Involvement of transient receptor potential vanilloid 1 in ectopic pain following inferior alveolar nerve transection in rats

Oxytocin-Dependent Regulation of TRPs Expression in Trigeminal Ganglion Neurons Attenuates Orofacial Neuropathic Pain Following Infraorbital Nerve Injury in Rats

We evaluated the mechanisms underlying the oxytocin (OXT)-induced analgesic effect on orofacial neuropathic pain following infraorbital nerve injury (IONI). IONI was established through tight ligation of one-third of the infraorbital nerve thickness. Subsequently, the head withdrawal threshold for mechanical stimulation (MHWT) of the whisker pad skin was measured using a von Frey filament. Trigeminal ganglion (TG) neurons innervating the whisker pad skin were identified using a retrograde labeling technique. OXT receptor-immunoreactive (IR), transient receptor potential vanilloid 1 (TRPV1)-IR, and TRPV4-IR TG neurons innervating the whisker pad skin were examined on post-IONI day 5. The MHWT remarkably decreased from post-IONI day 1 onward. OXT application to the nerve-injured site attenuated the decrease in MHWT from day 5 onward. TRPV1 or TRPV4 antagonism significantly suppressed the decrement of MHWT following IONI. OXT receptors were expressed in the uninjured and Fluoro-Gold (FG)-labeled TG neurons. Furthermore, there was an increase in the number of FG-labeled TRPV1-IR and TRPV4-IR TG neurons, which was inhibited by administering OXT. This inhibition was suppressed by co-administration with an OXT receptor antagonist. These findings suggest that OXT application inhibits the increase in TRPV1-IR and TRPV4-IR TG neurons innervating the whisker pad skin, which attenuates post-IONI orofacial mechanical allodynia.

Trigeminal Nerve Transection-Induced Neuroplastic Changes in the Somatosensory and Insular Cortices in a Rat Ectopic Pain Model

The primary sensory cortex processes competitive sensory inputs. Ablation of these competitive inputs induces neuroplastic changes in local cortical circuits. However, information concerning cortical plasticity induced by a disturbance of competitive nociceptive inputs is limited. Nociceptive information from the maxillary and mandibular molar pulps converges at the border between the ventral secondary somatosensory cortex (S2) and insular oral region (IOR); therefore, S2/IOR is a suitable target for examining the cortical changes induced by a disturbance of noxious inputs, which often causes neuropathic pain and allodynia. We focused on the plastic changes in S2/IOR excitation in a model of rats subjected to inferior alveolar nerve transection (IANX). Our optical imaging using a voltage-sensitive dye (VSD) revealed that the maxillary molar pulp stimulation-induced excitatory propagation was expanded one to two weeks after IANX at the macroscopic level. At the cellular level, based on Ca²⁺ imaging using two-photon microscopy, the amplitude of the Ca²⁺ responses and the number of responding neurons in S2/IOR increased in both excitatory and inhibitory neurons. The in vitro laser scanning photostimulation (LSPS) revealed that Layer II/III pyramidal and GABAergic fast-spiking neurons in S2/IOR received larger excitatory inputs from Layer IV in the IANX models, which supports the findings obtained by the macroscopic and microscopic optical imaging. Furthermore, the inhibitory postsynaptic inputs to the pyramidal neurons were decreased in the IANX models, suggesting suppression of inhibitory synaptic transmission onto excitatory neurons. These results suggest that IANX induces plastic changes in S2/IOR by changing the local excitatory and inhibitory circuits.

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.

