The role of sensory fiber demography in trigeminal and postherpetic neuralgias

Fibroblast-derived IL-33 exacerbates orofacial neuropathic pain via the activation of TRPA1 in trigeminal ganglion neurons

Damage to the peripheral nerves of trigeminal ganglion (TG) neurons leads to intractable orofacial neuropathic pain through the induction of neuroinflammation. However, the details of this process are not yet fully understood. Here, we found that fibroblast-derived interleukin (IL)-33 was required for the development of mechanical allodynia in whisker pad skin following infraorbital nerve injury (IONI). The amount of IL-33 in the TG increased after IONI when the mice exhibited mechanical allodynia. Neutralization of IL-33 in the TG inhibited the development of IONI-induced mechanical allodynia. Conversely, intra-TG administration of recombinant human IL-33 (rhIL-33) elicited mechanical allodynia in naïve mice. IL-33 and its receptor were exclusively expressed in fibroblasts and neurons, respectively, in the TG. Fibroblast ablation caused the loss of IL-33 in the TG and delayed the development of mechanical allodynia after IONI. rhIL-33 elicited an increase in intracellular Ca2+ concentration and subsequent enhancement of Ca2+ influx via transient receptor potential ankyrin 1 (TRPA1) in primary cultured TG neurons. Additionally, rhIL-33 facilitated membrane translocation of TRPA1 in the TG. Mechanical allodynia caused by intra-TG administration of rhIL-33 was significantly inhibited by pharmacological blockade or gene silencing of TRPA1 in the TG. Inhibition of protein kinase A abrogated TRPA1 membrane translocation and delayed mechanical allodynia after IONI. Substance P stimulation caused upregulation of IL-33 expression in primary cultured fibroblasts. Preemptive administration of a neurokinin-1 receptor antagonist in the TG attenuated mechanical allodynia and IL-33 expression following IONI. Taken together, these results indicate that fibroblast-derived IL-33 exacerbates TG neuronal excitability via suppression of tumorigenicity 2 (ST2)-TRPA1 signaling, ultimately leading to orofacial neuropathic pain.

Trigeminal Postherpetic Neuralgia: From Pathophysiology to Treatment

PURPOSE OF REVIEW

Trigeminal postherpetic neuralgia (TG-PHN) is a neuropathic pain condition complicating herpes zoster (HZ) attributed to the trigeminal nerve. It poses significant challenges due to its persistent and debilitating nature. This review explores the clinical characteristics of TG-PHN, analyzes its pathophysiological underpinnings, and addresses existent and potential therapies.

RECENT FINDINGS

TG-PHN is one of the most common and complex PHN locations. It has distinguishing clinical and pathophysiological characteristics, starting with viral triggered injuries to the trigeminal ganglion (TG) and peripheral tissue and involving the ascending and descending brain modulation pathways. Current therapies include vaccines, oral and topical medications, and interventional approaches, like nerve blocks and neurostimulation. This review covers TG-PHN's clinical and physiological components, treatment options, and potential future targets for improved management. By exploring the complexities of this condition, we aim to contribute to developing more effective and targeted therapies for patients suffering from trigeminal PHN.

Neuralgia in the occipital region associated with ipsilateral trigeminal herpes zoster: Three case reports

BACKGROUND

Nerve fibers related to pain and temperature sensation in the trigeminal nerve territory converge with the upper cervical spinal nerves from the level of the lower medulla oblongata to the upper cervical cord. This structure is called the trigemino-cervical complex and may cause referred pain in the territory of the trigeminal or upper cervical spinal nerves.

CASE SERIES

Here, we report three cases of paroxysmal neuralgia in the occipital region with mild conjunctivitis or a few reddish spots in the ipsilateral trigeminal nerve territory. The patients exhibited gradual progression of these reddish spots evolving into vesicles over the course of several days, despite the absence of a rash in the occipital region. The patients were diagnosed with trigeminal herpes zoster and subsequently received antiherpetic therapy. Remarkably, the neuralgia in the occipital region showed gradual amelioration or complete resolution before the treatment, with no sequelae reported in the occipital region.

DISCUSSION

The trigemino-cervical complex has the potential to cause neuralgia in the occipital region, as referred pain, caused by trigeminal herpes zoster. These cases suggest that, even if conjunctivitis or reddish spots appear to be trivial in the trigeminal nerve territory, trigeminal herpes zoster should be considered when neuralgia occurs in the ipsilateral occipital region.

Transplantation of olfactory ensheathing cells can alleviate neuroinflammatory responses in rats with trigeminal neuralgia

Trigeminal neuralgia (TN) is a common form of facial pain, which primarily manifests as severe pain similar to facial acupuncture and electric shock. Olfactory ensheathing cells (OECs) are glial cells with high bioactivity; these cells are essential for the periodic regeneration of the olfactory nerve and have been utilized for the repair of nerve injuries. A member of the P2X receptor family, P2X7R, is an ion channel type receptor that has been confirmed to participate in various pain response processes. In this study, we transplanted OECs into trigeminal nerve-model rats with distal infraorbital nerve ligation to observe the therapeutic effect of transplanted OECs in rats. Additionally, we utilized the P2X7R-specific inhibitor brilliant blue G (BBG) to study the therapeutic mechanisms of cell transplantation. The facial mechanical pain threshold of these rats significantly increased following cell transplantation. The immunohistochemistry, immunoblotting, and RT-qPCR results demonstrated that the levels of P2X7R, (NOD)-like receptor protein-3 (NLRP3), nuclear factor-κB (NF-κB), interleukin (IL)-1β, and IL-18 in the trigeminal ganglion of rats treated with OEC transplantation or BBG treatment were significantly lower than those in the injured group without treatment. Overall, our results demonstrate that OEC transplantation can alleviate TN in rats, and it can reduce the expression of P2X7R related inflammatory factors in TN rats, reducing neuroinflammatory response in TG.

Predictive value of preoperative magnetic resonance imaging structural and diffusion indices for the results of trigeminal neuralgia microvascular decompression surgery

PURPOSE

To explore the predictive value of preoperative magnetic resonance imaging structural and diffusion indices of the spinal trigeminal tract (SpTV) on the results of microvascular decompression (MVD) in patients with trigeminal neuralgia (TN).

METHODS

This retrospective study included patients diagnosed with TN and treated with MVD in the Jining First People's Hospital between January 2020 and January 2021. The patients were divided into good and poor results groups according to postoperative pain relief. Logistic regression analysis was performed to explore independent risk factors for poor results of MVD, and their predictive value was examined using receiver operating characteristic (ROC) curves.

RESULTS

A total of 97 TN cases were included, 24 cases with a poor result and 73 with a good result. They were comparable in demographic characteristics. Fractional anisotropy (FA) was lower (P < 0.001), and radial diffusivity (RD) was higher (P < 0.001) in the poor result group compared to the good result group. Patients in the good result group showed a higher proportion of grade 3 neurovascular contact (NVC) (39.7% vs. 16.7%, P = 0.001) and a lower RD (P < 0.001). The multivariate analysis showed that the RD of SpTV (OR = 0.000016, 95% CI: 0.000-0.004, P < 0.001) and NVC (OR = 8.07, 95% CI: 1.67-38.93, P = 0.009) were independently associated with poor results. The area under the curve (AUC) of RD and NVC were 0.848 and 0.710, and their combination achieved an AUC of 0.880.

