Neuropeptide Y in trigeminal ganglion following chronic constriction injury of the rat infraorbital nerve: is there correlation to somatosensory parameters?

Strategies for measuring non-evoked pain in preclinical models of neuropathic pain: Systematic review

The development of new analgesics for neuropathic pain treatment is crucial. The failure of promising drugs in clinical trials may be related to the over-reliance on reflex-based responses (evoked pain) in preclinical drug testing, which may not fully represent clinical neuropathic pain, characterized by spontaneous non-evoked pain (NEP). Hence, strategies for assessing NEP in preclinical studies emerged. This systematic review identified 443 articles evaluating NEP in neuropathic pain models (mainly traumatic nerve injuries in male rodents). An exponential growth in NEP evaluation was observed, which was assessed using 48 different tests classified in 12 NEP-related outcomes: anxiety, exploration/locomotion, paw lifting, depression, conditioned place preference, gait, autotomy, wellbeing, facial grooming, cognitive impairment, facial pain expressions and vocalizations. Although most of these outcomes showed clear limitations, our analysis suggests that conditioning-associated outcomes, pain-related comorbidities, and gait evaluation may be the most effective strategies. Moreover, a minimal part of the studies evaluated standard analgesics. The greater emphasis on evaluating NEP aligning with clinical pain symptoms may enhance analgesic drug development, improving clinical translation.

The use of medicaments in the management of symptomatic irreversible pulpitis: A community-based cohort study

AIM

To investigate patient outcomes from either pulpotomy or pulpectomy for the management of symptomatic irreversible pulpitis, with and without application of antibiotic/corticosteroid pastes in urgent primary dental care settings in the United Kingdom.

METHODOLOGY

All patients receiving intervention for symptomatic irreversible pulpitis in three different primary care settings were invited to participate. Pre-operatively, data regarding patients' numerical ratings scale (NRS), pain score (0-10), analgesic use, oral-health impact profile-14 (OHIP-14) and need for time away from work were collected. For 7 days post-operatively, participants recorded their NRS pain score, global rating of change score, medication use and their ability to work. Analysis used a mixed-effects model with post hoc Tukey's multiple comparisons test for continuous data and chi-squared or Fisher's exact test for categorical data. To test the effect of the corticosteroid/antibiotic paste, pulpectomy and pulpotomy groups were combined following Mantel-Haenszel stratified analysis or a weighted average of the difference between pulpotomy and pulpectomy with and without the use of corticosteroid/antibiotic paste. A binary composite score was constructed using pre- and post-operative data, whereby overall treatment success was defined as: (i) patients did not return for treatment due to pain by day seven; (ii) at day three, there was a 33% (or 2-points) reduction in NRS pain score; (iii) there was a change score of +3 in global rating; (iv) the patient was no longer using analgesia and able to return to work.

RESULTS

Eighty-five participants were recruited, with 83 completing follow up. Overall treatment success was 57%, with 25% of participants returning for more treatment due to inadequate pain relief. Overall treatment success did not differ between the two groups (p = .645), although patients self-reported greater improvement with an antibiotic/corticosteroid dressing for global rating of change (p = .015).

CONCLUSIONS

This study identified limited evidence of improved outcomes using antibiotic/corticosteroid dressings in the management of symptomatic irreversible pulpitis in the emergency setting. Further clinical research is needed to understand if these medications are beneficial in affording pain relief, above that of simple excision of irreversibly inflamed pulp tissue.

Description of Novel Molecular Factors in Lumbar DRGs and Spinal Cord Factors Underlying Development of Neuropathic Pain Component in the Animal Model of Osteoarthritis

Osteoarthritis (OA) is one of the most common joint disorder, with pain accompanied by functional impairment, as the most pronounced clinical symptom. Currently used pharmacotherapy involves symptomatic treatment that do not always provide adequate pain relief. This may be due to concomitance of central sensitization and development of neuropathic features in OA patients. Here we performed studies in the animal model of OA to investigate of the neuropathic component. Intraarticular injection of monoiodoacetate (MIA, 1 mg) was used to induce OA in Wistar male rats. Development of pain phenotype was assessed by behavioral testing (PAM test and von Frey's test), while corresponding changes in dorsal root ganglia (DRGs L3-L5) and spinal cord (SC) gene expression were assessed by means of qRT-PCR technique. We also performed microtomography of OA-affected knee joints to correlate the level of bone degradation with observed behavioral and molecular changes. We observed gradually developing remote allodynia after MIA treatment, indicating the presence of neuropathic component. Our results showed that, among DRGs innervating knee joint, development of central sensitization is most likely due to peripheral input of stimuli through DRG L5. In SC, development of secondary hypersensitivity correlated with increased expression of TAC1 and NPY. Our studies provided molecular records on abnormal activation of pain transmission markers in DRG and SC during development of OA that are responsible for the manifestation of neuropathic features. The obtained results increase insight into molecular changes occurring in the neuronal tissue during OA development and may contribute to readdressing treatment paradigms.

