Role of brainstem serotonin in analgesia produced by low-intensity exercise on neuropathic pain after sciatic nerve injury in mice

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

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Activation of mesolimbic dopamine system underlies exercise-induced hypoalgesia.

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Interaction between mesolimbic system and hypothalamus determines physical activity.

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Changing the lifestyle inactive to active may attenuate and prevent chronic pain.

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.

Deep Brain Stimulation Improves the Symptoms and Sensory Signs of Persistent Central Neuropathic Pain from Spinal Cord Injury: A Case Report

Central neuropathic pain (CNP) is a significant problem after spinal cord injury (SCI). Pharmacological and non-pharmacological approaches may reduce the severity, but relief is rarely substantial. While deep brain stimulation (DBS) has been used to treat various chronic pain types, the technique has rarely been used to attenuate CNP after SCI. Here we present the case of a 54-year-old female with incomplete paraplegia who had severe CNP in the lower limbs and buttock areas since her injury 30 years prior. She was treated with bilateral DBS of the midbrain periaqueductal gray (PAG). The effects of this stimulation on CNP characteristics, severity and pain-related sensory function were evaluated using the International SCI Pain Basic Data Set (ISCIPBDS), Neuropathic Pain Symptom Inventory (NPSI), Multidimensional Pain Inventory and Quantitative Sensory Testing before and periodically after initiation of DBS. After starting DBS treatment, weekly CNP severity ratings rapidly decreased from severe to minimal, paralleled by a substantial reduction in size of the painful area, reduced pain impact and reversal of pain-related neurological abnormalities, i.e., dynamic-mechanical and cold allodynia. She discontinued pain medication on study week 24. The improvement has been consistent. The present study expands on previous findings by providing in-depth assessments of symptoms and signs associated with CNP. The results of this study suggest that activation of endogenous pain inhibitory systems linked to the PAG can eliminate CNP in some people with SCI. More research is needed to better-select appropriate candidates for this type of therapy. We discuss the implications of these findings for understanding the brainstem's control of chronic pain and for future progress in using analgesic DBS in the central gray.

Prior voluntary wheel running attenuates neuropathic pain

Exercise is known to exert a systemic anti-inflammatory influence, but whether its effects are sufficient to protect against subsequent neuropathic pain is underinvestigated. We report that 6 weeks of voluntary wheel running terminating before chronic constriction injury (CCI) prevented the full development of allodynia for the ∼3-month duration of the injury. Neuroimmune signaling was assessed at 3 and 14 days after CCI. Prior exercise normalized ipsilateral dorsal spinal cord expression of neuroexcitatory interleukin (IL)-1β production and the attendant glutamate transporter GLT-1 decrease, as well as expression of the disinhibitory P2X4R-BDNF axis. The expression of the macrophage marker Iba1 and the chemokine CCL2 (MCP-1), and a neuronal injury marker (activating transcription factor 3), was attenuated by prior running in the ipsilateral lumbar dorsal root ganglia. Prior exercise suppressed macrophage infiltration and/or injury site proliferation, given decreased presence of macrophage markers Iba1, iNOS (M1), and Arg-1 (M2; expression was time dependent). Chronic constriction injury-driven increases in serum proinflammatory chemokines were suppressed by prior running, whereas IL-10 was increased. Peripheral blood mononuclear cells were also stimulated with lipopolysaccharide ex vivo, wherein CCI-induced increases in IL-1β, nitrite, and IL-10 were suppressed by prior exercise. Last, unrestricted voluntary wheel running, beginning either the day of, or 2 weeks after, CCI, progressively reversed neuropathic pain. This study is the first to investigate the behavioral and neuroimmune consequences of regular exercise terminating before nerve injury. This study suggests that chronic pain should be considered a component of "the diseasome of physical inactivity," and that an active lifestyle may prevent neuropathic pain.

Role of the Lateral Habenula in Pain-Associated Depression

Patients with chronic pain have significantly higher incidences of depression and anxiety than the average person. However, the mechanism underlying this link has not been elucidated in terms of how chronic pain causes significant mood changes and further develops into severe anxiety or depression. The serotonergic system in the raphe nuclei is an important component in both pain processing and the pathogenesis of depression. Since the lateral habenular nucleus (LHb) controls the raphe nuclei, it may participate in the regulation of pain-associated depression. Thus, the aim of the current study was to investigate the role of the LHb in this pathophysiological process. We used chronic constriction injury (CCI) of the sciatic nerve in rats as a model for neuropathic pain and assessed the changes potentially related to the mood disorders. The forced swim test (FST) and sucrose preference test (SPT) were performed to determine the behavioral changes 28 days after pain surgery. Expression of β calmodulin-dependent protein kinase type II (βCaMKII) in the LHb, cytochrome-c oxidase (COX) activity in the LHb and dorsal raphe nucleus (DRN) and serotonin (5-HT) levels in the DRN were measured. We found an increasing in LHb activity and βCaMKII expression, and a decrease in neuronal activity in the DRN and 5-hydroxyindoleacetic acid (5-HIAA)/5-HT ratios in the CCI rats. These effects were accompanied by the depression-like behaviors. Lesions in the LHb improved the pain threshold and depression-like behavior in the rats. These results suggest that the LHb may play a role in pain-associated depression by affecting the activity of 5-HT neurons in the DRN. Furthermore, we showed that increases in the LHb-DRN pathway activity were a common neurobiological mechanisms for pain and depression, which may explain the coexistence of pain and depression.

