Tooth pulp inflammation increases brain-derived neurotrophic factor expression in rodent trigeminal ganglion neurons

Neurotransmission, Vasculogenesis, and Osteogenesis Activities are Altered in the Aging Temporomandibular Joint of the Senescence-Accelerated Prone 8 Mouse Model

BACKGROUND

Alterations in neurotransmission, vasculogenesis, and osteogenesis pathways that may play pivotal roles in age-related changes in the temporomandibular joint (TMJ) are poorly understood.

PURPOSE

This study aimed to measure the associations between gene and protein profiles in senescence-accelerated prone 8 (SAMP8) mice.

STUDY DESIGN

The investigators designed and used 3 groups of 2 mouse models: 1) early aging SAMP8 at 24 weeks of age and control SAMR1 at 12 and 24 weeks (each stage n = 12).

PREDICTOR/EXPOSURE/INDEPENDENT VARIABLE

The independent variable was investigated using 3 mouse models: an early aging mouse model and a control mouse model (12 and 24 weeks).

MAIN OUTCOME VARIABLE(S)

The primary outcome variables were CGRP, VEGF-A, CD31, LYVE-1, osteocalcin, osteopontin, type I and II collagen, and MMP-2. The secondary outcome variables were histological characteristics.

COVARIATES

Not applicable.

ANALYSES

The gene and protein expression profiles of neurotransmitters, vasculogenesis, and osteogenesis were identified by quantitative real-time polymerase chain reaction and dot blot analysis, respectively. The cellular localization of these events was verified by in situ hybridization and immunohistochemistry. Bivariate statistics were computed for each of the outcome variables. Statistical significance was set to a P value < .05.

RESULTS

The expression of CGRP mRNA in the bony mandibular condyle (BMC) of SAMP8 mice (SAMP8, 3.3 ± 0.39 vs SAMR1, 0.001 ± 0.0001) was high at 24 weeks of age (24 weeks) (P < .001). Higher numbers of cells positive percentage for CGRP (MF, SAMP8, 28.67 ± 1.60 vs SAMR 1, 6.36 ± 1.10; CMC, 27.5 ± 2.12 vs 9.00 ± 1.21; BMC, 31.31 ± 2.81 vs 7.85 ± 1.14) and VEGF-A (MF, 34.43 ± 2.45 vs 14.01 ± 1.28; MD, 32.69 ± 1.86 vs 8.00 ± 0.91; CMC, 36.60 ± 2.05 vs 14.19 ± 1.25 BMC 36.49 vs 12.59 ± 1.41) antibodies were found in the 24 weeks TMJ (P < .01).

CONCLUSIONS AND RELEVANCE

The neurotransmitter, vasculogenesis, and osteogenesis pathways are associated with TMJ aging in the SAMP8 mouse model. In the future, the SAMP8 mouse model may prove to be a robust model for identifying molecular and biochemical events underlying the effects of feeding, occlusal changes, and tooth loss in the aging TMJ.

Acute pulpitis promotes purinergic signaling to induce pain in rats via P38MAPK/NF-κB signaling pathway

Toothache is one of the most common types of pain, but the mechanisms underlying pulpitis-induced pain remain unknown. The ionotropic purinergic receptor family (P2X) is reported to mediate nociception in the nervous system. This study aims to investigate the involvement of P2X3 in the sensitisation of the trigeminal ganglion (TG) and the inflammation caused by acute pulpitis. An acute tooth inflammation model was established by applying LPS to the pulp of SD rats. We found that the increased expression of P2X3 was induced by acute pulpitis. A selective P2X3 inhibitor (A-317491) reduced pain-like behavior in the maxillofacial region of rats and depressed the activation of neurons in the trigeminal ganglion induced by pulpitis. The upregulated MAPK signaling (p-p38, p-ERK1/2) expression in the ipsilateral TG induced by pulpitis could also be depressed by the application of the P2X3 inhibitor. Furthermore, the expression of markers of inflammatory processes, such as NF-κB, TNF-α and IL-1β, could be induced by acute pulpitis and deduced by the intraperitoneal injection of P2X3 antagonists. Our findings demonstrate that purinergic P2X3 receptor signaling in TG neurons contributes to pulpitis-induced pain in rats and that P2X3 signaling may be a potential therapeutic target for tooth pain.

Investigating In Situ Expression of Neurotrophic Factors and Partner Proteins in Irreversible Pulpitis

INTRODUCTION

In situ assessments of neurotrophic factors and their associated molecular partners have not been explored to date, particularly in humans. The present investigation aimed to explore the expressional dysregulation of neurotrophic factors (nerve growth factor [NGF], brain derived neurotrophic factor [BDNF], and NT4/5), their receptors (TrkA and TrkB), and their modulators (USP36 and Nedd4-2) directly in irreversibly inflamed human pulp tissues.

METHODS

Forty samples each of healthy and irreversibly inflamed pulp were extirpated for the study. Immunohistochemical examinations were carried out for the anatomic changes and expression of neurotrophic factors and partner proteins. Expression was digitally quantified using the IHC profiler module of ImageJ and deduced as optical density. Statistical analyses were carried out by GraphPad Prism.

RESULTS

Decrease in nuclear and vessel diameters was observed in irreversibly inflamed pulp tissues. NGF and BDNF were found to be significantly upregulated in symptomatic irreversible pulpitis (SIP), whereas no significant difference was observed in the expression of TrkA and TrkB. Expression of Nedd4-2, USP36, and TrkA was found positively correlated with the NGF in healthy pulp tissues. However, in SIP, positive correlation was only observed between the expression of USP36 and NGF. Among the ligands, BDNF expression was found positively correlated with NGF in healthy pulp but not with NT4/5. In the case of SIP, no correlation was observed between any neurotrophic factors.

