Sensory mechanisms in mammalian teeth and their supporting structures

The inferior alveolar nerve and its relationship to the mandibular canal

Previous work on the mandibular canal, mental foramen, and mandibular foramen has focused on humans and some other non-primate mammals (with small sample sizes), but little work has investigated the mandibular canal and inferior alveolar nerve (IAN) across primates. However, it is important to understand the relationship between the IAN and mandibular canal due to the IAN's close relationship to the teeth and mastication, and thus dietary adaptations. While it is assumed that most bony canals within the skull grow around and form to pre-existing nervous structures, this relationship has never been validated for the IAN and mandibular canal. MicroCT scans of 273 individuals (131 females, 134 males, and 8 unknown sex) from 68 primate species and three mammalian outgroups, and diceCT scans of 66 individuals (35 females, 23 males, and 8 unknown sex) from 33 primate species and the same mammalian outgroups were used to create 3D models of the IAN and mandibular canal from which cross-sectional areas were taken at various points on the structures. Using qualitative descriptions, phylogenetic generalized least squares analysis, and phylogenetic ANOVAs, we were able to establish three main conclusions: (1) the mandibular canal is most often not a defined canal within the mandible of primates, (2) when the canal can be identified, the IAN does not comprise most of the space within, and (3) there are significant relationships between the IAN and the corresponding canals, with most showing isometry and the mental foramen/nerve showing negative allometry.

From teeth to pad: tooth loss and development of keratinous structures in sirenians

Sirenians are a well-known example of morphological adaptation to a shallow-water grazing diet characterized by a modified feeding apparatus and orofacial morphology. Such adaptations were accompanied by an anterior tooth reduction associated with the development of keratinized pads, the evolution of which remains elusive. Among sirenians, the recently extinct Steller's sea cow represents a special case for being completely toothless. Here, we used μ-CT scans of sirenian crania to understand how motor-sensor systems associated with tooth innervation responded to innovations such as keratinized pads and continuous dental replacement. In addition, we surveyed nine genes associated with dental reduction for signatures of loss of function. Our results reveal how patterns of innervation changed with modifications of the dental formula, especially continuous replacement in manatees. Both our morphological and genomic data show that dental development was not completely lost in the edentulous Steller's sea cows. By tracing the phylogenetic history of tooth innervation, we illustrate the role of development in promoting the innervation of keratinized pads, similar to the secondary use of dental canals for innervating neomorphic keratinized structures in other tetrapod groups.

Grit and consequence

Grit is implicated in several biological phenomena-it wears teeth, it fractures teeth, it drives tooth evolution, it elicits complex manual manipulations-any one of which could be described as a central topic in evolutionary anthropology. But what is grit? We hardly know because we tend to privilege the consequences of grit (it is abrasive) over its formal features, all but ignoring crucial variables such as mineral composition, material properties, and particle geometry (size, angularity), not to mention natural variation in the habitats of primates and their food surfaces. Few topics have animated so much debate and invited such cool indifference at the same time. Our goal here is to shine a light on grit, to put a philosophical lens on the nature of our discourse, and to call attention to large empirical voids that should be filled and folded into our understanding of primate natural history and evolution.

Multiple complex somatosensory systems in mature rat molars defined by immunohistochemistry

OBJECTIVE

Intradental sensory receptors trigger painful sensations and unperceived mechanosensitivity, but the receptor bases for those functions are only partly defined. We present new evidence here concerning complex endings of myelinated axons in rat molars.

DESIGN

We sectioned mature rat jaws in sagittal and transverse planes to analyze neural immunoreactivity (IR) for parvalbumin, peripherin, neurofilament protein, neurotrophin receptors, synaptophysin, calcitonin gene-related peptide (CGRP), or mas-related g-protein-receptor-d (Mrgprd).

RESULTS

We found two complex sensory systems in mature rat molar dentin that labeled with neurofilament protein-IR, plus either parvalbumin-IR or peripherin-IR. The parvalbumin-IR system made extensively branched, beaded endings focused into dentin throughout each pulp horn. The peripherin-IR system primarily made unbeaded, fork-shaped dentinal endings scattered throughout crown including cervical regions. Both of these systems differed from neuropeptide CGRP-IR. In molar pulp we found peripherin- and parvalbumin-IR layered endings, either near special horizontal plexus arrays or in small coiled endings near tangled plexus, each with specific foci for specific pulp horns. Parvalbumin-IR nerve fibers had Aβ axons (5-7μm diameter), while peripherin-IR axons were thinner Aδ size (2-5μm). Mechano-nociceptive Mrgprd-IR was only found in peripherin-IR axons.

CONCLUSIONS

Complex somatosensory receptors in rat molars include two types of dentinal endings that both differ from CGRP-IR endings, and at least two newly defined types of pulpal endings. The PV-IR neurons with their widely branched, synaptophysin-rich, intradentinal beaded endings are good candidates for endodontic non-nociceptive, low threshold, unperceived mechanoreceptors. The complex molar dentinal and pulpal sensory systems were not found in rat incisors.

Scaffold-free microtissues: differences from monolayer cultures and their potential in bone tissue engineering

OBJECTIVES

Cell-based therapies for bone augmentation after tooth loss and for the treatment of periodontal defects improve healing defects. Usually, osteogenic cells or stem cells are cultivated in 2D primary cultures, before they are combined with scaffold materials, even though this means a loss of the endogenous 3D microenvironment for the cells. Moreover, the use of single-cell suspensions for the inoculation of scaffolds or for the direct application into an area of interest has the disadvantages of low initial cell numbers and susceptibility to unwanted cellular distribution, respectively.

MATERIALS AND METHODS

We addressed the question whether an alternative to monolayer cultures, namely 3D microtissues, has the potential to improve osteogenic tissue engineering and its clinical outcome.

RESULTS

By contrast, to monolayer cultures, osteogenic differentiation of 3D microtissues is enhanced by mimicking in vivo conditions. It seems that the osteogenic differentiation in microtissues is enhanced by strong integrin-extracellular matrix interaction and by stronger autocrine BMP2 signaling. Moreover, microtissues are less prone to wash out by body fluids and allow the precise administration of large cell numbers.

