Introduction

The prophylactic and treatment effects of exercise on various chronic diseases is well established [1]. Additionally, exercise can also help to prevent a host of cardiometabolic-related conditions (e.g., diabetes, early mortality) [2, 3]. Emerging work also demonstrates that, exercise, prior to a psychophysiological stressor [herein focused on toxic stress (not eustress)] [4], can mitigate the negative effects of the stressor. For example, we showed that acute exercise, occurring immediately before viewing emotionally charged, negatively valenced images, helped facilitate emotional regulation [5]. This “exercise preventive paradigm” has also been corroborated with other emotional regulation studies [6, 7].

This exercise preventive paradigm effect may also have implications in memory function. Psychophysiological stressors, such as forced social participation in a verbal presentation task, may have a negative effect on memory function [8,9,10,11,12,13,14]. Notably, this stress-memory relationship is thought to follow an inverted U-shaped relationship [15]. See Fig. 1 (and the Discussion section) for a schematic on the potential underlying mechanisms through which stress (both acute and chronic) may influence memory function. To our knowledge, however, there are no published reviews comprehensively evaluating the literature regarding the potential protective and/or therapeutic effects of exercise on mitigating stress-induced memory impairment [16], which was the purpose of this brief systematic review. The plausibility for exercise to attenuate stress-induced memory impairment is also shown in Fig. 1 (and further addressed in the Discussion section).

Fig. 1
figure 1

Schematic indicating potential mechanisms through which acute stress may enhance memory, detrimental effects of chronic stress on memory, and how exercise may attenuate stress-induced memory impairment. The circular crossed symbol denotes that chronic exercise attenuates the chronic stress mechanisms

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Methods

Studies were identified using electronic databases, including PubMed, PsychInfo, Sports Discus, and Google Scholar. We employed the computerized searches on April 25, 2018, identifying articles published prior to this date (no restriction was placed on how far back the study was published). The search terms included exercise, physical activity, stress, psychophysiological, rescue, preventive, treatment, therapeutic, psychological, memory, and cognition. To be eligible for inclusion in this systematic review, studies had to be published in English, employ an experimental design (cross-sectional designs on this topic were not eligible) [17], have the acute or chronic bout of exercise occur prior to, during, or after the stressor, implement a toxic psychophysiological stressor (pharmaceutical agent or ischemia-induction were not eligible [18,19,20,21,22], mild forms of the stressor were not eligible [23], and evaluating individuals without experimentally manipulating stress were not eligible) [24], and have an assessment of memory function that occurred after the stressor. To provide a comprehensive assessment on this topic, we applied no restriction on whether the study was conducted in humans or animal models. In total, 24 studies met our criteria. However, two appeared to be duplicate studies [25, 26], and thus, one was removed. As noted in Table 1, 23 studies were evaluated herein.

Table 1 Extraction table of the evaluated studies
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Results

Table 1 displays the extraction table for the evaluated studies. All were experimental studies conducted in an animal model. The stress protocol across the studies varied, including maternal separation, loud noise exposure, immobilization/restraint, social defeat/competition and exposure to cat odor. All studies, except one (single session acute stress protocol), employed a chronic stress protocol (e.g., multiple repeated exposures over 1–2 weeks). All 23 studies employed a chronic exercise protocol (e.g., daily exercise from 2 to 8 weeks; either forced treadmill exercise or voluntary wheel access). Among the 23 studies, the commonly assessed memory tasks included the Morris water maze, object recognition test, or inhibitory avoidance task.

Eight studies evaluated a preventive model (i.e., exercise occurring prior to stress-induction), three employed a concurrent model (exercise bout occurred during or around the same time as the stress protocol), ten studies employed a therapeutic model (i.e., exercise occurring after stress-induction), and two studies evaluated both a preventive and therapeutic model within the same study.

Among the eight studies employing a preventive model, all eight demonstrated that the stress regimen impaired memory function. In all eight of these studies, when exercise occurred prior to the stressor, exercise attenuated the stress-induced memory impairment effect.

Among the ten studies employing a therapeutic model, one study showed that the stress protocol enhanced memory function, one showed that the stress protocol did not influence memory, and eight demonstrated that the stress regimen impaired memory function. Among the eight studies showing that the stress protocol impaired memory function, all eight studies demonstrated that exercise, after the stressor, attenuated stress-induced memory impairment.

Among the three concurrent models, and the two studies that evaluated both preventive and therapeutic effects, all showed that the stress protocol impaired memory function. Among the three concurrent models, two demonstrated a beneficial effect of exercise in mitigating stress-induced memory impairment. Among the two studies employing both a preventive and therapeutic model, both demonstrated attenuation effects of exercise on stress-induced memory impairment.

Discussion

The objective of this systematic review was to evaluate the potential preventive and therapeutic effects of exercise in attenuating stress-induced memory impairment. There was consistent evidence that chronic exercise had both a preventive and therapeutic effect in mitigating chronic stress-induced memory impairment. The narrative that follows will discuss these mechanistic pathways, as displayed in Fig. 1. For additional discussion on these mechanisms, the reader is referred elsewhere [16, 27, 28].