CONCLUSIONS

Upregulation of NGF, BDNF, USP36 and Nedd4-2 in SIP indicates dysregulation in the molecular events underlying the disease biology and could be exploited as potential markers for the disease diagnosis.

Anti-Inflammatory Effects of Melatonin and 5-Methoxytryptophol on Lipopolysaccharide-Induced Acute Pulpitis in Rats

Aim

The aim of this study was to investigate the possible therapeutic impacts of two pineal hormones, melatonin and 5-methoxytryptophol (5-MTX), in a rat model of acute pulpitis by analyzing biochemical and histopathological parameters.

Methods

This research was done using 32 male and female Wistar albino rats with weight between 200 and 250 g. The rats were randomly divided into four groups: a control group (rats without any treatment), acute pulpitis (AP) group, AP+melatonin group, and AP+5-MTX group. In the AP-induced groups, the crowns of the upper left incisors were removed horizontally. Lipopolysaccharide solution was applied to the exposed pulp tissue before the canal orifices were sealed with a temporary filling material. Melatonin (10 mg/kg) and 5-MTX (5 mg/kg) were administered intraperitoneally. The rats were sacrificed 24 hours after pulp injury, and trunk blood and pulp samples were collected. The concentrations of TNF-α, IL-1β, MMP-1, and MMP-2 in sera and pulp samples were determined using ELISA assay kits.

Results

TNF-α, IL-1β, MMP-1, and MMP-2 levels in the serum and pulp tissues were considerably higher in the AP group than the control group (p < 0.01‐0.001). In the AP+melatonin and AP+5-MTX groups, TNF-α, IL-1β, MMP-1, and MMP-2 levels in the serum and pulp tissues were significantly lower than in the AP group (p < 0.05‐0.001).

Conclusions

Both melatonin and 5-MTX provided protective effects on acute pulpitis, which indicates they may be promising as a therapeutic strategy for oral disease.

Periodontitis and diabetes reshape neuronal dendritic arborization in the thalamus and nucleus oralis in the rat

Diabetes is a metabolic disorder resulting in long-term hyperglycemia that could induce oxidative stress as well as neural modifications in the central nervous system. Periodontal disease is highly comorbid with diabetes and in some cases, with exacerbated pain responses. Periodontal tissue is innervated by trigeminal afferents which extend to the nucleus oralis (NO) that sends input to the ventral posterior lateral thalamic nuclei (VPL). The present study aimed to evaluate the consequences of periodontitis, diabetes and both conditions on the dendritic morphology, spine type, and density in neurons of the NO and VPL in male and female rats. A quantitative neuromorphological analysis was performed using the Cox-Golgi staining in male and female rats in four groups: naïve control, after a periodontitis procedure, diabetic, and diabetic with periodontitis. Periodontitis decreased the total dendritic length (TDL) in the NO of the male rat but no change in the female rat and no neuronal alterations were observed in the VPL of both male and female rats. In contrast, diabetes increased the number of spines in the NO and VPL and decreased TDL in the NO in both male and female rats. We observed that periodontitis induced a dimorphic effect in the NO, whereas diabetes induced a strong neuromorphological effect regardless of sex. Moreover, while periodontitis had a limited effect on the neuronal morphology, it dramatically modified the neural consequences in the VPL and NO when comorbid with diabetes. In conclusion, these neuroplastic modifications may be relevant to understand how diabetes exacerbates the outcome of periodontitis in humans, particularly in the female population.

Tooth pulp injury induces sex-dependent neuronal reshaping in the ventral posterolateral nucleus of the rat thalamus

Orofacial injuries often result in persistent pain and are therefore considered a common health problem worldwide. Considerable evidence suggests that peripheral sensory nerve injury results in diverse plastic changes in the central nervous system (CNS). Tooth pulp is innervated by trigeminal afferents which extend to the trigeminal brainstem sensory nuclear complex and send input to higher level neurons in the CNS, including the ventral posterolateral nucleus of the thalamus (VPL). In the present study, we examined the long term effects of pulpal injury on neuronal arborization in the VPL using morphological analysis via Golgi-Cox staining. In addition, we examined these effects in both male and female rats due to the major prevalence of oral pain in women. Quantitative morphological analysis revealed that pulpal injury induced neuronal hypertrophy in VPL neurons of female rats. In clear contrast, pulpal injury increased arborization close to the soma and reduced arborization distal to the soma without modification of total dendritic length in male rats. As a result, we show, for the first time, sex-dependent morphological alterations in VPL neurons after orofacial peripheral injury. Since dental injuries are readily reproducible in rat dental molars and closely mimic the clinical setting in humans, this model represents a useful tool to further understand mechanisms of orofacial pain.

Orexin-A inhibits capsaicin-induced changes in cyclooxygenase-2 and brain-derived neurotrophic factor expression in trigeminal nucleus caudalis of rats

Background

The trigeminal nucleus caudalis (Vc) is a primary central site for trigeminal transmitting. Noxious stimulation of the trigeminal nociceptors alters the central synaptic releases and neural expression of some inflammatory and trophic agents. Orexin-A and the orexin 1 receptor (OX1R) are expressed in pain pathways including trigeminal pain transmission. However, the the mechanism(s) underling orexin-A effects on trigeminal pain modulation have not been fully clarified.

Methods

Trigeminal pain was induced by subcutaneous injection of capsaicin in the upper lip in rats. The effect of trigeminal pain on cyclooxygenase-2 (COX-2) and brain-derived neurotrophic factor (BDNF) expression in the Vc of animals was determined by immunofluorescence. Subsequently, OX1R agonist (orexin-A) and antagonist (SB-334867-A) was administrated in the Vc to investigate the possible roles of the Vc OX1R on changes in COX-2 and BDNF levels following pain induction.

