This chapter explores how pain can be powerfully amplified by the activation of peripheral immune cells associated with peripheral nerves and by the activation of immune-like glial cells (microglia and astrocytes) within the central nervous system. After review of the basics of pain and pain modulation, the focus will shift to chronic pain, which is poorly if at all controlled by currently available drugs. The thesis will be developed that such pain arises as a consequence of immune and glial activation. The involvement of peripheral immune cells and glia in such pain states is explored. Finally, the very recent finding that glia also oppose the pain-suppressive effects of drugs like morphine is discussed, including the implications of these findings for clinical pain control.

Most neuropathic pain conditions are caused by damage to peripheral nociceptive neurons known as “small-fibers.” Examination of punch skin biopsies immunolabeled with pan-axonal markers has identified degeneration of distal nociceptive axons as a pathological hallmark of most neuralgias including numerous polyneuropathies, postherpetic neuralgia, and complex regional pain syndrome. Similar axonopathy is present in several rodent models of neuralgia (chronic constriction injury and spared nerve injury). Here we have measured the density of distal cutaneous small-fiber axons in rats with persistent mechanical allodynia caused by the sciatic inflammatory neuropathy (SIN), a model of painful neuritis. We have evaluated whether perineural immune activation sufficient to cause evoked-pain behaviors is also associated with loss of distal epidermal innervation.SIN was created in adult male Sprague-Dawley rats by perineural zymosan administration using established methods. Unoperated rats provided controls. Two weeks of sensory testing established the presence of static mechanical allodynia. Full-thickness punches were then collected from ipsilesional sural-innervated plantar hindpaw skin. These were immunolabeled against PGP9.5 using standard methods, and the density of epidermal neurites quantitated microscopically by a single examiner who was unaware of rats' experimental group. Neurite densities were similar in SIN and control samples (P = 0.96), suggesting that mechanical allodynia can occur from inflammation near a nerve, without distal small-fiber degeneration.

Patients suffering from chronic pain often report that pain onset was preceded by an acute episode of peripheral inflammation or trauma. Previous findings demonstrate peripheral challenge can result in long-term changes in central nervous system astrocytes and microglia, possibly indicative of glial priming. By definition, primed cells can respond in an exaggerated manner to subsequent challenge. If priming were to occur in spinal cord glia, chronic pain may result on account of enhanced release of glial inflammatory products. In the current series of experiments, evidence for spinal glial priming was evaluated in two different animal models: subcutaneous inflammation via intraplantar injection of formalin and peripheral injury/inflammation associated with abdominal surgery (laparotomy). Evidence for spinal glial priming was assessed in each model following secondary challenge (2 weeks later) with intrathecal HIV-1 gp120, to directly stimulate spinal glia and induce glially-mediated pain enhancement. Changes in glial proinflammatory cytokine production and behavioral response thresholds to tactile stimuli were examined. Prior laparotomy followed by a later challenge does indeed lead to enhanced proinflammatory cytokine protein levels in the spinal cord. Examination of prior laparotomy on response thresholds are ongoing. Bilateral enhancement of pain behavior was seen in response to gp120 subsequent to unilateral subcutaneous inflammation 2 weeks before. Analyses of glial activation markers are underway to determine whether prior laparotomy or formalin result in exaggerated microglial and/or astrocytic response to gp120 injection. Taken together, these data suggest that spinal cord glia may be primed for extended periods of by a variety of stimuli, possibly leading to subsequent over-response to challenge.

Despite many decades of drug development, effective therapies for neuropathic pain remain elusive. The recent recognition of spinal cord glia and glial pro-inflammatory cytokines as important contributors to neuropathic pain suggests an alternative therapeutic strategy; that is, targeting glial activation or its downstream consequences. While several glial-selective drugs have been successful in controlling neuropathic pain in animal models, none are optimal for human use. Thus the aim of the present studies was to explore a novel approach for controlling neuropathic pain. Here, an adeno-associated viral (serotype II; AAV2) vector was created that encodes the anti-inflammatory cytokine, interleukin-10 (IL-10). This anti-inflammatory cytokine is known to suppress the production of pro-inflammatory cytokines. Upon intrathecal administration, this novel AAV2-IL-10 vector was successful in transiently preventing and reversing neuropathic pain. Intrathecal administration of an AAV2 vector encoding beta-galactosidase revealed that AAV2 preferentially infects meningeal cells surrounding the CSF space. Taken together, these data provide initial support that intrathecal gene therapy to drive the production of IL-10 may prove to be an efficacious treatment for neuropathic pain.