CONCLUSION

Microtissue cultures have closer characteristics with cells in vivo and their enhanced osteogenic differentiation makes scaffold-free microtissues a promising concept in osteogenic tissue engineering.

CLINICAL RELEVANCE

Microtissues are particularly suitable for tissue engineering because they improve seeding efficiency of biomaterials by increasing the cell load of a scaffold. This results in accelerated osteogenic tissue formation and could contribute to earlier implant stability in mandibular bone augmentation.

Scaffold-free microtissues: differences from monolayer cultures and their potential in bone tissue engineering

Objectives

The paper’s aim is to review dentin hypersensitivity (DHS), discussing pain mechanisms and aetiology.

Materials and methods

Literature was reviewed using search engines with MESH terms, DH pain mechanisms and aetiology (including abrasion, erosion and periodontal disease).

Results

The many hypotheses proposed for DHS attest to our lack of knowledge in understanding neurophysiologic mechanisms, the most widely accepted being the hydrodynamic theory. Dentin tubules must be patent from the oral environment to the pulp. Dentin exposure, usually at the cervical margin, is due to a variety of processes involving gingival recession or loss of enamel, predisposing factors being periodontal disease and treatment, limited alveolar bone, thin biotype, erosion and abrasion.

Conclusions

The current pain mechanism of DHS is thought to be the hydrodynamic theory. The initiation and progression of DHS are influenced by characteristics of the teeth and periodontium as well as the oral environment and external influences. Risk factors are numerous often acting synergistically and always influenced by individual susceptibility.

Clinical relevance

Whilst the pain mechanism of DHS is not well understood, clinicians need to be mindful of the aetiology and risk factors in order to manage patients’ pain and expectations and prevent further dentin exposure with subsequent sensitivity.

Unmyelinated nerve fibers in the human dental pulp express markers for myelinated fibers and show sodium channel accumulations

BACKGROUND

The dental pulp is a common source of pain and is used to study peripheral inflammatory pain mechanisms. Results show most fibers are unmyelinated, yet recent findings in experimental animals suggest many pulpal afferents originate from fibers that are myelinated at more proximal locations. Here we use the human dental pulp and confocal microscopy to examine the staining relationships of neurofilament heavy (NFH), a protein commonly expressed in myelinated afferents, with other markers to test the possibility that unmyelinated pulpal afferents originate from myelinated axons. Other staining relationships studied included myelin basic protein (MBP), protein gene product (PGP) 9.5 to identify all nerve fibers, tyrosine hydroxylase (TH) to identify sympathetic fibers, contactin-associated protein (caspr) to identify nodal sites, S-100 to identify Schwann cells and sodium channels (NaChs).

RESULTS

Results show NFH expression in most PGP9.5 fibers except those with TH and include the broad expression of NFH in axons lacking MBP. Fibers with NFH and MBP show NaCh clusters at nodal sites as expected, but surprisingly, NaCh accumulations are also seen in unmyelinated fibers with NFH, and in fibers with NFH that lack Schwann cell associations.

CONCLUSIONS

The expression of NFH in most axons suggests a myelinated origin for many pulpal afferents, while the presence of NaCh clusters in unmyelinated fibers suggests an inherent capacity for the unmyelinated segments of myelinated fibers to form NaCh accumulations. These findings have broad implications on the use of dental pulp to study pain mechanisms and suggest possible novel mechanisms responsible for NaCh cluster formation and neuronal excitability.

A novel computerized system for thermal stimulation of tooth in ferrets

A dual thermal and electrical stimulator was developed to examine the central pathways that transmit noxious stimuli for intact dentition. This system allows computer-controlled stimulation of the canines of ferrets with either noxious heat or electrical stimuli. A set of in vitro studies demonstrated that the application of thermal stimuli to an intact tooth can produce pulpal temperatures above 43 °C, which is perceived as a painful stimulus in humans. In a set of in vivo studies, it was demonstrated that heating an intact tooth at temperatures of at least 40 °C, excited trigeminal brainstem neurons. Only 15% of the neurons activated by electrical stimulation responded to noxious heat applied to the canine. Eight of the 23 neurons were classified as nociceptive specific neurons and responded only to noxious stimulation of their cutaneous receptive fields. Fifteen of the 23 neurons were classified as wide dynamic range neurons and responded to both noxious and non-noxious stimulation applied to their cutaneous receptive fields. This new device can accurately deliver both thermal and electrical stimuli to an intact tooth, which allows an evaluation of the central neural circuits that respond to noxious stimulation of the dentition.

Nav1.7 expression is increased in painful human dental pulp

BACKGROUND

Animal studies and a few human studies have shown a change in sodium channel (NaCh) expression after inflammatory lesions, and this change is implicated in the generation of pain states. We are using the extracted human tooth as a model system to study peripheral pain mechanisms and here examine the expression of the Nav1.7 NaCh isoform in normal and painful samples. Pulpal sections were labeled with antibodies against: 1) Nav1.7, N52 and PGP9.5, and 2) Nav1.7, caspr (a paranodal protein used to identify nodes of Ranvier), and myelin basic protein (MBP), and a z-series of optically-sectioned images were obtained with the confocal microscope. Nav1.7-immunofluorescence was quantified in N52/PGP9.5-identified nerve fibers with NIH ImageJ software, while Nav1.7 expression in myelinated fibers at caspr-identified nodal sites was evaluated and further characterized as either typical or atypical as based on caspr-relationships.

RESULTS

Results show a significant increase in nerve area with Nav1.7 expression within coronal and radicular fiber bundles and increased expression at typical and atypical caspr-identified nodal sites in painful samples. Painful samples also showed an augmentation of Nav1.7 within localized areas that lacked MBP, including those associated with atypical caspr-identified sites, thus identifying NaCh remodeling within demyelinating axons as the basis for a possible pulpal pain mechanism.

CONCLUSION

This study identifies the increased axonal expression and augmentation of Nav1.7 at intact and remodeling/demyelinating nodes within the painful human dental pulp where these changes may contribute to constant, increased evoked and spontaneous pain responses that characterize the pain associated with toothache.

