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The safety and efficacy of opioids are compromised as analgesic tolerance develops. Opioids are also ineffective against neuropathic pain. Recent reports have suggested that inhibitors of the epidermal growth factor receptor (EGFR), a receptor tyrosine kinase (RTK), may have analgesic effects in cancer patients suffering from neuropathic pain. It has been shown that the platelet-derived growth factor receptor-beta (PDGFR-↓), an RTK that has been shown to interact with the EGFR, mediates opioid tolerance but does not induce analgesia. Therefore, we sought to determine whether EGFR signaling was involved in opioid tolerance and if EGFR and PDGFR signaling could induce pain in rats.We found that gefitinib, an EGFR antagonist, eliminated morphine tolerance. In addition, repeated epidermal growth factor (EGF) administration rendered animals unresponsive to subsequent analgesic doses of morphine, a phenomenon we call 'pre-tolerance'. Using a nerve injury model, we found that gefitinib alone was not analgesic. Rather, it reversed insensitivity to morphine analgesia ('pre-tolerance') caused by the release of EGF by injured nerves. We also showed that repeated, but not acute EGF or platelet-derived growth factor-BB (PDGF-BB) administration induced mechanical hypersensitivity in rats. EGFR and PDGFR-↓ signaling interacted to produce this sensitization. EGFR was widely expressed in primary sensory afferent cell bodies, demonstrating a neuroanatomical substrate for our findings.Taken together, our results suggest a direct mechanistic link between opioid tolerance and mechanical sensitization. EGFR antagonism could eventually play an important clinical role in the treatment of opioid tolerance and neuropathic pain that is refractory to opioid treatment. Opioid tolerance and associated reduced effectiveness of opioids against neuropathic pain are two major clinical problems that are prime contributors to the opioid epidemic. However, the mechanisms underlying these phenomena are not clearly understood. Here we show that EGFR antagonism not only blocks morphine tolerance but also restores the effectiveness of opioids against neuropathic pain. Chronic EGF or PDGF administration induces mechanical sensitization, a prominent component of neuropathic pain, and renders animals 'pre-tolerant' to subsequent analgesic doses of morphine. Taken together, these results suggest a direct mechanistic link between opioid tolerance and neuropathic pain. EGFR antagonism could eventually play an important role in the treatment of opioid tolerance and severe neuropathic pain that requires ever increasing doses of opioids.
Learn More >Endometriosis is an estrogen-dependent inflammatory disease that affects approximately 10% of women. Debilitating pelvic or abdominal pain is one of its major clinical features. Current animal models of endometriosis-associated pain require surgery either to implant tissue or to remove the ovaries. Moreover, existing models do not induce spontaneous pain, which is the primary symptom of patients with chronic pain, including endometriosis. A lack of models that accurately recapitulate the disease phenotype must contribute to the high failure rate of clinical trials for analgesic drugs directed at chronic pain, including those for endometriosis. We set out to establish a murine model of endometriosis-associated pain. Endometriosis was induced non-surgically by injecting a dissociated uterine horn into a recipient mouse. The induced lesions exhibited histological features that resemble human lesions along with an increase in pro-inflammatory cytokines and recruitment of immune cells. We also observed the presence of CGRP-, TRPA1-, and TRPV1-expressing nerve fibers in the lesions. This model induced mechanical allodynia, spontaneous abdominal pain, and changes in thermal selection behavior that indicate discomfort. These behavioral changes were reduced by drugs used clinically for endometriosis, specifically letrozole (aromatase inhibitor) and danazol (androgen). Endometriosis also induced neuronal changes as evidenced by activation of the NF-κB signaling pathway in TRPA1- and TRPV1-expressing DRG neurons. In conclusion, we have established a model of endometriosis-associated pain that responds to clinically active drugs and can, therefore, be used to identify novel therapies.