Effects of intra-nasal melanocortin-4 receptor antagonist on trigeminal neuropathic pain in male and female rats

Treatment of chronic orofacial pain remains a major therapeutic challenge despite available medications. Melanocortins have been implicated in pathologic pain. Intrathecal administration of MC4R antagonists has been shown to alleviate neuropathic pain (NP) in male rats. However, intrathecal delivery is very invasive and requires surgeon's intervention. Intra-nasal rout offers a non-invasive drug delivery method that can be self-administered making it very attractive clinically. In this study, we investigated the effects of intra-nasally delivered MC4R antagonist (HS014) on trigeminal neuropathic pain (TNP) in male and female rats. We also measured the MC4R protein levels in the trigeminal ganglia (TG) and infraorbital nerve (ION) of rats. We used ION chronic constriction injury (ION-CCI) to induce TNP in rats. We used von Frey and pinprick assays to measure the development of hypersensitivity in the face following ION-CCI. At 22 days post-ION-CCI, we delivered HS014 intra-nasally to measure its effects on TNP in rats. We used enzyme linked immunosorbent assay to measure MC4R protein levels in the TG and ION. ION-CCI resulted in a significant increase of MC4R protein levels in the ipsilateral TG and ION of male and female rats. Intra-nasal delivered HS014 resulted in a significant reduction of ION-CCI induced hypersensitivity in male and female rats. These results demonstrate that intranasal delivery of MC4R antagonist alleviated TNP in male and female rats and suggest that such treatment could be beneficial therapeutically for individuals with chronic NP.

An Algorithmic Approach to the Treatment of Kienböck Disease

The algorithm and rationale described is a reflection of our own surgical experience for this challenging disorder and can be compared with other publications. Our algorithm has evolved from treatment of a large volume of patients with Kienböck disease in a referral practice. However, it is limited to the management that we have found logical, effective, and within our scope of experience. The treatment guidelines for our specialty as a whole will evolve as our understanding of the etiology and our ability to quantify efficacy improves.

Chemogenetic inhibition of trigeminal ganglion neurons attenuates behavioural and neural pain responses in a model of trigeminal neuropathic pain

BACKGROUND

Nerve injury can lead to ectopic activation of injured nociceptorsand central sensitization characterized by allodynia and hyperalgesia. Reduction in the activity of primary afferent neurons has been shown to be sufficient in alleviating peripherally generated pain. The cell bodies of such trigeminal nociceptors are located in the trigeminal ganglia (TG) with central processes that terminate in the brainstem trigeminal nucleus caudalis (TNC). The TG is therefore a strategic locus where afferent input can be manipulated. We hypothesized that chemogenetic inhibition of TG would suppress TNC neuronal activity and attenuate pain behaviour in a rat model of painful traumatic trigeminal neuropathy (PTTN).

METHODS

Trigeminal neuropathic pain was induced in adult male Sprague-Dawley rats (n = 24) via chronic constriction injury to the infraorbital nerve (ION-CCI). Naïve and sham rats were used as controls (n = 20/group). Rats within each group received TG-directed microinjections of AAV virus containing either the inhibitory hM4Di-DREADD construct or EGFP.

RESULTS

In the ION-CCI group, systemic administration of the DREADD agonist clozapine N-oxide (CNO) reversed the hypersensitivity phenotype in animals expressing hM4Di but not EGFP. CNO-mediated activation of hM4Di DREADD in ION-CCI animals was also associated with reduced Fos expression in the TNC elicited by repeated mechanical stimulation of the dermatome ipsilateral to the injury. There was no effect of CNO on pain behaviour or TNC Fos expression in eGFP animals.

CONCLUSION

Our results indicate that DREADDs may offer an effective therapeutic approach for treatment of trigeminal neuropathic pain.

SIGNIFICANCE

Trigeminal neuropathic pain is highly resistant to therapy and we are in dire need of novel approaches. This study provides further evidence for the successful application of DREADDs as an effective tool for modulating central nervous system function. CNO mediated activation of hM4Di-DREADDs in the trigeminal ganglion (TG) attenuates nerve injury induced neuropathic pain by acting on hyperactive TG cells. It also establishes the TG as an effective target to manage pain in the face and head. Accessing the TG in clinical populations is a relatively simple and safe procedure, making this approach highly significant. Moreover, the methodology described here has applications in trigeminal neuropathic pain from traumatic other etiologies and in spinal neuropathic pain. Chronic pain syndromes are characterized by a progressive failure of brain centers to adequately inhibit pain and as these are identified, we may be able to target them for therapy. Therefore, our findings might have wide application in chronic pain syndromes.

Neuropeptide Y: a potential theranostic biomarker for diabetic peripheral neuropathy in patients with type-2 diabetes

Background:

Diabetic peripheral neuropathy (DPN), the most common microvascular complication of type-2 diabetes mellitus (T2DM), results in nontraumatic lower-limb amputations. When DPN is not detected early, disease progression is irreversible. Thus, biomarkers for diagnosing DPN are needed.

Methods:

We analyzed three data sets of T2DM DPN: two for mouse models (GSE70852 and GSE34889) and one for a human model (GSE24290). We found common differentially expressed genes (DEGs) in the two mouse data sets and validated them in the human data set. To identify the phenotypic function of the DEGs, we overexpressed them in zebrafish embryos. Clinical information and serum samples of T2DM patients with and without DPN were obtained from the Korea Biobank Network. To assess the plausibility of DEGs as biomarkers of DPN, we performed an enzyme-linked immunosorbent assay.

