Gila Moalem-Taylor

The neuro-immune balance in neuropathic pain: Involvement of inflammatory immune cells, immune-like glial cells and cytokines

In a large proportion of individuals nervous system damage may lead to a debilitating chronic neuropathic pain. Such pain may now be considered a neuro-immune disorder, since recent data indicate a critical involvement of innate and adaptive immune responses following nerve injury. Activation of immune and immune-like glial cells in the injured nerve, dorsal root ganglia and spinal cord results in the release of both pro- and anti-inflammatory cytokines, as well as algesic and analgesic mediators, the balance of which determines whether pain chronicity is established. This review will critically examine the role of the immune system in modulating chronic pain in animal models of nervous system injury, and highlight the possible therapeutic opportunities to intervene in the development and maintenance of neuropathic pain.

Interleukin-17 Contributes to Neuroinflammation and Neuropathic Pain Following Peripheral Nerve Injury in Mice

UNLABELLED Cytokines, essential mediators of inflammatory and immune responses, play an important role in the pathophysiological processes associated with neuropathic pain following peripheral nerve injury. Recently, a novel proinflammatory cytokine, the interleukin (IL)-17, was found to orchestrate inflammatory responses in a wide range of inflammatory and autoimmune diseases of the nervous system. Here, we investigated the role of IL-17 in mediating neuroinflammation and pain hypersensitivity using the neuropathic pain model of partial ligation of the sciatic nerve in mice. Compared to wild-type, IL-17 knockout (KO) mice displayed significantly decreased mechanical pain hypersensitivity as well as decreased infiltration of T cells and macrophages to the injured sciatic nerves and the L3-L5 dorsal root ganglia and decreased activation of microglia and astrocytes in the L3-5 dorsal and ventral horns of the spinal cord. Further, intraplantar and intraneural injection of recombinant IL-17 into the hind paw and the sciatic nerve, respectively, induced both mechanical allodynia and thermal hyperalgesia, whereas intrathecal injection produced thermal hyperalgesia. IL-17 administration was associated with a significant increase in the numbers of infiltrating neutrophils and activated dendritic cells in the injected hind paws and infiltrating neutrophils in the injected sciatic nerves. Taken together, our results demonstrate that IL-17 contributes to the regulation of immune cell infiltration and glial activation after peripheral nerve injury and the ensuing neuropathic pain. PERSPECTIVE IL-17 is an important regulator of immune responses and is involved in inducing and mediating proinflammatory reactions. Using IL-17 KO mice, we have demonstrated that IL-17 contributes to neuroinflammatory responses and pain hypersensitivity following neuropathic injury. This work identifies IL-17 as a potential therapeutic target in neuropathic pain.

Detailed characterization of neuro-immune responses following neuropathic injury in mice.

Partial sciatic nerve injury is a common model of neuropathic pain in rodents, and produces both mechanical and thermal pain hypersensitivity. Several types of immune cells have been implicated in the pathogenesis of neuropathic pain due to nerve injury; however, the timing of their appearance has not been fully elucidated. Here, using immunohistochemistry, we characterized the time course and magnitude of inflammatory cell infiltration and resident immune cell activation in the sciatic nerves, L3-5 dorsal root ganglia (DRGs) and spinal segments following partial ligation of the sciatic nerve (PSNL) in C57BL/6J mice. PSNL markedly decreased paw withdrawal threshold to mechanical stimuli and paw withdrawal latency to thermal stimuli in the injured side. No changes were observed in the uninjured contralateral side. Mechanical allodynia persisted, and thermal hyperalgesia resolved by 2weeks after injury. We found a significant increase in the numbers of infiltrating neutrophils, macrophages, dendritic cells and lymphocytes in the injured sciatic nerve and ipsilateral DRGs in comparison to sham-operated controls, with different timelines of recruitment for each cell type. Expression of ATF3 in the cell bodies of DRG neurons indicated about 30-40% neuronal damage. No neutrophils, dendritic cells, or lymphocytes were found in the spinal cord. However, a significant increase in the level of microglial and astrocytic activation was observed in the spinal dorsal horn and to a lesser extent in the ventral horn, peaking on days 7 and 14 after nerve injury. These changes corresponded with a significant increase in immunoreactivity for phosphorylated NR1 subunit of the NMDA receptor, and a significant decrease in IB4-labeled non-peptidergic nociceptive terminals in the ipsilateral dorsal horn. Our findings suggest differential roles for peripheral and central neuroimmune interactions in the production of neuropathic pain.

