Alternation of gene expression in trigeminal ganglion neurons following complete Freund's adjuvant or capsaicin injection into the rat face

Transcriptomic Analysis of Trigeminal Ganglion and Spinal Trigeminal Nucleus Caudalis in Mice with Inflammatory Temporomandibular Joint Pain

Background

Persistent facial pain heavily impacts the quality of life in patients with temporomandibular joint (TMJ) disorders. Previous studies have demonstrated that long non-coding ribonucleic acid (lncRNA) is an important regulator of pain. In this study, we aimed to analyze lncRNA expression in the whole transcriptome of trigeminal ganglia (TG) and spinal trigeminal nucleus caudalis (Sp5C) in a chronic inflammatory TMJ pain mouse model.

Methods

Chronic inflammatory TMJ pain was induced by intra-TMJ injection of complete Freund’s adjuvant (CFA). Mouse TG and Sp5C tissues were harvested on day 4 after CFA injection. The lncRNA expression patterns in the whole transcriptome of TG and Sp5C were profiled with RNA sequencing.

Results

We observed that 38 lncRNAs and 849 mRNAs were differentially expressed after CFA treatment. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analysis further revealed relationships among those differentially expressed lncRNAs and mRNAs and their potential functions. Specific categories of biological process, cellular processes, and molecular function of the differentially expressed transcripts were ascertained.

Conclusion

Our results suggest that lncRNA expression in the whole transcriptome of trigeminal nociceptive system could contribute to the molecular mechanisms that underlie chronic inflammatory TMJ pain.

Models of Inflammation: Carrageenan- or Complete Freund's Adjuvant (CFA)-Induced Edema and Hypersensitivity in the Rat

Animal models of inflammation are used to assess the production of inflammatory mediators at sites of inflammation, the processing of pain sensation at CNS sites, the anti-inflammatory properties of agents such as nonsteroidal anti-inflammatory drugs (NSAIDs), and the efficacy of putative analgesic compounds in reversing cutaneous hypersensitivity. Detailed in this article are methods to elicit and measure carrageenan- and complete Freund's adjuvant (CFA)-induced cutaneous inflammation. Due to possible differences between the dorsal root sensory system and the trigeminal sensory system, injections into either the footpad or vibrissal pad are described. In this manner, cutaneous inflammation can be assessed in tissue innervated by the lumbar dorsal root ganglion neurons (footpad) or by the trigeminal ganglion neurons (vibrissal pad). © 2021 Wiley Periodicals LLC.

Neurotransmitter and tryptophan metabolite concentration changes in the complete Freund's adjuvant model of orofacial pain

BACKGROUND

The neurochemical background of the evolution of headache disorders, still remains partially undiscovered. Accordingly, our aim was to further explore the neurochemical profile of Complete Freund's adjuvant (CFA)-induced orofacial pain, involving finding the shift point regarding small molecule neurotransmitter concentrations changes vs. that of the previously characterized headache-related neuropeptides. The investigated neurotransmitters consisted of glutamate, γ-aminobutyric acid, noradrenalin and serotonin. Furthermore, in light of its influence on glutamatergic neurotransmission, we measured the level of kynurenic acid (KYNA) and its precursors in the kynurenine (KYN) pathway (KP) of tryptophan metabolism.

METHODS

The effect of CFA was evaluated in male Sprague Dawley rats. Animals were injected with CFA (1 mg/ml, 50 μl/animal) into the right whisker pad. We applied high-performance liquid chromatography to determine the concentrations of the above-mentioned compounds from the trigeminal nucleus caudalis (TNC) and somatosensory cortex (ssCX) of rats. Furthermore, we measured some of these metabolites from the cerebrospinal fluid and plasma as well. Afterwards, we carried out permutation t-tests as post hoc analysis for pairwise comparison.

RESULTS

Our results demonstrated that 24 h after CFA treatment, the level of glutamate, KYNA and that of its precursor, KYN was still elevated in the TNC, all diminishing by 48 h. In the ssCX, significant concentration increases of KYNA and serotonin were found.

CONCLUSION

This is the first study assessing neurotransmitter changes in the TNC and ssCX following CFA treatment, confirming the dominant role of glutamate in early pain processing and a compensatory elevation of KYNA with anti-glutamatergic properties. Furthermore, the current findings draw attention to the limited time interval where medications can target the glutamatergic pathways.

