A voltage-dependent transient K(+) current in rat dental pulp cells

Ion Channels Involved in Tooth Pain

The tooth has an unusual sensory system that converts external stimuli predominantly into pain, yet its sensory afferents in teeth demonstrate cytochemical properties of non-nociceptive neurons. This review summarizes the recent knowledge underlying this paradoxical nociception, with a focus on the ion channels involved in tooth pain. The expression of temperature-sensitive ion channels has been extensively investigated because thermal stimulation often evokes tooth pain. However, temperature-sensitive ion channels cannot explain the sudden intense tooth pain evoked by innocuous temperatures or light air puffs, leading to the hydrodynamic theory emphasizing the microfluidic movement within the dentinal tubules for detection by mechanosensitive ion channels. Several mechanosensitive ion channels expressed in dental sensory systems have been suggested as key players in the hydrodynamic theory, and TRPM7, which is abundant in the odontoblasts, and recently discovered PIEZO receptors are promising candidates. Several ligand-gated ion channels and voltage-gated ion channels expressed in dental primary afferent neurons have been discussed in relation to their potential contribution to tooth pain. In addition, in recent years, there has been growing interest in the potential sensory role of odontoblasts; thus, the expression of ion channels in odontoblasts and their potential relation to tooth pain is also reviewed.

Potassium Currents Activated by Depolarization in Odontoblasts

Increased intracellular free Ca²⁺ concentrations elicit plasma membrane depolarization, which leads to the activation of K⁺ currents. However, the precise properties of K⁺ currents activated by depolarization in odontoblasts remain to be elucidated. The present study identified biophysical and pharmacological characteristics of time-dependent and voltage-activated K⁺ currents in freshly dissociated rat odontoblasts using patch-clamp recordings in a whole-cell configuration. Using a holding potential of −70 mV, outwardly rectifying time- and voltage-dependent currents were activated by depolarizing voltage. To record pure K⁺ conductance, we substituted Cl⁻ in both the extracellular and intracellular solutions with gluconate⁻. Under these conditions, observation of K⁺ concentration changes in the extracellular solution showed that reversal potentials of tail currents shifted according to the K⁺ equilibrium potential. The activation kinetics of outward K⁺ currents were relatively slow and depended on the membrane potential. Kinetics of steady-state inactivation were fitted by a Boltzmann function. The half-maximal inactivation potential was −38 mV. Tetraethylammonium chloride, 4-aminopyridine, and α-dendrotoxin inhibited outward currents in odontoblasts in a concentration-dependent manner, suggesting that rat odontoblasts express the α-subunit of the time- and voltage-dependent K⁺ channel (Kv) subtypes Kv1.1, 1.2, and/or 1.6. We further examined the effects of Kv activity on mineralization by alizarin red and von Kossa staining. Continuous application of tetraethylammonium chloride to human odontoblasts grown in a mineralization medium over a 21-day period exhibited a dose-dependent decrease in mineralization efficiency compared to cells without tetraethylammonium chloride. This suggests that odontoblasts functionally express voltage-dependent K⁺ channels that play important roles in dentin formation.

Functional expression of TRPM8 and TRPA1 channels in rat odontoblasts

Odontoblasts produce dentin during development, throughout life, and in response to pathological conditions by sensing stimulation of exposed dentin. The functional properties and localization patterns of transient receptor potential (TRP) melastatin subfamily member 8 (TRPM8) and ankyrin subfamily member 1 (TRPA1) channels in odontoblasts remain to be clarified. We investigated the localization and the pharmacological, biophysical, and mechano-sensitive properties of TRPM8 and TRPA1 channels in rat odontoblasts. Menthol and icilin increased the intracellular free Ca(2+) concentration ([Ca(2+)]i). Icilin-, WS3-, or WS12-induced [Ca(2+)]i increases were inhibited by capsazepine or 5-benzyloxytriptamine. The increase in [Ca(2+)]i elicited by allyl isothiocyanate (AITC) was inhibited by HC030031. WS12 and AITC exerted a desensitizing effect on [Ca(2+)]i increase. Low-temperature stimuli elicited [Ca(2+)]i increases that are sensitive to both 5-benzyloxytriptamine and HC030031. Hypotonic stimulation-induced membrane stretch increased [Ca(2+)]i; HC030031 but not 5-benzyloxytriptamine inhibited the effect. The results suggest that TRPM8 channels in rat odontoblasts play a role in detecting low-temperature stimulation of the dentin surface and that TRPA1 channels are involved in sensing membrane stretching and low-temperature stimulation. The results also indicate that odontoblasts act as mechanical and thermal receptor cells, detecting the stimulation of exposed dentin to drive multiple cellular functions, such as sensory transduction.

