Sympathetic nervous system contributes to orthodontic tooth movement by central neural regulation from hypothalamus.

Orthodontic tooth movement (OTM) is a specific treatment of malocclusion, whose regulation mechanism is still not clear. This study aimed to reveal the relationship between the sympathetic nervous system (SNS) and OTM through the construction of an OTM rat model through the utilization of orthodontic nickeltitanium coiled springs. The results indicated that the stimulation of SNS by dopamine significantly promote the OTM process represented by the much larger distance between the first and second molar compared with mere exertion of orthodontic force. Superior cervical ganglionectomy (SCGx) can alleviate this promotion effect, further proving the role of SNS in the process of OTM. Subsequently, the ability of orthodontic force to stimulate the center of the SNS was visualized by the tyrosin hydroxylase (TH) staining of neurons in ventromedial hypothalamic nucleus (VMH) and arcuate nucleus (ARC) of the hypothalamus, as well as the up-regulated expression of norepinephrine in local alveolar bone. Moreover, we also elucidated that the stimulation of SNS can promote osteoclast differentiation in periodontal ligament cells (PDLCs) and bone marrow-derived cells (BMCs) through regulation of receptor activator of nuclear factor-κB ligand (RANKL)/osteoprotegerin (OPG) system, thus promoting the OTM process. In conclusion, this study provided the first evidence for the involvement of the hypothalamus in the promotion effect of SNS on OTM. This work could provide a novel theoretical and experimental basis for further understanding of the molecular mechanism of OTM.

Pivotal Role of Tenascin-W (-N) in Postnatal Incisor Growth and Periodontal Ligament Remodeling.

The continuously growing mouse incisor provides a fascinating model for studying stem cell regulation and organ renewal. In the incisor, epithelial and mesenchymal stem cells assure lifelong tooth growth. The epithelial stem cells reside in a niche known as the cervical loop. Mesenchymal stem cells are located in the nearby apical neurovascular bundle and in the neural plexus. So far, little is known about extracellular cues that are controlling incisor stem cell renewal and guidance. The extracellular matrix protein tenascin-W, also known as tenascin-N (TNN), is expressed in the mesenchyme of the pulp and of the periodontal ligament of the incisor, and is closely associated with collagen 3 fibers. Here, we report for the first time the phenotype of tenascin-W/TNN deficient mice, which in a C57BL/6N background exhibit a reduced body weight and lifespan. We found major defects in the alveolar bone and periodontal ligament of the growing rodent incisors, whereas molars were not affected. The alveolar bone around the incisor was replaced by a dense scar-like connective tissue, enriched with newly formed nerve fibers likely leading to periodontal pain, less food intake and reduced body weight. Using soft food to reduce mechanical load on the incisor partially rescued the phenotype. In situ hybridization and Gli1 reporter mouse experiments revealed decreased hedgehog signaling in the incisor mesenchymal stem cell compartment, which coordinates the development of mesenchymal stem cell niche. These results indicate that TNN deficiency in mice affects periodontal remodeling and increases nerve fiber branching. Through periodontal pain the food intake is reduced and the incisor renewal and the neurovascular sonic hedgehog secretion rate are reduced. In conclusion, tenascin-W/TNN seems to have a primary function in rapid periodontal tissue remodeling and a secondary function in mechanosensation.

Mechanobiology of the tooth movement during the orthodontic treatment: a literature review.

Orthodontic tooth movement differs significantly from the physiological tooth movement, as it determines a biological response of the surrounding tissues of the teeth, resulting in a remodelling of the periodontal ligament and the alveolar bone. The result is a biochemical adaptive response to the application of the orthodontic force with the reorganization of the intracellular and the extracellular matrix, in addition to a change of the local vascularization. This in turn leads to the synthesis and the release of arachidonic acid, growth factors, metabolites, cytokines and various enzymes. Biologically, not only the intensity of the force, but also its duration and the tissue response to the application of the same are important for tooth movement. Having these insights it will possible to examine the concept of optimal orthodontic force, a determining factor for the success of orthodontic treatment. The purpose of this revision was to describe the biological processes and future perspective of the application of orthodontic force, by providing relevant information to understand the changes at the molecular and cellular level occurring when the tissues are subjected to such forces. Knowledge on the subject of mechanics and biology in orthodontics is constantly growing, producing an increasingly strong basis for clinical success.