Clinical and scanning electron microscopic features of invasive cervical resorption in a maxillary molar

This case report presents clinical and morphological features of severe invasive cervical resorption (ICR) involving a maxillary first molar. Surface morphology of the ICR defect that extended into enamel was studied using a scanning electron microscope. The topography and morphology of resorbed enamel and pulpal surfaces are described. The pulpal space was characterized by proliferation of fibrovascular tissue invaded by a number of > or = 15-microm-large dentinoclastlike cells. The vertical orientation and hexagonal geometric outlines of enamel rods indicated that the resorptive defect had extended into the outer surface of enamel. Preferential odontoclastic dissolution of interprismatic enamel was noted. Deposits of a cementumlike substance were noticed on enamel surfaces, indicating that resorptive process was coupled with repair. Complete destruction of dentin was associated with no response by the tooth to cold stimulus, supporting the hydrodynamic theory of dentinal sensitivity. The results of this study indicate that in addition to predentin and cementum, the outer surface enamel may be resistant to the resorptive process as well.

Peripheral mechanisms of odontogenic pain

In this article, we review the key basic mechanisms associated with this phenomena and more recently identified mechanisms that are current areas of interest. Although many of these pain mechanisms apply throughout the body, we attempt to describe these mechanisms in the context of trigeminal pain.

High-Resolution Three-Dimensional Computer Simulation of Hominid Cranial Mechanics

In vivo data demonstrates that strain is not distributed uniformly on the surface of the primate skull during feeding. However, in vivo studies are unable to identify or track changes in stress and strain throughout the whole structure. Finite element (FE) analysis, a powerful engineering tool long used to predict the performance of man-made devices, has the capacity to track stress/strain in three dimensions (3-D) and, despite the time-consuming nature of model generation, FE has become an increasingly popular analytical device among biomechanists. Here, we apply the finite element method using sophisticated computer models to examine whether 3-D stress and strain distributions are nonuniform throughout the primate skull, as has been strongly suggested by 2-D in vivo strain analyses. Our simulations document steep internal stress/strain gradients, using models comprising up to three million tetrahedral finite elements and 3-D reconstructions of jaw adducting musculature with both cranium and mandible in correct anatomical position. Results are in broad concurrence with the suggestion that few regions of the hominid cranium are clearly optimized for routine feeding and also show that external stress/strain does not necessarily reflect internal distributions. Findings further suggest that the complex heterogeneity of bone in the skull may act to dissipate stress, but that consequently higher strain must be offset by additional strain energy. We hypothesize that, despite energetic costs, this system may lend adaptive advantage through enhancing the organism's ability to modify its behavior before reaching catastrophic failure in bony or dental structures.

Substance P and swallowing after stroke

In the field of digestive tract surgery, QOL can be significantly decreased in patients who experience recurrence or metastasis of a cancer, not only by digestive tract disorders that prevent the patient from taking a meal, but also by installation of gastric or ileus tubes. For such patients, surgical procedures aimed to palliate their symptoms are necessary, rather than radical surgeries. We examined 24 cases at our department, for which operations were performed with the aim of palliating their symptoms, during the 4-year period from October 2001 to December 2005. Sixteen (89%) out of 18 symptomatic cases (i.e. patients who could not take a meal, or who had undergone tracheal intubation) exhibited confirmed improvement in their symptoms after the operations. On the other hand, we also experienced one case in which installation of an ileus tube was necessary after the operation (1 case, 5.5%). Five patients (20.9%) died after the operation, before they were discharged from the hospital. For such patients, additional minimally invasive procedures should have been taken into consideration.

Mechanical properties of foods used in experimental studies of primate masticatory function

In vivo studies of jaw-muscle behavior have been integral factors in the development of our current understanding of the primate masticatory apparatus. However, even though it has been shown that food textures and mechanical properties influence jaw-muscle activity during mastication, very little effort has been made to quantify the relationship between the elicited masticatory responses of the subject and the mechanical properties of the foods that are eaten. Recent work on human mastication highlights the importance of two mechanical properties-toughness and elastic modulus (i.e., stiffness)-for food breakdown during mastication. Here we provide data on the toughness and elastic modulus of the majority of foods used in experimental studies of the nonhuman primate masticatory apparatus. Food toughness ranges from approximately 56.97 Jm(-2) (apple pulp) to 4355.45 Jm(-2) (prune pit). The elastic modulus of the experimental foods ranges from 0.07 MPa for gummy bears to 346 MPa for popcorn kernels. These data can help researchers studying primate mastication select among several potential foods with broadly similar mechanical properties. Moreover, they provide a framework for understanding how jaw-muscle activity varies with food mechanical properties in these studies.

Passive tactile sensibility in edentulous subjects treated with dental implants: A pilot study

STATEMENT OF PROBLEM

Edentulous patients treated with implant-supported prostheses have shown increased passive tactile sensibility compared with those using conventional complete dentures. This is thought to be due to the close mechanical coupling between the implant and bone via the osseointegrated interface, yet the phenomenon has received little attention.

PURPOSE

The purpose of this study was to measure passive tactile sensibility in a group of edentulous subjects treated with dental implants, and to relate the measured sensibility to a range of factors thought to be of possible relevance, namely, patient age, gender, time since implant placement, implant length, and implant separation.

MATERIAL AND METHODS

Twenty edentulous subjects successfully treated with 2 or more Nobel Biocare dental implants in the anterior mandible were studied. The inclusion criteria were : (1) age of less than 50 years, (2) a period of at least 12 months since implant placement, (3) implant length of at least 10 mm and of standard diameter (excluding narrow and wide platform designs), and (4) implant separation of at least 18 mm. Using a computer-controlled custom-made device, pushing forces (2.1, 2.4, 2.7, and 3.0 N/s) were applied directly and perpendicular to the long axes of the implant abutments until the subjects felt the first sensation of pressure. The magnitude of these forces was measured with an integral transducer. The applied force had a ramped staircase pattern, and force application rates were varied between 2.1 and 3.0 N/s. Multilevel modeling was used to analyze the collected data (alpha=.05).

