Behavioral testing in rodent models of orofacial neuropathic and inflammatory pain

Infraorbital nerve injury triggers sex-specific neuroimmune responses in the peripheral trigeminal pathway and common pain behaviours

Trigeminal neuropathic pain is emotionally distressing and disabling. It presents with allodynia, hyperalgesia and dysaesthesia. In preclinical models it has been assumed that cephalic nerve constriction injury shows identical molecular, cellular, and sex dependent neuroimmune changes as observed in extra-cephalic injury models. This study sought empirical evidence for such assumptions using the infraorbital nerve chronic constriction model (ION-CCI). We compared the behavioural consequences of nerve constriction with: (i) the temporal patterns of recruitment of macrophages and T-lymphocytes at the site of nerve injury and in the trigeminal ganglion; and (ii) the degree of demyelination and axonal reorganisation in the injured nerve. Our data demonstrated that simply testing for allodynia and hyperalgesia as is done in extra-cephalic neuropathic pain models does not provide access to the range of injury-specific nociceptive responses and behaviours reflective of the experience of trigeminal neuropathic pain. Similarly, trigeminal neuroimmune changes evoked by nerve injury are not the same as those identified in models of extra-cephalic neuropathy. Specifically, the timing, magnitude, and pattern of ION-CCI evoked macrophage and T-lymphocyte activity differs between the sexes.

A review of animal models for burning mouth syndrome: Mechanistic insights and knowledge gaps

OBJECTIVES

The underlying mechanisms of burning mouth syndrome (BMS) remain unclear leading to challenges and unsatisfactory management. Current treatments focus primarily on symptom relief, with few consistently achieving a 50% reduction in pain. This review aims to explore animal models of BMS to gain a better understanding of the underlying mechanisms and to discuss potential and existing knowledge gaps.

METHODS

A comprehensive review of PubMed® , Google Scholar, and Scopus was performed to assess advances and significant gaps of existing rodent models that mimic BMS-related symptoms.

RESULTS

Rodent models of BMS involve reproduction of dry-tongue, chorda tympani transection, or overexpression of artemin protein. Existing preclinical models tend to highlight one specific etiopathogenesis and often overlook sex- and hormone-specific factors.

CONCLUSION

Combining aspects from various BMS models could prove beneficial in developing comprehensive experimental designs and outcomes encompassing the multifaceted nature of BMS.

Progress in study on animal models of trigeminal neuralgia

Trigeminal neuralgia is a manifestation of orofacial neuropathic pain disorder, always deemed to be an insurmountable peak in the field of pain research and treatment. The pain is recurrent, abrupt in onset and termination similar to an electric shock or described as shooting. A poor quality of life has been attributed to trigeminal neuralgia, as the paroxysms of pain may be triggered by innocuous stimuli on the face or inside the oral cavity, such as talking, washing face, chewing and brushing teeth in daily life. The pathogenesis of trigeminal neuralgia has not been fully elucidated, although the microvascular compression in the trigeminal root entry zone is generally considered to be involved in the emergence and progression of the pain disorder. In addition, orofacial neuropathic pain restricted to one or more divisions of the trigeminal nerve might be secondary to peripheral nerve injury. Based on current hypotheses regarding the potential causes, a variety of animal models have been designed to simulate the pathogenesis of trigeminal neuralgia, including models of compression applied to the trigeminal nerve root or trigeminal ganglion, chronic peripheral nerve injury, peripheral inflammatory pain and center-induced pain. However, it has not yet been possible to determine which model can be perfectly employed to explain the mechanisms. The selection of appropriate animal models is of great significance for the study of trigeminal neuralgia. Therefore, it is necessary to discuss the characteristics of the animal models in terms of animal strains, materials, operation methods and behavior observation, in order to gain insight into the research progress in animal models of trigeminal neuralgia. In the future, animal models that closely resemble the features of human trigeminal neuralgia pathogenesis need to be developed, with the aim of making valuable contributions to the relevant basic and translational medical research.

Perineural Treatment with High Mobility Group Box-1 Monoclonal Antibody Prevents Initiation of Pain-Like Behaviors in Female Mice with Trigeminal Neuropathy

Post-traumatic trigeminal neuropathy (PTTN) is a type of chronic pain caused by damage to the trigeminal nerve. A previous study reported that pretreatment with anti-high mobility group box-1 (HMGB1) neutralizing antibodies (nAb) prevented the onset of PTTN following distal infraorbital nerve chronic constriction injury (dIoN-CCI) in male mice. Clinical evidence indicates a high incidence of PTTN in females. Although our previous study found that perineural HMGB1 is crucial in initiation of PTTN in male mice, it is currently unknown whether HMGB1 is also involved in the pathogenesis of PTTN in female mice. Therefore, in the current study, we examined the effect of anti-HMGB1 nAb on pain-like behavior in female mice following dIoN-CCI surgery. We found that dIoN-CCI surgery enhanced reactivity to mechanical and cold stimuli in female mice, which was suppressed by treatment with anti-HMGB1 nAb. Moreover, the increase in macrophages after dIoN-CCI was significantly attenuated by pretreatment with anti-HMGB1 nAb. Furthermore, anti-HMGB1 nAb treatment inhibited microglial activation in the trigeminal spinal tract nucleus. These data suggest that HMGB1 also plays a crucial role in the onset of PTTN after nerve injury in female mice. Thus, anti-HMGB1 nAb could be a novel therapeutic agent for inhibiting the onset of PTTN in female and male mice.

