Androgen and Estrogen Actions on Male Physical Activity: a Story Beyond Muscle

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Journal of F Jardí et al. Androgens and physical activity 238:1 R31–R52 Endocrinology REVIEW Androgen and estrogen actions on male physical activity: a story beyond muscle

Ferran Jardí1, Michaël R Laurent2,3, Vanessa Dubois2,†, Nari Kim1, Rougin Khalil1, Brigitte Decallonne1, Dirk Vanderschueren1 and Frank Claessens2

1Clinical and Experimental Endocrinology, Department of Chronic Diseases, Metabolism and Ageing (CHROMETA), KU Leuven, Leuven, Belgium 2Molecular Endocrinology Laboratory, Department of Cellular and Molecular Medicine, KU Leuven, Leuven, Belgium 3Gerontology and Geriatrics, Department of Chronic Diseases, Metabolism and Ageing (CHROMETA), KU Leuven, Leuven, Belgium

Correspondence should be addressed to F Jardí: [email protected]

†(V Dubois is now at INSERM UMR1011, University of Lille and Institut Pasteur de Lille, Lille, France)

Abstract

Physical inactivity is a pandemic that contributes to several chronic diseases and poses a Key Words significant burden on health care systems worldwide. The search for effective strategies ff physical activity to combat sedentary behavior has led to an intensification of the research efforts to ff exercise unravel the biological substrate controlling activity. A wide body of preclinical evidence ff muscle makes a strong case for sex steroids regulating physical activity in both genders, albeit ff brain

Endocrinology the mechanisms implicated remain unclear. The beneficial effects of androgens on ff androgens of muscle as well as on other peripheral functions might play a role in favoring adaptation to exercise. Alternatively or in addition, sex steroids could act on specific brain circuitries

Journal to boost physical activity. This review critically discusses the evidence supporting a role for androgens and estrogens stimulating male physical activity, with special emphasis on the possible role of peripheral and/or central mechanisms. Finally, the potential Journal of Endocrinology translation of these findings to humans is briefly discussed. (2018) 238, R31–R52

Introduction

Physical inactivity has emerged as a global health crisis, according to self-reported measures (Hallal et al. 2012). being the fourth leading cause of death worldwide Accelerometer data show even lower rates of adherence according to the World Health Organization (WHO) among adolescents, with only 8% of 12- to 15-year-old (WHO 2009). Not meeting the minimum physical individuals attaining sufficient physical activity (Troiano activity recommendations (WHO 2010) is estimated to et al. 2008). Taking into consideration the prevalence and cause 6% of the burden of disease from coronary heart risks associated with physical inactivity, as well as the disease, 7% of type 2 diabetes and 10% of both breast and fact that it can be easily and effectively prevented (Heath colon cancer (Lee et al. 2012). In contrast, 15 min of daily et al. 2012), it is without surprise that the promotion moderate-intensity exercise reduces all-cause mortality of physical activity has become a public health priority by 14% and extends by 3 years the life expectancy (Kohl et al. 2012). (Wen et al. 2011). Notwithstanding, one-third of adults The predisposition to engage in physical activity is worldwide are physically inactive and the prevalence a highly complex trait in humans, probably determined increases dramatically among adolescents, with four out by the interaction of biological, psychological, social and of five not reaching the minimum recommended levels environmental factors (Bauman et al. 2012). The role for

-18-0125 Journal of F Jardí et al. Androgens and physical activity 238:1 R32 Endocrinology a biological substrate influencing the levels of physical running in females is abolished upon gonadectomy and activity is a relatively recent concept supported by several can be restored following treatment with E2 (Berchtold lines of evidence, including human studies but mostly et al. 2001). Based on these and other preclinical data, preclinical investigations (Bouchard & Rankinen 2006). sex steroids have been suggested as strong candidates Based on a cohort of 37,000 twin pairs, Stubble et al. for contributing to the biological substrate influencing described that the heritability of exercise participation physical activity (Lightfoot 2008). Androgens are well ranged between 48% and 71% (Stubbe et al. 2006). known for their potent anabolic actions on muscle and Nevertheless, the extent to which genetic factors explain we have previously reviewed the cellular targets and adult exercise behavior is highly heterogenic between molecular pathways implicated (Dubois et al. 2012). studies, most likely due to differences in the methodology Therefore, it is conceivable that testosterone increases used to assess physical activity (Simonen et al. 2002, activity in males by favoring adaptation to exercise Eriksson et al. 2006). In addition, candidate gene studies (peripheral mechanisms). In addition or alternatively have pointed out several hormonal- and neurotransmitter- thereto, testosterone could act on the specific brain neural related genes associated with physical activity (Stefan et al. circuitries that have been identified to control physical 2002, Salmen et al. 2003, Simonen et al. 2003, Loos et al. activity levels (central mechanisms). Our understanding 2005). However, the two genome-wide association studies of the neural basis underpinning the effects of sex steroids (GWAS) published so far do not support a prominent role on activity has advanced markedly over the last decade. for most of these candidate genes (De Moor et al. 2009, Nevertheless, most of these studies have focused on the Kim et al. 2014). This inconsistency might be explained female brain (reviewed in Xu et al. 2017), whilst the in part by the limitations of physical activity data based central mechanisms implicated in male activity are less on questionnaires or diaries, which show poor accuracy documented. in detecting light-to-moderate activities and are prone to The recent publication of large clinical trials of error because of individual biases (Strath et al. 2013). The testosterone therapy in elderly men has reignited latter caveat is overcome by recent efforts to implement the debate on the risks and benefits of testosterone the use of automated monitors in physical activity replacement (Basaria et al. 2010, Snyder et al. 2016, research. Nonetheless, the strongest body of evidence Storer et al. 2016). In this context, the positive impact supporting that physical activity is biologically regulated of testosterone stimulating physical activity and the still comes from preclinical studies. underlying mechanisms of action represent an additional Androgens and estrogens are key regulators of consideration for testosterone replacement in elderly, reproductive organs but also exert multiple effects on mobility-impaired, frail or sarcopenic men. Here, we non-reproductive tissues, including brain, muscle, adipose review the data supporting a role for androgens and tissue and bone. Testosterone, the principal circulating estrogens regulating male physical activity and provide androgen, may stimulate the androgen receptor (AR) an overview of the potential mechanisms implicated. either directly, or indirectly following 5 alpha-reduction How sex steroids control physical activity in females is to dihydrotestosterone (DHT), as well as the estrogen reviewed in Lightfoot (2008) and Xu et al. (2017). We receptors (ERα and ERβ) after testosterone aromatization will first review some basic concepts regarding physical into estradiol (E2) (Callewaert et al. 2010). Even though activity and then discuss several lines of preclinical research into the biological control of physical activity evidence supporting the notion. In the following sections, has intensified in recent years, early findings in the 1920s we will review the impact of androgens in the physical already showed that wheel running in male rats was ability to partake in exercise and examine the specific abolished after orchiectomy (ORX) (Gans 1927). Later brain circuits that could be underlying the regulation of studies pinpointed testosterone as the gonadal product activity by testosterone. In the last section, we will briefly stimulating wheel running, albeit the extent to which discuss what is known regarding the effects of androgens prior aromatization into E2 was required remained debated and estrogens on physical activity in men. (see discussion in the ‘Replacement and pharmacological studies in adult rodents’ section). Physical activity is a sexually dimorphic trait in rodents (Eikelboom & Mills Categories of physical activity 1988), with females being more active than males and with fluctuations according to the estrous cycle (Wollnik Physical activity is defined as any bodily movement & Turek 1988). Similar to what is observed in males, wheel produced by skeletal muscles that results in energy