RESULTS

The threshold values of passive tactile sensibility ranged between 3.1 and 15.7 N (mean 10.9; SD 3.9). Analysis failed to show any significant association between passive tactile sensibility and the variables studied.

CONCLUSION

Within the limitations of this study, which included a small sample size, no relationship was found between passive tactile sensibility associated with long-standing implants and any of the variables studied (age, gender, time since implant placement, implant length, and implant separation).

Dental neuroplasticity, neuro-pulpal interactions, and nerve regeneration

This review covers current information about the ability of dental nerves to regenerate and the role of tooth pulp in recruitment of regenerating nerve fibers. In addition, the participation of dental nerves in pulpal injury responses and healing is discussed, especially concerning pulp regeneration and reinnervation after tooth replantation. The complex innervation of teeth is highly asymmetric and guided towards specific microenvironments along blood vessels or in the crown pulp and dentin. Pulpal products such as nerve growth factor are distributed in the same asymmetric gradients as the dentinal sensory innervation, suggesting regulation and recruitment of those nerve fibers by those specific factors. The nerve fibers have important effects on pulpal blood flow and inflammation, while their sprouting and cytochemical changes after tooth injury are in response to altered pulpal cytochemistry. Thus, their pattern and neuropeptide intensity are indicators of pulp status, while their local actions continually affect that status. When denervated teeth are injured, either by pulp exposure on the occlusal surface or by replantation, they have more pulpal necrosis than occurs for innervated teeth. However, small pulp exposures on the side of denervated crowns or larger lesions in germ-free animals can heal well, showing the value of postoperative protection from occlusal trauma or from infection. Current ideas about dental neuroplasticity, neuro-pulpal interactions, and nerve regeneration are related to the overall topics of tooth biomimetics and pulp/dentin regeneration.

Differences in the axonal compositions of the human mandibular nerve between dentulous and edentulous jaws

We examined the human mandibular nerve to find differences in the composition of nerve fiber axons between dentulous and edentulous jaws Using Goto's modification of Masson-Goldner's method. We discovered that the edentulous jaw did not contain any large size axons, compared with the dentulous jaw. This can be considered as evidence that the larger fibers innervating the periodontal ligament decreased degenerated after tooth loss.

Correlations between response properties of periodontal mechanosensitive neurones in the primary somatosensory cortex of the rabbit and cortically induced rhythmical jaw movements

The response properties of incisor- and molar-sensitive periodontal mechanosensitive (PM) neurones in the primary somatosensory (SI) cortex of rabbits were examined and rhythmical jaw movements induced by repetitive electrical stimulation of the recording sites of cortical PM neurones were observed. PM units were recorded from the rostromedial (RM) and rostrolateral (RL) areas of the SI cortex. In the RM area, most PMs (85%) were lower incisor-sensitive. Electrical stimulation of the RM area produced chopping-type rhythmical jaw movements. In the RL area, both incisor- and molar-sensitive PM units were recorded, and molar-sensitive units were located more rostromedially than incisor-sensitive units. More than half (66%) of the incisor-sensitive PM units were upper incisor-sensitive. The incidences of sustained-response type units were 8 and 10% for upper incisor- and lower incisor-sensitive units and 28 and 34% for upper molar- and lower molar-sensitive units, respectively. The optimal stimulus directions for the upper molar-sensitive units were predominantly labial or lingual, whereas those for most of the lower molar-sensitive units were lingual. Electrical stimulation of the PM unit-recording sites in the RL area induced grinding-type rhythmical jaw movements. Based on these findings, the lower incisor-sensitive neurones in the RM area of the SI cortex might mainly contribute to a neural network that controls jaw movements during ingestion. Furthermore, the response properties of molar-sensitive cortical neurones might be useful for discriminating the magnitude and direction of the biting force during grinding. Further studies are needed to clarify the role of upper incisor-sensitive neurones in the RL area in triggering grinding-type rhythmical jaw movements.

An in vitro temporomandibular joint-nerve preparation for pain study in rats

A novel in vitro TMJ-nerve preparation was developed to quantitatively study peripheral sensory mechanisms of temporomandibular joint (TMJ). The TMJ region on one side (including mandibular head, disc, retrodiscal tissue and mandibular fossa) of adult Wistar albino rats was excised together with the auriculo-temporal nerve. The block was preserved in a modified Krebs-Henseleit solution saturated with O(2)/CO(2) (95/5%) gas mixture. Using a calibrated von Frey type apparatus, mechanical noxious stimulation was applied directly to various sites within the TMJ region. In addition, thermal and chemical noxious stimuli were also attempted. Stable recordings of single unit activities from the auriculo-temporal nerve could be obtained for as long as 5 h, which was sufficient to analyze the response properties of the TMJ units to various stimuli. This new preparation would be useful for investigating TMJ peripheral sensory mechanisms, especially pain, and potentially makes it possible to reveal neural mechanisms of temporomandibular arthralgia, a syndrome that has recently shown an increased incidence in clinical dentistry.

A longitudinal study of the association between tooth loss and age-related hearing loss

The purpose of this study was to investigate cross-sectional and longitudinal associations between hearing acuity and tooth loss in 1156 US veterans taking part in the Veterans Affairs' Normative Aging (NAS) and Dental Longitudinal (DLS) Studies in the Boston, MA, area. The mean age was 48 years (SD = 8.9), 5.3% were edentulous, and 15.4% had < 17 teeth at baseline. Hearing acuity was determined by puretone, air- and bone-conduction audiometry, and speech discrimination tests at triennial examinations over a 20-year follow-up period. Hearing decline was defined as a change from baseline in the average puretone air-conduction thresholds of > or = 20 dB at 0.25, 0.5, 1, 2, 3, 4, 6, and 8 kHz. The explanatory variables of interest were change since baseline in dentate status (cut points at < 1, < 17, and < 20 teeth), and in the number of teeth lost (linear). Linear and logistic regression models--which controlled for baseline audiological status, age, air-bone gap, and otoscopic examination at current visit--showed that subjects who went from having > or = 17 to < 17 teeth had 1.64 times (95% CI, 1.24-2.17) as high odds of having hearing decline as those with no change in their dentate status. For every tooth lost since baseline, there was a 1.04 times as high odds (95% CI, 1.02-1.06) for hearing decline, when additional baseline and time-varying covariates were taken into account in the model.

