A new aspect of chronic pain as a lifestyle-related disease

Abstract

Physical exercise has been established as a low-cost, safe, and effective way to manage chronic intractable pain. We investigated the underlying mechanisms of exercise-induced hypoalgesia (EIH) using a mouse model of neuropathic pain (NPP). Epigenetic changes in activated microglia and maintained GABA synthesis in the spinal dorsal horn may contribute to EIH. Voluntary exercise (VE), a strong reward for animals, also induced EIH, which may be due in part to the activation of dopamine (DA) neurons in the ventral tegmental area (VTA). VE increases the expression of pCREB in dopaminergic neurons in the VTA, which would enhance dopamine production, and thereby contributes to the activation of the mesolimbic reward system in NPP model mice. We demonstrated that neurons in the laterodorsal tegmental and pedunculopontine tegmental nuclei, a major input source of rewarding stimuli to the VTA, were activated by exercise. Chronic pain is at least partly attributed to sedentary and inactive lifestyle as indicated by the Fear-avoidance model. Therefore, chronic pain could be recognized as a lifestyle-related disease. Physical activity/inactivity may be determined by genetic/epigenetic and neural factors encoded in our brain. The hypothalamus and reward system is closely related in the axis of food intake, energy metabolism and physical activity. Understanding the interactions between the mesolimbic DA system and the hypothalamus that sense and regulate energy balance is thus of significant importance. For example, proopiomelanocortin neurons and melanocortin 4 receptors may play a role in connecting these two systems. Therefore, in a certain sense, chronic pain and obesity may share common behavioral and neural pathology, i.e. physical inactivity, as a result of inactivation of the mesolimbic DA system. Exercise and increasing physical activity in daily life may be important in treating and preventing chronic pain, a life-style related disease.

Keywords: CBP, chronic low back pain; Chronic pain; DA, dopamine; Dopamine; Exercise-induced hypoalgesia; FM, fibromyalgia; GABA, gamma-aminobutyric acid; HDAC, histone deacetylase; LDT, laterodorsal tegmental nucleus; LH, lateral hypothalamus; LHb, lateral habenula; Laterodorsal tegmental nucleus; NAc, nucleus accumbens; NPP, neuropathic pain; PPTg, pedunculopontine tegmental nucleus; PSL, partial sciatic nerve ligation; Physical activity/inactivity; RMTg, rostromedial tegmental nucleus; TH, tyrosine hydroxylase; TMD, temporomandibular disorder; VTA, ventral tegmental area; VWR, voluntary wheel running; Ventral tegmental area; delta FosB, delta FBJ murine osteosarcoma viral; mPFC, medial prefrontal cortex; pCREB, phosphorylated cyclic AMP response element-binding protein.

Graphical abstract

Fig. 1

Protocols for voluntary wheel running (VWR). (A) The mice were divided into six groups: 1) Naive-Sedentary mice, 2) Naive-Runner mice, 3) Sham-Sedentary mice, 4) Sham-Runner mice, 4) PSL-Sedentary mice and 5) PSL-Runner mice. (B) Naive-, Sham, and PSL-Runner mice were allowed to run freely on the running wheel, (C) while Sedentary mice were reared in the cages with the locked running wheel. At 15 days after the surgeries, mice were transcardially perfused with 4% paraformaldehyde in 0.1 M PBS, and the brain was removed. Adapted with permission from Kami et al. (2016c).

Fig. 2

Changes of daily running distances in Naive, Sham and PSL-Runner mice throughout the experimental period. Naive-Runner (n = 5), Sham-Runner (n = 5) and PSL-Runner (n = 5) mice were placed in individual cages equipped with the low-profile wireless mouse running wheel, and daily running distances (m/day) were recorded throughout the experimental period. The distance traveled on the running wheel was monitored using a magnetic reed switch attached to a computerized exercise-monitoring system (SOF-860 wheel manager software, MED associates, Inc). Although in PSL-Runner mice, PSL surgery dramatically reduced the running distance at 1 day post-surgery, these levels gradually returned to nearly pre-surgical level at 15 days after PSL surgery. Adapted with permission from Kami et al. (2016c).

Fig. 3

Changes of pain behaviors in mice and relationships between pain behavior thresholds and total running distances in PSL-Runner mice. (A) von Frey test and (B) plantar test were performed in VE (Voluntary exercise for 14 days)-PSL-Runner (closed circles with solid lines, n = 6), FE (Forced exercise for 14 days)-PSL-Runner (closed circles with broken lines, n = 6), VE-PSL-Sedentary (closed triangles with solid lines, n = 6) and FE-PSL-Sedentary (closed triangles with broken lines, n = 6) mice. Mechanical withdrawal thresholds and thermal withdrawal latencies were significantly higher in PSL-Runner mice compared to PSL-Sedentary mice. VE-PSL-Runner vs VE-PSL-Sedentary = †p < 0.05, ††p < 0.01, †††p < 0.001; FE-PSL-Runner vs FE-PSL-Sedentary = ^*p < 0.05, ^**p < 0.01, ^***p < 0.001. Quantitative data are presented as the mean ± standard error of the mean (SEM). The significance of differences between groups was determined by Student’s t-test. Differences were considered significant at p < 0.05. A significant positive correlation was observed between total running distances during 15 days after PSL surgery and (C) the thresholds of von Frey (R = 0.933, p < 0.001, n = 9) or (D) the latencies of plantar tests (R = 0.818, p < 0.05, n = 8) in PSL-Runner mice. Adapted with permission from Kami et al. (2016c).

