Vanilloid receptor 1-like receptor-immunoreactive primary sensory neurons in the rat trigeminal nervous system

Exploring biological mechanisms in orthodontic tooth movement: Bridging the gap between basic research experiments and clinical applications - A comprehensive review

OBJECTIVES

The molecular mechanisms behind orthodontic tooth movements (OTM) were investigated by clarifying the role of chemical messengers released by cells.

METHODS

Using the Cochrane library, Google scholar, and PubMed databases, a literature search was conducted, and studies published from 1984 to 2024 were considered.

RESULTS

Both bone growth and remodeling may occur when a tooth is subjected to mechanical stress. These chemicals have a significant effect on the stimulation and regulation of osteoblasts, osteoclasts, and osteocytes during alveolar bone remodeling. This regulation can take place in pathological conditions, such as periodontal diseases, or during OTM alone. This comprehensive review outlines key molecular mechanisms underlying OTM and explores various clinical assumptions associated with specific molecules and their functional domains during this process. Furthermore, clinical applications of certain molecules such as relaxin, prostaglandin E (PGE), and interleukin-1β (IL-1β) in accelerating OTM have been reported. Our findings underscore the existing gap between OTM clinical applications and basic research investigations.

CONCLUSION

A comprehensive understanding of orthodontic treatment is enriched by insights into biological systems. We reported the activation of osteoblasts, osteoclast precursor cells, osteoclasts, and osteocytes in response to mechanical stress, leading to targeted cellular and molecular interventions and facilitating rapid and regulated alveolar bone remodeling during tooth movement. Despite the shortcomings of clinical studies in accelerating OTM, this review highlights the crucial role of biological agents in this process and advocates for prioritizing high-quality human studies in future research to gain further insights from clinical trials.

The role of TRPV2 as a regulator on the osteoclast differentiation during orthodontic tooth movement in rats

When orthodontic forces are applied to teeth, bone remodeling, which consists of bone resorption and bone formation, occurs around the teeth. Transient receptor potential vanilloid 2 (TRPV2) is a cation channel expressed in various cell types that responds to various stimuli, including mechanical stress, and involved in calcium oscillations during the early stages of osteoclast differentiation. However, in vivo expression of TRPV2 in osteoclasts has not yet been reported, and temporo-spatial expression of TRPV2 during osteoclast differentiation is unclear. In this study, we examined the TRPV2 expression during experimental tooth movement and assessed the effect of TRPV2 on osteoclast differentiation. TRPV2 was detected on day 1 after experimental tooth movement on the compression side, and the number of TRPV2-expressing cells significantly increased on day 7. These TRPV2-expressing cells had a single, or multiple nuclei and were positive for TRAP activity. Consistent with these in vivo findings, in vitro experiments using RAW264.7 osteoclast progenitor cells showed that TRPV2 mRNA was increased at the early stage of osteoclast differentiation and maintained until the late stage. Furthermore, a TRPV2 channel selective antagonist significantly inhibited osteoclast differentiation. These findings suggest that TRPV2 may have a regulatory role in osteoclast differentiation during orthodontic tooth movement.

Transient receptor potential melastatin-7 in the rat dorsal root ganglion

AIMS

Transient receptor potential melastatin-7 (TRPM7) is a selective cation permeable channel which plays important roles in cellular and developmental biology such as cell proliferation, survival, differentiation and migration. This channel is also known to be necessary for transmitter release in the peripheral nervous system. In this study, immunohistochemistry for TRPM7 was conducted in the rat lumbar dorsal root ganglion (DRG).

METHODS

Triple immunofluorescence methods were used to demonstrate distribution of TRPM7 and its relationship to other TRP channels in the DRG. Retrograde tracing and double immunofluorescence methods were also performed to know peripheral targets of DRG neurons containing TRPM7 and TRP vanilloid 1 (TRPV1). In addition, transection of the sciatic nerve was conducted to demonstrate an effect of the nerve injury on TRPM7expression in the DRG.

RESULTS

TRPM7-immunoreactivity was expressed by 53.9% of sensory neurons in the 4th lumbar DRG. TRPM7-immunoreactive (-IR) DRG neurons mostly had small (<600 µm²) and medium-sized (600-1200 µm²) cell bodies. By triple and double immunofluorescence methods, approximately 70% of TRPM7-IR DRG neurons contained TRPV1-immunoreactivity. Although the number of DRG neurons co-expressing TRPM7 and TRPM8 was small in the DRG, almost all of TRPM8-IR DRG neurons co-expressed TRPM7-immunoreactivity. By combination of retrograde tracing method and immunohistochemistry, TRPM7 was expressed by half of DRG neurons innervating the plantar skin (61.9%) and gastrocnemius muscle (51.2%), and 79.6% of DRG neurons innervating the periosteum. Co-expression of TRPM7 and TRPV1 among periosteum DRG neurons (75.7%) was more abundant than among cutaneous (53.2%) and muscular (40.4%) DRG neurons. DRG neurons which co-expressed these ion channels in the periosteum had smaller cell bodies compared to the skin and muscle. In addition, the sciatic nerve transection decreased the number of TRPM7-IR neurons in the DRG (approximately 60% reduction). The RT-qPCR analysis also demonstrated reduction of TRPM7 mRNA in the injured DRG.

CONCLUSION

The present study suggests that TRPM7 is mainly located in small nociceptors in the DRG. The content of TRPM7 in DRG neurons is probably different among their peripheral targets. TRPM7 in DRG neurons may be able to respond to noxious stimulation from their peripheral tissues. The nerve injury can decrease the level of TRPM7 mRNA and protein in DRG neurons.

Insight into the Role of Psychological Factors in Oral Mucosa Diseases

With the development of psychology and medicine, more and more diseases have found their psychological origins and associations, especially ulceration and other mucosal injuries, within the digestive system. However, the association of psychological factors with lesions of the oral mucosa, including oral squamous cell carcinoma (OSCC), burning mouth syndrome (BMS), and recurrent aphthous stomatitis (RAS), have not been fully characterized. In this review, after introducing the association between psychological and nervous factors and diseases, we provide detailed descriptions of the psychology and nerve fibers involved in the pathology of OSCC, BMS, and RAS, pointing out the underlying mechanisms and suggesting the clinical indications.

Pivotal role of Transient Receptor Potential Channels in oral physiology

BACKGROUND

Transient Receptor Potential (TRP) Channels constitute a large family of non-selective permeable ion channels involved in the perception of environmental stimuli with a central and continuously expanding role in oral tissue homeostasis. Recent studies indicate the regulatory role of TRPs in pulp physiology, oral mucosa sensation, dental pain nociception and salivary gland secretion. This review provides an update on the diverse functions of TRP channels in the physiology of oral cavity, with emphasis on their cellular location, the underlying molecular mechanisms and clinical significance.

