The change in body weight after surgery is shown in Figure 1. ANOVA for repeated measures (including control, SO, SOTE, CCI, and CCITE groups) in Figure 1A and ANOVA for repeated measures (including control, SO, SOSE, CCI, and CCISE groups) in Figure 1B showed significant main effects for groups (F 4,45 = 22.13, P < 0.0001; F 4,45 = 20.34, P < 0.0001), time (F 9405 = 10.31, P < 0.0001; F 9405 = 8.77, P < 0.0001), and significant interaction (F 36,405 = 5.89, P < 0.0001; F 36,405 = 4.72, P < 0.0001), respectively. Post hoc comparisons demonstrated significant differences between control (or SO) and SOTE (or CCI, CCITE) groups (P < 0.05, Tukey–Kramer, Fig. 1A) and between control (or SO) and SOSE (or CCI, CCISE) groups (P < 0.05, Tukey–Kramer, Fig. 1B), respectively. There was no significant difference between the SO and control groups (Fig. 1A and 1B). The CCISE (Fig. 1B) and CCITE (Fig. 1A) groups had no significant change in body weight when compared with the CCI group, respectively. Furthermore, the animals' grooming, sleep–wake cycles, and social interaction in the cage were not obviously affected (data not shown).
Thermal and Mechanical Sensitivity
ANOVA of repeated measures (including control, SO, SOTE, CCI, and CCITE groups) in Figure 2A and ANOVA of repeated measures (including control, SO, SOSE, CCI, and CCISE groups) in Figure 3A for thermal withdrawal latency demonstrated a significant main effect for groups (F 4,45 = 58.88, P < 0.0001; F 4,45 = 47.39, P < 0.0001), time (F 9405 = 41.58, P < 0.0001; F 9405 = 39.46, P < 0.0001), and significant interaction (F 36,405 = 8.74, P < 0.0001; F 36,405 = 9.56, P < 0.0001), respectively. Post hoc comparisons showed no significant differences among control, SO, and SOSE (or SOTE) for thermal withdrawal latency (P > 0.05, Tukey–Kramer).
In SO and CCI (or CCITE) groups (Fig. 2A), the significant difference in thermal withdrawal latency was maintained from day 3 to day 21 (P < 0.05, Tukey–Kramer). Post hoc comparisons demonstrated significant differences between CCITE and CCI groups from day 3 to day 21 (P < 0.05, Tukey–Kramer). In the CCI group in comparison with SO group (Fig. 3A), the significant difference in thermal withdrawal latency was maintained from day 3 to day 28 (P < 0.05, Tukey–Kramer), while in the CCISE group only from day 7 to day 14. Post hoc comparisons demonstrated significant differences between CCISE and CCI groups from day 3 to day 21 (P < 0.05, Tukey–Kramer).
ANOVA of repeated measures (including control, SO, SOTE, CCI, and CCITE groups) in Figure 2B and ANOVA of repeated measures (including control, SO, SOSE, CCI, and CCISE groups) in Figure 3B for mechanical withdrawal threshold showed a significant main effect for groups (F 4,45 = 66.91, P < 0.0001; F 4,45 = 81.28, P < 0.0001), time (F 9405 = 51.76, P < 0.0001; F 9405 = 67.82, P < 0.0001), and significant interaction (F 36,405 = 9.45, P < 0.0001; F 36,405 = 8.41, P < 0.0001), respectively. Post hoc comparisons demonstrated no significant differences among control, SO, and SOSE (or SOTE) for mechanical withdrawal threshold (P > 0.05, Tukey–Kramer).
Post hoc comparisons showed significant differences in mechanical withdrawal threshold between CCI and SO groups from day 1 to day 35, between CCITE and SO groups from day 1 to day 28, and between CCITE and CCI groups from day 21 to day 35 (Fig. 2B; P < 0.05, Tukey–Kramer). In SO and CCI (or CCISE) groups (Fig. 3B), the significant difference in mechanical withdrawal threshold was maintained from day 1 to day 35 (P < 0.05, Tukey–Kramer). Furthermore, the CCI group was again significantly different from the CCISE group from day 14 to day 21, showing an increase in mechanical withdrawal threshold (P < 0.05, Tukey–Kramer).
Furthermore, on day 21 after CCI, data showed a significant increase in thermal withdrawal latency and mechanical withdrawal threshold in the CCITE or CCISE group when compared with the CCI group. Therefore, we selected day 21 after CCI to evaluate TNF-α, IL-1β, and Hsp72 expression in sciatic nerve. There was no significant difference in thermal withdrawal latency and mechanical withdrawal threshold between CCITE (or CCISE) and CCI groups after exercise training for 39 days (day 39 after CCI).
