The effect of dentinal stimulation on pulp nerve function and pulp morphology in the dog

Thermal Pain in Teeth: Electrophysiology Governed by Thermomechanics

Thermal pain arising from the teeth is unlike that arising from anywhere else in the body. The source of this peculiarity is a long-standing mystery that has begun to unravel with recent experimental measurements and, somewhat surprisingly, new thermomechanical models. Pain from excessive heating and cooling is typically sensed throughout the body through the action of specific, heat sensitive ion channels that reside on sensory neurons known as nociceptors. These ion channels are found on tooth nociceptors, but only in teeth does the pain of heating differ starkly from the pain of cooling, with cold stimuli producing more rapid and sharper pain. Here, we review the range of hypotheses and models for these phenomena, and focus on what is emerging as the most promising hypothesis: pain transduced by fluid flowing through the hierarchical structure of teeth. We summarize experimental evidence, and critically review the range of heat transfer, solid mechanics, fluid dynamics, and electrophysiological models that have been combined to support this hypothesis. While the results reviewed here are specific to teeth, this class of coupled thermomechanical and neurophysiological models has potential for informing design of a broad range of thermal therapies and understanding of a range of biophysical phenomena.

Mechanotransducers in rat pulpal afferents

The hydrodynamic theory suggests that pain associated with stimulation of a sensitive tooth ultimately involves mechanotransduction as a consequence of fluid movement within exposed dentinal tubules. To determine whether putative mechanotransducers could underlie mechanotransduction in pulpal afferents, we used a single-cell PCR approach to screen retrogradely labeled pulpal afferents. The presence of mRNA encoding BNC-1, ASIC3, TRPV4, TRPA1, the alpha, beta, and gamma subunits of ENaC, and the two pore K+ channels (TREK1, TREK2) and TRAAK were screened in pulpal neurons from rats with and without pulpal inflammation. ASIC3, TRPA1, TREK1, and TREK2 were present in approximately 67%, 64%, 14%, and 10% of pulpal neurons, respectively. There was no detectable influence of inflammation on the proportion of neurons expressing these mechanotransducers. Given that the majority of pulpal afferents express ASIC3 and TRPA1, our results raise the possibility that these channels may be novel targets for the treatment of dentin sensitivity.

The relationship between the discharge of intradental nerves and the rate of fluid flow through dentine in the cat

OBJECTIVE

To investigate further the relationship between dentinal tubular flow and the discharge evoked in intradental nerves.

DESIGN

In anaesthetised cats, recordings were made of fluid flow through dentine during the application of hydrostatic pressure stimuli of 5 s duration in the range +500 to -500 mm Hg to exposed dentine and of the nerve impulses evoked by these stimuli. Single unit recordings were obtained from filaments dissected from the inferior alveolar nerve and multi-unit recordings, from the exposed dentine.

RESULTS

Of 20 single units tested, 10 (conduction velocities: 2.4-36.2 m s(-1)) responded to negative pressures and four of these, also to positive pressures. None responded to only positive pressures. The pressure thresholds of the units (single and multi-unit preparations) ranged from -100 to -500 and +100 to +500. In terms of flow (measured 1 s after the start of a stimulus) the thresholds ranged from 0.4 to 2.2 nl s(-1) mm(-2) exposed dentine with outward flow, and 0.4-2.1 nl s(-1) mm(-2) with inward flow. The outward flow per tubule at the threshold of the most sensitive units was estimated to be 21 fl s(-1) and the corresponding mean velocity of the contents of the dentinal tubules at their pulpal ends, 27 microm s(-1). Although the thresholds to outward and inward flow were similar, with outward flow the mean discharge rate increased with stimulus intensity; whereas with inward flow few impulses were evoked and the number was little affected by the stimulus intensity.

CONCLUSION

The transduction mechanism that generates impulses in hydrodynamic intradental afferents is much more responsive to outward than inward flow through the dentinal tubules, although the thresholds in both directions are similar.