METHODS

A structured search of bibliographic databases (PubMed and MEDLINE) was performed for peer reviewed studies on TRP channels function on oral cavity physiology the last ten years. A qualitative content analysis was performed in screened papers and a critical discussion of main findings is provided.

RESULTS

TRPs expression has been detected in major cell types of the oral cavity, including odontoblasts, periodontal ligament, oral epithelial, salivary gland cells, and chondrocytes of temporomandibular joints, where they mediate signal perception and transduction of mechanical, thermal, and osmotic stimuli. They contribute to pulp physiology through dentin formation, mineralization, and periodontal ligament formation along with alveolar bone remodeling in dental pulp and periodontal ligament cells. TRPs are also involved in oral mucosa sensation, dental pain nociception, saliva secretion, swallowing reflex and temporomandibular joints' development.

CONCLUSION

Various TRP channels regulate oral cavity homeostasis, playing an important role in the transduction of external stimuli to intracellular signals in a cell type-specific manner and presenting promising drug targets for the development of pharmacological strategies to manage oral diseases.

Distribution of alpha-synuclein in the rat cranial sensory ganglia, and oro-cervical regions

BACKGROUND

Alpha-synuclein (Syn), an unfolded soluble cytosolic protein, is known as a disease-associated protein in the brain. However, little is known about distribution of this protein in the peripheral nervous system. In this study, expression of Syn was investigated in the sensory ganglia of the cranial nerves V, IX and X.

METHODS

To analyze distribution of Syn and its co-expression with calcitonin gene-related peptide (CGRP) or the transient receptor potential cation channel subfamily V member 1 (TRPV1), immunohistochemical techniques were used in the rat cranial sensory ganglia and their peripheral tissues.

RESULTS

Syn-immunoreactive (-ir) neurons were abundant in the sensory ganglia of the petrosal (56.7%), jugular (28.3%) and nodose ganglia (82.5%). These neurons had small to medium-sized cell bodies (petrosal, mean ± S.D. = 667.4 ± 310.8 μ m²; jugular, 625.1 ± 318.4 μ m²; nodose, 708.3 ± 248.3 μ m²), and were distributed throughout the ganglia. However, the trigeminal ganglion was mostly free of Syn-ir neurons. By double and triple immunofluorescence staining, Syn-ir neurons co-expressed CGRP and TRPV1 in the petrosal and jugular ganglia. Syn-immunoreactivity was expressed by nerve fibers in the epithelium and taste bud of oral and cervical viscerae. These nerve fibers were abundant in the naso-pharynx, epiglottis and laryngeal vestibule. Some taste bud cells were also immunoreactive for Syn. In addition, Syn-ir nerve fibers were detected in the vicinity of macrophages, dendritic cells and Langerhans cells.

CONCLUSIONS

Syn was abundant in the visceral sensory neurons but not in somatic sensory neurons. This protein may play a role in nociceptive and chemosensory transduction in the glossopharyngeal and vagal sensory ganglia. It is possible that Syn has a function about the immune mechanism of the upper air way.

Parasympathetic neurons in the human submandibular ganglion

The submandibular ganglion (SMG) contains parasympathetic neurons which innervate the submandibular gland. In this study, immunohistochemistry for vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), choline acetyltransferase (ChAT), dopamine β-hydroxylase (DBH), tyrosine hydroxylase (TH), and the transient receptor potential cation channel subfamily V members 1 (TRPV1) and 2 (TRPV2) was performed on the human SMG. In the SMG, 17.5 % and 8.9 % of parasympathetic neurons were immunoreactive for VIP and TRPV2, respectively. SMG neurons mostly contained ChAT- and DBH-immunoreactivity. In addition, subpopulations of SMG neurons were surrounded by VIP (69.6 %)-, TRPV2 (54.4 %)- and DBH (9.5 %)-immunoreactive (-ir) nerve fibers. SMG neurons with pericellular VIP- and TRPV2-ir nerve fibers were significantly larger than VIP- and TRPV2-ir SMG neurons, respectively. Other neurochemical substances were rare in the SMG. In the human submandibular gland, TRPV1- and TRPV2-ir nerve fiber profiles were seen around blood vessels. Double fluorescence method also demonstrated that TRPV2-ir nerve fiber profiles were located around myoepithelial and acinar cells in the submandibular gland. VIP and TRPV2 are probably expressed by both pre- and post-ganglionic neurons innervating the submandibular and sublingual glands. VIP, DBH and TRPV2 may have functions about regulation of salivary components in the salivary glands and neuronal activity in the SMG.

Transient Receptor Potential (TRP) Ion Channels in Orofacial Pain

Orofacial pain, including temporomandibular joint disorders pain, trigeminal neuralgia, dental pain, and debilitating headaches, affects millions of Americans each year with significant population health impact. Despite the existence of a large body of information on the subject, the molecular underpinnings of orofacial pain remain elusive. Two decades of research has identified that transient receptor potential (TRP) ion channels play a crucial role in pathological pain. A number of TRP ion channels are clearly expressed in the trigeminal sensory system and have critical functions in the transduction and pathogenesis of orofacial pain. Although there are many similarities, the orofacial sensory system shows some distinct peripheral and central pain processing and different sensitivities from the spinal sensory system. Relative to the extensive review on TRPs in spinally-mediated pain, the summary of TRPs in trigeminally-mediated pain has not been well-documented. This review focuses on the current experimental evidence involving TRP ion channels, particularly TRPV1, TRPA1, TRPV4, and TRPM8 in orofacial pain, and discusses their possible cellular and molecular mechanisms.

Ion Channels Involved in Tooth Pain

The tooth has an unusual sensory system that converts external stimuli predominantly into pain, yet its sensory afferents in teeth demonstrate cytochemical properties of non-nociceptive neurons. This review summarizes the recent knowledge underlying this paradoxical nociception, with a focus on the ion channels involved in tooth pain. The expression of temperature-sensitive ion channels has been extensively investigated because thermal stimulation often evokes tooth pain. However, temperature-sensitive ion channels cannot explain the sudden intense tooth pain evoked by innocuous temperatures or light air puffs, leading to the hydrodynamic theory emphasizing the microfluidic movement within the dentinal tubules for detection by mechanosensitive ion channels. Several mechanosensitive ion channels expressed in dental sensory systems have been suggested as key players in the hydrodynamic theory, and TRPM7, which is abundant in the odontoblasts, and recently discovered PIEZO receptors are promising candidates. Several ligand-gated ion channels and voltage-gated ion channels expressed in dental primary afferent neurons have been discussed in relation to their potential contribution to tooth pain. In addition, in recent years, there has been growing interest in the potential sensory role of odontoblasts; thus, the expression of ion channels in odontoblasts and their potential relation to tooth pain is also reviewed.