Results

The data indicated an increase in COX-2 and decrease in BDNF immuno-reactivity in the Vc of capsaicin, and capsaicin- pretreated with SB-334867-A (80 nM), groups of rat. However, the effect of capsaicin on COX-2 and BDNF expressions was reversed by a Vc microinjection of orexin-A (100 pM).

Conclusions

Overall, the present data reveals that orexin-A can attenuate capsaicin-induced trigeminal pain through the modulation of pain effects on COX-2 and BDNF expressions in the Vc of rats.

Activation of satellite glial cells in trigeminal ganglion following dental injury and inflammation

Satellite glial cells (SGCs), a peripheral neuroglial cell, surround neurons and form a complete envelope around individual sensory neurons in the trigeminal ganglia (TG), which may be involved in modulating neurons in inflammation. The purpose of this study was to determine the effect of dental injury and inflammation on SGCs in the TG. Pulp exposure (PX) was performed on the first maxillary molar of 28 rats. The neurons innervating injured tooth in TG were labeled by the retrograde transport of fluoro-gold (FG). Specimens were collected at 1, 3, 7, 14, 21 and 28 days after PX and stained immunohistochemically for glial fibrillary acid protein (GFAP), a marker of SGCs activation, in the TG. We observed that GFAP-immunoreactivity (IR) SGCs enclosed FG-labeled neurons increased in a time-dependent manner after PX. The neurons surrounded by GFAP-IR SGCs were mainly small and medium in size. The GFAP-IR SGCs encircled neurons increased significantly in the maxillary nerve region of the TG at 7-28 days following PX. The results show that dental injury and inflammation induced SGCs activation in the TG. It indicates that activation of SGCs might be implicated in the peripheral mechanisms of pain following dental injury and inflammation.

Role of Neuron-Glial Interaction Mediated by IL-1β in Ectopic Tooth Pain

Although many reports have demonstrated that ectopic pain develops in the orofacial region following tooth pulp inflammation, which often causes misdiagnosis and inappropriate treatment for patients with pulpitis, the precise mechanism remains unknown. In the present study, we hypothesized that the functional interaction between satellite glial cells and neurons mediated by interleukin 1β (IL-1β) in the trigeminal ganglion (TG) is involved in ectopic orofacial pain associated with tooth pulp inflammation. The digastric muscle electromyogram (D-EMG) activity elicited by capsaicin administration into the maxillary second molar tooth pulp was analyzed to evaluate the noxious reflex and was significantly increased in rats with inflammation of the maxillary first molar (M1) versus rats injected with saline. A significant increase in the expression of connexin43 (Cx43), a gap junction containing protein, was observed in activated satellite glial cells surrounding second molar-innervating neurons in the TG after M1 pulpitis. Daily administration of Gap26, a Cx43 mimetic peptide and inhibitor, in the TG significantly suppressed the enhancement of capsaicin-induced D-EMG activity and the percentage of Fluoro-Gold (FG)-labeled cells encircled by glial fibrillary acid protein-immunoreactive (IR) + Cx43-IR cells after M1 pulp inflammation ( P < 0.01). The percentage of FG-labeled cells encircled by glial fibrillary acid protein-IR + IL-1β-IR cells, IL-1 type I receptor-IR cells labeled with FG, and TRPV1-IR cells labeled with FG significantly increased after M1 pulp inflammation ( P < 0.01). Daily administration of IL-1ra, an IL-1 receptor antagonist, into the TG significantly reduced the enhancement of capsaicin-induced D-EMG activity and the percentage of TRPV1-IR neurons labeled with FG after M1 pulp inflammation ( P < 0.01). The present findings suggest that satellite glial cell is activated in the TG via activated gap junctions composed of Cx43 following tooth pulp inflammation, which leads to the hyperactivation of remote neurons via IL-1β mechanisms and results in ectopic tooth pulp pain in the adjacent tooth.

Inhibition of TNF reduces mechanical orofacial hyperalgesia induced by Complete Freund's Adjuvant by a TRPV1-dependent mechanism in mice

BACKGROUND

Inflammation in the orofacial region results in pain and is associated with many pathological states, including migraine, neuralgias and temporomandibular disorder. Although extensively studied, the mechanisms responsible for these conditions are not known and effective treatments are lacking. We reported earlier that the proinflammatory cytokine tumor necrosis factor (TNF) plays an important role in regulation of trigeminal ganglion (TG) neuron function in vitro. In the present study we investigated the role of TNF in mechanical hypersensitivity in mice.

METHODS

We employed the Complete Freund's Adjuvant (CFA)-induced model of orofacial pain and evaluated the effect of blocking of soluble TNF activity by peripheral administration of the novel dominant negative TNF biologic, XPro1595.

RESULTS

We show that CFA administration into the lower lip causes hyperalgesia and an increase in both expression of transient receptor potential vanilloid subfamily member 1 (TRPV1) mRNA and in the average intensity of TRPV1 protein immunoreactivity in TG neurons. We also show that intraperitoneal administration of XPro1595 prevents both CFA-induced mechanical hypersensitivity and, as shown in immunohistochemical staining - upregulation of TRPV1 protein expression in TG neurons.

CONCLUSIONS

We conclude that one of the possible regulatory mechanisms of TNF in pain involves upregulation of the nociceptor TRPV1, and that peripheral treatment with a selective anti-soluble TNF biologic can prevent hyperalgesia caused by inflammation in the orofacial region. Therefore, these new findings suggest that XPro1595 may serve as a novel treatment for orofacial pain disorders.

