Regular voluntary exercise reduces anxiety-related behaviour and impulsiveness in mice

Behavioural Brain Research 155 (2004) 197–206

Research report

Regular voluntary exercise reduces anxiety-related behaviour and impulsiveness in mice Elke Binder a , Susanne K. Droste b,c , Frauke Ohl a , Johannes M.H.M. Reul b,c,∗ a

Section of Behavioral Phenotyping, Max Planck Institute of Psychiatry, Munich D-80804, Germany Section of Neuropsychopharmacology, Max Planck Institute of Psychiatry, Munich D-80804, Germany Henry Wellcome Laboratories for Integrative Neuroscience and Endocrinology, The Dorothy Hodgkin Building, University of Bristol, Whitson Street, Bristol BS1 3NY, UK b

c

Received 3 February 2004; received in revised form 14 April 2004; accepted 15 April 2004 Available online 24 June 2004

Abstract We embarked on a study to delineate the behavioural changes in mice after 4 weeks of voluntary exercise. As an initial behavioural characterization, we exposed the control and exercising mice to a modified hole board and an open field test. As compared to control mice, exercising animals showed clear signs of increased behavioural inhibition (e.g. a longer latency to enter unprotected areas), suggesting increased anxiety in these animals. In addition, the exercising mice were reluctant to spend time in the open field’s centre during the beginning of the 30-min open field test, but compensated for this at later times. Paradoxically, the exercising animals showed more rearings on the board of the modified hole board, indicating decreased anxiety. Thus, the behavioural inhibition seen in exercising mice is likely to represent decreased stress responsiveness at the behavioural level which can also be interpreted as reduced impulsiveness. To clarify whether voluntary exercise evolves in more or less anxiety-related behaviour, we exposed animals to the elevated plus-maze and the dark–light box, two selective tests for unconditioned anxiety. Clearly, compared to the control animals, exercising mice spent significantly more time on the open arm of the plus-maze and spent double the amount of time in the light compartment of the dark–light box. Taken together, we conclude that long-term voluntary exercise appears to result in decreased anxiety-related behaviour and impulsiveness. Thus, our observations fit into the concept that regular exercise strengthens endogenous stress coping mechanisms, thereby protecting the organism against the deleterious effects of stress. © 2004 Elsevier B.V. All rights reserved. Keywords: Modified hole board; Open field; Dark–light box; Elevated plus-maze; Brain; Stress; Depression

1. Introduction The regular performance of exercise has vast beneficial effects on a variety of biological systems. From the perspective of intensity of research activities and interest of the public, the main emphasis has been on weight control [1–4] and benefits for the cardiovascular system [5–8]. However, evidence is accumulating that regular exercise also impacts positively on the brain. It was shown that long-term voluntary exercise in mice—by allowing access to a running wheel—resulted in increased neurogenesis in the dentate gyrus [9] as well as improved cognitive abilities [10,11]; for review, see [12]. Lancel et al. [13] found that exercising mice show increased sleep consolidation ∗ Corresponding author. Tel.: +44 117 331 3137; fax: +44 117 331 3139. E-mail address: [email protected] (J.M.H.M. Reul).

0166-4328/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2004.04.017

and reduced rapid-eye-movement (REM) sleep, suggesting improved sleep quality. Moreover, the social stress-induced increase in REM sleep observed in control mice was found to be reversed (i.e. decreased) in exercising mice [13]. Recently, we also reported that long-term voluntary exercise in mice leads to significant changes at different levels of the hypothalamic–pituitary–adrenal (HPA) axis [14]. These changes included reduced HPA hormone responses to emotional stimuli, i.e. exposure to a novel environment [14]. Thus, long-term voluntary exercise appears to strengthen endogenous stress-coping mechanisms protecting the organism against the potentially deleterious effects of stress. The here mentioned form of regular exercise, however, is not to be confused with the high-demand endurance training (e.g. marathon running) in humans or forced—e.g. treadmill—running in rodents. Endurance training, because of its excessive (eccentric) physical demand, has been observed to cause injuries [15,16], reproductive distur-