Journal of F Jardí et al. Androgens and physical activity 238:1 R33 Endocrinology expenditure (Caspersen et al. 1985). Although the term animals, thus inducing a substantial psychological stress ‘exercise’ is often interchanged with physical activity, (Sasaki et al. 2016). In addition, rodents do not always exercise is indeed a subset of physical activity that is display consistent swimming behaviors and some animals planned and structured and implies a purpose to achieve will just float, bob and/or dive American( Physiological health or fitness benefits (Caspersen et al. 1985). In our Association 2007). Therefore, we focused on voluntary daily life, we also engage in incidental physical activity wheel running, in which animals have a complete control when performing home- or work-related activities or on the amount and intensity of exercise. during transportation (Strath et al. 2013). Nevertheless, the latter activities can be performed in an exercise manner and thus it is not always straightforward to Preclinical evidence supporting a role for discern between the voluntary (planned) and incidental androgens and estrogens regulating male (spontaneous) domains of physical activity (Garland physical activity et al. 2011). Notably, an increase in energy expenditure associated with non-exercise activity has been shown Most of our knowledge regarding the role of biological to prevent fat gain in humans (Levine 1999). Therefore, factors controlling physical activity derives from rodent physical activity derived from daily tasks may also play a studies and thus the overall significance to human role in disease prevention and should be included together health remains to be determined. Nevertheless, evidence with exercise when assessing physical activity. from preclinical studies in rats and mice clearly shows With regard to preclinical animal studies, wheel the importance of androgens and normal AR and ERα running in rodents has been extensively used as a signaling in the regulation of male activity. model of human exercise (Garland et al. 2011, Novak et al. 2012). Similar to the ‘runner’s high’ experienced Replacement and pharmacological studies in during endurance exercise in humans, wheel running adult rodents is rewarding for rodents and both rats and mice show preference for environments previously associated with The first observations showing that ORX abrogates wheel running (Lett et al. 2001, Fernandes et al. 2015). When running activity in rats date from almost a century ago given free access to running wheels, mice reduce the time (Gans 1927). Subsequent studies extended these findings spent in cage floor activity at the expense of wheel running to mice as well as other rodent species (Daan et al. 1975, time, the net result being an increase in their total activity Morin & Cummings 1981, Dark & Zucker 1984, Jechura levels (De Visser et al. 2005, Silvennoinen et al. 2014). et al. 2000) and demonstrated the implication of testicular Spontaneous physical activity (SPA) refers to the non- testosterone production (Daan et al. 1975, Roy & Wade exercise component of activity and therefore should not 1975). It is now well established that replacement with be used as interchangeable for wheel running, although testosterone rescues wheel running activity in a gradual very often these two parameters correlate reasonably dose-response manner following ORX (Butler et al. 2012). well. There are several methods to assess SPA in rodents, For instance, the average daily distance run by ORX mice including infrared photobeams, radiotelemetry, video increases from 1 to almost 4 km following 2 weeks of tracking systems or force plates (Silvennoinen et al. 2014, treatment with physiological doses of testosterone (Jardí Teske et al. 2014, Chabert et al. 2016). The main difference et al. 2018). Consequently, the combination of testosterone between these approaches is their accuracy in detecting and wheel running mediates an adaptational shift to a ambulation and climbing as well as more subtle stationary more oxidative fiber profile in the skeletal muscle of ORX movements, such as grooming or feeding. Regardless of the (Allen et al. 2001, Ibebunjo et al. 2011). In gonadally method used, a long (i.e., hours) period of acclimatization intact male rats, administration of supraphysiological to the activity chamber is required to discharge the doses of testosterone also increases running distances novelty effects on locomotion (Teske et al. 2014). Studies (McGinnis et al. 2007). Similar to humans, aging in male failing to meet the former methodological requirement mice is associated with a progressive decline in activity were excluded from this review. It is important to note (Fig. 1; Hamrick et al. 2006) and this can be partly that we focus here on volitional activity, and therefore attenuated by the combination of exogenous testosterone do not include preclinical studies using forced exercise and low-intensity physical training (Guo et al. 2012). It is capacity tests. Forced treadmill running or swimming important to note, however, that the concentrations of require the use of aversive stimuli to motivate the testosterone in male mice appear to remain constant with

(2.3 km vs. 3.8 km; Jardí et al. 2018) and both testosterone and DHT were equally effective restoring SPA (Jardí et al. 2018). It is noteworthy that DHT metabolites 5α-androstane-3α, 17β-diol (3α-diol) and 5α-androstane- 3β, 17β-diol (3β -diol) may initiate behavioral processes through AR-independent mechanisms, namely by acting on gamma-aminobutyric acid (GABA) type A (Frye et al. 1996) or ERs (Frye et al. 2008a). Nevertheless, we showed that DHT failed to rescue wheel running in AR knockout (ARKO) mice, ruling out the implication of AR-independent Figure 1 mechanisms (Jardí et al. 2018). Additionally, the activity Changes in SPA with age in male C57BL/6 mice. Mean values are shown with standard error means as vertical bars. (Data from Hamrick et al. of ORX+testosterone-treated mice was partially lowered 2006.) by the AR antagonist enzalutamide (Wu et al. 2016), further supporting the involvement of AR signaling aging (Hamrick et al. 2006), in contrast to what is observed in the stimulation of wheel running (Jardí et al. 2018). in elderly men (Wu et al. 2008). Thus, it is unlikely that Taken together, these data make a strong case for the a reduced androgen bioactivity is the main driver of the contribution of AR to the effects of androgens on activity, decreased SPA in old mice, even when this is responsive at least in male mice, though its importance is somewhat to testosterone supplementation in the setting of physical less compared to aromatization and ER signaling. training. In an attempt to investigate the estrogenic component Many of the actions of testosterone in male behavior of testosterone actions, ORX rats and mice have been are mediated by estrogen receptors (ERs). For instance, treated with systemic E2 (Roy & Wade 1975, Ogawa et al. testosterone acts predominantly as a prohormone for E2 in 2003, Bowen et al. 2011, 2012, Blattner & Mahoney 2015, the programming of male-typical and territorial behavior Royston et al. 2016). It should be noted, however, that the in mice, although execution of this behavior relies on physiological relevance of this approach is uncertain and the AR (Wu et al. 2009). It remains debated whether this deserves special consideration. Serum E2 concentrations dual androgenic and estrogenic action of testosterone in both gonadally intact and ORX+testosterone-, but not also applies to physical activity. Initially, testosterone was E2-treated mice, are below the limit of detection of sensitive suggested to stimulate wheel running exclusively through analysis methods using mass spectrometry (McNamara aromatase-dependent mechanisms (Stern & Murphy 1971, et al. 2010, Nilsson et al. 2015, Laurent et al. 2016b), the Roy & Wade 1975). Treatment with E2 was more effective ‘gold standard’ for sex steroids measurements. Circulating than testosterone in stimulating wheel running in ORX levels of E2 are also extremely low in male rats, at the level rats and the non-aromatizable androgen DHT showed no of a few picograms per milliliter (Quignot et al. 2012). effects (Roy & Wade 1975). Also in ORX rats, biologically Therefore, a ‘physiological’ E2 dose is virtually impossible effective doses of the antiandrogen cyproterone acetate to define in male rodents, and most ‘replacement’ doses failed to inhibit the stimulatory actions of testosterone are almost by definition supraphysiological. Remarkably, on wheel running (Stern & Murphy 1971). In accordance, aromatase is highly expressed in several brain regions treatment with the 5α-reductase inhibitor dutasteride did of male rodents, including the bed nucleus of the stria not alter the levels of wheel running in male Zucker obese terminalis, the olfactory tubercle, the medial amygdala rats (Sato et al. 2013). In the latter study, however, the lack (MeA) and the hypothalamus, suggesting a role for the of effects could be potentially explained by the already autocrine and paracrine actions of estrogens (Zhao low levels of wheel running of these animals (Stern & et al. 2007, Stanić et al. 2014). Overall, we cannot draw Johnson 1977) and/or by a compensatory effect related conclusions about the physiological role of E2 on physical to the secondary increases in testosterone concentrations activity levels based on studies treating male rodents with (Amory et al. 2007). Later studies challenged the notion systemic E2, even when pharmacological doses of E2 of aromatase being indispensable for testosterone effects restore wheel running following ORX (Roy & Wade 1975, on wheel running. In male mice, treatment with DHT Ogawa et al. 2003, Bowen et al. 2011, 2012, Blattner & partly restored the drop in wheel running following Mahoney 2015, Royston et al. 2016). Although treatment ORX (Karatsoreos et al. 2007, Iwahana et al. 2008), with the aromatase inhibitors letrozole and exemestane although to a lesser extent compared with testosterone did not affect wheel running either in gonadally intact