Pulpal regeneration after cavity preparation, with special reference to close spatio-relationships between odontoblasts and immunocompetent cells

The regeneration process of the odontoblast cell layer incident to tooth injury, especially its relationship with immunocompetent cells in pulp healing, has not been fully understood. The purpose of the present study was to clarify this relationship between odontoblasts and immunocompetent cells in the process of pulp regeneration following cavity preparation in rat molars by immunocytochemistry for heat shock protein (Hsp) 25 as well as class II major histocompatibility complex (MHC) molecules. In untreated control teeth, intense Hsp 25-immunoreactivity was found in the cell bodies of odontoblasts and their processes within the predentin, whereas class II MHC-positive cells were predominantly located beneath the odontoblast cell layer. Cavity preparation caused the destruction of the odontoblast layer to form an edematous lesion and the shift of class II MHC-positive cells with the injured odontoblasts toward the pulp core at the affected site. Some damaged odontoblasts without apparent cytoplasmic processes, round in profile, retained the immunoreactivity for Hsp25, suggesting the survival of a part of the odontoblasts against artificial external stimuli. Twelve hours after cavity preparation, numerous class II MHC-positive cells appeared along the pulp-dentin border and extended their processes deep into the exposed dentinal tubules. By postoperative 72 hours, newly differentiated odontoblasts with Hsp 25-immunoreactivity were arranged at the pulp-dentin border, but the class II MHC-positive cells moved from the pulp-dentin border to the subodontoblastic layer. These findings indicate that the time course of changes in the expression of Hsp 25-immunoreactivity reflects the regeneration process of odontoblasts. The functional roles of Hsp 25-positive odontoblasts and immunocompetent cells such as class II MHC-positive cells in the process of pulp regeneration after cavity preparation are discussed in conjunction with our previous experimental data.

Use of noninvasive dental dolorimetry to evaluate analgesic effects of intravenous and intrathecal administration of morphine in anesthetized dogs

OBJECTIVE

To determine whether changes in amplitude of the reflex-evoked muscle action potential (REMP) elicited by noninvasive dental dolorimetry (electrical stimulation of the tooth pulp) in anesthetized dogs may be used to objectively evaluate the effectiveness of IV and intrathecal (IT) administration of morphine.

ANIMALS

6 male Beagles that were 2 to 6 years old.

PROCEDURE

Dogs were used in a crossover design with at least a 5-day washout period between treatments. Each dog received morphine, saline (0.9% NaCl) solution, and oxytocin via the IV and IT routes of administration; however, only results for morphine and saline treatments were reported here. Dogs were anesthetized and prepared for noninvasive dental dolorimetry. After IV or IT administration, electrical stimulation was applied to a tooth, and REMPs of the digastricus muscle were recorded at 5-minute intervals for 60 minutes. To determine differences in REMP amplitude between treatments, a linear regression line was fitted for each dog-treatment combination.

RESULTS

The IV administration of morphine significantly inhibited REMP amplitude, compared with IV administration of saline solution. Intrathecal administration of morphine significantly inhibited REMP amplitude, compared with IT administration of saline solution.

CONCLUSIONS AND CLINICAL RELEVANCE

Noninvasive dental dolorimetry in anesthetized dogs has promise as a technique for use in evaluating the analgesic potential of drugs administered IV and IT through evaluation of their effect on REMP amplitude recorded for the digastricus muscle.

Nociceptive behaviour induced by dental application of irritants to rat incisors: a new model for tooth inflammatory pain

Animal models simulating acute human pulpitis are still lacking. The rat incisors present a particular situation where most of their innervation is considered to be unmyelinated and concentrated mainly in the tooth pulp. This study reports on a new model for dental pain induced by inflammatory agents applied to the tooth pulps of incisors. In different groups of rats, artificial crowns were fixed on the lower incisors, after cutting 1-2mm of their distal extremities. A volume of 7-10 microl of solutions of saline, capsaicin (1-10mg/ml) or formalin (2.5% or 5%) was injected in the crown cavity, and the nociceptive behaviour was quantitated following a devised scoring method of four scales. Intradental application of capsaicin produced nociceptive scores in the form of one plateau for 1-2h depending on the concentration used. Similar results were obtained with intradental application of formalin 2.5%. The one plateau of nociceptive scores obtained with formalin contrasts with the biphasic aspect of nociceptive behaviour described with the intradermal formalin test. This discrepancy could be attributed to a difference in the types of afferent fibres involved in each situation. Pretreatment with morphine (2 mg/kg) attenuated, in a naloxone-reversible manner, the nociceptive behaviour observed following intradental application of capsaicin. Pretreatment with meloxicam (a cyclo-oxygenase-2 inhibitor) exerted a less pronounced attenuation of the nociceptive scores when compared with morphine. These results provide evidence for the validity of the described model for the simulation of tooth pulp inflammatory pain in awake animals.

