Chronic oropharyngeal pain and medical nomadism in an Eagle's syndrome patient: a case report

Background

Elongation of the styloid process associated with oropharyngeal pain and dysphagia is known as Eagle’s syndrome, a condition whose pathophysiology is still a matter of debate. Given its low prevalence and complex symptomatology, this syndrome is often misdiagnosed, leading to chronic pain and medical nomadism.

Case summary

A 51-year-old woman of African origin with 3-year history of left-side oropharyngeal pain and worsening dysphagia consulted several health professionals. Medical and surgical treatments, including a sinus surgery and the extraction of three healthy teeth, did not improve her symptoms. Evaluation in an orofacial pain clinic revealed an asymmetrically elongated styloid process. Surgical shortening of the elongated styloid process provided complete pain relief and recovery of normal swallowing function.

Conclusion

Based on this case report, the pathophysiology of Eagle’s syndrome is discussed, and the need for specific follow-up in a subpopulation of patients with asymptomatic styloid process elongation is highlighted.

Misdiagnosis of Eagle’s syndrome often leads to unnecessary and expansive medical nomadism.

Eagle’s syndrome is a multifactorial syndrome.

All physicians or clinicians (dentists, radiologists, etc.) should be sensitized and trained to detect and report styloid process elongation.

Activation of translocator protein alleviates mechanical allodynia and bladder dysfunction in cyclophosphamide-induced cystitis through repression of BDNF-mediated neuroinflammation

BACKGROUND

Bladder pain syndrome/interstitial cystitis (BPS/IC) is a refractory disease accompanied by bladder-related pain and hyperactivity. Studies have shown that the translocator protein (TSPO) modulates neuroinflammation and central sensitisation associated with pain. Moreover, we previously demonstrated that brain-derived neurotrophic factor (BDNF) regulates neuroinflammation and mechanical allodynia in cyclophosphamide (CYP)-induced cystitis through activation of glial cells. Here, we aimed to explore whether activation of TSPO attenuates mechanical allodynia and bladder dysfunction by regulating BDNF induced neuroinflammation in a CYP-induced cystitis model.

METHODS

Injection of CYP was performed to form a rat model of BPS/IC. The expression of TSPO was regulated by intrathecal injection of the TSPO agonist Ro5-4864. The von Frey filament test was applied to evaluate suprapubic allodynia. Bladder function was assessed using filling cystometry. Western blotting was used to detect the expression of TSPO, BDNF, GFAP, Iba-1, p-p38, p-JNK, TNF-α, and IL-1β, and double immunofluorescence was performed to localise TSPO in the L6-S1 spinal dorsal horn (SDH).

RESULTS

TSPO was activated in the SDH after CYP injection and was primarily colocalised with astrocytes. Ro5-4864 reversed mechanical allodynia and bladder dysfunction induced by CYP. Moreover, the upregulation of BDNF and activation of astrocytes and microglia was suppressed by Ro5-4864, resulting in downregulation of p-p38, p-JNK, TNF-α, and IL-1β.

CONCLUSIONS

Ro5-4864 alleviated mechanical allodynia and bladder dysfunction in the CYP model, possibly by inhibiting the elevation of BDNF and consequent activation of astrocytes and microglia induced neuroinflammation. TSPO may be a potential target for the treatment of BPS/IC.

Targeting the A3 adenosine receptor to prevent and reverse chemotherapy-induced neurotoxicities in mice

Cisplatin is used to combat solid tumors. However, patients treated with cisplatin often develop cognitive impairments, sensorimotor deficits, and peripheral neuropathy. There is no FDA-approved treatment for these neurotoxicities. We investigated the capacity of a highly selective A₃ adenosine receptor (AR) subtype (A₃AR) agonist, MRS5980, to prevent and reverse cisplatin-induced neurotoxicities. MRS5980 prevented cisplatin-induced cognitive impairment (decreased executive function and impaired spatial and working memory), sensorimotor deficits, and neuropathic pain (mechanical allodynia and spontaneous pain) in both sexes. At the structural level, MRS5980 prevented the cisplatin-induced reduction in markers of synaptic integrity. In-situ hybridization detected Adora3 mRNA in neurons, microglia, astrocytes and oligodendrocytes. RNAseq analysis identified 164 genes, including genes related to mitochondrial function, of which expression was changed by cisplatin and normalized by MRS5980. Consistently, MRS5980 prevented cisplatin-induced mitochondrial dysfunction and decreased signs of oxidative stress. Transcriptomic analysis showed that the A₃AR agonist upregulates genes related to repair pathways including NOTCH1 signaling and chromatin modification in the cortex of cisplatin-treated mice. Importantly, A₃AR agonist administration after completion of cisplatin treatment resolved cognitive impairment, neuropathy and sensorimotor deficits. Our results highlight the efficacy of a selective A₃AR agonist to prevent and reverse cisplatin-induced neurotoxicities via preventing brain mitochondrial damage and activating repair pathways. An A₃AR agonist is already in cancer, clinical trials and our results demonstrate management of neurotoxic side effects of chemotherapy as an additional therapeutic benefit.