Expression of Vesicular Nucleotide Transporter in Rat Odontoblasts

Several theories have been proposed regarding pain transmission mechanisms in tooth. However, the exact signaling mechanism from odontoblasts to pulp nerves remains to be clarified. Recently, ATP-associated pain transmission has been reported, but it is unclear whether ATP is involved in tooth pain transmission. In the present study, we focused on the vesicular nucleotide transporter (VNUT), a transporter of ATP into vesicles, and examined whether VNUT was involved in ATP release from odontoblasts. We examined the expression of VNUT in rat pulp by RT-PCR and immunostaining. ATP release from cultured odontoblast-like cells with heat stimulation was evaluated using ATP luciferase methods. VNUT was expressed in pulp tissue, and the distribution of VNUT-immunopositive vesicles was confirmed in odontoblasts. In odontoblasts, some VNUT-immunopositive vesicles were colocalized with membrane fusion proteins. Additionally P2X3, an ATP receptor, immunopositive axons were distributed between odontoblasts. The ATP release by thermal stimulation from odontoblast-like cells was inhibited by the addition of siRNA for VNUT. These findings suggest that cytosolic ATP is transported by VNUT and that the ATP in the vesicles is then released from odontoblasts to ATP receptors on axons. ATP vesicle transport in odontoblasts seems to be a key mechanism for signal transduction from odontoblasts to axons in the pulp.

Vitamin B complex attenuated heat hyperalgesia following infraorbital nerve constriction in rats and reduced capsaicin in vivo and in vitro effects

Vitamins of the B complex attenuate some neuropathic pain sensory aspects in various animal models and in patients, but the mechanisms underlying their effects remain to be elucidated. Herein it was investigated if the treatment with a vitamin B complex (VBC) reduces heat hyperalgesia in rats submitted to infraorbital nerve constriction and the possibility that TRPV1 receptors represent a target for B vitamins. In the present study, the VBC refers to a combination of vitamins B1, B6 and B12 at low- (18, 18 and 1.8mg/kg, respectively) or high- (180, 180 and 18mg/kg, respectively) doses. Acute treatment of rats with either the low- or the high-doses combination reduced heat hyperalgesia after nerve injury, but the high-doses combination resulted in a long-lasting effect. Repeated treatment with the low-dose combination reduced heat hyperalgesia on day four after nerve injury and showed a synergist effect with a single injection of carbamazepine (3 or 10mg/kg), which per se failed to modify the heat threshold. In naïve rats, acute treatment with the high-dose of VBC or B1 and B12 vitamins independently reduced heat hyperalgesia evoked by capsaicin (3µg into the upper lip). Moreover, the VBC, as well as, each one of the B vitamins independently reduced the capsaicin-induced calcium responses in HEK 293 cells transiently transfected with the human TRPV1 channels. Altogether, these results indicate that B vitamins can be useful to control heat hyperalgesia associated with trigeminal neuropathic pain and that modulation of TRPV1 receptors may contribute to their anti-hyperalgesic effects.

Is there a relationship between brain-derived neurotrophic factor for driving neuronal auditory circuits with onset of auditory function and the changes following cochlear injury or during aging?

Brain-derived neurotrophic factor, BDNF, is one of the most important neurotrophic factors acting in the peripheral and central nervous system. In the auditory system its function was initially defined by using constitutive knockout mouse mutants and shown to be essential for survival of neurons and afferent innervation of hair cells in the peripheral auditory system. Further examination of BDNF null mutants also revealed a more complex requirement during re-innervation processes involving the efferent system of the cochlea. Using adult mouse mutants defective in BDNF signaling, it could be shown that a tonotopical gradient of BDNF expression within cochlear neurons is required for maintenance of a specific spatial innervation pattern of outer hair cells and inner hair cells. Additionally, BDNF is required for maintenance of voltage-gated potassium channels (KV) in cochlear neurons, which may form part of a maturation step within the ascending auditory pathway with onset of hearing and might be essential for cortical acuity of sound-processing and experience-dependent plasticity. A presumptive harmful role of BDNF during acoustic trauma and consequences of a loss of cochlear BDNF during aging are discussed in the context of a partial reversion of this maturation step. We compare the potentially beneficial and harmful roles of BDNF for the mature auditory system with those BDNF functions known in other sensory circuits, such as the vestibular, visual, olfactory, or somatosensory system.

Brain-derived neurotrophic factor enhances the excitability of small-diameter trigeminal ganglion neurons projecting to the trigeminal nucleus interpolaris/caudalis transition zone following masseter muscle inflammation

Background

The trigeminal subnuclei interpolaris/caudalis transition zones (Vi/Vc) play an important role in orofacial deep pain, however, the role of primary afferent projections to the Vi/Vc remains to be determined. This study investigated the functional significance of hyperalgesia to the brain-derived neurotrophic factor (BDNF)-tyrosine kinase B (trkB) signaling system in trigeminal ganglion (TRG) neurons projecting to the Vi/Vc transition zone following masseter muscle (MM) inflammation.

Results

The escape threshold from mechanical stimulation applied to skin above the inflamed MM was significantly lower than in naïve rats. Fluorogold (FG) labeling was used to identify the TRG neurons innervating the MM, while microbeads (MB) were used to label neurons projecting to the Vi/Vc region. FG/MB-labeled TRG neurons were immunoreactive (IR) for BDNF and trkB. The mean number of BDNF/trkB-IR small/medium-diameter TRG neurons was significantly higher in inflamed rats than in naïve rats. In whole-cell current-clamp experiments, the majority of dissociated small-diameter TRG neurons showed a depolarization response to BDNF that was associated with spike discharge, and the concentration of BDNF that evoked a depolarizing response was significantly lower in the inflamed rats. In addition, the relative number of BDNF-induced spikes during current injection was significantly higher in inflamed rats. The BDNF-induced changes in TRG neuron excitability was abolished by tyrosine kinase inhibitor, K252a.