Journal of F Jardí et al. Androgens and physical activity 238:1 R35 Endocrinology or ORX+testosterone-treated mice (Bowen et al. 2011), The reduction of SPA following ORX, however, was only this study needs further technical validations. The partially rescued by MK-4541 (Chisamore et al. 2016). In ultimate proof of the role of ERs on male physical activity a similar pattern, treatment with SARM-2f completely comes from studies in knockout mice lacking ERα or restored muscle mass in ORX mice but was less effective the aromatase enzyme (see discussion in the ‘Global KO increasing their wheel running activity (Morimoto mouse models’ section). et al. 2018). In a preclinical mouse model of muscular dystrophy, an enobosarm analog extended the survival of Androgenic-anabolic steroids (AAS) and selective mice but failed to increase their low SPA (Ponnusamy et al. androgen receptor modulators (SARMS) 2017). Nevertheless, the severity of the pathology of these AAS are synthetic derivatives of testosterone that were mice may have impeded an increase in their SPA levels. developed in an attempt to maximize the anabolic The effects of SARMs on physical activity are worthy of activity of testosterone. The prolonged abuse of AAS further study and could represent an additional benefit in in men is related to several psychiatric adverse effects, the treatment of muscle wasting conditions. including irritability, mood swings, mania (‘steroid rage’ at supraphysiological doses) and depression Perinatal studies (Pope & Katz 1994, Bjørnebekk et al. 2017). Although scarce, a few preclinical studies have explored whether So far, we discussed how androgens stimulate physical doses of AAS that model human abuse patterns affect activity in adult rodents. It is noteworthy that behavioral physical activity levels. In contrast to testosterone, both responses to sex steroids during adulthood are influenced nandrolone (McGinnis et al. 2007, Tanehkar et al. 2013) by sex steroids acting on the nervous system during early and stanozolol (McGinnis et al. 2007) reduced wheel critical periods. In their seminal work in 1959, Phoenix running activity in gonadally intact male rats. Similarly, et al. were the first to dichotomize hormonal effects in the combination of nandrolone and methandrostenolone two groups: organizational vs. activational (Phoenix et al. decreased voluntary wheel running in mice (Onakomaiya 1959). The organizational-activational theory states that et al. 2014). Paradoxically, despite the detrimental effects hormones sculpt neural tissues during development so of nandrolone on wheel running in male rats (McGinnis that they respond differentially to hormonal activation et al. 2007, Tanehkar et al. 2013), the combination of both in adulthood (Phoenix et al. 1959). The study of the interventions improved their running endurance (Vanzyl organizational effects of sex steroids has mainly focused et al. 1995). As for the mechanisms of action, AAS and on the programming of mating behavior (Arnold 2009). endogenous testosterone differ in both the affinity to Although less evidence is available for non-sexual bind AR and the ability to be aromatized (Bergink et al. behavior, the results from several preclinical studies 1985, Fragkaki et al. 2009). Possibly, AAS regimes reduce suggest that the perinatal sex steroid milieu might also wheel running by interfering with the endogenous determine physical activity in adulthood. testosterone production and hence E2 (Barone et al. The perinatal surges of testosterone in male rodents 2017). Alternatively, AAS might act directly on several during the late embryogenic period and the first hours neurotransmitter systems in the brain that affect physical of life are critical for the male brain to become capable activity (Mhillaj et al. 2015). of producing male sexual behavior (masculinization) The need to improve both the benefit/risk profile and whilst losing its ability to respond in a female-like pattern pharmacokinetics of androgens spurred the development (defeminization; Lenz & McCarthy 2010 for review on this of selective androgen receptor modulators (SARMs) topic). Apart from the perinatal period, a growing body of (Mohler et al. 2009). SARMs received initially a great evidence shows that during puberty, testosterone is also deal of clinical attention for their potential use in the required for the full masculinization and defeminization treatment of muscle wasting conditions. To date, yet, none of male brain (Schulz et al. 2004, De Lorme & Sisk 2016, of these compounds has shown unequivocal efficacy in Sano et al. 2016). Paradoxically, the organizational clinical trials (Almeida et al. 2017). Preclinical data show effects of testosterone on the developing male brain are that SARMs display a better myotrophic index compared actually driven by ERs, suggesting the implication of to their steroidal analogues. In ORX mice, treatment brain aromatase activity (Ogawa et al. 2000, Wu et al. with the SARM MK-4541 led to an increase in muscle 2009, Juntti et al. 2010). In females, prepubertal estrogen mass and function comparable to DHT but without the levels are very low, and the traditional dogma states sparing effects on seminal vesicles (Chisamore et al. 2016). that sexual behavior is merely the result of the absence