The Role of Transient Receptor Potential (TRP) Channels in the Transduction of Dental Pain

Dental pain is a common health problem that negatively impacts the activities of daily living. Dentine hypersensitivity and pulpitis-associated pain are among the most common types of dental pain. Patients with these conditions feel pain upon exposure of the affected tooth to various external stimuli. However, the molecular mechanisms underlying dental pain, especially the transduction of external stimuli to electrical signals in the nerve, remain unclear. Numerous ion channels and receptors localized in the dental primary afferent neurons (DPAs) and odontoblasts have been implicated in the transduction of dental pain, and functional expression of various polymodal transient receptor potential (TRP) channels has been detected in DPAs and odontoblasts. External stimuli-induced dentinal tubular fluid movement can activate TRP channels on DPAs and odontoblasts. The odontoblasts can in turn activate the DPAs by paracrine signaling through ATP and glutamate release. In pulpitis, inflammatory mediators may sensitize the DPAs. They could also induce post-translational modifications of TRP channels, increase trafficking of these channels to nerve terminals, and increase the sensitivity of these channels to stimuli. Additionally, in caries-induced pulpitis, bacterial products can directly activate TRP channels on DPAs. In this review, we provide an overview of the TRP channels expressed in the various tooth structures, and we discuss their involvement in the development of dental pain.

Transient receptor potential melastatin-3 in the rat sensory ganglia of the trigeminal, glossopharyngeal and vagus nerves

Transient receptor potential melastatin-3 (TRPM3) is a nonselective cation channel, has permeability of Ca²⁺, and probably participates in thermosensitive nociception. In this study, immunohistochemistry for TRPM3 was conducted in the rat trigeminal, glossopharyngeal and vagal sensory ganglia. TRPM3-immunoreactivity was expressed by half of sensory neurons in the trigeminal (TG), petrosal (PG) and jugular ganglia (JG), and by about 80% of sensory neurons in the nodose ganglion (NG). They mostly had small to medium-sized cell bodies. A trichrome immunofluorescence method showed co-existence of TRPM3 with TRP vanilloid 1 (TRPV1) and calcitonin gene-related peptide (CGRP). Approximately 70% of TRPM3-immunoreactive (-IR) neurons contained TRPV1-immunoreactivity in all the examined ganglia. More than 40% of TRPM3-IR neurons exhibited CGRP-immunoreactivity in the TG, PG and JG. Only a few sensory neurons co-expressed TRPM3- and CGRP-immunoreactivity in the NG. In addition, more than 40% of TRPM3-IR neurons bound to isolectin B4 in all the examined ganglia. By combination of retrograde tracing method and immunohistochemistry, half of TG neurons innervating the facial skin and incisive papilla expressed TRPM3-immunoreactivity whereas approximately 20% of those innervating the tooth pulp contained TRPM3-immunoreactivity. Co-expression of TRPM3-immunoreactivity with TRPV1- or CGRP-immunoreactivity was common among cutaneous and papillary TG neurons but not among pulpal TG neurons. More than 60% of PG and JG neurons innervating the external ear canal skin and circumvallate papilla contained TRPM3-immunoreactivity. Co-expression of TRPM3 with TRPV1 or CGRP was common among PG and JG neurons innervating the external ear canal skin. However, a smaller number of TRPM3-IR neurons co-expressing TRPV1- or CGRP-immunoreactivity innervate the circumvallate papilla in the PG. The present study suggests that expression of TRPM3 and its co-existence with TRPV1 and CGRP in sensory neurons depend on the variety of their peripheral targets in the trigeminal, glossopharyngeal and vagal nervous systems.

Innervation of the Human Incisive Papilla: Comparison with Other Oral Regions

Immunohistochemistry for several neurochemical substances was performed on the human incisive papilla and other oral structures. Sodium channel alpha subunit 7 (SCN7A) protein-immunoreactive (IR) Schwann cells and protein gene product 9.5 (PGP 9.5)-IR nerve fibers made nerve plexuses beneath the epithelium of the palate, including the incisive papilla, tongue, and lip. SCN7A immunoreactivity could also be detected in lamellated and nonlamellated capsules of corpuscle endings. Lamellated SCN7A-IR corpuscle endings were mostly restricted to the mucous and cutaneous lips. These endings had thick and spiral-shaped PGP 9.5-IR axons without ramification. Nonlamellated SCN7A-IR corpuscle endings were most numerous in the incisive papilla among the oral regions. On the basis of axonal morphology, the nonlamellated endings were divided into simple and complex types. PGP 9.5-IR terminal axons in the simple type ran straight or meandered with slight ramification, whereas those in the complex type were densely entangled with abundant ramification. Substance P (SP)-, calcitonin gene-related peptide (CGRP)-, and transient receptor potential cation channel subfamily V member 2 (TRPV2)-IR varicose fibers were rarely seen beneath the epithelium of oral structures. The present study indicates that the human incisive papilla has many low-threshold mechanoreceptors with nonlamellated capsules. SP-, CGRP-, and TRPV2-containing nociceptors may be infrequent in the incisive papilla and other oral regions.

Nerve Injury Increases the Expression of Alpha-2/Delta-1 Subunit of L-Type Calcium Channel in Sensory Neurons of Rat Spinal and Trigeminal Nerves

By immunohistochemistry, an effect of nerve injury on distribution of alpha-2/delta-1 subunit of L-type calcium channel was investigated in rat's 4th and 5th lumbar dorsal root ganglia (DRGs), trigeminal ganglion (TG), and mesencephalic trigeminal nucleus (Mes5). The immunoreactivity was expressed by 52.2% of DRG neurons and 31.4% of TG neurons in intact animals. These neurons mostly had small-to-medium-sized cell bodies. In the DRG and TG, alpha-2/delta-1 subunit-positive neurons were lightly or moderately stained. However, the number of alpha-2/delta-1 subunit-immunoreactive (-IR) neurons dramatically increased in the ipsilateral DRG at 3-28 days after sciatic nerve transection (75.3-79.5%) and in the ipsilateral TG at 7 days after infraorbital nerve transection (66.3%). The IR density of alpha-2/delta-1 subunit in DRG and TG neurons was also elevated by the transection. In the injured DRG and TG, many sensory neurons with small-to-medium-sized cell bodies were strongly stained. Some large DRG and TG neurons showing strong staining intensity also appeared after the treatment. In the intact Mes5, sensory neurons were mostly devoid of alpha-2/delta-1 subunit-immunoreactivity (0.4%). However, alpha-2/delta-1-IR sensory neurons on the ipsilateral side of the Mes5 dramatically increased at 7 days after masseteric nerve transection (31.3%). A double immunofluorescence method also demonstrated that c-Jun activating transcription factor 3 (ATF3)-positive DRG (98.3-99.9%) and Mes5 (81.8%) neurons mostly co-expressed alpha-2/delta-1 subunit after the nerve injuries. However, alpha-2/delta-1 subunit immunoreactivity was relatively infrequent among ATF3-immunonegative DRG neurons (51.6-74.1%) and Mes5 neurons (<1%). The present study indicates that the nerve injury increases the protein level of alpha-2/delta-1 subunit among several types of axotomized sensory neurons in the spinal and trigeminal nervous systems.

