An electrophysiological investigation of the sensitivity of intradental nerves in the cat to changes in the ionic composition of extracellular fluid

Effect of HeNe and pulsed Nd:YAG laser irradiation on intradental nerve responses to mechanical stimulation of dentine

BACKGROUND AND OBJECTIVE

Our study aimed to determine how lasing affected intradental nerve responses to dentine stimulation. Study Design/Materials and Methods Intradental nerve activity was recorded from canine teeth of anaesthetised ferrets. Dentine exposed at the tip of the tooth was stimulated with a glass probe. After determining baseline responses to mechanical stimulation, dentine was lased using a pulsed Nd:YAG laser at 60-150mJ/pulse and 10-30 pulses/sec (total power = 0.3-3.0 W).

RESULTS

The HeNe aiming beam alone and Nd:YAG laser at 0.3 W (+ HeNe) had no effect on intradental nerve responses to dentine stimulation. Lasing at 0.6-1. 5 W could either enhance or suppress intradental nerve responses. Lasing at more than or equal to 2.0 W or repeated lasing at lower intensities depressed intradental nerve responses. Lasing often induced intradental nerve firing.

CONCLUSION

HeNe lasing had no effect on intradental nerve excitability. The Nd:YAG laser could depress intradental nerve responses to dentine stimulation.

Effects of potassium ions on action potential conduction in A- and C-fibers of rat spinal nerves

Potassium ions in dentifrices for treating 'hypersensitive' dentin are believed to act directly on intradental nerves by raising extracellular potassium ion concentration ([K+]o) sufficiently to prevent action potential generation by axonal accommodation. However, the [K+]o necessary to block nerve conduction is not precisely known, nor is it certain that K+ can diffuse from a dentifrice in sufficient amounts to inactivate intradental nerves. To establish more accurately the [K+]o required to block nerve conduction under controlled conditions, we studied the effects of increased [K+]o on the sizes of compound action potentials (CAP) recorded from rat spinal nerves in vitro. [K+]o was increased by the addition of either KCl or KNO3 to Krebs' solutions applied to the central portion of the nerves. CAP attenuation increased in a dose-dependent manner as [K+]o was raised in the 8 to 64 mmol/L range, and complete block was generally produced with solutions containing at least 32 mmol/L K+. CAP attenuation was reversible, and recovery times increased with increasing [K+]o. The effects of KCl and KNO3 solutions were the same for all [K+]o tested. Half-maximal (50%) reduction in the A beta-fiber component of the CAP occurred with 17.4 mmol/L K+, and with 17.8 mmol/L and 19.3 mmol/L K+, respectively, for the A delta- and C-fiber components. Control experiments with glucose and choline chloride confirmed that the conduction block observed with increased [K+]o was not due to increased solution osmolarity or ionic strength.(ABSTRACT TRUNCATED AT 250 WORDS)

Neural form of voltage-dependent sodium current in human cultured dental pulp cells

Intradental, i.e. pulpal, cells may play an important part in sensory transduction in teeth, although the cellular mechanisms and the identity of the specific cell types involved are still unclear. Because the majority of cells in dental pulp are derived from neural crest, it seemed likely that these might have the membrane properties of other neural-derived cells found in the peripheral or central nervous system. The patch-clamp recording technique was used to show that cells in explant cultures from human dental pulp contain a voltage-gated, tetrodotoxin-sensitive inward current. Mean activation potential of the current was -42 +/- 2.5 mV and the voltage at half-inactivation was -79.4 +/- 5.3 mV, suggesting a neural-like sodium conductance. In addition, these cells were immunoreactive to glial acidic fibrillary protein, growth-associated protein (GAP-43), and vimentin, further suggesting that dental pulp contains a population of cells with membrane properties similar to neuronal satellite cells. These cells may contribute, either directly or indirectly, to somatosensation in teeth.

Factors affecting nerve excitability and conduction as a basis for desensitizing dentine

Nerve excitability is principally determined by the state of membrane sodium channels, which can be influenced by factors such as drugs or the ionic composition of the extracellular fluids. The excitability of isolated nerves is increased by lowered extracellular calcium and hydrogen ion concentrations, and similar effects are seen on intradental nerve excitability. This is of interest as some agents used to desensitize hypersensitive dentine are believed to act by reducing intradental nerve excitability. A variety of ions, including potassium and divalent cations, have been shown to reduce intradental nerve excitability when applied in relatively high concentrations to dentine close to the pulp. However, these conditions do not mimic the clinical situation, where agents are applied to outer dentine, up to 2 mm from the pulp. Also, when agents are applied to dentine it is difficult to quantify the precise changes in extracellular fluid composition, and there is the additional complication of possible interactions between the agents and dentine. These problems can be minimized by using isolated nerves, which allow better control over the conditions and ionic concentrations necessary to achieve nerve conduction block. Such methods can provide a convenient way to screen potential desensitizing agents before evaluation in vivo or in clinical trials.