Changes in the Distribution of Periodontal Nerve Fibers during Dentition Transition in the Cat.

The periodontal ligament has a rich sensory nerve supply which originates from the trigeminal ganglion and trigeminal mesencephalic nucleus. Although various types of mechanoreceptors have been reported in the periodontal ligament, the Ruffini ending is an essential one. It is unknown whether the distribution of periodontal nerve fibers in deciduous teeth is identical to that in permanent teeth or not. Moreover, morphological changes in the distribution of periodontal nerve fibers during resorption of deciduous teeth and eruption of successional permanent teeth in diphyodont animals have not been reported in detail. Therefore, in this study, we examined changes in the distribution of periodontal nerve fibers in the cat during changes in dentition (i.e., deciduous, mixed and permanent dentition) by immunohistochemistry of protein gene product 9.5. During deciduous dentition, periodontal nerve fibers were concentrated at the apical portion, and sparsely distributed in the periodontal ligament of deciduous molars. During mixed dentition, the periodontal nerve fibers of deciduous molars showed degenerative profiles during resorption. In permanent dentition, the periodontal nerve fibers of permanent premolars, the successors of deciduous molars, increased in number. Similar to permanent premolars, the periodontal nerve fibers of permanent molars, having no predecessors, increased in number, and were densely present in the apical portion. The present results indicate that the distribution of periodontal nerve fibers in deciduous dentition is almost identical to that in permanent dentition although the number of periodontal nerve fibers in deciduous dentition was low. The sparse distribution of periodontal nerve fibers in deciduous dentition agrees with clinical evidence that children are less sensitive to tooth stimulation than adults.

Distribution of transient receptor potential melastatin-8-containing nerve fibers in rat oral and craniofacial structures.

The transient receptor potential melastatin-8 (TRPM8) is a cold and menthol receptor located in the sensory ganglia. Immunohistochemistry for TRPM8 was performed on oral and craniofacial structures of the rat. TRPM8-immunoreactive (-IR) nerve fibers were detected in the oral mucous membrane. In the gingiva, TRPM8-IR nerve fibers were abundant beneath and within crestal and outer epithelia. Such nerve fibers were also common beneath and within taste buds in the incisive papilla. In addition, TRPM8-immunoreactivity was expressed by some taste bud cells in the papilla. Lips, periodontal ligaments and salivary glands as well as masticatory muscles and temporomandibular joints were mostly devoid of TRPM8-IR nerve fibers. A double immunofluorescence study indicated different distribution patterns of nerve fibers containing TRPM8 and calcitonin gene-related peptide in oral and craniofacial tissues. Retrograde tracing method also indicated that TRPM8-IR nerve fibers in the gingiva and incisive papilla originate from small sensory neurons in the trigeminal ganglion. TRPM8 may be associated with cool, cold nociceptive (

Selective ß2-adrenergic Antagonist Butoxamine Reduces Orthodontic Tooth Movement.

Recently, involvement of the sympathetic nervous system in bone metabolism has attracted attention. ß2-Adrenergic receptor (ß2-AR) is presented on osteoblastic and osteoclastic cells. We previously demonstrated that ß-AR blockers at low dose improve osteoporosis with hyperactivity of the sympathetic nervous system via ß2-AR blocking, while they may have a somewhat inhibitory effect on osteoblastic activity at high doses. In this study, the effects of butoxamine (BUT), a specific ß2-AR antagonist, on tooth movement were examined in spontaneously hypertensive rats (SHR) showing osteoporosis with hyperactivity of the sympathetic nervous system. We administered BUT (1 mg/kg) orally, and closed-coil springs were inserted into the upper-left first molar. After sacrifice, we calculated the amount of tooth movement and analyzed the trabecular microarchitecture and histomorphometry. The distance in the SHR control was greater than that in the Wistar-Kyoto rat group, but no significant difference was found in the SHR treated with BUT compared with the Wistar-Kyoto rat control. Analysis of bone volume per tissue volume, trabecular number, and osteoclast surface per bone surface in the alveolar bone showed clear bone loss by an increase of bone resorption in SHR. In addition, BUT treatment resulted in a recovery of alveolar bone loss. Furthermore, TH-immunoreactive nerves in the periodontal ligament were increased by tooth movement, and BUT administration decreased TH-immunoreactive nerves. These results suggest that BUT prevents alveolar bone loss and orthodontic tooth movement via ß2-AR blocking.