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bances [15,17], impaired immunity [18], accelerated wear of the movement apparatus [15,19], and chronic stress-like changes in the HPA axis [20–22]. Studies in healthy humans and patients have revealed behavioural and neuropsychological changes associated with regular physical exercise. Exercise training was shown to increase mood in normal subjects and to evoke anxiolytic and antidepressant effects in phobic and depressed patients [23–29]. Anxiety and major depressive disorders present an enormous burden for the inflicted patients and the society as a whole. Since regular voluntary exercise appears to strengthen endogenous stress coping mechanisms, it clearly should be of interest to study the neurobiological basis of these mechanisms as well as to investigate comprehensively stress-related behavioural changes. However, until now only a few studies have been conducted on rodents which came to different results. One study reported reduced exploratory behaviour of rats in an open field test after regular voluntary exercise [30], whereas another one discovered the opposite [31]. Given the scarce amount of information available on exercise-induced behavioural changes, we embarked on an extensive characterization of the behavioural alterations in mice after 4 weeks of voluntary exercise with a focus on changes in anxiety-related behaviour. First, a comprehensive ethological analysis was performed by use of the modified hole board. This test has been shown to reveal a wide range of behavioural patterns as well as the interaction of the different behavioural dimensions [32]. Thereafter, animals were examined in more selective tests for locomotion (including novelty seeking and exploration) and anxiety-related behaviour to: (i) assess context specificity of potential findings from the modified hole board tests and (ii) to investigate the behaviour under more challenging conditions as presented by unprotected heights in the elevated plus-maze and bright light in the dark–light box.

2. Materials and methods 2.1. Animals Studies were performed in naive, 12- to 14-week-old male C57BL/6N mice (n = 57; Charles River WIGA GmbH, Sulzfeld, Germany). They were singly housed in Macrolon® type III cages (43 cm × 24 cm × 20 cm, l × w × h) under standard conditions (temperature: 22 ± 1 ◦ C; humidity: 55 ± 5%; free access to water and food; 12-h light:12-h dark cycle, lights on at 06:00 a.m.). 2.2. Voluntary exercise paradigm After habituation to the housing conditions for 5 days, the experimental group was allowed free access to a running wheel (∅: 14 cm) in their home cages for a period of 4 weeks. Using an infrared video camera and a wheel-turning count-

ing system, it was observed that the mice were mainly using the running wheel during the dark phase of the diurnal cycle [13]. The mice ran approximately 4 km per night which is in agreement with other reports [33], although strain differences have been observed [34]. This running performance was reached within a few days after providing the wheels to the animals (data not shown). Importantly, wheel running is not regarded as a form of stereotypic behaviour [33] because it is not expressed at the cost of resting behaviour as is the case with other reported locomotor stereotypies [35,36]. The housing of the sedentary (i.e. control) animals remained unchanged. All animal experiments were approved by the Ethical Committee on Animal Care and Use of the government of Bavaria, Germany. 2.3. Modified hole board The modified hole board paradigm [32] combines the features of an open field and a hole board, originally designed by File and Wardill [37] to analyse exploratory behaviour of rodents [38,39]. The experimental set up consists of an opaque grey PVC (i.e. polyvinylchloride) box (150 cm × 50 cm × 50 cm, l × w × h, light intensity: 200–250 lx). This box is divided into a test compartment (100 cm × 50 cm × 50 cm, l × w × h) and a group compartment (50 cm ×50 cm ×50 cm) by a transparent PVC partition perforated with 120 holes (1 cm in diameter). The hole board (60 cm × 20 cm × 2 cm), which is made of the same material as the box, is positioned in the middle of the test compartment, thus, representing an unprotected area comparable with the Center of an open field. On the board, 23 holes (1.5 cm × 0.5 cm) are staggered in three lines which are covered by movable lids of the same material. The area surrounding the hole board is divided by white lines into 12 rectangles (8.3 cm × 15 cm). On the 3 days before the day of the experiment each animal received one piece of almond (about 0.05 g) for habituation as almonds were used as familiar food in the modified hole board test. The almonds were put in one corner of the protected area of the experimental box, i.e. outside the board. Each animal was tested on the modified hole board for 10 min. As in previous studies [32,40], several parameters for anxiety-related behaviour (e.g. latency until the first board entry, number of rearings on the board), locomotion (e.g. number of line crossings), exploration (e.g. number of rearings on the board), risk assessment (e.g. number of stretched attends), arousal and food intake inhibition were measured. For sake of clarity and conciseness, only results on those parameters will be presented which showed statistically significant differences between control and exercising mice. 2.4. Open field/object recognition The used open field consists of a round area (∅: 82 cm) evenly lit (250–300 lx). One familiar and one unfamiliar ob-