Learn More >Low back pain (LBP) is a highly prevalent and disabling condition whose initiating factors are poorly understood. It is known that psychological and physical stress is associated with LBP but the causal relationship, mechanisms and mediators have not been elucidated, and a preclinical model enabling the investigation of causality and thereby critically contributing to clinical translation does not exist.In the present study, we first established and characterized a myofascial LBP model in mice based on NGF injection into the low back muscles. Secondly, we investigated the effect of two different stress paradigms on this mouse LBP model by applying the chronic unpredictable stress (CUS) and vertical chronic restraint stress (vCRS) paradigms, to mimic psychological and psychophysical stress, respectively. In these studies, we combined longitudinal behavioral tests with gene and protein expression analysis in the muscle, dorsal root ganglia and spinal cord. NGF-induced LBP was characterized by long-lasting local and plantar mechanical hypersensitivity, cold hyperalgesia, decreased grip strength and wheel running activity, and time-dependent changes of neuropeptide and glial markers in the spinal cord. Interestingly, the exposure to CUS slightly worsened pain behavior, whereas vCRS primed and highly aggravated pain in this LBP model, by causing per se the intramuscular upregulation of endogenous NGF and increased spinal astrocyte expression.Our mouse model, particularly the combination of NGF injection and vCRS suggest that similar mechanisms are important in non-specific LBP and might help to investigate certain aspects of stress-induced exacerbation of pain.
Learn More >Pharmacological evaluation of the mu-opioid receptor (MOR) agonist properties of NKTR-181 in rodent models.
Learn More >EphrinB-EphB receptor tyrosine kinases have been demonstrated to play important roles in pain processing after peripheral nerve injury. We have previously reported that ephrinB-EphB receptor signaling can regulate excitability and plasticity of neurons in spinal dorsal horn, and thus contribute to spinal central sensitization in neuropathic pain. How EphB receptor activation influences excitability of primary neurons in dorsal root ganglion (DRG), however, remains unknown. Here we report that EphB receptor activation facilitates calcium influx through NMDA receptor dependent and independent manners. In cultured DRG cells from adult rats, EphB1 and EphB2 receptors were expressed in neurons, but not the glial cells. Bath application of EphB receptor agonist ephrinB2-Fc induced NMDA-independent Ca influx, which was from the extracellular space rather than endoplasmic reticulum. EphB receptor activation also greatly enhanced NMDA-dependent Ca influx and NR2B phosphorylation, which was prevented by pre-treatment of Src kinase inhibitor PP2. In nerve-injured DRG neurons, elevated expression and activation of EphB1 and EphB2 receptors contributed to the increased intracellular Ca concentration and NMDA-induced Ca influx. Repetitive intrathecal administration of EphB2-Fc inhibited the increased phosphorylation of NR2B and Ca-dependent subsequent signals Src, ERK, and CaMKII as well as behaviorally expressed pain after nerve injury. These findings demonstrate that activation of EphB receptors can modulate DRG neuron excitability by facilitating Ca influx directly or through Src kinase activation-mediated NMDA receptor phosphorylation and that EphB receptor activation is critical to DRG neuron hyperexcitability, which has been considered critical to the subsequent spinal central sensitization and neuropathic pain.
Learn More >Endothelin-1 (ET-1), an endogenous vasoactive peptide, has been found to play an important role in peripheral pain signaling. Acid-sensing ion channels (ASICs) are key sensors for extracellular protons and contribute to pain caused by tissue acidosis. It remains unclear whether an interaction exists between ET-1 and ASICs in primary sensory neurons. In this study, we reported that ET-1 enhanced the activity of ASICs in rat dorsal root ganglia (DRG) neurons. In whole-cell voltage-clamp recording, ASIC currents were evoked by brief local application of pH 6.0 external solution in the presence of TRPV1 channel blocker AMG9810. Pre-application with ET-1 (1-100 nM) dose-dependently increased the proton-evoked ASIC currents with an EC value of 7.42 ± 0.21 nM. Pre-application with ET-1 (30 nM) shifted the concentration-response curve of proton upwards with a maximal current response increase of 61.11% ± 4.33%. We showed that ET-1 enhanced ASIC currents through endothelin-A receptor (ETR), but not endothelin-B receptor (ETR) in both DRG neurons and CHO cells co-expressing ASIC3 and ETR. ET-1 enhancement was inhibited by blockade of G-protein or protein kinase C signaling. In current-clamp recording, pre-application with ET-1 (30 nM) significantly increased acid-evoked firing in rat DRG neurons. Finally, we showed that pharmacological blockade of ASICs by amiloride or APETx2 significantly alleviated ET-1-induced flinching and mechanical hyperalgesia in rats. These results suggest that ET-1 sensitizes ASICs in primary sensory neurons via ETR and PKC signaling pathway, which may contribute to peripheral ET-1-induced nociceptive behavior in rats.