Response properties of slowly and rapidly adapting periodontal mechanosensitive neurones in the primary somatosensory cortex of the cat

Periodontal mechanosensitive neurones in the primary somatosensory (SI) cortex are classified as either slowly or rapidly adapting. The responses of cortical neurones and their projection pathways were studied using mechanical and electrical stimulation of the teeth and electrical stimulation of the thalamic posteromedial ventral (VPM) nuclei and contralateral SI cortex. A total of 247 periodontal mechanosensitive units were recorded from the SI cortex in 35 anaesthetized cats, distributed mainly in area 3b: 14% were slowly adapting and 86% rapidly adapting units; 62% of the slowly adapting and 9% of the rapidly adapting units were single-tooth units sensitive to stimulation of only one tooth. The incidence of slowly adapting units with an ipsilateral receptive field was almost equal to that of slowly adapting units with a contralateral receptive field, and more than half of the units were directionally selective to mechanical tooth stimulation. The majority of rapidly adapting units had their receptive field in the contralateral teeth and were directionally non-selective to tooth stimulation. The latencies of the cortical responses of the slowly and rapidly adapting units were 7.3 and 10.7 ms, respectively, on electrical stimulation of the contralateral teeth, and 1.8 and 2.0 ms, respectively, on electrical stimulation of the ipsilateral VPM nucleus. From these findings, it is inferred that slowly adapting neurones are useful for discriminating the tooth stimulated, the stimulus direction, the stimulus intensity and the change of pressure applied to the tooth, while rapidly adapting neurones could function to signal initial contact with food or the opposing teeth.

Histochemical studies of glycosaminoglycans in developing periodontal ligaments of ICR mice

Although the periodontal ligament (PL) contains an abundance of glycosaminoglycans (GAGs), there are only a few histochemical studies describing GAGs in the developing PL. In the present study, the relationship between the formation of principal fibers and the molecular species of GAGs in the developing PL was examined by light microscopic histochemistry. Jcl:ICR mice were killed on day 0 to day 28 after birth. Paraffin-embedded tissue sections were routinely made and stained with hematoxylin-eosin (H&E), Azan, or the sensitized high iron diamine (S-HID) procedure combined with enzyme digestions. Before tooth eruption, thin threads of collagen fibers in the PL assembled and constructed principal fibers, which projected from both the side of the alveolar bone and the root of the tooth. After tooth eruption, the principal fibers from both sides were tightly entangled. In the developing PL, the molecular species of GAGs was mainly dermatan sulfate. Moreover, the relative amount of dermatan sulfate increased together with the maturation of the principal fibers, while the principal fibers adjacent to the alveolar bone and cementum contained chondroitin sulfate. These results suggest that dermatan sulfate contributes to collagen fiber assembly in the PL and that chondroitin sulfate relates to PL adhesion to the alveolar bone and to the cementum of the root.

Fos expression in the ferret trigeminal nuclear complex following tooth pulp stimulation

The aim of this study was to establish which regions of the trigeminal nucleus are activated by tooth pulp stimulation in the normal ferret. The distribution of Fos-like immunoreactivity was examined following electrical stimulation of the tooth pulp in the awake and anaesthetized ferret. Stimulus-specific labelling was found in subnuclei caudalis and oralis of the trigeminal spinal nucleus. Three groups of chronically prepared animals; conscious, anaesthetized (alphaxolone/alphadolone) and anaesthetized-paralysed (alphaxolone/alphadolone with gallamine triethiodide), received electrical stimuli to both the upper and lower left canine teeth (1 Hz train of 3 x 0.5 ms at 200 Hz) at an amplitude of 10 times the threshold of the jaw opening reflex. Three control groups were treated identically except no stimulus was given. In stimulated anaesthetized and anaesthetized-paralysed animals, Fos-positive profiles were seen in laminae I and II of subnucleus caudalis and in the medial part of subnucleus oralis. There was no labelling evident in subnucleus interpolaris or the main sensory nucleus, or contralaterally in any of the subnuclei. In all conscious stimulated animals there was additional bilateral Fos-positive labelling, mainly in the deeper laminae of subnucleus caudalis. This bilateral labelling was not stimulus-specific as it was also seen in conscious non-stimulated animals. After correction for this bilateral labelling no significant difference was found between conscious, anaesthetized and anaesthetized-paralysed groups of stimulated animals or between the different groups of control animals. These results support the concept that the rostral parts of the trigeminal spinal nucleus are involved in processing of nociceptive input. They also demonstrate that light alphaxolone/alphadolone anaesthesia has no effect on stimulus-specific Fos expression following tooth pulp stimulation. The second aim of this study was to develop a clearly defined model for future studies in which Fos expression is no different to that seen in the conscious state. As in the conscious animal, labelling not associated with the stimulus is difficult to distinguish from stimulus specific labelling, further studies using this model of trigeminal nociceptive pathways would be best carried out in lightly anaesthetized animals.

NADPH-diaphorase activity in nerves and Schwann cells in the periodontal ligament of rat incisor teeth

The lingual portion of the incisor periodontal ligament demonstrated activity for nicotinamide adenosine dinucleotide phosphate (NADPH)-diaphorase. Schwann cells surrounding Ruffini-like endings coexpressed NADPH-diaphorase activity and immunoreactivity for inducible nitric oxide synthase. NADPH-diaphorase-positive nerve fibres which coexpressed immunoreactivity for neuronal nitric oxide synthase were in contact with Schwann cells surrounding Ruffini-like endings or terminated as free nerve endings. Neural NADPH-diaphorase activity could not be found in the tissues covering the labial portion of incisor tooth root. It is possible that nitric oxide in Schwann cells and nerves has functions specific to the incisor periodontal ligament.