TRPV1 receptor in the human trigeminal ganglion and spinal nucleus: immunohistochemical localization and comparison with the neuropeptides CGRP and SP

This work presents new data concerning the immunohistochemical occurrence of the transient receptor potential vanilloid type-1 (TRPV1) receptor in the human trigeminal ganglion (TG) and spinal nucleus of subjects at different ontogenetic stages, from prenatal life to postnatal old age. Comparisons are made with the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP). TRPV1-like immunoreactive (LI) material was detected by western blot in homogenates of TG and medulla oblongata of subjects at prenatal and adult stages of life. Immunohistochemistry showed that expression of the TRPV1 receptor is mostly restricted to the small- and medium-sized TG neurons and to the caudal subdivision of the spinal trigeminal nucleus (Sp5C). The extent of the TRPV1-LI TG neuronal subpopulation was greater in subjects at early perinatal age than at late perinatal age and in postnatal life. Centrally, the TRPV1 receptor localized to fibre tracts and punctate elements, which were mainly distributed in the spinal tract, lamina I and inner lamina II of the Sp5C, whereas stained cells were rare. The TRPV1 receptor colocalized partially with CGRP and SP in the TG, and was incompletely codistributed with both neuropeptides in the spinal tract and in the superficial laminae of the Sp5C. Substantial differences were noted with respect to the distribution of the TRPV1-LI structures described in the rat Sp5C and with respect to the temporal expression of the receptor during the development of the rat spinal dorsal horn. The distinctive localization of TRPV1-LI material supports the concept of the involvement of TRPV1 receptor in the functional activity of the protopathic compartment of the human trigeminal sensory system, i.e. the processing and neurotransmission of thermal and pain stimuli.

La(3+) Alters the Response Properties of Neurons in the Mouse Primary Somatosensory Cortex to Low-Temperature Noxious Stimulation of the Dental Pulp

Although dental pain is a serious health issue with high incidence among the human population, its cellular and molecular mechanisms are still unclear. Transient receptor potential (TRP) channels are assumed to be involved in the generation of dental pain. However, most of the studies were conducted with molecular biological or histological methods. In vivo functional studies on the role of TRP channels in the mechanisms of dental pain are lacking. This study uses in vivo cellular electrophysiological and neuropharmacological method to directly disclose the effect of LaCl₃, a broad spectrum TRP channel blocker, on the response properties of neurons in the mouse primary somatosensory cortex to low-temperature noxious stimulation of the dental pulp. It was found that LaCl₃ suppresses the high-firing-rate responses of all nociceptive neurons to noxious low-temperature stimulation and also inhibits the spontaneous activities in some nonnociceptive neurons. The effect of LaCl₃ is reversible. Furthermore, this effect is persistent and stable unless LaCl₃ is washed out. Washout of LaCl₃ quickly revitalized the responsiveness of neurons to low-temperature noxious stimulation. This study adds direct evidence for the hypothesis that TRP channels are involved in the generation of dental pain and sensation. Blockade of TRP channels may provide a novel therapeutic treatment for dental pain.

RNA-Seq Analysis of Human Trigeminal and Dorsal Root Ganglia with a Focus on Chemoreceptors

The chemosensory capacity of the somatosensory system relies on the appropriate expression of chemoreceptors, which detect chemical stimuli and transduce sensory information into cellular signals. Knowledge of the complete repertoire of the chemoreceptors expressed in human sensory ganglia is lacking. This study employed the next-generation sequencing technique (RNA-Seq) to conduct the first expression analysis of human trigeminal ganglia (TG) and dorsal root ganglia (DRG). We analyzed the data with a focus on G-protein coupled receptors (GPCRs) and ion channels, which are (potentially) involved in chemosensation by somatosensory neurons in the human TG and DRG. For years, transient receptor potential (TRP) channels have been considered the main group of receptors for chemosensation in the trigeminal system. Interestingly, we could show that sensory ganglia also express a panel of different olfactory receptors (ORs) with putative chemosensory function. To characterize OR expression in more detail, we performed microarray, semi-quantitative RT-PCR experiments, and immunohistochemical staining. Additionally, we analyzed the expression data to identify further known or putative classes of chemoreceptors in the human TG and DRG. Our results give an overview of the major classes of chemoreceptors expressed in the human TG and DRG and provide the basis for a broader understanding of the reception of chemical cues.

Thermosensitive transient receptor potential (TRP) channel agonists and their role in mechanical, thermal and nociceptive sensations as assessed using animal models

INTRODUCTION

The present paper summarizes research using animal models to investigate the roles of thermosensitive transient receptor potential (TRP) channels in somatosensory functions including touch, temperature and pain. We present new data assessing the effects of eugenol and carvacrol, agonists of the warmth-sensitive TRPV3, on thermal, mechanical and pain sensitivity in rats.

METHODS

Thermal sensitivity was assessed using a thermal preference test, which measured the amount of time the animal occupied one of two adjacent thermoelectric plates set at different temperatures. Pain sensitivity was assessed as an increase in latency of hindpaw withdrawal away from a noxious thermal stimulus directed to the plantar hindpaw (Hargreaves test). Mechanical sensitivity was assessed by measuring the force exerted by an electronic von Frey filament pressed against the plantar surface that elicited withdrawal.

RESULTS

Topical application of eugenol and carvacrol did not significantly affect thermal preference, although there was a trend toward avoidance of the hotter surface in a 30 vs. 45°C preference test for rats treated with 1 or 10% eugenol and carvacrol. Both eugenol and carvacrol induced a concentration-dependent increase in thermal withdrawal latency (analgesia), with no significant effect on mechanosensitivity.

CONCLUSIONS

The analgesic effect of eugenol and carvacrol is consistent with previous studies. The tendency for these chemicals to increase the avoidance of warmer temperatures suggests a possible role for TRPV3 in warmth detection, also consistent with previous studies. Additional roles of other thermosensitive TRP channels (TRPM8 TRPV1, TRPV2, TRPV4, TRPM3, TRPM8, TRPA1, TRPC5) in touch, temperature and pain are reviewed.