Fig. 4

Changes of TH immunoreactivities in the latVTA by PSL with or without VWR. Brain sections (−2.92 and −3.08 mm from the bregma) in (A) Naïve-Runner (B) PSL-Sedentary and (C) PSL-Runner mice were immunostained with TH antibody. The right and left sides of the pictures indicate the ipsilateral and contralateral sides of PSL surgery, respectively. fr: fasciculus retroflexus, Bars = 300 μm. As shown in (A), a square of 200 μm x 200 μm in size was placed on the lateral region of VTA (latVTA) on microscopic images, and the immunofluorescence intensity of TH within it was quantified. (D) A bar chart showing intensities of TH-immunoreactivity in the ipsilateral and contralateral sides of latVTA in Naive and PSL mice. The intensities of TH-immunoreactivity were significantly increased by VWR (#p < 0.01 vs Naive-Sedentary, n = 5; $p < 0.01 vs PSL-Sedentary, n = 5), while the intensities of TH-immunoreactivity were significantly weaker in the contralateral side of the latVTA in PSL-Sedentary mice than those of the other groups (^*p < 0.01, n = 5). Quantitative data are presented as the mean ± standard error of the mean (SEM). The significance of differences among groups was determined by a one-way ANOVA and Tukey-Kramer post hoc test. (E) Mouse brain atlas showing area in the latVTA in which the immunofluorescence intensity of TH has been analyzed (red square).

Fig. 5

Changes of pCREB⁺/TH⁺ neurons in the lVTA by VWR. Double immunostainings with pCREB and TH antibodies were performed on the brain sections containing latVTA in each group. Photomicrographs shows localization of pCREB + cells (A, C, E) and pCREB + /TH + /DAPI + cells (B, D, F) in the contralateral side in the latVTA of Naive-Sedentary (A, B), PSL-Sedentary (C, D) and PSL-Runner mice (E, F). VWR resulted in up-regulation of pCREB in TH⁺ neurons. Arrows and arrowheads indicate TH-/pCREB + and TH + /pCREB + cells, respectively. ml: medial lemniscus. Bars = 50 μm. (G) A bar chart showing the numbers of pCREB⁺/TH⁺ cells in the latVTA of each group. The numbers of pCREB⁺/TH⁺ cells were significantly increased by VWR (# p < 0.01 vs Naive-Sedentary, n = 5; ∫∫ p < 0.01 vs the contralateral side of Sham-Sedentary, n = 5; ∫ p < 0.05 vs the ipsilateral side of Sham-Sedentary, n = 5; † p < 0.01 vs PSL-Sedentary, n = 5), while the numbers of pCREB⁺/TH⁺ cells in the contralateral side in PSL-Sedentary was significantly decreased compared with those of Naive- and Sham-Sedentary (^*p < 0.05). Quantitative data are presented as the mean ± standard error of the mean (SEM). The significance of differences among groups was determined by a one-way ANOVA and Tukey-Kramer post hoc test. A significant positive correlation was observed between the number of pCREB⁺/TH⁺ neurons in the contralateral side and (H) the thresholds of von Frey (R = 0.885, p < 0.05, n = 5) or (I) the thermal withdrawal latencies of plantar tests (R = 0.932, p < 0.05, n = 5) in PSL-Runner mice.

Fig. 6

Simplified schematic drawing of the major neural circuit connections involved in EIH. The network displays the complex interplay in regulating cellular activity within the reward system and several nuclei projecting to the VTA. GABA: gamma-aminobutyric acid, LDT: laterodorsal tegmental nucleus, LHb: lateral habenula, mPFC: medial prefrontal cortex, NAc: nucleus accumbens, PPTg: pedunculopontine tegmental nucleus, RMTg: rostromedial tegmental nucleus, VTA: ventral tegmental area.

Fig. 7

Voluntary exercise increases ΔFosB/FosB expression in the LDT/PPTg neurons. Double immunostaining with nNOS and ΔFosB/FosB antibodies was performed on the brain stem sections containing LDT/PPTg. The locations of the LDT and PPTg containing nNOS + (cholinergic) neurons in the midbrain are indicated as squares in (A). Aq: aqueduct. (B) Mouse brain atlas showing areas of LDT/PPTg (blue squares). No ΔFosB/FosB is expressed in LDT (C, D, E)/PPTg (F, G, H) neurons in Sham-Sedentary mice, while numerous ΔFosB/FosB positive nuclei were observed in both nNOS + (arrowheads) and nNOS- neurons in the LDT (I, J, K)/PPTg (L, M, N) in Sham-Runner mice. Bars = 50 μm.

Fig. 8

Schematic representation of the interaction between hypothalamic POMC neurons and the reward system. Although this schematic representation highlights the importance of POMC neurons located in the ARC and projecting to MC4R containing second-order neurons in the paraventricular nucleus and lateral hypothalamic area to control appetite and energy metabolism, they also project to MC4R containing neurons in the reward system, such as VTA and NAc, to positively control physical activity. Thus, hypothalamus and reward system control energy metabolism in a concerted manner. Synthesis of MC4R is under the control of a transcription factor NHLH2, a key molecule that control physical activity and fitness. ARC: arcuate nucleus of the hypothalamus, cAMP: cyclic adenosine monophosphate, LR: leptin receptor; MC4R: melanocortin 4 receptor; POMC: proopiomelanocortin protein; α-MSH: α-melanocyte stimulating hormone; Modified from Fig. 2 of da Silva et al. (2013).