Force production in the primate masticatory system: electromyographic tests of biomechanical hypotheses

Studies of the influence of dietary selection pressures in living and extinct primate taxa frequently interpret cranial diversity using a simple lever model. When this model is applied to functional or evolutionary questions, it is commonly assumed that the muscles of mastication vary little in activity during biting at points along the tooth row. A pattern of smoothly increasing maximum bite force magnitudes is therefore predicted as the bite point is moved posteriorly along the dental arcade. Diverse adaptive explanations are mapped onto this pattern. In this study, the activity of the superficial masseter and anterior temporalis muscles in humans was quantified during high magnitude bite force production at points along the tooth row. These data indicate that there are substantial changes in muscle activity with bite point, and that the standard lever model is therefore an incomplete description of masticatory force production. Maximum muscle force magnitudes were found to be greatest during first molar biting and to decrease as the bite point moved anteriorly and posteriorly. Additionally, relative balancing and working side muscle activity changed by bite point. This latter pattern is consistent with the predictions of Greaves' constrained lever model, which assumes that masticatory muscle activity is restricted by the need to maintain compressive forces at both temporomandibular joints. However, these results also imply that additional factors influence muscle activity--such as dental morphology, mandibular kinematics, and the need to safeguard against joint distraction during diverse loading conditions--and that the constrained lever model of Greaves is therefore also incomplete. These considerations suggest that masticatory system morphology in primates will respond differently to dietary selection pressures than is commonly hypothesized. Intepretations of cranial morphology in fossil taxa may therefore also differ.

Guidelines for the design and conduct of clinical trials on dentine hypersensitivity

Clinical trials on dentine hypersensitivity have been numerous and protocols varied. To date there is little consensus as to the conduct of studies on this poorly-understood yet common and painful dental condition. A committee of interested persons from academia and industry was convened to discuss the subject of clinical trials on dentine hypersensitivity and a consensus report is presented. A double-blind randomized parallel groups design is recommended, although cross-over designs may be used for the preliminary screening of agents. Subjects may have multiple sites scored. Sample size will be determined by estimating the variability in the study population, the effect to be detected and the power of the statistical test to be used. Subject selection is based on a clinical diagnosis of dentine hypersensitivity, excluding those with conflicting characteristics such as currently-active medical or dental therapy. The vestibular surfaces of incisors, cuspids and bicuspids are preferred as sites to be tested. A range of sensitivity levels should be included. Tactile, cold and evaporative air stimuli should be applied. Negative and benchmark controls should be incorporated. Most trials should last 8 weeks. Sensitivity may be assessed either in terms of the stimulus intensity required to evoke pain or the subjective evaluation of pain produced by a stimulus using a visual analog or other appropriate scale. The subject's overall assessment may be determined by questionnaire. Outcomes should be expressed in terms of clinically significant changes in symptoms. Follow-up evaluation is required to determine the persistence of changes. At least 2 independent trials should be conducted before a product receives approval.

Dental innervation and CGRP in adult p75-deficient mice

Adult dental tissues have unusual neurotrophin biology. Pulpal fibroblasts express nerve growth factor (NGF) and the low-affinity p75 neurotrophin receptor, their sensory nerve fibers express p75 and trk A, and pulpal sympathetic fibers lack p75. Following tooth injury, there is increased pulpal NGF, sprouting of sensory nerve endings, and increased immunoreactivity for the sensory neuropeptide calcitonin gene-related peptide (CGRP). In the present study, we have analyzed tooth structure and innervation of pulp and periodontal ligament in young (6-8 weeks, 3 months) and older (5-12 months) adult mice carrying a null mutation in the p75 gene and compared the results with those of age-matched wild-type controls. Our hypotheses were that tooth structure would be abnormal and that pulpal innervation would be greatly reduced because it consists primarily of nociceptive fibers that have been found to be severely depleted in skin of p75(-/-) mice. Tissues were fixed, X-rayed for gross dental morphology, decalcified, and analyzed for immunoreactivity for CGRP and for a general nerve marker, protein gene product 9.5. Radiographs showed worn-down molar crowns in p75-deficient mice. Light microscopy confirmed the accelerated molar wear and showed intense CGRP immunoreactivity in pulp nerve endings of mutant mice, compared with a gradual decrease in CGRP intensity in controls during normal aging. The CGRP intensity in 5-12-month-old pairs of mice was threefold greater in the mutants (P < 0.03), and in younger mice the mutant always had more CGRP than its matched control. The innervation of molar ligament in all p75-deficient mice was similar to that of controls except there was nerve sprouting near bone loss in mutants. The incisors of mutant mice did not have unusual wear and their pulpal CGRP immunoreactivity remained normal, but their periodontal ligament had fewer thin branched nerve endings at all ages. Thus, most innervation of teeth and their supporting tissues developed normally, and the only neural changes in p75(-/-) mutant mice were the reduction of incisor ligament sensory receptors and increased molar CGRP. Sensory nerves in teeth gradually lose neuropeptide intensity during aging, but that did not happen in the mutant mice, suggesting that the accelerated molar wear stimulated persistent high levels of CGRP.

Immunohistochemical localization of nerve fibres during development of embryonic rat molar using peripherin and protein gene product 9.5 antibodies

Nerve fibres were localized during the initiation and early morphogenesis of the first molar tooth in rat embryos by immunoperoxidase detection of the intermediate-filament protein peripherin and protein gene product 9.5 (PGP 9.5). Nerve fibres from the trigeminal ganglion were detected in the developing first branchial arch of E12-14 embryos. Nerves were not seen in the vicinity of the developing tooth germ before the buid stage (E15), when they were seen around the condensed dental mesenchyme. During transition from the bud to the cap stage (E15), nerve fibres were detected not only in the area of the future dental follicle but also in the mesenchyme next to dental epithelium on the buccal side of the tooth germ. During later cap and bell stages nerve fibres persisted in the dental follicle, but they were not seen in the epithelial dental organ or dental papilla mesenchyme. Absence of trigeminal nerve fibres from the presumptive tooth-bearing area indicates that they are not involved in the initiation of rat tooth development. In addition, the localization of nerve fibres shows that there are some differences in the innervation of rat teeth compared with human and mouse teeth. These results provide data for further studies on the regulation of embryonic rat tooth innervation.

