An immunohistochemical and monastral blue-vascular labelling study on the involvement of capsaicin-sensitive sensory innervation of the junctional epithelium in neurogenic plasma extravasation in the rat gingiva

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 (<around 25°C) and chemoreceptive transmission in the oral mucosa.

Merkel cells and permanent disesthesia in the oral mucosa after soft tissue grafts

Connective tissue grafts are routinely procedures in the treatment of gingival defects. The clinical success of the gingival tissue graft procedures anyway should ensure not only the aesthetic integration between the tissues but also the physiological activity of the graft in terms of sensitivity and immunity because the skin and the mucosae constitute the first natural aspecific borders against pathogens. The aim of this paper was to investigate nervous net recovery after connective graft procedure, in relation with sensorial alteration in the injured area. Results showed that there is a close link among the number of Merkel cells and the alteration of sensations. Merkel cells can be found isolated standing in the basal layer, supposed to have neuroendocrine functions in the epithelia or in larger group not associated with nerves; when found in association with nerves they are named Merkel complexes, acting as slow adapter mechanical receptor. Our data can be explained in two ways: Merkel cells increase as a consequence of tissue injury, a sort of "SOS cells" that secrete neuroendocrine signals to guide tissue healing; as an alternative the presence of the Merkel cells could be read as a derailment of tissue regeneration with the stop of cellular differentiation in the direction of an abnormal proliferation, a sort of mad stem cell.

TRPV2 expression in rat oral mucosa

The oral mucosa is a highly specialised, stratified epithelium that confers protection from infection and physical, chemical and thermal stimuli. The non-keratinised junctional epithelium surrounds each tooth like a collar and is easily attacked by foreign substances from the oral sulcus. We found that TRPV2, a temperature-gated channel, is highly expressed in junctional epithelial cells, but not in oral sulcular epithelial cells or oral epithelial cells. Dual or triple immunolabelling with immunocompetent cell markers also revealed TRPV2 expression in Langerhans cells and in dendritic cells and macrophages. Electron microscopy disclosed TRPV2 immunoreactivity in the unmyelinated and thinly myelinated axons within the connective tissue underlying the epithelium. TRPV2 labelling was also observed in venule endothelial cells. The electron-dense immunoreaction in junctional epithelial cells, macrophages and neural axons occurred on the plasma membrane, on invaginations of the plasma membrane and in vesicular structures. Because TRPV2 has been shown to respond to temperature, hypotonicity and mechanical stimuli, gingival cells expressing TRPV2 may act as sensor cells, detecting changes in the physical and chemical environment, and may play a role in subsequent defence mechanisms.

Immunocytochemical localization of the neurokinin 1 receptor in rat dental pulp

The dentin-pulp complex is a peripheral end-organ supplied by dense sensory nerve fibers. Substance P, a representative neuropeptide widely distributed in the dental pulp, has been reported to play roles in pain transmission and the amplification of inflammation. We analyzed here the expression of the neurokinin 1 (NK1) receptor, preferentially activated by substance P, using immunocytochemistry in rat dental pulp at both the light and electron microscopic levels. Conspicuous NK1 receptor immunoreactivity was found in the odontoblasts; immunolabelings were present at their plasma membrane and endosomal structures, especially in their cytoplasmic processes. Immunoreactions for NK1 receptor were also detectable in a part of the nerve terminals associated with the cytoplasmic processes of the odontoblasts. Furthermore, the endothelial cells of capillaries and post-capillary venules and the fibroblasts were labeled with the NK1 receptor in the subodontoblast layer. These findings suggest that pulpal cells and nerve fibers are targets for substance P that mediate multiple functions, including a vasoactive function and the regulation of vascular permeability as well as the modulation of pain transmission.

Unilateral ligature-induced periodontitis influences the expression of neuropeptides in the ipsilateral and contralateral trigeminal ganglion in rats

OBJECTIVES

Expression of neuronal neuropeptides in inflammatory conditions is altered. The changes in expression of substance P (SP) and calcitonin gene-related peptide (CGRP) in ipsilateral and contralateral trigeminal ganglion (TG) neurons were investigated by immunohistochemistry one week after unilateral ligature-induced periodontitis in rats.