Learn More >Upon transient musculoskeletal diseases, some patients develop persistent pain while others recover from pain. Here we studied whether such heterogeneity also occurs in rats after recovery from unilateral antigen-induced arthritis (AIA) in the knee joint, and which pain phenotype may predict the course of pain. Typically inflammatory swelling lasts about three weeks. Pain-related behaviors were monitored for 84 days after AIA induction. Unbiased cluster analysis of intra-group-differences at day 84 of AIA revealed that about one third of the rats (cluster 1) showed persistent mechanical hyperalgesia at the injected knee joint, whereas the other rats (cluster 2) had recovered from pain. Retrograde analysis of pain-related behaviors revealed that cluster 1 rats exhibited more severe mechanical hyperalgesia at the injected knee from day 3 of AIA, and mechanical hyperalgesia at the contralateral knee. Cluster 1 and 2 rats did not show different inflammatory swelling, secondary mechanical and thermal hyperalgesia at the ipsilateral hindpaw, guarding score and asymmetry of weight bearing during AIA. Thus in particular early severe mechanical hyperalgesia in the inflamed joint and segmental contralateral mechanical hyperalgesia seem to be a risk factor for the development of persistent mechanical hyperalgesia pointing to the importance of spinal mechanisms. However, none of the rats showed an expression of ATF3 in DRG neurons, nor morphological spinal microglia activation thus not suggesting development of neuropathic pain. Both clusters showed a persistent upregulation of pCREB in DRG neurons, inversely correlated with mechanical hyperalgesia at the knee. The role of pCREB needs to be further explored.
Learn More >IL-4 is a pleiotropic antiinflammatory cytokine, which can be neuroprotective after nervous system injury. The beneficial actions of IL-4 are thought to result from the blunting of action of inflammatory mediators, such as proinflammatory cytokines. Here, we demonstrate that IL-4 induces M2 macrophages to continuously produce opioid peptides and ameliorate pain. IL-4 application at injured nerves in mice shifted F4/80+ macrophages from the proinflammatory M1 to the antiinflammatory M2 phenotype, which synthesized opioid peptides (Met-enkephalin, β-endorphin, and dynorphin A 1-17). These effects were accompanied by a long-lasting attenuation of neuropathy-induced mechanical hypersensitivity, beyond the IL-4 treatment. This IL-4-induced analgesia was decreased by opioid peptide antibodies and opioid receptor (δ, μ, κ) antagonists applied at injured nerves, which confirms the involvement of the local opioid system. The participation of M2 macrophages was supported by analgesia in recipient mice injected at injured nerves with F4/80+ macrophages from IL-4-treated donors. Together, IL-4-induced M2 macrophages at injured nerves produced opioid peptides, which activated peripheral opioid receptors to diminish pain. Fostering the opioid-mediated actions of intrinsic M2 macrophages may be a strategy to tackle pathological pain.
Learn More >Morphine-3-glucuronide (M3G), the main metabolite of morphine, has been implicated in the development of tolerance and of opioid-induced hyperalgesia, both limiting the analgesic use of morphine. We evaluated the acute and chronic effects of M3G and morphine as well as development of antinociceptive cross-tolerance between morphine and M3G after intrathecal administration and assessed the expression of pain-associated neurotransmitter substance P in the spinal cord. Sprague-Dawley rats received intrathecal M3G or morphine twice daily for 6 days. Nociception and tactile allodynia were measured with von Frey filaments after acute and chronic treatments. Substance P levels in the dorsal horn of the spinal cord were determined by immunohistochemistry after 4-day treatments. Acute morphine caused antinociception as expected, whereas acute M3G caused tactile allodynia, as did both chronic M3G and morphine. Chronic M3G also induced antinociceptive cross-tolerance to morphine. M3G and morphine increased substance P levels similarly in the nociceptive laminae of the spinal cord. This study shows that chronic intrathecal M3G sensitises animals to mechanical stimulation and elevates substance P levels in the nociceptive laminae of the spinal cord. Chronic M3G also induces antinociceptive cross-tolerance to morphine. Thus, chronic M3G exposure might contribute to morphine-induced tolerance and opioid-induced hyperalgesia.
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