Transient receptor potential ion channels in primary sensory neurons as targets for novel analgesics

The last decade has witnessed an explosion in novel findings relating to the molecules involved in mediating the sensation of pain in humans. Transient receptor potential (TRP) ion channels emerged as the greatest group of molecules involved in the transduction of various physical stimuli into neuronal signals in primary sensory neurons, as well as, in the development of pain. Here, we review the role of TRP ion channels in primary sensory neurons in the development of pain associated with peripheral pathologies and possible strategies to translate preclinical data into the development of effective new analgesics. Based on available evidence, we argue that nociception-related TRP channels on primary sensory neurons provide highly valuable targets for the development of novel analgesics and that, in order to reduce possible undesirable side effects, novel analgesics should prevent the translocation from the cytoplasm to the cell membrane and the sensitization of the channels rather than blocking the channel pore or binding sites for exogenous or endogenous activators.

Distribution of TRPVs, P2X3, and parvalbumin in the human nodose ganglion

Immunohistochemistry for several neurochemical substances, the transient receptor potential cation channel subfamily V member 1 (TRPV1) and 2 (TRPV2), P2X3 receptor, and parvalbumin (PV), was performed on the nodose ganglion, pharynx, and epiglottis in human cadavers. The nodose ganglion was situated beneath the jugular foramen, and had a spindle shape with the long rostrocaudal axis. The pharyngeal branch (PB) issued from a rostral quarter of the nodose ganglion, whereas the superior laryngeal nerve (SLN) usually originated from a caudal half of the ganglion. In the nodose ganglion, sensory neurons were mostly immunoreactive for TRPV1 (89 %) or P2X3 (93.9 %). About 30 % of nodose neurons contained TRPV2 (35.7 %)-or PV (29.9 %)-immunoreactivity (-IR). These neurons mainly had small to medium-sized cell bodies, and were distributed throughout the ganglion. Neurodegenerative profiles such as shrinkage or pyknosis could not be detected in the examined ganglion. Occasionally, TRPV2-IR nerve fibers surrounded blood vessels in the epiglottis as well as in the nasal and oral parts of the pharynx. Isolated TRPV2-IR nerve fibers were also located beneath the epithelium. TRPV1-, P2X3-, or PV-IR nerve endings could not be detected in the pharynx or epiglottis. In the PB and SLN, however, numerous nerve fibers contained TRPV1-, TRPV2-, P2X3-, and PV-IR. The present study suggests that TRPV1-, TRPV2-, P2X3-, and PV-IR neurons in the human nodose ganglion innervate the pharynx and epiglottis through the PB and SLN. These neurons may respond to chemical, thermal, and mechanical stimuli during respiration and swallowing.

Sensory TRP channel interactions with endogenous lipids and their biological outcomes

Lipids have long been studied as constituents of the cellular architecture and energy stores in the body. Evidence is now rapidly growing that particular lipid species are also important for molecular and cellular signaling. Here we review the current information on interactions between lipids and transient receptor potential (TRP) ion channels in nociceptive sensory afferents that mediate pain signaling. Sensory neuronal TRP channels play a crucial role in the detection of a variety of external and internal changes, particularly with damaging or pain-eliciting potentials that include noxiously high or low temperatures, stretching, and harmful substances. In addition, recent findings suggest that TRPs also contribute to altering synaptic plasticity that deteriorates chronic pain states. In both of these processes, specific lipids are often generated and have been found to strongly modulate TRP activities, resulting primarily in pain exacerbation. This review summarizes three standpoints viewing those lipid functions for TRP modulations as second messengers, intercellular transmitters, or bilayer building blocks. Based on these hypotheses, we discuss perspectives that account for how the TRP-lipid interaction contributes to the peripheral pain mechanism. Still a number of blurred aspects remain to be examined, which will be answered by future efforts and may help to better control pain states.

An insight into neurophysiology of pulpal pain: facts and hypotheses

Pain and pain control are important to the dental profession because the general perception of the public is that dental treatment and pain go hand in hand. Successful dental treatment requires that the source of pain be detected. If the origin of pain is not found, inappropriate dental care and, ultimately, extraction may result. Pain experienced before, during, or after endodontic therapy is a serious concern to both patients and endodontists, and the variability of discomfort presents a challenge in terms of diagnostic methods, endodontic therapy, and endodontic knowledge. This review will help clinicians understand the basic neurophysiology of pulpal pain and other painful conditions of the dental pulp that are not well understood.

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.

What do we know about the transient receptor potential vanilloid 2 (TRPV2) ion channel?

Transient receptor potential (TRP) ion channels are emerging as a new set of membrane proteins involved in a vast array of cellular processes and regulated by a large number of physical and chemical stimuli, which involves them with sensory cell physiology. The vanilloid TRP subfamily (TRPV) named after the vanilloid receptor 1 (TRPV1) consists of six members, and at least four of them (TRPV1-TRPV4) have been related to thermal sensation. One of the least characterized members of the TRP subfamily is TRPV2. Although initially characterized as a noxious heat sensor, TRPV2 now seems to have little to do with temperature sensing but a much more complex physiological profile. Here we review the available information and research progress on the structure, physiology and pharmacology of TRPV2 in an attempt to shed some light on the physiological and pharmacological deorphanization of TRPV2.

The distribution of TRPV1 and TRPV2 in the rat pharynx

Immunohistochemistry for two nociceptive transducers, the transient receptor potential cation channel subfamily V members 1 (TRPV1) and 2 (TRPV2), was performed on the pharynx and its adjacent regions. TRPV1-immunoreactivity (IR) was detected in nerve fibers beneath and within the epithelium and/or taste bud-like structure. In the pharynx, these nerve fibers were abundant in the naso-oral part and at the border region of naso-oral and laryngeal parts. They were also numerous on the laryngeal side of the epiglottis and in the soft palate. TRPV2-IR was expressed by dendritic cells in the pharynx and epiglottis, as well as in the root of the tongue and soft palate. These cells were located in the epithelium and lamina propria. TRPV2-immunoreactive (IR) dendritic cells were numerous in the naso-oral part of the pharynx, epiglottis, and tongue. Abundance of TRPV2-IR dendritic processes usually obscured the presence of TRPV2-IR nerve fibers in these portions. However, some TRPV2-IR nerve fibers could be observed in the epithelium of the soft palate. Retrograde tracing method also revealed that sensory neurons which innervate the pharynx or soft palate were abundant in the jugular-petrosal ganglion complex and relatively rare in the nodose ganglion. In the jugular-petrosal ganglion complex, TRPV1- and TRPV2-IR were expressed by one-third of pharyngeal and soft palate neurons. TRPV2-IR was also detected in 11.5 % pharyngeal and 30.9 % soft palate neurons in the complex. Coexpression of TRPV1 and CGRP was frequent among pharyngeal and soft palate neurons. The present study suggests that TRPV1- and TRPV2-IR jugular-petrosal neurons may be associated with the regulation of the swallowing reflex.

