object was presented (‘when’ memory) and the place it was presented (‘where’ memory) because the integrated memory for an event, a time and a place constitutes the basis of episodic-like memory (Dere et al., 2005). The ‘when’ and ‘where’ memories are more complex than recognition memories for familiar objects and locations and involve greater hippocampal demand (Albasser et al., 2012, Castilla-Ortega et al., 2012). A role for the medial prefrontal cortex (mPFC) has also been established, as the mPFC interacts with the hippocampus to solve the ‘where’ memory (DeVito & Eichenbaum, 2010). On the other hand, functional imaging has revealed that, compared to resting levels, performing the Www-Task increases functional activity in the hippocampus, mPFC, basolateral amygdala (BLA), paraventricular hypothalamic nucleus (PVN), accumbens (Acb) and perirhinal cortex (PRh) (Castilla-Ortega et al., 2012 and unpublished observations). In addition to their potential role in the Www-Task, these extrahippocampal areas are relevant because of their involvement in stress and reward processing (Herman et al., 2003, Tzschentke and Schmidt, 2000). Ninety minutes after the completion of the Www-Task, animals were sacrificed and perfused. The hippocampal and extrahippocampal (mFPC, BLA, PVN, Acb and PRh) c-Fos expression was then assessed as a measure of the functional activity induced by behavior (Barry & Commins, 2011). Moreover, following each experimental treatment, c-Fos expression in the newly born cells was examined to explore their potential involvement in the behavioral task.