Figure 4A and 4B depict the levels of TNF-α and IL-1β in sciatic nerve of CCI, CCISE, and CCITE rats after exercise training for 21 days (day 21 after CCI). The expression of TNF-α was decreased in the CCITE group (59.4 ± 3.2 pg/mg protein, P < 0.05) or CCISE (60.9 ± 2.9 pg/mg protein, P < 0.05) group in comparison with the CCI group (75.3 ± 2.1 pg/mg protein) on day 21 after CCI (Fig. 4A). The expression of IL-1β was decreased in the CCITE group (69.3 ± 8.8 pg/mg protein, P < 0.01) and CCISE group (92.3 ± 24.0 pg/mg protein, P < 0.05), in comparison with the CCI group (162.9 ± 19.8 pg/mg protein) on day 21 after CCI, as shown in Figure 4B.
Figure 5 demonstrated the expression of Hsp72 in sciatic nerve after CCI in 3 different groups. The Hsp72 level in sciatic nerve was significantly increased 3.2-fold in the CCITE group (P < 0.01) and 2.1-fold in the CCISE group (P < 0.05) on day 21 after CCI when compared with the CCI group (Fig. 5).
The main finding of this study is that swimming and treadmill exercises appeared to retard peripheral neuropathic pain after CCI of the sciatic nerve in rats. After CCI, rats with swimming or treadmill training had decreased TNF-α and IL-1β expression and increased levels of HSP 72 in sciatic nerve when compared with rats after CCI without exercise training.
Effects of Exercise on Thermal and Mechanical Sensitivity
Physical exercise is often recommended to patients who have chronic pain. For instance, it has been shown that treadmill and swimming exercises were found to ameliorate spinal cord injury–induced allodynia and restore normal sensation after spinal cord contusion in rats.23 Bement and Sluka's study demonstrated that low-intensity exercise reversed mechanical hyperalgesia in a chronic muscle pain rat model through the activation of opioid receptors.24 This study also indicated that the CCI rats in the swimming or treadmill exercise group had attenuated thermal hyperalgesia and mechanical allodynia on day 21 after CCI when compared with the CCI rats not in exercise groups (Figs. 2 and 3). Our results are in agreement with those of previous studies, which reported that swimming exercise attenuated behavioral hypersensitivity in formalin- and nerve injury–induced animal models of persistent pain.1
The more recently published information suggests that intense exercise may exacerbate hyperalgesia25,26; for example, the spread of hyperalgesia was enhanced via fatigue by having mice spontaneously run in a running wheel for 2 hours, but not hyperalgesia at the site of insult.26 It has been shown that moderate-intensity exercise training cannot treat but can significantly decrease deep and cutaneous tissue mechanical hypersensitivity induced via acidic saline injection.27 In our study, the degree of reduction in decreased thermal withdrawal latencies (maximal 30%) and mechanical von Frey thresholds (<50%) by exercise was quite small and demonstrates the relevance of the findings in relation to neuropathic pain that is still present. Our study is consistent with the findings of animal studies, in that exercise does not completely reverse the painful condition.27
In this study, SOSE, SOTE, CCI, CCISE, or CCITE rats showed decreases in body weight in comparison with control or SO rats. We speculate that rats suffered from stress (e.g., CCI) and exercise, and therefore a similar trend of decreasing body weight in the SOSE, SOTE, CCI, CCISE, or CCITE rats was found. Weight reduction is one of the health benefits of regular exercise that should be emphasized and reinforced by every mental health professional to their patients.28
Our previous study demonstrated that progressive exercise training for at least 3 weeks induces Hsp72 overexpression in many vital organs and attenuates overproduction of tissue cytokines, including TNF-α, and arterial hypotension during heatstroke.29 This present study also showed that 3 weeks (21 days) of exercise training decreases heat hyperalgesia and mechanical allodynia in rats after CCI (Figs. 2 and 3). Therefore, we tested sciatic nerve Hsp72, IL-1beta, and TNF-α expression on day 21 after CCI.