Emerging peripheral receptor targets for deep-tissue craniofacial pain therapies

While effective therapies are available for some types of craniofacial pain, treatments for deep-tissue craniofacial pain such as temporomandibular disorders are less efficacious. Several ion channels and receptors which are prominent in craniofacial nociceptive mechanisms have been identified on trigeminal primary afferent neurons. Many of these receptors and channels exhibit unusual distributions compared with extracranial regions. For example, expression of the ATP receptor P2X(3) is strongly implicated in nociception and is more abundant on trigeminal primary afferent neurons than analogous extracranial neurons, making them potentially productive targets specifically for craniofacial pain therapies. The initial part of this review therefore focuses on P2X(3) as a potential therapeutic target to treat deep-tissue craniofacial pain. In the trigeminal ganglion, P2X(3) receptors are often co-expressed with the nociceptive neuropeptides CGRP and SP. Therefore, we discuss the role of CGRP and SP in deep-tissue craniofacial pain and suggest that neuropeptide antagonists, which have shown promise for the treatment of migraine, may have wider therapeutic potential, including the treatment of deep-tissue craniofacial pain. P2X(3), TRPV1, and ASIC3 are often co-expressed in trigeminal neurons, implying the formation of functional complexes that allow craniofacial nociceptive neurons to respond synergistically to altered ATP and pH in pain. Future therapeutics for craniofacial pain thus might be more efficacious if targeted at combinations of P2X(3), CGRP, TRPV1, and ASIC3.

TRPV2 is activated by cannabidiol and mediates CGRP release in cultured rat dorsal root ganglion neurons

Transient receptor potential V2 (TRPV2) has been proposed to be a high-threshold thermosensor. However, further elucidation of the channel properties and physiological role of TRPV2 have been hindered by the lack of selective pharmacological tools as well as by the species-dependent differences in the activation of this channel. In the present study, we have used cell-based calcium mobilization and electrophysiological assays to identify and characterize several novel cannabinoid TRPV2 agonists. Among these, cannabidiol was found to be the most robust and potent (EC(50) = 3.7 microM), followed by Delta(9)-tetrahydrocannabinol (EC(50) = 14 microM) and cannabinol (EC(50) = 77.7 microM). We also demonstrated that cannabidiol evoked a concentration-dependent release of calcitonin gene-related peptide (CGRP) from cultured rat dorsal root ganglion neurons in a cannabinoid receptor- and TRPV1-independent manner. Moreover, the cannabidiol-evoked CGRP release depended on extracellular calcium and was blocked by the nonselective TRP channel blocker, ruthenium red. We further provide evidence through the use of small interfering RNA knockdown and repetitive stimulation studies, to show that cannabidiol-evoked CGRP release is mediated, at least in part, by TRPV2. Together, these data suggest not only that TRPV2 may comprise a mechanism whereby cannabidiol exerts its clinically beneficial effects in vivo, but also that TRPV2 may constitute a viable, new drug target.

TRPV2 is activated by cannabidiol and mediates CGRP release in cultured rat dorsal root ganglion neurons

Transient receptor potential V2 (TRPV2) has been proposed to be a high-threshold thermosensor. However, further elucidation of the channel properties and physiological role of TRPV2 have been hindered by the lack of selective pharmacological tools as well as by the species-dependent differences in the activation of this channel. In the present study, we have used cell-based calcium mobilization and electrophysiological assays to identify and characterize several novel cannabinoid TRPV2 agonists. Among these, cannabidiol was found to be the most robust and potent (EC(50) = 3.7 microM), followed by Delta(9)-tetrahydrocannabinol (EC(50) = 14 microM) and cannabinol (EC(50) = 77.7 microM). We also demonstrated that cannabidiol evoked a concentration-dependent release of calcitonin gene-related peptide (CGRP) from cultured rat dorsal root ganglion neurons in a cannabinoid receptor- and TRPV1-independent manner. Moreover, the cannabidiol-evoked CGRP release depended on extracellular calcium and was blocked by the nonselective TRP channel blocker, ruthenium red. We further provide evidence through the use of small interfering RNA knockdown and repetitive stimulation studies, to show that cannabidiol-evoked CGRP release is mediated, at least in part, by TRPV2. Together, these data suggest not only that TRPV2 may comprise a mechanism whereby cannabidiol exerts its clinically beneficial effects in vivo, but also that TRPV2 may constitute a viable, new drug target.

Histochemistry of nerve fibres double labelled with anti-TRPV2 antibodies and sensory nerve marker AM1-43 in the dental pulp of rat molars

AM1-43 can label sensory nerve fibres and sensory neurons. Permeation of non-selective cation channels of the nerve cell membrane is suggested to be the mechanism responsible for labelling. To identify these channels, two candidates, TRPV1 and TRPV2 were examined by immunocytochemistry in the dental pulp and trigeminal ganglion of rats injected with AM1-43. A part of AM1-43-labelled nerve fibres was also positive for anti-TRPV2 antibody but negative for anti-TRPV1 antibody in the dental pulp. In the trigeminal ganglion, a part of the neuron showed both bright AM1-43 labelling and anti-TRPV2 immunolabelling, but neurons double labelled with AM1-43 and TRPV1 were rare. These results suggest that TRPV2 channels, but not TRPV1 channels, contribute to the fluorescent labelling of AM1-43 in the dental pulp.

Transient receptor potential V2 expressed in sensory neurons is activated by probenecid

Temperature-activated transient receptor potential ion channels (thermoTRPs) are known to function as ambient temperature sensors and are also involved in peripheral pain sensation. The thermoTRPs are activated by a variety of chemicals, of which specific activators have been utilized to explore the physiology of particular channels and sensory nerve subtypes. The use of capsaicin for TRPV1 is an exemplary case for nociceptor studies. In contrast, specific agents for another vanilloid subtype channel, TRPV2 have been lacking. Here, we show that probenecid is able to activate TRPV2 using electrophysiological and calcium imaging techniques with TRPV2-expressing HEK293T cells. Five other sensory thermoTRPs-TRPV1, TRPV3, TRPV4, TRPM8 and TRPA1-failed to show a response to this drug in the same heterologous expression system, suggesting that probenecid is a specific activator for TRPV2. Probenecid-evoked responses were also reproduced in a distinct subset of cultured trigeminal neurons that were responsive to 2-aminoethoxydiphenyl borate, a TRPV1-3 activator. The probenecid-sensitive neurons were mainly distributed in a medium to large-diameter population, in agreement with previous observations with TRPV2 immunolocalization. Under inflammation, probenecid elicited nociceptive behaviors in in vivo assays. These results suggest that TRPV2 is specifically activated by probenecid and that this chemical might be useful for investigation of pain-related TRPV2 function.