Nonpharmacological Interventions in Targeting Pain-Related Brain Plasticity

Chronic pain is a highly prevalent and debilitating condition that is frequently associated with multiple comorbid psychiatric conditions and functional, biochemical, and anatomical alterations in various brain centers. Due to its widespread and diverse manifestations, chronic pain is often resistant to classical pharmacological treatment paradigms, prompting the search for alternative treatment approaches that are safe and efficacious. The current review will focus on the following themes: attentional and cognitive interventions, the role of global environmental factors, and the effects of exercise and physical rehabilitation in both chronic pain patients and preclinical pain models. The manuscript will discuss not only the analgesic efficacy of these therapies, but also their ability to reverse pain-related brain neuroplasticity. Finally, we will discuss the potential mechanisms of action for each of the interventions.

Involvement of mesolimbic dopaminergic network in neuropathic pain relief by treadmill exercise: A study for specific neural control with Gi-DREADD in mice

Background

Exercise alleviates pain and it is a central component of treatment strategy for chronic pain in clinical setting. However, little is known about mechanism of this exercise-induced hypoalgesia. The mesolimbic dopaminergic network plays a role in positive emotions to rewards including motivation and pleasure. Pain negatively modulates these emotions, but appropriate exercise is considered to activate the dopaminergic network. We investigated possible involvement of this network as a mechanism of exercise-induced hypoalgesia.

Methods

In the present study, we developed a protocol of treadmill exercise, which was able to recover pain threshold under partial sciatic nerve ligation in mice, and investigated involvement of the dopaminergic reward network in exercise-induced hypoalgesia. To temporally suppress a neural activation during exercise, a genetically modified inhibitory G-protein-coupled receptor, hM4Di, was specifically expressed on dopaminergic pathway from the ventral tegmental area to the nucleus accumbens.

Results

The chemogenetic-specific neural suppression by Gi-DREADD system dramatically offset the effect of exercise-induced hypoalgesia in transgenic mice with hM4Di expressed on the ventral tegmental area dopamine neurons. Additionally, anti-exercise-induced hypoalgesia effect was significantly observed under the suppression of neurons projecting out of the ventral tegmental area to the nucleus accumbens as well.

Conclusion

Our findings suggest that the dopaminergic pathway from the ventral tegmental area to the nucleus accumbens is involved in the anti-nociception under low-intensity exercise under a neuropathic pain-like state.

Exploring the Mechanisms of Exercise-Induced Hypoalgesia Using Somatosensory and Laser Evoked Potentials

Exercise-induced hypoalgesia is well described, but the underlying mechanisms are unclear. The aim of this study was to examine the effect of exercise on somatosensory evoked potentials, laser evoked potentials, pressure pain thresholds and heat pain thresholds. These were recorded before and after 3-min of isometric elbow flexion exercise at 40% of the participant's maximal voluntary force, or an equivalent period of rest. Exercise-induced hypoalgesia was confirmed in two experiments (Experiment 1–SEPs; Experiment 2–LEPs) by increased pressure pain thresholds at biceps brachii (24.3 and 20.6% increase in Experiment 1 and 2, respectively; both d > 0.84 and p < 0.001) and first dorsal interosseous (18.8 and 21.5% increase in Experiment 1 and 2, respectively; both d > 0.57 and p < 0.001). In contrast, heat pain thresholds were not significantly different after exercise (forearm: 10.8% increase, d = 0.35, p = 0.10; hand: 3.6% increase, d = 0.06, p = 0.74). Contrasting effects of exercise on the amplitude of laser evoked potentials (14.6% decrease, d = −0.42, p = 0.004) and somatosensory evoked potentials (10.9% increase, d = −0.02, p = 1) were also observed, while an equivalent period of rest showed similar habituation (laser evoked potential: 7.3% decrease, d = −0.25, p = 0.14; somatosensory evoked potential: 20.7% decrease, d = −0.32, p = 0.006). The differential response of pressure pain thresholds and heat pain thresholds to exercise is consistent with relative insensitivity of thermal nociception to the acute hypoalgesic effects of exercise. Conflicting effects of exercise on somatosensory evoked potentials and laser evoked potentials were observed. This may reflect non-nociceptive contributions to the somatosensory evoked potential, but could also indicate that peripheral nociceptors contribute to exercise-induced hypoalgesia.

Neurobiology of fibromyalgia and chronic widespread pain

Fibromyalgia is the current term for chronic widespread musculoskeletal pain for which no alternative cause can be identified. The underlying mechanisms, in both human and animal studies, for the continued pain in individuals with fibromyalgia will be explored in this review. There is a substantial amount of support for alterations of central nervous system nociceptive processing in people with fibromyalgia, and that psychological factors such as stress can enhance the pain experience. Emerging evidence has begun exploring other potential mechanisms including a peripheral nervous system component to the generation of pain and the role of systemic inflammation. We will explore the data and neurobiology related to the role of the CNS in nociceptive processing, followed by a short review of studies examining potential peripheral nervous system changes and cytokine involvement. We will not only explore the data from human subjects with fibromyalgia but will relate this to findings from animal models of fibromyalgia. We conclude that fibromyalgia and related disorders are heterogenous conditions with a complicated pathobiology with patients falling along a continuum with one end a purely peripherally driven painful condition and the other end of the continuum is when pain is purely centrally driven.