Responses of feline intradental sensory nerves to hyperosmotic stimulation of dentin

Sensory ability in the narwhal tooth organ system

The erupted tusk of the narwhal exhibits sensory ability. The hypothesized sensory pathway begins with ocean water entering through cementum channels to a network of patent dentinal tubules extending from the dentinocementum junction to the inner pulpal wall. Circumpulpal sensory structures then signal pulpal nerves terminating near the base of the tusk. The maxillary division of the fifth cranial nerve then transmits this sensory information to the brain. This sensory pathway was first described in published results of patent dentinal tubules, and evidence from dissection of tusk nerve connection via the maxillary division of the fifth cranial nerve to the brain. New evidence presented here indicates that the patent dentinal tubules communicate with open channels through a porous cementum from the ocean environment. The ability of pulpal tissue to react to external stimuli is supported by immunohistochemical detection of neuronal markers in the pulp and gene expression of pulpal sensory nerve tissue. Final confirmation of sensory ability is demonstrated by significant changes in heart rate when alternating solutions of high-salt and fresh water are exposed to the external tusk surface. Additional supporting information for function includes new observations of dentinal tubule networks evident in unerupted tusks, female erupted tusks, and vestigial teeth. New findings of sexual foraging divergence documented by stable isotope and fatty acid results add to the discussion of the functional significance of the narwhal tusk. The combined evidence suggests multiple tusk functions may have driven the tooth organ system's evolutionary development and persistence.

Pain in the trigeminal system: irritation of the nasal mucosa using short- and long-lasting stimuli

The paper describes methods which allow intranasal irritation using short- and long-lasting painful stimuli in humans. Short-lasting pain is induced by gaseous CO(2), while long-lasting pain is induced by a stream of dry air. Both models have been explored regarding their major determinants, e.g. stimulus duration, stimulus intensity, or repeated stimulation. Short-lasting, non-inflammatory pain stimuli seem to provide specific indicators of A(delta)-fiber function, while responses to long-lasting, inflammatory pain appear to be indicative of C-fiber function. Responses to both types of painful stimuli are modulated by analgesic drugs. As these well-investigated models allow the detailed and precise analysis of modulatory effects on intranasal nociception, they appear to be suited for the investigation of subtle changes of intranasal irritation, e.g. induced by environmental agents.

Permeability changes of dentine treated with titanium tetrafluoride

OBJECTIVES

The aim of this research was to evaluate the effect of titanium tetrafluoride on dentine permeability in comparison with sodium fluoride and acidulated phosphate fluoride to determine their contribution to acid resistance of dentine.

METHODS

Thirty-two dentine discs were prepared from extracted mandibular molars. The hydraulic conductance of all discs was measured before and after the formation of smear layer. The discs were then randomly divided into five experimental and one-control groups. Coronal surfaces of dentine discs were treated with either fluoride solutions of NaF, APF, 1% TiF4, 0.5% TiF4 and 0.1% TiF4 or de-ionised water. Following the measurement of hydraulic conductance, treated surfaces were subjected to 25% citric acid application. Then, final permeability measurements were made. The data were statistically analysed using ANOVA and Tukey's HSD multiple comparisons.

RESULTS

While smear layer formation considerably reduced dentinal permeability of dentine discs, fluoride or de-ionised water application to smeared surfaces did not cause any significant change in hydraulic conductance (p > 0.05). After citric acid application, control and NaF groups showed higher permeability values than 1%, 0.5% and 0.1% TiF4 and APF groups (p < 0.01).

CONCLUSION

Clinical use of acidic solutions of titanium tetrafluoride in dentine cavities may be considered since smeared dentine surfaces are modified to a stabile and acid-resistant state.

The neurophysiological basis and the role of inflammatory reactions in dentine hypersensitivity

Recent studies indicate that intradental A-type nerve fibres are responsible for the sensitivity of dentine and are activated by fluid movements in dentinal tubules (hydrodynamic mechanism). The patency of the tubules affects dentine sensitivity to a great extent. Both A delta- and A beta-type nerve fibres respond to dentinal (hydrodynamic) stimulation in a similar way. Only a few studies have been made on the regional sensitivity of dentine or the receptive areas of intradental nerve fibres. The results indicate that the fibres innervating different parts of coronal dentine are equally sensitive to dentinal stimulation but those in the cervical area may be less responsive. Inflammation in the pulp can considerably alter dentine sensitivity. In dog teeth with chronically exposed dentine, nerve responses to hydrodynamic stimulation were reduced although other functional changes indicated nerve sensitization. This may be due to spontaneously occurring changes in the exposed dentine that block the tubules. In acute experiments on cat and dog teeth with open dentinal tubules, certain inflammatory mediators increase the sensitivity of the responding nerve fibres. It seems that intradental C-fibres do not respond to hydrodynamic stimulation of dentine. They are polymodal and activated when external stimuli reach the pulp proper. They could perhaps mediate the dull pain connected with pulpitis. However, they might also have an important modifying effect on dentine sensitivity because they can release neuropeptides, which function in the inflammatory reactions.