CONCLUSION

NVC and RD of SpTV are independent risk factors for poor results after MVD surgery, and combining the NVC and RD might achieve relatively high predictive value for poor results.

Characterization of trigeminal C-fiber reactivity through capsaicin-induced release of calcitonin gene-related peptide

OBJECTIVE

We hypothesized that the response of trigeminal dermal blood flow (DBF) in the trigeminal system and consecutive expansion of flare response to capsaicin would differ from the somatosensory system (arm). We also investigated whether there are differences between patients with migraine and healthy controls (HC).

BACKGROUND

Functional differences between the trigeminal and extracephalic somatosensory systems may partly explain the susceptibility for headaches in patients with migraine. Capsaicin-induced activation of nociceptive C-fibers in the skin is mainly mediated by calcitonin gene-related peptide (CGRP) and induces cutaneous vessel dilatation and flare response.

METHODS

Female patients with migraine (n = 38) and age-matched HC (n = 35) underwent DBF measurement at baseline and after topical capsaicin administration using laser speckle imaging. DBF before and after capsaicin stimulation was analyzed over ophthalmic nerve/maxillary nerve/mandibular nerve (V1/V2/V3) dermatomes and the forearm as an extracephalic control.

RESULTS

Capsaicin-induced DBF increased more in the trigeminal dermatomes than on the forearm. The V1 dermatome showed a smaller increase of DBF in patients with migraine compared to HC.

CONCLUSION

Our results suggest that the trigeminovascular system reacts differently from extracephalic areas, which may explain the trigeminal susceptibility to CGRP-mediated pain attacks. By demonstrating a different reactivity of the V1 dermatome in patients with migraine, our finding suggests that the first trigeminal branch is functionally different from the second and third branches; however, only in patients with migraine.

PAQR8 and PAQR9 expression is altered in the ventral tegmental area of aged rats infected with varicella zoster virus

Infection with varicella zoster virus (VZV) results in chicken pox and reactivation of VZV results in herpes zoster (HZ) or what is often referred to as shingles. Patients with HZ experience decreased motivation and increased emotional distress consistent with functions of the ventral tegmental area (VTA) of the brain. In addition, activity within the ventral tegmental area is altered in patients with HZ. HZ primarily affects individuals that are older and the VTA changes with age. To begin to determine if the VTA has a role in HZ symptoms, a screen of 10,000 genes within the VTA in young and old male rats was completed after injecting the whisker pad with VZV. The two genes that had maximal change were membrane progesterone receptors PAQR8 (mPRβ) and PAQR9 (mPRε). Neurons and non-neuronal cells expressed both PAQR8 and PAQR9. PAQR8 and PAQR9 protein expression was significantly reduced after VZV injection of young males. In old rats PAQR9 protein expression was significantly increased after VZV injection and PAQR9 protein expression was reduced in aged male rats versus young rats. Consistent with previous results, pain significantly increased after VZV injection of the whisker pad and aged animals showed significantly more pain than young animals. Our data suggests that PAQR8 and PAQR9 expression is altered by VZV injection and that these changes are affected by age.

Trigeminal Neuralgia

Trigeminal neuralgia (TN) is a rare neuropathic pain disorder characterized by recurrent, paroxysmal episodes of short-lasting severe electric shock-like pain along the sensory distribution of the trigeminal nerve. Recent classification systems group TN into 3 main categories depending on the underlying pathophysiology. This article will present a case history and review the epidemiology, diagnostic criteria, classification, clinical features, diagnostic investigations, pathophysiology, and management of TN.

Trigeminal ganglion itself can be a viable target to manage trigeminal neuralgia

Excruciating trigeminal neuralgia (TN) management is very difficult and severely affects the patient's quality of life. Earlier studies have shown that the trigeminal ganglion (TG) comprises several receptors and signal molecules that are involved in the process of peripheral sensitization, which influences the development and persistence of neuropathic pain. Targeting TG can modulate this sensitization pathway and mediate the pain-relieving effect. So far,there are few studies in which modulation approaches to TG itself have been suggested so far. "Trigeminal ganglion modulation" and "trigeminal neuralgia" were used as search phrases in the Scopus Index and PubMed databases to discover articles that were pertinent to the topic. In this review, we address the role of the trigeminal ganglion in TN and underlying molecules and neuropeptides implicated in trigeminal pain pathways in processing pathological orofacial pain. We also reviewed different modulation approaches in TG for TN management. Furthermore, we discuss the prospect of targeting trigeminal ganglion to manage such intractable pain.

Repetitive nociceptive stimulation increases spontaneous neural activation similar to nociception-induced activity in mouse insular cortex

Recent noninvasive neuroimaging technology has revealed that spatiotemporal patterns of cortical spontaneous activity observed in chronic pain patients are different from those in healthy subjects, suggesting that the spontaneous cortical activity plays a key role in the induction and/or maintenance of chronic pain. However, the mechanisms of the spontaneously emerging activities supposed to be induced by nociceptive inputs remain to be established. In the present study, we investigated spontaneous cortical activities in sessions before and after electrical stimulation of the periodontal ligament (PDL) by applying wide-field and two-photon calcium imaging to anesthetized GCaMP6s transgenic mice. First, we identified the sequential cortical activation patterns from the primary somatosensory and secondary somatosensory cortices to the insular cortex (IC) by PDL stimulation. We, then found that spontaneous IC activities that exhibited a similar spatiotemporal cortical pattern to evoked activities by PDL stimulation increased in the session after repetitive PDL stimulation. At the single-cell level, repetitive PDL stimulation augmented the synchronous neuronal activity. These results suggest that cortical plasticity induced by the repetitive stimulation leads to the frequent PDL stimulation-evoked-like spontaneous IC activation. This nociception-induced spontaneous activity in IC may be a part of mechanisms that induces chronic pain.

Sumatriptan prevents central sensitisation specifically in the trigeminal dermatome in humans

BACKGROUND

The exact mechanism and site of action of triptans in aborting migraine attacks remain under debate. We hypothesized that the clinical efficacy of triptans lies in aborting central sensitization and focused on the question of why triptans are headache-specific, i.e. highly effective in migraine and cluster headache and ineffective in extracephalic pain.

METHODS

Forty healthy participants were enrolled in this double-blinded, randomized, placebo-controlled study. The effect of sumatriptan (n=20) vs placebo (n=20) was investigated in a cephalic (V1) vs an extracephalic dermatome (forearm) using a topical capsaicin sensitization model. Capsaicin-induced primary and secondary hyperalgesia were evaluated using quantitative sensory testing.