Noxious stimuli do not determine reflex cardiorespiratory effects in anesthetized rabbits

The main purpose of this study is to examine whether the stimulation of an exclusively pain-sensing receptive field (dental pulp) could determine cardiorespiratory effects in animals in which the cortical integration of the peripheral information is abolished by deep anesthesia. In 15 anesthetized (alpha-chloralose and urethan) rabbits, low (3-Hz)- and high-frequency (100-Hz) electrical dental pulp stimulation was performed. Because this stimulation caused dynamic and static reflex contractions of the digastric muscles leading to jaw opening jaw-opening reflex (JOR); an indirect sign of algoceptive fiber activation], experimentally induced direct dynamic and static contractions of the digastric muscle were also performed. The low- and high-frequency stimulation of the dental pulp determined cardiovascular [systolic arterial pressure (SAP): -21.7 +/- 4.6 and 10.8 +/- 4.7 mmHg, respectively] and respiratory [pulmonary ventilation (VE): 145.1 +/- 44.9 and 109.3 +/- 28.4 ml/min, respectively] reflex responses similar to those observed during experimentally induced dynamic (SAP: -17.5 +/- 4.2 mmHg; VE: 228.0 +/- 58.5 ml/min) and static (SAP: 5.8 +/- 1.5 mmHg; VE: 148.0 +/- 75.3 ml/min) muscular contractions. The elimination of digastric muscular contraction (JOR) obtained by muscular paralysis did away with the cardiovascular changes induced by dental pulp stimulation, the effectiveness of which in stimulating dental pulp receptors has been shown by recording trigeminal-evoked potentials in six additional rabbits. The main conclusion was that, in deeply anesthetized animals, an algesic stimulus is unable to determine cardiorespiratory effects, which appear to be exclusively linked to the stimulation of ergoreceptors induced by muscular contraction.

Fluid shifts across human dentine in vitro in response to hydrodynamic stimuli

Most authorities agree that the hydrodynamic theory of dentine sensitivity best explains the stimulus-response relations of most painful stimuli. However, as the usual hydrodynamic stimuli are so different, it has been impossible to compare them. The equivalency of hydrodynamic stimuli can be evaluated from measurements of the fluid movement induced in vitro and relating this to the hydraulic conductance (Lp) of the same dentine specimen. From this determination, a common denominator is obtained which is equivalent to the hydrostatic pressure that would be required to cause the same magnitude of fluid movement. The purpose of this study was to measure the direction and magnitude of fluid shifts across dentine in extracted human crown segments with a flat, dentine occlusal surface in response to the following hydrodynamic stimuli; air blast, 56 degrees C water, 2 degrees C water, tactile and osmotic. In acid-etched superficial dentine, which simulates hypersensitive dentine, the largest to the smallest fluid flows obtained were: hot > cold > air blast > osmotic > tactile. When these were converted to equivalency units, the ranking of stimuli from strongest to weakest was hot > cold > air blast > osmotic > tactile. This new approach to comparing hydrodynamic stimuli should be verified in vivo.

Parvalbumin-immunoreactive nerve endings in the periodontal ligaments of rat teeth

Parvalbumin-immunoreactive nerve fibres were most abundant in the lingual periodontal ligaments of incisor teeth. In the part of the ligament adjacent to the alveolar bone, thick and smooth parvalbumin-immunoreactive nerve fibres left main nerve bundles and passed towards the part of the ligament adjacent to the tooth. In the ligament, halfway between the bone and tooth surfaces, these nerve fibres repeatedly branched and extended one to four twigs to produce bush-like endings. Parvalbumin-immunoreactive fibres and terminals were infrequent in the periodontal ligaments of the molar teeth. It is possible that parvalbumin-immunoreactive endings are periodontal mechanoreceptors, but at present it is difficult to account for the different representations around rat molar and incisor teeth.

Hypertension-associated hypalgesia. Evidence in experimental animals and humans, pathophysiological mechanisms, and potential clinical consequences

A behavioral hypalgesia (increased response threshold to noxious stimuli) has been consistently, although not invariably, reported in spontaneous and experimental acute and chronic hypertension in the rat. Studies in human hypertension have also demonstrated a diminished perception of pain, assessed as pain thresholds or ratings. The sensitivity to painful stimuli correlated inversely with blood pressure levels, and this relationship extended into the normotensive range. Evidence in humans and rats points to a role of the baroreflex system in modulating nociception. In the rat, blood pressure-related antinociception may be due to attenuated transmission of noxious stimuli at the spinal level secondary to descending inhibitory influences that are projected from brain stem sites involved in cardiovascular regulation and that may depend on baroreceptor activation and/ or on a central "drive." Both endorphinergic and noradrenergic central neurons (the latter acting through postsynaptic alpha 2-receptors) have been shown to be involved, and other mediators probably also play a role. Functionally, blood pressure-related antinociception may represent an aspect of a more-complex coordinated adaptive response of the body to "stressful" situations. It is still uncertain whether in human essential hypertension hypalgesia is secondary to elevated blood pressure or whether both depend on some common mechanism. Studies on the effect of hypotensive treatment are too few to allow conclusions. According to one hypothesis, the reduction in pain perception caused by baroreceptor activation secondary to blood pressure elevation may represent a rewarding mechanism that may be reinforced with repeated stress and may be involved in the development of hypertension in some individuals. Hypertension-associated hypalgesia may have clinically relevant consequences, especially in silent myocardial ischemia and unrecognized myocardial infarction, both of which are more prevalent in hypertensive individuals.

Periodontal and gastric convergences within the hypothalamic ventromedial nucleus area--single unit study on anesthetized rats

Unitary activities were recorded in the ventromedial nucleus of the hypothalamus (VHM) of anesthetized rats. Cells responding to periodontal stimulation (100-200 g disto-mesial traction applied to an upper incisive) were selected. The effects of gastric stimulation (2-5 ml distension) were then investigated. Out of the 40 cells activated (22 cells) or inhibited (18 cells) by periodontal stimulation, only seventeen were influenced by gastric stimulation. Eight of them responded in the same way and nine in the opposite way. Unlike the periodontal stimulation, which elicited specific spatio-temporal patterns, the gastric stimulation had only weak effects. These data nevertheless demonstrate that periodontal-gastric convergences exist in the VHM nucleus, which is consistent with the role previously ascribed to this area in alimentary behaviour.

