Nociceptors and Chronic Pain

Chronic pain lasting months or longer is very common, poorly treated, and sometimes devastating. Nociceptors are sensory neurons that usually are silent unless activated by tissue damage or inflammation. In humans their peripheral activation evokes conscious pain, and their spontaneous activity is highly correlated with spontaneous pain. Persistently hyperactive nociceptors mediate increased responses to normally painful stimuli (hyperalgesia) in chronic conditions and promote the sensitization of central pain pathways that allows low-threshold mechanoreceptors to elicit painful responses to innocuous stimuli (allodynia). Investigations of rodent models of neuropathic pain and hyperalgesic priming have revealed many alterations in nociceptors and associated cells that are implicated in the development and maintenance of chronic pain. These include chronic nociceptor hyperexcitability and spontaneous activity, sprouting, synaptic plasticity, changes in intracellular signaling, and modified responses to opioids, along with alterations in the expression and translation of thousands of genes in nociceptors and closely linked cells.

Role of GABAAR in the Transition From Acute to Chronic Pain and the Analgesic Effect of Electroacupuncture on Hyperalgesic Priming Model Rats

Chronic pain is a costly health problem that impairs health-related quality of life when not effectively treated. Regulating the transition from acute to chronic pain is a new therapeutic strategy for chronic pain that presents a major clinical challenge. The underlying mechanisms of pain transition are not entirely understood, and strategies for preventing this transition are lacking. Here, a hyperalgesic priming model was used to study the potential mechanism by which γ-aminobutyric acid receptor type A (GABAAR) in the dorsal root ganglion (DRG) contributes to pain transition. Furthermore, electroacupuncture (EA), a modern method of acupuncture, was administered to regulate pain transition, and the mechanism underlying EA’s regulatory effect was investigated. Hyperalgesic priming was induced by intraplanar injection of carrageenan (Car)/prostaglandin E2 (PGE2). The decrease in mechanical withdrawal threshold (MWT) induced by PGE2 returned to baseline 4 h after injection in NS + PGE2 group, and still persisted 24 h after injection in Car + PGE2 group. Lower expression of GABAAR in the lumbar DRG was observed in the model rats. Furthermore, activating or blocking GABAAR could reversed the long-lasting hyperalgesia induced by Car/PGE2 injection or produced a persistent hyperalgesia. In addition, GABAAR may be involved in Protein Kinase C epsilon (PKCε) activation in the DRG, a mark molecular of pain transition. EA considerably increased the mechanical pain thresholds of hyperalgesic priming model mammals in both the acute and chronic phases. Furthermore, EA upregulated the expression of GABAAR and inhibited the activation of PKCε in the DRG. In addition, peripheral administration of picrotoxin blocked the analgesic effect of EA on the model rats and abolished the regulatory effect of EA on PKCε activation. These findings suggested that GABAAR plays a key role in both the transition from acute to chronic pain and the analgesic effect of EA on hyperalgesic priming.

A female-specific role for Calcitonin Gene-Related Peptide (CGRP) in rodent pain models

We aimed to investigate a potentially sexually dimorphic role of Calcitonin Gene-Related Peptide (CGRP) in mouse and rat models of pain. Based on findings in migraine where CGRP has a preferential pain-promoting effect in female rodents, we hypothesized that CGRP antagonists and antibodies would attenuate pain sensitization more efficaciously in female than male mice and rats. In hyperalgesic priming induced by activation of interleukin 6 (IL-6) signaling, CGRP receptor antagonists, olcegepant and CGRP8-37, both given intrathecally, blocked and reversed hyperalgesic priming only in females. A monoclonal antibody against CGRP, given systemically, blocked priming specifically in female rodents but failed to reverse it. In the spared nerve injury (SNI) model, there was a transient effect of both CGRP antagonists, given intrathecally, on mechanical hypersensitivity in female mice only. Consistent with these findings, intrathecally applied CGRP caused a long-lasting, dose-dependent mechanical hypersensitivity in female mice but more transient effects in males. This CGRP-induced mechanical hypersensitivity was reversed by the KCC2 activator, CLP257 suggesting a role for anionic plasticity in the dorsal horn in the pain-promoting effects of CGRP in females. In spinal dorsal horn slices, CGRP shifted GABAA reversal potentials to significantly more positive values but, again, only in female mice. Therefore, CGRP may regulate KCC2 expression and/or activity specifically in females. However, KCC2 hypofunction promotes mechanical pain hypersensitivity in both sexes because CLP257 alleviated hyperalgesic priming in male and female mice. We conclude that CGRP promotes pain plasticity in female mice, but has a limited impact in male mice. Significance Statement: The majority of patients impacted by chronic pain are women. Mechanistic studies in rodents are creating a clear picture that molecular events promoting chronic pain are different in male and female animals. Far more is known about chronic pain mechanisms in male animals. We sought to build on recent evidence showing that CGRP is a more potent and efficacious promoter of headache pain in female than in male rodents. To test this, we used hyperalgesic priming and the spared nerve injury (SNI) neuropathic pain models in mice. Our findings show a clear sex dimorphism wherein CGRP promotes pain in female but not male mice. Our work suggests that CGRP antagonists could be tested for efficacy in women for a broader variety of pain conditions.