RESULTS

After capsaicin application, primary hyperalgesia developed in both sumatriptan and placebo groups in both dermatomes. However, sumatriptan exclusively prevented secondary hyperalgesia in the V1 dermatome but not on the forearm. Placebo exerted no effects on secondary hyperalgesia in both trigeminal and extracephalic dermatomes. Additionally, sumatriptan reduced the flare size exclusively in the V1 dermatome.

CONCLUSIONS

Our data suggest that sumatriptan reduces central sensitization (secondary hyperalgesia) without modulating peripheral sensitization (primary hyperalgesia) in a human pain model of capsaicin-induced sensitization. Moreover, despite a systemic administration of sumatriptan, the modulatory effects are trigeminal-specific, echoing the clinical effect of triptans in aborting headaches, but not extracephalic pain.

COVID-19 causes neuronal degeneration and reduces neurogenesis in human hippocampus

Recent investigations of COVID-19 have largely focused on the effects of this novel virus on the vital organs in order to efficiently assist individuals who have recovered from the disease. In the present study we used hippocampal tissue samples extracted from people who died after COVID-19. Utilizing histological techniques to analyze glial and neuronal cells we illuminated a massive degeneration of neuronal cells and changes in glial cells morphology in hippocampal samples. The results showed that in hippocampus of the studied brains there were morphological changes in pyramidal cells, an increase in apoptosis, a drop in neurogenesis, and change in spatial distribution of neurons in the pyramidal and granular layer. It was also demonstrated that COVID-19 alter the morphological characteristics and distribution of astrocyte and microglia cells. While the exact mechanism(s) by which the virus causes neuronal loss and morphology in the central nervous system (CNS) remains to be determined, it is necessary to monitor the effect of SARS-CoV-2 infection on CNS compartments like the hippocampus in future investigations. As a result of what happened in the hippocampus secondary to COVID-19, memory impairment may be a long-term neurological complication which can be a predisposing factor for neurodegenerative disorders through neuroinflammation and oxidative stress mechanisms.

Sex Differences in the Role of Neurexin 3α in Zoster Associated Pain

Varicella zoster virus (VZV) induces orofacial pain and female rats show greater pain than male rats. During the proestrus phase of the estrous cycle the VZV induce pain response is attenuated in female rats. A screen of gene expression changes in diestrus and proestrus female rats indicated neurexin 3α (Nrxn3α) was elevated in the central amygdala of proestrus rats vs. diestrus rats. GABAergic neurons descend from the central amygdala to the lateral parabrachial region and Nrxn3α is important for presynaptic γ-Aminobutyric acid (GABA) release. Thus, we hypothesized that the reduced orofacial pain in male rats and proestrus female rats is the result of increased Nrxn3α within the central amygdala that increases GABA release from axon terminals within the parabrachial and inhibits ascending pain signals. To test this hypothesis Nrxn3 α expression was knocked-down by infusing shRNA constructs in the central amygdala. Then GABA release in the parabrachial was quantitated concomitant with measuring the pain response. Results revealed that knockdown of Nrxn3α expression significantly increases the pain response in both male rats and proestrus female rats vs. diestrus rats. GABA release was significantly reduced in the parabrachial of male and proestrus female rats after Nrxn3α knockdown. Neuronal activity of excitatory neurons was significantly inhibited in the parabrachial after Nrxn3α knockdown. These results are consistent with the idea that Nrxn3 within the central amygdala controls VZV associated pain by regulating GABA release in the lateral parabrachial that then modulates ascending orofacial pain signals.

Neurexin 3α in the central amygdala has a role orofacial varicella zoster pain

Varicella zoster virus (VZV) is responsible for chronic pain. VZV injection has similarities to herpes zoster (HZ) "shingles" pain in humans. In this study orofacial pain was induced by injecting male rats with the human VZV. The amygdala and parabrachial have been implicated to control affective/motivational orofacial pain. Recently our lab reported neurexin 3α (Nrxn3α) is expressed in the central amygdala and parabrachial. GABAergic neurons descend from the central amygdala to the lateral parabrachial region and Nrxn3α is important for presynaptic (γ-Aminobutyric acid) GABA release. Thus, we hypothesized that lateral parabrachial neuronal activity and orofacial pain are controlled by Nrxn3α within the central amygdala. To test the hypothesis Nrxn3α expression was knocked down (i.e., using short hairpin RNA or shRNA) in the central amygdala and GABA release and neuronal activity were quantitated in the parabrachial concomitant with measurement of the VZV induced pain response. Results revealed that attenuating Nrxn3 expression within the amygdala reduces GABA release in the parabrachial and increases neuronal activity within the lateral parabrachial region. Attenuating Nrxn3 expression also increases VZV associated orofacial pain. Activating GABAergic neurons within the central amygdala with opsins increase GABA release in the parabrachial and reduced the pain response after Nrxn3 shRNA treatment. These results are consistent with the idea that Nrxn3 within the central amygdala controls VZV associated pain by regulating GABA release in the lateral parabrachial that then controls the activity of ascending pain neurons.

Trigeminal sensory modulatory effects of galcanezumab and clinical response prediction

ABSTRACT

Galcanezumab, a monoclonal antibody against calcitonin gene-related peptide, is an emerging migraine preventative. We hypothesized that the preventive effects are conveyed via modulation of somatosensory processing and that certain sensory profiles may hence be associated with different clinical responses. We recruited migraine patients (n=26), who underwent quantitative sensory tests (QST) over the right V1 dermatome and forearm at baseline (T0), 2-3 weeks (T1), and one year (T12) after monthly galcanezumab treatment. The clinical response was defined as a reduction of ≥30% in headache frequency based on the headache diary. Predictors for clinical response were calculated using binary logistical regression models. After galcanezumab (T1 vs. T0), the heat pain threshold (HPT) (°C, 44.9 ± 3.4 vs. 43.0 ± 3.3, p=0.013) and mechanical pain threshold (MPT) (log mN, 1.60 ± 0.31 vs. 1.45 ± 0.26, p=0.042) were increased exclusively in the V1 dermatome, but not the forearm. These changes were immediate, did not differ between responders and non-responders, and did not last in one year of follow-up (T12 vs. T0). However, baseline HPT (OR: 2.13, 95% CI: 1.08-4.19, p = 0.029) on the forearm was a robust predictor for a clinical response three months later. In summary, our data demonstrated that galcanezumab modulates pain thresholds specifically in the V1 dermatome, but this modulation is short-lasting and irrelevant to clinical response. Instead, the clinical response may be determined by individual sensibility even before the administration of medication.

Role of voltage-gated sodium channels in axonal signal propagation of trigeminal ganglion neurons after infraorbital nerve entrapment

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Infraorbital nerve entrapment (IoNE) induces mechanical allodynia and enhances signal propagation in primary afferent A- and C-fibers.

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IoNE increases sensitivity of A- and C-fibers to conduction block by tetrodotoxin (TTX) and selective voltage-gated sodium channel 1.8 (NaV1.8) inhibitor, A-803467.

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IoNE increases signal propagation in vibrissal pad Ad -, but not Aβ-fibers, and their sensitivity to conduction block by the selective NaV1.8 inhibitor.

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IoNE increases membrane excitability of dissociated small and medium sized trigeminal neurons.

