A Runner’s High for New Neurons? Potential Role for Endorphins in Exercise Effects on Adult Neurogenesis

hysical exercise has wide-ranging bene ts to cognitive functioning and mental state, ef- fects very closely resembling enhancements to hippocampal functioning. Hippocampal neurogenesis has been implicated in many of these mental bene ts of exercise. However, precise mechanisms behind these effects are not well known. Released peripherally during exercise, beta-endorphins are an intriguing candidate for moderating increases in neurogenesis and the related behavioral bene ts of exercise. Although historically ignored due to their peripheral release and status as a peptide hormone, this review highlights reasons for further exploring beta-endorphin as a key mediator of hippocampal neurogenesis. This includes possible routes for beta-endorphin signaling into the hippocampus during exercise, direct effects of beta-endorphin on cell proliferation and neurogenesis, and behavioral effects of manipulating endogenous opioid signaling. Together, beta-endorphin appears to be a promising mechanism for understanding the speci c ways that exercise promotes adult neurogenesis speci cally and brain health broadly. Keywords: exercise; adult neurogenesis; beta-endorphin; hippocampus; stress; dentate gyrus; spatial memory; depression 1. Exercise Effects on Brain and Mental Health Aerobic exercise has widely been prescribed to bene t many physical health condi- tions, such as cardiovascular disease, obesity, diabetes, and immune functioning [1–3], but growing evidence points to chronic aerobic exercise as vital for brain health. Exercise increases blood ow to the brain, reduces

hippocampus; stress; dentate gyrus; spatial memory; depression 1. Exercise Effects on Brain and Mental Health Aerobic exercise has widely been prescribed to bene t many physical health condi- tions, such as cardiovascular disease, obesity, diabetes, and immune functioning [1–3], but growing evidence points to chronic aerobic exercise as vital for brain health. Exercise increases blood ow to the brain, reduces risk of stroke, prevents age-associated reductions in brain volume, and is protective against the progression of various neurodegenerative disorders, including Parkinson's disease, Alzheimer's disease and other dementias, and progressive multiple sclerosis [4–9]. In addition, exercise has wide-ranging positive effects on cognitive functioning and mental health across the lifespan [10–12]. Although physi- cal exercise shows strong effectiveness as a prescribed treatment or prevention for many conditions, it is not always practical due to physical and mental barriers behind many neurological and psychiatric conditions [13]. Therefore, having a deeper understanding of the brain mechanisms behind its therapeutic effects is vital to offering the promise of exercise treatment to many. 2. Hippocampus and Adult Neurogenesis—Potential Involvement in Many of the Exercise-Related Effects Many of the cognitive and emotional effects of aerobic exercise involve normalized or enhanced functioning of the hippocampus. Speci cally, exercise enhances spatial learn- ing and memory processes, pattern separation and mnemonic discrimination, attention shifting, and negative feedback to stress, in addition to decreasing feelings of anxiety and depression [14–19]. Using rodent lesion models, behaviors modeling all of these are dependent on normal hippocampal functioning [20], suggesting that the hippocampus may be a key target to effects of exercise on cognition and mental health. Plastic changes Biomolecules2021,11, 1077.

Biomolecules2021,11, 1077 2 of 15 within the hippocampus are likely responsible for producing long-term improvements in brain functioning following chronic exercise, which can be studied in many ways, in- cluding dendritic growth of neurons, synapse formation, and long-term potentiation or other changes to physiological strength of synaptic connections. One additional process relatively unique to the hippocampus in the adult mammalian brain is the continual pro- duction of new neurons in the dentate gyrus region of the hippocampus. New neurons are born throughout adulthood in the dentate gyrus of mammals, from rodents to primates and humans [21,22], and have the unique potential to dynamically alter hippocampal processing by their introduction to dentate networks. Recent work utilizing models to ablate neurogenesis in the hippocampus of adult rodents has demonstrated a functional role for adult-born neurons in many of the hip- pocampal functions that exercise improves. In learning and memory tasks, new neurons in the hippocampus are important for normal adoption of spatial strategies when solving new environments [23] and initial acquisition of contextual fear conditioning [24]. On tasks assessing speci c areas of cognitive functioning, deletion of new neurons impairs normal pattern separation abilities [25] and attention shifting [26]. Lastly, with stress physiology and related behaviors, ablating adult neurogenesis impairs negative feedback of the normal corticosterone response to stress and leaves rodents at higher risk for developing anxiety or depression [27], although many studies show that losing new neurons does not simply produce anxiety or depression [28]. Overall, there is much circumstantial evidence to suggest that adult neurogenesis is a possible mechanism behind the bene ts of exercise on mental functioning. The purpose of this review is to describe the effects of exercise on adult neurogenesis, discuss the functional relevance of changing adult neurogenesis on ex- ercise's effects, and highlight -endorphin as one interesting and underlooked mechanism mediating these effects. 3. Exercise Effects on Adult Neurogenesis Initially, the basic characterization of adult neurogenesis in the hippocampus of rodents occurred in tandem to experiments showing manipulations that reduced cell pro- liferation and neurogenesis, such as stress. Conversely, living in an enriched environment was