Identification and validation of Rab11a in Rat orofacial inflammatory pain model induced by CFA

Orofacial pain (OFP) is a clinically very common and the most troubling condition; however, there is few effective way to relieve OFP. Rab11a, a small molecule guanosine triphosphate enzyme, is one of the Rab member family playing a vital role in intracellular endocytosis and the pain process. Therefore, we investigated the hub genes of rat OFP model induced by Complete Freund's Adjuvant (CFA) via re-analyzing microarray data (GSE111160). We found that Rab11a acted as a key hub gene in the process of OFP. During the validation of Rab11a, the OFP model was established by peripheral injection of CFA, which decreased the head withdrawal threshold (HWT) and head withdrawal lantency (HWL). Rab11a was observed in NeuN of Sp5C instead of GFAP/IBA-1, and double-IF of Rab11a and Fos positive cells were increased on the 7th day after CFA modeling statistically. Rab11a protein expression in TG and Sp5C of CFA group was also significantly increased. Interestingly, injection of Rab11a-targeted short hairpin RNA (Rab11a-shRNA) into Sp5C could reverse the decrease in HWT and HWL and reduce the expression level of Rab11a. Electrophysiological recording further demonstrated that the activity of Sp5C neuron was improved in CFA group, while Rab11a-shRNA considerably decreased the enhancement of Sp5C neuronal activity. Finally, we detected the expression level of p-PI3K, p-AKT, and p-mTOR in Sp5C of rats after injecting the Rab11a-shRNA virus. To our surprise, CFA upregulated the phosphorylation of PI3K, AKT and mTOR in Sp5C, and Rab11a-shRNA downregulated these molecules' expression. Our data suggest that CFA activates the PI3K/AKT signaling pathway through up-regulating Rab11a expression, which can induce OFP hyperalgesia development furtherly. Targeting Rab11a may be a novel treatment strategy for OFP.

Standardized Centella asiatica (ECa 233) extract decreased pain hypersensitivity development in a male mouse model of chronic inflammatory temporomandibular disorder

Chronic inflammatory temporomandibular disorder (TMD) pain has a high prevalence, and available nonspecific treatments have adverse side effects. ECa 233, a standardized Centella asiatica extract, is highly anti-inflammatory and safe. We investigated its therapeutic effects by injecting complete Freund's adjuvant (CFA) into right temporomandibular joint of mice and administering either ibuprofen or ECa 233 (30, 100, and 300 mg/kg) for 28 days. Inflammatory and nociceptive markers, bone density, and pain hypersensitivity were examined. CFA decreased ipsilateral bone density, suggesting inflammation localization, which ipsilaterally caused immediate calcitonin gene-related peptide elevation in the trigeminal ganglia (TG) and trigeminal subnucleus caudalis (TNC), followed by late increase of NaV1.7 in TG and of p-CREB and activation of microglia in TNC. Contralaterally, only p-CREB and activated microglia in TNC showed delayed increase. Pain hypersensitivity, which developed early ipsilaterally, but late contralaterally, was reduced by ibuprofen and ECa 233 (30 or 100 mg/kg). However, ibuprofen and only 100-mg/kg ECa 233 effectively mitigated marker elevation. This suggests 30-mg/kg ECa 233 was antinociceptive, whereas 100-mg/kg ECa 233 was both anti-inflammatory and antinociceptive. ECa 233 may be alternatively and safely used for treating chronic inflammatory TMD pain, showing an inverted U-shaped dose-response relationship with maximal effect at 100 mg/kg.

Oral Health-Related Quality of Life among Chinese Chronic Orofacial Pain Patients with Psychological Health Problems: A Moderated Mediation Model

Psychological therapies are important for comprehensive chronic orofacial pain (COFP) treatment. This study is to validate the effects of psychological factors on oral health-related quality of life (OHRQoL) among COFP patients in China. Pain catastrophizing, which is a subjective cognitive emotion used to manage the psychological aspects of pain among COFP patients, was examined in relation to COFP severity and OHRQoL. All 479 participants were recruited in Changsha, Hunan Province, China. Cronbach's alpha coefficients (0.868-0.960), composite reliability scores (0.924-0.969), and average variance extracted from each construct (0.555-0.753) all indicated a good model fit. Pearson's correlation analysis showed that age and education status have a positive correlation with COFP severity, pain catastrophizing, and anxiety. COFP severity was related to anxiety, depression, and COFP-OHRQoL. Pain catastrophizing was related to employment status. Anxiety and depression symptoms indirectly mediated the correlation between COFP severity and COFP-OHRQoL. As a second-stage moderator, pain catastrophizing moderated the mediating effects of anxiety symptoms and depression symptoms. Our findings suggest that anxiety, depression, and pain catastrophizing should be evaluated jointly to improve COFP-OHRQoL among COFP patients. This evidence will help therapists to comprehensively treat patients for the best treatment effect.

