The present study evaluated the ability of forced or voluntary exercise to overcome the inhibitory effects of MPTP lesions on motor and affective behavior, as well as dopamine and serotonin content in male C57BL/6 mice. Because exercise has been shown to enhance motor function in patients with PD [13
] and also improves mood in non-PD patients [3
], the present study determined if exercise could manifest similar benefits in a mouse model of PD. For 6 wks after injection of MPTP or saline, mice were subject to vigorous exercise 5 d/wk, up to 1h/day on a treadmill or running wheel, or remained sedentary. MPTP did not reduce voluntary exercise, except during the first week after lesioning. Nonetheless, MPTP lesion impaired performance on the Rotarod. MPTP-induced deficits in motor behavior were paralleled by a reduction in striatal content of DA and 5HT, as reported previously [82
]. MPTP was anxiogenic, as measured by increased marble burying. This was accompanied by 5HT depletion in BLA. In contrast, exercise was anxiolytic, as reflected by open arm entries on the elevated plus maze. However, exercise had no effect on brain levels of DA or 5HT. As determined by sucrose preference or tail suspension, there was no effect of MPTP or exercise on depression. Although mice exercising on a running wheel ran further than those exposed to the treadmill, performance on the Rotarod improved only in mice subject to forced exercise. These results demonstrate anxiogenic effects of MPTP lesioning, and anxiolytic effects of exercise. Similarly, MPTP reduces and forced exercise selectively improves motor function. Thus, exercise has the potential to improve both affective and motor behavior in PD patients.
At face value, MPTP is an appropriate animal model for PD because MPTP lesions replicate the loss of dopamine neurons in SNpc that occurs spontaneously in PD [37
]. Indeed, MPTP lesions in monkeys and humans also demonstrate key symptoms of motor dysfunction seen in PD patients, including rigidity, bradykinesia, and resting tremor [27
]. Although MPTP faithfully and selectively lesions nigrostriatal DA neurons in mice and reduces striatal DA content, the resulting behavioral phenotype demonstrates relatively subtle motor deficits, many of which resolve rapidly [48
]. In the present study, there were no differences in running velocity or distance traveled during voluntary wheel-running exercise in control and MPTP-lesioned mice, except during the first week. Likewise, MPTP did not impair voluntary motor activity in the elevated plus maze, as measured by the number of closed arm entries.
Nonetheless, MPTP lesions do impair motor function. Previous studies have demonstrated lasting deficits in performance on the Rotarod [68
], as well as on tests of balance, gait [49
], and swimming [34
]. Different lesioning protocols with MPTP make it difficult to directly compare among published studies. In particular, studies of Rozas et al [68
], Haobam et al [34
], Liebetanz et al [49
], and Pothakos et al [63
] use repeated injection with MPTP at intervals of days or weeks to mimic chronic Parkinsonism. By contrast, acute MPTP lesions in the present study (4 injections of 20 mg/kg free base MPTP, each 2h apart) allow behavioral and neurochemical recovery over time [38
]. In this manner, our injection paradigm represents a dynamic model, but in the opposite direction from PD, where symptoms become progressively worse. Despite the potential for recovery during 44 days after MPTP, motor and neurochemical deficits persist in the present study, as demonstrated by reductions in both ORP and dopamine content in CPu and Acb.
Fewer studies have examined the effects of MPTP lesions on affective behavior. Previous studies have demonstrated increased immobility in the tail suspension test 30 days post-MPTP [55
], as well as mild cognitive impairment in the social transmission of food preference test [82
]. In a similar manner, anxiety is increased in parkin-deficient mice, a knockout model of early-onset familial PD [88
]. The present study expands upon these findings, revealing a selective increase in anxiety as demonstrated with marble burying, along with decreased levels of 5HT in BLA. 5HT in BLA is strongly implicated in fear and anxiety [10
]. The MPTP-induced reduction in 5HT content is consistent with this role. Importantly, anxiogenic responses in the marble burying test are not confounded by any MPTP-induced motor deficits, since anxious mice bury more marbles. However, 5HT content was selectively decreased by MPTP in BLA and CPu. By contrast, 5HT content in Acb of MPTP-lesioned mice was increased above that in saline-treated animals, as reported previously [39
Interestingly, MPTP failed to elicit similar anxiogenic responses in the elevated plus maze. In this regard, defensive burying represents an active coping strategy in response to a discrete threat. By contrast, anxious behavior in the elevated plus maze is expressed as a passive avoidance response to a potential threat. Different brain circuits may regulate responses to these two challenges [33
]. Our findings suggest that DA depletion in SNpc selectively increases anxiety in response to proximal discrete threat stimuli. As reflected in performance on the elevated plus maze, exploratory behavior involving reward circuits through the ventral striatum and lateral hypothalamus is unaffected by MPTP, at least when tested 44 days after MPTP. The absence of anhedonia in the sucrose preference test also supports this conclusion. Alternatively, it is possible that faster recovery of the ventral striatal circuits post-lesion accounts for the lack of anxiety symptoms in the elevated plus maze. In this regard, sucrose preference was tested weekly throughout the study, and we did not observe evidence of transient depressive symptoms that resolved before mice were tested at 44 days post-lesion.