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IoNE increases nerve, but not ganglion, levels of NaV1.3, NaV1.7, and NaV1.8 mRNAs, and NaV1.8 protein.

Chronic pain arising from peripheral nerve injuries represents a significant clinical challenge because even the most efficacious anticonvulsant drug treatments are limited by their side effects profile. We investigated pain behavior, changes in axonal signal conduction and excitability of trigeminal neurons, and expression of voltage-gated sodium channels (NaVs) in the infraorbital nerve and trigeminal ganglion (TG) after infraorbital nerve entrapment (IoNE). Compared to Sham, IoNE rats had increased A- and C-fiber compound action potentials (CAPs) and Aδ component of A-CAP area from fibers innervating the vibrissal pad. After IoNE, A- and C-fiber CAPs were more sensitive to blockade by tetrodotoxin (TTX), and those fibers that were TTX-resistant were more sensitive to blockade by the NaV1.8 selective blocker, A-803467. Although NaV1.7 blocker, ICA-121431 alone, did not affect Aδ-fiber signal propagation, cumulative application with A-803467 and 4,9-anhydro-TTX significantly reduced the Aδ-fiber CAP in IoNE rats. In patch clamp recordings from small- and medium-sized TG neurons, IoNE resulted in reduced action potential (AP) depolarizing current threshold, hyperpolarized AP voltage threshold, increased AP duration, and a more depolarized membrane potential. While the transcripts of most NaVs were reduced in the ipsilateral TG after IoNE, NaV1.3, NaV1.7, and NaV1.8 mRNAs, and NaV1.8 protein, were significantly increased in the nerve. Altogether, our data suggest that axonal redistribution of NaV1.8, and to a lesser extent NaV1.3, and NaV1.7 contributes to enhanced nociceptive signal propagation in peripheral nerve after IoNE.

Photoaversion in inherited retinal diseases: clinical phenotypes, biological basis, and qualitative and quantitative assessment

Severe light sensitivity is a feature common to a range of ophthalmological and neurological diseases. In inherited retinal diseases (IRDs) particularly, this may be accompanied by significant visual disruption. These symptoms are extremely debilitating for affected individuals and have significant implications in terms of day-to-day activities. Underlying mechanisms remain to be fully elucidated. Currently, there are many assessments of photoaversion (PA), however, all have limitations, with quantitative measurement in particular needing further evaluation. To understand the complexities associated with photoaversion from different pathologies, qualitative and quantitative assessments of the light aversion response must be standardized. There is no treatment to date, and strategies to alleviate symptoms focus on light avoidance. With respect to IRDs, however, gene therapy is currently being investigated in clinical trials and promising and further treatments may be on the horizon. The better characterization of these symptoms is an important end point measure in IRD gene therapy trials.

Comprehensive Review on Neuro-COVID-19 Pathophysiology and Clinical Consequences

Aside from the respiratory distress as the predominant clinical presentation of SARS-CoV-2 infection, various neurological complications have been reported with the infection during the ongoing pandemic, some of which cause serious morbidity and mortality. Herein, we gather the latest anatomical evidence of the virus's presence within the central nervous system. We then delve into the possible SARS-CoV-2 entry routes into the neurological tissues, with the hematogenous and the neuronal routes as the two utmost passage routes into the nervous system. We then give a comprehensive review of the neurological manifestations of the SARS-CoV-2 invasion in both the central and peripheral nervous system and its underlying pathophysiology via investigating large studies in the field and case reports in cases of study scarcity.

Involvement of the BNP/NPR-A/BKCa pathway in rat trigeminal ganglia following chronic constriction injury

Accumulating evidence indicates that the brain natriuretic peptide (BNP) and its receptor (natriuretic peptide receptor, NPR) are widely distributed in a variety of tissues including trigeminal ganglion (TG). Furthermore, recent studies support the involvement of the BNP-NPR-A pathway in acute and chronic pain. To investigate the role of this pathway in chronic pain, an infraorbital nerve-chronic constriction injury (ION-CCI) model of trigeminal neuralgia (TN) was produced in the rat. The time course of changes in mechanical pain threshold was examined. We observed an upregulation of BNP and NPR-A and a downregulation of large-conductance Ca²⁺-activated K⁺ (BKCa) mRNA and protein in rats subjected to ION-CCI. Patch clamping experiments in vitro found that BKCa currents were significantly reduced in rats subjected to ION-CCI. BNP increased BKCa currents in ION-CCI rats. These results suggest that BNP and NPR-A might serve as endogenous pain relievers in ION-CCI rats. Modulation of the BNP/NPR-A/BKCa channel pathway in TG may be a viable strategy for the treatment of TN.NEW & NOTEWORTHY BNP has been known to activate its receptor, NPR-A, to modulate inflammatory pain. However, the potential modulatory roles of BNP in TN have not been investigated in detail. We established an ION-CCI model of TN in the rat and observed an upregulation of BNP and NPR-A and a downregulation of BKCa in rats subjected to ION-CCI. Moreover, BNP can increase BKCa currents in ION-CCI rats. Thus, BNP and NPR-A might have inhibitory effects on trigeminal neuralgia through activating the BNP/NPR-A/BKCa channel pathway.

Atypical triggers in trigeminal neuralgia: the role of A-delta sensory afferents in food and weather triggers

Background

Trigeminal neuralgia is a debilitating craniofacial pain syndrome that is characterized by paroxysms of intense, short-lived electric shock-like pains in the trigeminal nerve distribution. Recently, the presence of triggers has become one of the key diagnostic criteria in the 3rd edition of the International Classification of Headache Disorders. Light touch is the most common trigger, however other non-mechanical triggers, such as cold weather and certain foods, have been thought to provoke trigeminal neuralgia anecdotally. We aimed to characterize the prevalence and characteristics of these atypical triggers.

Methods

We conducted a retrospective, cross-sectional study of atypical triggers in trigeminal neuralgia patients seen in a tertiary pain clinic in Singapore. Patients were recruited via clinic records, and study data were identified from physician documentation.

Results

A total of 60 patients met the inclusion criteria. Weather triggers were observed in 12 patients (20%), of which five patients (8%) reported strong winds, 4 patients (7%) reported cold temperatures, and 3 patients (5%) reported cold winds as triggers. Fifteen patients (25%) had a specific food trigger, of which 10 patients (17%) reported hard or tough food, 5 patients (8%) reported hot/cold food, 4 patients (7%) reported spicy food, and 2 patients (3%) reported sweet food as triggers.

Conclusions

Although trigeminal neuralgia is most commonly triggered by mechanical stimuli, atypical triggers such as cold temperatures and certain foods are seen in a significant proportion of patients. These atypical triggers may share a common pathway of sensory afferent Aδ fiber activation.

Temporal disconnection between pain relief and trigeminal nerve microstructural changes after Gamma Knife radiosurgery for trigeminal neuralgia

OBJECTIVE

Gamma Knife radiosurgery (GKRS) is a noninvasive surgical treatment option for patients with medically refractive classic trigeminal neuralgia (TN). The long-term microstructural consequences of radiosurgery and their association with pain relief remain unclear. To better understand this topic, the authors used diffusion tensor imaging (DTI) to characterize the effects of GKRS on trigeminal nerve microstructure over multiple posttreatment time points.