A novel implantable device for sensory and affective assessment of orofacial pain in rats

Background and objective

Orofacial pain, in particular, chronic orofacial pain remains a great challenge in clinical practice. To better understand the underlying mechanism of disease, it is essential to apply a feasible and stable preclinical measurement of facial pain. Here, we introduced a novel electrical noxious stimulator in freely behavioral rodents and examined its validation in both naïve and chronic orofacial pain animals.

Methods

One subcutaneous device of electrical stimulator was implanted in the facial region for delivery of the nociceptive input. The sensory component of orofacial pain was assessed by response scoring tool, and conditioned place aversion (CPA) paradigm for pain affect respectively. To confirm its usage in chronic pain state, the chronic constriction injury of the infraorbital nerve (ION-CCI) model was then applied.

Results

We found that responsive scores increased with stimulation intensity, and acted in a dosage-dependent manner, which can be attenuated by the administration of morphine intraperitoneally. Naïve rats displayed significant aversive reaction to the noxious electrical stimulation (25V) in the CPA testing. In addition, an obvious sensory hypersensitivity to electrical stimulation was confirmed by the increased response scores in ION-CCI rats. Furthermore, ION-CCI animal showed significant avoidance to electrical stimulation at relatively low intensity (10V), which was innoxious to naïve rats.

Conclusion

Our findings may provide an alternative pre-clinical measurement of orofacial pain, to quantitively assess both sensory and affective component of orofacial pain.

The Impact of Activity-Based Interventions on Neuropathic Pain in Experimental Spinal Cord Injury

Physical activity-based rehabilitative interventions represent the main treatment concept for people suffering from spinal cord injury (SCI). The role such interventions play in the relief of neuropathic pain (NP) states is emerging, along with underlying mechanisms resulting in SCI-induced NP (SCI-NP). Animal models have been used to investigate the benefits of activity-based interventions (ABI), such as treadmill training, wheel running, walking, swimming, and bipedal standing. These activity-based paradigms have been shown to modulate inflammatory-related alterations as well as induce functional and structural changes in the spinal cord gray matter circuitry correlated with pain behaviors. Thus far, the research available provides an incomplete picture of the cellular and molecular pathways involved in this beneficial effect. Continued research is essential for understanding how such interventions benefit SCI patients suffering from NP and allow the development of individualized rehabilitative therapies. This article reviews preclinical studies on this specific topic, goes over mechanisms involved in SCI-NP in relation to ABI, and then discusses the effectiveness of different activity-based paradigms as they relate to different forms, intensity, initiation times, and duration of ABI. This article also summarizes the mechanisms of respective interventions to ameliorate NP after SCI and provides suggestions for future research directions.

Animal models - Mimicking the pain of trigeminal neuralgia

Trigeminal neuralgia (TN) is an episodic facial pain which feels like an electric shock of unilateral origin. This neuropathic disorder is an intensely stressful to bear for patient and impacts the quality of life. Most of the cases of TN arise when the root of fifth cranial nerve, i.e., trigeminal nerve is compressed after a few millimeters of its entry into the pons. This article describes various animal models and the role of biomarkers to study the underlying mechanisms of neuropathic pain in animal models as well as different modes of management of TN.

A novel rat model of temporomandibular disorder with improved face and construct validities

AIMS

Temporomandibular disorders are a cluster of orofacial conditions that are characterized by pain in the temporomandibular joint (TMJ) and surrounding muscles/tissues. Animal models of painful temporomandibular dysfunction (TMD) are valuable tools to investigate the mechanisms responsible for symptomatic temporomandibular joint and associated structures disorders. We tested the hypothesis that a predisposing and a precipitating factor are required to produce painful TMD in rats, using the ratgnawmeter, a device that determines temporomandibular pain based on the time taken for the rat to chew through two obstacles.

MATERIALS AND METHODS

Increased time in the ratgnawmeter correlated with nociceptive behaviors produced by TMJ injection of formalin (2.5%), confirming chewing time as an index of painful TMD. Rats exposed only to predisposing factors, carrageenan-induced TMJ inflammation or sustained inhibition of the catechol-O-methyltransferase (COMT) enzyme by OR-486, showed no changes in chewing time. However, when combined with a precipitating event, i.e., exaggerated mouth opening produced by daily 1-h jaw extension for 7 consecutive days, robust function impairment was produced.

KEY FINDINGS

These results validate the ratgnawmeter as an efficient method to evaluate functional TMD pain by evaluating chewing time, and this protocol as a model with face and construct validities to investigate symptomatic TMD mechanisms.

SIGNIFICANCE

This study suggests that a predisposition factor must be present in order for an insult to the temporomandibular system to produce painful dysfunction. The need for a combined contribution of these factors might explain why not all patients experiencing traumatic events, such as exaggerated mouth opening, develop TMDs.