DESIGN

A retrograde nerve tracer Fluorogold (FG) was applied into the gingival sulcus of the second maxillary molar to identify the neurons in TG that specifically innervate the inflamed gingivomucosa. In addition, neurons from the corresponding maxillary and the adjacent mandibular-ophthalmic regions in TG were analysed.

RESULTS

Statistically significantly higher frequencies of CGRP-positive neurons, regardless of their size, were found in TG ipsilateral to the periodontitis (83% and 73% in FG-labelled and maxillary regions, respectively) than in the control group without periodontitis (52% and 42% in FG-labelled and maxillary regions, respectively). The frequency of small FG-labelled SP-positive neurons in the ipsilateral TG (60%) was significantly higher than in the control TG (25%). In the contralateral TG the frequency of CGRP-positive neurons in maxillary region (66%) was significantly higher than in the control group. Surprisingly, the number of SP-positive neurons in all regions of contralateral TG decreased when compared to control and ipsilateral TGs.

CONCLUSIONS

Taken together, these results implicate a role of neurogenic component in the pathogenesis of periodontitis. The contralateral response in the TG could be mediated through the transmedian neurological pathways crossing in the trigeminal nuclear complex or through the systemic inflammatory reaction and the activation of the so called "neuro-immune axis".

Expression of TrkA receptor for neurotrophins in trigeminal neurons innervating the rat gingivomucosal tissue

The purpose of this study was to characterize and evaluate the expression of TrkA receptor in trigeminal ganglion (TG) neurons that innervate the rat gingivomucosal tissue. A retrograde nerve tracer Fluorogold (FG) was injected into the gingiva (group 1) or applied into the gingival sulcus (group 2) of the first right maxillary molar to identify the neurons in TG that innervate the gingivomucosa. After 10 days TG were dissected and FG fluorescence in neurons was observed under UV light microscope. To draw a comparison, approximately 1000 neurons per ganglion from the entire TG (group 3) and approximately 350 neurons per ganglion from the maxillary region in TG (group 4), were analyzed. Expression of TrkA receptor in TG neurons was investigated by immunohistochemistry. About 70% of neurons in groups 1 and 2 contained TrkA receptor, which was statistically significantly more than in groups 3 (41%) and 4 (38%). FG-labeled TrkA-immunopositive neurons were predominantly small or medium-sized (less than 1200microm(2)). However, the neurons innervating the rat gingivomucosa were on average larger than the neurons in the entire TG or in the maxillary region. In conclusion, the majority of neurons in TG that innervate the rat gingivomucosa are small or medium-sized, contain TrkA receptor and are most probably nociceptive.

Neuropeptide release from dental pulp cells by RgpB via proteinase-activated receptor-2 signaling

Dental pulp inflammation often results from dissemination of periodontitis caused mostly by Porphyromonas gingivalis infection. Calcitonin gene-related peptide and substance P are proinflammatory neuropeptides that increase in inflamed pulp tissue. To study an involvement of the periodontitis pathogen and neuropeptides in pulp inflammation, we investigated human dental pulp cell neuropeptide release by arginine-specific cysteine protease (RgpB), a cysteine proteinase of P. gingivalis, and participating signaling pathways. RgpB induced neuropeptide release from cultured human pulp cells (HPCs) in a proteolytic activity-dependent manner at a range of 12.5-200 nM. HPCs expressed both mRNA and the products of calcitonin gene-related peptide, substance P, and proteinase-activated receptor-2 (PAR-2) that were also found in dental pulp fibroblast-like cells. The PAR-2 agonists, SLIGKV and trypsin, also induced neuropeptide release from HPCs, and HPC PAR-2 gene knockout by transfection of PAR-2 antisense oligonucleotides inhibited significantly the RgpB-elicited neuropeptide release. These results indicated that RgpB-induced neuropeptide release was dependent on PAR-2 activation. The kinase inhibitor profile on the RgpB-neuropeptide release from HPC revealed a new PAR-2 signaling pathway that was mediated by p38 MAPK and activated transcription factor-2 activation, in addition to the PAR-2-p44/42 p38MAPK and -AP-1 pathway. This new RgpB activity suggests a possible link between periodontitis and pulp inflammation, which may be modulated by neuropeptides released in the lesion.