Enhancement of ERK phosphorylation and photic responses in Vc/C1 neurons of a migraine model

Although it is well known that migraine pain is enhanced by photic stimulation of the eye, the mechanisms underlying this response are not yet understood. Noxious stimulation to the dura is known to activate trigeminal spinal subnucleus caudalis and upper cervical spinal cord (Vc/C1) neurons, causing migraine pain. Intense photic stimulation to the eye is also known to activate certain Vc/C1 neurons, thus increasing migraine pain. In this study, we hypothesized that Vc/C1 neurons receiving noxious dural input would be further activated by intense photic stimulation, resulting in the enhancement of migraine pain. However, mechanisms underlying the interactions between dural and photic sensory information in Vc/C1 neurons is unknown. To evaluate the above hypothesis, we studied phosphorylated extracellular signal-regulated kinase (pERK) -immunoreactive (IR) cells in Vc/C1 in dural mustard oil (DMO)-administrated rats. The change in neuronal excitability of Vc/C1 nociceptive neurons receiving input from the dura in DMO rats was examined and tested if those neurons were modulated by intense flush light stimulation. There were many pERK-IR cells in the lateral portion of Vc/C1 after MO administration to the dura. Flashlight presentation to the eye in DMO rats caused an enhancement of ERK phosphorylation in Vc/C1 neurons and pERK-IR cells were significantly suppressed after intracisternal administration of MEK1 inhibitor PD98059. Dura-light sensitive (DL) neurons were recorded in the lateral portion of Vc/C1 and photic responses of DL neurons were significantly enhanced following dural MO administration. These findings indicate that DL Vc/C1 neurons in DMO rats intensified their responses to intense photic stimulation and that ERK phosphorylation in Vc/C1 neurons receiving noxious dural input increased with intense photic stimulation, suggesting that Vc/C1 nociceptive neurons are involved in the enhancement of dural nociception associated with intense light stimulation.

Distinct TRPV1- and TRPA1-based mechanisms underlying enhancement of oral ulcerative mucositis-induced pain by 5-fluorouracil

In many patients with cancer, chemotherapy-induced severe oral ulcerative mucositis causes intractable pain, leading to delays and interruptions in therapy. However, the pain mechanism in oral ulcerative mucositis after chemotherapy has not been extensively studied. In this study, we investigated spontaneous pain and mechanical allodynia in a preclinical model of oral ulcerative mucositis after systemic administration of the chemotherapy drug 5-fluorouracil, using our proprietary pain assay system for conscious rats. 5-Fluorouracil caused leukopenia but did not induce pain-related behaviors. After 5-fluorouracil administration, oral ulcers were developed with topical acetic acid treatment. Compared with saline-treated rats, 5-fluorouracil-exposed rats showed more severe mucositis with excessive bacterial loading due to a lack of leukocyte infiltration, as well as enhancements of spontaneous pain and mechanical allodynia. Antibacterial drugs, the lipid A inhibitor polymyxin B and the TRPV1/TRPA1 channel pore-passing anesthetic QX-314, suppressed both the spontaneous pain and the mechanical allodynia. The cyclooxygenase inhibitor indomethacin and the TRPV1 antagonist SB-366791 inhibited the spontaneous pain, but not the mechanical allodynia. In contrast, the TRPA1 antagonist HC-030031 and the N-formylmethionine receptor FPR1 antagonist Boc MLF primarily suppressed the mechanical allodynia. These results suggest that 5-fluorouracil-associated leukopenia allows excessive oral bacterial infection in the oral ulcerative region, resulting in the enhancement of spontaneous pain through continuous TRPV1 activation and cyclooxygenase pathway, and mechanical allodynia through mechanical sensitization of TRPA1 caused by neuronal effects of bacterial toxins. These distinct pain mechanisms explain the difficulties encountered with general treatments for oral ulcerative mucositis-induced pain in patients with cancer and suggest more effective approaches.

Connexin 43 contributes to ectopic orofacial pain following inferior alveolar nerve injury

Background

Clinically, it is well known that injury of mandibular nerve fiber induces persistent ectopic pain which can spread to a wide area of the orofacial region innervated by the uninjured trigeminal nerve branches. However, the exact mechanism of such persistent ectopic orofacial pain is not still known. The present study was undertaken to determine the role of connexin 43 in the trigeminal ganglion on mechanical hypersensitivity in rat whisker pad skin induced by inferior alveolar nerve injury. Here, we examined changes in orofacial mechanical sensitivity following inferior alveolar nerve injury. Furthermore, changes in connexin 43 expression in the trigeminal ganglion and its localization in the trigeminal ganglion were also examined. In addition, we investigated the functional significance of connexin 43 in relation to mechanical allodynia by using a selective gap junction blocker (Gap27).