adult neurogenesis on ex- ercise's effects, and highlight -endorphin as one interesting and underlooked mechanism mediating these effects. 3. Exercise Effects on Adult Neurogenesis Initially, the basic characterization of adult neurogenesis in the hippocampus of rodents occurred in tandem to experiments showing manipulations that reduced cell pro- liferation and neurogenesis, such as stress. Conversely, living in an enriched environment was shown to enhance adult neurogenesis in mice and follow-up studies demonstrated that access to a running wheel was the most important factor in producing these effects [29]. This important rst study and similar ones that followed consistently showed that rodents given free access to running wheels to run whenever they wanted would gain a boost in hippocampal neurogenesis [30,31]. Limiting the timing of wheel access, though, showed that hippocampal neurogenesis would be most promoted when wheel access was granted for multiple hours during the active dark phase of their circadian cycle, when rodents would naturally be most active on running wheels anyway [32]. In general, running longer distances results in increased neurogenesis [33]. However, use of resistance-embedded running wheels results in shorter distances but similar increases to adult neurogenesis [34], suggesting that amount of effort is more importantly associated with neurogenic effects rather than simply distance ran, but to a point. In long-term running conditions (one month or longer), limiting daily access to running wheels results in higher cell proliferation than unlimited access [35,36], suggesting that neurogenesis effects are more nuanced and likely balanced by energy demands exerted by excessive exercise. Relatively newer research looking at different types of exercise on cell proliferation and neurogenesis supports the idea that physical fatigue can offset exercise-induced increases in adult neurogenesis. Adding strength training to a regular treadmill paradigm through body weights and incline walking depresses hippocampal cell proliferation back to baseline levels [37]. However, just performing strength training, as has been conducted with resistance-based ladder climbing, increases cell proliferation on its own [38,39], suggesting that type of exercise—aerobic or anaerobic, endurance or high intensity—may not matter, as long as exercise load does not become excessive. Other resistance-based exercise paradigms show the opposite

incline walking depresses hippocampal cell proliferation back to baseline levels [37]. However, just performing strength training, as has been conducted with resistance-based ladder climbing, increases cell proliferation on its own [38,39], suggesting that type of exercise—aerobic or anaerobic, endurance or high intensity—may not matter, as long as exercise load does not become excessive. Other resistance-based exercise paradigms show the opposite effect—no change in cell proliferation but decreases in neurogenesis [40],

Biomolecules2021,11, 1077 3 of 15 suggesting that disparities in duration and intensity of workouts matter. By contrast, allowing rodents to voluntarily run at their own pace may be the simplest way to keep fatigue in check and maximize hippocampal plasticity. Voluntary exercise does not universally enhance all facets of adult neurogenesis, however. Transiently, cell proliferation from progenitor cells in the hippocampus is initially suppressed in the rst two days of running, although by one week, there are characteristic increases in cell proliferation [41]. New cells are proliferated, but also more new neurons are differentiated and survive to fully integrate into hippocampal circuitry within a matter of weeks [42]. There is even evidence of increased number of new neurons within two days of running [41], likely re ecting enhanced survival or hastened maturation of preexisting neurons [42,43]. However, these effects do not continually persist for the duration of long-term running. By the third week of exercise, running rats no longer have increased cell proliferation [42]. Cell proliferation may initially dip following initiation of running due to stress. Exercise is a physical stressor in that it activates the hypothalamic-pituitary-adrenal (HPA) axis resulting in peripheral increases in corticosterone [44]. Despite consistently inhibitory effects of various stressors, both physical and psychological, on adult neuroge- nesis(reviewed in [45]) , running seems to serve as a `positive' stressor that can maintain bene cial effects on brain plasticity, such as increased adult neurogenesis. One of the ways harmful effects of running stress may be buffered is through social housing. Exercise produces either no change in adult neurogenesis or uncharacteristic decreases in adult neurogenesis in socially isolated rats and mice [46–48], and intentionally depressing the level of corticosterone circulating in socially isolated runners restores the bene cial effects of exercise on adult neurogenesis [46]. In addition, the voluntary nature of wheel running is most often utilized as it gives rodents agency in choosing to run and is considered less stressful, compared to prolonged forced exercise on treadmills. Although many stud- ies utilizing treadmills show enhancements in adult neurogenesis [49], intense exercise paradigms using treadmills show diminished increases in