http://joe.endocrinology-journals.org © 2018 Society for Endocrinology https://doi.org/10.1530/JOE-18-0125 Published by Bioscientifica Ltd. Printed in Great Britain Downloaded from Bioscientifica.com at 09/26/2021 07:39:28PM via free access Journal of F Jardí et al. Androgens and physical activity 238:1 R36 Endocrinology of perinatal testosterone organizational actions. Recent treatment (Royston et al. 2016). From these observations, data challenge the latter hypothesis demonstrating that it is reasonable to assume that the neonatal testosterone E2 actions outside the perinatal period are required for surge programs male physical activity and that this the normal development of female sexual behavior (Brock might involve aromatase-dependent processes (Fig. 2). et al. 2011). The neural substrate underlying the persistent In support of this premise, female rats treated neonatally and irreversible effects of sex steroid actions on the with androgens develop a resistance to E2 stimulation of developing brain remains mostly unknown. Although not wheel running in adulthood, which can be prevented by an exhaustive list, some suggested mechanisms include treating pups with antisense oligodeoxynucleotides to epigenetic modulation (Nugent et al. 2015), regulation of ER mRNA (Blizard 1983, McCarthy et al. 1993, Royston cell death and proliferation (Wu et al. 2009) and changes et al. 2016). Whether the neural mechanisms underlying in synaptic patterning (Schwarz et al. 2008). the organizational effects of testosterone on activity are Activity is a sexually dimorphic behavior in rodents, similar to those described for mating behavior remains with females showing higher levels of wheel running and to be investigated. Similarly, the organizational effects SPA compared to males (Eikelboom & Mills 1988). Gender of testosterone outside the perinatal window on physical differences in activity remain after gonadectomy and activity (i.e. puberty) await confirmation. thus cannot be explained by hormonal dimorphism in adulthood (Gentry & Wade 1976, Blizard 1983, Broida & Global KO mouse models Svare 1984, Bowen et al. 2012, Kuljis et al. 2013). Differences in sex chromosomes as such are also insufficient to explain Studies in transgenic mice lacking either steroid hormone the dimorphism in activity, as evidenced by studies using receptors or the aromatase enzyme provide further insights the mouse model of 4 core genotypes, in which sex into the role of sex steroids regulating physical activity. chromosome complement is independent of gonadal However, the interpretation of the results obtained from phenotype (De Vries et al. 2002, Kuljis et al. 2013). In a these mouse models is hampered by alterations of the similar pattern to XX females, ovariectomy of mice with a hypothalamic–pituitary–gonadal (HPG) axis. In addition, deletion of the testis-determining gene Sry results in running it is reasonable to expect changes in the sex-steroid- longer distances than ORX males (Kuljis et al. 2013). dependent brain organization (see previous section) Accumulating literature suggests a role for sex steroid caused by the gene deletion during development. actions during the perinatal period determining gender differences in activity. ORX at postnatal day 0 (P0) Androgen receptor knockout (ARKO) mice results in higher levels of SPA than that at P5, 10 or 25 Several ARKO mouse models targeting either exon 1, 2 or 3 in male mice (Broida & Svare 1984). Also in male mice, have been generated using the Cre-LoxP system (Fan et al. estrogen deficiency during early life leads to a higher 2005, Ophoff et al. 2009a, Rana et al. 2011). Our group wheel running response in adulthood following E2 reported an almost complete abrogation of wheel running

Figure 2 Schematic overview of the overall proposed mechanisms of action of testosterone on male activity (see text). A, androgens; CNS, central nervous system; E2, estradiol; PNS, peripheral nervous system; +, stimulatory effect; ++, dominant stimulatory effect.

Journal of F Jardí et al. Androgens and physical activity 238:1 R37 Endocrinology as well as a reduced SPA in exon 2-deleted male ARKO mice show increased levels of serum testosterone (Callewaert (Ophoff et al. 2009a). A similar low activity phenotype was et al. 2009), are less active than their wild-type (WT) observed in mice with a targeted disruption of AR exon 1 littermates when given access to running wheels (Blattner (Fan et al. 2005) and in a DNA-binding domain ARKO mouse & Mahoney 2012), and develop elevated adiposity in late- model that only retains non-genomic actions (Rana et al. life (Heine et al. 2000). As shown first in the seminal work of 2011). The fact that global ARKO mice are less physically Ogawa et al. and later confirmed by others, the stimulatory active might predispose them to develop late-onset actions of E2 treatment on wheel running activity are adiposity (Fan et al. 2005, Rana et al. 2011). Global ARKO abrogated in ERαKO mice (Ogawa et al. 2003, Dworatzek mice appear to suffer from an impaired gonadal testosterone et al. 2014, Blattner & Mahoney 2015). This is not the case production (Callewaert et al. 2009) and might therefore not for ERβKO mice in which the response to E2 is unaffected be useful to discern between purely AR-mediated effects and (Ogawa et al. 2003). Therefore, the estrogenic regulation those secondary to E2 deficiency. Indeed, treatment with of wheel running in males is primarily mediated by ERα, testosterone completely restored wheel running in male though this does not exclude the possibility that ERβ plays ARKO mice, an effect associated with a normalization of a role in modulating the circadian-rhythm patterns of their E2 concentrations in brain (Jardí et al. 2018). Thus, it activity, as shown for females (Royston et al. 2014). is reasonable to assume that the low activity phenotype of ARKO mice is derived in large part from a lack of testosterone ‘Non-classical’ ER knock-in (NERKI) mice substrate for aromatization into E2. Transgenic mice with a knock-in mutation in the ERα selectively abolishing estrogen response element (ERE) Aromatase knockout mice binding were originally generated to distinguish between Mice in which E2 biosynthesis is disrupted by deletion classical and non-classical ERα actions in vivo (Jakacka of the Cyp19a1 gene represent a useful tool to study the et al. 2002). Male ‘non-classical’ ER knock-in mice (NERKI) implication of E2 in the regulation of male physiological mice show a reduced wheel running activity that does not processes (Cooke et al. 2017). Most studies in male increase following challenge with E2 (Blattner & Mahoney aromatase KO mice agree that both wheel running (Watai 2012, 2015). Based on these observations, it is tempting to et al. 2007, Brockman et al. 2011) and SPA (Hill et al. speculate that ERα actions on wheel running activity are 2007) are diminished in the absence of E2. Nevertheless, dependent on ERα binding to ERE elements. one study reported increased levels of wheel running in male aromatase KO mice, as part of a broader spectrum of G-protein-coupled receptor 30 knockout compulsive behaviors (Hill et al. 2007). It should be noted (GPR30KO) mice that the authors analyzed pre-plateau phases of activity, The G-protein-coupled receptor 30 (GPR30) has been which could have led to a mistaken estimation of the proposed as a membrane ER (Langer et al. 2010). There ‘true’ levels of wheel running. Be that as it may, aromatase are important phenotypic differences between the four KO mice also show an impaired negative feedback of the independent GPR30KO mouse models available, most HPG axis, with higher levels of serum testosterone and likely reflecting that other factors apart from deletion of gonadotropins (Fisher et al. 1998, Jones et al. 2000, Öz GPR30 are contributing to the observed effects (Langer et al. 2000). The fact that aromatase KO mice display a et al. 2010). In two different GPR30KO mouse models, low activity phenotype even in the presence of excess males exhibited normal SPA levels (Sharma et al. 2013, testosterone indicates a crucial role for aromatization in Kastenberger & Schwarzer 2014), but another study reported the testosterone-induced stimulation of male physical that GPR30KO male mice displayed a mild reduction activity. Supporting this notion, male aromatase KO mice in wheel running as well as an impaired left-ventricular do not show an ORX-induced reduction of wheel running cardiac function (Delbeck et al. 2011). It therefore remains (Brockman et al. 2011). uncertain whether GPR30 is physiologically relevant for physical activity, and if so, if it contributes to E2-induced Estrogen receptor α (ERα-) and β knockout responses and/or initiates independent mechanisms. (ERβKO) mice The generation of ERα and ERβKO mice has advanced Sex-hormone-binding globulin transgenic our understanding of the mechanisms implicated in the (SHBG-tg) mice estrogenic regulation of male physiology (Cooke et al. Sex-hormone-binding globulin (SHBG) is a high-affinity 2017). Similar to male aromatase KO mice, ERαKO mice binding protein for testosterone, DHT and E2 that is found