Results:

Among the DEGs, only NPY and SLPI were validated in the human data set. As npy is conserved in zebrafish, its mRNA was injected into zebrafish embryos, and it was observed that the branches of the central nervous system became thicker and the number of dendritic branches increased. Baseline characteristics between T2DM patients with and without DPN did not differ, except for the sex ratio. The mean serum NPY level was higher in T2DM patients with DPN than in those without DPN (p = 0.0328), whereas serum SLPI levels did not differ (p = 0.9651).

Conclusion:

In the pathogenesis of DPN, NPY may play a protective role in the peripheral nervous system and may be useful as a biomarker for detecting T2DM DPN.

The Persistent Pain Transcriptome: Identification of Cells and Molecules Activated by Hyperalgesia

During persistent pain, the dorsal spinal cord responds to painful inputs from the site of injury, but the molecular modulatory processes have not been comprehensively examined. Using transcriptomics and multiplex in situ hybridization, we identified the most highly regulated receptors and signaling molecules in rat dorsal spinal cord in peripheral inflammatory and post-surgical incisional pain models. We examined a time course of the response including acute (2 hours) and longer term (2 day) time points after peripheral injury representing the early onset and instantiation of hyperalgesic processes. From this analysis, we identify a key population of superficial dorsal spinal cord neurons marked by somatotopic upregulation of the opioid neuropeptide precursor prodynorphin, and 2 receptors: the neurokinin 1 receptor, and anaplastic lymphoma kinase. These alterations occur specifically in the glutamatergic subpopulation of superficial dynorphinergic neurons. In addition to specific neuronal gene regulation, both models showed induction of broad transcriptional signatures for tissue remodeling, synaptic rearrangement, and immune signaling defined by complement and interferon induction. These signatures were predominantly induced ipsilateral to tissue injury, implying linkage to primary afferent drive. We present a comprehensive set of gene regulatory events across 2 models that can be targeted for the development of non-opioid analgesics. PERSPECTIVE: The deadly impact of the opioid crisis and the need to replace morphine and other opioids in clinical practice is well recognized. Embedded within this research is an overarching goal of obtaining foundational knowledge from transcriptomics to search for non-opioid analgesic targets. Developing such analgesics would address unmet clinical needs.

Exploring the roles of neuropeptides in trigeminal neuropathic pain: A systematic review and narrative synthesis of animal studies

OBJECTIVE

This systematic review aims to explore the changes in expression of neuropeptides and/or their receptors following experimental trigeminal neuropathic pain in animals.

DESIGN

MEDLINE, Embase, and Scopus were searched for publications up to 31st March 2021. Study selection and data extraction were completed by two independent reviewers based on the eligibility criteria. The quality of articles was judged based on the Systematic Review Centre for Laboratory Animal Experimentation (SYRCLE) risk-of-bias tool.

RESULTS

A total of 19 studies satisfied the eligibility criteria and were included for narrative synthesis. Methods of trigeminal neuropathic pain induction were nerve ligation, nerve compression/crush, nerve transection and dental pulp injury. Animal behaviours used for pain verification were evoked responses to mechanical and thermal stimuli. Non-evoked behaviours, including vertical exploration, grooming and food consumption, were also employed in some studies. Calcitonin gene-related peptide (CGRP) and substance P were the most frequently reported neuropeptides. Overall, unclear to high risk of bias was identified in the included studies.

CONCLUSIONS

Limited evidence has suggested the pro-nociceptive role of CGRP in trigeminal neuropathic pain. In order to further translational pain research, animal models of trigeminal neuropathic pain and pain validation methods need to be optimised. Complete reporting of future studies based on available guidelines to improve confidence in research is encouraged.

Sigma-1 receptor modulates neuroinflammation associated with mechanical hypersensitivity and opioid tolerance in a mouse model of osteoarthritis pain

BACKGROUND AND PURPOSE

Osteoarthritic pain is a chronic disabling condition lacking effective treatment. Continuous use of opioid drugs during osteoarthritic pain induces tolerance and may result in dose escalation and abuse. Sigma-1 (σ1) receptors, a chaperone expressed in key areas for pain control, modulates μ-opioid receptor activity and represents a promising target to tackle these problems. The present study investigates the efficacy of the σ1 receptor antagonist E-52862 to inhibit pain sensitization, morphine tolerance, and associated electrophysiological and molecular changes in a murine model of osteoarthritic pain.

EXPERIMENTAL APPROACH

Mice received an intra-knee injection of monoiodoacetate followed by 14-day treatment with E-52862, morphine, or vehicle, and mechanical sensitivity was assessed before and after the daily doses.

KEY RESULTS

Monoiodoacetate-injected mice developed persistent mechanical hypersensitivity, which was dose-dependently inhibited by E-52862. Mechanical thresholds assessed before the daily E-52862 dose showed gradual recovery, reaching complete restoration by the end of the treatment. When repeated treatment started 15 days after knee injury, E-52862 produced enhanced short-term analgesia, but recovery to baseline threshold was slower. Both a σ1 receptor agonist and a μ receptor antagonist blocked the analgesic effects of E-52862. An acute, sub-effective dose of E-52862 restored morphine analgesia in opioid-tolerant mice. Moreover, E-52862 abolished spinal sensitization in osteoarthritic mice and inhibited pain-related molecular changes.