Regulatory T cells attenuate neuropathic pain following peripheral nerve injury and experimental autoimmune neuritis.

Summary Increasing the number of regulatory T cells (Tregs) reduces neuroinflammation and alleviates mechanical pain hypersensitivity, while depleting Tregs potentiates pain in animal models of neuropathy. Abstract Neuroimmune crosstalk in neuropathic pain is a key contributor to pain hypersensitivity following nervous system injury. CD4+CD25+Foxp3+ regulatory T cells (Tregs) are endogenous immune suppressors, reducing T‐cell proliferation and proinflammatory cytokine production. Currently, the role of Tregs in neuropathic pain is unknown. In this study, we tested the effects of expanding Tregs on pain hypersensitivity and neuroinflammation in 2 models of neuropathy; sciatic nerve chronic constriction injury and experimental autoimmune neuritis in rats. Following chronic constriction injury, treatment with CD28 superagonist (CD28SupA), a Treg population expander, significantly increased Tregs in the lymphoid tissues, injured sciatic nerve, and lumbar spinal cord of rats. CD28SupA treatment led to a significant reduction in mechanical pain hypersensitivity, alongside a decrease in the numbers of infiltrating T cells, macrophages, and antigen‐presenting cells in the sciatic nerve and dorsal root ganglia. In experimental autoimmune neuritis‐affected rats, CD28SupA treatment resulted in a significant improvement in disease severity and in mechanical pain hypersensitivity. This was associated with a reduction in the numbers of T cells, macrophages, and antigen‐presenting cells in the sciatic nerve and dorsal root ganglia, and reduced activation of microglia and infiltration of T cells in the spinal cord. Furthermore, depletion of Tregs by a CD25 antibody in mice with a partial sciatic nerve ligation resulted in prolonged mechanical pain hypersensitivity. These findings suggest that Tregs play a role in endogenous recovery from neuropathy‐induced pain. Thus, this T‐cell subset may be specifically targeted to alleviate chronic neuropathic pain.

A preconditioning nerve lesion inhibits mechanical pain hypersensitivity following subsequent neuropathic injury

BackgroundA preconditioning stimulus can trigger a neuroprotective phenotype in the nervous system - a preconditioning nerve lesion causes a significant increase in axonal regeneration, and cerebral preconditioning protects against subsequent ischemia. We hypothesized that a preconditioning nerve lesion induces gene/protein modifications, neuronal changes, and immune activation that may affect pain sensation following subsequent nerve injury. We examined whether a preconditioning lesion affects neuropathic pain and neuroinflammation after peripheral nerve injury.ResultsWe found that a preconditioning crush injury to a terminal branch of the sciatic nerve seven days before partial ligation of the sciatic nerve (PSNL; a model of neuropathic pain) induced a significant attenuation of pain hypersensitivity, particularly mechanical allodynia. A preconditioning lesion of the tibial nerve induced a long-term significant increase in paw-withdrawal threshold to mechanical stimuli and paw-withdrawal latency to thermal stimuli, after PSNL. A preconditioning lesion of the common peroneal induced a smaller but significant short-term increase in paw-withdrawal threshold to mechanical stimuli, after PSNL. There was no difference between preconditioned and unconditioned animals in neuronal damage and macrophage and T-cell infiltration into the dorsal root ganglia (DRGs) or in astrocyte and microglia activation in the spinal dorsal and ventral horns.ConclusionsThese results suggest that prior exposure to a mild nerve lesion protects against adverse effects of subsequent neuropathic injury, and that this conditioning-induced inhibition of pain hypersensitivity is not dependent on neuroinflammation in DRGs and spinal cord. Identifying the underlying mechanisms may have important implications for the understanding of neuropathic pain due to nerve injury.