The effect of orofacial complete Freund's adjuvant treatment on the expression of migraine-related molecules

Background

Migraine is a neurovascular primary headache disorder, which causes significant socioeconomic problems worldwide. The pathomechanism of disease is enigmatic, but activation of the trigeminovascular system (TS) appears to be essential during the attack. Migraine research of recent years has focused on neuropeptides, such as calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide 1–38 (PACAP1–38) as potential pathogenic factors and possible therapeutic offensives. The goal of present study was to investigate the simultaneous expression of CGRP and precursor of PACAP1–38 (preproPACAP) in the central region of the TS in a time-dependent manner following TS activation in rats.

Methods

The right whisker pad of rats was injected with 50 μl Complete Freund’s Adjuvant (CFA) or saline. A mechanical allodynia test was performed with von Frey filaments before and after treatment. Transcardial perfusion of the animals was initiated 24, 48, 72 and 120 h after injection, followed by the dissection of the nucleus trigeminus caudalis (TNC). After preparation, the samples were stored at − 80 °C until further use. The relative optical density of CGRP and preproPACAP was analyzed by Western blot. One-way ANOVA and Kruskal-Wallis followed by Tukey post hoc test were used to evaluate the data. Regression analysis was applied to explore the correlation between neuropeptides expression and hyperalgesia.

Results

Orofacial CFA injection resulted in significant CGRP and preproPACAP release in the TNC 24, 48, 72 and 120 h after the treatment. The level of neuropeptides reached its maximum at 72 h after CFA injection, corresponding to the peak of facial allodynia. Negative, linear correlation was detected between the expression level of neuropeptides and value of mechanonociceptive threshold.

Conclusion

This is the first study which suggests that the expression of CGRP and preproPACAP simultaneously increases in the central region of activated TS and it influences the formation of mechanical hyperalgesia. Our results contribute to a better understanding of migraine pathogenesis and thereby to the development of more effective therapeutic approaches.

Chronic orofacial pain animal models - progress and challenges

INTRODUCTION

Chronic orofacial pain is one of the most common pain conditions experienced by adults. Animal models are often selected as the most useful scientific methodology to explore the pathophysiology of the disorders that cause this disabling pain to facilitate the development of new treatments. The creation of new models or the improvement of existing ones is essential for finding new ways to approach the complex neurobiology of this type of pain. Areas covered: The authors describe and discuss a variety of animal models used in chronic orofacial pain (COFP). Furthermore, they examine in detail the mechanisms of action involved in orofacial neuropathic pain and orofacial inflammatory pain. Expert opinion: The use of animal models has several advantages in chronic orofacial pain drug discovery. Choosing an animal model that most closely represents the human disease helps to increase the chances of finding effective new therapies and is key to the successful translation of preclinical research to clinical practice. Models using genetically modified animals seem promising but have not yet been fully developed for use in chronic orofacial pain research. Although animal models have provided significant advances in the pharmacological treatment of orofacial pain, several barriers still need to be overcome for better treatment options.

Transcriptional Alterations in the Trigeminal Ganglia, Nucleus and Peripheral Blood Mononuclear Cells in a Rat Orofacial Pain Model

Orofacial pain and headache disorders are among the most debilitating pain conditions. While the pathophysiological basis of these disorders may be diverse, it is generally accepted that a common mechanism behind the arising pain is the sensitization of extra- and intracranial trigeminal primary afferents. In the present study we investigated gene expression changes in the trigeminal ganglia (TRG), trigeminal nucleus caudalis (TNC) and peripheral blood mononuclear cells (PBMC) evoked by Complete Freund's Adjuvant (CFA)-induced orofacial inflammation in rats, as a model of trigeminal sensitization. Microarray analysis revealed 512 differentially expressed genes between the ipsi- and contralateral TRG samples 7 days after CFA injection. Time-dependent expression changes of G-protein coupled receptor 39 (Gpr39), kisspeptin-1 receptor (Kiss1r), kisspeptin (Kiss1), as well as synaptic plasticity-associated Lkaaear1 (Lkr) and Neurod2 mRNA were described on the basis of qPCR results. The greatest alterations were observed on day 3 ipsilaterally, when orofacial mechanical allodynia reached its maximum. This corresponded well with patterns of neuronal (Fosb), microglia (Iba1), and astrocyte (Gfap) activation markers in both TRG and TNC, and interestingly also in PBMCs. This is the first description of up- and downregulated genes both in primary and secondary sensory neurones of the trigeminovascular system that might play important roles in neuroinflammatory activation mechanisms. We are the first to show transcriptomic alterations in the PBMCs that are similar to the neuronal changes. These results open new perspectives and initiate further investigations in the research of trigeminal pain disorders.