METHODS

Ninety-two sets of 3-T anatomical and diffusion-weighted MR images from 55 patients with TN treated by GKRS were divided within 6-, 12-, and 24-month posttreatment time points into responder and nonresponder subgroups (≥ 75% and < 75% reduction in posttreatment pain intensity, respectively). Within each subgroup, posttreatment pain intensity was then assessed against pretreatment levels and followed by DTI metric analyses, contrasting treated and contralateral control nerves to identify specific biomarkers of successful pain relief.

RESULTS

GKRS resulted in successful pain relief that was accompanied by asynchronous reductions in fractional anisotropy (FA), which maximized 24 months after treatment. While GKRS responders demonstrated significantly reduced FA within the radiosurgery target 12 and 24 months posttreatment (p < 0.05 and p < 0.01, respectively), nonresponders had statistically indistinguishable DTI metrics between nerve types at each time point.

CONCLUSIONS

Ultimately, this study serves as the first step toward an improved understanding of the long-term microstructural effect of radiosurgery on TN. Given that FA reductions remained specific to responders and were absent in nonresponders up to 24 months posttreatment, FA changes have the potential of serving as temporally consistent biomarkers of optimal pain relief following radiosurgical treatment for classic TN.

Reduced activity of GAD67 expressing cells in the reticular thalamus enhance thalamic excitatory activity and varicella zoster virus associated pain

Within the reticular thalamic nucleus neurons express gamma aminobutyric acid (GABA) and these cells project to the ventral posteromedial thalamic nucleus. When GABA activity decreases the activity of excitatory cells in the ventral posteromedial nucleus would be expected to increase. In this study, we addressed the hypothesis that attenuating GABAergic cells in the reticular thalamic nucleus increases excitatory activity in the ventral posteromedial nucleus increasing varicella zoster virus (VZV) associated pain in the orofacial region. Adeno-associated virus (AAV) was infused in the reticular thalamic nucleus of Gad1-Cre rats. This virus transduced a G inhibitory designer receptor exclusively activated by designer drugs (DREADD) gene that was Cre dependent. A dose of estradiol that was previously shown to reduce VZV pain and increase GABAergic activity was administered to castrated and ovariectomized rats. Previous studies suggest that estradiol attenuates herpes zoster pain by increasing the activity of inhibitory neurons and decreasing the activity of excitatory cells within the lateral thalamic region. The ventral posteromedial nucleus was infused with AAV containing a GCaMP6f expression construct. A glass lens was implanted for miniscope imaging. Our results show that the activity of GABA cells within the reticular thalamic region decreased with clozapine N-oxide treatment concomitant with increased calcium activity of excitatory cells in the ventral posteromedial nucleus and an increased orofacial pain response. The results suggest that estradiol attenuates herpes zoster pain by increasing the activity of inhibitory neurons within the reticular thalamus that then inhibit excitatory activity in ventral posteromedial nucleus causing a reduction in orofacial pain.

Neuromechanisms of SARS-CoV-2: A Review

Recent studies have suggested the neuroinvasive potential of severe acute respiratory coronavirus 2 (SARS-CoV-2). Notably, neuroinvasiveness might be involved in the pathophysiology of coronavirus disease 2019 (COVID-19). Some studies have demonstrated that synapse-connected routes may enable coronaviruses to access the central nervous system (CNS). However, evidence related to the presence of SARS-CoV-2 in the CNS, its direct impact on the CNS, and the contribution to symptoms suffered, remain sparse. Here, we review the current literature that indicates that SARS-CoV-2 can invade the nervous system. We also describe the neural circuits that are potentially affected by the virus and their possible role in the progress of COVID-19. In addition, we propose several strategies to understand, diagnose, and treat the neurological symptoms of COVID-19.

Microstructural changes in the trigeminal nerve of patients with episodic migraine assessed using magnetic resonance imaging

BACKGROUND

There is histological evidence of microstructural changes in the zygomaticotemporal branch of the trigeminal nerve in migraineurs. This raises the possibility that altered trigeminal nerve properties contribute to migraine pathophysiology. Whilst it is not possible to explore the anatomy of small trigeminal nerve branches it is possible to explore the anatomy of the trigeminal root entry zone using magnetic resonance imaging in humans. The aim of this investigation is to assess the microstructure of the trigeminal nerve in vivo to determine if nerve alterations occur in individuals with episodic migraine.

METHODS

In 39 migraineurs and 39 matched controls, T1-weighted anatomical images were used to calculate the volume (mm³) and maximal cross-sectional area of the trigeminal nerve root entry zone; diffusion tensor images were used to calculate fractional anisotropy, mean diffusion, axial diffusion and radial diffusion.

RESULTS

There were significant differences between the left and right nerve of controls and migraineurs with respect to volume and not cross-sectional area. Migraineurs displayed reduced axial diffusion in the right nerve compared to the left nerve, and reduced fractional anisotropy in the left nerve compared to left controls. Furthermore, although there were no differences in mean diffusion or radial diffusion, regional analysis of the nerve revealed significantly greater radial diffusion in the middle and rostral portion of the left trigeminal nerve in migraineurs compared with controls.

CONCLUSIONS

Migraine pathophysiology is associated with microstructural abnormalities within the trigeminal nerve that are consistent with histological evidence of altered myelin and/or organization. These peripheral nerve changes may provide further insight into migraine pathophysiology and enable a greater understanding for targeted treatments of pain alleviation.

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.

Estradiol Acts in Lateral Thalamic Region to Attenuate Varicella Zoster Virus Associated Affective Pain

Varicella zoster virus (VZV) results in chicken pox and herpes zoster. Female rats show a higher level of herpes zoster associated pain than males, consistent with human studies. In this study, we addressed the novel hypothesis that sex difference in herpes zoster associated pain is due, in part, to estradiol modulating activity in the thalamus. To test this hypothesis a high and low physiological dose of estradiol was administered to castrated and ovariectomized rats and the affective pain response was measured after injection of VZV into the whisker pad. Thalamic infusion of the estrogen receptor antagonist ICI 182,780 concomitant with a high dose of estradiol addressed the role of estradiol binding to its receptor to effect pain. Phosphorylated extracellular signal-regulated protein kinase (pERK) positive cells were measured in excitatory (glutaminase positive) and inhibitory (glutamate decarboxylase 67 positive) cells of the lateral thalamic region. Our results show that a high dose of estradiol significantly reduced the pain response in both males and females. pERK significantly increased in excitatory cells after treatment with a low dose of estradiol and increased in inhibitory cells after treatment with a high dose of estradiol. Administration of ICI 182,780 significantly increased the pain response, reduced expression of GABA related genes in the thalamic region and significantly reduced the number of inhibitory cells expressing pERK. The results suggest that estradiol attenuates herpes zoster pain by increasing the activity of inhibitory neurons within the thalamus and that this reduction includes an estrogen receptor dependent mechanism.