Expression of pituitary adenylate cyclase-activating peptide (PACAP) and PAC1 in the periodontal ligament after tooth luxation.

Pituitary adenylate cyclase-activating peptide (PACAP) is widely distributed throughout the nervous system. PACAP not only acts as a neurotransmitter but also elicits a broad spectrum of biological action via the PACAP-specific receptor, PAC1. However, no studies have investigated PACAP and PAC1 in the periodontal ligament (PDL), so we aimed to perform this investigation in rats after tooth luxation. In the PDL of an intact first molar, there are few osteoclasts and osteoblasts. However, at days 3 and 5 after luxation, large PAC1-positive cells, thought to be osteoclasts because of their expression of the osteoclast marker, tartrate-resistant acid phosphatase, were detected in appreciable numbers. Osteoblast numbers increased dramatically on day 7 after luxation, and PAC1-positive mononuclear small cells were increased at day 14, many of which expressed the osteoblast marker, alkaline phosphatase. PACAP-positive nerve fibers were rarely detected in the PDL of intact first molars, but were increasingly evident at this site on days 5 and 7 after luxation. Double-immunofluorescence analysis demonstrated the relationship between PACAP-positive nerve fibers and PAC1-positive osteoclasts/-blasts in the PDL. At 5 days after luxation, PACAP-positive nerve fibers appeared in close proximity to PAC1-positive osteoclasts. At 7 days after luxation, PACAP-positive nerve fibers appeared in close proximity to PAC1-positive osteoblasts. These results suggest that PACAP may have effects on osteoclasts and osteoblasts in the PDL after tooth luxation and thus regulate bone remodeling after these types of injury.

Changes of myelinated nerve and myelin basic protein expression in rats' periodontal ligaments after experimental tooth movement.

INTRODUCTION: Information about the effect of tooth movement on the myelinated nerve in the periodontal ligament is limited. In this study, we aimed to investigate what responses of the periodontal myelinated nerve can be evoked during experimental tooth movement. METHODS: In experimental-I group, the maxillary left and mandibular right third molars were moved distally. In experimental-II group, the maxillary left third molar but not the right one was moved, and the bilateral mandibular third molars were extracted. The ultrastructures of the myelinated nerve in the periodontal ligament of the bilateral maxillary third molars were observed under a transmission electron microscope. The expression of myelin basic protein was evaluated by immunohistochemistry. RESULTS: Degenerative ultrastructural changes of the myelinated nerve in the periodontal ligament were noticed mainly in the myelin sheath; these were observed earlier and were recoverable in the experimental-I group. In contrast, the ultrastructural changes of the myelinated nerve occurred mainly in the axons, were observed later, and were unrecoverable in the experimental-II group. A concomitant decrease of myelin basic protein expression was observed in both groups. CONCLUSIONS: Both experimental tooth movement and occlusal changes accompanying it caused changes of the myelinated nerve in the periodontal ligament.

Immunoexpression of aquaporin-1 in the rat periodontal ligament during experimental tooth movement.

This study examined the immunoexpression pattern of aquaporin-1 (AQP1), first identified as a water channel protein, in the periodontal ligament of rat molars during experimental tooth movement to clarify its role in periodontal responses in an overloaded model by the insertion of a piece of elastic band. In the control group without any treatment, the cementoblasts and osteogenic cells as well as the vascular endothelial cells showed AQP1 immunoreaction. In the experimental group, hyalinized tissue and intensely AQP1 positive amorphous structures which were identified as degenerated endothelial cells by immunoelectron microscopy, occurred at the compression side on Days 1 and 3. AQP1 immunoreaction came to be stronger in the intact endothelial cells around the hyalinized tissue. The hyalinized tissue had almost disappeared by Day 5 when many macrophages reactive to acid phosphatase activity appeared. The periodontal width on Day 7 became almost the same as that in the control group. These findings indicate that the hyalinized tissue and damaged AQP1 positive endothelial cells are phagocytized by macrophages which have temporally migrated, and suggest that the surviving endothelial cells with intense AQP1 reaction are involved in periodontal regeneration by capillary sprouting.

[Orthodontic tooth movement and expression of calcium regulating hormone].