The impact of foot shock-induced stress on pain-related behavior associated with burn injury

Acute pain is prevalent following burn injury and can often transition to chronic pain. Prolonged acute pain is an important risk factor for chronic pain and there is little preclinical research to address this problem. Using a mouse model of second-degree burn, we investigated whether pre-existing stress influences pain(sensitivity) after a burn injury. We introduced a contribution of stress in two different ways: (1) the use of foot-shock as a pre-injury stressor or (2) the use of A/J mice to represent higher pre-existing stress compared to C57Bl/6 mice. C57Bl/6 and A/J mice were exposed to repeated mild foot shock to induce stress for 10 continuous days and mice underwent either burn injury or sham burn injury of the plantar surface of the right hind paw. Assessments of mechanical and thermal sensitivities of the injured and uninjured paw were conducted during the shock protocol and at intervals up to 82-day post-burn injury. In both strains of mice that underwent burn injury, thermal hypersensitivity and mechanical allodynia appeared rapidly in the ipsilateral paw. Mice that were stressed took much longer to recover their hind paw mechanical thresholds to baseline compared to non-stressed mice in both burn and non-burn groups. Analysis of the two mouse strains revealed that the recovery of mechanical thresholds in A/J mice which display higher levels of baseline anxiety was shorter than C57Bl/6 mice. No differences were observed regarding thermal sensitivities between strains. Our results support the view that stress exposure prior to burn injury affects mechanical and thermal thresholds and may be relevant to as a risk factor for the transition from acute to chronic pain. Finally, genetic differences may play a key role in modality-specific recovery following burn injury.

Metformin attenuates diabetic neuropathic pain via AMPK/NF-κB signaling pathway in dorsal root ganglion of diabetic rats

Neuropathic pain is a common complication of diabetes mellitus with poorly relieved by conventional analgesics. Metformin, a first-line drug for type 2 diabetes, reduces blood glucose by activating adenosine monophosphate protein kinase (AMPK) signalling system. However, the effect of Metformin on diabetic neuropathic pain is still unknown. In the present study, we showed that Metformin was capable of attenuating diabetes induced mechanical allodynia, and the analgesia effect could be blocked by Compound C (an AMPK inhibitor). Importantly, Metformin enhanced the phosphorylation level of AMPK in L4-6 DRGs of diabetic rats but not affect the expression of total AMPK. Intrathecal injection of AICAR (an AMPK agonist) could activate AMPK and alleviate the mechanical allodynia of diabetic rats. Additionally, phosphorylated AMPK and NF-κB was co-localized in small and medium neurons of L4-6 DRGs. Interestingly, the regulation of NF-κB in diabetic rats was obviously reduced when AMPK was activated by AICAR. Notably, Metformin could decrease NF-κB expression in L4-6 DRGs of diabetic rats, but the decrease was blocked by Compound C. In conclusion, Metformin alleviates diabetic mechanical allodynia via activation of AMPK signaling pathway in L4-6 DRGs of diabetic rats, which might be mediated by the downregulation of NF-κB, and this providing certain basis for Metformin to become a potential drug in the clinical treatment of diabetic neuropathic pain.

Cisplatin induces TRPA1-mediated mechanical allodynia in the oral mucosa

OBJECTIVE

Cisplatin, a platinum-based anticancer drug, produces reactive oxygen species (ROS) in many cell types and induces mechanical allodynia in the hands and/or feet (chemotherapy-induced painful neuropathy: CIPN). In this study, we examined the possibility of inducing neuropathy in the oral region using oral keratinocytes and rats.

METHODS

Human oral keratinocytes (HOKs) were used to evaluate ROS generation after cisplatin application by a ROS-reactive fluorescent assay. In rats, after cisplatin administrations (two times), the trigeminal ganglion (TG) was investigated by electron microscopy and quantitative RT-PCR. Using our proprietary assay system, oral pain-related behaviors were observed in cisplatin-treated rats.

RESULTS

In rats, cisplatin administration reduced food intake and body weight. In electron microscopic analysis, glycogen granules in the TG were depleted following administration, although organelles were intact. In HOK cells, cisplatin significantly increased ROS generation with cell death, similar to glycolysis inhibitors. Cisplatin administration did not show any effects on Trpa1 mRNA levels in the TG. However, the same procedure induced hypersensitivity to mechanical stimulation and the TRPA1 agonist allyl isothiocyanate in the oral mucosa. Mechanical hypersensitivity was inhibited by the antioxidative drug α-lipoic acid and the TRPA1 antagonist HC-030031, similar to that of the hind paw.

CONCLUSION

The present findings suggest that cisplatin induces TRPA1-mediated CIPN due to ROS generation in the oral region. This study will provide a better understanding of persistent oral pain in cancer patients.