http://joe.endocrinology-journals.org © 2018 Society for Endocrinology https://doi.org/10.1530/JOE-18-0125 Published by Bioscientifica Ltd. Printed in Great Britain Downloaded from Bioscientifica.com at 09/26/2021 07:39:28PM via free access Journal of F Jardí et al. Androgens and physical activity 238:1 R38 Endocrinology circulating in humans but is almost completely lacking in et al. 2012) and, in turn, the capacity to exercise. In other rodent serum. We recently showed in a humanized mouse words, testosterone might influence both the motivation model expressing human SHBG that testosterone needs and/or the ability to partake in activity, referred here as to be biologically available (i.e. not bound to SHBG) to central and peripheral actions, respectively (Fig. 2). Note produce its stimulatory effects on wheel running (Jänne that the term ‘peripheral’ as applied to this classification et al. 1998, Jardí et al. 2018). In SHBG-tg mice, the does not preclude the implication of the central nervous majority of testosterone is bound by SHBG and cannot system (CNS) in the observed effects. enter target tissues by the canonical way (Laurent et al. 2016a,b). Male SHBG-tg mice display features of a mild Peripheral actions androgen deficiency phenotype, including a 50% decrease in wheel running activity (Jardí et al. 2018). As the decline In the next section, we will examine whether androgens of androgen bioactivity in elderly men is also only mild provide greater tolerance to exercise by improving muscle (Wu et al. 2008), SHBG-tg mice may be a more suitable strength, motor skills, metabolism and/or other peripheral animal model to study late-onset hypogonadism than parameters in male rodents. ORX. The interpretation of studies using constitutive KO Muscle mass and strength mice for steroid hormone receptors or the aromatase Both the mass and the ex vivo contractility of hindlimb enzyme is subject to several limitations, including muscles show a mild reduction following ORX or AR alterations of the HPG axis, developmental compensatory deletion in male mice (Axell et al. 2006, MacLean et al. mechanisms and the impossibility to distinguish tissue- 2008, De Naeyer et al. 2014, Dubois et al. 2014). The specific contributions. Nonetheless, findings in both differences in strength disappear when normalized for aromatase and ERKO mice indicate that testosterone muscle weight, suggesting that the intrinsic contractile actions on physical activity require normal aromatase and properties of muscle remain unaltered (Axell et al. 2006, ERα functions. In contrast, we cannot draw conclusions MacLean et al. 2008, Hourdé et al. 2009, De Naeyer et al. about the implication of AR from studies in global ARKO 2014). However, in five different myoblast or satellite- mice because of their severe hypogonadism. However, cell-specific ARKO models, the mass and contractility replacement and pharmacological studies made clear of hindlimb muscles were not, or marginally, affected that the contribution of AR to testosterone effects cannot (Ophoff et al. 2009b, Chambon et al. 2010, Dubois et al. be dismissed as minimal or trivial (see discussion in 2014, Ferry et al. 2014, Rana et al. 2016), whilst all five the ‘Replacement and pharmacological studies in adult models showed a partial decrease in the weight of the rodents’ section). androgen-sensitive perineal muscles. Nevertheless, the expression of AR in the muscle cell lineage was required for the full gain in muscle mass induced by mechanical

Mechanisms implicated in the effects of loading (Ferry et al. 2014), an effect that could be related androgens and estrogens on male to the modulation of a reserve pool of adult muscle stem physical activity cells by androgens (Kim et al. 2016). An overview of the main features of the different muscle-specific ARKO Despite the large amount of preclinical data showing models is presented in Table 1. Overall, the former studies that androgens and estrogens control physical activity suggest a role for non-myocytic AR mediating the modest in males, the mechanisms involved are not well actions of androgens on murine muscle. In this regard, understood. Due to their pleiotropic nature, sex steroids a recent study points toward neuronal AR contributing could stimulate physical activity by acting on multiple to maintain hindlimb muscle mass but not contractility organs or systems. Of particular interest, the use of in male mice (Davey et al. 2017). The cellular targets genetic techniques to manipulate neuronal pools in a underlying the anabolic actions of androgens on muscle selective manner has allowed the identification of several have been reviewed by Dubois et al. (2012). sex-steroid-responsive neural circuitries implicated in The ex vivo contractility studies referred to in the the drive to engage in physical activity (Musatov et al. earlier sections do not support a role for an intrinsic 2007, Xu et al. 2011, 2015, Correa et al. 2015). Besides muscle dysfunction impairing the physical ability of ORX these central mechanisms, androgens could also increase and ARKO mice. Yet, these findings do not exclude the activity levels by improving muscle function (Dubois possibility that androgens regulate the neuromuscular

Journal of F Jardí et al. Androgens and physical activity 238:1 R39 Endocrinology

) function in vivo. Exercise training potentiates the

↑ ↓ ( ND ND ND ND ND ND ND ND ND SPA stimulatory effects of androgens on muscle mass and strength (Guo et al. 2012, Cozzoli et al. 2013). Indeed, the rotarod test; f combination of both interventions benefits endurance ↓ = = = = ND ND ND ND ND ND

Wheel capacity in rodents (Vanzyl et al. 1995, Georgieva & running Boyadjiev 2004, Cozzoli et al. 2013), presumably through increasing muscle function. However, ORX per se f d d diminishes grip strength by only about 5–15% in some = = = ND ND ND ND ND ND ND ND Motor determined by DXA; but not all studies (Borst et al. 2007, Windahl et al. 2011, e

performance White et al. 2013, Chisamore et al. 2016). The discrepancies between studies might be due to the lack of sensitivity of the conventional forelimb grip strength test (Takeshita ↓ = (=) (=) ND ND ND ND ND ND ND et al. 2017). Additionally, the motivation to hold onto the function Contractile treadmill running; grid may vary between mice, causing inconsistencies in d the results. Be that as it may, a modest reduction (~10%) in grip strength was also found in two muscle-specific ARKO ↓ ↓ ↓ = ND ND ND ND ND ND ND Grip mouse models (Chambon et al. 2010, Dubois et al. 2014) strength , decreased; =, unchanged; (=), minor impact on fatigue but no

↓ (Table 1). However, this did not impede muscle-specific ARKO mice from displaying similar performances to their e ↓ ↓ ↓ ======ND ND WT littermates in both the voluntary wheel running and mass as assessed by SV weight; Muscle c the forced treadmill tests (Table 1). Overall, we conclude that it is unlikely that the mild loss of muscle function following androgen deficiency in male mice is the main driver of their low activity phenotype. )), not statistically significant; ↑ )) c c c c Motor skills

↑ ↑ ↑ ↑ = ↑ ↑ = = ND ND (( Preclinical data show that testosterone deficiency exerts detrimental effects on male CNS health (Khasnavis et al. 2013, Jayaraman et al. 2014, Atallah et al. 2017). In mice, prepubertal ORX led to a poor performance in the rotarod Serum testosterone/SV weight Serum testosterone/SV test, which the authors associated with a loss of midbrain , AR expression also reduced in brain and fat; dopaminergic neurons (Khasnavis et al. 2013). Conversely, b also in the rotarod test, male testicular-feminized (Tfm) ) 2015) b ,

) mice expressing a non-functional AR performed better b (2010) (2010) than their WT littermates (Rizk et al. 2005). Although (2009 (2014 (2009) (2017) (2018) (2010) this might seem unexpected at first, the increased rotarod (2016) (2016) (2017 et al. et al. (2013) et al.

et al. performance of Tfm mice could be due to their lower et al. et al. et al. et al. et al. et al. et al. body weight, since these two variables share an inverse et al.

