Activation of oxytocin receptor in the trigeminal ganglion attenuates orofacial ectopic pain attributed to inferior alveolar nerve injury

Role of neuropeptides in orofacial pain: A literature review

BACKGROUND

Neuropeptides play a critical role in regulating pain and inflammation. Despite accumulating evidence has further uncovered the novel functions and mechanisms of different neuropeptides in orofacial pain sensation and transmission, there is deficient systematic description of neuropeptides' pain modulation in the orofacial region, especially in the trigeminal system.

OBJECTIVES

The present review aims to summarise several key neuropeptides and gain a better understanding of their major regulatory roles in orofacial inflammation and pain.

METHODS

We review and summarise current studies related to calcitonin gene-related peptide (CGRP), substance P (SP), opioid peptide (OP), galanin (GAL) and other neuropeptides' functions and mechanisms as well as promising targets for orofacial pain control.

RESULTS

A number of neuropeptides are clearly expressed in the trigeminal sensory system and have critical functions in the transduction and pathogenesis of orofacial pain. The functions, possible cellular and molecular mechanisms have been introduced and discussed. Neuropeptides and their agonists or antagonists which are widely studied to be potential treatment options of orofacial pain has been evaluated.

CONCLUSIONS

Various neuropeptides play important but distinct (pro-nociceptive or analgesic) roles in orofacial pain with different mechanisms. In summary, CGRP, SP, NPY, NKA, HK-1, VIP mainly play proinflammatory and pro-nociceptive effects while OP, GAL, OXT, OrxA mainly have inhibitory effects on orofacial pain.

How Sex Hormones Affect Migraine: An Interdisciplinary Preclinical Research Panel Review

Sex hormones and migraine are closely interlinked. Women report higher levels of migraine symptoms during periods of sex hormone fluctuation, particularly during puberty, pregnancy, and perimenopause. Ovarian steroids, such as estrogen and progesterone, exert complex effects on the peripheral and central nervous systems, including pain, a variety of special sensory and autonomic functions, and affective processing. A panel of basic scientists, when challenged to explain what was known about how sex hormones affect the nervous system, focused on two hormones: estrogen and oxytocin. Notably, other hormones, such as progesterone, testosterone, and vasopressin, are less well studied but are also highlighted in this review. When discussing what new therapeutic agent might be an alternative to hormone therapy and menopause replacement therapy for migraine treatment, the panel pointed to oxytocin delivered as a nasal spray. Overall, the conclusion was that progress in the preclinical study of hormones on the nervous system has been challenging and slow, that there remain substantial gaps in our understanding of the complex roles sex hormones play in migraine, and that opportunities remain for improved or novel therapeutic agents. Manipulation of sex hormones, perhaps through biochemical modifications where its positive effects are selected for and side effects are minimized, remains a theoretical goal, one that might have an impact on migraine disease and other symptoms of menopause. This review is a call to action for increased interest and funding for preclinical research on sex hormones, their metabolites, and their receptors. Interdisciplinary research, perhaps facilitated by a collaborative communication network or panel, is a possible strategy to achieve this goal.

Intranasal oxytocin alleviates comorbid depressive symptoms in neuropathic pain via elevating hippocampal BDNF production in both female and male mice

The comorbidity of pain and depression is frequently observed in patients suffering from chronic pain and depression. However, the comorbid mechanism is not well elucidated and the therapeutic medication is still inadequate. Oxytocin is a neuropeptide synthesized in the hypothalamus. It has been reported to relieve chronic pain and depressive symptoms. However, the analgesic action and mechanisms of oxytocin have mainly been investigated using peripheral or spinal administration. Because of the advantage of intranasal delivery of oxytocin in crossing the blood-brain barrier, we investigated the effect of intranasal application of oxytocin on neuropathic pain and comorbid depressive symptoms in both female and male mice. In female and male mice receiving spared nerve injury (SNI) surgery, intranasal oxytocin (2.4 μg, daily for 28 days) attenuated depression-like behavior, but did not alleviate mechanical hyperalgesia. Intranasal oxytocin not only inhibited the activation of microglia and astrocytes, but also increased the downregulated oxytocin receptor (OTR) expression, reversed the elevated GluN2A, and restored the decreased BDNF expression in the hippocampus. SNI also decreased OTR expression in the spinal cord and increased spinal GluN2A and BDNF. However, intranasal oxytocin treatment did not change the expression levels of OTR, GluN2A, or BDNF in the spinal cord of neuropathic mice. The results suggest that the oxytocin signaling in the hippocampus is involved in the comorbidity of pain and depression, and intranasal oxytocin may have the potential to treat depressive symptoms in neuropathic pain patients.