An in vitro jaw-nerve preparation for oral sensory study in the rat

We have developed an in vitro jaw-nerve preparation in Wistar albino rats, suitable for the quantitative study of peripheral sensory mechanisms in the oral region. The mandible on one side was excised together with the inferior alveolar nerve and was kept in a modified Krebs-Henseleit solution saturated with O2/CO2 (95:5) gas mixture. Recordings were made from single afferent fibers in the inferior alveolar nerve in response to mechanical stimulation of various oral sites (oral mucosa, periodontal ligament and tooth pulp) by calibrated von Frey hairs. In the case of tooth pulp stimulation, heat and chemical (bradykinin) stimuli were also used. Stable unitary recordings could be made for up to 5 h which was long enough to record the activities evoked by various stimuli applied to the oral area. This paper will describe the procedure for making this preparation together with some responses of single units in the inferior alveolar nerve to mechanical, thermal and chemical stimulations applied to this preparation.

Nerve growth factor depletion reduces collateral sprouting of cutaneous mechanoreceptive and tooth-pulp axons in ferrets

1. Electrophysiological experiments were carried out to determine whether or not collateral sprouting of cutaneous low-threshold mechanoreceptive fibres could be detected and to investigate the effect of nerve growth factor (NGF) deprivation on the sprouting of these fibres and the fibres innervating tooth pulps. 2. In twenty-one ferrets (eleven of which had been autoimmunized against NGF) the right inferior alveolar nerve (IAN) was sectioned and prevented from regenerating. After 12 weeks, transmedian innervation from the left IAN was determined by stimulating the nerve whilst recording from electrodes implanted in the contralateral anterior teeth and also by single unit recordings from the nerve whilst mechanically and electrically stimulating the skin. The results were compared with those from ten control animals. 3. Transmedian innervation of contralateral teeth was found in none of the control animals; in all ten of the animals which had undergone denervation without immunization (4/10 canines, 17/20 incisors); but in only six of the eleven immunized and denervated animals (0/11 canines, 7/22 incisors). 4. Of 270 cutaneous mechanoreceptive units sampled in the controls, only four units had transmedian receptive fields, extending a maximum of 1 mm across the mid-line. After denervation, significantly more units (42 of 274) crossed the mid-line and extended up to 4 mm. After immunization and denervation only eleven of 305 units crossed the midline by a maximum of 1 mm. 5. These data show that cutaneous low-threshold mechanoreceptive A beta and A delta fibres, as well as A delta tooth pulp fibres, are able to undergo collateral sprouting. This sprouting is partially blocked by NGF depletion, suggesting that NGF plays an essential role in the process.

Convergence of afferents from superior sagittal sinus and tooth pulp on cells in the upper cervical spinal cord of the cat

Units in the dorsolateral area of the upper cervical cord respond to craniovascular stimulation. This study examined tooth pulp responses in this area in cats. Eleven of 21 units tested in the dorsolateral area had convergent inputs from superior sagittal sinus and tooth pulp; while 10 units had sagittal sinus, but not tooth pulp, input. Mean response latency to tooth pulp stimulation (25.8 ms) was significantly longer than to superior sagittal sinus stimulation (9.8 ms). Half of the units had cutaneous receptive fields; and in five units, action potentials could be evoked by electrical stimulation in the posterior complex of the thalamus.

Morphology and synaptic connections of slowly adapting periodontal afferent terminals in the trigeminal subnuclei principalis and oralis of the cat

Previous studies suggest that sensory information from primary afferent fibers is processed in a distinct manner in the individual subnuclei of trigeminal sensory nuclear complex. The present study has addressed this issue by using intra-axonal labeling with horseradish peroxidase to examine the ultrastructure and synaptic organization of axon terminals from slowly adapting (SA) periodontal afferents in the ventral subdivision (Vpv) of principalis and the rostro-dorsomedial part (Vo.r) of oralis. Our observations are based on complete or near-complete reconstructions of 139 synaptic boutons in Vpv and 105 in Vo.r. All the labeled boutons contained clear, spherical, synaptic vesicles and were presynaptic to unlabeled dendrites, and they were frequently postsynaptic to unlabeled axon terminals containing pleomorphic synaptic vesicles (P-endings). The P-endings frequently formed axodendritic synapses on dendrites which received axodendritic synapses from labeled boutons (synaptic triads). On the basis of the number of contacts, synaptic arrangements associated with the labeled boutons could be subgrouped into simple (one or two contacts), intermediate (three or four contacts), and complex (five or more contacts) types. The labeled boutons varied from round to elongated forms with smooth to more irregular or scalloped contours. The boutons with scalloped contour were much more frequent in the complex type. The boutons of the intermediate type were significantly smaller than the complex type and larger than the simple type. The SA periodontal afferent terminals participated in each type of synaptic arrangements in Vpv, but were mostly of the simple type in Vo.r. The size of labeled boutons was significantly larger in Vpv than in Vo.r. The total number of axodendritic and axoaxonic contacts per labeled bouton was significantly higher in Vpv than in Vo.r. Another difference was the more frequent occurrence of synaptic triads in Vpv than in Vo.r. These observations provide evidence that sensory information from primary afferent fibers is processed in a different manner in the two subnuclei.

The effect of temperature on the discharge of periodontal ligament mechanoreceptors in the cat canine tooth

The response of periodontal ligament mechanoreceptors to mechanical stimuli was studied at different temperatures in anaesthetized cats. Recordings were made from functionally single fibres in the inferior alveolar nerve from mechanoreceptors which were located in the ligament by dissection and punctate stimulation and electrical stimulation. A thermode was used to alter and maintain small areas of periodontal ligament at temperatures ranging from thermode tip temperatures of 3 degrees C to 57 degrees C. The response to ramp-plateau mechanical stimuli applied to the tip of the crown of the canine tooth was greatest when the thermode temperature was 37.5 degrees C +/- 6.5 degrees C. The effect was similar for all receptors studied.