RNA Sequencing of Trigeminal Ganglia in Rattus Norvegicus after Glyceryl Trinitrate Infusion with Relevance to Migraine

INTRODUCTION

Infusion of glyceryl trinitrate (GTN), a donor of nitric oxide, induces immediate headache in humans that in migraineurs is followed by a delayed migraine attack. In order to achieve increased knowledge of mechanisms activated during GTN-infusion this present study aims to investigate transcriptional responses to GTN-infusion in the rat trigeminal ganglia.

METHODS

Rats were infused with GTN or vehicle and trigeminal ganglia were isolated either 30 or 90 minutes post infusion. RNA sequencing was used to investigate transcriptomic changes in response to the treatment. Furthermore, we developed a novel method for Gene Set Analysis Of Variance (GSANOVA) to identify gene sets associated with transcriptional changes across time.

RESULTS

15 genes displayed significant changes in transcription levels in response to GTN-infusion. Ten of these genes showed either sustained up- or down-regulation in the 90-minute period after infusion. The GSANOVA analysis demonstrate enrichment of pathways pointing towards an increase in immune response, signal transduction, and neuroplasticity in response to GTN-infusion. Future functional in-depth studies of these mechanisms are expected to increase our understanding of migraine pathogenesis.

Lack of evidence for ectopic sprouting of genetically labeled Aβ touch afferents in inflammatory and neuropathic trigeminal pain

Background

Mechanical and in particular tactile allodynia is a hallmark of chronic pain in which innocuous touch becomes painful. Previous cholera toxin B (CTB)-based neural tracing experiments and electrophysiology studies had suggested that aberrant axon sprouting from touch sensory afferents into pain-processing laminae after injury is a possible anatomical substrate underlying mechanical allodynia. This hypothesis was later challenged by experiments using intra-axonal labeling of A-fiber neurons, as well as single-neuron labeling of electrophysiologically identified sensory neurons. However, no studies have used genetically labeled neurons to examine this issue, and most studies were performed on spinal but not trigeminal sensory neurons which are the relevant neurons for orofacial pain, where allodynia oftentimes plays a dominant clinical role.

Findings

We recently discovered that parvalbumin::Cre (Pv::Cre) labels two types of Aβ touch neurons in trigeminal ganglion. Using a Pv::CreER driver and a Cre-dependent reporter mouse, we specifically labeled these Aβ trigeminal touch afferents by timed taxomifen injection prior to inflammation or infraorbital nerve injury (ION transection). We then examined the peripheral and central projections of labeled axons into the brainstem caudalis nucleus after injuries vs controls. We found no evidence for ectopic sprouting of Pv::CreER labeled trigeminal Aβ axons into the superficial trigeminal noci-receptive laminae. Furthermore, there was also no evidence for peripheral sprouting.

Conclusions

CreER-based labeling prior to injury precluded the issue of phenotypic changes of neurons after injury. Our results suggest that touch allodynia in chronic orofacial pain is unlikely caused by ectopic sprouting of Aβ trigeminal afferents.

A novel operant-based behavioral assay of mechanical allodynia in the orofacial region of rats

BACKGROUND

Detecting behaviors related to orofacial pain in rodent models often relies on subjective investigator grades or methods that place the animal in a stressful environment. In this study, an operant-based behavioral assay is presented for the assessment of orofacial tactile sensitivity in the rat.

NEW METHODS

In the testing chamber, rats are provided access to a sweetened condensed milk bottle; however, a 360° array of stainless steel wire loops impedes access. To receive the reward, an animal must engage the wires across the orofacial region. Contact with the bottle triggers a motor, requiring the animal to accept increasing pressure on the face during the test. To evaluate this approach, tolerated bottle distance was measured for 10 hairless Sprague Dawley rats at baseline and 30 min after application of capsaicin cream (0.1%) to the face. The experiment was repeated to evaluate the ability of morphine to reverse this effect.

RESULTS

The application of capsaicin cream reduced tolerated bottle distance measures relative to baseline (p<0.05). As long as morphine did not cause reduced participation due to sedation, subcutaneous morphine dosing reduced the effects of capsaicin (p<0.001). Comparison with existing method: For behavioral tests, experimenters often make subjective decisions of an animal's response. Operant methods can reduce these effects by measuring an animal's selection in a reward-conflict decision. Herein, a method to measure orofacial sensitivity is presented using an operant system.