Responses of odontoblasts to cavity preparation in rat molars as demonstrated by immunocytochemistry for heat shock protein (Hsp) 25

Responses of odontoblasts to cavity preparation in rat molars were investigated by immunocytochemistry for heat shock protein (Hsp) 25. In untreated control teeth, intense Hsp 25-immunoreactivity was found in the cell bodies of odontoblasts and their processes within the predentin. Confocal microscopy of Hsp 25-immunostained and rhodamine-labeled sections revealed that the immunoreactive odontoblasts were intensely labeled for phalloidin at the periphery of their cytoplasm and throughout their processes, but the reaction for phalloidin was limited within the inner half of the dentin. Cavity preparation caused an edematous reaction between the injured odontoblasts and predentin as well as a beaded swelling and successive destruction of the odontoblast processes. Immediately after cavity preparation, the odontoblasts beneath the edematous lesion showed an immunoreactivity for Hsp 25, which subsequently disappeared completely from the pulp-dentin border by 12 h after the operation. However, round cells without apparent cytoplasmic processes continued to be immunoreactive, suggesting the survival of a part of the odontoblasts against preparation stimuli. Numerous phalloidin-reactive but Hsp 25-immunonegative cells appeared along the pulp-dentin border and extended their processes deep into the exposed dentinal tubules, probably categorized in a lineage of immunocompetent cells. By postoperative 72 h, newly differentiated odontoblasts with Hsp 25-immunoreactivity were arranged at the pulp-dentin border. These findings indicate that the time course of changes in the expression of Hsp 25-immunoreactivity reflects the regeneration process of odontoblasts, and suggest that this protein is a useful marker substance for differentiated odontoblasts.

Dental injury models: experimental tools for understanding neuroinflammatory interactions and polymodal nociceptor functions

Recent research has shown that peripheral mechanisms of pain are much more complex than previously thought, and they differ for acutely injured normal tissues compared with chronic inflammation or neuropathic (nerve injury) pain. The purpose of the present review is to describe uses of dental injury models as experimental tools for understanding the normal functions of polymodal nociceptive nerves in healthy tissues, their neuroinflammatory interactions, and their roles in healing. A brief review of normal dental innervation and its interactions with healthy pulp tissue will be presented first, as a framework for understanding the changes that occur after injury. Then, the different types of dental injury that allow gradation of the extent of tissue damage will be described, along with the degree and duration of inflammation, the types of reactions in the trigeminal ganglion and brainstem, and the type of healing. The dental injury models have some unique features compared with neuroinflammation paradigms that affect other peripheral tissues such as skin, viscera, and joints. Peripheral inflammation models can all be contrasted to nerve injury studies that produce a different kind of neuroplasticity and neuropathic pain. Each of these models provides different insights about the normal and pathologic functions of peripheral nerve fibers and their effects on tissue homeostasis, inflammation, and wound healing. The physical confinement of dental pulp and its innervation within the tooth, the high incidence of polymodal A-delta and C-fibers in pulp and dentin, and the somatotopic organization of the trigeminal ganglion provide some special advantages for experimental design when dental injury models are used for the study of neuroinflammatory interactions.

Fibrinogen/fibrin and fibronectin in the dentin-pulp complex after cavity preparation in rat molars

OBJECTIVE

The purpose of this study was to examine changes in distribution of fibrinogen/fibrin and fibronectin in the dentin-pulp complex after cavity preparation.