CONCLUSION AND IMPLICATIONS

These findings show dual effects of σ1 receptor antagonism alleviating both short- and long-lasting antinociception during chronic osteoarthritis pain. They identify E-52862 as a promising pharmacological agent to treat chronic pain and avoid opioid tolerance.

Differential gene expression in trigeminal ganglia of male and female rats following chronic constriction of the infraorbital nerve

BACKGROUND

The mechanisms underlying sex-based differences in pain and analgesia are poorly understood. In this study, we investigated gene expression changes in trigeminal ganglia (TG) of male and female rats exposed to infraorbital nerve chronic constriction injury (IoN-CCI).

METHODS

Somatosensory assessments were performed prior to IoN-CCI and at selected time points postsurgery. Selected gene expression changes were examined with real-time quantitative polymerase chain reaction (RT-PCR) in ipsilateral TG at 21 days postsurgery.

RESULTS

Rats exposed to IoN-CCI developed significant mechanical allodynia and hyperalgesia on days 19 and 21 postsurgery. During this period, females developed significantly more allodynia but not hyperalgesia compared to males. At 21 days postsurgery, expression levels of 44 of the 84 investigated pain-related genes in ipsilateral TG were significantly regulated relative to naïve rats in either sex. Csf1 and Cx3cr1 were up-regulated in both sexes, but the magnitude of regulation was significantly higher in females (p = 0.02 and p = 0.001, respectively). Htr1a and Scn9a were down-regulated in both sexes, but the down-regulation was significantly more pronounced in males (p = 0.04 and p = 0.02, respectively). Additionally, Cck, Il1a, Pla2g1b and Tnf genes were significantly regulated in females but not in males, and Chrna4 gene was significantly down-regulated in males but not in females.

CONCLUSIONS

Our findings suggest sex-dependent gene regulation in response to nerve injury, which may contribute to sex dimorphism of trigeminal neuropathic pain. Further studies are needed to establish gene expression changes over time and correlate these with hormonal and other physiological parameters in male and female.

SIGNIFICANCE

We present novel sex-specific transcriptional regulation in trigeminal ganglia that may contribute to male-/female-based differences in trigeminal neuropathic pain. These findings are expected to open new research horizons, particularly in male versus female targeted therapeutic regimens.

Diabetes mellitus influences the expression of NPY and VEGF in neurons of rat trigeminal ganglion

BACKGROUND

Diabetes mellitus (DM) influences the trigeminal nerve function by changing the pain response and transduction of the orofacial sensory pathways. It affects the inflammatory response via neuropeptide Y (NPY) and vascular endothelial growth factor (VEGF), which could potentially have a relevant role in the pathophysiology of diabetic neuropathy. The aim was to investigate expression of VEGF and NPY in subpopulations of trigeminal ganglion (TG) neurons in rat models of early DM1 and DM2.

METHODS

DM1 model was induced by an intraperitoneal (i.p.) injection of streptozotocin (STZ) (55mg/kg). DM2 rats were fed with a high fat diet (HFD) for two weeks and then received 35mg/kg of STZ i.p. Two weeks and 2months after the STZ-diabetes induction, rats were sacrificed and TG was immunohistochemically analyzed for detection of VEGF and NPY expression, and also double immunofluorescence labeling with isolectin (IB4) was completed.

RESULTS

An increased percentage of NPY+ neurons was observed 2weeks after DM1 and 2months post DM2 induction. NPY immunoreactivity was restricted to IB4-negative small-diameter and IB4+ neurons. Two weeks post induction, DM1 rats showed an increased percentage of VEGF/IB4- large neurons and DM2 rats showed an increased percentage of VEGF/IB4+ neurons. Two months after DM induction, the DM1 group showed a reduced percentage of VEGF/IB4- small neurons.

CONCLUSION

The observed changes may play a critical role in the modulation of nociceptor activity and plasticity of primary sensory trigeminal neurons. The results contribute to the understanding of the basic pathophysiology of trigeminal diabetic neuropathy.

Perspectives on next steps in classification of oro-facial pain - Part 3: biomarkers of chronic oro-facial pain - from research to clinic

The purpose of this study was to review the current status of biomarkers used in oro-facial pain conditions. Specifically, we critically appraise their relative strengths and weaknesses for assessing mechanisms associated with the oro-facial pain conditions and interpret that information in the light of their current value for use in diagnosis. In the third section, we explore biomarkers through the perspective of ontological realism. We discuss ontological problems of biomarkers as currently widely conceptualised and implemented. This leads to recommendations for research practice aimed to a better understanding of the potential contribution that biomarkers might make to oro-facial pain diagnosis and thereby fulfil our goal for an expanded multidimensional framework for oro-facial pain conditions that would include a third axis.