Syndecan-1 Expression During the Formation of Junctional Epithelium

BACKGROUND

Syndecans are cell surface heparan sulfate proteoglycans (PG) which can bind to and modulate the action of growth factors and extracellular matrix components (ECM). Syndecan- 1 has been shown to play important roles during early tooth development and wound healing and repair. Among diverse cells and tissues that comprise the periodontium, the junctional epithelium (JE) constitutes a region of significant anatomic and clinical importance, but the nature of inductive signals and molecules involved in its formation is still unclear. Therefore, this work examines if syndecan-1 is associated with formation of JE, and the distribution of other syndecan family members in the epithelium.

METHODS

In situ hybridization and immunohistochemical techniques were performed using oral tissues from 4-day-old to 10-week-old mice to investigate the expression of syndecan- 1, -2, -3 and -4 mRNAs and their corresponding proteins.

RESULTS

Based on in situ hybridization experiments, all syndecan mRNAs were detected in sulcular epithelium (SE), gingival epithelium (GE), and JE with varying intensity and distribution. Syndecan-1 immunostaining was localized on the cell surface while that of syndecan-2 did not show clear membrane localization. Our experiments in the developing tooth demonstrated that syndecan-1 protein followed characteristic patterns of expression during JE formation and that immunoreactivity for syndecan-1 protein decreased with age when JE cells underwent terminal differentiation.

CONCLUSION

Results of syndecan-1 mRNA and protein expression patterns suggested that this proteoglycan might be an important molecule during the formation of JE.

Cementum onSmilodon sabers

The maxillary canines of Smilodon californicus Bovard, 1907 have a deeply curved cementoenamel junction. The gingiva of modern cats is attached to the tooth at the cementoenamel junction and provides tactile and other dental information to the animal. The presence of cementum at the cervix of the maxillary canines, also called sabers, would indicate that the gingiva in Smilodon was attached in this region. Such an attachment would be advantageous, providing stability and sensory input for the large tooth. Also, gingiva at the cervix would impact the manner in which the teeth were used. Previous study using scanning electron microscopy of dental casts was indirect. The purpose of this study was to confirm by direct methods the presence of cementum at the cervix of Smilodon californicus sabers. Parts of three Smilodon californicus sabers were sectioned and examined with light and scanning electron microscopy (EDS). In addition, percent weight of calcium and phosphorus was measured in enamel, dentin, and cementum using electron dispersive spectroscopy. Cementum was identified in the cervical region of each saber. Spectroscopy confirmed that the tissue is calcified and the mineral is hydroxyapatite. Percent calcium and percent phosphorus of individual tissues were highly variable between specimens. However, the ratios of calcium to phosphorus were not significantly different from the hydroxyapatite standard. In the future, bite models will have to take the presence of soft tissues into account.

A model of periodontitis in the rat: effect of lipopolysaccharide on bone resorption, osteoclast activity, and local peptidergic innervation

OBJECTIVE

To establish and characterise a rat model of periodontitis that reiterates the features of human disease.

METHODS

Periodontal inflammation was induced by a single injection of 10 microg liposaccharide (LPS) (Salmonella typhimurium) in 1 microl saline into rat mandibular gingiva at the buccomesial aspect of the second molar. Animals were killed after 3, 7 and 10 days, mandibles dissected and sectioned for histological and immunocytochemical analysis.

RESULTS

LPS injection resulted in a significant gingival and periodontal inflammation with inflammatory infiltrate, apical migration of the junctional epithelium, interdental bone loss, and activation of osteoclasts at the site of injection 7 and 10 days after injection. At 10 days post injection, there was a significant trend for bone loss on both sides of the mandible. Periodontal inflammation was associated with alteration in the levels of calcitonin gene-related peptide-like immunoreactivity in nerve terminals innervating the inflamed gingival papilla.

CONCLUSION

Intragingival injection of LPS in the rat provides an easily induced reproducible experimental model of periodontal inflammation that reiterates features of human disease.