STUDY DESIGN

Class V cavity preparations were prepared on maxillary first molars of 12 rats. The dentin and pulps were observed histologically and immunohistochemically for fibrinogen and fibronectin at 6 hours and 1, 2, and 3 days after the preparation.

RESULTS

At 6 hours and 1 day after cavity preparation, positive staining for fibrinogen was noted in the exudative lesion and in the dentinal tubules under the cavity preparation. Fibronectin staining in the exudate showed a pattern with close similarity to the fibrinogen staining. At 3 days after cavity preparation, the irregularly shaped predentin under the cavity preparation showed strong positive staining for fibronectin.

CONCLUSIONS

Fibrinogen/fibrin and fibronectin are present during the healing process of the dentin-pulp complex after cavity preparation.

Teeth and tooth nerves

(1) Although our knowledge on teeth and tooth nerves has increased substantially during the past 25 years, several important issues remain to be fully elucidated. As a result of the work now going on at many laboratories over the world, we can expect exciting new findings and major break-throughs in these and other areas in a near future. (2) Dentin-like and enamel-like hard tissues evolved as components of the exoskeletal bony armor of early vertebrates, 500 million years ago, long before the first appearance of teeth. It is possible that teeth developed from tubercles (odontodes) in the bony armor. The presence of a canal system in the bony plates, of tubular dentin, of external pores in the enamel layer and of a link to the lateral line system promoted hypotheses that the bony plates and tooth precursors may have had a sensory function. The evolution of an efficient brain, of a head with paired sense organs and of toothed jaws concurred with a shift from a sessile filter-feeding life to active prey hunting. (3) The wide spectrum of feeding behaviors exhibited by modern vertebrates is reflected by a variety of dentition types. While the teeth are continuously renewed in toothed non-mammalian vertebrates, tooth turnover is highly restricted in mammals. As a rule, one set of primary teeth is replaced by one set of permanent teeth. Since teeth are richly innervated, the turnover necessitates a local neural plasticity. Another factor calling for a local plasticity is the relatively frequent occurrence of age-related and pathological dental changes. (4) Tooth development is initiated through interactions between the oral epithelium and underlying neural crest-derived mesenchymal cells. The interactions are mediated by cell surface molecules, extracellular matrix molecules and soluble molecules. The possibility that the initiating events might involve a neural component has been much discussed. With respect to mammals, the experimental evidence available does not support this hypothesis. In the teleost Tilapia mariae, on the other hand, tooth germ formation is interrupted, and tooth turnover ceases after local denervation. (5) Prospective dental nerves enter the jaws well before onset of tooth development. When a dental lamina has formed, a plexus of nerve branches is seen in the subepithelial mesenchyme. Shortly thereafter, specific branches to individual tooth primordia can be distinguished. In bud stage tooth germs, axon terminals surround the condensed mesenchyme and in cap stage primordia axons grow into the dental follicle.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of immunocompetent cells to cavity preparation in rat molars: an immunohistochemical study using OX6-monoclonal antibody

Responses of immunocompetent cells, especially class II major histocompatibility complex (MHC) antigen-expressing cells, were investigated after cavity preparation in the erupted upper first molar teeth of rats, by immunohistochemistry using OX6-monoclonal antibody. In control teeth, OX6-immunopositive cells were predominantly located beneath the odontoblast layer in the dental pulp. Cavity preparation caused an acute edematous reaction between the injured odontoblasts and predentin, and most of OX6-immunopositive cells in the affected site shifted away from the pulp-dentin border. After 12-24 hours, many OX6-immunopositive cells accumulated along the pulp-dentin border and extended their cytoplasmic processes into the exposed dentinal tubules. After 72 hours, newly differentiated odontoblasts replaced the degenerated odontoblasts, and few OX6-immunopositive cells remained along the pulp-dentin border. Our data suggest that some of the class II MHC antigen-expressing cells in the dental pulp participate in the initial defense reaction and presumably serve as a biological sensor for the external stimuli arriving through the exposed dentinal tubules.