Targeting sensory neuron GPCRs for peripheral neuropathic pain

Despite the high prevalence of peripheral neuropathic pain (NP) conditions and significant progress in understanding its underlying mechanisms, the management of peripheral NP remains inadequate. Existing pharmacotherapies for NP act primarily on the central nervous system (CNS) and are often associated with CNS-related adverse effects, limiting their clinical effectiveness. Mounting preclinical evidence indicates that reducing the heightened activity in primary sensory neurons by targeting G-protein-coupled receptors (GPCRs), without activating these receptors in the CNS, relieves pain without central adverse effects. In this review, we focus on recent advancements in GPCR-mediated peripheral pain relief and discuss strategies to advance the development of more effective and safer therapies for peripheral NP by shifting from traditional CNS modulatory approaches toward selective targeting of GPCRs on primary sensory neurons.

Oxytocin as neuro-hormone and neuro-regulator exert neuroprotective properties: A mechanistic graphical review

BACKGROUND

Neurodegeneration is progressive cell loss in specific neuronal populations, often resulting in clinical consequences with significant medical, societal, and economic implications. Because of its antioxidant, anti-inflammatory, and anti-apoptotic properties, oxytocin has been proposed as a potential neuroprotective and neurobehavioral therapeutic agent, including modulating mood disturbances and cognitive enchantment.

METHODS

Literature searches were conducted using the following databases Web of Science, PubMed, Elsevier Science Direct, Google Scholar, the Core Collection, and Cochrane from January 2000 to February 2023 for articles dealing with oxytocin neuroprotective properties in preventing or treating neurodegenerative disorders and diseases with a focus on oxidative stress, inflammation, and apoptosis/cell death.

RESULTS

The neuroprotective effects of oxytocin appears to be mediated by its anti-inflammatory properties, inhibition of neuro inflammation, activation of several antioxidant enzymes, inhibition of oxidative stress and free radical formation, activation of free radical scavengers, prevent of mitochondrial dysfunction, and inhibition of apoptosis.

CONCLUSION

Oxytocin acts as a neuroprotective agent by preventing neuro-apoptosis, neuro-inflammation, and neuronal oxidative stress, and by restoring mitochondrial function.

Metabotropic glutamate receptor 5-mediated inhibition of inward-rectifying K+ channel 4.1 contributes to orofacial ectopic mechanical allodynia following inferior alveolar nerve transection in male mice

Inward-rectifying K⁺ channel 4.1 (Kir4.1), which regulates the electrophysiological properties of neurons and glia by affecting K⁺ homeostasis, plays a critical role in neuropathic pain. Metabotropic glutamate receptor 5 (mGluR5) regulates the expression of Kir4.1 in retinal Müller cells. However, the role of Kir4.1 and its expressional regulatory mechanisms underlying orofacial ectopic allodynia remain unclear. This study aimed to investigate the biological roles of Kir4.1 and mGluR5 in the trigeminal ganglion (TG) in orofacial ectopic mechanical allodynia and the role of mGluR5 in Kir4.1 regulation. An animal model of nerve injury was established via inferior alveolar nerve transection (IANX) in male C57BL/6J mice. Behavioral tests indicated that mechanical allodynia in the ipsilateral whisker pad lasted at least 14 days after IANX surgery and was alleviated by the overexpression of Kir4.1 in the TG, as well as intraganglionic injection of an mGluR5 antagonist (MPEP hydrochloride) or a protein kinase C (PKC) inhibitor (chelerythrine chloride); Conditional knockdown of the Kir4.1 gene downregulated mechanical thresholds in the whisker pad. Double immunostaining revealed that Kir4.1 and mGluR5 were co-expressed in satellite glial cells in the TG. IANX downregulated Kir4.1 and upregulated mGluR5 and phosphorylated PKC (p-PKC) in the TG; Inhibition of mGluR5 reversed the changes in Kir4.1 and p-PKC that were induced by IANX; Inhibition of PKC activation reversed the downregulation of Kir4.1 expression caused by IANX (p < .05). In conclusion, activation of mGluR5 in the TG after IANX contributed to orofacial ectopic mechanical allodynia by suppressing Kir4.1 via the PKC signaling pathway.

