Animal models of trigeminal neuralgia: A commentary

Effect of autophagy blockage on trigeminal neuropathic pain in rats: Role of microglia

Microglia activation and autophagy changes are associated with the regulation of pain, but no study to date has been designed to address whether these features apply to trigeminal neuropathic pain. This study aimed to investigate how alterations in autophagy affect nociceptive behaviors may be associated with microglia activation in the caudal part of the spinal trigeminal nucleus (SpVC) in a rat model of trigeminal neuropathic pain. This model was established by chronic constriction injury of the infraorbital nerve. Autophagy inhibitors and agonists were injected into the lateral ventricle to regulate autophagy. The autophagy markers microtubule-associated protein light chain 3 I (LC3-I), LC3-II, sequestosome1 (p62), and LC-3 were examined by western blotting and/or immunofluorescence. The microglia marker ionized calcium binding adapter molecule 1 (Iba-1) was examined by immunohistochemistry. Nociceptive behavior changes were detected by measuring the mechanical thresholds and face-grooming duration. The results showed that microglia in SpVC were activated, and autophagy flux was blocked in the trigeminal neuropathic pain model. Autophagy agonists inhibited microglia activation and alleviated nociceptive behaviors. In contrast, autophagy inhibitors further activated microglia and exacerbated nociceptive behaviors. In a rat model of trigeminal neuropathic pain, autophagy blockage leads to microglia activation, which significantly influences nociceptive processes.

Emerging Psychotropic Drug for the Treatment of Trigeminal Pain: Salvinorin A

Trigeminal neuralgia (TN) is chronic pain caused by damage to the somatosensorial system on the trigeminal nerve or its branches, which involves peripheral and central dysfunction pain pathways. Trigeminal pain triggers disruptive pain in regions of the face, including within and around the mouth. Besides clinical experiences, translating the language of suffering into scientific terminology presents substantial challenges. Due to the complex and multifactorial pathophysiology underlying trigeminal pain, elucidating its social impact presents significant difficulties. Carbamazepine and oxcarbazepine are first-line treatments for TN, achieving approximately 50% pain reduction in 60-70% of treated patients. However, their efficacy is often limited by common side effects, such as dizziness, vertigo, nausea, seizures, and cognitive symptoms. In some cases, patients experience severe side effects, including myelosuppression, hyponatremia, hormonal imbalances, liver toxicity, suicidal ideation, teratogenicity, and other adverse reactions. Given these clinical limitations, the search for new painkiller candidates continues. Hence, we focused this review on salvinorin A (SalA), a natural agonist of κ-opioid receptors (KORs), which demonstrated anti-nociceptive, anti-inflammatory, and anti-neuropathic properties in various experimental models of the spinal sensory system. Furthermore, preclinical evidence indicates that SalA does not induce dependence and demonstrates a favorable toxicological and safety profile in comparison with currently marketed opioid drugs. We propose Salvinorin A as a promising candidate for treating trigeminal neuralgia, offering the potential for reduced adverse effects.

Alterations in the spinal cord, trigeminal nerve ganglion, and infraorbital nerve through inducing compression of the dorsal horn region at the upper cervical cord in trigeminal neuralgia

BACKGROUND

Idiopathic trigeminal neuralgia (TN) cases encountered frequently in daily practice indicate significant gaps that still need to be illuminated in the etiopathogenesis. In this study, a novel TN animal model was developed by compressing the dorsal horn (DH) of the upper cervical spinal cord.

METHODS

Eighteen rabbits were equally divided into three groups, namely control (CG), sham (SG), and spinal cord compression (SCC) groups. External pressure was applied to the left side at the C3 level in the SCC group. Dorsal hemilaminectomy was performed in the SG, and the operative side was closed without compression. No procedure was implemented in the control group. Samples from the SC, TG, and ION were taken after seven days. For the histochemical staining, damage and axons with myelin were scored using Hematoxylin and Eosin and Toluidine Blue, respectively. Immunohistochemistry, nuclei, apoptotic index, astrocyte activity, microglial labeling, and CD11b were evaluated.