CONCLUSIONS

This operant device allows for consistent measurement of heightened tactile sensitivity in the orofacial regions of the rat.

P2X3 receptor mediates ectopic mechanical allodynia with inflamed lower lip in mice

Ectopic pain in other orofacial regions develops with local inflammation in separated orofacial structures. However, the basis for the spreading of pain to adjacent orofacial areas after local inflammation is still unknown. In the present study, we determined if the P2X(3) receptor (P2X(3)R) was associated with altered mechanical sensitivity of the whisker pad skin following complete Freund's adjuvant (CFA) injection into the lower lip. Mice with local inflammation induced by CFA injection into the lower lip demonstrated significant mechanical allodynia of whisker pad skin. The mechanical allodynia was reversed by P2X(3)R antagonist, A-317491 administration into whisker pad skin. The number of P2X(3)R and calcitonin gene-related peptide (CGRP) positive trigeminal ganglion (TG) neurons that innervates the whisker pad skin and lower lip was increased after CFA injection into the lower lip. CGRP protein expression in TG ipsilateral to CFA injection was also significantly greater than that of the saline-injected mice. The present findings suggest that induced CGRP by local inflammation in the lower lip increases P2X(3)R in TG neurons, the increased P2X(3)Rs are involved in the sensitization of primary afferent neurons in the whisker pad skin. This P2X(3)R overexpression may underlie ectopic mechanical allodynia in the whisker pad skin after CFA injection into the lower lip.

Involvement of peripheral ionotropic glutamate receptors in orofacial thermal hyperalgesia in rats

Background

The purpose of the present study was to elucidate the mechanisms that may underlie the sensitization of trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord (C1-C2) neurons to heat or cold stimulation of the orofacial region following glutamate (Glu) injection.

Results

Glu application to the tongue or whisker pad skin caused an enhancement of head-withdrawal reflex and extracellular signal-regulated kinase (ERK) phosphorylation in Vc-C2 neurons. Head-withdrawal reflex and ERK phosphorylation were also enhanced following cold stimulation of the tongue but not whisker pad skin in Glu-injected rats, and the head-withdrawal reflex and ERK phosphorylation were enhanced following heat stimulation of the tongue or whisker pad skin. The enhanced head-withdrawal reflex and ERK phosphorylation after heat stimulation of the tongue or whisker pad skin, and those following cold stimulation of the tongue but not whisker pad skin were suppressed following ionotropic glutamate receptor antagonists administration into the tongue or whisker pad skin. Furthermore, intrathecal administration of MEK1/2 inhibitor PD98059 caused significant suppression of enhanced head-withdrawal reflex in Glu-injected rats, heat head-withdrawal reflex in the rats with Glu injection into the tongue or whisker pad skin and cold head-withdrawal reflex in the rats with Glu injection into the tongue.

Conclusions

The present findings suggest that peripheral Glu receptor mechanisms may contribute to cold hyperalgesia in the tongue but not in the facial skin, and also contribute to heat hyperalgesia in the tongue and facial skin, and that the mitogen-activated protein kinase cascade in Vc-C2 neurons may be involved in these Glu-evoked hyperalgesic effects.

Patterns of pain: meta-analysis of microarray studies of pain

Existing microarray gene expression profiling studies of tonic/chronic pain were subjected to meta-analysis to identify genes found to be regulated by these pain states in multiple, independent experiments. Twenty studies published from 2002 to 2008 were identified, describing the statistically significant regulation of 2254 genes. Of those, a total of 79 genes were found to be statistically significant "hits" in 4 or more independent microarray experiments, corresponding to a conservative P<0.01 overall. Gene ontology-based functional annotation clustering analyses revealed strong evidence for regulation of immune-related genes in pain states. A multi-gene quantitative real-time polymerase chain reaction experiment was run on dorsal root ganglion (DRG) and spinal cord tissue from rats and mice given nerve (sciatic chronic constriction; CCI) or inflammatory (complete Freund's adjuvant) injury. We independently confirmed the regulation of 43 of these genes in the rat-CCI-DRG condition; the genetic correlates in all other conditions were largely and, in some cases, strikingly, independent. However, a handful of genes were identified whose regulation bridged etiology, anatomical locus, and/or species. Most notable among these were Reg3b (regenerating islet-derived 3 beta; pancreatitis-associated protein) and Ccl2 (chemokine [C-C motif] ligand 2), which were significantly upregulated in every condition in the rat.