Investigating potential mood effects of MPTP in mice has clinical relevance for understanding neurologic function in PD patients. Disturbance of mood is a common condition in PD, with an average prevalence of 25–40% in outpatient settings for major depressive disorders [46
], and up to 40% for anxiety disorders [85
]. During early stages of illness, when DA cell loss and/or dysfunction are insufficient to elicit prominent motor deficits, mood symptoms may be the first sign of brain degeneration [45
]. Later, with the initiation of DA replacement therapy, mood symptoms are seen in as many as 75% of patients with motor fluctuations. In particular, depression or anxiety is reported in the ‘off’ period (when there is an absence of the normal motor response to antiparkinsonian medications) and a neutral or elevated mood in the ‘on’ state (the subjective sense that they are having a motor response to their medication). In addition, depressive and anxiety disorders occur frequently independent of motor fluctuations.
Since PD is a movement disorder, it stands to reason that exercise training improves motor function in PD patients and MPTP-lesioned mice [25
]. Clinical trials and epidemiologic studies have supported the potential benefits of exercise in PD patients [12
]. Epidemiologic studies suggest a role of exercise in lowering the risk for PD, while clinical trials in patients with PD have shown functional benefits, including the unified PD rating scale, functional limitations (walking, stair climbing), gait training, daily activities, and breathing [13
]. Using MPTP lesions in mice, previous studies have shown improvement in gait and balance, with exercise training [63
]. However, in the present study, we were surprised to find that ORP improved only with treadmill exercise training, and not after voluntary wheel-running. This finding was particularly unexpected since both MPTP-lesioned mice and saline controls ran further and faster during voluntary exercise on a running wheel than when subject to forced treadmill exercise. Vigorous running wheel activity after MPTP lesion is consistent with the large number of closed arm entries and lack of sucrose anhedonia in these mice, and suggests that exploratory drive and basic locomotor activity is not compromised by MPTP. However, performance on the Rotarod reflects more than exercise capacity, and may have a learning component. In fact, the Rotarod ressembles a round treadmill. To avoid falling, mice must maintain a constant and consistent pace, similar to forced exercise. Ultimately, tests of exercise on motor function must take into account the specific exercise training regimen.
The present study supports and extends previous studies suggesting the potential for exercise to improve affective behavior in animal models of PD. In particular, our study is the first to compare forced vs voluntary exercise after MPTP lesion. It is interesting to note that voluntary wheel-running reduced anxiety, as measured by an increase in percent of open arm entries in the elevated plus maze. Treadmill exercise was without effect. Circulating levels of stress-induced corticosterone were elevated in mice subject to treadmill exercise, compared with those exposed to voluntary wheel-running. Thus, the stress of forced exercise may diminish the potential benefits of exercise on affective behavior.
The potential benefits of exercise on mental health are widely accepted, even in the popular press. A variety of studies have reported beneficial effects of exercise on depression and anxiety in humans, including improvements in mood disturbances associated with chronic disease [3
]. Furthermore, many studies show that the effects of exercise on mood may also be sustained. Considering the number of studies investigating exercise effects on anxiety and depression in humans, it is remarkable that we know so little about the underlying mechanisms. It has been hypothesized that the beneficial effects of exercise may reflect increased levels of brain-derived neurotrophic factor in the hippocampus [6
] or improvement in circadian rhythmicity [73
]. It seems likely that exercise also has a variety of effects on neurotransmitters and neurotrophic factors, including glutamate [81
], 5-HT, and DA. An earlier study in aged mice subject to MPTP lesion found an increase in striatal DA with exercise compared to sedentary lesioned animals [78
]. However, exercise was initiated 12h post-MPTP, thereby including both neuroprotective and neurorehabilitative effects. In the present study, exercise beginning 5 days after MPTP had minimal impact on DA or 5HT content in brain regions important for mood or motor function. Similar results have been reported recently with forced exercise after MPTP in mice [2
], and with treadmill exercise in humans [83
]. Nonetheless, at some level exercise improves mood and motor behavior through dynamics of neurotransmitter content and/or release.