Aging and miR-155 in mice influence survival and neuropathic pain after spinal cord injury

Spinal cord injury (SCI) elicits chronic pain in 65% of individuals. In addition, SCI afflicts an increasing number of aged individuals, and those with SCI are predisposed to shorter lifespan. Our group previously identified that deletion of the microRNA miR-155 reduced neuroinflammation and locomotor deficits after SCI. Here, we hypothesized that aged mice would be more susceptible to pain symptoms and death soon after SCI, and that miR-155 deletion would reduce pain symptoms in adult and aged mice and improve survival. Adult (2 month-old) and aged (20 month-old) female wildtype (WT) and miR-155 knockout (KO) mice received T9 contusion SCI. Aged WT mice displayed reduced survival and increased autotomy - a symptom of spontaneous pain. In contrast, aged miR-155 KO mice after SCI were less susceptible to death or spontaneous pain. Evoked pain symptoms were tested using heat (Hargreaves test) and mechanical (von Frey) stimuli. At baseline, aged mice showed heightened heat sensitivity. After SCI, adult and aged WT and miR-155 KO mice all exhibited heat and mechanical hypersensitivity at all timepoints. miR-155 deletion in adult (but not aged) mice reduced mechanical hypersensitivity at 7 and 14 d post-SCI. Therefore, aging predisposes mice to SCI-elicited spontaneous pain and expedited mortality. miR-155 deletion in adult mice reduces evoked pain symptoms, and miR-155 deletion in aged mice reduces spontaneous pain and expedited mortality post-SCI. This study highlights the importance of studying geriatric models of SCI, and that inflammatory mediators such as miR-155 are promising targets after SCI for improving pain relief and longevity.

Anti-allodynic and promotive effect on inhibitory synaptic transmission of riluzole in rat spinal dorsal horn

Riluzole (2-amino-6-(trifluoromethoxy)benzothiazole) is a drug known for its inhibitory effect on glutamatergic transmission and its anti-nociceptive and anti-allodynic effects in neuropathic pain rat models. Riluzole also has an enhancing effect on GABAergic synaptic transmission. However, the effect on the spinal dorsal horn, which plays an important role in modulating nociceptive transmission, remains unknown. We investigated the ameliorating effect of riluzole on mechanical allodynia using the von Frey test in a rat model of neuropathic pain and analyzed the synaptic action of riluzole on inhibitory synaptic transmission in substantia gelatinosa (SG) neurons using whole-cell patch clamp recordings. We found that single-dose intraperitoneal riluzole (4 mg/kg) administration effectively attenuated mechanical allodynia in the short term in a rat model of neuropathic pain. Moreover, 300 μM riluzole induced an outward current in rat SG neurons. The outward current induced by riluzole was not suppressed in the presence of tetrodotoxin. Furthermore, we found that the outward current was suppressed by simultaneous bicuculline and strychnine application, but not by strychnine alone. Altogether, these results suggest that riluzole enhances inhibitory synaptic transmission monosynaptically by potentiating GABAergic synaptic transmission in the rat spinal dorsal horn.

The primary motor cortex electrical and chemical stimulation attenuates the chronic neuropathic pain by activation of the periaqueductal grey matter: The role of NMDA receptors

BACKGROUND

Motor cortex stimulation (MCS) is proper as a non-pharmacological therapy for patients with chronic and neuropathic pain (NP).

AIMS

This work aims to investigate if the MCS in the primary motor cortex (M₁) produces analgesia and how the MCS could interfere in the MCS-induced analgesia. Also, to elucidate if the persistent activation of N-methyl-d-aspartic acid receptor (NMDAr) in the periaqueductal grey matter (PAG) can contribute to central sensitisation of the NP.

METHODS

Male Wistar rats were submitted to the von Frey test to evaluate the mechanical allodynia after 21 days of chronic constriction injury (CCI) of the sciatic nerve. The MCS was performed with low-frequency (20 μA, 100 Hz) currents during 15 s by a deep brain stimulation (DBS) device. Moreover, the effect of M₁-treatment with an NMDAr agonist (at 2, 4, and 8 nmol) was investigated in CCI rats. The PAG dorsomedial column (dmPAG) was pretreated with the NMDAr antagonist LY 235959 (at 8 nmol), followed by MCS.

RESULTS

The MCS decreased the mechanical allodynia in rats with chronic NP. The M₁-treatment with an NMDA agonist at 2 and 8 nmol reduced the mechanical allodynia in CCI rats. In addition, dmPAG-pretreatment with LY 235959 at 8 nmol attenuated the mechanical allodynia evoked by MCS.

CONCLUSION

The M₁ cortex glutamatergic system is involved in the modulation of chronic NP. The analgesic effect of MCS may depend on glutamate signaling recruitting NMDAr located on PAG neurons in rodents with chronic NP.

Improved potency of pyridin-2(1H)one derivatives for the treatment of mechanical allodynia

Mechanical allodynia, a painful sensation caused by innocuous touch, is a major chronic pain symptom, which often remains without an effective treatment. There is thus a need for new anti-allodynic treatments based on new drug classes. We recently synthetized new 3,5-disubstituted pyridin-2(1H)-one derivatives. By substituting the pyridinone at the 3-position by various aryl/heteroaryl moieties and at the 5-position by a phenylamino group, we discovered that some derivatives exhibited a strong anti-allodynic potency in rats. Here, we report that varying the substitution of the pyridinone 5-position, the 3-position being substituted by an indol-4-yl moiety, further improves such anti-allodynic potency. Compared with 2, one of the two most active compounds of the first series, eleven out of nineteen newly synthetized compounds showed higher anti-allodynic potency, with two of them completely preventing mechanical allodynia. In the first series, hit compounds 1 and 2 appeared to be inhibitors of p38α MAPK, a protein kinase known to underlie pain hypersensitivity in animal models. Depending on the substitution at the 5-position, some newly synthetized compounds were also stronger p38α MAPK inhibitors. Surprisingly, though, anti-allodynic effects and p38α MAPK inhibitory potencies were not correlated, suggesting that other biological target(s) is/are involved in the analgesic activity in this series. Altogether, these results confirm that 3,5-disubstituted pyridine-2(1H)-one derivatives are of high interest for the development of new treatment of mechanical allodynia.