Nociception by orthodontic tooth movement stimulate Trigeminal nerve free endings in periodontal ligament (PDL) , and neuropeptides such as substance P and CGRP are synthesized in Trigeminal ganglion sensory cells and released both centrally and peripherally around blood vessels in PDL and pulp. Neuropeptides such as CGRP and substance P are the signal transmitter of pain and might modulate vascular enlargement, blood flow or vascular permeability. CGRP receptor for its subunit, receptor activity modifying protein 1 (RAMP 1) distributed on osteoclasts and osteoblasts in PDL. CGRP may have effects on bone remodeling due to not only inhibiting bone resorption like calcitonin but also directly stimulating bone formation in the luxated PDL and during experimental tooth movement.

Developmentally regulated expression of Sema3A chemorepellant in the developing mouse incisor.

OBJECTIVE: Semaphorin 3A (Sema3A) is an essential chemorepellant controlling peripheral axon pathfinding and patterning, but also serves non-neuronal cellular functions. Incisors of rodent are distinctive from molars as they erupt continuously, have only one root and enamel is present only on the labial side. The aim of this study is to address putative regulatory roles of Sema3A chemorepellant in the development of incisor innervation and formation. MATERIALS AND METHODS: This study analyzed expression of Sema3A mRNAs during embryonic and early post-natal stages of mouse mandibular incisor using sectional radioactive in situ hybridization. RESULTS: Although Sema3A mRNAs were observed in condensed dental mesenchyme during the early bud stage, they were absent in dental papilla or pulp at later stages. Sema3A mRNAs were observed in the dental epithelium including the cervical loops and a prominent expression was also seen in alveolar bone. Interestingly, transcripts were absent from the mesenchymal dental follicle target area (future periodontal ligament) throughout the studied stages. CONCLUSION: The expression patterns of Sema3A indicate that it may control the timing and patterning of the incisor innervation. In particular, Sema3A appears to regulate innervation of the periodontal ligament, while nerve penetration into the incisor dental pulp appears not to be dependent on Sema3A. Moreover, Sema3A may regulate the functions of cervical loops and the development of alveolar bone. Future study with Sema3A deficient mice will help to elucidate the putative neuronal and non-neuronal functions of Sema3A in incisor tooth development.

Vesicular glutamate transporter immunoreactivity in the periodontal ligament of the rat incisor.

The distribution of three vesicular glutamate transporter (VGluT) isoforms, VGluT1, VGluT2, and VGluT3, were investigated in the trigeminal ganglion of the periodontal ligament in the rat incisor-a receptive field of trigeminal ganglion neurons. In the trigeminal ganglion, mRNAs for all VGluT isoforms were detected and proteins were observed in the cytoplasm of trigeminal ganglion cells. VGluT1 immunoreactions were localized within the cytoplasm for all sizes of trigeminal neurons, although predominately in medium-large trigeminal neurons. Double-labeling showed that most VGluT1 contained both VGluT2 and VGluT3. In the periodontal ligament of the incisor, the Ruffini endings, principal periodontal mechanoreceptors, displayed VGluT1 and VGluT2 immunoreactivities. However, lacked immunoreactions for VGluT3. At the electron microscopic level, VGluT1 immunoreactions were localized around the vesicle membranes at the axon terminal of Ruffini endings. The present results indicate that VGluT is expressed in the sensory nerve endings where apparent synapses are not present. Thus, glutamate in the sensory nerve endings is thought to be used in metabotropic functions. This is because glutamate is a general metabolic substrate, and/or acts as a neurotransmitter as proposed in muscle spindles.

Electromyographic evaluation of the 'vertical' dimension: the Learreta TMJ decompression test.

The clinical observation of the incisors overbite is the most common form used to evaluate the occlusal vertical dimension (OVD); however, this technique offers poor information about the compression state of the TMJ. In order to obtain such information, it is necessary to evaluate the electrical activity of the elevator muscles using surface electromyography (EMG). In case of a compressive irritation of the joint receptors, the trigeminal nucleus returns an inhibitory motor response of the elevator muscles that can be measured. The Learreta's EMG decompression test is done by measuring the EMG response of the masticatory muscles at maximal occlusion in four different OVD positions in such a way that the reduction of the TMJ pressure, and subsequently, relief of the inhibitory motor response can be studied. The aim of this study is to illustrate this technique, its clinical use and its limitations.

Periodontal ligament and intraosseous anesthetic injection techniques: alternatives to mandibular nerve blocks.