A high-threshold heat-activated channel in cultured rat dorsal root ganglion neurons resembles TRPV2 and is blocked by gadolinium

Heat-activated ion channels from the vanilloid-type TRP group (TRPV1-4) seem to be central for heat-sensitivity of nociceptive sensory neurons. Displaying a high-threshold (> 52 degrees C) for activation, TRPV2 was proposed to act as a sensor for intense noxious heat in mammalian sensory neurons. However, although TRPV2 is expressed in a distinct population of thinly myelinated primary afferents, a widespread expression in a variety of neuronal and non-neuronal tissues suggests a more diverse physiological role of TRPV2. In its role as a heat-sensor, TRPV2 has not been thoroughly characterized in terms of biophysical and pharmacological properties. In the present study, we demonstrate that the features of heterologously expressed rat TRPV2 closely resemble those of high-threshold heat-evoked currents in medium- and large-sized capsaicin-insensitive rat dorsal root ganglion (DRG) neurons. Both in TRPV2-expressing human embryonic kidney (HEK)293t cells and in DRGs, high-threshold heat-currents were sensitized by repeated activation and by the TRPV1-3 agonist, 2-aminoethoxydiphenyl borate (2-APB). In addition to a previously described block by ruthenium red, we identified the trivalent cations, lanthanum (La(3+)) and gadolinium (Gd(3+)) as potent blockers of TRPV2. Thus, we present a new pharmacological tool to distinguish between heat responses of TRPV2 and the closely related capsaicin-receptor, TRPV1, which is strongly sensitized by trivalent cations. We demonstrate that self-sensitization of heat-evoked currents through TRPV2 does not require extracellular calcium and that TRPV2 can be activated in cell-free membrane patches in the outside-out configuration. Taken together our results provide new evidence for a role of TRPV2 in mediating high-threshold heat responses in a subpopulation of mammalian sensory neurons.

Brain-derived neurotrophic factor-immunoreactive neurons in the rat vagal and glossopharyngeal sensory ganglia; co-expression with other neurochemical substances

Immunohistochemistry for brain-derived neurotrophic factor (BDNF) was performed on the rat vagal and glossopharyngeal sensory ganglia. In the jugular, petrosal and nodose ganglia, 56.1+/-5.5%, 52.4+/-9.4% and 80.0+/-3.0% of sensory neurons, respectively, were immunoreactive for BDNF. These neurons were small- to medium-sized and observed throughout the ganglia. In the solitary tract nucleus, the neuropil showed BDNF immunoreactivity. A double immunofluorescence method demonstrated that BDNF-immunoreactive neurons were also immunoreactive for calcitonin gene-related peptide (CGRP), P2X3 receptor, the capsaicin receptor (VR1) or vanilloid receptor 1-like receptor (VRL-1) in the jugular (CGRP, 43.5%; P2X3 receptor, 51.1%; VR1, 71.7%; VRL-1, 0.5%), petrosal (CGRP, 33.2%; P2X3 receptor, 58.4%; VR1, 54.2%; VRL-1, 23.3%) and nodose ganglia (CGRP, 1.8%; P2X3 receptor, 49.1%; VR1, 70.7%; VRL-1, 11.5%). The co-expression with tyrosine hydroxylase was also detected in the petrosal (2.9%) and nodose ganglia (2.2%). However, BDNF-immunoreactive neurons were devoid of parvalbumin in these ganglia. The present findings suggest that BDNF-containing vagal and glossopharyngeal sensory neurons have nociceptive and chemoreceptive functions.

A novel technique to perform direct intraganglionar injections in rats

The present work describes a simple method for direct drug administration into the dorsal root ganglion (DRG) in anesthetized rats. This technique does not involve surgery, is easy to learn and allows behavioral testing within minutes after the injection. Based on landmarks that target the L5 DRG, an orifice was created with a guide needle through which a specially designed needle was inserted for solution injection. Its introduction into the ganglia was ensured by the triggering of an ipsilateral hindpaw reflex. The precision of the technique was checked by injections of the biological dye Pontamine Sky Blue (PSB) or C14-labeled arginine. There was no leakage of the dye to the surrounding tissues after a single 4 microl or three successive 2.5 microl injections (at 30-min intervals). Moreover, identical effects were observed with prostaglandin E2 (PGE2), morphine or glibenclamide injected intraplantarly or in the DRG, thus confirming the precision of the method and suggesting that the ganglion cells and peripheral nociceptors may display similar receptor population.

ASIC3-immunoreactive neurons in the rat vagal and glossopharyngeal sensory ganglia

ASIC3-immunoreactivity (ir) was examined in the rat vagal and glossopharyngeal sensory ganglia. In the jugular, petrosal and nodose ganglia, 24.8%, 30.8% and 20.6% of sensory neurons, respectively, were immunoreactive for ASIC3. These neurons were observed throughout the ganglia. A double immunofluorescence method demonstrated that many ASIC3-immunoreactive (ir) neurons co-expressed calcitonin gene-related peptide (CGRP)- or vanilloid receptor subtype 1 (VRL-1)-ir in the jugular (CGRP, 77.8%; VRL-1, 28.0%) and petrosal ganglia (CGRP, 61.7%; VRL-1, 21.5%). In the nodose ganglion, however, such neurons were relatively rare (CGRP, 6.3%; VRL-1, 0.4%). ASIC3-ir neurons were mostly devoid of tyrosine hydroxylase in these ganglia. However, some ASIC3-ir neurons co-expressed calbindin D-28k in the petrosal (5.5%) and nodose ganglia (3.8%). These findings may suggest that ASIC3-containing neurons have a wide variety of sensory modalities in the vagal and glossopharyngeal sensory ganglia.

Brain-derived neurotrophic factor-immunoreactive primary sensory neurons in the rat trigeminal ganglion and trigeminal sensory nuclei

Immunohistochemistry for brain-derived neurotrophic factor (BDNF) was performed on the rat trigeminal ganglion (TG). The immunoreactivity (IR) was detected in 46% of TG neurons. These neurons were mostly small- or medium-sized (range, 149.7-1246.3 microm2; mean +/- SD = 373.4 +/- 151.6 microm2). A double immunofluorescence method also revealed that 54% of BDNF-immunoreactive (IR) neurons were immunoreactive for calcitonin-gene-related peptide. In addition, 93% of BDNF-IR TG neurons contained vanilloid receptor subtype 1. However, the co-expression of BDNF and vanilloid receptor 1-like receptor was very rare (less than 1%). In the trigeminal sensory nuclei, laminae II of the medullary dorsal horn was abundant in presumed BDNF-IR axon terminals. Such profiles were also detected in the dorsolateral part of the subnucleus oralis. The retrograde tracing and immunohistochemical methods demonstrated that BDNF-IR was common among cutaneous TG neurons (47%) but not tooth pulp TG neurons (13%). The present study indicates that BDNF-IR TG neurons have unmyelinated axons and project to the superficial medullary dorsal horn. It is likely that BDNF-containing neurons in both the trigeminal and spinal sensory systems have similarities in morphology and function. However, the content of BDNF in TG neurons probably depends on their peripheral targets. BDNF seems to convey nociceptive cutaneous input to the trigeminal sensory nuclei.