Results

Long-lasting mechanical allodynia in the whisker pad skin and the upper eyelid skin, and activation of satellite glial cells in the trigeminal ganglion, were induced after inferior alveolar nerve injury. Connexin 43 was expressed in the activated satellite glial cells encircling trigeminal ganglion neurons innervating the whisker pad skin, and the connexin 43 protein expression was significantly increased after inferior alveolar nerve injury. Administration of Gap27 in the trigeminal ganglion significantly reduced satellite glial cell activation and mechanical hypersensitivity in the whisker pad skin. Moreover, the marked activation of satellite glial cells encircling trigeminal ganglion neurons innervating the whisker pad skin following inferior alveolar nerve injury implies that the satellite glial cell activation exerts a major influence on the excitability of nociceptive trigeminal ganglion neurons.

Conclusions

These findings indicate that the propagation of satellite glial cell activation throughout the trigeminal ganglion via gap junctions, which are composed of connexin 43, plays a pivotal role in ectopic mechanical hypersensitivity in whisker pad skin following inferior alveolar nerve injury.

Morphological and functional changes in regenerated primary afferent fibres following mental and inferior alveolar nerve transection

BACKGROUND

It is important to know the mechanisms underlying pain abnormalities associated with inferior alveolar nerve (IAN) regeneration in order to develop the appropriate treatment for orofacial neuropathic pain patients. However, peripheral mechanisms underlying orofacial pain abnormalities following IAN regeneration are not fully understood.

METHODS

Head withdrawal threshold (HWT), jaw opening reflex (JOR) thresholds, single-fibre recordings of the regenerated mental nerve (MN) fibres, calcitonin gene-related peptide (CGRP), isolectin B4 (IB4), peripherin, neurofilament-200 (NF-200) and transient receptor potential vanilloid 1 (TRPV1) expression in trigeminal ganglion (TG) cells, and electron microscopic (EM) observations of the regenerated MN fibres were studied in MN- and IAN-transected (M-IANX) rats.

RESULTS

HWT to mechanical or heat stimulation of the mental skin was significantly lower in M-IANX rats compared with sham rats. Mean conduction velocity of action potentials recorded from MN fibres (n = 124) was significantly slower in M-IANX rats compared with sham rats. The percentage of Fluoro-Gold (FG)-labelled CGRP-, peripherin- or TRPV1-immunoreactive (IR) cells was significantly larger in M-IANX rats compared with that of sham rats, whereas that of FG-labelled IB4- and NF-200-IR cells was significantly smaller in M-IANX rats compared with sham rats. Large-sized myelinated nerve fibres were rarely observed in M-IANX rats, whereas large-sized unmyelinated nerve fibres were frequently observed and were aggregated in the bundles at the distal portion of regenerated axons.

CONCLUSIONS

These findings suggest that the demyelination of MN fibres following regeneration may be involved in peripheral sensitization, resulting in the orofacial neuropathic pain associated with trigeminal nerve injury.

Toll-like receptor 4 signaling in trigeminal ganglion neurons contributes tongue-referred pain associated with tooth pulp inflammation

Background

The purpose of the present study is to evaluate the mechanisms underlying tongue-referred pain associated with tooth pulp inflammation.

Method

Using mechanical and temperature stimulation following dental surgery, we have demonstrated that dental inflammation and hyperalgesia correlates with increased immunohistochemical staining of neurons for TLR4 and HSP70.