Preclinical models of deep craniofacial nociception and temporomandibular disorder pain

Chronic pain in temporomandibular disorder (TMD) is a common health problem. Cumulating evidence indicates that the etiology of TMD pain is complex with multifactorial experience that could hamper the developments of treatments. Preclinical research is a resource to understand the mechanism for TMD pain, whereas limitations are present as a disease-specific model. It is difficult to incorporate multiple risk factors associated with the etiology that could increase pain responses into a single animal. This article introduces several rodent models which are often employed in the preclinical studies and discusses their validities for TMD pain after the elucidations of the neural mechanisms based on the clinical reports. First, rodent models were classified into two groups with or without inflammation in the deep craniofacial tissues. Next, the characteristics of each model and the procedures to identify deep craniofacial pain were discussed. Emphasis was directed on the findings of the effects of chronic psychological stress, a major risk factor for chronic pain, on the deep craniofacial nociception. Preclinical models have provided clinically relevant information, which could contribute to better understand the basis for TMD pain, while efforts are still required to bridge the gap between animal and human studies.

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.

Microglia and Inhibitory Circuitry in the Medullary Dorsal Horn: Laminar and Time-Dependent Changes in a Trigeminal Model of Neuropathic Pain

Craniofacial neuropathic pain affects millions of people worldwide and is often difficult to treat. Two key mechanisms underlying this condition are a loss of the negative control exerted by inhibitory interneurons and an early microglial reaction. Basic features of these mechanisms, however, are still poorly understood. Using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic pain in mice, we have examined the changes in the expression of GAD, the synthetic enzyme of GABA, and GlyT2, the membrane transporter of glycine, as well as the microgliosis that occur at early (5 days) and late (21 days) stages post-CCI in the medullary and upper spinal dorsal horn. Our results show that CCI-IoN induces a down-regulation of GAD at both postinjury survival times, uniformly across the superficial laminae. The expression of GlyT2 showed a more discrete and heterogeneous reduction due to the basal presence in lamina III of 'patches' of higher expression, interspersed within a less immunoreactive 'matrix', which showed a more substantial reduction in the expression of GlyT2. These patches coincided with foci lacking any perceptible microglial reaction, which stood out against a more diffuse area of strong microgliosis. These findings may provide clues to better understand the neural mechanisms underlying allodynia in neuropathic pain syndromes.

Evolution of mirror-image pain in temporomandibular joint osteoarthritis mouse model

OBJECTIVE

Mirror-image pain is a kind of pain that occurs on the contralateral side, but its pathogenesis remains unclear. To develop an osteoarthritis mouse model for investigating mirror-image pain through observing nocifensive behaviors, histological changes, and nociceptive activity at days 3, 7, 14, 21, and 28 after the chemical induction of unilateral temporomandibular joint (TMJ) osteoarthritis.

METHODOLOGY

We randomly divided 6-week-old mice into sham and complete Freund adjuvant groups. To induce nocifensive behaviors, we applied 0.04 g of von Frey filament, 10 psi of air puff, and cold acetone on both sides of whisker pads at different days. The histology of TMJ on both sides was observed by hematoxylin/eosin staining and microcomputed tomography scanning. Furthermore, the nociceptive activity was evaluated using the phosphorylated cyclic AMP response element binding protein (pCREB) and a microglia marker at different days in the trigeminal subnucleus caudalis.

RESULTS

Nocifensive behaviors against mechanical and temperature stimuli on the contralateral side became stronger than the baseline on day 28, in agreement with the elevation of the pCREB and the microglia marker in the trigeminal subnucleus caudalis. Thus, hypernociception on the contralateral side occurred at day 28.

CONCLUSIONS

Clearly, the TMJ model with unilateral osteoarthritis exhibited mirror-image pain. Therefore, this model is useful in investigating the pathogenesis of pain and in developing treatments.

Assessment of nociception and related quality-of-life measures in a porcine model of neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disorder resulting from germline mutations in the NF1 gene, which encodes neurofibromin. Patients experience a variety of symptoms, but pain in the context of NF1 remains largely underrecognized. Here, we characterize nociceptive signaling and pain behaviors in a miniswine harboring a disruptive NF1 mutation (exon 42 deletion). We present the first characterization of pain-related behaviors in a pig model of NF1, identifying unchanged agitation scores, lower tactile thresholds (allodynia), and decreased response latencies to thermal laser stimulation (hyperalgesia) in NF1 (females only) pigs. Male NF1 pigs with tumors showed reduced sleep quality and increased resting, 2 health-related quality-of-life symptoms found to be comorbid in people with NF1 pain. We explore these phenotypes in relationship to suppression of the increased activity of the N-type voltage-gated calcium (CaV2.2) channel by pharmacological antagonism of phosphorylation of a regulatory protein-the collapsin response mediator protein 2 (CRMP2), a known interactor of neurofibromin, and by targeting the interface between the α subunit of CaV2.2 and the accessory β-subunits with small molecules. Our data support the use of NF1 pigs as a large animal model for studying NF1-associated pain and for understanding the pathophysiology of NF1. Our findings demonstrate the translational potential of 2 small molecules in reversing ion channel remodeling seen in NF1. Interfering with CaV2.2, a clinically validated target for pain management, might also be a promising therapeutic strategy for NF1-related pain management.

Sensory, Affective, and Cognitive Effects of Trigeminal Injury in Mice

PURPOSE

To characterize adequate study of chronic neuropathic orofacial pain induced by a mental nerve injury in a mouse model, we propose a behavioral assessment of its dimensions: sensory, affective, and cognitive.

MATERIALS AND METHODS

Trigeminal injury was induced by a chronic mental nerve constriction (MnC). Behavioral tests were conducted to assess the different dimensions of pain and to evaluate the general well-being of mice.