Novel expression pattern of neuropeptide Y immunoreactivity in the peripheral nervous system in a rat model of neuropathic pain

Background

Neuropeptide Y (NPY) has been implicated in the modulation of pain. Under normal conditions, NPY is found in interneurons in the dorsal horn of the spinal cord and in sympathetic postganglionic neurons but is absent from the cell bodies of sensory neurons. Following peripheral nerve injury NPY is dramatically upregulated in the sensory ganglia. How NPY expression is altered in the peripheral nervous system, distal to a site of nerve lesion, remains unknown. To address this question, NPY expression was investigated using immunohistochemistry at the level of the trigeminal ganglion, the mental nerve and in the skin of the lower lip in relation to markers of sensory and sympathetic fibers in a rat model of trigeminal neuropathic pain.

Results

At 2 and 6 weeks after chronic constriction injury (CCI) of the mental nerve, de novo expression of NPY was seen in the trigeminal ganglia, in axons in the mental nerve, and in fibers in the upper dermis of the skin. In lesioned animals, NPY immunoreactivity was expressed primarily by large diameter mental nerve sensory neurons retrogradely labelled with Fluorogold. Many axons transported this de novo NPY to the periphery as NPY-immunoreactive (IR) fibers were seen in the mental nerve both proximal and distal to the CCI. Some of these NPY-IR axons co-expressed Neurofilament 200 (NF200), a marker for myelinated sensory fibers, and occasionally colocalization was seen in their terminals in the skin. Peptidergic and non-peptidergic C fibers expressing calcitonin gene-related peptide (CGRP) or binding isolectin B4 (IB4), respectively, never expressed NPY. CCI caused a significant de novo sprouting of sympathetic fibers into the upper dermis of the skin, and most, but not all of these fibers, expressed NPY.

Conclusions

This is the first study to provide a comprehensive description of changes in NPY expression in the periphery after nerve injury. Novel expression of NPY in the skin comes mostly from sprouted sympathetic fibers. This information is fundamental in order to understand where endogenous NPY is expressed, and how it might be acting to modulate pain in the periphery.

Inhibition of SNL-induced upregulation of CGRP and NPY in the spinal cord and dorsal root ganglia by the 5-HT(2A) receptor antagonist ketanserin in rats

Our previous study has demonstrated that topical and systemic administration of the 5-HT(2A) receptor antagonist ketanserin attenuates neuropathic pain. To explore the mechanisms involved, we examined whether ketanserin reversed the plasticity changes associated with calcitonin gene-related peptides (CGRP) and neuropeptide Y (NPY) which may reflect distinct mechanisms: involvement and compensatory protection. Behavioral responses to thermal and tactile stimuli after spinal nerve ligation (SNL) at L5 demonstrated neuropathic pain and its attenuation in the vehicle- and ketanserin-treated groups, respectively. SNL surgery induced an increase in CGRP and NPY immunoreactivity (IR) in laminae I-II of the spinal cord. L5 SNL produced an expression of NPY-IR in large, medium and small diameter neurons in dorsal root ganglion (DRG) only at L5, but not adjacent L4 and L6. Daily injection of ketanserin (0.3 mg/kg, s.c.) for two weeks suppressed the increase in CGRP-IR and NPY-IR in the spinal cord or DRG. The present study demonstrated that: (1) the expression of CGRP was enhanced in the spinal dorsal horn and NPY was expressed in the DRG containing injured neurons, but not in the adjacent DRG containing intact neurons, following L5 SNL; (2) the maladaptive changes in CGRP and NPY expression in the spinal cord and DRG mediated the bioactivity of 5-HT/5-HT(2A) receptors in neuropathic pain and (3) the blockade of 5-HT(2A) receptors by ketanserin reversed the evoked upregulation of both CGRP and NPY in the spinal cord and DRG contributing to the inhibition of neuropathic pain.

PKCgamma in Vc and C1/C2 is involved in trigeminal neuropathic pain

The aim of the present study was to clarify the involvement of protein kinase Cγ (PKCγ) in the facial neuropathic pain following infraorbital nerve injury. We analyzed the change in PKCγ expression in the trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord (C1/C2) following chronic constriction injury of the infraorbital nerve (ION-CCI). We also studied ION-CCI-mediated mechanical nocifensive behavior in rats. The mechanical head-withdrawal threshold significantly decreased 1 to 14 days after ION-CCI compared with that before ION-CCI and in sham rats. The expression of PKCγ was significantly larger in the ipsilateral Vc compared with the contralateral side in ION-CCI rats 3, 7, and 14 days after ION-CCI. Intrathecal (i.t.) administration of the PKCγ inhibitor chelerythrine prevented an increase in the PKCγ expression in the ipsilateral Vc. Moreover, i.t. administration of chelerythrine annulled ION-CCI-mediated reduction in the head-withdrawal threshold. Taken together, these findings suggest that PKCγ expression in the Vc played an important role in the mechanism of orofacial static mechanical allodynia following trigeminal nerve injury.