of exercise on adult neurogenesis [46]. In addition, the voluntary nature of wheel running is most often utilized as it gives rodents agency in choosing to run and is considered less stressful, compared to prolonged forced exercise on treadmills. Although many stud- ies utilizing treadmills show enhancements in adult neurogenesis [49], intense exercise paradigms using treadmills show diminished increases in neurogenesis [50,51], possibly related to allostatic demands of those paradigms. 4. Functional Role of Neurogenesis on Exercise-Induced Changes to Mental Functioning Although running-increased adult neurogenesis may mediate many of the broad behavioral changes produced by exercise, most studies directly studying the function of new neurons have focused on memory functions of the hippocampus, namely spa- tial/environmental learning and memory. Overall, effects are mixed when asking whether increased neurogenesis is necessary for improved cognition following exercise. Using irradiation or pharmacogenetic models of neurogenesis inhibition, partial reduction in adult neurogenesis following exercise has been shown to decrease spatial performance in the Morris water maze [52,53]. However, no changes to exercise-enhanced performance were observed in similar studies [54–57], or following treatment with the anti-mitotic drug, Ara-C [58]. Perhaps surprisingly, one of the papers that found effects of irradiation on exercise-enhanced spatial memory failed to nd effects on contextual fear conditioning [52], whereas a paper that failed to nd spatial memory de cits did see a reduction in contextual fear conditioning following irradiation [54]. Stress does seem to be an important variable when examining the functional role of new neurons in exercise-induced changes to mental functioning. Although ablation of new neurons does not prevent anxiolysis following exercise in basal conditions [59], loss of new neurons does diminish the reduction in anxiety-like behavior produced by exercise in a chronic pain model [60]. Likewise, even when absence of adult-born neurons in rats fails to impact spatial learning in the Morris water maze following running, it does make running less bene cial when rats were injected with corticosterone [58]. One potential reason why stress may affect hippocampal processing is that stress can saturate hippocampal long-term potentiation (LTP) [61], leading to reduced capacity for future learning. Adult neurogenesis

absence of adult-born neurons in rats fails to impact spatial learning in the Morris water maze following running, it does make running less bene cial when rats were injected with corticosterone [58]. One potential reason why stress may affect hippocampal processing is that stress can saturate hippocampal long-term potentiation (LTP) [61], leading to reduced capacity for future learning. Adult neurogenesis has been proposed to help combat LTP saturation in the hippocampus and

Biomolecules2021,11, 1077 4 of 15 maintain hippocampal memory capacity [62]. Important for this discussion, exercise was speci cally shown to aid in the recovery of contextual fear learning following arti cial LTP saturation, but not when neurogenesis was inhibited by irradiation [62]. It should be noted that many of the cognitive and affective bene ts of exercise may be due to other factors outside of changes to cell proliferation and neurogenesis. Long-term running enhances LTP in the dentate gyrus [63] and increases downstream molecular pathways involved in synaptic plasticity [64]. In addition, exercise impacts hippocampal structure in a variety of ways. Exercise increases dendritic length and spine density in hippocampal neurons [65] and increases overall hippocampal volume [66]. Exercise results in the release of many trophic factors, and although there is signi cant evidence that these growth factors enhance neurogenesis in adult rodents (see below), they are also likely to mediate other physiological and structural changes from exercise [64,67]. Therefore, exercise produces many changes within the hippocampus, and while evidence points to neurogenesis being important for many of the mental bene ts of exercise, it is likely one type of plasticity among many that have a functional role. 5. Potential Mechanisms Involved in Exercise Effects on Neurogenesis—Endorphins? Early studies showed various neurotrophic factors important for developmental neu- rogenesis and neuroprotection mediate exercise effects on adult neurogenesis. Vascular endothelial growth factor (VEGF), released by skeletal muscle cells, is increased by run- ning, promotes cell proliferation in adult rodent neural progenitor pools [68,69] and its activity is necessary for increases in adult neurogenesis [70]. As VEGF promotes angiogen- esis within the brain, further studies have shown that only blocking angiogenic activity, through angiotensin II receptor antagonists, is suf cient to block exercise effects on adult neurogenesis [71]. Likewise, insulin-like growth factor I (IGF-I), which is important for cell proliferation and neuronal differentiation and survival in the embryonic brain [72], is necessary for exercise to increase neurogenesis in the adult rodent [73]. The most work, however, has been performed circling the neurotrophic factor BDNF (brain-derived neurotrophic factor). BDNF is expressed relatively weakly during