Dynamic plasticity of dental sensory nerve structure and cytochemistry

Hypersensitive dentine responds to normal changes in touch or temperature with abnormal pain sensations. This paper reviews studies that have shown dynamic changes in sensory nerve structure, cytochemistry and location after tooth injury, suggesting that those changes contribute to dentine hypersensitivity. Nerve fibres containing calcitonin gene-related peptide (CGRP) are the main type of sensory fibre to innervate dentine. Evidence that many of those dentinal nerve endings originate from small myelinated fibres is presented here. The location of CGRP nerve terminals correlates with the pulpal gradients of nerve growth factor that have been demonstrated in normal teeth by in situ hybridization histochemistry. When shallow cavities are drilled into the outer dentine of rat molars a five-to-eight-fold increase in pulpal nerve growth factor precedes the extensive structural changes in the sensory nerve reactions eventually subside if healing occurs, but both continue if inflammation continues. Evidence correlating pulpal inflammation with long-term changes in central trigeminal pain pathways is reviewed. There can be extensive neuroplasticity after tooth injury, both within dental pain fibres and in central pain pathways. The timing of those alterations of nerve structure, location, and cytochemistry is consistent with their involvement in mechanisms of dentine hypersensitivity.

Pulpal effects of argon: fluoride excimer laser irradiation and acid-etching of rat molar enamel

The reaction of enamel, dentine and pulpal tissues to exposure from a laser beam has been shown to depend on the type of laser medium used. The objective of this study was to examine the pulpal response in rat molars after external enamel surface treatment with either an Ar:F excimer laser or acid-gel application. Maxillary right molar occlusal surfaces in 22 animals were irradiated (energy density = 45.0 J/cm2). Maxillary left molar occlusal surfaces were treated with 37 per cent phosphoric acid for 30 s. Untreated mandibular right molars served as controls. At two postoperative time periods (1 and 6 weeks), molars were removed, sectioned, stained (H&E) and scored. Data analysis indicated no significant difference between Ar:F irradiation and controls at 1 week. Treatment with laser or acid-etching left a similar degree of pathosis at 1 and 6 weeks. Although the Ar:F excimer laser produced a more exaggerated pulp response than controls at 6 weeks, tissue vitality was maintained. The Ar:F excimer laser may be useful for ablating vital tooth structure since pulpal tissue in rat molars exhibited no damage in response to low-power irradiation.

Involvement of afferent nerves in pulpal blood-flow reactions in response to clinical and experimental procedures in the cat

A unilateral resection of the mandibular nerve (n = 20) was made 10-14 days before investigation of the contribution of afferent nerves in vasodilator reactions in the dental pulp. Lower canine teeth were subjected to various stimuli and pulp blood-flow responses monitored by laser Doppler flowmetry. An absence of response to bipolar electrical (5 impulses, 50 microA, 5 ms, 2 Hz) stimulation on the tooth surface was used to demonstrate a successful chronic nerve lesion. Local application of capsaicin (10(-4) M) in a deep dentinal cavity induced a long-lasting increase in pulpal blood flow in control teeth only. Bradykinin (10(-3) M) induced significantly larger responses in control than in denervated teeth (58.3 +/- 9.8% and 24.5 +/- 4.9%, respectively, p less than 0.005, n = 8); in addition, the onset was slower and the duration of the response significantly (60%) shorter than in control teeth. Intermittent grinding of surface dentine instantly increased flow in control teeth by 53.0 +/- 12.5% (n = 12) whereas in denervated teeth the response was delayed and significantly (70%) smaller. Deeper preparation produced responses of similar magnitude in control and denervated teeth (69 and 50%, respectively) but the onset was delayed in denervated teeth. Low-intensity ultrasonic stimulation caused vasodilation in intact teeth (38% increase) but had no effect in denervated teeth. This effect was abolished after local anaesthetic (mepivacaine) injection. Sympathectomy (n = 3) did not influence stimulation-induced blood-flow responses in the dental pulp.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration of physiological barrier between pulpal odontoblasts and its perturbation following routine restorative procedures: a horseradish peroxidase tracing study in the rat