TREK-1 potassium channels participate in acute and long-lasting nociceptive hypersensitivity induced by formalin in rats

TREK-1 channels are expressed in small nociceptive dorsal root ganglion (DRG) neurons where they participate in acute inflammatory and neuropathic pain. However, the role of TREK-1 in persistent pain is not well understood. The aim of this study was to investigate the local peripheral and spinal participation of TREK-1 in formalin-induced acute and long-lasting nociceptive hypersensitivity. Local peripheral or intrathecal pre-treatment with spadin, selective blocker of TREK-1, increased acute flinching behavior and secondary mechanical allodynia and hyperalgesia behavior observed 6 days after formalin injection. Local peripheral or intrathecal pre-treatment with BL-1249, selective opener of TREK-1, decreased long-lasting secondary mechanical allodynia and hyperalgesia induced by formalin. Pre-treatment with BL-1249 prevented the pro-nociceptive effect of spadin on acute nociception and long-lasting mechanical allodynia and hyperalgesia in rats. Pre-treatment with two recombinant channels that produce a high TREK-1 current, S300A and S333A (non-phosphorylated state of TREK-1), reduced formalin-induced acute pain and long-lasting mechanical allodynia and hyperalgesia. Besides, post-treatment with S300A, S333A or BL-1249 reversed long-lasting mechanical allodynia and hyperalgesia induced by formalin. Formalin increased TREK-1 expression at 1 and 6 days in DRG and dorsal spinal cord in rats, whereas that it increased c-fos expression at the DRG. Intrathecal repeated transfection of rats with S300A and S333A or injection with BL-1249 reduced formalin-induced enhanced c-fos expression. Data suggest that TREK-1 activity at peripheral and spinal sites reduces neuronal excitability in the process of acute and long-lasting nociception induced by formalin in rats.

Cannabidiol in the prelimbic cortex modulates the comorbid condition between the chronic neuropathic pain and depression-like behaviour in rats: The role of medial prefrontal cortex 5-HT1A and CB1 receptors

The prelimbic division (PrL) of the medial prefrontal cortex (mPFC) is a cerebral division that is putatively implicated in the chronic pain and depression. We investigated the activity of PrL cortex neurons in Wistar rats that underwent chronic constriction injury (CCI) of sciatic nerve and were further subjected to the forced swimming (FS) test and mechanical allodynia (by von Frey test). The effect of blockade of synapses with cobalt chloride (CoCl₂), and the treatment of the PrL cortex with cannabidiol (CBD), the CB₁ receptor antagonist AM251 and the 5-HT_1A receptor antagonist WAY-100635 were also investigated. Our results showed that CoCl₂ decreased the time spent immobile during the FS test but did not alter mechanical allodynia. CBD (at 15, 30 and 60 nmol) in the PrL cortex also decreased the frequency and duration of immobility; however, only the dose of 30 nmol of CBD attenuated mechanical allodynia in rats with chronic NP. AM251 and WAY-100635 in the PrL cortex attenuated the antidepressive and analgesic effect caused by CBD but did not alter the immobility and the mechanical allodynia when administered alone. These data show that the PrL cortex is part of the neural substrate underlying the comorbidity between NP and depression. Also, the previous blockade of CB₁ cannabinoid receptors and 5-HT_1A serotonergic receptors in the PrL cortex attenuated the antidepressive and analgesics effect of the CBD. They also suggest that CBD could be a potential medicine for the treatment of depressive and pain symptoms in patients with chronic NP/depression comorbidity.

Mast cell stabilizer ketotifen fumarate reverses inflammatory but not neuropathic-induced mechanical pain in mice

Supplemental Digital Content is Available in the Text.

Our preclinical findings indicate that ketotifen fumarate's analgesic effects are MC-dependent, and the case series report presented supports its use for the treatment of chronic pain.

Introduction:

Mast cell (MC) activation could establish a positive feedback loop that perpetuates inflammation and maintains pain. Stabilizing MCs with ketotifen fumarate (KF) may disrupt this loop and relieve pain.

Objective:

We aimed to test the effect of treatment with KF in pain assays in mice and in a case series of patients with chronic widespread pain.

Methods:

The analgesic effect of KF was tested in CD-1 mice injected with formalin, complete Freund's adjuvant, or subjected to spared nerve injury. In addition, wild-type (C57BL/6) and MC-deficient (C57BL/6-Kit^W-sh/W-sh) mice were injected with formalin or complete Freund's adjuvant and treated with KF. Patients with chronic widespread pain (n = 5; age: 13–16 years) who failed to respond to standard of care participated in a 16-week treatment trial with KF (6 mg/d). Ketotifen fumarate's therapeutic effect was evaluated using the patient global impression of change.

Results:

In the mouse experiments, KF produced dose- and MC-dependent analgesic effects against mechanical allodynia in the acute and chronic inflammatory pain but not neuropathic pain assays. In the patient case series, 4 patients reported that activity limitations, symptoms, emotions, and overall quality of life related to their pain condition were “better” or “a great deal better” since beginning treatment with KF. This was accompanied by improvements in pain comorbid symptoms.