Aromatase Derived Estradiol Within the Thalamus Modulates Pain Induced by Varicella Zoster Virus

Herpes zoster or shingles is the result of varicella zoster virus (VZV) infection and often results in chronic pain that lasts for months after visible symptoms subside. Testosterone often attenuates pain in males. Previous work demonstrates ovarian estrogen effects γ-aminobutyric acid (GABA) signaling in the thalamus, reducing pain but the role of testosterone within the thalamus is currently unknown. Because aromatase affects pain and is present in the thalamus we tested a hypothesis that testosterone converted to estrogen in the thalamus attenuates herpes zoster induced pain. To address this hypothesis, male Sprague-Dawley rats received whisker pad injection of either MeWo cells or MeWo cells containing VZV. To reduce aromatase derived estrogen in these animals we injected aromatase inhibitor letrozole systemically or infused it into the thalamus. To test if estrogen was working through the estrogen receptor (ER) agonist, 4, 4′, 4″-(4-Propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) was infused concomitant with letrozole. Motivational and affective pain was measured after letrozole and/or PPT treatment. Vesicular GABA transporter (VGAT) is important in pain signaling. Because estrogen effects VGAT expression we measured its transcript and protein levels after letrozole treatment. Virus injection and letrozole significantly increased the pain response but thalamic infusion of PPT reduced zoster pain. Letrozole increased the number of thalamic neurons staining for phosphorylated ERK (pERK) but decreased VGAT expression. The results suggest in male rats aromatase derived estradiol interacts with the ER to increase VGAT expression and increase neuronal inhibition in the thalamus to attenuate VZV induced pain.

PPARγ Agonists Attenuate Trigeminal Neuropathic Pain

OBJECTIVES

The aim of this study is to investigate the role of peroxisome proliferator-activated receptor-gamma isoform (PPARγ), in trigeminal neuropathic pain utilizing a novel mouse trigeminal inflammatory compression (TIC) injury model.

RESULTS

The study determined that the PPARγ nuclear receptor plays a significant role in trigeminal nociception transmission, evidenced by: 1) Intense PPARγ immunoreactivity is expressed 3 weeks after TIC nerve injury in the spinal trigeminal caudalis, the termination site of trigeminal nociceptive nerve fibers. 2) Systemic administration of a PPARγ agonist, pioglitazone (PIO), attenuates whisker pad mechanical allodynia at doses of 300 mg/kg i.p. and 600 mg/kg p.o. 3) Administration of a PPARγ antagonist, GW9662 (30 mg/kg i.p.), prior to providing the optimal dose of PIO (300 mg/kg i.p.) blocked the analgesic effect of PIO.

DISCUSSION

This is the first study localizing PPARγ immunoreactivity throughout the brainstem trigeminal sensory spinal nucleus (spV) and its increase three weeks after TIC nerve injury. This is also the first study to demonstrate that activation of PPARγ attenuates trigeminal hypersensitivity in the mouse TIC nerve injury model. The findings presented here suggest the possibility of utilizing the FDA approved diabetic treatment drug, PIO, as a new therapeutic that targets PPARγ for treatment of patients suffering from orofacial neuropathic pain.

Neurophysiological aspects of the trigeminal sensory system: an update

The trigeminal system is one of the most complex cranial nerve systems of the human body. Research on it has vastly grown in recent years and concentrated more and more on molecular mechanisms and pathophysiology, but thorough reviews on this topic are lacking, certainly on the normal physiology of the trigeminal sensory system. Here we review the current literature on neurophysiology of the trigeminal nerve from peripheral receptors up to its central projections toward the somatosensory cortex. We focus on the most recent scientific discoveries and describe historical relevant research to substantiate further. One chapter on new insights of the pathophysiology of pain at the level of the trigeminal system is added. A database search of Medline, Embase and Cochrane was conducted with the search terms ‘animal study’, ‘neurophysiology’, ‘trigeminal’, ‘oral’ and ‘sensory’. Articles were manually selected after reading the abstract and where needed the article. Reference lists also served to include relevant research articles. Fifty-six articles were included after critical appraisal. Physiological aspects on mechanoreceptors, trigeminal afferents, trigeminal ganglion and central projections are reviewed in light of reference works. Embryologic and anatomic insights are cited where needed. A brief description of pathophysiology of pain pathways in the trigeminal area and recent advances in dental stem cell research are also discussed. Neurophysiology at the level of the central nervous system is not reviewed. The current body of knowledge is mainly based on animal and cadaveric studies, but recent advancements in functional imaging and molecular neuroscience are elucidating the pathways and functioning of this mixed nerve system. Extrapolation of animal studies or functioning of peripheral nerves should be warranted.

Changes in the expression of voltage-gated sodium channels Nav1.3, Nav1.7, Nav1.8, and Nav1.9 in rat trigeminal ganglia following chronic constriction injury

Voltage-gated sodium channels (VGSCs), especially the tetrodotoxin-sensitive Nav1.3 and Nav1.7, and the tetrodotoxin-resistant Nav1.8 and Nav1.9, have been implicated in acute and chronic neuropathic pain. The aim of this study was to investigate the expression of VGSC Nav1.3, Nav1.7, Nav1.8, and Nav1.9 after nerve injury and their roles in the development of trigeminal neuralgia (TN). We used the infraorbital nerve-chronic constriction injury model of TN in the rat. The time course of changes in the mechanical pain threshold was examined. In addition, real-time PCR and double immunofluorescence staining of VGSC α subunits were used to evaluate messenger RNA and protein expression, respectively, in the trigeminal ganglion. Behavioral tests showed that the mechanical pain threshold decreased significantly 4-42 days after surgery and reached the lowest observed value by day 12. Compared with sham-operated controls, we found that trigeminal ganglion in rats subjected to an infraorbital nerve-chronic constriction injury showed upregulation of Nav1.3 and downregulation of Nav1.7, Nav1.8, and Nav1.9 messenger RNA and protein levels. Our findings suggest that VGSC may participate in the regulation of TN.

Electrophysiological characteristics of the frontal nerve in patients with herpetic ophthalmic neuralgia

INTRODUCTION

The aim of this study was to explore a method for obtaining sensory nerve action potentials (SNAPs) of the supratrochlear (STN) and supraorbital (SON) nerves and evaluate the function of affected nerves in patients with herpetic ophthalmic neuralgia (HON).

METHODS

Thirty healthy volunteers and 40 subjects with subacute HON participated in this study.

RESULTS

The amplitudes and sensory conduction velocities (SCVs) that predicted HON were identified. The corresponding cutoff values for the amplitudes ranged from 11.10 μV to 12.45 μV. The corresponding cutoff values for the SCVs ranged from 43.14 m/s to 44.64 m/s. SCVs were markedly lower on the affected side compared with healthy volunteers (P < 0.05), and the amplitudes of SNAPs on the affected side were decreased by 36% compared with healthy volunteers (P < 0.05).