RESULTS

Mechanical allodynia was observed on the ipsilateral side in the SCC group. In addition, both the TG and ION were partially damaged from SC compression, which resulted in significant histopathological changes and increased the expression of all markers in both the SG and SCC groups compared to that in the CG. There was a notable increase in tissue damage, an increase in the number of apoptotic nuclei, an increase in the apoptotic index, an indication of astrocytic gliosis, and an upsurge in microglial cells. Significant increases were noted in the SG group, whereas more pronounced significant increases were observed in the SCC group. Transmission electron microscopy revealed myelin damage, mitochondrial disruption, and increased anchoring particles. Similar changes were observed to a lesser extent in the contralateral spinal cord.

CONCLUSION

Ipsilateral trigeminal neuropathic pain was developed due to upper cervical SCC. The clinical finding is supported by immunohistochemical and ultrastructural changes. Thus, alterations in the DH due to compression of the upper cervical region should be considered as a potential cause of idiopathic TN.

Trigeminal neuralgia

Trigeminal neuralgia (TN) is a facial pain disorder characterized by intense and paroxysmal pain that profoundly affects quality of life and presents complex challenges in diagnosis and treatment. TN can be categorized as classical, secondary and idiopathic. Epidemiological studies show variable incidence rates and an increased prevalence in women and in the elderly, with familial cases suggesting genetic factors. The pathophysiology of TN is multifactorial and involves genetic predisposition, anatomical changes, and neurophysiological factors, leading to hyperexcitable neuronal states, central sensitization and widespread neural plasticity changes. Neurovascular compression of the trigeminal root, which undergoes major morphological changes, and focal demyelination of primary trigeminal afferents are key aetiological factors in TN. Structural and functional brain imaging studies in patients with TN demonstrated abnormalities in brain regions responsible for pain modulation and emotional processing of pain. Treatment of TN involves a multifaceted approach that considers patient-specific factors, including the type of TN, with initial pharmacotherapy followed by surgical options if necessary. First-line pharmacological treatments include carbamazepine and oxcarbazepine. Surgical interventions, including microvascular decompression and percutaneous neuroablative procedures, can be considered at an early stage if pharmacotherapy is not sufficient for pain control or has intolerable adverse effects or contraindications.

Progress in animal models of trigeminal neuralgia

OBJECTIVE

This review aims to systematically summarize the methods of establishing various models of trigeminal neuralgia (TN), the scope of application, and current animals used in TN research and the corresponding pain measurements, hoping to provide valuable reference for researchers to select appropriate TN animal models and make contributions to the research of pathophysiology and management of the disease.

DESIGN

The related literatures of TN were searched through PubMed database using different combinations of the following terms and keywords including but not limited: animal models, trigeminal neuralgia, orofacial neuropathic pain. To find the maximum number of eligible articles, no filters were used in the search. The references of eligible studies were analyzed and reviewed comprehensively.

RESULTS

This study summarized the current animal models of TN, categorized them into the following groups: chemical induction, photochemical induction, surgery and genetic engineering, and introduced various measurement methods to evaluate animal pain behaviors.

CONCLUSIONS

Although a variety of methods are used to establish disease models, there is no ideal TN model that can reflect all the characteristics of the disease. Therefore, there is still a need to develop more novel animal models in order to further study the etiology, pathological mechanism and potential treatment of TN.

Foramen lacerum impingement of trigeminal nerve root as a rodent model for trigeminal neuralgia

Trigeminal neuralgia (TN) is a classic neuralgic pain condition with distinct clinical characteristics. Modeling TN in rodents is challenging. Recently, we found that a foramen in the rodent skull base, the foramen lacerum, provides direct access to the trigeminal nerve root. Using this access, we developed a foramen lacerum impingement of trigeminal nerve root (FLIT) model and observed distinct pain-like behaviors in rodents, including paroxysmal asymmetric facial grimaces, head tilt when eating, avoidance of solid chow, and lack of wood chewing. The FLIT model recapitulated key clinical features of TN, including lancinating pain-like behavior and dental pain-like behavior. Importantly, when compared with a trigeminal neuropathic pain model (infraorbital nerve chronic constriction injury [IoN-CCI]), the FLIT model was associated with significantly higher numbers of c-Fos-positive cells in the primary somatosensory cortex (S1), unraveling robust cortical activation in the FLIT model. On intravital 2-photon calcium imaging, synchronized S1 neural dynamics were present in the FLIT but not the IoN-CCI model, revealing differential implication of cortical activation in different pain models. Taken together, our results indicate that FLIT is a clinically relevant rodent model of TN that could facilitate pain research and therapeutics development.