E. Binder et al. / Behavioural Brain Research 155 (2004) 197–206

ject were placed across from the starting point in the second half of the circle, i.e. in the inner area of the open field. The familiar object (a hexagonal cylinder made out of brass sized 2 cm × 1.5 cm) had been put in the home cage of the animal for 48 h and was removed 24 h before the test. The unfamiliar object consisted of a red rubber triangle of approximately the same size as the brass object. The behaviour of each individual animal was observed for 30 min. Several parameters for anxiety-related behaviour (e.g. frequency of entries into the centre, latency until the first entry into the centre, percentage of time spent in the centre), locomotion, exploration (e.g. latency until first exploration of a familiar or an unknown object, number of rearings), risk assessment, and arousal (e.g. percentage of time spent grooming) were measured. 2.5. Elevated plus-maze The used elevated plus-maze comprises two open arms (50 cm×10 cm, l × w) and two closed arms (50 cm×10 cm× 25 cm, l × w × h) extending from a common central platform (10 cm × 10 cm). The apparatus was positioned 70 cm from the floor. The central platform and the floor of the maze are made of grey PVC, while the side walls of the closed arms are made of clear Plexiglas. Closed and open arms were evenly lit (250–300 lx). Each individual animal was tested for 5 min starting from the platform facing the closed arm. The times spent in the closed arms, on the platform and in the open arms as well as the entries into these three compartments were measured.

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had been assigned to specific keys of the computer keyboard (Observer, Noldus, Wageningen, The Netherlands). Separate groups of mice were tested on the modified hole board (control: n = 11, exercise: n = 10), in the dark–light box (both groups, n = 10), and on the elevated plus-maze (both groups, n = 8). Animals to be used for the open field test, had been tested before in the light-dark box. 2.8. Statistical analysis Data are presented as mean values + standard error of the mean (S.E.M.). Data were analysed with Student’s t-test or by (one-way) analysis of variance (ANOVA; STATISTICA 6.0, StatSoft, Tulsa, USA) with exercise as between subject factor. Comparison of data derived from different time intervals in the open field test was performed with ANOVA with repeated measures with exercise as between subject factor and time as within subject factor. In case of a significant main factor, a Fisher LSD post-hoc test was performed. As level of significance, P < 0.05 was accepted. For all post-hoc tests, in appropriate cases, the level of significance was reduced according to the Bonferroni procedure to keep the probability of a type 1 error less than 5%.

3. Results 3.1. Modified hole board

The used dark–light box consists of two compartments connected by a tunnel (4 cm × 7 cm × 10 cm, l × w × h). The dark compartment (10–20 lx; 15 cm × 20 cm × 25 cm, l × w × h) was made of black opaque PVC whereas the light compartment (30 cm × 20 cm × 25 cm, l × w × h) was made of white opaque PVC. To increase the aversive character of the light compartment, it was brightly illuminated (680–700 lx). Each animal was observed individually for 5 min starting in the dark compartment facing the tunnel. The times spent in the dark compartment, in the tunnel and in the light compartment as well as the entries into these three compartments were measured.

Exercising mice showed a significantly higher latency until the first entry on the board than the control animals (P = 0.009, Student’s t-test, Fig. 1A). There were no differences between the two groups regarding the percentage of time spent on the board and the number of entries to the board. Furthermore, the exercising mice showed significantly more rearings on the board (P = 0.04, Student’s t-test, Fig. 1B), but rearings in the protected area of the hole board set-up (i.e. outside the board) did not differ between the experimental groups (data not shown). The exercising mice also showed more stretched attends (P = 0.01, Student’s t-test, Fig. 1C). The number of line crossings performed by the exercising animals was lower compared to the control group (P = 0.04, Student’s t-test, Fig. 1D). There were no differences in the distance travelled in the modified hole board between the groups of mice (data not shown).