RESULTS

Rodents who went through MnC showed signs of mechanical hyperalgesia and increased escape/avoidance behavior. They showed no alterations in general well-being behaviors, yet the injury was sufficient to induce impairment in the ability to adapt to the environmental requirements.

CONCLUSIONS

MnC injury is an efficient model for the study of orofacial pain in mice, capable of inducing impairment in the different dimensions of pain. Intensity and temporality of its effects make our model less aggressive, yet effective to generate cognitive impairment. This work provides a solid foundation for the study of the neural circuits involved in the processing of neuropathic orofacial pain.

Role of Dopaminergic Receptors Within the Ventral Tegmental Area in Antinociception Induced by Chemical Stimulation of the Lateral Hypothalamus in an Animal Model of Orofacial Pain

Introduction

The ventral tegmental area (VTA), as one of the classical components of the brain reward circuitry, shares large neural networks with the pain processing system. We previously showed the role of VTA dopamine receptors in modulation of lateral hypothalamus (LH)-induced antinociception in acute pain conditions. However, considering the fact that the neural systems involved in the mediation of tonic pain are not the same as those that mediate phasic pain. In the present study, we aimed to examine the role of intra-VTA dopamine receptors in LH-induced antinociceptive responses during tonic orofacial pain conditions.

Methods

Male Wistar rats weighing 230-250 g were implanted with two separate cannulae into the LH and VTA on the same side. Different solutions of carbachol (62.5, 125 and 250 nM), as a non-selective cholinergic receptor agonist that activates the LH projecting neurons, were microinjected into the LH. In the other groups, D1-like dopamine receptor antagonist, SCH-23390 (0.25, 1 and 4 µg/03 µL saline) or D2-like dopamine receptor antagonist, Sulpiride (0.25, 1 and 4 µg/0.3 µL DMSO 12%) were microinjected into VTA, 5 min prior intra-LH carbachol (250 nM), then subjected to orofacial formalin test. Intra-LH carbachol microinjection dose-dependently attenuated biphasic orofacial pain.

Results

Intra-VTA administration of SCH-23390 or Sulpiride dose-dependently decreased intra-LH carbachol-induced antinociception during both phases of orofacial formalin test with further effects in the late phase.

Discussion

The findings suggest that chemical stimulation of the LH by carbachol possibly activates the orexin projecting neurons and subsequently, the VTA dopaminergic neurons involved in the orofacial pain modulation. Detecting such neural circuitry offers an alternative approach in the development of more efficient therapies for such debilitating pain conditions.

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 anti-nociceptive effects of Porphyromonas gingivalis lipopolysaccharide

OBJECTIVE

The objective of this study was to assess the effect of Porphyromonas gingivalis lipopolysaccharide (PG LPS) on acute pain-related behaviour induced in rats and to measure its impact on the levels of pro-inflammatory cytokines (IL-1β, IL-6) and anti-inflammatory (IL-10) cytokines.

DESIGN

The Brennan model was used to induce acute pain like signs in rats' hind paw. Twenty-four hours following the surgery the rats were divided into 5 groups and the affected paws were injected with 0.2 m l of one of three commercialized forms PG LPS doses (high - 1 mg/ml, medium - 0.6 mg/m l and low - 0.2 mg/m l), diclofenac sodium (1 mg/kg) or saline. Tactile allodynia, mechanical hyperalgesia, body temperature and paw swelling were assessed at baseline, 24 h postoperatively and 2 h after the paw injection. The affected and contra-lateral paw tissue was assessed for the mentioned above cytokines levels employing enzyme-linked immunosorbent assay.

RESULTS

This study may suggest that PG LPS can reduce pain like behaviour via increased levels of anti-inflammatory cytokine IL-10 (5900 ± 748, p < 0.05). The high PG LPS dose and diclofenac reduced the tactile allodynia and mechanical hyperalgesia significantly (42.2 ± 4 and1.6 ± 0.3, p < 0.05). PG LPS high dose increase IL-10 levels while diclofenac reduces IL-1β levels significantly (5900 ± 748 and 1760 ± 271.2). The LPS administration had no effect on paw swelling and did not increase rat's body temperature.

CONCLUSION

The results demonstrated that PG LPS local application could possess anti- nociceptive properties, which at least in part is mediated by an increase in IL-10 levels.

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.

Molecular mechanism underlying orofacial antinociceptive activity of Vanillosmopsis arborea Baker (Asteraceae) essential oil complexed with β-cyclodextrin

BACKGROUND

Vanillosmopsis arborea Baker has recognized economic value owing to the high content of (-)-α-bisabolol (BISA) in the essential oil of its stem (EOVA). The antinociceptive effect of EVOA has already been demonstrated, and β-cyclodextrin (βCD) is known to improve the analgesic effect of various substances.

PURPOSE

Thus, we aimed to evaluate the orofacial antinociceptive effect of a complex containing EOVA-βCD in rodents.