Jardí correlation (Mao et al. 2015). Confirming the latter results, Chambon Chambon Ophoff Dubois Raskin Juntti Rana Rana Yu Davey Jardí Ref male ARKO mice showed normal motor skills despite their loss in striatal dopamine concentrations, a fact that might be explained by compensatory adaptations during Cre α ontogeny (Jardí et al. 2018). In contrast to prepubertal castration, ORX adult rodents showed an altered skm;HSACre skm;HSACreER skm;MCKCre sat+skm;MyoD Neu+glia;NesCre Neu+glia;NesCre skm;HSACre(a,b) skm;MCKCre(a) Neu;SynapsinICre Neu;CamKII Neu;Thy1CreER(g) performance in neither the rotarod test nor a nigrostriatal AR AR AR AR AR AR AR AR AR AR AR Mouse strain pathology (Kritzer et al. 2001, Frye et al. 2008b, Khasnavis ), SPA restored to control values following normalization of testosterone levels; (( ), SPA ↑

Overview of muscle- and neuronal-specific ARKO mouse models. et al. 2013). In the beam-walking test, which detects more subtle motor coordination deficits, one study showed that ORX mice performed worse than sham-operated controls , increased; ( AR expression retained in the hypothalamus. AR lox hemizygous mice show a phenotype of hyperandrogenization; Cell type Myofibers Neurons g ↑ seminal vesicles; testosterone, spontaneous physical activity; SV, changes in maximal muscle tension; ND, not determined; neu, neurons; sat, satellite cells; skm, skeletal myofibers; SPA, testosterone. Table 1 Table a (McDermott et al. 1994). However, the performance in http://joe.endocrinology-journals.org © 2018 Society for Endocrinology https://doi.org/10.1530/JOE-18-0125 Published by Bioscientifica Ltd. Printed in Great Britain Downloaded from Bioscientifica.com at 09/26/2021 07:39:28PM via free access Journal of F Jardí et al. Androgens and physical activity 238:1 R40 Endocrinology the latter test is in part influenced by motivational factors maintaining the capacity of male rodents for endurance (McDermott et al. 1994, Curzon et al. 2009, Deacon 2013). activities. In contrast to what is observed in the voluntary Regarding peripheral nerve health, both ORX and wheel running test, both ARKO mice and ORX rats treatment with the antiandrogen flutamide reduced perform as well as control animals when forced to run the synthesis of myelin proteins in the sciatic nerve of in a treadmill (Ophoff et al. 2009a, Petroianu et al. 2010). male rodents (Magnaghi et al. 1999, 2004, Jayaraman Therefore, we conclude that androgens must act through et al. 2014). In male mice, the decrease in myelin sheath other mechanisms beyond muscle to stimulate physical was associated with macrophage infiltration (Jayaraman activity. Nevertheless, it is still possible that the increase et al. 2014). Also in male mice, however, nerve function in activity over time potentiates the peripheral actions of as assessed by sciatic nerve conductance studies was not androgens so that they acquire a greater importance in altered following either ORX or ARKO (Jardí et al. 2018). sustaining activity (Fig. 2). Taken together, we conclude that the adverse actions of androgen deficiency on male neural health do not seem Central actions to translate into a deterioration of motor performance that would limit physical activity, at least in adult rodents. In the next section, we will review the available findings This does not preclude a role for androgens improving supporting the notion that testosterone is mainly acting motor functional recovery in animal models of neural centrally to boost physical activity in male rodents. disease (Uchida et al. 2009, Yoo & Ko 2012, Ponnusamy et al. 2017). Central-nervous-system-specific KOs In the mouse brain, AR and ERα display a moderate-to- Other peripheral actions high regionalized expression profile, both showing a high Skeletal muscle metabolism is critical to maintain fuel expression in the hypothalamus and brainstem, whilst AR homeostasis during exercise. In both male rats and mice, levels are also abundant in the hippocampus and cerebral androgen deficiency decreases muscle glycogen stores cortex (Gofflot et al. 2007, Lein et al. 2007, Mahfouz et al. (Ramamani et al. 1999, Dubois et al. 2016), a suggested 2016). Four different whole brain ARKO mouse models risk factor for fatigue development (Xirouchaki et al. have been generated so far, allowing to explore the impact 2016). Conversely, testosterone promotes glucose uptake of AR signaling in the nervous tissue without interfering in skeletal muscle, although it is not clear to what extent with its peripheral functions (Raskin et al. 2009, Juntti these effects are dependent on local AR actions (Sato et al. et al. 2010, Yu et al. 2013, Davey et al. 2017). An overview 2008, Ibebunjo et al. 2011, Dubois et al. 2016, Kelly et al. of the main features of the different neuronal ARKO 2016). In male mice, deletion of myogenic AR did not models is presented in Table 1. All four models show affect either muscle glycogen stores (Ophoff et al. 2009b) an intact peripheral masculinization but unfortunately or glucose tolerance (Dubois et al. 2016), in contrast to that have an impaired negative feedback of the HPG axis in observed in both liver- (Lin et al. 2008) and adipose-tissue- males, resulting in higher serum levels of testosterone specific ARKO mice McInnes( et al. 2012). The mechanisms and increased seminal vesicle weights (Raskin et al. 2009, by which testosterone acts on metabolic pathways have Juntti et al. 2010, Yu et al. 2013, Davey et al. 2017). It is been reviewed elsewhere (Kelly & Jones 2013). Besides nevertheless clear that the neuronal AR is required for the metabolism, long-term ORX might affect exercise full expression of both male-typical sexual and aggressive capacity in rodents by attenuating cardiac contractility, behaviors (Raskin et al. 2009, Juntti et al. 2010, Marie-Luce as suggested by both in vivo echocardiographic and ex et al. 2013). Regarding physical activity, one study reported vivo studies (Sebag et al. 2011, Eleawa et al. 2013). These an increase in SPA in Nestin-Cre neuronal ARKO mice, alterations in cardiac contractile function could arise from although levels were restored to control values following testosterone modulating Ca2+ handling in ventricular normalization of testosterone levels, pointing to an myocytes, as recently reviewed (Ayaz & Howlett 2015). implication of the altered HPG axis in the observed effects Additionally, androgens might influence adaptation to (Raskin et al. 2009). Contrarily, both SPA and wheel running exercise by affecting hemoglobin and hematocrit levels were reduced by 60% in CamKII-Cre neuronal ARKO (Bhasin et al. 2012), bone (Vanderschueren et al. 2014) mice, which retains only non-DNA-binding-dependent and/or lung function (Townsend et al. 2012). However, AR actions (Davey et al. 2017). The latter results support overall, the actions of androgens on these parameters or the interpretation that AR exerts a predominant role in on muscle do not appear to play an indispensable role in the regulation of male physical activity by testosterone.