2.7. Test procedure

3.2. Open field

All tests were performed and videotaped between 08:00 a.m. and 13:00 p.m. Mice in the open field experiment were tracked by an automated video tracking system (Ethovision, Noldus, Wageningen, The Netherlands). Object exploration was measured by visual analysis of the video tapes by a trained observer blind to the treatment. The other experiments were directly monitored by a trained observer using software in which behavioural parameters

During the first 10 min of the open field test, the control mice seemed to explore the centre more frequently than the exercising mice (ANOVA with repeated measures, interaction between exercise and time: F2,32 = 3.53, P = 0.041; Fig. 2A). Furthermore, the latency until the first entry into the centre (P = 0.02, Student’s t-test, Fig. 2B) as well as the latency until the first exploration of the familiar, (P < 0.05, Student’s t-test, Fig. 2C) and the unknown

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Fig. 1. Behaviour of exercising and control mice in the modified hole board test. Results are shown regarding the latency to enter the board (A), the number of rearings on the board (B), the number of stretched attends (C), and the number of line crossings (D). Animals were tested on the board for 10 min. For more information, see Section 2. ∗ P < 0.05, Student’s t-test.

object (P < 0.05, Student’s t-test, Fig. 2C), was significantly shorter in the control mice than in the exercising animals. Although no differences were found between the two groups of mice regarding the exploration of the centre of the open field over the complete 30-min test interval (control: 21.0 ± 2.2%, exercise: 22.2 ± 2.4%, P > 0.05, Student’s t-test), a time course analysis (by splitting the 30-min test time into three equal time intervals) of the time spent in the centre revealed marked time- and group-dependent differences in exploratory behaviour (Fig. 2D). Overall, the data showed a time-dependent increase in the percentage of time spent in the centre (ANOVA with repeated measures, effect of time: F2,32 = 3.88, P = 0.031). Although the repeated measures ANOVA did not reveal a significant effect of exercise, a significant interaction between exercise and time was found. Thus, exercising and control mice showed differences in exploration of the centre of the open

field over the three time intervals (ANOVA with repeated measures, interaction between exercise and time: F2,32 = 4.21, P = 0.024). Whereas control mice did not show any difference in the percentage of time spent in the centre between the first and the second time interval, the exercising animals spent significantly more time in the centre during the second interval than during the first one (P = 0.004; Fig. 2D). Both groups of mice displayed no differences in exploration of the centre between the second and the third time interval. Fig. 3 presents activity patterns of a representative control and exercising mouse indeed showing the increased latency of the exercising animal during the first 10 min to enter the centre of the open field. Finally, the control and exercising mice show differences in the time they spent on grooming during their presence in the open field (ANOVA with repeated measures, interaction between exercise and time: F2,32 = 3.570, P =

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Fig. 2. Behaviour of exercising and control mice during a 30-min open field test. In (A), (D), (E) and (F), the number of entries of the centre of the open field, the time spent in the centre, the time spent grooming, and the number of rearings are presented, respectively, split in 10-min time bins. (B) and (C) show results on the latency to enter the centre and the latency until the mice explored the familiar and unknown objects. Data in (A), (D), (E) and (F) were tested with ANOVA with repeated measures (for ANOVA results, see text) followed by Fisher LSD post hoc test (*, indicates significant difference; significance level corrected according to the Bonferroni procedure). Data in (B) and (C) were tested with Student’s t-test (∗ P < 0.05).

0.040; Fig. 2E). Post-hoc testing revealed the main difference (i.e. increased grooming in the exercising animals) to be attained during the last 10 min of the open field test (P = 0.005). The data presented show that, as compared to control mice, exercising mice show differences in their behaviour on

the modified hole board and in the open field which may involve changes in anxiety. Therefore, we tested the behaviour of control and exercising mice on the elevated plus-maze and in the dark–light box to determine particularly changes in anxiety-related behaviour after long-term voluntary exercise.

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3.4. Dark–light box

Fig. 3. Representative tracking pattern of a control and an exercising mouse in the 30-min open field. The testing time is split in threetime bins. Whereas the exercising mice rarely explored the centre of the arena during the first 10-min interval, they showed a similar activity pattern as the control mice during the second and third interval. For more information, see Section 2 and the text of the Section 3.

3.3. Elevated plus-maze Exercising mice spent significantly more time in the open arm (P = 0.02, Student’s t-test, Fig. 4A) and less time in the closed arm than the control animals (P = 0.009, Student’s t-test). Furthermore, exercising mice showed less entries into the closed arm than the control animals (P = 0.004, Student’s t-test), whereas there were no differences regarding the entries into the open arms (Fig. 4B). The exercising mice showed less rearings within the closed arms (data not shown). However, this difference was lost when the number of rearings was divided by the amount of time the animals were present in the closed arm (data not shown). There were no differences in the distance travelled in the elevated plus-maze between the groups of mice (data not shown).