METHODS

EOVA was obtained by simple hydrodistillation, and the essential oil was complexed with βCD. The animals (n = 6/group) were treated orally with EOVA-βCD (10 or 50 mg/kg), or vehicle (control), and subjected to cutaneous orofacial nociception (formalin, capsaicin, acidic saline or glutamate), corneal (hypertonic saline) or temporomandibular (formalin) tests. The expression of FOS protein was analyzed in the spinal cord. Molecular docking was performed using the 5-HT₃ and M₂ receptors and BISA.

RESULTS

The oral administration of EOVA-βCD reduced nociceptive behaviour. Moreover, EOVA-βCD decreased FOS expression. The molecular docking study indicates that BISA interacts with 5-HT3 and M2 receptors, indicating the potential mechanism of action of the tested compound.

CONCLUSIONS

Our results indicate that EOVA-βCD possesses orofacial antinociceptive effect, indicating that this complex can be used in analgesic drug development.

Focal localization of inflammatory cytokines and neurotrophins in a tongue chronic injury model

OBJECTIVES

Chronic injury in tongue causes the variety of reactions in the oral cavity, frequently leading to its functional and structural disintegrity including inflammation and sensory dysfunction, but its detailed profiles were not elucidated yet. One of the chronically injured tongue such as tongue piercing, as a pathological aspect, is currently popular among younger people but may be associated with severe side effects, leading to pathophysiological complications. However, the pathophysiological aspects and related cellular and molecular mechanisms underlying tongue injury are not clearly understood.

DESIGN

In this study, we designed an experimental model system using C57BL/6 male mice that mimics a chronically injured situation by penetrating the middle part of tongue with silk suture. After 5 and 10 days mice were sacrificed and tongues were collected and processed for histological evaluation and immunohistochemistry.

RESULTS

We found that the anterior tongue showed localization of neuro-inflammatory signaling molecules such as myeloperoxidase (MPO), matrix metalloproteinase 2 (MMP2), tumor necrosis factor-α (TNF-α), nerve growth factor, and transient receptor potential cation channel subfamily V member 1 (TRPV1) without any apparent inflammation in temporal manner. In addition, the signal for AM1-43, an activity-dependent nerve terminal probe, decreased within the fungiform papillae on the anterior tongue after injury.

CONCLUSIONS

These results implied that the distinct localizations of inflammatory cytokines and neurotrophin would contribute altered sensory function in anterior tongue following the chronic injury. Our study indicates the possible pathophysiologic mechanism underlying neuro-inflammation following chronically injury of tongue. In addition, it could be cautiously postulated that mechanical injury should be avoided to prevent chronic pain disorders from being triggered.

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.

Grading facial expression is a sensitive means to detect grimace differences in orofacial pain in a rat model

Although pre-clinical models of pain are useful for defining relationships between biological mechanisms and pain, common methods testing peripheral sensitivity do not translate to the human pain experience. Facial grimace scales evaluate affective pain levels in rodent models by capturing and scoring spontaneous facial expression. But, the Rat Grimace Scale (RGS) has not assessed the common disorder of temporomandibular joint (TMJ) pain. A rat model of TMJ pain induced by jaw loading (1 hr/day for 7 days) was used to investigate the time course of RGS scores and compare them between different loading magnitudes with distinct peripheral sensitivity profiles (0N–no sensitivity, 2N–acute sensitivity, 3.5N–persistent sensitivity). In the 3.5N group, RGS is elevated over baseline during the loading period and one day after loading and is correlated with peripheral sensitivity (ρ = −0.48, p = 0.002). However, RGS is not elevated later when that group exhibits peripheral sensitivity and moderate TMJ condylar cartilage degeneration. Acutely, RGS is elevated in the 3.5N loading group over the other loading groups (p < 0.001). These findings suggest that RGS is an effective tool for detecting spontaneous TMJ pain and that spontaneous pain is detectable in rats that develop persistent TMJ sensitivity, but not in rats with acute resolving sensitivity.

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.

Spared nerve injury model to study orofacial pain

BACKGROUND & OBJECTIVES

There are many difficulties in generating and testing orofacial pain in animal models. Thus, only a few and limited models that mimic the human condition are available. The aim of the present research was to develop a new model of trigeminal pain by using a spared nerve injury (SNI) surgical approach in the rat face (SNI-face).

METHODS

Under anaesthesia, a small incision was made in the infraorbital region of adult male Wistar rats. Three of the main infraorbital nerve branches were tightly ligated and a 2 mm segment distal to the ligation was resected. Control rats were sham-operated by exposing the nerves. Chemical hyperalgesia was evaluated 15 days after the surgery by analyzing the time spent in face grooming activity and the number of head withdrawals in response to the orofacial formalin test.

RESULTS

SNI-face rats presented a significant increase of the formalin-induced pain-related behaviours evaluated both in the acute and tonic phases (expected biphasic pattern), in comparison to sham controls.

INTERPRETATION & CONCLUSIONS

The SNI-face model in the rat appears to be a valid approach to evaluate experimental trigeminal pain. Ongoing studies will test the usefulness of this model to evaluate therapeutic strategies for the treatment of orofacial pain.