Conclusion:

Treatment with KF is capable of reducing established inflammatory-induced mechanical nociception in an MC-dependent manner in mice, and it may be beneficial for the treatment of chronic pain conditions.

Downregulation of metallothionein-2 contributes to oxaliplatin-induced neuropathic pain

Background

We previously reported a correlation between small doses of oxaliplatin penetrating onto the spinal cord and acute pain after chemotherapy. Here, we propose that MT2 within the spinal dorsal horns participates in the development of oxaliplatin-induced neuropathic pain and may be a pharmacological target for the prevention and treatment of chemotherapy-induced peripheral neuropathy (CIPN).

Methods

The rat model of CIPN was established by 5 consecutive injections of oxaliplatin (0.4 mg/100 g/day). Genetic restoration of neuron-specific metallothionein-2 was implemented 21 days before oxaliplatin treatment, and also, genetic inhibition by metallothionein-2 siRNA was performed. Mechanical allodynia and locomotor activity were assayed. Cell-specific expression of metallothionein-2, the mRNA levels of pro-inflammatory cytokines, nuclear translocation of NF-κB, the protein levels of expression of IκB-α, and interaction between IκB-α and P65 were evaluated in the spinal dorsal horns. Also, in vitro interaction of sequentially deleted IκB-α promoter with metallothionein-2 was used to assess the signal transduction mechanism.

Results

We found that oxaliplatin induced downregulation of metallothionein-2 in rat spinal cord neurons. By contrast, genetic restoration of metallothionein-2 in the spinal dorsal horn neuron blocked and reversed neuropathic pain in oxaliplatin-treated rats of both sexes, whereas genetic inhibition of metallothionein-2 triggered neuropathic pain in normal rats. Overall locomotor activity was not impaired after the genetic alterations of metallothionein-2. At the molecular level, metallothionein-2 modulated oxaliplatin-induced neuroinflammation, activation of NF-κB, and inactive transcriptional expression of IκB-α promoter, and these processes could be blocked by genetic restoration of metallothionein-2 in the spinal dorsal horn neurons.

Conclusions

Metallothionein-2 is a potential target for the prevention and treatment of CIPN. A reduction of NF-κB activation and inflammatory responses by enhancing the transcription of IκB-α promoter is proposed in the mechanism.

Inhibition of angiotensin converting enzyme induces mechanical allodynia through increasing substance P expression in mice

Although emerging evidence shows that angiotensin converting enzyme (ACE) is associated with pain, it is not clear whether inhibition of ACE could affect to nociceptive transmission and which mediators are involved in this process. Here we investigated whether administration of the ACE inhibitors, captopril and enalapril increases the expression of substance P (SP) and whether this increase contributes to the induction of mechanical allodynia in mice. ACE was expressed in the lumbar dorsal root ganglion (DRG) and the superficial dorsal horn (SDH) region of the spinal cord in mice. Either intraperitoneal or intrathecal administration of the ACE inhibitors, captopril and enalapril for 10 days significantly increased the paw withdrawal frequency to innocuous mechanical stimuli and the levels of SP in both the lumbar DRG and the SDH region of the spinal cord dorsal horn. In addition, intraperitoneal administration of the SP receptor (neurokinin-1 receptor) antagonist, L-733,060 suppressed mechanical allodynia that was induced by pretreatment of captopril and enalapril. Intraplantar administration of SP for 3 days induces mechanical allodynia, and this effect was reduced by exogenous ACE administration. These findings demonstrate that inhibition of ACE increases the levels of SP in both the lumbar DRG and spinal cord dorsal horn, ultimately contributing to the induction of mechanical allodynia in mice.

The implication of transient receptor potential canonical 6 in BDNF-induced mechanical allodynia in rat model of diabetic neuropathic pain

AIMS

Brain-derived neurotrophic factor (BDNF) is vital in the pathogenesis of mechanical allodynia with a paucity of reports available regarding diabetic neuropathy pain (DNP). Herein we identified the involvement of BDNF in driving mechanical allodynia in DNP rats via the activation of transient receptor potential canonical 6 (TRPC6) channel.

MATERIALS AND METHODS

The DNP rat model was established via streptozotocin (STZ) injection, and allodynia was assessed by paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL). The expression profiles of BDNF and TRPC6 in dorsal root ganglia (DRG) and spinal cord were illustrated by immunofluorescence and Western blotting. Intrathecal administration of K252a or TrkB-Fc was performed to inhibit BNDF/TrkB expression, and respective injection of GsMTX-4, BTP2 and TRPC6 antisense oligodeoxynucleotides (TRPC6-AS) was likewise conducted to inhibit TRPC6 expression in DNP rats. Calcium influx in DRG was monitored by calcium imaging.

KEY FINDINGS

The time-dependent increase of BDNF and TRPC6 expression in DRG and spinal cord was observed since the 7th post-STZ day, correlated with the development of mechanical allodynia in DNP rats. Intrathecal administration of K252a, TrkB-Fc, GsMTX-4 and BTP2 prevented mechanical allodynia in DNP rats. Pre-treatment of TRPC6-AS reversed the BDNF-induced pain-like responses in DNP rats rather than the naïve rats. In addition, the TRPC6-AS reversed BDNF-induced increase of calcium influx in DRG neurons in DNP rats.