Results

Mechanical or heat hyperalgesia significantly enhanced in the ipsilateral tongue at 1 to 9 days after complete Freund’s adjuvant (CFA) application to the left lower molar tooth pulp compared with that of sham-treated or vehicle-applied rats. The number of fluorogold (FG)-labeled TLR4-immunoreactive (IR) cells was significantly larger in CFA-applied rats compared with sham-treated or vehicle-applied rats to the molar tooth. The number of heat shock protein (Hsp) 70-IR neurons in trigeminal ganglion (TG) was significantly increased on day 3 after CFA application compared with sham-treated or vehicle-applied rats to the molar tooth. About 9.2% of TG neurons were labeled with DiI applied to the molar tooth and FG injected into the tongue, and 15.4% of TG neurons were labeled with FG injected into the tongue and Alexa-labeled Hsp70-IR applied to the tooth. Three days after Hsp70 or lipopolysaccharide (LPS) application to the tooth in naive rats, mechanical or heat hyperalgesia was significantly enhanced compared with that of saline-applied rats. Following successive LPS-RS, an antagonist of TLR4, administration to the TG for 3 days, the enhanced mechanical or heat hyperalgesia was significantly reversed compared with that of saline-injected rats. Noxious mechanical responses of TG neurons innervating the tongue were significantly higher in CFA-applied rats compare with sham rats to the tooth. Hsp70 mRNA levels of the tooth pulp and TG were not different between CFA-applied rats and sham rats.

Conclusions

The present findings indicate that Hsp70 transported from the tooth pulp to TG neurons or expressed in TG neurons is released from TG neurons innervating inflamed tooth pulp, and is taken by TG neurons innervating the tongue, suggesting that the Hsp70-TLR4 signaling in TG plays a pivotal role in tongue-referred pain associated with tooth pulp inflammation.

Nitric Oxide Signaling Contributes to Ectopic Orofacial Neuropathic Pain

Inferior alveolar nerve (IAN) injury induces persistent ectopic pain which spreads to a wide area in the orofacial region. Its exact mechanism remains unclear. We investigated the involvement of nitric oxide (NO) in relation to ectopic orofacial pain caused by IAN transection (IANX). We assessed the changes in mechanical sensitivity of the whisker pad skin following IANX, neuronal nitric oxide synthase (nNOS) expression in the trigeminal ganglion (TG), and the functional significance of NO in relation to the mechanical allodynia following intra-TG administration of a chemical precursor to NO and selective nNOS inhibitors. IANX induced mechanical allodynia, which was diminished by intra-TG administration of selective nNOS inhibitors. NO metabolites and nNOS immunoreactive neurons innervating the lower lip were also increased in the TG. Intra-TG administration of nNOS substrate induced the mechanical allodynia. The present findings suggest that NO released from TG neurons regulates the excitability of TG neurons innervating the whisker pad skin, and the enhancement of TG neuronal excitability may underlie ectopic mechanical allodynia.

Enhanced excitability of thalamic sensory neurons and slow-wave EEG pattern after stimuli that induce spinal long-term potentiation

Spinal nociceptive neurons are well known to undergo a process of long-term potentiation (LTP) following conditioning by high-frequency sciatic nerve stimulation (HFS) at intensities recruiting C-fibers. However, little if any information exists as to whether such HFS conditioning that produces spinal LTP affects sensory transmission at supraspinal levels. The present study explored this possibility. Conventional extracellular recording methods were used to examine the consequences of HFS versus sham HFS conditioning on individual wide-dynamic range thalamic neurons located in the ventro-postero-lateral (VPL) nucleus in isoflurane-anesthetized rats. Following HFS, the ongoing firing rate and stimulus-evoked (brush, pinch, sciatic nerve) responses were markedly enhanced as were responses to juxtacellular, microiontophoretic applications of glutamate. These HFS-induced enhancements lasted throughout the recording period. Sham stimuli had no effect on VPL neuron excitability. Cortical electroencephalographic (EEG) wave activities were also measured around HFS in conjunction with VPL neuron recordings. The cortical EEG pattern under baseline conditions consisted of recurring short duration bursts of high-amplitude slow waves followed by longer periods of flat EEG. Following HFS, the EEG shifted to a continuous large-amplitude, slow-wave pattern within the 0.5-8.0 Hz bandwidth lasting throughout the recording period. Sham HFS did not alter EEG activity. Sciatic nerve conditioning at A-δ fiber strength, known to reverse spinal LTP, did not alter enhanced neuronal excitability or the EEG slow-wave pattern induced by HFS. These data support the concept that HFS conditioning of the sciatic nerve, which leads to spinal LTP, is associated with distinct, long-lasting changes in the excitability of neurons comprising thalamocortical networks.