Orofacial antinociceptive effect of the ethanolic extract of Annona vepretorum Mart. (Annonaceae)

Annona vepretorum Mart. (Annonaceae) is a species popularly known in Brazil as “araticum” and “pinha da Caatinga”. We have evaluated the antinociceptive effects of A. vepretorum in formalin-, capsaicin-, and glutamate-induced orofacial nociception in mice. Male Swiss mice were pretreated with either saline (p.o.), A. vepretorum ethanol extract (Av-EtOH 25, 50 and 100 mg/kg, p.o.), or morphine (10 mg/kg, i.p.), before formalin, capsaicin, or glutamate was injected into the right upper lip. Pre-treatment with Av-EtOH at all doses produced a reduction in face-rubbing behavior induced by formalin in both phases, and these pre-treatments also produced a significant antinociceptive effect in the capsaicin and glutamate tests. Pre-treatment with naloxone (1.5 mg/kg, i.p.) did not reverse the antinociceptive activity of the extract at the dose of 100 mg/kg in the first phase of this test. Our results suggest that Av-EtOH might be useful in the treatment of orofacial pain.

Ilex paraguariensis Promotes Orofacial Pain Relief After Formalin Injection: Involvement of Noradrenergic Pathway

Background:

Drinking mate or chimarrão, a hot infusion of Ilex paraguariensis (ILEX) leaves, is a common habit in Southern South America that has a social and almost ritualistic role. It has been used as a stimulant beverage in South America and analgesic in regions of Argentina for treatment of headache and others painful inflammatory conditions such as arthritis and rheumatism.

Objective:

The aim of this study was to evaluate the pharmacological activity of I. paraguariensis infusion (ILEX) on orofacial nociception model induced by formalin, and study its mechanism of action.

Materials and Methods:

The analgesic effect of ILEX was assessed through writhing test, paw formalin test, paw edema induced by carrageenan, and orofacial pain induced by formalin. To study the action mechanism of ILEX, opioidergic, dopaminergic, nitrergic, and adrenergic pathways were investigated.

Results:

The high-performance liquid chromatography analysis of ILEX infusion revealed caffeine and theobromine. The treatment with ILEX reduced the number of writhing. However, it was effective neither in the formalin paw test nor in the paw edema induced by carrageenan. Different from formalin paw test, ILEX was able to reduce the orofacial reactivity to formalin in 31.8% (70.4 ± 2.5 s; first phase), and 20% (127.3 ± 18.9 s; second phase). The analgesic effect of ILEX results from the modulation of noradrenergic pathways since prazosin (α₁-adrenoceptor antagonist, 0.15 mg/kg; intraperitoneal) reversed the analgesic effect of ILEX.

Conclusions:

The present report demonstrates that analgesic effect of ILEX in orofacial formalin test is due mainly to modulation of noradrenergic pathways.

SUMMARY

Ilex paraguariensis (ILEX) has been used as a stimulant beverage in South America and analgesic in regions of Argentina for the treatment of headache and others painful inflammatory conditions such arthritis and rheumatism.

The aim of this study was to evaluate the pharmacological activity of ILEX on orofacial nociception model induced by formalin, and study its mechanism of action.

ILEX reduced the number of writhing and orofacial reactivity to formalin in mice. However, it was effective neither in the formalin paw test nor in the paw edema induced by carrageenan.

The analgesic effect of ILEX results from the modulation of noradrenergic pathways.

Abbreviation Used: ILEX: Infusion of Ilex paraguariensis, NSAIDs: Nonsteroidal anti-inflammatory drugs, L-NOARG: L-NG-nitro-arginine, UV: Ultraviolet, i.p.: Intraperitoneal, NOS: Nitric Oxide Synthase, ANOVA: Analysis of variance, S.E.M.: Standard error of mean, HPLC: High-performance liquid chromatography, NO: Nitric Oxide, v.o.: Oral route, DCQ: dicaffeoylquinic acid, BMS: Burning mouth syndrome, PBS: Phosphate-buffered saline.

The role of Nav1.9 channel in the development of neuropathic orofacial pain associated with trigeminal neuralgia

BACKGROUND

Trigeminal neuralgia is accompanied by severe mechanical, thermal and chemical hypersensitivity of the orofacial area innervated by neurons of trigeminal ganglion (TG). We examined the role of the voltage-gated sodium channel subtype Nav1.9 in the development of trigeminal neuralgia.

RESULTS

We found that Nav1.9 is required for the development of both thermal and mechanical hypersensitivity induced by constriction of the infraorbital nerve (CION). The CION model does not induce change on Nav1.9 mRNA expression in the ipsilateral TG neurons when evaluated 9 days after surgery.

CONCLUSIONS

These results demonstrate that Nav1.9 channels play a critical role in the development of orofacial neuropathic pain. New routes for the treatment of orofacial neuropathic pain focussing on regulation of the voltage-gated Nav1.9 sodium channel activity should be investigated.