SIGNIFICANCE

The intrathecal inhibition of TRPC6 alleviated the BDNF-induced mechanical allodynia in DNP rat model. This finding may validate the application of TRPC6 antagonists as interesting strategy for DNP management.

Influence of Preoperative Mechanical Allodynia on Predicting Postoperative Pain after Root Canal Treatment: A Prospective Clinical Study

INTRODUCTION

Odontogenic pain can manifest as pulpal pain, periapical pain (mechanical allodynia), or both. This study aimed to assess the changes in the intensity of mechanical allodynia (MA) and to identify predictors of postoperative pain after root canal treatment (RCT).

METHODS

In total, 579 consecutive patients who required RCT were enrolled; we included patients with asymptomatic pulpal diagnoses to avoid any effects of preoperative spontaneous pain on postoperative pain and to evaluate MA independently. Using a visual analog scale (VAS), patients separately indicated the intensity of spontaneous pain, tenderness to percussion, and pain on biting; these measurements were performed before treatment (preoperative pain), at the beginning of each visit (postpreparation pain), and daily for 1 week after RCT (postobturation pain). For analytical purposes, patients were subdivided into 2 groups based on the intensity of preoperative MA (none to mild [VAS <4] or moderate to severe [VAS ≥4]) to evaluate changes in MA and predictive factors of moderate to severe postoperative pain. A generalized estimating equation, repeated-measures analysis of variance, and logistic regression analysis were used.

RESULTS

Although the intensity of MA was significantly higher in the moderate to severe group after the initiation of RCT (P < .05), 93% of them experienced alleviation in MA, and 30% of patients in the none to mild group experienced an increase in MA. After adjusting for clinical variables, moderate to severe preoperative MA and the presence of necrotic pulp were significantly correlated with moderate to severe postoperative pain with an odds ratio of 4.107 and 0.286, respectively.

CONCLUSIONS

Moderate to severe preoperative MA was a predictive factor of postoperative pain in patients undergoing RCT.

Involvement of selective GABA-A receptor subtypes in amelioration of cisplatin-induced neuropathic pain by 2'-chloro-6-methyl flavone (2'-Cl-6MF)

Cisplatin-induced peripheral neuropathic pain is a common adverse effect of chemotherapy. The present study evaluated the effects of 2'-chloro-6-methylflavone (2'-Cl-6MF) at recombinant α1β2γ2L, α2β1-3γ2L, and α3β1-3γ2L GABA-A receptor subtypes expressed in Xenopus oocytes and subsequently evaluated its effectiveness in cisplatin-induced neuropathic pain. The results showed that 2'-Cl-6MF potentiated GABA-elicited currents at α2β2/3γ2L and α3β2/3γ2L GABA-A receptor subtypes. The potentiation was blocked by the co-application of flumazenil (a benzodiazepine (BDZs) site antagonist). In behavioral studies, mechanical allodynia was induced by intraplantar injection of cisplatin (40 μg/paw) in Sprague Dawley rats, and behavioral assessments were made 24 h after injection. 2'-Cl-6MF (1, 10, 30, and 100 mg/kg, i.p.), was administered 1 h before behavioral evaluation. Administration of 2'-Cl-6MF (30 and 100 mg/kg, i.p) significantly enhanced the paw withdrawal threshold and decreased mechanical allodynia. The standard drugs, gabapentin (GBP) at the dose of 70 mg/kg, and HZ 166 (16 mg/kg), i.p. also significantly enhanced the paw withdrawal threshold in mechanical allodynia. Pretreatment with pentylenetetrazole (PTZ) (15 mg/kg, i.p.) and flumazenil reversed the antinociceptive effect of 2'-Cl-6MF in mechanical allodynia indicating GABAergic mechanisms. Moreover, the binding mechanism of 2'-Cl-6MF was rationalized by in silico modeling tools. The 3D-coordinates of α2β2γ2L and α2β3γ2L were generated after homology modeling of the α2 subtype and 2'-Cl-6MF was at predicted binding sites of the developed models. The α2 model was compared with the α1 and α3 subunits via structural and sequence alignment. Molecular docking depicted that the compound binds efficiently at the neuromodulator binding site of the receptors. The findings of this study revealed that 2'-Cl-6MF ameliorated the manifestations of cisplatin-induced neuropathic pain in rats. Furthermore, we also conclude that GABAergic mechanisms may contribute to the antinociceptive effect of 2'-Cl-6MF. The molecular docking studies also confirm the involvement of the BDZs site of GABA-A receptors. It was observed that Ile230 of α2 stabilize the chlorophenyl ring of 2'-Cl-6MF through hydrophobic interactions, which is replaced by Val203 in α1 subunit. However, the smaller side chain of Val203 does not provide hydrophobic interaction to the compound due to high conformational flexibility of α1 subunit.