Differential distribution of vanilloid receptors in the primary sensory neurons projecting to the dorsal skin and muscles

We examined transient receptor potential (TRP) V1 and TRPV2 expression in calcitonin gene-related peptide (CGRP) positive (+) primary sensory neurons projecting to the skin and skeletal muscles of the rat dorsum. Among the dorsal root ganglia at the levels from C2 to Th1, 34.9% of neurons projecting to the skin were positive for CGRP, and 32.6% or 21.6% of neurons projecting to the trapezius muscle or the longissimus muscle were positive for CGRP. Of the small CGRP+ neurons projecting to the skin, 53.5% were positive for TRPV1, 11.6% were positive for TRPV2. Of the small CGRP+ neurons projecting to the trapezius or the longissimus, 53.1 or 53.2% were positive for TRPV1, 8.8 or 8.3% were positive for TRPV2, respectively. In the periphery, 29.3% of CGRP+ nerve fibers were positive for TRPV1 in the skin, whereas 65.0 or 59.8% were positive in the trapezius or the longissimus. Therefore, the present study showed that the percentage of CGRP+ neurons projecting to the trapezius is higher than that to the longissimus, and that the co-localization percentage of CGRP and TRPV1 on the sensory nerves was also higher in the trapezius than in the longissimus and the skin.

Participation of TRPV1 and TRPV2 in the rat laryngeal sensory innervation

Laryngeal sensory innervation is essential to the laryngeal defense system. We investigated the participation of TRPV1 and its homologue TRPV2 in the rat laryngeal sensory innervation using immunohistochemistry and the neuronal tracer, fluoro-gold (FG). After injection of FG into the internal branch of the superior laryngeal nerve, FG-labeled neurons were seen in the rostral part of the nodose ganglion (NG). Neurons immunoreactive for TRPV1 or TRPV2 were distributed throughout the NG. TRPV1 immunoreactivity was seen in 49.0+/-4.5% of the FG-labeled neurons, while TRPV2 immunoreactivity was seen in 12.5+/-4.1% of the FG-labeled neurons. These findings suggest that both TRPV1 and TRPV2 participate in laryngeal nociception, but that TRPV1 may have a particularly important role.

Mechanisms of orofacial pain control in the central nervous system

Recent advances in the study of pain have revealed somatotopic- and modality-dependent processing and the integration of nociceptive signals in the brain and spinal cord. This review summarizes the uniqueness of the trigeminal sensory nucleus (TSN) in structure and function as it relates to orofacial pain control. The oral nociceptive signal is primarily processed in the rostral TSN above the obex, the nucleus principalis (Vp), and the subnuclei oralis (SpVo) and interpolaris (SpVi), while secondarily processed in the subnucleus caudalis (SpVc). In contrast, the facial nociceptive signal is primarily processed in the SpVc. The neurons projecting to the thalamus are localized mostly in the Vp, moderately in the SpVi, and modestly in the ventrolateral SpVo and the SpVc. Orofacial sensory inputs are modulated in many different ways: by interneurons in the TSN proper, through reciprocal connection between the TSN and rostral ventromedial medulla, and by the cerebral cortex. A wide variety of neuroactive substances, including substance P, gamma-aminobutyric acid, serotonin and nitric oxide (NO) could be involved in the modulatory functions of these curcuits. The earliest expression of NO synthase (NOS) in the developing rat brain is observed in a discrete neuronal population in the SpVo at embryonic day 15. NOS expression in the SpVc is late at postnatal day 10. The neurons receiving intraoral signals are intimately related with the sensorimotor reflexive function through the SpVo. In summary, a better understanding of the trigeminal sensory system--which differs from the spinal system--will help to find potential therapeutic targets and lend to developing new analgesics for orofacial-specific pain with high efficacy and fewer side effects.

Immunohistochemical localization of gamma and beta subunits of epithelial Na+ channel in the rat molar tooth pulp

The distribution of gamma and beta subunits of epithelial Na(+) channel (ENaC), markers for low-threshold mechanoreceptors in peripheral tissues, was examined in the tooth pulp. In the root pulp, gammaENaC- and betaENaC-immunoreactive (IR) nerve fibers showed a thick smooth appearance. These nerve fibers ascended toward the pulp horn and formed subodontoblastic nerve plexuses. Immunoelectron microscopic method revealed that 63% of axons were immunoreactive for gammaENaC in the root pulp. Virtually all myelinated axons showed gammaENaC-IR (97%), whereas unmyelinated axons were mostly devoid of it (12%). These findings suggest that myelinated tooth pulp nociceptors respond to mechanical stimuli.

Extensive co-localization and heteromultimer formation of the vanilloid receptor-like protein TRPV2 and the capsaicin receptor TRPV1 in the adult rat cerebral cortex

The capsaicin receptor TRPV1, a member of the transient receptor potential (TRP) family of calcium-selective ion channels, responds to noxious stimuli and is predominantly expressed in nociceptive neurons. The homologous receptor TRPV2 shows wide tissue distribution including some sensory neurons, where it is proposed to function as a heat sensor or a growth-factor-activated channel. Members of the TRP family of channels have been shown to interact, resulting in hybrid channels with new properties. We examined the possibility of multimer formation between TRPV1 and TRPV2, using biochemical techniques. We present evidence that TRPV1 and TRPV2 can heteromultimerize efficiently in vitro. By using immunohistochemistry we detected co-localization of the two receptors in rat dorsal root ganglia. TRPC4 transcripts are also detected in capsaicin-sensitive dorsal root ganglia neurons. We extended the search for TRPV1-TRPV2 co-localization in the brain, where we detected extensive co-expression of the two receptors in the IV, V and VI layer neurons of the adult rat cerebral cortex. Co-immunoprecipitation experiments confirmed the interaction of the two receptors in vivo, indicating heteromultimer formation in native tissue. Formation of heteromultimers between vanilloid receptors may increase the functional diversity of this receptor family.

Satellite glial cells in sensory ganglia: from form to function

Current information indicates that glial cells participate in all the normal and pathological processes of the central nervous system. Although much less is known about satellite glial cells (SGCs) in sensory ganglia, it appears that these cells share many characteristics with their central counterparts. This review presents information that has been accumulated recently on the physiology and pharmacology of SGCs. It appears that SGCs carry receptors for numerous neuroactive agents (e.g., ATP, bradykinin) and can therefore receive signals from other cells and respond to changes in their environment. Activation of SGCs might in turn influence neighboring neurons. Thus SGCs are likely to participate in signal processing and transmission in sensory ganglia. Damage to the axons of sensory ganglia is known to contribute to neuropathic pain. Such damage also affects SGCs, and it can be proposed that these cells have a role in pathological changes in the ganglia.