Exercise Prevents the Increase in Sciatic TNF-α and IL-1β
The appearance of cytokines in plasma or in the tissues (e.g., nerves, muscles, bones) has been reported in work-related musculoskeletal disorders in humans30 and in animal models.31 However, the relationships among cytokines, pain, and exercises have not been analyzed. Our results showed that treadmill or swimming exercise training attenuated TNF-α and IL-1β expression on day 21 after CCI. This evidence may provide a reasonable explanation for why exercise training can partly alleviate neuropathic pain after CCI in rats.
Evidence has been presented that neuropathic pain consequent to peripheral nerve injury is associated with local inflammation and overexpression of inflammatory cytokines.32 It is well known that CCI induces axons to become hypersensitive and enhances retrograde transmission to cell bodies in the dorsal root ganglia and spinal cord with subsequent release of some mediators. These mediators were able to activate the microglia cells via specific receptors and induce phosphorylation of p38 mitogen-activated protein kinase in spinal cord, where they may alter gene expression of the neurons.33 – 35 Moreover, the hyperactive microglia result in the release of bioactive substances, including cytokines, prostaglandin E2, and excitatory amino acids (such as glutamate and aspartate) that alter the responses of dorsal horn cells and maintain neuropathic pain states.8,36
Effects of Exercise on Hsp72
Voluntary exercise for 7 days up-regulates the small heat shock protein Hsp27 in the hippocampus,37 and forced long-term exercise in mice has been reported to increase heat-shock protein/cognate 70 (Hsp/C 70).29 Our previous study demonstrated that a 3-week, but not a 1- or 2-week, exercise training regimen conferred significant protection against hyperthermia, decreased cardiac output, arterial hypotension, and increased serum or tissue levels of TNF-α, and improved survival during heatstroke.29 A 3-week exercise training treatment is able to maintain a high level of HSP72 in several vital organs for only 3 to 4 days,29 and a >3-fold overexpression of Hsp72 in the nucleus tractus solitarii may play an important role in protecting against hemodynamic dysfunction during heatstroke onset.18 In this study, we noted that swimming or treadmill exercise retarded mechanical allodynia and heat hyperaglesia and significantly increased Hsp72 expression on day 21 after CCI. Therefore, HSP72 expression in the sciatic nerve after 3 weeks of progressive exercise may play an important role throughout the sciatic nerve injury period.
In agreement with our results, exercise has been shown to be beneficial for an anti-inflammatory effect and neuropathic pain resolution.1,7 Several studies demonstrated that exercise-induced modulation of heat shock factor-1 (HSP's transcription factor) aggregation subsequently affects expression of Hsp72 in multiple organs or neurons of rats.18,29,37,38 In addition, the effect of treatment with BRX-220 (coinducer of HSPs) on the expression of Hsp70 leads to either slowly developing analgesic actions or enhancement of recovery processes in rats after L5 spinal nerve ligation.39 Moreover, it has been proven that the increase in the expression of HSPs can decrease the production of proinflammatory cytokines.40 Our results revealed that swimming or treadmill exercise training significantly promoted Hsp72 expression and ameliorated the CCI-induced expression of proinflammatory cytokines (TNF-α and IL-1β) in rat sciatic nerve. Furthermore, we also demonstrated that swimming or treadmill exercise training decreases CCI-induced neuropathic pain.
Although we did not provide direct evidence of the mechanism of Hsp72 that attenuated proinflammatory cytokine expression in this study, accumulated evidence shows that HSPs can decrease the production of the proinflammatory cytokines.40 However, we did note that the observations in this study on thermal hyperalgesia, mechanical allodynia, TNF-α, IL-1β, and Hsp72 are, at present, merely coincident.
Our study demonstrated that swimming and treadmill exercises increase HSP 72 expression in sciatic nerve of CCI rats and ameliorate thermal hyperalgesia, mechanical allodynia, and the expressions of TNF-α and IL-1β in sciatic nerve.
Name: Yu-Wen Chen, PhD.
Contribution: This author helped design the study, conduct the study, and analyze the data.
Attestation: Yu-Wen Chen has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Yung-Tsung Li, MS.
Contribution: This author helped conduct the study and analyze the data.
Attestation: Yung-Tsung Li has seen the study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Yu Chung Chen, MS.
Contribution: This author helped design the study, conduct the study, and analyze the data.
Attestation: Yu Chung Chen has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Zong-Ying Li, BS.
Contribution: This author helped conduct the study and analyze the data.
Attestation: Zong-Ying Li has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Ching-Hsia Hung, PhD.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Ching-Hsia Hung has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
This manuscript was handled by: Tony L. Yaksh, PhD.
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© 2012 International Anesthesia Research Society
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