Pain and endometriosis

Endometriosis is the commonest cause of chronic pelvic pain in women (Fauconnier and Chapron, 2005). It is characterized by the presence of uterine endometrial tissue outside of the uterus, most commonly in the pelvic cavity. The disorder mainly affects women of reproductive age. Symptoms of endometriosis include recurrent painful periods, painful intercourse, painful defecation during menstruation, chronic lower abdominal pain and hypersensitivity, chronic lower back pain and infertility (Farquhar, 2007). For many women, endometriosis has a negative impact on the ability to work, on family relationships and self-esteem (Huntington and Gilmour, 2005). Many women with endometriosis describe a progression of symptoms over their menstrual life, which may include a mix of different pains and abnormal visceral sensations, indicative of viscero-visceral hyperalgesia and suggestive of neuropathic pain (Horowitz, 2007). Current medical treatments for endometriosis include oral contraceptives, progestogens, androgenic agents, gonadotrophin releasing hormone analogues, as well as laparoscopic surgical excision of the endometriotic lesions. However, management of pain in women with endometriosis is currently insufficient for many women. Here we review the involvement of the nervous system, immune cells and inflammatory response, and hormones in endometriosis as well as current practice in pain management. We suggest that

The Contribution of Immune and Glial Cell Types in Experimental Autoimmune Encephalomyelitis and Multiple Sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system characterised by widespread areas of focal demyelination. Its aetiology and pathogenesis remain unclear despite substantial insights gained through studies of animal models, most notably experimental autoimmune encephalomyelitis (EAE). MS is widely believed to be immune-mediated and pathologically attributable to myelin-specific autoreactive CD4+ T cells. In recent years, MS research has expanded beyond its focus on CD4+ T cells to recognise the contributions of multiple immune and glial cell types to the development, progression, and amelioration of the disease. This review summarises evidence of T and B lymphocyte, natural killer cell, macrophage/microglial, astrocytic, and oligodendroglial involvement in both EAE and MS and the intercommunication and influence of each cell subset in the inflammatory process. Despite important advances in the understanding of the involvement of these cell types in MS, many questions still remain regarding the various subsets within each cell population and their exact contribution to different stages of the disease.

Role of Histamine H3 and H4 Receptors in Mechanical Hyperalgesia following Peripheral Nerve Injury

Objective: Histamine is a chemical mediator that acts at four known types of histamine receptors and has been widely implicated in the development of nociception and neuropathic pain. Blocking histamine H1 and H2 receptors has been shown to reduce hyperalgesia following nerve injury, but the role of histamine H3 and H4 receptors in neuropathic pain has not been studied. Here, we used blockers of histamine H3 and H4 receptors to assess their effects on neuropathic pain behavior and mast cell numbers following peripheral nerve injury. In addition, we assessed the effect of activating H4 receptors on neuropathic pain behavior. Methods: Rats were subjected to a partial ligation of the sciatic nerve, a model of neuropathic pain, and were treated either systemically or locally (hindpaw) with the H3/H4 receptor inverse agonist thioperamide, the specific H4 receptor antagonist JNJ 7777120, or the H4 receptor agonist VUF 8430. Measurements of mechanical hyperalgesia were carried out by Randall-Selitto test for 1–3 weeks, and sciatic nerve tissues were analyzed for numbers of intact mast cells by histology at 9 h after surgery. Results: Rats treated with thioperamide or JNJ 7777120 showed significantly enhanced mechanical hyperalgesia after partial ligation of the sciatic nerve. The number of intact mast cells in the injured nerve of these rats was higher than in control rats suggesting reduced mast cell degranulation, but was still significantly lower than in intact nerves. Rats treated with VUF 8430 showed significantly reduced mechanical hyperalgesia. Conclusion: We propose that the increase in mechanical hyperalgesia produced by thioperamide and JNJ 7777120 and the decrease in mechanical hyperalgesia produced by VUF 8430 may represent a direct effect of these agents on mechanospecific primary afferents, or an indirect effect of these agents via injury-induced inflammation.