A Src family kinase maintains latent sensitization in rats, a model of inflammatory and neuropathic pain

Latent sensitization is a long-term model of chronic pain in which hyperalgesia is continuously suppressed by opioid receptors, as demonstrated by the induction of mechanical allodynia by opioid antagonists. Different intracellular signals may mediate the initiation, maintenance and expression of latent sensitization. Our criterion for the involvement of a signal in the maintenance of latent sensitization is that inhibitors should permanently eliminate the allodynia produced by an opioid antagonist. We hypothesized that Src family kinases (SFKs) maintain latent sensitization and tested this hypothesis by inducing latent sensitization in rats with complete Freund's adjuvant (CFA) or spared nerve injury. After measures of mechanical allodynia returned to baseline, vehicle or the SFK inhibitor PP2 were injected intrathecally. The opioid antagonist naltrexone injected intrathecally 15 min later produced allodynia in control rats but not in rats injected with PP2. Vehicle or PP2 were injected daily for two more days and naltrexone was injected five days later. Again, naltrexone induced allodynia in the control rats but not in the rats injected with PP2. Results were similar when latent sensitization was induced with CFA or spared nerve injury. We concluded that an SFK, likely Fyn, maintains latent sensitization induced by inflammation or nerve injury.

Potentiation of P2X3 receptor mediated currents by endothelin-1 in rat dorsal root ganglion neurons

Endothelin-1 (ET-1), an endogenous vasoconstrictor, has been known as a pro-nociceptive agent involved in multitude of pain. ET-1 acts on endothelin receptors on vascular endothelial cells, sensitizes release of ATP, which then acts on P2X3 receptors on nociceptors and results in mechanical hyperalgesia. Both endothelin receptors and P2X3 receptors are present in primary sensory neuron, where it remains unclear whether there is an interaction between them. Herein, we reported that ET-1 potentiated the electrophysiological activity of P2X3 receptors in rat dorsal root ganglia (DRG) neurons. ET-1 concentration-dependently increased α,β-methylene-ATP (α,β-meATP)-evoked inward currents, which were mediated by P2X3 receptors. ET-1 shifted the α,β-meATP concentration-response curve upwards, with an increase of 34.38 ± 4.72% in the maximal current response to α,β-meATP in the presence of ET-1. ET-1 potentiation of α,β-meATP-evoked currents was voltage-independent. ET-1 potentiated P2X3 receptor-mediated currents through endothelin-A receptors (ET_AR), but not endothelin-B receptors (ET_BR). ET-1 potentiation was supressed by blockade of intracellular G-protein or protein kinase C (PKC) signaling. Moreover, there is a synergistic effect on mechanical allodynia induced by intraplantar injection of ET-1 and α,β-meATP in rats. Pharmacological blockade of P2X3 receptors also alleviated ET-1-induced mechanical allodynia. These results suggested that ET-1 sensitized P2X3 receptors in primary sensory neurons via an ET_AR and PKC signaling pathway. Our data provide evidence that cutaneous ET-1 induced mechanical allodynia not only by increasing the release of ATP from vascular endothelial cells, but also by sensitizing P2X3 receptors on nociceptive DRG neurons.

Pain and Activity Measurements

Musculoskeletal pain contributes significantly to chronic pain experienced by adults and to health care use. This chapter details several methods to evaluate pain and physical activity in mice that can be applied to preclinical orthopedic models. These methods include the von Frey filament assay that measures mechanical allodynia, open-field activity assays for evaluation of ambulation, and incapacitance measurements to determine static weight bearing.

Aqueous extract from Luehea divaricata Mart. Leaves reduces nociception in rats with neuropathic pain

ETHNOPHARMACOLOGICAL RELEVANCE

Luehea divaricata, popularly known in Brazil as "açoita-cavalo", has been widely explored by different ethnic groups native to Brazil to treat different pathologic conditions, including inflammatory pain. However, no report could be found on the effect that extract of L. divaricata has on neuropathic pain. This is an important topic because convergent and divergent mechanisms underlie inflammatory vs. neuropathic pain indicate that there may not always be a clear mechanistic delineation between these two conditions.

AIM OF THE STUDY

The study aimed to determine antioxidant activity and macronutrient composition of aqueous extract from leaves of L. divaricata, and the effect of oral administration on nociception in rats with chronic constriction injury (CCI) of sciatic nerve-induced neuropathic pain, one of the most commonly employed animal models of neuropathic pain.

MATERIALS AND METHODS

The antioxidant activity of the extract was evaluated by total phenolic content and DPPH, ABTS^●+ and ORAC methods. Vitexin was determined by HPLC to show that the composition of the extract of the present study is similar to that used in previous studies with this genus. Total sugar and sucrose concentrations were assessed by the anthrone method, while glucose and triacilglycerides were determined using commercially available kits. Fructose concentration was calculated from values for total sugars, glucose and sucrose. Total protein was determined by Bradford assay. The effect on DNA strand breaking was investigated by inhibition of strand breaking of supercoiled DNA by hydroxyl radical. The antinociceptive effects of aqueous extract (100, 300, 500, and 1000 mg/kg, i.g.) were evaluated on thermal and mechanical thresholds for neuropathic pain induced by chronic constriction injury (CCI) of the sciatic nerve in rats. We also compared the antinociceptive effect of the extract (500 mg/kg, i.g.) with that induced by gabapentin (50 mg/kg, i.g.), a first-line clinical treatment for neuropathic pain. The effect of co-administration of extract (500 mg/kg, i.g.) and low-dose gabapentin (30 mg/kg, i.g.) was also assessed. In addition, the effect of the extract on body weight, and blood and hepatic parameters were investigated to reveal possible side effects of treatment.