Vascular injection of the macromolecular tracer, horseradish peroxidase (HRP), was used to study the permeability of the odontoblast cell layer in developing and mature rat molar teeth, and to investigate the effect of cavity preparations on the permeability of this epithelioid cell layer in adult animals. HRP injected into the vascular system of normal animals 28 days of age and older was localized histochemically (from 5 to 90 min after injection) throughout the extracellular spaces of the maxillary dental pulps; however, the tracer did not penetrate beyond the tight junctions at the apical region of the odontoblast cell layer, and was absent from the predentin and dentin. In contrast, HRP injected into very young neonatal animals (e.g., day 3) resulted in free passage of HRP between odontoblasts and into the overlying predentin and dentin. When Class V cavities had been prepared in adult maxillary molars after HRP was injected into the blood stream, HRP reaction product penetrated the predentin and dentin immediately beneath the cavity preparation; however, adjacent, untraumatized areas of predentin and dentin in the operated teeth were devoid of reaction product. These results provide evidence that: (1) a physiological barrier develops between the distal segments of odontoblast cell bodies in normal rat molar teeth between days 15 and 28 of postnatal life, and this barrier prevents the passage of macromolecules from the pulp into the predentin and dentin; and (2) this barrier is perturbed following routine restorative procedures in adult animals.

Acute and chronic reactions of dental sensory nerve fibers to cavities and desiccation in rat molars

We have studied the effects of air drying of exposed, acid etched dentin on the sensory innervation of rat molars. In the acute series of experiments, trigeminal nerve fibers were labeled by axonal transport of radioactive protein prior to the dentin exposure and desiccation, the anesthetized rats were fixed by aldehyde perfusion 10 min later, and the teeth were prepared for autoradiography. The results confirmed the hydrodynamic theory by showing outward movement of labeled nerve material in response to dentinal drilling and desiccation. It also showed that some odontoblasts could be separated from the dentinal nerve fibers. In the chronic series, teeth were injured 25 h, 5-7 days, or 21 days prior to fixation and nerves were labeled during the last 24 hours; the surviving vital nerve fibers were evident because of their axonal transport of the radioactive label. In that series, sensory nerve fibers were found to have been lost from areas with newly-formed reparative dentin, or from dentinal tubules that had lost their odontoblasts. In the teeth injured 25 h, 5-7 days, or 21 days earlier, an abnormal nonneuronal labeling occurred 0.2-0.3 mm into injured dentin. Our results are discussed in relation to the hydrodynamic theory, nerve-odontoblast interactions, differences between shallow and deep cavity injuries, altered nerve location in response to pulpal or dentinal injury, and characteristics of the pulp-dentin border.

The response of dog intradental nerves to hypertonic solutions of CaCl2 and NaCl, and other stimuli, applied to exposed dentine

Responses of single nerve units from the canine and incisor pulps of anaesthetized beagles to CaCl2 (3.5 M, 4.9 M and saturated) and NaCl (2.5 M and 4.0 M) were recorded. The sensitivity of these nerve units to drilling and probing of dentine, and to drying with air blasts, was also studied. Twenty-one out of 22 units responded immediately, either with a few spikes or, sometimes, with a 2-5 s train of impulses, to hypertonic CaCl2 when applied to superficial dentine. Deep in dentine, CaCl2 induced immediately responses in 15 out of 21 units. There were responses to hypertonic NaCl in 15 out of 19 units, but only when applied deep in dentine. This firing had a latency of 15-300 s (mean 94 s) and continued until the solution was washed away. Units sensitive to CaCl2 also responded to drilling, probing and drying. When applied to the exposed pulp, CaCl2 never induced nerve activity, but hypertonic NaCl induced responses in all units tested (n = 19); the latencies were 0-300 s (mean 34 s). The mechanism of nerve activation in response to hypertonic CaCl2 is probably hydrodynamic, and common to several other stimuli as in drilling, probing and air drying. Responses to hypertonic NaCl may have been induced by a direct excitatory effect of Na+-ions on the nerve endings or axons in the pulp-dentine border.