Intracisternal administration of mitogen-activated protein kinase inhibitors reduced mechanical allodynia following chronic constriction injury of infraorbital nerve in rats

The present study investigated the role of mitogen-activated protein kinase (MAPK) in orofacial neuropathic pain following chronic constriction injury of the infraorbital nerve (ION-CCI). Experiments were carried out on male Sprague-Dawley rats weighing between 200 and 230 g. The ION was separated from adhering tissue, and two ligatures (5-0 chromic gut) were tied loosely around it. We examined the air-puff thresholds (mechanical allodynia), scores of pinprick (mechanical hyperalgesia), and face grooming frequency for acetone application (hypersensitivity for cold stimulation) - 3, 3, 6, 9, 12, 15, 20, 25, 30, and 40 days after surgery. ION-CCI produced mechanical allodynia, hyperalgesia, and cold hypersensitivity. We investigated whether administration of MAPKs inhibitors blocks ION-CCI-induced mechanical allodynia. Intracisternal administration with PD98059 or SB203580, a MEK inhibitor or a p38 MAPK inhibitor, respectively, significantly inhibited ION-CCI-induced mechanical allodynia in the orofacial area. These results indicate that the ION-CCI produced behavioral alterations in the orofacial area and those central MAPKs pathways contribute to orofacial neuropathic pain. Our findings suggest that MAPKs inhibitors have a potential role in treatment for orofacial neuropathic pain.

Management of neuropathic orofacial pain

Current management of painful trigeminal neuropathies relies on pharmacological (topical and systemic), surgical, and complementary modalities. There is, however, a lack of quality research relating to the effectiveness of these modalities. In this review we analyze the available data that relates to the therapy of trigeminal neuralgia, postherpetic neuralgia, and posttraumatic neuropathies and provide clinical guidelines. The review focuses on medical management, as well as surgical and other interventions for painful neuropathies.

Spinal mechanisms of NPY analgesia

We review previously published data, and present some new data, indicating that spinal application of neuropeptide Y (NPY) reduces behavioral and neurophysiological signs of acute and chronic pain. In models of acute pain, early behavioral studies showed that spinal (intrathecal) administration of NPY and Y2 receptor agonists decrease thermal nociception. Subsequent neurophysiological studies indicated that Y2-mediated inhibition of excitatory neurotransmitter release from primary afferent terminals in the substantia gelatinosa may contribute to the antinociceptive actions of NPY. As with acute pain, NPY reduced behavioral signs of inflammatory pain such as mechanical allodynia and thermal hyperalgesia; however, receptor antagonist studies indicate an important contribution of spinal Y1 rather than Y2 receptors. Interestingly, Y1 agonists suppress inhibitory synaptic events in dorsal horn neurons (indeed, well known mu-opioid analgesic drugs produce similar cellular actions). To resolve the behavioral and neurophysiological data, we propose that NPY/Y1 inhibits the spinal release of inhibitory neurotransmitters (GABA and glycine) onto inhibitory neurons, e.g. disinhibition of pain inhibition, resulting in hyporeflexia. The above mechanisms of Y1- and Y2-mediated analgesia may also operate in the setting of peripheral nerve injury, and new data indicate that NPY dose-dependently inhibits behavioral signs of neuropathic pain. Indeed, neurophysiological studies indicate that Y2-mediated inhibition of Ca(2+) channel currents in dorsal root ganglion neurons is actually increased after axotomy. We conclude that spinal delivery of Y1 agonists may be of use in the treatment of chronic inflammatory pain, and that the use of Y1 and Y2 agonists in neuropathic pain warrants further consideration.

Neuropeptide Y modulates effects of bradykinin and prostaglandin E2 on trigeminal nociceptors via activation of the Y1 and Y2 receptors

BACKGROUND AND PURPOSE

Although previous studies have demonstrated that neuropeptide Y (NPY) modulates nociceptors, the relative contributions of the Y1 and Y2 receptors are unknown. Therefore, we evaluated the effect of Y1 and Y2 receptor activation on nociceptors stimulated by bradykinin (BK) and prostaglandin E2 (PGE2).

EXPERIMENTAL APPROACH

Combined immunohistochemistry (IHC) with in situ hybridization (ISH) demonstrated that Y1- and Y2-receptors are collocated with bradykinin (2) (B2)-receptors in rat trigeminal ganglia (TG). The relative functions of the Y1 and Y2 receptors in modulating BK/PGE2-evoked CGRP release and increased intracellular calcium levels in cultured TG neurons were evaluated.

KEY RESULTS

The Y1 and Y2 receptors are co-expressed with B2 in TG neurons, suggesting the potential for direct NPY modulation of BK responses. Pretreatment with the Y1 agonist [Leu31,Pro34]-NPY, inhibited BK/PGE2-evoked CGRP release. Conversely, pretreatment with PYY(3-36), a Y2 agonist, increased BK/PGE2 evoked CGRP release. Treatment with NPY evoked an overall inhibitory effect, although of lesser magnitude. Similarly, [Leu31,Pro34]-NPY inhibited BK/PGE2-evoked increases in intracellular calcium levels whereas PYY(3-36) increased responses. NPY inhibition of BK/PGE2-evoked release of CGRP was reversed by the Y1 receptor antagonist, BIBO3304, and higher concentrations of BIBO3304 significantly facilitated CGRP release. The Y2 receptor antagonist, BIIE0246, enhanced the inhibitory NPY effects.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that NPY modulation of peptidergic neurons is due to net activation of inhibitory Y1 and excitatory Y2 receptor systems. The relative expression or activity of these opposing receptor systems may mediate dynamic responses to injury and pain.