RESULTS

The extract showed high content of total phenol; good reducing capacity for DPPH, ABTS^●+ and ORAC assays; presence of vitexin; and a high capacity to inhibit strand breaking of supercoiled DNA. The predominant sugar was sucrose, followed by glucose and fructose. Total protein was greater than triacylglycerides, with the latter being present in a trace amount in the extract. The extract increased the thermal and mechanical thresholds, which was reduced by CCI. The antinociceptive effect was comparable to gabapentin and was also found after co-administration of extract and low-dose gabapentin. No significant change was found in body weight and blood and hepatic indicators after extract treatment.

CONCLUSIONS

Aqueous extract from L. divaricata leaves was as effective as gabapentin at attenuating CCI-induced neuropathic pain, indicating for first time the therapeutic potential of this species for this type of pain.

Expression and functional characterization of transient receptor potential vanilloid 4 in the dorsal root ganglion and spinal cord of diabetic rats with mechanical allodynia

Diabetic mechanical allodynia (DMA) is a common manifestation in patients with diabetes mellitus, and currently, no effective treatment is available. Transient receptor potential vanilloid 4 (TRPV4) is involved in mechanical hypersensitivity resulting from varying aetiologies in animal, but its expression pattern during DMA and whether it contributes to this condition are still unclear. We investigated the spatial and temporal expression patterns of TRPV4 in the dorsal root ganglion (DRG) and spinal dorsal horn (SDH) by qRT-PCR, Western blotting and immunofluorescence assays. The pathophysiological role of TRPV4 in DMA was also investigated by intrathecal application of the TRPV4 selective antagonist HC-067047 or the agonist GSK1016790A. The results showed that both the mRNA and protein levels of TRPV4 were strikingly upregulated on day 14 in the rats with DMA. The increase in TRPV4 was mainly observed in the soma and central processes of calcitonin gene-related peptide (CGRP)- or neurofilament 200 kDa (NF200)-containing DRG neurons. Both single and repetitive intrathecal applications of HC-067047 (400 ng/kg) significantly alleviated mechanical allodynia in the rats with DMA, whereas a single application of GSK1016790A (200 ng/kg) aggravated mechanical allodynia. The present data suggest that TRPV4 undergoes expression changes that are associated with mechanical hypersensitivity in diabetic rats. TRPV4 may be a new molecular target for developing a clinical strategy to treat this intractable neuropathic pain.

Development and persistence of neuropathic pain through microglial activation and KCC2 decreasing after mouse tibial nerve injury

Gamma-amino butyric acid (GABA) is an inhibitory neurotransmitter in the mature brain, but is excitatory during development and after motor nerve injury. This difference in GABAergic action depends on the intracellular chloride ion concentration ([Cl-]i), primarily regulated by potassium chloride co-transporter 2 (KCC2). To reveal precise processes of the neuropathic pain through changes in GABAergic action, we prepared tibial nerve ligation and severance models using male mice, and examined temporal relationships amongst changes in (1) the mechanical withdrawal threshold in the sural nerve area, (2) localization of the molecules involved in GABAergic transmission and its upstream signaling in the dorsal horn, and (3) histology of the tibial nerve. In the ligation model, tibial nerve degeneration disappeared by day 56, but mechanical allodynia, reduced KCC2 localization, and increased microglia density remained until day 90. Microglia density was higher in the tibial zone than the sural zone before day 21, but this result was inverted after day 28. In contrast, in the severance model, all above changes were detected until day 28, but were simultaneously and significantly recovered by day 90. These results suggested that in male mice, allodynia may be caused by reduced GABAergic synaptic inhibition, resulting from elevated [Cl-]i after the reduction of KCC2 by activated microglia. Furthermore, our results suggested that factors from degenerating nerve terminals may diffuse into the sural zone, whereby they induced the development of allodynia in the sural nerve area, while other factors in the sural zone may mediate persistent allodynia through the same pathway.

PKCγ interneurons, a gateway to pathological pain in the dorsal horn

Chronic pain is a frequent and disabling condition that is significantly maintained by central sensitization, which results in pathological amplification of responses to noxious and innocuous stimuli. As such, mechanical allodynia, or pain in response to a tactile stimulus that does not normally provoke pain, is a cardinal feature of chronic pain. Recent evidence suggests that the dorsal horn excitatory interneurons that express the γ isoform of protein kinase C (PKCγ) play a critical role in the mechanism of mechanical allodynia during chronic pain. Here, we review this evidence as well as the main aspects of the development, anatomy, electrophysiology, inputs, outputs, and pathophysiology of dorsal horn PKCγ neurons. Primary afferent high-threshold neurons transmit the nociceptive message to the dorsal horn of the spinal cord and trigeminal system where it activates second-order nociceptive neurons relaying the information to the brain. In physiological conditions, low-threshold mechanoreceptor inputs activate inhibitory interneurons in the dorsal horn, which may control activation of second-order nociceptive neurons. During chronic pain, low-threshold mechanoreceptor inputs now activate PKCγ neurons that forward the message to second-order nociceptive neurons, turning thus tactile inputs into pain. Several mechanisms may contribute to opening this gate, including disinhibition, activation of local astrocytes, release of diffusible factors such as reactive oxygen species, and alteration of the descending serotoninergic control on PKCγ neurons through 5-HT_2A serotonin receptors. Dorsal horn PKCγ neurons, therefore, appear as a relevant therapeutic target to alleviate mechanical allodynia during chronic pain.