Assessment of pain in cervical dentinal sensitivity studies. A review

Traditionally cervical dentinal sensitivity (CDS) has been evaluated mainly subjectively on the basis of the individual patient's subjective response, e.g., in the form of verbal rating and visual analogue scales and questionnaires. The stimuli used for evaluating this response can be grouped into 4 main categories: mechanical, chemical, electrical and thermal. This review of the literature, however, indicates that there are problems in evaluating patient subjective response to these various test stimuli used in the assessment and treatment of CDS. Opinions also vary as to the reliability of some of these methods of assessment, although recently, efforts have been made to develop controlled reproducible stimuli more suited to the evaluation of CDS. Currently no single method of eliciting and assessing CDS may be considered ideal. Further research is required to evaluate suitable methodology for the quantification of realistic test stimuli under controlled clinical conditions, whereby the subjective response may be objectively measured by the investigator.

Sensitizing effects of leukotriene B4 on intradental primary afferents

Previous studies have established that leukotriene B4 (LTB4) sensitizes cutaneous nociceptors. In this study the effects of LTB4 on spontaneous and stimulus-evoked nerve activity from primary afferents innervating the dentin of canines in adult cats were examined. LTB4 treatment (25 micrograms/ml) significantly enhanced stimulus-evoked intradental nerve activity (INA) for at least 20 min after the removal of the compound from the recording preparation. Teeth treated with LTB4 demonstrated enhanced spontaneous nerve activity following the removal of hypertonic saline used to evoked INA. These findings provide additional evidence that LTB4 is able to sensitize nociceptors and may be a long-lasting hyperalgesic factor which may contribute to pain of pulpal origin.

An in vivo study of dentin sensitivity: the relation of dentin sensitivity and the patency of dentin tubules

Dentin hypersensitivity can be a major problem for periodontal patients. The relationship between dentin hypersensitivity and the patency of dentin tubules in vivo has been established. Thirteen adult patients with teeth scheduled for extraction were selected and a stent fabricated to confirm location of the root surface being examined. Response to a constant air blast was recorded on a numeric pain rating scale (from 0 to 4) during the course of treatment. The tooth surface was initially treated with 0.5 molar EDTA (pH = 7.4), to remove the smear layer and expose tubules. The region was then treated with either a 3% monopotassium-monohydrogen oxalate solution or a 3% sodium chloride solution, both at pH = 2.4. Solutions were prepared to be indistinguishable to the examiner. Response to air was evaluated before and after EDTA treatment and after treatment with a desensitizing agent. The patient was anesthetized and the treated tooth extracted. Specimens were sectioned, critical point dried, sputter coated with gold, and examined under the scanning electron microscope. Photomicrographs were analyzed by computer assisted digital analysis to evaluate the degree of tubule occlusion. Statistical analysis by repeated measures ANOVA for univariate tests of hypothesis for within subject effects showed the sodium chloride solution was more effective in reducing dentin sensitivity than the potassium oxalate solution. Scanning electron micrographic analysis revealed a mean dentin tubule aperture size of 1.720 square microns following EDTA treatment alone, 0.564 square micron following potassium oxalate treatment, and 0.386 square micron following sodium chloride treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Dentinal tubule occlusion and root hypersensitivity

Previous studies have established that root sensitivity is due in part to open dentinal tubules at the root surface. The purpose of this study was to longitudinally evaluate the occlusion of dentinal tubules by various clinical procedures including scaling and root planing and the application of potassium oxalate. A model was developed to evaluate dentinal surfaces in vivo. Six 2 mm x 3 mm sections were taken from the roots of extracted teeth immediately below the CEJ. One half of the treated dentin samples from each donor was incorporated into the removable denture worn by the donor and the other half served as baseline. The dentin samples were evaluated at 1 week by scanning electron microscopy. At day zero no open tubules were observed in any of the samples due to the presence of a smear layer or oxalate crystals. Evaluation of root planed samples revealed that by 7 days the tubules had re-opened. The samples which had been treated with potassium oxalate showed few oxalate crystals by 7 days revealing open tubules. Control samples were etched with EDTA and evaluated in the same manner. Although the number of tubules did not significantly change in the EDTA etched control samples, the diameter of the tubules dramatically decreased by 4 weeks. It can be concluded that the creation of a smear layer or application of oxalates to occlude dentinal tubules to reduce sensitivity are relatively short-lived. These procedures may provide patient comfort prior to natural occlusion of the tubules.