Role of Gap Junctions in Chronic Pain

Gap junctions are specialized transmembrane channels that allow rapid electrical signalling and direct intercellular communication for maintenance and coordination of normal cellular activities and homeostasis. Although gap junction channels in the nervous system mediate intercellular coupling between glial cells and between neurons, they also contribute to the spread of secondary damage and inflammation under pathological conditions. There is now evidence of the involvement of gap junctions in chronic pain caused by nervous system damage or tissue inflammation. In this Mini‐Review, we highlight recent studies demonstrating the dynamic plasticity of gap junctions in response to nervous system injury and the effects of gap junction blockade on neuronal survival and modulation of pain in animal models of neuropathic and inflammatory pain. The involvement of dorsal root ganglia and spinal cord gap junctions in mediating chronic pain and the potential for targeting connexins as a novel modality for the treatment of intractable pain syndromes arising from nervous system injury and disorders are discussed.

Chronic constriction of the sciatic nerve and pain hypersensitivity testing in rats.

Chronic neuropathic pain, resulting from damage to the central or peripheral nervous system, is a prevalent and debilitating condition, affecting 7-18% of the population(1,2). Symptoms include spontaneous (tingling, burning, electric-shock like) pain, dysaesthesia, paraesthesia, allodynia (pain resulting from normally non-painful stimuli) and hyperalgesia (an increased response to painful stimuli). The sensory symptoms are co-morbid with behavioural disabilities, such as insomnia and depression. To study chronic neuropathic pain several animal models mimicking peripheral nerve injury have been developed, one of the most widely used is Bennett and Xies (1988) unilateral sciatic nerve chronic constriction injury (CCI)(3) (Figure 1). Here we present a method for performing CCI and testing pain hypersensitivity. CCI is performed under anaesthesia, with the sciatic nerve on one side exposed by making a skin incision, and cutting through the connective tissue between the gluteus superficialis and biceps femoris muscles. Four chromic gut ligatures are tied loosely around the sciatic nerve at 1 mm intervals, to just occlude but not arrest epineural blood flow. The wound is closed with sutures in the muscle and staples in the skin. The animal is then allowed to recover from surgery for 24 hrs before pain hypersensitivity testing begins. For behavioural testing, rats are placed into the testing apparatus and are allowed to habituate to the testing procedure. The area tested is the mid-plantar surface of the hindpaw (Figure 2), which falls within the sciatic nerve distribution. Mechanical withdrawal threshold is assessed by mechanically stimulating both injured and uninjured hindpaws using an electronic dynamic plantar von Frey aesthesiometer or manual von Frey hairs(4). The mechanical withdrawal threshold is the maximum pressure exerted (in grams) that triggers paw withdrawal. For measurement of thermal withdrawal latency, first described by Hargreaves et al (1988), the hindpaw is exposed to a beam of radiant heat through a transparent glass surface using a plantar analgesia meter(5,6). The withdrawal latency to the heat stimulus is recorded as the time for paw withdrawal in both injured and uninjured hindpaws. Following CCI, mechanical withdrawal threshold, as well as thermal withdrawal latency in the injured paw are both significantly reduced, compared to baseline measurements and the uninjured paw (Figure 3). The CCI model of peripheral nerve injury combined with pain hypersensitivity testing provides a model system to investigate the effectiveness of potential therapeutic agents to modify chronic neuropathic pain. In our laboratory, we utilise CCI alongside thermal and mechanical sensitivity of the hindpaws to investigate the role of neuro-immune interactions in the pathogenesis and treatment of neuropathic pain.