A231 THE MICROBIOTA-NOCICEPTORS-MICROGLIA AXIS CONTROLS THE DEVELOPMENT AND MAINTENANCE OF CHRONIC VISCERAL HYPERSENSITIVITY

Background Pain is the most common cause of disability in inflammatory bowel disease (IBD). Current medical interventions control the debilitating clinical symptoms by reducing gastrointestinal (GI) inflammation. Despite successful treatment of active disease, abdominal pain persists during remission, suggesting a high level of plasticity in pain-sensing circuits (hyperalgesic priming) caused by inflammation. What drives this remodelling has remained elusive. We have identified microglia as active players of hyperalgesic priming in IBD. Furthermore, it was recently shown that commensal bacteria control the maturation of microglia in the CNS, suggesting that dysbiosis could influence visceral sensitivity through regulating colonic nociceptors-microglia interaction. Here we test the hypothesis that microbiome-nociceptors-microglia interactions control visceral sensitivity and pain in IBD. Aims We investigated the role of the microbiota in the developmental regulation of colonic nociceptors that express the pain receptor TRPV1. We will identify the microbial factors that control neuron-microglia interactions during bacterial colonization and post-inflammatory dysbiosis. Methods We have developed a germ-free TRPV1-GFP reporter mouse to be used for a combination of behavioural tests and phenotypic characterization of TRPV1+ nociceptors. RNA-sequencing of FACS isolated TRPV1+ neurons of germ-free mice will be used to identify genes that are under the control of the microbiota. We will restore discrepancies observed in germ-free mice by recolonization to assess the impact of the microbiota. Furthermore, we will investigate the regulation of Ahr in TRPV1+ neurons by the microbiota and the effect of its ligands on microglial activation and post-inflammatory visceral pain. Results Measuring somatic pain sensation in naive germ-free and SPF mice, we showed a 15% reduction in thermal pain threshold, as measured by the Hargreaves test, and a 50% reduction in mechanical pain threshold, as measured by the Von Frey test, in germ-free mice. When looking at the dorsal root ganglia of germ-free and SPF mice, we saw a 15% increase in the percentage of neurons that were TRPV1-GFP positive in germ-free mice. Conclusions Our results thus far highlight the importance of the microbiota in regulating the lineage of nociceptive neurons and the threshold of mechanical and thermal pain responses. These findings suggest a major contribution of the microbiota in shaping the neuro-immune axis, with major implications for visceral sensitization in the context of dysbiosis. My project will be looking further into the phenotype of nociceptors in germ-free mice and the effect of microbial-derived Ahr agonists on the maturation and function of colonic TRPV1+ nociceptors. My work will advance our understanding of mechanisms by which commensal bacteria regulate GI pain. Funding Agencies CIHR

De novo protein synthesis is necessary for priming in preclinical models of migraine

Background Migraine attacks are often triggered by normally innocuous stimuli, suggesting that sensitization within the nervous system is present. One mechanism that may contribute to neuronal sensitization in this context is translation regulation of new protein synthesis. The goal of this study was to determine whether protein synthesis contributes to behavioral responses and priming in preclinical models of migraine. Methods Mice received a dural injection of interleukin-6 in the absence or presence of the protein synthesis inhibitor anisomycin or the translation initiation inhibitor 4EGI-1 and were tested for facial hypersensitivity. Upon returning to baseline, mice were given a second, non-noxious dural injection of pH 7.0 to test for priming. Additionally, eIF4ES209Amice lacking phosphorylation of mRNA cap-binding protein eIF4E received dural interleukin-6 or were subjected to repeated restraint stress and then tested for facial hypersensitivity. After returning to baseline, mice were given either dural pH 7.0 or a systemic sub-threshold dose of the nitric oxide donor sodium nitroprusside and tested for priming. Results Dural injection of interleukin-6 in the presence of anisomycin or 4EGI-1 or in eIF4ES209Amice resulted in the partial attenuation of acute facial hypersensitivity and complete block of hyperalgesic priming. Additionally, hyperalgesic priming following repeated restraint stress was blocked in eIF4ES209Amice. Conclusions These studies show that de novo protein synthesis regulated by activity-dependent translation is critical to the development of priming in two preclinical models of migraine. This suggests that targeting the regulation of protein synthesis may be a novel approach for new migraine treatment strategies.