Divergence in endothelin-1- and bradykinin-activated store-operated calcium entry in afferent sensory neurons

Endothelin-1 (ET-1) and bradykinin (BK) are endogenous peptides that signal through Gαq/11-protein coupled receptors (GPCRs) to produce nociceptor sensitization and pain. Both peptides activate phospholipase C to stimulate Ca²⁺ accumulation, diacylglycerol production, and protein kinase C activation and are rapidly desensitized via a G-protein receptor kinase 2-dependent mechanism. However, ET-1 produces a greater response and longer lasting nocifensive behavior than BK in multiple models, indicating a potentially divergent signaling mechanism in primary afferent sensory neurons. Using cultured sensory neurons, we demonstrate significant differences in both Ca²⁺ influx and Ca²⁺ release from intracellular stores following ET-1 and BK treatments. As intracellular store depletion may contribute to the regulation of other signaling cascades downstream of GPCRs, we concentrated our investigation on store-operated Ca²⁺ channels. Using pharmacological approaches, we identified transient receptor potential canonical channel 3 (TRPC3) as a dominant contributor to Ca²⁺ influx subsequent to ET-1 treatment. On the other hand, BK treatment stimulated Orai1 activation, with only minor input from TRPC3. Taken together, data presented here suggest that ET-1 signaling targets TRPC3, generating a prolonged Ca²⁺ signal that perpetuates nocifensive responses. In contrast, Orai1 dominates as the downstream target of BK receptor activation and results in transient intracellular Ca²⁺ increases and abridged nocifensive responses.

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.

Trigeminal intersubnuclear neurons: morphometry and input-dependent structural plasticity in adult rats

Intersubnuclear neurons in the caudal division of the spinal trigeminal nucleus that project to the principal nucleus (Pr5) play an active role in shaping the receptive fields of other neurons, at different levels in the ascending sensory system that processes information originating from the vibrissae. By using retrograde labeling and digital reconstruction, we investigated the morphometry and topology of the dendritic trees of these neurons and the changes induced by long-term experience-dependent plasticity in adult male rats. Primary afferent input was either eliminated by transection of the right infraorbital nerve (IoN), or selectively altered by repeated whisker clipping on the right side. These neurons do not display asymmetries between sides in basic metric and topologic parameters (global number of trees, nodes, spines, or dendritic ends), although neurons on the left tend to have longer terminal segments. Ipsilaterally, both deafferentation (IoN transection) and deprivation (whisker trimming) reduced the density of spines, and the former also caused a global increase in total dendritic length and a relative increase in more complex arbors. Contralaterally, deafferentation reduced more complex dendritic trees, and caused a moderate decline in dendritic length and spatial reach, and a loss of spines in number and density. Deprivation caused a similar, but more profound, effect on spines. Our findings provide original quantitative descriptions of a scarcely known cell population, and show that denervation- or deprivation-derived plasticity is expressed not only by neurons at higher levels of the sensory pathways, but also by neurons in key subcortical circuits for sensory processing.

Gene therapy for trigeminal pain in mice

The aim of this study was to test the efficacy of a single direct injection of viral vector encoding for encephalin to induce a widespread expression of the transgene and potential analgesic effect in trigeminal behavioral pain models in mice. After direct injection of HSV-1 based vectors encoding for human preproenkephalin (SHPE) or the lacZ reporter gene (SHZ.1, control virus) into the trigeminal ganglia in mice, we performed an orofacial formalin test and assessed the cumulative nociceptive behavior at different time points after injection of the viral vectors. We observed an analgesic effect on nociceptive behavior that lasted up to 8 weeks after a single injection of SHPE into the trigeminal ganglia. Control virus injected animals showed nociceptive behavior similar to naïve mice. The analgesic effect of SHPE injection was reversed/attenuated by subcutaneous naloxone injections, a μ-opioid receptor antagonist. SHPE injected mice also showed normalization in withdrawal latencies upon thermal noxious stimulation of inflamed ears after subdermal complete Freund’s adjuvans injection indicating widespread expression of the transgene. Quantitative immunohistochemistry of trigeminal ganglia showed expression of human preproenkephalin after SHPE injection.

Direct injection of viral vectors proved to be useful for exploring the distinct pathophysiology of the trigeminal system and could also be an interesting addition to the pain therapists’ armamentarium.

Systemic pregabalin attenuates facial hypersensitivity and noxious stimulus-evoked release of glutamate in medullary dorsal horn in a rodent model of trigeminal neuropathic pain

Pregabalin is effective in treating many neuropathic pain conditions. However, the mechanisms of its analgesic effects remain poorly understood. The aim of the present study was to determine whether pregabalin suppresses facial mechanical hypersensitivity and evoked glutamate release in the medullary dorsal horn (MDH) in a rodent model of trigeminal neuropathic pain. Nociceptive mechanical sensitivity was assessed pre-operatively, and then post-operatively 1h following pregabalin or vehicle (saline) treatment on post-operative days 2 and 5 following infraorbital nerve transection (IONX). In addition, an in vivo microdialysis probe was inserted into the exposed medulla post-operatively and dialysate samples were collected. Glutamate release was then evoked by mustard oil (MO) application to the tooth pulp, and the effects of pregabalin or vehicle were examined on the MDH glutamate release. Glutamate concentrations in the dialysated samples were determined by HPLC, and data analyzed by ANOVA. IONX animals (but not control animals) showed facial mechanical hypersensitivity for several days post-operatively. In addition, tooth pulp stimulation with MO evoked a transient release of glutamate in the MDH of IONX animals. Compared to vehicle, administration of pregabalin significantly attenuated the facial mechanical hypersensitivity as well as the MO-evoked glutamate release in MDH. This study provides evidence in support of recent findings pointing to the usefulness of pregabalin in the treatment of orofacial neuropathic pain.