Acute stress and memory

Acute moderate levels of stress may enhance memory, particularly emotional-based information (vs neutral stimuli). Specifically, enhanced encoding and consolidation of stimuli is more likely to occur for information perceived as “high priority” [29,30,31,32,33]. The stressor, occurring prior to the memory task, may help to augment attentional resources (via, for example, the prefrontal and parietal structures) to the memory stimuli and, in turn, enhance encoding of the information [34, 35]. In addition to psychological stress, emerging work also suggests that exercise-induced physiological arousal may help to subserve stress-related memory function (emotional memory) [36]. Additional work is needed to determine whether there is an additive effect of exercise and acute stress on memory function.

Additionally, the stressor (including exercise) [36], occurring before, or shortly after, the memory task can help to facilitate the consolidation of the memory trace. For example, cortisol crosses the blood–brain barrier and binds to mineralocorticoid or glucocorticoid receptors. After which, PKA activation may help to facilitate exocytosis of AMPA receptors (and activation of NMDA receptors) [37], subserving hippocampal LTP [38]. Acute stress may also induce levels of epinephrine, activating the vagus nerve and, in turn, facilitating LTP via neurotransmitter (e.g., norepinephrine, dopamine, serotonin, and acetylcholine) production to the hippocampus [39,40,41,42]. To illustrate, the vagus nerve may stimulate the production of norepinephrine from the locus coeruleus, which then binds to adrenergic receptors, ultimately facilitating a cascade of intracellular signaling to induce synaptic plasticity [43]. Moreover, cortisol may augment endocannabinoid levels, binding to CB1 receptors in GABA interneurons and, ultimately, inhibiting GABA neurotransmitter levels [44]. This, in turn, may help to preserve memory, as GABA inhibition may help facilitate LTP [45] and GABA receptor activation may impair memory [46].

Although acute stress, occurring before encoding or during the early stages of consolidation, can facilitate encoding and consolidation of the prioritized stimuli, it can have the opposite effect on non-prioritized stimuli. The encoding of the stressful event may compete with the encoding of non-relevant or non-prioritized stimuli. Further, if the stressor occurs around the period of retrieving a memory, this memory retrieval process can be impaired, as attentional resources are shifted away from retrieval processes to encoding the stressful event. Additionally, inactive synapses, representing previously encoded stimuli, may become de-potentiated when LTP increases in other synapses [28]. Moreover, during the stressor, reduced neuronal firing may occur in the prefrontal cortex, which may impair memory retrieval since the prefrontal cortex plays an important role in such retrieval processes [47]. It would be worthwhile to investigate whether acute exercise can attenuate these effects by, for example, attenuating the stress response and facilitating emotional regulation [5].

Taken together, acute moderate levels of stress may help to facilitate encoding and consolidation of prioritized stimuli (particularly emotional stimuli), whereas extreme acute stressors may detrimentally influence retrieval of memories when the stressor occurs around the time of retrieving an unrelated memory. Notably, and as discussed next, chronic elevations in cortisol, lasting more than a few hours, can impair memory function (inducing LTD) [38].

Chronic stress and memory

Chronic stress may detrimentally influence stress through various mechanisms, including enhanced HPA axis activity. Over time, this may impair cell survival and neuronal morphology (e.g., loss of spines, shrinkage of dendrites) [4]. Regarding cell survival, astrocytes, which support the survival of neurons, possess glucocorticoid receptors and are significantly affected by chronic psychosocial stress [48]. Considering neuronal morphology, reduced synaptic firing, via LTD for example, causes actin loss and dendritic spine shrinkage [49]. Further, chronic stress may reduce BDNF levels [50], which play an important role in facilitating signaling pathways (e.g., RAC1) that stabilize dendritic spines [51]. Additionally, chronic stress may reduce neurotransmitter levels (e.g., dopamine) [52], decrease AMPA receptor expression [53], and downregulate mineralocorticoid and glucocorticoid receptor expression [54]. This downregulation and desensitization of these receptors may prevent activation of some of the above-mentioned cellular pathways (e.g., PKA) that may facilitate LTP. Further, chronic stress may inhibit neurogenesis, and ultimately, hippocampal volume loss, via, for example, apoptosis of progenitor cells to cell cycle arrest [49].

Exercise mitigates negative effects of chronic stress

Exercise may attenuate the memory-related consequences of chronic stress via various pathways. Ultimately, exercise may help to facilitate LTP through induced neuronal excitability, via stimulation of the vagus nerve as well as muscle afferent nerve fibers [55], which have direct projections to the brainstem and, ultimately, the hippocampus. Further, exercise-induced alterations in hormones (e.g., epinephrine and cortisol) can also influence neuronal excitability. Facilitating these effects, exercise has been shown to enhance neurotrophic factors (e.g., BDNF), induce transcription factors (e.g., CREB) expression, and increase AMPA trafficking [55]. Exercise may also help attenuate chronic stress-induced memory impairment via attenuation of HPA axis activity, suppress oxidative stress, facilitate neurogenesis, and regulate mineralocorticoid and glucocorticoid receptor expression [27].

Conclusion

This review demonstrates that, within animal models, chronic stress is associated with memory impairment and chronic exercise has both a preventive and therapeutic effect in attenuating stress-induced memory impairment. Given the paucity of work among human studies, future work on this topic among humans should investigate, specifically, whether exercise has a preventive and therapeutic effect in mitigating memory impairment caused from psychophysiological stress. Such work should also consider models that evaluate acute exercise and acute stress protocols. Further, work should also evaluate varying parameters of exercise, such as the intensity, duration, and type of exercise, as variations of these dimensions may have a unique influence on potentially attenuating stress-induced memory impairment.