Up-regulation of oxytocin receptors on peripheral sensory neurons mediates analgesia in chemotherapy-induced neuropathic pain

BACKGROUND AND PURPOSE

Chemotherapy-induced neuropathic pain (CINP) currently has limited effective treatment. Although the roles of oxytocin (OXT) and the oxytocin receptor (OXTR) in central analgesia have been well documented, the expression and function of OXTR in the peripheral nervous system remain unclear. Here, we evaluated the peripheral antinociceptive profiles of OXTR in CINP.

EXPERIMENTAL APPROACH

Paclitaxel (PTX) was used to establish CINP. Quantitative real-time polymerase chain reaction (qRT-PCR), in situ hybridization, and immunohistochemistry were used to observe OXTR expression in dorsal root ganglia (DRG). The antinociceptive effects of OXT were assessed by hot-plate and von Frey tests. Whole-cell patch clamp was performed to record sodium currents, excitability of DRG neurons, and excitatory synapse transmission.

KEY RESULTS

Expression of OXTR in DRG neurons was enhanced significantly after PTX treatment. Activation of OXTR exhibited antinociceptive effects, by decreasing the hyperexcitability of DRG neurons in PTX-treated mice. Additionally, OXTR activation up-regulated the phosphorylation of protein kinase C (pPKC) and, in turn, impaired voltage-gated sodium currents, particularly the voltage-gated sodium channel 1.7 (Na_V 1.7) current, that plays an indispensable role in PTX-induced neuropathic pain. OXT suppressed excitatory transmission in the spinal dorsal horn as well as excitatory inputs from primary afferents in PTX-treated mice.

CONCLUSION AND IMPLICATIONS

The OXTR in small-sized DRG neurons is up-regulated in CINP and its activation relieved CINP by inhibiting the neural excitability by impairment of Na_V 1.7 currents via pPKC. Our results suggest that OXTR on peripheral sensory neurons is a potential therapeutic target to relieve CINP.

Pathophysiology of Post-Traumatic Trigeminal Neuropathic Pain

Trigeminal nerve injury is one of the causes of chronic orofacial pain. Patients suffering from this condition have a significantly reduced quality of life. The currently available management modalities are associated with limited success. This article reviews some of the common causes and clinical features associated with post-traumatic trigeminal neuropathic pain (PTNP). A cascade of events in the peripheral and central nervous system function is involved in the pathophysiology of pain following nerve injuries. Central and peripheral processes occur in tandem and may often be co-dependent. Due to the complexity of central mechanisms, only peripheral events contributing to the pathophysiology have been reviewed in this article. Future investigations will hopefully help gain insight into trigeminal-specific events in the pathophysiology of the development and maintenance of neuropathic pain secondary to nerve injury and enable the development of new therapeutic modalities.

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.

Interactions Among Non-Coding RNAs and mRNAs in the Trigeminal Ganglion Associated with Neuropathic Pain

Background

Recent studies have demonstrated the contribution of non-coding RNAs (ncRNAs) to neuropathic pain. However, the expression profile of ncRNAs in the trigeminal ganglion (TG) and their functional mechanism underlying trigeminal neuropathic pain are still unclear.

Methods

In the present study, the trigeminal neuropathic pain model induced by chronic constriction injury of the infraorbital nerve (CCI-ION) was used to study the expression profile and potential regulatory mechanism of miRNAs, lncRNAs, circRNAs, and mRNAs in the TG by RNA-sequencing (RNA-seq) and bioinformatics analysis. CCI-ION mice suffered from mechanical allodynia from 3 days to 28 days after surgery.

Results

The RNA-seq results discovered 67 miRNAs, 216 lncRNAs, 14 circRNAs, 595 mRNAs, and 421 genes were differentially expressed (DE) in the TG of CCI-ION mice 7 days after surgery. And 39 DEGs were known pain genes. Besides, 5 and 35 pain-related DE mRNAs could be targeted by 6 DE miRNAs and 107 DE lncRNAs, respectively. And 23 pain-related DEGs had protein–protein interactions (PPI) with each other. GO analysis indicated membrane-related cell components and binding-related molecular functions were significantly enriched. KEGG analysis showed that nociception-related signaling pathways were significantly enriched for DE ncRNAs and DEGs. Finally, the competing endogenous RNA (ceRNA) regulatory network of DE lncRNA/DE circRNA-DE miRNA-DE mRNA existed in the TG of mice with trigeminal neuropathic pain.

Conclusion

Our findings demonstrate ncRNAs are involved in the development of trigeminal neuropathic pain, possibly through the ceRNA mechanism, which brings a new bright into the study of trigeminal neuropathic pain and the development of novel treatments targeting ncRNAs.