Preclinical orofacial pain assays and measures and chronic primary orofacial pain research: where we are and where we need to go

Chronic primary orofacial pain (OFP) conditions such as painful temporomandibular disorders (pTMDs; i.e., myofascial pain and arthralgia), idiopathic trigeminal neuralgia (TN), and burning mouth syndrome (BMS) are seemingly idiopathic, but evidence support complex and multifactorial etiology and pathophysiology. Important fragments of this complex array of factors have been identified over the years largely with the help of preclinical studies. However, findings have yet to translate into better pain care for chronic OFP patients. The need to develop preclinical assays that better simulate the etiology, pathophysiology, and clinical symptoms of OFP patients and to assess OFP measures consistent with their clinical symptoms is a challenge that needs to be overcome to support this translation process. In this review, we describe rodent assays and OFP pain measures that can be used in support of chronic primary OFP research, in specific pTMDs, TN, and BMS. We discuss their suitability and limitations considering the current knowledge of the etiology and pathophysiology of these conditions and suggest possible future directions. Our goal is to foster the development of innovative animal models with greater translatability and potential to lead to better care for patients living with chronic primary OFP.

Identification of the NRF2 transcriptional network as a therapeutic target for trigeminal neuropathic pain

Trigeminal neuralgia, historically dubbed the "suicide disease," is an exceedingly painful neurologic condition characterized by sudden episodes of intense facial pain. Unfortunately, the only U.S. Food and Drug Administration (FDA)-approved medication for trigeminal neuralgia carries substantial side effects, with many patients requiring surgery. Here, we identify the NRF2 transcriptional network as a potential therapeutic target. We report that cerebrospinal fluid from patients with trigeminal neuralgia accumulates reactive oxygen species, several of which directly activate the pain-transducing channel TRPA1. Similar to our patient cohort, a mouse model of trigeminal neuropathic pain also exhibits notable oxidative stress. We discover that stimulating the NRF2 antioxidant transcriptional network is as analgesic as inhibiting TRPA1, in part by reversing the underlying oxidative stress. Using a transcriptome-guided drug discovery strategy, we identify two NRF2 network modulators as potential treatments. One of these candidates, exemestane, is already FDA-approved and may thus be a promising alternative treatment for trigeminal neuropathic pain.

CaV3.2 calcium channels contribute to trigeminal neuralgia

ABSTRACT

Trigeminal neuralgia (TN) is a rare but debilitating disorder characterized by excruciating facial pain, with a higher incidence in women. Recent studies demonstrated that TN patients present mutations in the gene encoding the CaV3.2 T-type calcium channel, an important player in peripheral pain pathways. Here we characterize the role of CaV3.2 channels in TN at two levels. First, we examined the biophysical properties of CACNA1H variants found in TN patients. Second, we investigated the role of CaV3.2 in an animal model of trigeminal neuropathic pain. Whole cell patch clamp recordings from four different mutants expressed in tsA-201 cells (E286K in the pore loop of domain I, H526Y, G563R and P566T in the domain I-II linker) identified a loss-of-function in activation in the E286K mutation and gain-of-function in the G563R and P566T mutations. Moreover, a loss-of-function in inactivation was observed with the E286K and H526Y mutations. Cell surface biotinylation revealed no difference in channel trafficking among the variants. The G563R mutant also caused a gain-of-function in the firing properties of transfected trigeminal ganglion neurons. In female and male mice, constriction of the infraorbital nerve (CION) induced facial thermal heat hyperalgesia. Block of T-type channels with Z944 resulted in antihyperalgesia. The effect of Z944 was absent in CaV3.2-/- mice, indicating that CaV3.2 is the molecular target of the antihyperalgesic Z944 effect. Finally, ELISA analysis revealed increased CaV3.2 channel expression in the spinal trigeminal subnucleus caudalis. Altogether, the present study demonstrates an important role of CaV3.2 channels in trigeminal pain.