Increased sodium channel immunofluorescence at myelinated and demyelinated sites following an inflammatory and partial axotomy lesion of the rat infraorbital nerve

The localization of sodium channels (NaChs) change following nerve lesions and this change may contribute to the development of increased pain states. Here we examine the change in distribution of NaChs within the rat infraorbital nerve (ION) two weeks after a combined inflammatory/partial axotomy lesion that results in behavior showing increased sensitivity to mechanical stimuli. Sections from experimental and normal control IONs were double-stained for indirect immunofluorescence using an antibody that identifies all NaCh isoforms and caspr-antibody to identify nodes of Ranvier, and a confocal microscope z-series of optically sectioned images were then obtained. ImageJ (NIH) software was used to quantify the area of pixels showing maximum NaCh intensity within both caspr and non-caspr associated accumulations. Analysis showed that the lesioned IONs had many more split nodes, heminodes and caspr-negative "naked" accumulations, a significantly increased area of NaCh staining within typical nodes and "naked" accumulations, as well as an increased density and size of significant accumulations when compared to normal IONs. This study demonstrates a dramatic redistribution and increased immunofluorescence of NaChs especially at myelinated and demyelinated sites in fibers located just proximal to the lesion. The remodeling of NaChs seen in this study may represent an important event associated with the development of increased nerve excitability after lesions.

Behavioural responses following tooth injury in rats

Early changes in spontaneous behaviour (exploration, grooming, freezing, rearing, jaw motion, yawning) and body weight were measured at two and three days after pulp exposure injury and implantation of Fluorogold (FG) into molar teeth of rats. Rats with FG and injuries to three teeth gained weight less rapidly, explored less frequently and froze more often than sham-operated rats. Yawning was not observed in any rats prior to surgery and it was seen more frequently in tooth-injured rats than in sham-operated rats. These results suggest that careful observation of spontaneous behaviour after tooth injuries can be used to assess dental pain in rats and may provide behavioural markers to correlate with anatomical changes after injury. The dental nerve cell bodies that had accumulated transported FG were medium to large, and they only co-localized calcitonin gene-related peptide (CGRP) in a subset of the medium neurons. Chromatolytic or moribund FG-labelled neurons were also found.

Effect of a graded single constriction of the rat sciatic nerve on pain behavior and expression of immunoreactive NPY and NPY Y1 receptor in DRG neurons and spinal cord

In the present study, the rat sciatic nerve was constricted to varying degrees using only one ligature with a very thin polyethylene sheath placed between nerve and ligature thread. Complete nerve transection was studied for comparison. With a 40-80% constriction of the nerve we observed allodynia to a similar extent as in the so-called Bennett model based on four loose ligatures. We also monitored changes in the expression of neuropeptide Y (NPY) and the NPY Y1 receptor (Y1R) in the lumbar 4-5 dorsal root ganglia (DRG) and dorsal horn and found upregulation of NPY and downregulation of the Y1R in DRG neurons after injury. These results indicate that similar peptide and receptor changes occur in this model as after axotomy and in other nerve injury models, although the immunohistochemical and behavioral changes seem to be dependent on the degree of constriction of the nerve. Thus, it seems relevant to monitor the degree of constriction when evaluating pain and other post-injury events. The possibility that some of the changes in NPY-ergic neurotransmission are related to the generation of allodynia is discussed; as well as the possibility to use this mononeuropathic model based on a single ligature nerve constriction (SLNC) as a complementary approach to other widely used pain models.

The antimigraine 5-HT 1B/1D receptor agonists, sumatriptan, zolmitriptan and dihydroergotamine, attenuate pain-related behaviour in a rat model of trigeminal neuropathic pain

1. Peripheral lesion to the trigeminal nerve may induce severe pain states. Several lines of evidence have suggested that the antimigraine effect of the triptans with 5-HT(1B/1D) receptor agonist properties may result from inhibition of nociceptive transmission in the spinal nucleus of the trigeminal nerve by these drugs. On this basis, we have assessed the potential antinociceptive effects of sumatriptan and zolmitriptan, compared to dihydroergotamine (DHE), in a rat model of trigeminal neuropathic pain. 2. Chronic constriction injury was produced by two loose ligatures of the infraorbital nerve on the right side. Responsiveness to von Frey filament stimulation of the vibrissal pad was used to evaluate allodynia. 3. Two weeks after ligatures, rats with a chronic constriction of the right infraorbital nerve displayed bilateral mechanical hyper-responsiveness to von Frey filament stimulation of the vibrissal pad with a mean threshold of 0.38+/-0.04 g on the injured side and of 0.43+/-0.04 g on the contralateral (left) side (versus > or =12.5 g on both sides in the same rats prior to nerve constriction injury). 4. Sumatriptan at a clinically relevant dose (100 microg kg(-1), s.c.) led to a significant reduction of the mechanical allodynia-like behaviour on both the injured and the contralateral sides (peak-effects 6.3+/-1.1 g and 4.4+/-0.7 g, respectively). A more pronounced effect was obtained with zolmitriptan (100 microg kg(-1), s.c.) (peak-effects: 7.4+/-0.9 g and 3.2+/-1.3 g) whereas DHE (50-100 microg kg(-1), i.v.) was less active (peak-effect approximately 1.5 g). 5. Subcutaneous pretreatment with the 5-HT(1B/1D) receptor antagonist, GR 127935 (3 mg kg(-1)), prevented the anti-allodynia-like effects of triptans and DHE. Pretreatment with the 5-HT(1A) receptor antagonist, WAY 100635 (2 mg kg(-1), s.c.), did not alter the effect of triptans but significantly enhanced that of DHE (peak effect 4.3+/-0.5 g). 6. In a rat model of peripheral neuropathic pain, which consisted of a unilateral loose constriction of the sciatic nerve, neither sumatriptan (50-300 microg kg(-1)) nor zolmitriptan (50-300 microg kg(-1)) modified the thresholds for paw withdrawal and vocalization in response to noxious mechanical stimulation. 7. These results support the rationale for exploring the clinical efficacy of brain penetrant 5-HT(1B/1D) receptor agonists as analgesics to reduce certain types of trigeminal neuropathic pain in humans.