Behavioral and receptor expression studies on the primary somatosensory cortex and anterior cingulate cortex oxytocin involvement in modulation of sensory and affective dimensions of neuropathic pain induced by partial sciatic nerve ligation in rats

BACKGROUND

Brain cortical areas are involved in processing of sensory, affective and cognitive aspects of pain. In the present study, microinjection effects of oxytocin and L-368,899 (an oxytocin receptor antagonist) into the primary somatosensory cortex (S1) and anterior cingulate cortex (ACC) were investigated on sensory and affective aspects of neuropathic pain.

METHODS

Neuropathic pain was induced by partial sciatic nerve ligation (PSNL). Seven days later, right and left sides of S1 and ACC were surgically implanted with guide cannulas. Sensory (day 14) and affective (day 17) dimensions were recorded using von Frey filaments and place escape avoidance paradigm, respectively. The S1 and ACC oxytocin receptor protein expression were also determined.

RESULTS

The S1 and ACC oxytocin suppressed PSNL-induced mechanical allodynia, whereas PSNL-induced aversion was attenuated by ACC oxytocin. In the S1, alone L-368,899 with no effect on aversion increased mechanical allodynia, whereas, in the ACC, this treatment increased both mechanical allodynia and aversion. Pre-treatment with L-368,899 prevented oxytocin-induced anti-allodynia and anti-aversion. Oxytocin and L-368,899 did not alter mechanical allodynia in intact and sham groups. All the above-mentioned treatments did not change crossing number. The density of oxytocin receptors in the S1 and ACC of PSNL group was increased 1.5-2 folds in comparison to intact and sham groups.

CONCLUSIONS

The results of the present study explained that the ACC and S1 oxytocin ameliorated sensory component of neuropathic pain, whereas affective component was attenuated only by ACC oxytocin. These effects might be related to the PSNL-increased oxytocin receptor expression in the S1 and ACC.

C-X-C motif chemokine ligand 1 and its receptor C-X-C motif chemokine receptor 2 in trigeminal ganglion contribute to nerve injury-induced orofacial mechanical allodynia

BACKGROUND

Orofacial ectopic pain induced by trigeminal nerve injury is a serious complication of dental treatment. C-X-C motif chemokine ligand 1 (CXCL1) and its primary receptor C-X-C motif chemokine receptor 2 (CXCR2) contribute to the development and maintenance of neuropathic pain in the spinal nervous system, but their roles in trigeminal neuropathic sensation are still poorly understood.

OBJECTIVES

This study aimed to investigate the exact role of CXCL1 and CXCR2 in the regulation of orofacial ectopic mechanical allodynia and their potential downstream mechanisms in the trigeminal ganglion (TG).

METHODS

The head withdrawal threshold (HWT) of C57BL/6 mice was evaluated after inferior alveolar nerve (IAN) transection (IANX). Then, the distribution and expression of CXCL1 and CXCR2, and their potential downstream mechanisms in the TG were further measured using immunohistochemistry, real-time reverse transcription-quantitative polymerase chain reaction and Western blotting. Moreover, the effect of SB225002 (an inhibitor of CXCR2) on mechanical allodynia was examined. The data were analysed using the Student's t test and a analysis of variance (ANOVA).

RESULTS

IANX triggered persistent (>21 days) mechanical allodynia and upregulation of CXCL1 and CXCR2 in the TG. In addition, exogenous CXCL1 also lowered the HWT, which was alleviated by CXCR2 and protein kinase C (PKC) antagonists (p < .05). In addition, IANX increased the phosphorylated PKC (p-PKC) levels and decreased the expression of voltage-gated potassium channels (Kv), and these effects were reversed by inhibition of CXCR2 (p < .05).

CONCLUSION

Our results demonstrated that CXCR2 participated in orofacial ectopic mechanical allodynia via downregulation of Kv1.4 and Kv1.1 through the PKC signalling pathway. This mechanism may be a potential target in developing a treatment strategy for ectopic orofacial pain.

The role of oxytocin, vasopressin, and their receptors at nociceptors in peripheral pain modulation

Oxytocin and vasopressin are neurohypophyseal hormones with sequence similarity and play a central role in bodily homeostatic regulation. Pain is currently understood to be an important phenotype that those two neurohormones strongly downregulate. Nociceptors, the first component of the ascending neural circuit for pain signals, have constantly been shown to be modulated by those peptides. The nociceptor modulation appears to be critical in pain attenuation, which has led to a gradual increase in scientific interest about their physiological processes and also drawn attention to their translational potentials. This review focused on what are recently understood and stay under investigation in the functional modulation of nociceptors by oxytocin and vasopressin. Effort to produce a nociceptor-specific view could help to construct a more systematic picture of the peripheral pain modulation by oxytocin and vasopressin.