BACKGROUND: and Overview. The provision of mandibular anesthesia traditionally has relied on nerve block anesthetic techniques such as the Halsted, the Gow-Gates and the Akinosi-Vazirani methods. The authors present two alternative techniques to provide local anesthesia in mandibular teeth: the periodontal ligament (PDL) injection and the intraosseous (IO) injection. The authors also present indications for and complications associated with these techniques. CONCLUSIONS: The PDL injection and the IO injection are effective anesthetic techniques for managing nerve block failures and for providing localized anesthesia in the mandible. CLINICAL IMPLICATIONS: Dentists may find these techniques to be useful alternatives to nerve block anesthesia.

Histomorphological study of myelinated nerve fibres in the periodontal ligament of human canine.

OBJECTIVE: This study aims to describe the human periodontal ligament (PDL) using serial sections, with a focus on mechanoreceptor distribution and morphology. MATERIALS AND METHODS: One permanent lower canine with surrounding PDL and alveolar bone tissues was retrieved from a human cadaver. After being embedded into paraffin block, the canine was horizontally cut in 6 µm thick serial sections. At root levels of 0.3, 1.5, 3, 4.5 and 6 mm from the apex, five slices each level were evaluated. Immunocytochemisty was performed on the same serial sections, enabling a more reliable description of neural structures. RESULTS: The distribution of myelinated fibres varied from apical to coronal level, with a total number of 38 at 0.3 mm from the apex, 25 at 1.5 mm, 25 at 3 mm, 31 at 4.5 mm and 32 at 6 mm. At all times, mesial and buccal regions were typically more densely innervated (p < 0.01) except at the 3 mm level. The average density of myelinated nerve fibres increased by arriving closer to the apex. However, the average diameter did not show any significant differences amongst quadrants or root levels (p > 0.05). The average diameter of myelinated fibres varied between 5.3-7.8 µm. Grouped myelinated axons were twice as common as isolated ones, with the innervation being rather close to the alveolar bone. Isolated myelinated axons showed a tendency to group around large blood vessels. CONCLUSION: The present results add to the understanding of human PDL innervation, indicating dense innervations by myelinated nerve fibres in close proximity to collagen fibres and alveolar bone. It also reveals that apical as well as mesial and buccal sites of the human canine are more densely innervated.

Differential TRPV1 and TRPV2 channel expression in dental pulp.

Hypersensitivity to thermal and mechanical stimuli can occur in painful pulpitis. To explore the neuro-anatomical basis of heat and mechanical sensitivity, we evaluated expression of TRPV1 (heat) and TRPV2 (heat/mechanical) channels in the cell bodies and terminal arborizations of neurons that innervate the dental pulp (DP) and periodontal tissues (PDL). We report that ~50% of trigeminal ganglion (TG) neurons retrogradely labeled from the DP express TRPV2, and this was significantly greater than the general expression of this channel in the TG (15%) and slightly more than what is expressed in the PDL by retrograde labeling (40%). The TRPV1 receptor, however, was less prevalent in neurons innervating the DP than their general expression in the TG (17% vs. 26%) and was more extensively expressed in neurons innervating the PDL (26%). Co-labeling studies showed that 70% of neurons that innervate the DP are myelinated. Approximately 1/3 of the retrogradely labeled neurons from the DP were calcitonin-gene-related-peptide-positive (peptide-expressing), but very few expressed the IB4 marker of non-peptidergic unmyelinated afferents. These findings suggest that the DP has a unique neurochemical innervation with regard to TRP receptor expression, which has significant implications for the mechanisms contributing to odontogenic pain and management strategies.

Detection of acid-sensing ion channel 3 (ASIC3) in periodontal Ruffini endings of mouse incisors.

The acid-sensing ion channel 3 (ASIC3), a member of the epithelial sodium channel/degenerin (ENaC/DEG) superfamily, has been reported to participate in acid sensing, mechanosensation, and nociception. However, no information is available regarding the precise localization and function of this molecule in the periodontal ligament, which contains abundant sensory nerves originating from the trigeminal ganglion. The present study examined the expression of ASIC3 in the lingual periodontal ligament of mouse incisors by immunohistochemistry. Furthermore, the expression of ASIC3 in the trigeminal ganglion - which innervates the periodontal ligament - was investigated at protein (immunohistochemistry and quantitative analysis) and mRNA levels (RT-PCR technique and in situ hybridization histochemistry). Immunohistochemistry for ASIC3 was able to demonstrate dendritic profiles of the periodontal Ruffini endings in the mouse incisors. No thin fibers terminating as nociceptive free nerve endings exhibited ASIC3 immunoreactivity. Double immunofluorescent staining revealed ASIC3 immunoreaction in the axoplasm but not in the ordinary Schwann cells - including the associated terminal Schwann cells. Observation of the trigeminal ganglia showed variously sized neurons expressing ASIC3 immunoreaction; the most intense immunopositivity was found in the small and medium-sized neurons, as confirmed by in situ hybridization histochemistry using a specific cRNA probe. Quantitative analysis on trigeminal ganglion neurons showed that 38.0% of ASIC3 neurons could be categorized as medium-sized neurons which mediate mechanotransduction. These findings suggest that ASIC3 functions as a molecule for mechanosensation in the periodontal Ruffini endings.