DISCUSSION

SCVs of STN and SONs can be obtained with the 3-channel method and used to evaluate myelinated fibers in patients with HON. Muscle Nerve 57: 973-980, 2018.

Postherpetic Neuralgia and Trigeminal Neuralgia

Postherpetic neuralgia (PHN) is an unpredictable complication of varicella zoster virus- (VZV-) induced herpes zoster (HZ) which often occurs in elderly and immunocompromised persons and which can induce psychosocial dysfunction and can negatively impact on quality of life. Preventive options for PHN include vaccination of high-risk persons against HZ, early use of antiviral agents, and robust management of pain during the early stage of acute herpes zoster. If it does occur, PHN may persist for months or even years after resolution of the HZ mucocutaneous eruptions, and treatment is often only partially effective. Classical trigeminal neuralgia is a severe orofacial neuropathic pain condition characterized by unilateral, brief but recurrent, lancinating paroxysmal pain confined to the distribution of one or more of the branches of the trigeminal nerve. It may be idiopathic or causally associated with vascular compression of the trigeminal nerve root. The anticonvulsive agents, carbamazepine or oxcarbazepine, constitute the first-line treatment. Microvascular decompression or ablative procedures should be considered when pharmacotherapy is ineffective or intolerable. The aim of this short review is briefly to discuss the etiopathogenesis, clinical features, and treatment of PHN and classical trigeminal neuralgia.

The Molecular Fingerprint of Dorsal Root and Trigeminal Ganglion Neurons

The dorsal root ganglia (DRG) and trigeminal ganglia (TG) are clusters of cell bodies of highly specialized sensory neurons which are responsible for relaying information about our environment to the central nervous system. Despite previous efforts to characterize sensory neurons at the molecular level, it is still unknown whether those present in DRG and TG have distinct expression profiles and therefore a unique molecular fingerprint. To address this question, we isolated lumbar DRG and TG neurons using fluorescence-activated cell sorting from Advillin-GFP transgenic mice and performed RNA sequencing. Our transcriptome analyses showed that, despite being overwhelmingly similar, a number of genes are differentially expressed in DRG and TG neurons. Importantly, we identified 24 genes which were uniquely expressed in either ganglia, including an arginine vasopressin receptor and several homeobox genes, giving each population a distinct molecular fingerprint. We compared our findings with published studies to reveal that many genes previously reported to be present in neurons are in fact likely to originate from other cell types in the ganglia. Additionally, our neuron-specific results aligned well with a dataset examining whole human TG and DRG. We propose that the data can both improve our understanding of primary afferent biology and help contribute to the development of drug treatments and gene therapies which seek targets with unique or restricted expression patterns.

Sex differences underlying orofacial varicella zoster associated pain in rats

BACKGROUND

Most people are initially infected with varicella zoster virus (VZV) at a young age and this infection results in chickenpox. VZV then becomes latent and reactivates later in life resulting in herpes zoster (HZ) or "shingles". Often VZV infects neurons of the trigeminal ganglia to cause ocular problems, orofacial disease and occasionally a chronic pain condition termed post-herpetic neuralgia (PHN). To date, no model has been developed to study orofacial pain related to varicella zoster. Importantly, the incidence of zoster associated pain and PHN is known to be higher in women, although reasons for this sex difference remain unclear. Prior to this work, no animal model was available to study these sex-differences. Our goal was to develop an orofacial animal model for zoster associated pain which could be utilized to study the mechanisms contributing to this sex difference.

METHODS

To develop this model VZV was injected into the whisker pad of rats resulting in IE62 protein expression in the trigeminal ganglia; IE62 is an immediate early gene in the VZV replication program.

RESULTS

Similar to PHN patients, rats showed retraction of neurites after VZV infection. Treatment of rats with gabapentin, an agent often used to combat PHN, ameliorated the pain response after whisker pad injection. Aversive behavior was significantly greater for up to 7 weeks in VZV injected rats over control inoculated rats. Sex differences were also seen such that ovariectomized and intact female rats given the lower dose of VZV showed a longer affective response than male rats. The phase of the estrous cycle also affected the aversive response suggesting a role for sex steroids in modulating VZV pain.

CONCLUSIONS

These results suggest that this rat model can be utilized to study the mechanisms of 1) orofacial zoster associated pain and 2) the sex differences underlying zoster associated pain.

The role of the blood-brain barrier in the development and treatment of migraine and other pain disorders

The function of the blood-brain barrier (BBB) related to chronic pain has been explored for its classical role in regulating the transcellular and paracellular transport, thus controlling the flow of drugs that act at the central nervous system, such as opioid analgesics (e.g., morphine) and non-steroidal anti-inflammatory drugs. Nonetheless, recent studies have raised the possibility that changes in the BBB permeability might be associated with chronic pain. For instance, changes in the relative amounts of occludin isoforms, resulting in significant increases in the BBB permeability, have been demonstrated after inflammatory hyperalgesia. Furthermore, inflammatory pain produces structural changes in the P-glycoprotein, the major efflux transporter at the BBB. One possible explanation for these findings is the action of substances typically released at the site of peripheral injuries that could lead to changes in the brain endothelial permeability, including substance P, calcitonin gene-related peptide, and interleukin-1 beta. Interestingly, inflammatory pain also results in microglial activation, which potentiates the BBB damage. In fact, astrocytes and microglia play a critical role in maintaining the BBB integrity and the activation of those cells is considered a key mechanism underlying chronic pain. Despite the recent advances in the understanding of BBB function in pain development as well as its interference in the efficacy of analgesic drugs, there remain unknowns regarding the molecular mechanisms involved in this process. In this review, we explore the connection between the BBB as well as the blood-spinal cord barrier and blood-nerve barrier, and pain, focusing on cellular and molecular mechanisms of BBB permeabilization induced by inflammatory or neuropathic pain and migraine.

Protein kinase C gamma interneurons in the rat medullary dorsal horn: distribution and synaptic inputs to these neurons, and subcellular localization of the enzyme

The γ isoform of protein kinase C (PKCγ), which is concentrated in interneurons in the inner part of lamina II (IIi ) of the dorsal horn, has been implicated in the expression of tactile allodynia. Lamina IIi PKCγ interneurons were shown to be activated by tactile inputs and to participate in local circuits through which these inputs can reach lamina I, nociceptive output neurons. That such local circuits are gated by glycinergic inhibition and that A- and C-fibers low threshold mechanoreceptors (LTMRs) terminate in lamina IIi raise the general issue of synaptic inputs to lamina IIi PKCγ interneurons. Combining light and electron microscopic immunochemistry in the rat spinal trigeminal nucleus, we show that PKCγ-immunoreactivity is mostly restricted to interneurons in lamina IIi of the medullary dorsal horn, where they constitute 1/3 of total neurons. The majority of synapses on PKCγ-immunoreactive interneurons are asymmetric (likely excitatory). PKCγ-immunoreactive interneurons appear to receive exclusively myelinated primary afferents in type II synaptic glomeruli. Neither large dense core vesicle terminals nor type I synaptic glomeruli, assumed to be the endings of unmyelinated nociceptive terminals, were found on these interneurons. Moreover, there is no vesicular glutamate transporter 3-immunoreactive bouton, specific to C-LTMRs, on PKCγ-immunoreactive interneurons. PKCγ-immunoreactive interneurons contain GABAA ergic and glycinergic receptors. At the subcellular level, PKCγ-immunoreactivity is mostly concentrated on plasma membranes, close to, but not within, postsynaptic densities. That only myelinated primary afferents were found to contact PKCγ-immunoreactive interneurons suggests that myelinated, but not unmyelinated, LTMRs play a critical role in the expression of mechanical allodynia.