Journal of F Jardí et al. Androgens and physical activity 238:1 R41 Endocrinology

Nonetheless, the above-mentioned studies did not include to estrogens in females, regulate male energy expenditure littermates carrying the Cre transgene as controls. This is by acting on AR and/or ER-positive hypothalamic particularly important since Cre-driven transgenes might neurons. Hypothalamic testosterone implants restored exert independent effects on behavioral outcomes (Harno wheel running activity in ORX mice (Model et al. 2015). et al. 2013, Chen et al. 2016). There is also the possibility However, since testosterone can diffuse to adjacent brain that brain AR deletion at early stages may have indirectly areas, these might also be implicated in the observed interfered with the organizational actions of testosterone effects. According to unpublished data of Ogawa et al. by altering the HPG axis. To circumvent this caveat, we (Sano et al. 2013), site-specific ERα knockdown in the recently generated a tamoxifen-inducible neuronal ARKO VMH completely abolished estrogen-induced facilitation mouse model by using the Thy1-CreER, which drives gene of wheel running in male mice, similar to that described deletion in extrahypothalamic regions of the brain (Jardí in females (Fig. 3). Notwithstanding the aforementioned et al. 2017a). In contrast to CamKII-Cre neuronal ARKO limitation of treating male mice with estrogens, these mice, deletion of AR in the CNS of pubertal male Thy1- results suggest that the VMH forms part of the neural CreER neuronal ARKO mice did not affect wheel running circuitries mediating testosterone-induced wheel running (Jardí et al. 2017b). The differences between studies could in male mice and highlights the implication of local be attributed to the residual expression of hypothalamic aromatization in this brain region (Roselli et al. 1985). AR in our mice (Jardí et al. 2017a). All in all, the study of In contrast to females, however, the regulation of male neuronal ARKO mice has not provided clear answers so far physical activity by ERα expression in the VMH may regarding the role of brain androgenic signaling in male involve a distinct module than the ventrolateral region physical activity. To the best of our knowledge, only one since stimulation of this neuronal cluster did not increase study reported a whole-brain ERαKO in male mice, but the SPA in male mice (Correa et al. 2015). authors did not assess physical activity in these animals (Xu et al. 2011). Medial amygdala The neuronal basis regulating physical activity Hypothalamic circuitries likely implies other neuronal circuitries beyond the The hypothalamus is a key brain region that integrates hypothalamic energy balance systems, such as brain nutritional, hormonal and neural information to regions involved in emotional and motivational orchestrate adaptive physiological responses aimed to processing (Fig. 3). MeA neurons projecting to the maintain the whole-body energy balance (Lenard & hypothalamus play a central role in generating emotional Berthoud 2008, Schneeberger et al. 2014). Technical responses to chemosensory signals (Keshavarzi et al. advances in viral-vector-mediated gene manipulation 2014, Takahashi 2014). Compared to other amygdalar as well as in opto- and chemogenetics have allowed the nuclei, the expression of sex steroid receptors is enriched identification of hypothalamic circuitries implicated in the MeA (Gofflot et al. 2007). In addition, a high in the control of energy homeostasis by sex steroids. proportion of MeA neurons are aromatase positive, and In particular, hypothalamic ERα signaling in female therefore this brain region might be exposed to relatively rodents has been shown to exert a holistic control of high levels of E2 (Wu et al. 2009, Unger et al. 2015). In their overall energy balance, influencing food intake as male mice, ERα signaling in the MeA is required for the well as energy expenditure (Xu et al. 2017 for review on pubertal organizational actions of testosterone on social this topic). Silencing of ERα in the ventromedial nucleus and sexual behaviors (Sano et al. 2016). In addition, Xu of the hypothalamus (VMH) by small hairpin RNA et al. provided compelling evidence that ERα signaling knockdown led to obesity in both female rats and mice, in the MeA stimulated SPA in males and secondarily an effect associated with a decline in physical activity prevented body weight gain in the setting of high-fat diet and thermogenesis (Musatov et al. 2007). Remarkably, (Xu et al. 2015). Particularly, deletion of ERα in the MeA the effects of estrogens on these two parameters of of adult male mice resulted in a low activity phenotype, energy expenditure seem to be driven by independent whilst the other components of energy expenditure, ERα modules of the VMH (Xu et al. 2011, Correa et al. namely thermogenesis and resting metabolic rate, 2015). In male mice, global deletion of either AR or ERα remained unaltered (Xu et al. 2015). Together with results in a decreased energy expenditure that might the VMH (unpublished data of Ogawa et al.), the MeA predispose them to increased adiposity (Heine et al. 2000, is the only brain region identified so far that could be Fan et al. 2005). It is not clear whether androgens, similar mediating the stimulatory effects of testosterone on

http://joe.endocrinology-journals.org © 2018 Society for Endocrinology https://doi.org/10.1530/JOE-18-0125 Published by Bioscientifica Ltd. Printed in Great Britain Downloaded from Bioscientifica.com at 09/26/2021 07:39:28PM via free access Journal of F Jardí et al. Androgens and physical activity 238:1 R42 Endocrinology male physical activity (Fig. 3). In addition, these findings receptors but also indirectly through the afferent neurons also imply a role for brain aromatization in testosterone projecting to them (Kritzer & Creutz 2008, Aubele & regulation of activity, similar to that described for male- Kritzer 2012, Purves-Tyson et al. 2012, Locklear et al. typical aggressive behavior (Unger et al. 2015). 2017). However, it is not clear if the main contribution of androgens to the overall dopaminergic tone is at the level Rewarding and motivational pathways of dopamine synthesis, transport, release, metabolization Besides the metabolic-regulatory circuitries of the and/or signaling (Aubele & Kritzer 2011, Khasnavis hypothalamus and the MeA, brain regions controlling et al. 2013, Purves-Tyson et al. 2014, Wang et al. 2016). rewarding and motivational functions might also be Testosterone restoration of basal extracellular dopamine involved in the regulation of physical activity (Garland levels in the mesocortical dopaminergic pathway implies et al. 2011) (Fig. 3). Endurance running generates a AR-dependent mechanisms, though this might not apply rewarding aftereffect in both humans and rodents to the rest of dopaminergic systems (Kritzer 2003, Aubele (Boecker et al. 2008, Fernandes et al. 2015). The traditional & Kritzer 2011, Locklear et al. 2017). Amphetamine view attributes reward functions to dopamine signaling injections induce a hyperlocomotion response in rodents, in the nucleus accumbens (Wise 2004, Palmiter 2008). which has been ascribed to the drug’s dopamine-releasing However, recent findings show that other dopamine actions (Salahpour et al. 2008). Notably, ORX increases, terminal fields are also components of the brain reward whilst testosterone restores the responsiveness to the system, including the dorsal striatum, amygdala and locomotor effects of amphetamine in both rats and mice frontal cortex (Wise 2004, Palmiter 2008). Following the (Purves-Tyson et al. 2015, Jardí et al. 2018). Overall, there withdrawal of running wheels, mice selectively bred for is enough evidence to conclude that androgens influence high wheel running showed a more pronounced increase the dopamine function in vivo. In addition, we recently in striatal activation compared to controls (Rhodes et al. showed that systemic administration of the dopamine 2003). Also in male mice, the relative volume of the receptor 2 antagonist L-741,626 reduced by almost 50% striatum predicted the levels of subsequent wheel running the distance ran by ORX+testosterone-treated mice, activity on in vivo MRI (Cahill et al. 2015). whilst it had no effect on castrated mice receiving vehicle Testosterone might act directly on male midbrain (Jardí et al. 2018). Further research is required to narrow dopamine neurons by binding to their AR and/or ERs down the relevant androgen-responsive components of

Figure 3 Schematic overview of the proposed neuronal mechanisms by which testosterone increases male physical activity (see text). A, androgens; DA, dopamine; E2, estradiol; MeA, medial amygdala; NAcc, nucleus accumbens; SPA, spontaneous physical activity; VMH, ventromedial hypothalamus.