Conclusion

The present study provided evidence that BDNF enhances the excitability of the small-diameter TRG neurons projecting onto the Vi/Vc following MM inflammation. These findings suggest that ganglionic BDNF-trkB signaling is a therapeutic target for the treatment of trigeminal inflammatory hyperalgesia.

Cellular and molecular mechanisms of dental nociception

Due, in part, to the unique structure of the tooth, dental pain is initiated via distinct mechanisms. Here we review recent advances in our understanding of inflammatory tooth pain and discuss 3 hypotheses proposed to explain dentinal hypersensitivity: The first hypothesis, supported by functional expression of temperature-sensitive transient receptor potential channels, emphasizes the direct transduction of noxious temperatures by dental primary afferent neurons. The second hypothesis, known as hydrodynamic theory, attributes dental pain to fluid movement within dentinal tubules, and we discuss several candidate cellular mechanical transducers for the detection of fluid movement. The third hypothesis focuses on the potential sensory function of odontoblasts in the detection of thermal or mechanical stimuli, and we discuss the accumulating evidence that supports their excitability. We also briefly update on a novel strategy for local nociceptive anesthesia via nociceptive transducer molecules in dental primary afferents with the potential to specifically silence pain fibers during dental treatment. Further understanding of the molecular mechanisms of dental pain would greatly enhance the development of therapeutics that target dental pain.

The mechanism of functional up-regulation of P2X3 receptors of trigeminal sensory neurons in a genetic mouse model of familial hemiplegic migraine type 1 (FHM-1)

A knock-in (KI) mouse model of FHM-1 expressing the R192Q missense mutation of the Cacna1a gene coding for the α1 subunit of Ca_V2.1 channels shows, at the level of the trigeminal ganglion, selective functional up-regulation of ATP -gated P2X3 receptors of sensory neurons that convey nociceptive signals to the brainstem. Why P2X3 receptors are constitutively more responsive, however, remains unclear as their membrane expression and TRPV1 nociceptor activity are the same as in wildtype (WT) neurons. Using primary cultures of WT or KI trigeminal ganglia, we investigated whether soluble compounds that may contribute to initiating (or maintaining) migraine attacks, such as TNFα, CGRP, and BDNF, might be responsible for increasing P2X3 receptor responses. Exogenous application of TNFα potentiated P2X3 receptor-mediated currents of WT but not of KI neurons, most of which expressed both the P2X3 receptor and the TNFα receptor TNFR2. However, sustained TNFα neutralization failed to change WT or KI P2X3 receptor currents. This suggests that endogenous TNFα does not regulate P2X3 receptor responses. Nonetheless, on cultures made from both genotypes, exogenous TNFα enhanced TRPV1 receptor-mediated currents expressed by a few neurons, suggesting transient amplification of TRPV1 nociceptor responses. CGRP increased P2X3 receptor currents only in WT cultures, although prolonged CGRP receptor antagonism or BDNF neutralization reduced KI currents to WT levels. Our data suggest that, in KI trigeminal ganglion cultures, constitutive up-regulation of P2X3 receptors probably is already maximal and is apparently contributed by basal CGRP and BDNF levels, thereby rendering these neurons more responsive to extracellular ATP.

Neurotrophins and nerve regeneration-associated genes are expressed in the cornea after lamellar flap surgery

PURPOSE

To determine the in vivo expression of neurotrophins (NTs) and nerve regeneration-associated genes (RAGs) after surgically creating a hinged lamellar corneal flap in thy1-YFP mice.

METHODS

Lamellar corneal flaps with multiple hinges were created in thy1-YFP mice. Mice were killed at weeks 2, 4, and 8. Quantitative polymerase chain reaction was performed to determine the expression of NTs and RAGs in the corneas after lamellar transection. Nerve growth factor (Ngf), brain-derived neurotrophic factor (Bdnf), glial cell-derived neurotrophic factor (Gdnf), neurotrophin 3, neurotrophin 5, small proline-rich repeat protein 1A (Sprr1a), growth-associated protein 43 (Gap43), and beta III tubulin (Tubb3) gene expressions were analyzed. Whole-mount confocal immunofluorescence and Western analyses were performed for localization and abundance of robustly expressed genes.

RESULTS

Sprouts of fine YFP-positive fronds emanating from transected (injured) nerve bundles were seen in the flap area at 2 weeks onward. Bdnf and Sprr1a were robustly and significantly expressed at 2 weeks postoperatively (>2-fold increase in expression; P<0.05). Bdnf localized to thy1-YFP+ cells in operated corneas. Sprr1a localized to corneal epithelial cell membranes. At 8 weeks, none of the NTs and RAGs had increased expression. Bdnf (ρ=0.73, P=0.001) and Sprr1a (ρ=0.76, P=0.001) showed a significant positive correlation with beta III tubulin.

CONCLUSIONS

The neurotrophin Bdnf and RAG Sprr1a are robustly and significantly expressed during corneal nerve regeneration in vivo.