Chronic abdominal pain is a common symptom of great clinical significance in several areas of medicine. In many cases no organic cause can be established resulting in the classification as functional gastrointestinal disorder. Irritable Bowel Syndrome (IBS) is the most common of these conditions and is considered an important public health problem because it can be disabling and constitutes a major social and economic burden given the lack of effective treatments. IBS aetiology is most likely multi-factorial involving biological, psychological and social factors. Visceral hyperalgesia (or hypersensitivity) and visceral hypervigilance, which could be mediated by peripheral, spinal, and/or central pathways, constitute key concepts in current research on pathophysiological mechanisms of visceral hyperalgesia. The role of central nervous system mechanisms along the “brain–gut axis” is increasingly appreciated, owing to accumulating evidence from brain imaging studies that neural processing of visceral stimuli is altered in IBS together with long-standing knowledge regarding the contribution of stress and negative emotions to symptom frequency and severity. At the same time, there is also growing evidence suggesting that peripheral immune mechanisms and disturbed neuro-immune communication could play a role in the pathophysiology of visceral hyperalgesia. This review presents recent advances in research on the pathophysiology of visceral hyperalgesia in IBS, with a focus on the role of stress and anxiety in central and peripheral response to visceral pain stimuli. Together, these findings support that in addition to lower pain thresholds displayed by a significant proportion of patients, the evaluation of pain appears to be altered in IBS. This may be attributable to affective disturbances, negative emotions in anticipation of or during visceral stimulation, and altered pain-related expectations and learning processes. Disturbed “top-down” emotional and cognitive pain modulation in IBS is reflected by functional and possibly structural brain changes involving prefrontal as well as cingulate regions. At the same time, there is growing evidence linking peripheral and mucosal immune changes and abdominal pain in IBS, supporting disturbed peripheral pain signalling. Findings in post-infectious IBS emphasize the interaction between centrally-mediated psychosocial risk factors and local inflammation in predicting long-term IBS symptoms. Investigating afferent immune-to-brain communication in visceral hyperalgesia as a component of the sickness response constitutes a promising future research goal.

It has become clear that spinal cord glia (microglia and astrocytes) importantly contribute to the creation of exaggerated pain responses. One model used to study this is peri-spinal (intrathecal, i.t.) administration of gp120, an envelope protein of HIV-1 known to activate glia. Previous studies demonstrated that i.t. gp120 produces pain facilitation via the release of glial proinflammatory cytokines. The present series of studies tested whether spinal nitric oxide (NO) contributes to i.t. gp120-induced mechanical allodynia and, if so, what effect NO has on spinal proinflammatory cytokines. gp120 stimulation of acutely isolated lumbar dorsal spinal cords released NO as well as proinflammatory cytokines (tumor necrosis factor-alpha, interleukin-1beta (IL1), interleukin-6 (IL6)), thus identifying NO as a candidate mediator of gp120-induced behavioral effects. Behaviorally, identical effects were observed when gp120-induced mechanical allodynia was challenged by i.t. pre-treatment with either a broad-spectrum nitric oxide synthase (NOS) inhibitor (L-NAME) or 7-NINA, a selective inhibitor of NOS type-I (nNOS). Both abolished gp120-induced mechanical allodynia. While the literature pre-dominantly documents that proinflammatory cytokines stimulate the production of NO rather than the reverse, here we show that gp120-induced NO increases proinflammatory cytokine mRNA levels (RT-PCR) and both protein expression and protein release (serial ELISA). Furthermore, gp120 increases mRNA for IL1 converting enzyme and matrix metalloproteinase-9, enzymes responsible for activation and release of proinflammatory cytokines.