Journal of F Jardí et al. Androgens and physical activity 238:1 R43 Endocrinology the dopamine system that might be mediating the effects Nevertheless, other studies using larger cohorts and activity- of testosterone on physical activity (Jardí et al. 2018). based questionnaires yielded conflicting observations Amin( To summarize, the control of activity and metabolism et al. 2000, Beutel et al. 2005, Tajar et al. 2012). Similarly, are regulated in a coupled manner by estrogen-responsive there is no clear evidence from interventional studies that hypothalamic pathways in females. The implication supports a causal effect of sex steroids on physical activity. of similar mechanisms in males remains to be proven. Several double-blind, placebo-controlled clinical trials have Extrahypothalamic inputs coming from brain regions examined the effects of testosterone replacement on both related to emotional and rewarding processes might muscle as well as functional mobility in large samples of modulate the homeostatic control exerted by the community-dwelling men with late-onset hypogonadism hypothalamus. Testosterone might act on the MeA to (Emmelot-Vonk et al. 2008, Srinivas-Shankar et al. 2010, increase physical activity in males following aromatization Snyder et al. 2016, Storer et al. 2016). Overall, testosterone into E2 (Fig. 3). The dopamine system is another potential supplementation in hypogonadal men results in modest candidate mediating the effects of testosterone on gains in muscle mass and strength, but the benefits on activity, but further research is required to determine the their overall physical function are inconsistent (Emmelot- mechanisms implicated (Fig. 3). Vonk et al. 2008, Srinivas-Shankar et al. 2010, Snyder et al. 2016, Storer et al. 2016). In addition, testosterone therapy in men is hampered by its potential adverse effects on the Human evidence cardiovascular system and the prostate (Basaria et al. 2010). Therefore, the clinical meaningfulness of testosterone After reviewing the available data, it is clear that androgens replacement in healthy elderly men with late-onset regulate physical activity in male rodents, but in men, hypogonadism remains debated. Remarkably, treatment it remains a question. The etiology of physical activity with testosterone for 6 months did not affect the self- in humans appears to be much more complex than in reported levels of activity in a cohort of 274 community- animals, being influenced by the social and cultural dwelling frail elderly men with low levels of testosterone environments (Bauman et al. 2012). Heritability studies (Srinivas-Shankar et al. 2010). Smaller studies in similar in twins and families suggest that our drive to engage in patient populations also showed a lack of testosterone effect exercise is also determined by intrinsic biological processes on activity scores (Kenny et al. 2001, 2010). Contrarily, a (Simonen et al. 2002, Bouchard & Rankinen 2006, Stubbe positive association between serum testosterone levels and et al. 2006). Nevertheless, the genetics of physical activity activity scores was found in a cohort of elderly men receiving remain currently unknown, with only limited genotype– a combined treatment of testosterone and recombinant phenotype associations found so far. A candidate gene human growth hormone (Sattler et al. 2011). The authors study in postmenopausal women showed an association hypothesized that this association might reflect a central between aromatase gene polymorphisms and time spent in action of testosterone on the individual’s mood (Sattler physical activity (Salmen et al. 2003). This association for et al. 2011). To date, there is no real evidence supporting the aromatase gene was replicated in a GWAS for leisure- the former premise, with the effects of testosterone on time exercise behavior but only in one out of the two vitality and mood parameters in elderly men being unclear populations of adult men included (De Moor et al. 2009). (Snyder et al. 1999, Srinivas-Shankar et al. 2010, Snyder et al. Polymorphisms in the AR or ERα gene were not associated 2016). More importantly, all previous studies were based on with the amount of physical activity per week in either activity questionnaires, which show limited reliability and young or middle-aged men or women (Lorentzon et al. 1999, accuracy (Strath et al. 2013). Thus, caution should be taken Salmén et al. 2000, Okura et al. 2003, Walsh et al. 2005). when interpreting the data. In conclusion, it is premature to Aging is associated with a decline in physical activity in both support or reject convincingly the implication of androgens genders (Jones et al. 2011) as well as with a gradual reduction in physical activity in men. in serum testosterone, especially its free fraction, in men (Wu et al. 2008). So far, studies of the association between late-onset hypogonadism and the age-related decline in Conclusion activity have produced inconsistent results. A small cross- sectional study in community-dwelling men showed an A wide body of preclinical evidence makes a strong case association between low testosterone levels and a decrease for androgens as potential candidates contributing to in pedometer-recorded step counts (Cobo et al. 2017). the biological basis regulating male physical activity.

Nonetheless, reliable human studies are lacking and References our present knowledge of the implicated processes is Allen DL, Harrison BC, Maass A, Bell ML, Byrnes WC & Leinwand LA very limited. This is explained in part by the fact that 2001 Cardiac and skeletal muscle adaptations to voluntary wheel the interest in the biological factors determining activity running in the mouse. Journal of Applied Physiology 90 1900–1908. levels is relatively recent. Measurement of physical activity (https://doi.org/10.1530/eje.0.1330361) Almeida M, Laurent MR, Dubois V, Claessens F, O’Brien CA, Bouillon R, in humans most often relies on self-reported instruments Vanderschueren D, Manolagas SC. 2017 Estrogens and androgens due to their practicality and low cost. Nevertheless, these in skeletal physiology and pathophysiology. Physiological Reviews 97 measures have severe limitations in terms of reliability 135–187. 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(https://doi. org/10.1016/j.yhbeh.2009.03.011) mice, the modest and slow gains in muscle mass induced Atallah A, Mhaouty-Kodja S & Grange-Messent V 2017 Chronic depletion by androgens contrast with their dramatic and fast actions of gonadal testosterone leads to blood-brain barrier dysfunction and increasing wheel running. Taken together, preclinical inflammation in male mice.Journal of Cerebral Blood Flow and Metabolism 37 3161–3175. (https://doi.org/10.1177/0271678X16683961) data do not support the notion that androgens stimulate Aubele T & Kritzer MF 2011 Gonadectomy and hormone replacement physical activity primarily by favoring muscle function. affects in vivo basal extracellular dopamine levels in the prefrontal On the other hand, the central nature of testosterone cortex but not motor cortex of adult male rats. Cerebral Cortex 21 222–232. 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(https://doi. required for the full action of testosterone on activity. org/10.1093/cercor/bhr258) In conclusion, the CNS appears as the primary target of Axell A-M, MacLean HE, Plant DR, Harcourt LJ, Davis JA, Jimenez M, Handelsman DJ, Lynch GS & Zajac JD 2006 Continuous testosterone action for androgens stimulating physical activity, whilst administration prevents skeletal muscle atrophy and enhances the multiple beneficial effects of testosterone on muscle resistance to fatigue in orchidectomized male mice. American Journal and other peripheral organs do not drive the increase in of Physiology: Endocrinology and Metabolism 291 E506–E516. (https:// doi.org/10.1152/ajpendo.00058.2006) activity but might contribute to the adaptation to exercise. Ayaz O & Howlett SE 2015 Testosterone modulates cardiac contraction A better understanding of the underlying biological and calcium homeostasis: cellular and molecular mechanisms. 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Received in final form 29 March 2018 Accepted 9 May 2018 Accepted Preprint published online 9 May 2018