Glial cell line-derived neurotrophic factor modulates the excitability of nociceptive trigeminal ganglion neurons via a paracrine mechanism following inflammation

Previous our report indicated that acute application of glial cell line-derived neurotrophic factor (GDNF) enhances the neuronal excitability of adult rat small-diameter trigeminal ganglion (TRG) neurons, which innervate the facial skin in the absence of neuropathic and inflammatory conditions. This study investigated whether under in vivo conditions, GDNF modulates the excitability of nociceptive Aδ-TRG neurons innervating the facial skin via a paracrine mechanism following inflammation. We used extracellular electrophysiological recording with multibarrel-electrodes in this study. Spontaneous Aδ-TRG neuronal activity was induced in control rats after iontophoretic application of GDNF into the trigeminal ganglia (TRGs). Noxious and non-noxious mechanical stimuli evoked Aδ-TRG neuronal firing rate were significantly increased by iontophoretic application of GDNF. The mean mechanical threshold of nociceptive TRG neurons was significantly decreased by GDNF application. The increased discharge frequency and decreased mechanical threshold induced by GDNF were antagonized by application of the protein tyrosine kinase inhibitor, K252b. The number of Aδ-TRG neurons with spontaneous firings and their firing rates in rats with inflammation induced by Complete Freund's Adjuvant were significantly higher than control rats. The firing rates of Aδ-TRG spontaneous neuronal activity were significantly decreased by iontophoretic application of K252b in inflamed rats. K252b also inhibited Aδ-TRG neuron activity evoked by mechanical stimulation in inflamed rats. These results suggest that in vivo GDNF enhances the excitability of nociceptive Aδ-TRG neurons via a paracrine mechanism within TRGs following inflammation. GDNF paracrine mechanism could be important as a therapeutic target for trigeminal inflammatory hyperalgesia.

Mechanisms underlying ectopic persistent tooth-pulp pain following pulpal inflammation

In order to clarify the peripheral mechanisms of ectopic persistent pain in a tooth pulp following pulpal inflammation of an adjacent tooth, masseter muscle activity, phosphorylated extracellular signal-regulated protein kinase (pERK) and TRPV1 immunohistochemistries and satellite cell activation using glial fibrillary acidic protein (GFAP) immunohistochemistry in the trigeminal ganglion (TG) were studied in the rats with molar tooth-pulp inflammation. And, Fluorogold (FG) and DiI were also used in a neuronal tracing study to analyze if some TG neurons innervate more than one tooth pulp. Complete Freund's adjuvant (CFA) or saline was applied into the upper first molar tooth pulp (M1) in pentobarbital-anesthetized rats, and capsaicin was applied into the upper second molar tooth pulp (M2) on day 3 after the CFA or saline application. Mean EMG activity elicited in the masseter muscle by capsaicin application to M2 was significantly larger in M1 CFA-applied rats compared with M1 vehicle-applied rats. The mean number of pERK-immunoreactive (IR) TG cells was significantly larger in M1 CFA-applied rats compared with M1 vehicle-applied rats. Application of the satellite cell inhibitor fluorocitrate (FC) into TG caused a significant depression of capsaicin-induced masseter muscle activity and a significant reduction of satellite cell activation. The number of TRPV1-IR TG cells innervating M2 was significantly larger in M1 CFA-applied rats compared with M1 vehicle-applied rats, and that was decreased following FC injection into TG. Furthermore, 6% of TG neurons innervating M1 and/or M2 innervated both M1 and M2. These findings suggest that satellite cell activation following tooth pulp inflammation and innervation of multiple tooth pulps by single TG neurons may be involved in the enhancement of the activity of TG neurons innervating adjacent non-inflamed teeth that also show enhancement of TRPV1 expression in TG neurons, resulting in the ectopic persistent tooth-pulp pain following pulpal inflammation of adjacent teeth.

Suppression of neurokinin-1 receptor in trigeminal ganglia attenuates central sensitization following inflammation

This study examined whether local application of a neurokinin-1 (NK1) receptor antagonist into the trigeminal ganglia (TRGs) modulates hyperexcitability of trigeminal spinal nucleus caudalis (SpVc) wide-dynamic range (WDR) neuron activity innervating both the temporomandibular joint (TMJ) region and facial skin following TMJ inflammation. Extracellular single unit recording combined with multibarrel electrodes was used. TMJ inflammation was induced by the injection of complete Freund's adjuvant (CFA). WDR neurons responding to electrical stimuli of the TMJ region and facial skin were recorded from the SpVc in anesthetized rats. The spontaneous and mechanical stimulation-induced discharge frequencies of WDR neurons were significantly larger in inflamed rats than in control rats. The spontaneous WDR activities were current-dependently decreased by local iontophoretic application of an NK1 receptor antagonist into the TRGs after 1 and 2 days of inflammation. The firing frequency of WDR neurons and threshold evoked by mechanical stimulation of facial skin returned to control levels by application of the NK1 receptor antagonist into TRGs after 1 day, but not 2 days, of inflammation. These results suggest that in the early stages of inflammation suppression of the NK1 receptor mechanism in TRGs may prevent central sensitization of SpVc nociceptive neurons.

Behavioral testing in rodent models of orofacial neuropathic and inflammatory pain

Orofacial pain conditions are often very debilitating to the patient and difficult to treat. While clinical interest is high, the proportion of studies performed in the orofacial region in laboratory animals is relatively low, compared with other body regions. This is partly due to difficulties in testing freely moving animals and therefore lack of reliable testing methods. Here we present a comprehensive review of the currently used rodent models of inflammatory and neuropathic pain adapted to the orofacial areas, taking into account the difficulties and drawbacks of the existing approaches. We examine the available testing methods and procedures used for assessing the behavioral responses in the face in both mice and rats and provide a summary of some pharmacological agents used in these paradigms to date. The use of these agents in animal models is also compared with outcomes observed in the clinic.