Brain signature of chronic orofacial pain: a systematic review and meta-analysis on neuroimaging research of trigeminal neuropathic pain and temporomandibular joint disorders

Brain neuroimaging has been widely used to investigate the bran signature of chronic orofacial pain, including trigeminal neuropathic pain (TNP) and pain related to temporomandibular joint disorders (TMD). We here systematically reviewed the neuroimaging literature regarding the functional and structural changes in the brain of TNP and TMD pain patients, using a computerized search of journal articles via PubMed. Ten TNP studies and 14 TMD studies were reviewed. Study quality and risk of bias were assessed based on the criteria of patient selection, the history of medication, the use of standardized pain/psychological assessments, and the model and statistics of imaging analyses. Qualitative meta-analysis was performed by examining the brain regions which showed significant changes in either brain functions (including the blood-oxygen-level dependent signal, cerebral blood flow and the magnetic resonance spectroscopy signal) or brain structure (including gray matter and white matter anatomy). We hypothesized that the neuroimaging findings would display a common pattern as well as distinct patterns of brain signature in the disorders. This major hypothesis was supported by the following findings: (1) TNP and TMD patients showed consistent functional/structural changes in the thalamus and the primary somatosensory cortex, indicating the thalamocortical pathway as the major site of plasticity. (2) The TNP patients showed more alterations at the thalamocortical pathway, and the two disorders showed distinct patterns of thalamic and insular connectivity. Additionally, functional and structural changes were frequently reported in the prefrontal cortex and the basal ganglia, suggesting the role of cognitive modulation and reward processing in chronic orofacial pain. The findings highlight the potential for brain neuroimaging as an investigating tool for understanding chronic orofacial pain.

Inhibition of G protein-coupled P2Y2 receptor induced analgesia in a rat model of trigeminal neuropathic pain

BACKGROUNDS

ATP and P2X receptors play important roles in the modulation of trigeminal neuropathic pain, while the role of G protein-coupled P2Y₂ receptors and the underlying mechanisms are less clear. The threshold and frequency of action potentials, fast inactivating transient K+ channels (IA) are important regulators of membrane excitability in sensory neurons because of its vital role in the control of the spike onset. In this study, pain behavior tests, QT-RT-PCR, immunohistochemical staining, and patch-clamp recording, were used to investigate the role of P2Y₂ receptors in pain behaviour.

RESULTS

In control rats: 1) UTP, an agonist of P2Y₂/P2Y₄ receptors, caused a significant decrease in the mean threshold intensities for evoking action potentials and a striking increase in the mean number of spikes evoked by TG neurons. 2) UTP significantly inhibited IA and the expression of Kv1.4, Kv3.4 and Kv4.2 subunits in TG neurons, which could be reversed by the P2 receptor antagonist suramin and the ERK antagonist U0126. In ION-CCI (chronic constriction injury of infraorbital nerve) rats: 1) mRNA levels of Kv1.4, Kv3.4 and Kv4.2 subunits were significantly decreased, while the protein level of phosphorylated ERK was significantly increased. 2) When blocking P2Y₂ receptors by suramin or injection of P2Y2R antisense oligodeoxynucleotides both led to a time- and dose-dependent reverse of allodynia in ION-CCI rats. 3) Injection of P2Y₂ receptor antisense oligodeoxynucleotides induced a pronounced decrease in phosphorylated ERK expression and a significant increase in Kv1.4, Kv3.4 and Kv4.2 subunit expression in trigeminal ganglia.

CONCLUSIONS

Our data suggest that inhibition of P2Y₂ receptors leads to down-regulation of ERK-mediated phosphorylation and increase of the expression of I(A)-related Kv channels in trigeminal ganglion neurons, which might contribute to the clinical treatment of trigeminal neuropathic pain.

Transcranial segment of the trigeminal nerve: macro-/microscopic anatomical study using sheet plastination

BACKGROUND

Trigeminal neuralgia (TN) may be caused by the mechanical compression of the trigeminal nerve. In the studies on the location of mechanical irritation and entrapment of the nerve, attention has been paid mostly to vascular structures in the subarachnoid space. Few studies have explored the relationship between the trigeminal nerve and its surrounding structures along its course in the skull base. The aim of this study was to examine and trace the root, ganglion and three divisions of the trigeminal nerve and their relationships with surrounding soft and bony structures in the skull base, and to identify the likely mechanical compression points.

METHODS

A total of 26 adult cadavers (ten females, 16 males; age range, 45-81 years) were examined in this study, eight for dissection and 16 for sheet plastination study.

RESULTS

Anatomical structures that may make the trigeminal nerve susceptible to entrapment in the skull base were located at (1) the inferolateral edge of the mouth of Meckel's cave, (2) the middle cranial fossa dura and the lateral wall of the anterior intracavernous portion of the internal carotid artery, (3) the ridge of the medial wall of the foramen rotundum, and (4) the twisted periosteum and venous plexus of the foramen ovale.

CONCLUSION

This study identified four likely mechanical compression points along the course of the trigeminal nerve in the skull base. Knowledge of these TN-susceptible sites may be useful to both skull base surgeon and TN-animal model researcher, particularly when they study TN without vascular compression.

Reduced basal ganglia μ-opioid receptor availability in trigeminal neuropathic pain: a pilot study

BACKGROUND

Although neuroimaging techniques have provided insights into the function of brain regions involved in Trigeminal Neuropathic Pain (TNP) in humans, there is little understanding of the molecular mechanisms affected during the course of this disorder. Understanding these processes is crucial to determine the systems involved in the development and persistence of TNP.

FINDINGS

In this study, we examined the regional μ-opioid receptor (μOR) availability in vivo (non-displaceable binding potential BPND) of TNP patients with positron emission tomography (PET) using the μOR selective radioligand [11C]carfentanil. Four TNP patients and eight gender and age-matched healthy controls were examined with PET. Patients with TNP showed reduced μOR BPND in the left nucleus accumbens (NAc), an area known to be involved in pain modulation and reward/aversive behaviors. In addition, the μOR BPND in the NAc was negatively correlated with the McGill sensory and total pain ratings in the TNP patients.

CONCLUSIONS

Our findings give preliminary evidence that the clinical pain in TNP patients can be related to alterations in the endogenous μ-opioid system, rather than only to the peripheral pathology. The decreased availability of μORs found in TNP patients, and its inverse relationship to clinical pain levels, provide insights into the central mechanisms related to this condition. The results also expand our understanding about the impact of chronic pain on the limbic system.