Decreasing intradental nerve activity in the cat with potassium and divalent cations

Nerve activity was recorded from deep dentinal cavities in the canine teeth to assess the possible influence of potassium and divalent cations in decreasing this activity in hypersensitive teeth. The decreased activity after the topical application of 0.756 mol/l KCl to the cavity was primarily due to the cation. KCl elicited a biphasic response from intradental nerves, an initial transient excitatory response followed by a prolonged inhibitory period. During the inhibitory period 3 mol/l NaCl, an effective excitatory stimulus, failed to evoke intradental nerve activity. However, with time the response to 3 mol/l NaCl eventually recovered to its previous control level. Close, intra-arterial injection of KCl showed the same biphasic response and time-course of intradental nerve activity as with topical application. Therefore, whether KCl was applied topically or injected its effectiveness in altering the nerve activity was similar. Pretreatment of the dentinal cavity with CaCl2, MgCl2 or SrCl2 greatly reduced the response of intradental nerves to KCl. Therefore these divalent cations seem to have a depressant action on pulpal nerve fibres. The mechanism of action of KCl seems to be an alteration of K+ concentration immediately surrounding the intradental nerves which presumably depolarizes the nerve fibre membrane and elicits an initial firing of action potentials. Because of the persisting high levels of extracellular potassium a sustained depolarized state occurs that results in an inactivation of the action potential. Divalent cations appear to depress the excitability of the nerve cell membrane without altering membrane potential. Such ionic agents could be used in conjunction with KCl as a possible treatment for hypersensitive teeth.

Effects of ionic and non-ionic solutions on intradental nerve activity in the cat

Intradental nerve activity (INA) was recorded from cat canine teeth to determine whether solutions altering intradental nerve sensitivity were strongly correlated to the osmotic concentration of the solution or via a more direct action on intradental nerve excitability. The effects of various ionic and non-ionic solutions were tested in both deep and shallow dentinal cavities. With saline in the deep dentinal cavity a very low firing rate or resting nerve spike (action potentials) activity was recorded. When 3 M NaCl was placed in the same or similar cavity a high discharge rate of nerve spike activity was obtained. This 3 M NaCl elicited activity was utilized to determine the inhibitory or excitatory effects of various test agents on the intradental nerves. The following agents: MgCl2, MgSO4, and CaCl2 were inhibitory to the INA response elicited by 3 M NaCl. Non-ionic solutions of urea or sucrose failed to evoke INA and they were also minimally effective in altering 3 M NaCl elicited activity. Shallow cavities were utilized to maintain the tubular structure of dentin relatively intact. In the shallow cavity preparations hypertonic sucrose or urea failed to evoke INA, even when dentin was etched with 50% citric acid for 2 min. The results suggest that the osmolarity of these solutions is a poor indicator of the INA.

The effects of burnishing NaF/kaolin/glycerin paste on dentin permeability

The effects on dentin permeability of burnishing NaF, kaolin, or glycerin, alone and in various combinations, were determined using an in vitro system. The results indicate that the important variable was not any of the constituents of the paste but the burnishing process itself. Burnishing dentin creates a partial smear layer on the dentin surface, thereby occluding the orifices of dentinal tubules sufficiently to reduce fluid movement across dentin 50 to 80%.

Perception of pulpal pain as a function of intradental nerve activity

The purpose of the present investigation was to find neurophysiological correlates of pain perception. The magnitude and time course of perceived pain was successfully related to the neural discharge evoked by rapid cooling of the tooth surface in 6 dental patients whose lower incisors were to be extracted for prosthodontic reasons. Two cavities were prepared on the facial surface of human lower incisors. The cavities were deepened using hand driven instruments until the pulp was visible through a thin layer of dentin. A metal tube was placed in contact with amalgam on each cavity bottom and fixed in place by composite filling material. The tubes were connected to standard equipment for electrophysiological recordings by a flexible circuit. The magnitude of perceived pain was assessed by a cross-modality matching to finger span in combination with sensory verbal pain descriptors and magnitude estimation. The striking agreement between the integrated nerve activity, probably of the A delta type and pain perception, is of great importance from the methodological point of view since it strongly argues in favor of the appropriateness of the techniques applied here to elucidate the neural substrate of some types of nociception and also to evaluate various means of relieving such pain.