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.

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.

The effect of some metal ions on the intradental sensory nerves of the cat

Intradental nerve activity from canine teeth of anesthetized cats was recorded during the application of different metal ion solutions in a dentinal cavity. The application of Ag2+, Ca2+, Cd2+, Cr3+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Sn2+, and Zn2+ reduced the nerve activity induced by NaCl 0.76 M and compound 48/80. The application of Cu2+ and Co2+ excited the intradental sensory nerves.

Responses of feline intradental sensory nerves to hyperosmotic stimulation of dentin

The responses of intradental sensory nerves to hyperosmotic solutions of sucrose (4M) and calcium chloride (6M) applied in dentinal cavities were studied in anesthetized cats. Nerve impulse activity was recorded from canine teeth after application of the test solutions in shallow and deep cavities. In shallow cavities (thickness of remaining dentin, about 500 micron) sucrose and calcium chloride caused an immediate and transient excitation of the nerves in 3 out of 15 teeth and in 8 of 12 teeth, respectively. Treatment of such cavities with lactic acid (1M) increased the frequency of nerve responses to 100%. When applied in deep cavities (thickness of remaining dentin, 0-50 micron), sucrose induced a burst of impulses followed by continuous nerve activity, whereas calcium chloride decreased the nerve excitability. Our results support the hypothesis that solutions exerting an effective osmotic pressure excite the intradental nerves by an indirect mechanism when applied on the dentin and that they exert a direct effect on nerves when in contact with the pulp. In addition, it is suggested that acids produced in carious dentin may facilitate the induction of pain by hyperosmotic stimuli.

Microscopy of the dentine of enamel-free areas of rat molar teeth

Dentine of the enamel-free areas (EFA) of first mandibular molar teeth of rats aged between 20-150 days was examined by scanning electron microscopy. Demineralized sections were stained with Gram-Twort's stain and examined for dental caries. In newly erupted teeth, a few dentinal tubules opened on the EFA surface. As teeth became worn down, extensive areas of EFA showed numerous patent tubules. In primary dentine, no occlusion or partial occlusion of tubules was apparent at any age except at the occlusal surface. It is postulated that this surface narrowing was caused either by dentine debris or salivary deposits. The only other covering of tubule openings appeared to be a thin salivary pellicle which in older animals was plaque-like. Patent tubules in primary dentine contained odontoblast processes which even in worn teeth extended to EFA surfaces. Processes in adjacent enamel-covered dentine also extended to the enamel-dentine junction. Only a few examples of bacterial invasion of EFA dentine were seen: in younger animals bacteria occupied cracks in the EFA surface, but in older animals they also occupied tubules. The EFA dentine does not appear to respond to attrition by infilling of the tubules; the persistence of vital odontoblast processes in worn teeth suggests that dead tract formation is not extensive. Continuous occlusal wear may restrict the progress of caries in EFA but odontoblasts in the rat may be also physiologically and immunologically involved in restricting the ingress of bacteria into otherwise poorly protected EFA.

A fine-structural investigation on the extent of perineurial investment of the nerve supply to the pulp in rat molar teeth

The fine structure of the tissues immediately adjacent to nerve fibres supplying the rat molar pulp has been examined and compared with that of perineurium of other peripheral nerves. In the periapical region, almost all nerve fibres were invested with typical perineurium; only a few Schwann cell units (unmyelinated axons) lacked perineurium. In the pulp at the apex of the distal root, most fibres were invested by less typical perineurium but some were not invested at all. In the pulp at the coronal end of the root, nerve fibres were completely invested or incompletely invested with attenuated perineurium or lacked investment altogether. In the coronal pulp, there was virtually no perineurium at all. Thus rat molar pulp nerve fibres lack perineurium for distances of about 0.5-3.0 mm, distances greater than in situations such as skin or muscle. In normal peripheral nerves, the perineurium behaves as a barrier to the passage of substances from the immediate environment of the nerve fibres (endoneurium) to the surrounding environment (epineurium) and vice versa. Its absence around pulp nerve fibres raises questions about the nature of the pulp environment and may help to explain the sensitivity of pulp nerves to chemical, osmotic and other stimuli.