Effects of CO2 laser irradiation of the gingiva during tooth movement.

Patients often feel pain or discomfort in response to orthodontic force. It was hypothesized that CO(2) laser irradiation may reduce the early responses to nociceptive stimuli during tooth movement. The distribution of Fos-immunoreactive (Fos-IR) neurons in the medullary dorsal horn of rats was evaluated. Two hrs after tooth movement, Fos-IR neurons in the ipsilateral part of the medullary dorsal horn increased significantly. CO(2) laser irradiation to the gingiva just after tooth movement caused a significant decrease of Fos-IR neurons. PGP 9.5- and CGRP-positive nerve fibers were observed in the PDL of all study groups. The maximum temperature below the mucosa during CO(2) laser irradiation was less than 40 degrees C. It was suggested that CO(2) laser irradiation reduced the early responses to nociceptive stimuli during tooth movement and might not have adverse effects on periodontal tissue.

A comparison of the postnatal development of muscle-spindle and periodontal-ligament neurons in the mesencephalic trigeminal nucleus of the rat.

The trigeminal mesencephalic nucleus (Vmes) is known to include primary afferent neurons of jaw muscle spindles (MS neurons) and periodontal ligament receptors (PL neurons). The aim of this study was to clarify the postnatal development of Vmes neurons by comparing MS neurons with PL neurons using horseradish peroxidase labeling. We measured somal diameter and somal shape of MS and PL neurons in rats from postnatal day (P)7 to P70. No significant changes were seen between postnatal day P7 and P70 in somal diameter or somal shape of MS neurons. Conversely, PL neurons showed a larger somal diameter at P7 than at P14, and in terms of somal profile, multipolar neurons comprised 0% at P7, but 4.8% at P14 and 16.9% at P70. These findings suggest that PL neurons develop with the eruption of teeth, taking into account the fact that tooth eruption occurs from around P14 in rats. Conversely, the lack of postnatal changes in MS neurons is due to the fact that these neurons have been active since the embryonic period, as swallowing starts in utero.

Immunohistochemical localization of SNARE proteins in dental pulp and periodontal ligament of the rat incisor.

Distribution of three soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) proteins, syntaxin-1, synaptosomal-associated protein of 25 kDa (SNAP-25), and vesicle-associated membrane protein-2 (VAMP-2), was examined in dental pulp and periodontal ligament of the rat incisor. In the trigeminal ganglion, syntaxin-1 and SNAP-25 immunoreactivity was predominately detected in medium- to large-sized neurons. Most syntaxin-1 immunoreactive neurons expressed SNAP-25. In contrast, VAMP-2 was localized in small- to medium-sized neurons and in slender-shaped cells surrounding SNAP-25-immunopositive neurons. When the inferior alveolar nerve, one of the mandibular nerve branches innervating the dental pulp and periodontal ligament, was ligated, SNARE proteins accumulated at the site proximal to the ligation. In the incisor dental pulp, all nerve fibers displayed immunoreactivity for syntaxin-1, SNAP-25, and VAMP-2. In the periodontal ligament of the incisor, almost all nerve fibers displayed both syntaxin-1 and SNAP-25 immunoreactivity, but lacked VAMP-2 immunoreactivity. SNAP-25 protein expression was localized around the vesicle membranes at the axon terminal of the periodontal mechanoreceptors. These present data suggest that these three SNARE proteins are synthesized at the trigeminal ganglion, transported centrally and peripherally, and expressed in sensory endings where apparent synapses are not present. Because those proteins participate in docking and exocytosis of synapse vesicles in the central nervous system, they might also contribute to vesicle exocytosis at receptive fields where apparent synapses are not present.