Fig. 5A shows that exercising mice made significantly more entries into the tunnel (P = 0.035, Student’s t-test) as well as into the light compartment than the control animals (P = 0.025, Student’s t-test). Moreover, the latency until the first entry into the light compartment as well as the latency until the first exploration of the end of the light compartment were significantly lower in exercising mice than in control animals (data not shown). Furthermore, the exercising mice spent a significantly higher percentage of time in the light compartment (P = 0.002, Student’s t-test, Fig. 5B), this solely at the cost of the time spent in the dark compartment (P = 0.001, Student’s t-test).

4. Discussion The aim of the present study was to characterize the impact of voluntary exercise on the behaviour of mice regarding general aspects as well as with particular emphasis on anxiety-related behaviour. Therefore, mice which had free access to a running wheel for 28 days were investigated in several behavioural tests. The modified hole board test was used to gain a general overview of the behavioural characteristics of the animals since this test allows the assessment of a variety of behavioural parameters such as exploration, anxiety-related behaviour, risk assessment, locomotion, and food-intake inhibition [32]. In this test, the exercising mice displayed a longer latency to the first board entry and more stretched attends than the control animals. At a first glance, these results could be interpreted as suggesting that exercising mice show more anxiety-related behaviour than the controls. However, other behavioural paradigms assessed by the modified hole board test (e.g. the increase in the number Entries into arms

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Fig. 4. Behaviour of exercising and control mice in the elevated plus-maze. The animals were tested for 5 min. The time spent in the open and closed arms and on the platform is shown (A) as well as the number of entries in these areas of the maze (B). For more details, see Section 2. ∗ P < 0.05 (Student’s t-test) as compared to the respective parameter in the control group.

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Fig. 5. Behaviour of exercising and control mice in the dark–light box. The animals were tested for 5 min. The number of entries in the dark compartment, the tunnel and light compartment is shown (A) as well as the time spent in these compartments (B). For more details, see Section 2. ∗ P < 0.05 (Student’s t-test) as compared to the respective parameter in the control group.

of rearings on the board and the absence of differences in the percentage of time spent on the board and the number of entries to the board in the exercised mice) indicated that this was not necessarily the case. It was observed that there were no differences between the groups of mice regarding the number of entries and the time spent on the board. Moreover, exercising animals made more rearings on the board. These observations oppose the concept of an increased anxiety level in these mice. The activity pattern of the mice as expressed in the open field largely corresponds with the observations made in the modified hole board test. During the first 10 min, the exercising mice spent less time in the centre of the open field for which they, however, made up during the remaining time in the test. Therefore, there was no group difference when the complete test duration was considered. The effects of exercise on open field behaviour have also been investigated by other researchers. However, when comparing the results of other studies with ours, one needs, beside test-specific differences (e.g. exposure time, illumination, used species), to consider that often studies have been conducted on rats or mice which were forced to run, for instance in a treadmill. In such forced exercise models and in humans conducting high intensity exhaustive exercise training (e.g. marathon runners), injuries [15,16], reproductive disturbances [15,17], accelerated wear of the movement apparatus [15,19], and chronic stress-like changes in the hypothalamic–pituitary–adrenal axis [20–22,41,42] have been observed. These changes oppose the anti-stress effects seen after voluntary exercise on biological systems such as neurogenesis [9,11] the HPA axis [14], sleep [13], and behaviour (present study, see below). A rather early report [30] describes that rats show less exploratory activity in an open field test after voluntary usage of a running wheel than animals which had been regularly forced to exercise (running