effects on adult neurogenesis [71]. Likewise, insulin-like growth factor I (IGF-I), which is important for cell proliferation and neuronal differentiation and survival in the embryonic brain [72], is necessary for exercise to increase neurogenesis in the adult rodent [73]. The most work, however, has been performed circling the neurotrophic factor BDNF (brain-derived neurotrophic factor). BDNF is expressed relatively weakly during embry- onic development, but expression increases postnatally and strongly within the hippocam- pus [74,75], suggesting it has a role in the process of postnatal neurogenesis. Exercise transiently increases BDNF mRNA expression [76] and BDNF protein increases are consis- tent across the lifespan within a week [77]. As these levels return to baseline after long-term running, it mirrors effects on cell proliferation that are strongest towards the beginning of running [77]. Through transgenic studies, BDNF is vital for normal neurogenesis in the adult hippocampus [78] and arti cially increasing its expression stimulates increases in neurogenesis [79]. In addition, BDNF is required for neurogenesis increases following en- riched environment including an exercise wheel [80] and transgenic deletion of the BDNF receptor, TrkB, prevents neurogenesis enhancements by exercise as well [81]. Overall, the consensus suggests BDNF as a key mediator of adult neurogenesis in the adult brain and thus a key mediator of the effects of exercise on neurogenesis as well. Because running is a physical stressor, it is also tempting to consider what factors may uniquely counteract the well-documented inhibitory effects of negative stressors on adult neurogenesis [45]. One intriguing culprit is the endogenous endorphins, known to be released during aerobic exercise and long thought to be responsible for the proposed `runner's high' [82], although this speci c role may be more suited for exercised-induced release of endocannabinoids [83]. During an acute stressor, hypothalamic corticotropin- releasing hormone (CRH) activates the synthesis of pro-opiomelanocortin (POMC), the precursor for adrenocorticotropic hormone (ACTH) and -endorphin, both simultaneously released by the anterior pituitary [84]. Traditionally, -endorphins have a peripheral role in analgesia; helping an organism continue to ght-or- ee despite potential harm and pain from a physical stressor [85]. Intriguingly, though, adult-born neurons have been suggested

stressor, hypothalamic corticotropin- releasing hormone (CRH) activates the synthesis of pro-opiomelanocortin (POMC), the precursor for adrenocorticotropic hormone (ACTH) and -endorphin, both simultaneously released by the anterior pituitary [84]. Traditionally, -endorphins have a peripheral role in analgesia; helping an organism continue to ght-or- ee despite potential harm and pain from a physical stressor [85]. Intriguingly, though, adult-born neurons have been suggested to be involved in chronic pain states [86] and -endorphins have also been speculated to be important for a wide array of behaviors and conditions related to hippocampal neuroge-

Biomolecules2021,11, 1077 5 of 15 nesis, including depression, anxiety, and stress physiology [87]. Thus, a more speci c role of -endorphins within the relationship between exercise and adult neurogenesis deserves investigation. Despite many unanswered questions, -endorphin remains an intriguing candidate for mediating exercise-induced increases to adult neurogenesis [88], as will be outlined here. 6. -Endorphin: Just for the Periphery? One potential problem with focusing on -endorphins is the presumed one-way di- rection of -endorphin transmission from brain to body. As a peptide hormone, it has long been postulated that -endorphin is transiently elevated in plasma by stress and exercise via its release from the anterior pituitary but because it does not readily cross the blood– brain barrier, these increases are speci c to the periphery. However, the evidence for this largely relies on measurements showing transient drops in -endorphin levels soon after acute stressors in the hypothalamus and pituitary, from where -endorphin would be mo- bilized for secretion to the periphery [89]. It should be noted, however, that -endorphin is elevated centrally in various brain areas following long-term exercise [90] and mediates be- haviors affected by stress [91], suggesting that -endorphin acts centrally in some capacity during stress and exercise as well. Although it is well known that related opioid peptides, the enkephalins and dynorphins, are synthesized and released locally within the dentate gyrus [92], hippocampal neurons do not functionally express POMC for -endorphin synthesis [93], suggesting that any -endorphin signaling would be coming from outside the hippocampus. It is unknown by which mechanisms -endorphin signaling within the hippocampus occurs, but there at least three possible routes:(1) -endorphin may be released centrally by axons projecting from POMC-expressing hypothalamic neurons, (2) -endorphin may be transported across the blood–brain barrier after being released peripherally, or (3) -endorphin may be secreted into cerebrospinal uid (CSF) and trans- ported to the hippocampus via volume transmission in cerebral ventricles. The most direct route of -endorphin activity within the dentate gyrus would be from direct innervations of -endorphin-releasing axon terminals in the dentate gyrus. Neurons that use -endorphin are typically labeled through their expression of POMC

This review explores the role of beta-endorphins in exercise-induced neurogenesis.