Antinociceptive interaction of (±)-CPP and propentofylline in monoarthritic rats

Introduction

Multiple studies have shown that glial cells of the spinal cord, such as astrocytes and microglia, have close contact with neurons, suggesting the term tripartite synapse. In these synapses, astrocytes surrounding neurons contribute to neuronal excitability and synaptic transmission, thereby increasing nociception and thus the persistence of chronic pain. Conversely, the N-methyl-D-aspartate (NMDA) receptor is crucial in the generation and maintenance of chronic pain. It has multiple sites of modulation. One is the site of recognition of extracellular neurotransmitter (glutamate), which can be blocked by competitive antagonists such as (3-(2-carboxipiperazin-4)1-propyl phosphonic acid), (±)-CPP, resulting in a blockade of the calcium current and thus the intracellular transduction process. In the present study, we investigated whether the potential antinociceptive effect of glial inhibition produced by propentofylline (PPF) can be enhanced when combined with an NMDA-receptor inhibitor such as (±)-CPP.

Methods

We used Sprague-Dawley monoarthritic rats. The monoarthritis was induced by injection of complete Freund adjuvant in the right tibiotarsal joint. Four weeks later, rats were treated with PPF (1, 10, 30, and 100 μg/10 μl) intrathecally (i.t.) for 10 days, injected once with (±)-CPP (2.5, 5, 12.5, 25, 50, and 100 μg/10 μl, i.t.), or both treatments combined. The antinociceptive effect was evaluated on day 11 for PPF and immediately to (±)-CPP, by assessing the vocalization threshold to mechanical stimulation of the arthritic paw.

Results

The data indicate that intrathecal administration of increasing concentrations of (±)-CPP or PPF produced a significant dose-dependent antinociceptive effect with respect to monoarthritic rats receiving saline. The linear regression analysis showed that the dose that produces 30% of maximal effect (ED₃₀) for i.t. (±)-CPP was 3.97 μg, and 1.42 μg for i.t. PPF. The administration of the PPF and (±)-CPP combination in fixed proportions of ED₃₀ produced a dose-dependent antinociceptive effect, showing an interaction of the supraadditive type.

Conclusions

The results suggest that glia inhibitors can synergically potentiate the effect of glutamate blockers for the treatment of chronic inflammatory pain.

Brain-derived neurotrophic factor in primary headaches

Brain derived neurotrophic factor (BDNF) is associated with pain modulation and central sensitization. Recently, a role of BDNF in migraine and cluster headache pathophysiology has been suspected due to its known interaction with calcitonin gene-related peptide. Bi-center prospective study was done enrolling four diagnostic groups: episodic migraine with and without aura, episodic cluster headache, frequent episodic tension-type headache, and healthy individuals. In migraineurs, venous blood samples were collected twice: outside and during migraine attacks prior to pain medication. In cluster headache patients serum samples were collected in and outside cluster bout. Analysis of BDNF was performed using enzyme-linked immunosorbent assay technique. Migraine patients revealed significantly higher BDNF serum levels during migraine attacks (n = 25) compared with headache-free intervals (n = 53, P < 0.01), patients with tension-type headache (n = 6, P < 0.05), and healthy controls (n = 22, P < 0.001). There was no significant difference between patients with migraine with aura compared with those without aura, neither during migraine attacks nor during headache-free periods. Cluster headache patients showed significantly higher BDNF concentrations inside (n = 42) and outside cluster bouts (n = 24) compared with healthy controls (P < 0.01, P < 0.05). BDNF is increased during migraine attacks, and in cluster headache, further supporting the involvement of BDNF in the pathophysiology of these primary headaches.

Aberrant TRPV1 expression in heat hyperalgesia associated with trigeminal neuropathic pain

Trigeminal neuropathic pain is a facial pain syndrome associated with trigeminal nerve injury. However, the mechanism of trigeminal neuropathic pain is poorly understood. This study aimed to determine the role of transient receptor potential vanilloid 1 (TRPV1) in heat hyperalgesia in a trigeminal neuropathic pain model. We evaluated nociceptive responses to mechanical and heat stimuli using a partial infraorbital nerve ligation (pIONL) model. Withdrawal responses to mechanical and heat stimuli to vibrissal pads (VP) were assessed using von Frey filaments and a thermal stimulator equipped with a heat probe, respectively. Changes in withdrawal responses were measured after subcutaneous injection of the TRP channel antagonist capsazepine. In addition, the expression of TRPV1 in the trigeminal ganglia was examined. Mechanical allodynia and heat hyperalgesia were observed in VP by pIONL. Capsazepine suppressed heat hyperalgesia but not mechanical allodynia. The number of TRPV1-positive neurons in the trigeminal ganglia was significantly increased in the large-diameter-cell group. These results suggest that TRPV1 plays an important role in the heat hyperalgesia observed in the pIONL model.

Nerve growth factor contribution via transient receptor potential vanilloid 1 to ectopic orofacial pain

It is well known that oral inflammation causes tenderness in temporomandibular joints or masseter muscles. The exact mechanism of such an orofacial ectopic hyperalgesia remains unclear. Here, we investigated the functional significance of interaction of nerve growth factor (NGF) and transient receptor potential vanilloid 1 (TRPV1) in relation to heat hyperalgesia in the whisker pad skin caused by complete Freund's adjuvant (CFA) injection into the lower lip. CFA injection induced heat hyperalgesia of the ipsilateral whisker pad skin. Moreover, it leads to enhancement of spontaneous activity and heat responses in trigeminal ganglion (TG) neurons that was elicited by heat stimulation of the whisker pad skin. The heat hyperalgesia was dose-dependently reversed by intraperitoneal TRPV1 antagonist administration, also diminished by neutralizing anti-NGF antibody administration into the lower lip and intraganglionic administration of K252a, a tyrosine kinase receptor inhibitor. Nerve fibers in bundle of mandibular nerve and TG neurons that innervates the whisker pad skin and lower lip both expressed labeled NGF, which was administrated into the lower lip. Moreover, the NGF concentrations in ophthalmic-maxillary and mandibular divisions of the TG increased after CFA injection into the lower lip. The number of TRPV1-positive neurons that innervates the whisker pad skin and lower lip was increased after CFA injection into the lower lip, and this increase was annulled by anti-NGF administration. The present findings suggest that inflammation in the lower lip induces release of NGF that regulates TRPV1 expression in TG neurons. This TRPV1 overexpression may underlie ectopic heat hyperalgesia in the whisker pad skin.