or swimming). In contrast to these findings, another study [31] shows that rats display a lower activity after forced exercise than animals which were allowed to voluntarily exercise. However, it is important to note that in neither of the two studies the open field test was run for longer than five min. If one considers only our open field data collected during the first 10 min, then a parallel result can be seen with the study of Weber and Lee [30]. However, due to its short test period, the latter study failed to detect the compensation in exploratory behaviour seen in exercising animals in our study at later time points of the open field test. After the rather general assessment of differences in the mice’s behavioural state, we were prompted to have a closer look at the putative changes in anxiety-related behaviour. This because the modified hole board and open field test results had provided, on the one hand, data suggesting an increased anxiety level in exercising mice, whereas, on the other hand, other data (e.g. rearings on the board, time on the board) spoke against this. Therefore, behavioural tests were conducted for unconditioned anxiety, i.e. the elevated plus-maze and the dark–light box. The aversive component in these tests was substantially stronger than in the modified hole board and open field tests because of the unprotected heights of the open arms and the brightly illuminated space of the light compartment, respectively. The exercising animals, however, explored the open arms of the plus-maze and the light compartment of the dark–light box much longer and/or more often than the control animals. These results strongly indicate that long-term voluntary exercise results in an improved coping with aversive situations, thus, leading to a reduced anxiety level. The question remains how to interpret the observations in the modified hole board test, suggesting increased anxiety, in the face of the decreased anxiety seen in the elevated plus-maze and the dark–light box. It should be noted, however, that naive C57BL/6N mice

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are known to show a strong behavioural activation when exposed to a novel environment [32,43]. Thus, exercising C57BL/6N mice show an increased latency both before entering the centre of the open field and before entering the unprotected area of the board in combination with an increased exploration (i.e. rearings) in this area, most likely because of a decreased stress responsiveness at the behavioural level; a phenomenon which has also been interpreted as decreased impulsiveness [44]. Presently, the neurobiological mechanisms underlying the decreased anxiety and impulsiveness after long-term voluntary exercise are still unclear. Given the substantial impact of voluntary exercise on many metabolic and physiological processes, the direct and indirect involvement of a variety of neurotransmitter and hormonal systems, including serotonin, atrial natriuretic peptide (ANP) and GABA, may be expected. Given that the release of serotonin is strongly associated with motor activity [45–47] and this neurotransmitter is thought to play an important role in impulsiveness [48–50] and risk assessment [51–54], serotonin might be implicated in the behavioural changes seen after long-term voluntary exercise. Indeed, an increased synthesis [55] and secretion [46,56] of serotonin (due to a rise in the brain tryptophan concentration), and downregulation of 5-HT1A receptors as well as 5-HT1B autoreceptors [57–59] have been observed after regular physical exercise. Interestingly, Gross et al. [44] showed that mice genetically deficient for 5-HT1A receptors explored a new environment with a higher latency than control animals, an observation which was interpreted as reduced impulsiveness. This reduction was abolished after reinstatement of the receptor [44]. Thus, a decrease in 5-HT1A receptor function could play a role in the reduced impulsiveness seen in our exercising mice. However, more research is required on the changes in 5-HT1A receptor function after exercise as well as on the neuroanatomical specificity of these changes before final statements can be made. ANP is a polypeptide which could be pertinent for explaining the emotional changes in our exercising mice. ANP was discovered as a peptide expressed in the heart [60–63] but the peptide and its receptor are also expressed in the central nervous system [62,64–68]. Its synthesis in atrial cardiomyocytes has been found to be raised by enhanced cardiac activity, as during exercise [69–71]. Importantly, it has been recently discovered that ANP has anxiolytic properties in rats [72,73] as well as humans [74,75]. Thus, it is tempting to speculate that an elevated ANP production in exercising mice could be involved in the observed anxiolysis, but this needs to be verified. Since the GABA receptors have often been shown to be implicated in anxiolysis, an increased GABAergic tonus and/or GABA receptor function in anxiety-relevant brain regions (i.e. amygdala, hippocampus) would be expected in exercising subjects to explain their reduced level of anxiety. However, until now, increases in GABA function have only been observed in the caudal and posterior hypothalamus in

the context of cardiovascular regulation [76,77]. Whether exercise also increases GABAergic control in anxiety-relevant brain structures awaits further investigations. In conclusion, voluntary exercise has major beneficial implications for molecular, cellular and physiological functioning [12] which, as shown in the present study, also precipitate at the behavioural level. The behavioural profiling of our exercising mice shows that long-term voluntary exercise leads to decreases in impulsiveness and emotionality. Increased attention, reduced impulsiveness, reduced panic-like behaviour are regarded as adaptive behavioural responses leading to enhanced stress resistance; in animals, increasing the chances of survival and, in humans, increasing general well-being and stress coping. In view of the vast percentage of the population suffering from psychosomatic and psychiatric disorders, the elucidation of the neurobiological mechanisms underlying the beneficial effects of exercise on physiology and behaviour should be an important track of future research.

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