Seeding of breast cancer cell line (MDA-MB-231LUC+) to the mandible induces overexpression of substance P and CGRP throughout the trigeminal ganglion and widespread peripheral sensory neuropathy throughout all three of its divisions

Some types of cancer are commonly associated with intense pain even at the early stages of the disease. The mandible is particularly vulnerable to metastasis from breast cancer, and this process has been studied using a bioluminescent human breast cancer cell line (MDA-MB-231^LUC+). Using this cell line and anatomic and neurophysiologic methods in the trigeminal ganglion (TG), we examined the impact of cancer seeding in the mandible on behavioral evidence of hypersensitivity and on trigeminal sensory neurons. Growth of cancer cells seeded to the mandible after arterial injection of the breast cancer cell line in Foxn1 animals (allogeneic model) induced behavioral hypersensitivity to mechanical stimulation of the whisker pad and desensitization of tactile and sensitization of nociceptive mechanically sensitive afferents. These changes were not restricted to the site of metastasis but extended to sensory afferents in all three divisions of the TG, accompanied by widespread overexpression of substance P and CGRP in neurons through the ganglion. Subcutaneous injection of supernatant from the MDA-MB-231^LUC+ cell culture in normal animals mimicked some of the changes in mechanically responsive afferents observed with mandibular metastasis. We conclude that released products from these cancer cells in the mandible are critical for the development of cancer-induced pain and that the overall response of the system greatly surpasses these local effects, consistent with the widespread distribution of pain in patients. The mechanisms of neuronal plasticity likely occur in the TG itself and are not restricted to afferents exposed to the metastatic cancer microenvironment.

Chronic Orofacial Pain: Models, Mechanisms, and Genetic and Related Environmental Influences

Chronic orofacial pain conditions can be particularly difficult to diagnose and treat because of their complexity and limited understanding of the mechanisms underlying their aetiology and pathogenesis. Furthermore, there is considerable variability between individuals in their susceptibility to risk factors predisposing them to the development and maintenance of chronic pain as well as in their expression of chronic pain features such as allodynia, hyperalgesia and extraterritorial sensory spread. The variability suggests that genetic as well as environmental factors may contribute to the development and maintenance of chronic orofacial pain. This article reviews these features of chronic orofacial pain, and outlines findings from studies in animal models of the behavioural characteristics and underlying mechanisms related to the development and maintenance of chronic orofacial pain and trigeminal neuropathic pain in particular. The review also considers the role of environmental and especially genetic factors in these models, focussing on findings of differences between animal strains in the features and underlying mechanisms of chronic pain. These findings are not only relevant to understanding underlying mechanisms and the variability between patients in the development, expression and maintenance of chronic orofacial pain, but also underscore the importance for considering the strain of the animal to model and explore chronic orofacial pain processes.

Potential Molecular Targets for Treating Neuropathic Orofacial Pain Based on Current Findings in Animal Models

This review highlights potential molecular targets for treating neuropathic orofacial pain based on current findings in animal models. Preclinical research is currently elucidating the pathophysiology of the disease and identifying the molecular targets for better therapies using animal models that mimic this category of orofacial pain, especially post-traumatic trigeminal neuropathic pain (PTNP) and primary trigeminal neuralgia (PTN). Animal models of PTNP and PTN simulate their etiologies, that is, trauma to the trigeminal nerve branch and compression of the trigeminal root entry zone, respectively. Investigations in these animal models have suggested that biological processes, including inflammation, enhanced neuropeptide-mediated pain signal transmission, axonal ectopic discharges, and enhancement of interactions between neurons and glial cells in the trigeminal pathway, are underlying orofacial pain phenotypes. The molecules associated with biological processes, whose expressions are substantially altered following trigeminal nerve damage or compression of the trigeminal nerve root, are potentially involved in the generation and/or exacerbation of neuropathic orofacial pain and can be potential molecular targets for the discovery of better therapies. Application of therapeutic candidates, which act on the molecular targets and modulate biological processes, attenuates pain-associated behaviors in animal models. Such therapeutic candidates including calcitonin gene-related peptide receptor antagonists that have a reasonable mechanism for ameliorating neuropathic orofacial pain and meet the requirements for safe administration to humans seem worth to be evaluated in clinical trials. Such prospective translation of the efficacy of therapeutic candidates from animal models to human patients would help develop better therapies for neuropathic orofacial pain.