Vanilloid receptor expression in the rat tongue and palate

Capsaicin, the pungent substance in hot peppers, evokes a sensation of burning pain by stimulating the vanilloid receptor 1 (VR1) on primary afferent neurons. Immunohistochemistry revealed that the taste papillae in the tongue and palate are richly innervated by VR1-immunoreactive nerve fibers. Furthermore, VR1 protein expression was seen in the epithelium facing the oral cavity, although taste cells seemed to be devoid of VR1. The most conspicuous VR1 expression was observed in the epithelial cells of the palatal rugae, although there were no VR1-immunoreactive nerves there. The finding that VR1 is expressed not only in primary afferents but also in oral epithelial cells suggests that it is of great importance in the perception of capsaicin, heat, and acid in the mouth. Since VR1 is known to play a key role in nociception and inflammatory pain, it may be a new target for the treatment of oral pain.

Differences between tooth stimulation and capsaicin-induced neurogenic vasodilatation in human gingiva

Animal experiments have shown that the application of capsaicin to oral mucosa leads to a neurogenic inflammation associated with blood flow elevations in gingivomucosal tissues. In this investigation, we measured the tooth stimulation and capsaicin-evoked blood flow responses in maxillary gingiva in humans to study whether axon-reflex-mediated vasodilatation crosses the midline of the maxilla. The vasoactive reactions were mapped by laser Doppler imaging. Unilateral stimulation of alveolar mucosa and attached gingiva by capsaicin evoked a distinct neurogenic vasodilatation in ipsilateral gingiva, which rapidly attenuated at the midline. Capsaicin stimulation of alveolar mucosa provoked clear inflammatory reactions. In contrast to capsaicin stimuli, tooth stimulation produced symmetrical vasodilatations bilaterally in the gingiva. The ipsilateral responses were significantly smaller during tooth stimulation than during capsaicin stimuli. Analysis of these data suggests that capsaicin-induced inflammatory reactions in gingivomucosal tissues do not cross the midline in the anterior maxilla. The enhanced reaction found during stimulation of alveolar mucosa indicates that alveolar mucosa is more sensitive to chemical irritants than attached gingiva.

Regeneration of nerve fibres in the peri-implant epithelium incident to implantation in the rat maxilla as demonstrated by immunocytochemistry for protein gene product 9.5 (PGP9.5) and calcitonin gene-related peptide (CGRP)

The response of nerve fibres in the peri-implant epithelium to titanium implantation was investigated with an experimental model using rat maxilla and immunohistochemical techniques. The latter employed antibodies to protein gene product 9.5 (PGP9.5), and to calcitonin gene-related peptide (CGRP). In control rats without an implantation, a dense innervation of PGP9.5- and CGRP-positive nerve fibres was recognized throughout the junctional epithelium, as has been previously reported. A titanium-implantation induced a remarkable inflammatory reaction, as well as the destruction of covering epithelial cells. By 3-5 days post-implantation, inflammatory reaction showed a tendency to disappear, and the peri-implant epithelium showed proliferation and down-growth along the implant. At this stage, no nerve fibres were found around the peri-implant epithelium. At 10 days, a few nerve fibres reached the basal cell layers of the peri-implant epithelium, and entered it 15 days after implantation when the peri-implant epithelial cells showed morphological features roughly resembling those of normal junctional epithelial cells. At the complete osseointegration stage (days 20-30), the PGP9.5- and CGRP-positive nerve fibres, thin and beaded in appearance, were found distributed in the peri-implant epithelium. After 20 days, the numerical density of the intraepithelial nerves in the peri-implant epithelium appeared the same as, or less than, that in the normal junctional epithelium. These findings indicate that the peri-implant epithelium shows the same innervation as that in normal junctional epithelium, and that the intraepithelial nerve fibres in the peri-implant epithelium might have diverse functions, which have been suggested in the literature.

Difference in penetration of horseradish peroxidase tracer as a foreign substance into the peri-implant or junctional epithelium of rat gingivae

Horseradish peroxidase (HRP) tracer was applied to the gingival sulcus of implants or natural teeth at 5, 25, or 50 mg/ml to investigate the sealing capacities of the peri-implant epithelium (PIE) and junctional epithelium (JE); the extent of HRP penetration was observed under electron microscopy. A Ti-6Al-4V implant was inserted either immediately (immediate implantation) or 2 weeks (delayed implantation) after extraction of the maxillary left first molar of rats. The JE of the right molar was used as a control. Although the whole PIE of undecalcified sections appeared to be attached to the implant surface, electron microscopically, the internal basement lamina (IBL) and hemidesmosomes were deficient in the coronal-middle region of the PIE. There were extensive extracellular deposits of HRP in the intercellular spaces between PIE cells, and HRP was blocked to some extent by the lamina lucida and lamina densa of the external basal lamina and basal cell junction. HRP was detected in the connective tissue under the PIE, but was not found in the connective tissue under the JE. Intracellularly, HRP was found in the vesicles and granules of PIE cells and JE cells. These were fewer in number in PIE cells than in JE cells. There were no differences between the findings for immediate and delayed implantation. The results indicate that a deficiency in the IBL permitted penetration of HRP from the gingival sulcus into the connective tissue under the PIE, and suggest that the endocytotic capacity of PIE cells is inferior to that of JE cells.