Application of a pro-inflammatory agent to the orbital portion of the rat infraorbital nerve induces changes indicative of ongoing trigeminal pain

The present experiments investigated the behavioral and immunocytchemical (ICC) effects of applying complete Freund's adjuvant (CFA) to the orbital portion of the infraorbital nerve (IOn). Two control groups, the first had saline applied to the IOn and the second underwent sham operation, were included in the study. In the CFA group, significant hyper-responsiveness to von Frey (analysis of variance <0.05) and to pinprick stimulation (Kruskal Wallis <0.05) in the vibrissal pad was observed on the fourth and the fifth days post-operative (dpo). This was accompanied by a reduced bite force and altered bite patterns of similar duration. Histology of the IOn in CFA rats revealed immune cell infiltration and edema around and in the nerve trunk with only mild axonal damage confirmed by neuropeptide Y immunoreactivity in trigeminal ganglion. Histological areas of inconsistent and mild inflammation were observed in the saline group that were accompanied by similarly attenuated behavioral and ICC changes. This model of inflammation-induced neuropathic pain is highly applicable to the study of neuroinflammatory orofacial pain.

Strain-dependent modification of neuropathic pain behaviour in the rat hindpaw by a priming painful trigeminal nerve injury

The aim of the present study was to test the behavioural effect of infraorbital (IO) chronic constriction injury (CCI) on the development of neuropathic pain in the rat hindpaw following sciatic nerve CCI performed 7 days later. Control groups consisted of rats that underwent sham infraorbital surgery followed by sciatic CCI at identical time points. Sensory testing of the rat's face and hindpaw was performed at baseline and at 4, 11, 14, 17 and 21 days postoperative (dpo) relative to the IOCCI, at which time all rats were euthanized. To test for strain differences the experiment was performed on Sprague-Dawley, Sabra and Lewis rats. In Lewis rats the trigeminal nerve injury significantly accelerated the development of hindpaw mechanoallodynia (11th, 14th and 17th dpo, unpaired t-test, P<0.05) and mechanohyperalgesia (14th and 17th dpo, unpaired t-test, P<0.05), following a second sciatic nerve CCI relative to the control group. This effect was not observed in Sprague-Dawley or Sabra rats.

Antihyperalgesic effects of intrathecal neuropeptide Y during inflammation are mediated by Y1 receptors

Inflammation induces an up-regulation of neuropeptide tyrosine (NPY) and its receptors in the dorsal horn, suggesting an important role in nociceptive transmission. Our initial studies revealed that NPY dose-dependently increased hotplate response latency, and to a lesser degree, thermal paw withdrawal latency (PWL); these effects occurred at doses that affect neither motor coordination (as assessed by the rotarod test) nor paw skin temperature. We next evaluated the behavioral effects of intrathecal administration of NPY and NPY antagonists with the aim of assessing the contribution of NPY to correlates of persistent nociception associated with the unilateral plantar injection of carrageenan or complete Freund's adjuvant (CFA). NPY robustly and dose-dependently increased PWL on the side ipsilateral to carrageenan injection, with only a small effect on the contralateral side. Similarly, NPY (30 microg) produced a large and long-lasting increase in PWL on the side ipsilateral to CFA injection (140% change), with only a small effect on the contralateral side (25% change). The ipsilateral effect of NPY was completely inhibited with the potent Y1 antagonist, BIBO 3304 (3 microg), but not the Y2 antagonist, BIIE 0246. When administered alone, BIBO 3304 (but not BIIE 0246) slightly decreased thermal PWL on the side ipsilateral (25% change), but not contralateral, to CFA injection; this suggests that inflammation strengthens inhibitory NPY tone. We conclude that spinal Y1 receptors contribute to the inhibitory effects of NPY on thermal hypersensitivity in the awake rat. Further studies are necessary to determine whether enhanced release of NPY and Y1-mediated inhibition of spinal nociceptive transmission ultimately results in a compensatory, adaptive inhibition of thermal hypersensitivity in the setting of inflammation.