TRPV1-mediated calcium signal couples with cannabinoid receptors and sodium-calcium exchangers in rat odontoblasts

Odontoblasts are involved in the transduction of stimuli applied to exposed dentin. Although expression of thermo/mechano/osmo-sensitive transient receptor potential (TRP) channels has been demonstrated, the properties of TRP vanilloid 1 (TRPV1)-mediated signaling remain to be clarified. We investigated physiological and pharmacological properties of TRPV1 and its functional coupling with cannabinoid (CB) receptors and Na(+)-Ca(2+) exchangers (NCXs) in odontoblasts. Anandamide (AEA), capsaicin (CAP), resiniferatoxin (RF) or low-pH evoked Ca(2+) influx. This influx was inhibited by capsazepine (CPZ). Delay in time-to-activation of TRPV1 channels was observed between application of AEA or CAP and increase in [Ca(2+)](i). In the absence of extracellular Ca(2+), however, an immediate increase in [Ca(2+)](i) was observed on administration of extracellular Ca(2+), followed by activation of TRPV1 channels. Intracellular application of CAP elicited inward current via opening of TRPV1 channels faster than extracellular application. With extracellular RF application, no time delay was observed in either increase in [Ca(2+)](i) or inward current, indicating that agonist binding sites are located on both extra- and intracellular domains. KB-R7943, an NCX inhibitor, yielded an increase in the decay time constant during TRPV1-mediated Ca(2+) entry. Increase in [Ca(2+)](i) by CB receptor agonist, 2-arachidonylglycerol, was inhibited by CB1 receptor antagonist or CPZ, as well as by adenylyl cyclase inhibitor. These results showed that TRPV1-mediated Ca(2+) entry functionally couples with CB1 receptor activation via cAMP signaling. Increased [Ca(2+)](i) by TRPV1 activation was extruded by NCXs. Taken together, this suggests that cAMP-mediated CB1-TRPV1 crosstalk and TRPV1-NCX coupling play an important role in driving cellular functions following transduction of external stimuli to odontoblasts.

Sodium-calcium exchangers in rat ameloblasts

Although the central role of ameloblasts in synthesis and resorption of enamel matrix proteins during amelogenesis is well documented, the Ca(2+)-transport/extrusion mechanism remains to be fully elucidated. To clarify Ca(2+)-transport in rat ameloblasts, we investigated expression and localization of Na(+)-Ca(2+) exchanger (NCX) isoforms and the functional characteristics of their ion transporting/pharmacological properties. RT-PCR and immunohistochemical analyses revealed expression of NCX1 and NCX3 in ameloblasts, localized in the apical membrane. In patch-clamp recordings, Ca(2+) efflux by Na(+)-Ca(2+) exchange showed dependence on external Na(+). Ca(2+) influx by Na(+)-Ca(2+) exchange, measured by fura-2 fluorescence, showed dependence on extracellular Ca(2+) concentration, and it was blocked by NCX inhibitors KB-R7943, SEA0400, and SN-6. These results showed significant expression of NCX1 and NCX3 in ameloblasts, indicating their involvement in the directional Ca(2+) extrusion pathway from cells to the enamel mineralizing front.