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.

Repeated oxytocin prevents central sensitization by regulating synaptic plasticity via oxytocin receptor in a chronic migraine mouse model

BACKGROUND

Central sensitization is one of the characters of chronic migraine (CM). Aberrant synaptic plasticity can induce central sensitization. Oxytocin (OT), which is a hypothalamic hormone, plays an important antinociceptive role. However, the antinociceptive effect of OT and the underlying mechanism in CM remains unclear. Therefore, we explored the effect of OT on central sensitization in CM and its implying mechanism, focusing on synaptic plasticity.

METHODS

A CM mouse model was established by repeated intraperitoneal injection of nitroglycerin (NTG). Von Frey filaments and radiant heat were used to measure the nociceptive threshold. Repeated intranasal OT and intraperitoneal L368,899, an oxytocin receptor (OTR) antagonist, were administered to investigate the effect of OT and the role of OTR. The expression of calcitonin gene-related peptide (CGRP) and c-fos were measured to assess central sensitization. N-methyl D-aspartate receptor subtype 2B (NR2B)-regulated synaptic-associated proteins and synaptic plasticity were explored by western blot (WB), transmission electron microscope (TEM), and Golgi-Cox staining.

RESULTS

Our results showed that the OTR expression in the trigeminal nucleus caudalis (TNC) of CM mouse was significantly increased, and OTR was colocalized with the postsynaptic density protein 95 (PSD-95) in neurons. Repeated intranasal OT alleviated the NTG-induced hyperalgesia and prevented central sensitization in CM mouse. Additionally, the OT treatment inhibited the overexpression of phosphorylated NR2B and synaptic-associated proteins including PSD-95, synaptophysin-1 (syt-1), and synaptosomal-associated protein 25 (snap25) in the TNC of CM mouse and restored the abnormal synaptic structure. The protective effect of OT was prevented by L368,899. Furthermore, the expression of adenylyl cyclase 1 (AC1)/ protein kinase A (PKA)/ phosphorylation of cyclic adenosine monophosphate response element-binding protein (pCREB) pathway was depressed by OT and restored by L368,899.

CONCLUSIONS

Our findings demonstrate that repeated intranasal OT eliminates central sensitization by regulating synaptic plasticity via OTR in CM. The effect of OT has closely associated with the down-regulation of AC1/PKA/pCREB signaling pathway, which is activated in CM model. Repeated intranasal OT may be a potential candidate for CM prevention.

NMDAR1-Src-Pannexin1 Signal Pathway in the Trigeminal Ganglion Contributed to Orofacial Ectopic Pain Following Inferior Alveolar Nerve Transection

The N-methyl-d-aspartate receptor (NMDAR) is a glutamate-gated receptor channel that plays a role in peripheral neuropathic pain. Src, a protein tyrosine kinase, can regulate the activation of NMDARs in chronic pain conditions. Pannexin 1 (Panx1), a plasma membrane channel, plays an important role in neuropathic pain and functionally interacts with NMDARs in the pathological condition of epilepsy. In this study, the roles of NMDAR1 (NR1), Src, and Panx1 and their interactions in the trigeminal ganglion (TG) in orofacial ectopic pain attributed to inferior alveolar nerve transection (IANX) were investigated. IANX induced mechanical allodynia in the whisker pad with increased expression levels of NR1, Src phosphorylation (p-Src), and Panx1 in the TG. Double immunostaining revealed that NR1, Src, and Panx1 all colocalized with glutamine synthetase (GS) and neuronal nuclei (NeuN), and they overlapped in the TG, suggesting that they might be structurally connected to one another. In addition, trigeminal injection of memantine, PP2, or ¹⁰Panx attenuated IANX-induced mechanical allodynia in the whisker pad. Continuous intraganglionic administration of memantine (an antagonist of NMDAR) decreased IANX-induced upregulated expression of p-Src and Panx1. Similarly, PP2 (an inhibitor of Src) also decreased Panx1 protein expression but had no effect on NR1. In addition, intraganglionic injection of ¹⁰Panx (a blocker of Panx1) decreased NR1 protein expression but did not affect Src. In general, our findings demonstrated that NR1, Src, and Panx1 all contributed to orofacial ectopic pain following IANX and that they composed a signalling pathway in the TG involved in mechanical allodynia.