Capsaicin-evoked CGRP release from rat buccal mucosa: development of a model system for studying trigeminal mechanisms of neurogenic inflammation

Many of the physiological hallmarks associated with neurogenic inflammatory processes in cutaneous tissues are similarly present within orofacial structures. Such attributes include the dependence upon capsaicin-sensitive sensory neurons and the involvement of certain inflammatory mediators derived therein, including calcitonin gene-related peptide (CGRP). However, there are also important differences between the trigeminal and spinal nervous systems, and the potential contributions of neurogenic processes to inflammatory disease within the trigeminal system have yet to be fully elucidated. We present here a model system that affords the ability to study mechanisms regulating the efferent functions of peptidergic terminals that may subserve neurogenic inflammation within the oral cavity. Freshly dissected buccal mucosa tissue from adult, male, Sprague-Dawley rats was placed into chambers and superfused with oxygenated, Krebs buffer. Serial aliquots of the egressing superfusate were acquired and analysed by radioimmunoassay for immunoreactive CGRP (iCGRP). Addition of the selective excitotoxin, capsaicin (10-300 microm), to the superfusion buffer resulted in a significant, concentration-dependent increase in superfusate levels of iCGRP. Similarly, release of iCGRP from the buccal mucosa could also be evoked by a depolarizing concentration of potassium chloride (50 mm) or by the calcium ionophore A23187 (1 microm). The specific, capsaicin receptor antagonist, capsazepine (300 microm), completely abolished the capsaicin-evoked release of iCGRP while having no effect whatsoever on the potassium-evoked release. Moreover, capsaicin-evoked release was dependent upon the presence of extracellular calcium ions and was significantly, though incompletely, attenuated by neonatal capsaicin denervation. Collectively, these data indicate that the evoked neurosecretion of iCGRP in response to capsaicin occurs via a vanilloid receptor-mediated, exocytotic mechanism. The model system described here should greatly facilitate future investigations designed to identify and characterize the stimuli that regulate the release of CGRP or other neurosecretory substances in isolated tissues. This system may also be used to elucidate the role of these mediators in the aetiology of inflammatory processes within the trigeminal field of innervation.

Immunocytochemical study of nerve fibers containing substance P in the junctional epithelium of rats

Nerve fibers with substance P-like immunoreactivity (SP-IR) in the junctional epithelium (JE) of 32-42-d-old rats were examined by both light and electron microscopy using the avidin-biotin-peroxidase complex method. The density of nerve fibers with SP-IR was highest in the middle portion of the JE; however, a few fibers were localized in the coronal portion of the JE and close to the enamel surface. Also, rich innervation was found especially in the basal cell layer of the JE. Unmyelinated axons with SP-IR in the connective tissue underlying the JE were enveloped by Schwann cells but lost their Schwann cell sheath almost completely in the JE. The axons often formed varicosities with SP-IR as terminals in various areas of the JE. The terminals contained numerous large granular vesicles, small clear vesicles and a few mitochondria, and were surrounded by the cytoplasmic processes of the junctional epithelial cells. These terminals were sometimes located close to neutrophils in the JE; the minimum gap distance between the terminals and the processes of junctional epithelial cells or neutrophils was about 20 nm. A few terminals with SP-IR came close to the enamel surface, and the minimal distance between the terminals and the enamel surface was about 5 microns. SP-IR in the nerve terminals in the JE fixed with 0.1% or 0.25% glutaraldehyde was distributed diffusely in the axoplasm or was confined to the granular vesicles. These findings show that substance P is contained in the large granular vesicles in the nerve terminals, and suggest that these terminals may function as modulators of junctional epithelial cells and neutrophils.