Tooth injury increases expression of the cold sensitive TRP channel TRPA1 in trigeminal neurons

OBJECTIVE

Transient receptor potential (TRP) channels, a family of structurally related proteins have been implicated in the sensation of pain and hyperalgesia caused by exogenous and endogenous agonists, as well as touch, pH, and temperature. The objective of this study was to determine the effects of tooth injury on the expression of the cold sensitive channel TRPA1, in the trigeminal ganglion, the primary source of sensory and nociceptive innervation of teeth.

DESIGN

We analyzed TRPA1 expression in a rodent model of tooth injury, by Western blot analyses of proteins extracted from trigeminal ganglia.

RESULTS

We found that TRPA1 was selectively increased in trigeminal ganglia innervating injured teeth when compared to TRPA1 expression in trigeminal ganglia innervating healthy teeth.

CONCLUSIONS

Our results provide the first evidence of increased expression of a cold-sensitive TRP channel in trigeminal ganglia after pulp exposure, and are consistent with the possibility that increased expression and function of TRPA1 in trigeminal neurons contributes to hyperalgesia and allodynia following tooth injury.

Lipopolysaccharide-induced pulpitis up-regulates TRPV1 in trigeminal ganglia

Tooth pain often accompanies pulpitis. Accumulation of lipopolysaccharides (LPS), a product of Gram-negative bacteria, is associated with painful clinical symptoms. However, the mechanisms underlying LPS-induced tooth pain are not clearly understood. TRPV1 is a capsaicin- and heat-gated nociceptive ion channel implicated in thermosensation and hyperalgesia under inflammation or injury. Although TRPV1 is expressed in pulpal afferents, it is not known whether the application of LPS to teeth modulates TRPV1 in trigeminal nociceptors. By assessing the levels of protein and transcript of TRPV1 in mouse trigeminal ganglia, we demonstrate that dentinal application of LPS increases the expression of TRPV1. Our results suggest that the up-regulation of TRPV1 in trigeminal nociceptors following bacterial infection could contribute to hyperalgesia under pulpitis conditions.

Tumor necrosis factor-α increases brain-derived neurotrophic factor expression in trigeminal ganglion neurons in an activity-dependent manner

Many chronic trigeminal pain conditions, such as migraine or temporo-mandibular disorders, are associated with inflammation within peripheral endings of trigeminal ganglion (TG) sensory neurons. A critical role in mechanisms of neuroinflammation is attributed to proinflammatory cytokines, such as interleukin-1β and tumor necrosis factor-α (TNFα) that also contribute to mechanisms of persistent neuropathic pain resulting from nerve injury. However, the mechanisms of cytokine-mediated synaptic plasticity and nociceptor sensitization are not completely understood. In the present study, we examined the effects of TNFα on neuronal expression of brain-derived neurotrophic factor (BDNF), whose role in synaptic plasticity and sensitization of nociceptive pathways is well documented. We show that 4- and 24-h treatment with TNFα increases BDNF mRNA and protein, respectively, in neuron-enriched dissociated cultures of rat TG. TNFα increases the phosphorylated form of the cyclic AMP-responsive element binding protein (CREB), a transcription factor involved in regulation of BDNF expression in neurons, and activates transcription of BDNF exon IV (former exon III) and, to a lesser extent, exon VI (former exon IV), but not exon I. TNFα-mediated increase in BDNF expression is accompanied by increase in calcitonin gene-related peptide (CGRP), which is consistent with previously published studies, and indicates that both peptides are similarly regulated in TG neurons by inflammatory mediators. The effect of TNFα on BDNF expression is dependent on sodium influx through TTX-sensitive channels and on p38-mitogen-activated protein kinase. Moreover, electrical stimulation and forskolin, known to increase intracellular cAMP, potentiate the TNFα-mediated upregulation of BDNF expression. This study provides new evidence for a direct action of proinflammatory cytokines on TG primary sensory neurons, and reveals a mechanism through which TNFα stimulates de novo synthesis of BDNF in these neurons. Thus, TNFα should be considered in mechanisms of BDNF-dependent neuronal plasticity.

Hippocampal FGF-2 and BDNF overexpression attenuates epileptogenesis-associated neuroinflammation and reduces spontaneous recurrent seizures

Under certain experimental conditions, neurotrophic factors may reduce epileptogenesis. We have previously reported that local, intrahippocampal supplementation of fibroblast growth factor-2 (FGF-2) and brain-derived neurotrophic factor (BDNF) increases neurogenesis, reduces neuronal loss, and reduces the occurrence of spontaneous seizures in a model of damage-associated epilepsy. Here, we asked if these possibly anti-epileptogenic effects might involve anti-inflammatory mechanisms. Thus, we used a Herpes-based vector to supplement FGF-2 and BDNF in rat hippocampus after pilocarpine-induced status epilepticus that established an epileptogenic lesion. This model causes intense neuroinflammation, especially in the phase that precedes the occurrence of spontaneous seizures. The supplementation of FGF-2 and BDNF attenuated various parameters of inflammation, including astrocytosis, microcytosis and IL-1β expression. The effect appeared to be most prominent on IL-1β, whose expression was almost completely prevented. Further studies will be needed to elucidate the molecular mechanism(s) for these effects, and for that on IL-1β in particular. Nonetheless, the concept that neurotrophic factors affect neuroinflammation in vivo may be highly relevant for the understanding of the epileptogenic process.