Rapid penetration of lucifer yellow into vital teeth and dye coupling between odontoblasts and neighbouring pulp cells in the cat

The location and long process of odontoblasts appear well suited to detection of external stimuli. The odontoblasts may transmit the information not individually but as a syncytium via gap junction, which functions as a mechanism for intercellular linking cells and as the route for dye coupling. The aims of the present study were to examine dye penetration through enamel and dentin, and to confirm dye coupling between odontoblasts (OBs) or between odontoblasts and other pulpal cells beneath the odontoblastic layer (PCs). Either lucifer yellow (LY) or borate buffer (control) was applied to etched enamel surface of feline canines for 30 min at atmospheric pressure. In the decalcified sections, lucifer yellow positive cells were found not only in but also beneath the odontoblastic layer (experiment 1). In the isolated pulp cells, all OBs (27/27) and some PCs (6/9) that were immunocytochemically differentiated using two monoclonal antibodies were labelled with LY (experiment 2). These results indicate the remarkably quick movement of LYE through enamel and dentin into the superficial pulp. In experiment 3, fresh OBs and PCs were isolated from feline canines to which LY had not been applied. LY was iontophoretically injected into an OB-like cell that had an oval cell body and a long monopolar process. Some PCs and OBs identified immunocytochemically were labelled with LY, with the exception of a few LY-negative cells. These findings indicate that dye coupling exists not only between OBs but also between OBs and PCs. Thus, the coupling provides evidence for a functional link via which information is transmitted between OBs and PCs.

The odontoblast as a sensory receptor cell? The expression of TRPV1 (VR-1) channels

Previous reports have shown the expression of several mechanosensitive ionic channels on the plasma membrane in odontoblasts, which are the cells responsible for dentin formation. The membrane characteristics of odontoblasts imply that they could play critical roles in the mechano-transduction of fluid displacement within dentinal tubules into the electrical cell signals, to carry dentin sensation to the central nervous system. However, the direct ionic mechanism underlying such a dentin nociceptive function remains unclear. In the present study, we investigated the expression of the transient receptor potential vanilloid subfamily member 1 (TRPV1) channel--which essentially contributes to the detection of pain sensation--in rat odontoblasts by immunohistochemical and nystatin perforated patch-clamp techniques. Immunohistochemical observation showed the localization of TRPV1-immunoreactions on the distal regions of odontoblast membranes. In the patch-clamp experiments, we observed capsaicin-induced inward currents that were inhibited by capsazepine, a TRPV1 channel antagonist. Our results indicate a significant expression of TRPV1 channels in odontoblasts, suggesting that odontoblasts may directly respond to noxious stimuli such as a thermal-heat stimulus, and point to the necessity for a reconsideration of the cellular mechanisms of dentin sensation based on the transmembrane ionic signals in odontoblasts.

Ca2+ signaling mediated by IP3-dependent Ca2+ releasing and store-operated Ca2+ channels in rat odontoblasts

In the phospholipase-C (PLC) signaling system, Ca2+ is mobilized from intracellular Ca2+ stores by an action of inositol 1,4,5-trisphosphate (IP3). The depletion of IP3-sensitive Ca2+ stores activates a store-operated Ca2+ entry (SOCE). However, no direct evidence has been obtained about these signaling pathways in odontoblasts. In this study, we investigate the characteristics of the SOCE and IP3-mediated Ca2+ mobilizations in rat odontoblasts using fura-2 microfluorometry and a nystatin-perforated patch-clamp technique. In the absence of extracellular Ca2+ ([Ca2+]o), thapsigargin (TG) evoked a transient rise in intracellular Ca2+ concentration ([Ca2+]i). After TG treatment to deplete the store, the subsequent application of Ca2+ resulted in a rapid rise in [Ca2+]i caused by SOCE. In the absence of TG treatment, no SOCE was evoked. The Ca2+ influx was dependent on [Ca2+]o (KD = 1.29 mM) and was blocked by an IP3 receptor inhibitor, 2-aminoethoxydiphenyl borate (2-APB), as well as La3+ in a concentration-dependent manner (IC50 = 26 microM). In TG-treated cells, an elevation of [Ca2+]o from 0 to 2.5 mM elicited an inwardly rectifying current at hyperpolarizing potentials with a positive reversal potential. The currents were selective for Ca2+ over the other divalent cations (Ca2+ > Ba2+ > Sr2+ >> Mn2+). In the absence of [Ca2+]o, carbachol, bradykinin, and 2-methylthioadenosine 5'triphosphate activated Ca2+ release from the store; these were inhibited by 2-APB. These results indicate that odontoblasts possessed Ca2+ signaling pathways through the activation of store-operated Ca2+ channels by the depletion of intracellular Ca2+ stores and through the IP3-induced Ca2+ release activated by PLC-coupled receptors.