Potassium currents in cells derived from human dental pulp

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.

An overview of the dental pulp: its functions and responses to injury

The dental pulp is a unique tissue and its importance in the long-term prognosis of the tooth is often ignored by clinicians. It is unique in that it resides in a rigid chamber which provides strong mechanical support and protection from the microbial rich oral environment. If this rigid shell loses its structural integrity, the pulp is under the threat of the adverse stimuli from the mouth, such as caries, cracks, fractures and open restoration margins, all of which provide pathways for micro-organisms and their toxins to enter the pulp. The pulp initially responds to irritation by becoming inflamed and, if left untreated, this will progress to pulp necrosis and infection. The inflammation will also spread to the surrounding alveolar bone and cause periapical pathosis. The magnitude of pulp-related problems should not be underestimated since their most serious consequence is oral sepsis, which can be life threatening, and hence correct diagnosis and management are essential. Clinicians must have a thorough understanding of the physiological and pathological features of the dental pulp as well as the biological consequences of treatment interventions.

Shear stress facilitates tissue-engineered odontogenesis

Numerous studies have demonstrated the effect of shear stress on osteoblasts, but its effect on odontogenic cells has never been reported. In this study, we focused on the effect of shear stress on facilitating tissue-engineered odontogenesis by dissociated single cells. Cells were harvested from the porcine third molar tooth at the early stage of crown formation, and the isolated heterogeneous cells were seeded on a biodegradable polyglycolic acid fiber mesh. Then, cell-polymer constructs with and without exposure to shear stress were evaluated by in vitro and in vivo studies. In in vitro studies, the expression of both epithelial and mesenchymal odontogenic-related mRNAs was significantly enhanced by shear stress for 2 h. At 12 h after exposure to shear stress, the expression of amelogenin, bone sialoprotein and vimentin protein was significantly enhanced compared with that of control. Moreover, after 7 days, alkaline phosphatase activity exhibited a significant increase without any significant effect on cell proliferation in vitro. In vivo, enamel and dentin tissues formed after 15 weeks of in vivo implantation in constructs exposure to in vitro shear stress for 12 h. Such was not the case in controls. We concluded that shear stress facilitates odontogenic cell differentiation in vitro as well as the process of tooth tissue engineering in vivo.

GFAP immunoreactivity and transcription in trigeminal and dental tissues of rats and transgenic GFP/GFAP mice

Sensory mechanisms in teeth are not well understood and may involve pulpal-neural interactions. Tooth cells that proliferate in vitro have polyclonal immunoreactivity (IR) for glial fibrillary acidic protein (GFAP), growth-associated protein (GAP-43), and vimentin, plus glial-like ion channels. Here, we analyzed GFAP-IR patterns in dental and trigeminal tissues of rats, for comparison with green fluorescent protein (GFP) associated with GFAP transcription in transgenic mice, in order to better characterize glial-like cells in dental tissues. Astrocytes, ganglion satellite cells, and epineurial Schwann cells were demonstrated by anti-GFAP antibodies and GFP-GFAP, as expected. Odontoblasts did not stain by any of these methods and cannot be the glial-like cells. Fibroblasts and undifferentiated mesenchymal cells in pulp had polyclonal GFAP-IR and vimentin-IR, while nerve fibers reacted only with polyclonal antibody. Some Schwann cell subtypes in trigeminal nerve and oral mucosa were positive for GFP and for polyclonal anti-GFAP, but not for monoclonal antibody. In pulp almost all Schwann cells were unstained, but many Schwann cells in periodontal ligament had polyclonal GFAP-IR. These results show greater heterogeneity for Schwann cells than expected, and suggest that the glial-like pulp cells are fibroblasts and/or undifferentiated mesenchymal cells or stem cells. We also found that polyclonal GFAP revealed intermediate filaments in preterminal sensory nerve fibers, thereby providing a useful marker for that neural subregion. GFP transcription by some Schwann cell subtypes in oral mucosae and trigeminal nerve, but not trigeminal root was a novel finding that reveals more complexity in peripheral glia than previously recognized.

Altered localization of Cav1.2 (L-type) calcium channels in nerve fibers, Schwann cells, odontoblasts, and fibroblasts of tooth pulp after tooth injury

We have determined the localization of Cav1.2 (L-Type) Ca2+ channels in the cells and nerve fibers in molars of normal or injured rats. We observed high levels of immunostaining of L-type Ca2+ channels in odontoblast cell bodies and their processes, in fibroblast cell bodies and in Schwann cells. Many Cav1.2-containing unmyelinated and myelinated axons were also present in root nerves and proximal branches in coronal pulp, but were usually missing from nerve fibers in dentin. Labeling in the larger fibers was present along the axonal membrane, localized in axonal vesicles, and in nodal regions. After focal tooth injury, there is a marked loss of Cav1.2 channels in injured teeth. Immunostaining of Cav1.2 channels was lost selectively in nerve fibers and local cells of the tooth pulp within 10 min of the lesion, without loss of other Cav channel or pulpal labels. By 60 min, Cav1.2 channels in odontoblasts were detected again but at levels below controls, whereas fibroblasts were labeled well above control levels, similar to upregulation of Cav1.2 channels in astrocytes after injury. By 3 days after the injury, Cav1.2 channels were again detected in nerve fibers and immunostaining of fibroblasts and odontoblasts had returned to control levels. These findings provide new insight into the localization of Cav1.2 channels in dental pulp and sensory fibers, and demonstrate unexpected plasticity of channel distribution in response to nerve injury.

Expression and localization of TREK-1 K+ channels in human odontoblasts

During tooth development, odontoblasts are the cells that form dentin and possibly mediate early stages of sensory processing in teeth. It is suggested that ion channels assist in these events. Indeed, mechanosensitive potassium currents, transducing mechanical stimuli into electrical cell signals, have been previously recorded in the human odontoblast cell membrane. Here, we show by RT-PCR that the mechanosensitive potassium channel TREK-1 (a member of the two-pore-domain potassium channel family) is overexpressed in these cultured cells compared with pulp cells in vitro. In situ hybridization showed that transcripts are detected in the odontoblast layer in vivo. The use of antibodies shows that TREK-1 is strongly expressed in the membrane of coronal odontoblasts and absent in the root. This distribution is related to the spatial distribution of nerve endings identified by labeling of the low-affinity nerve growth factor (NGF) receptor (p75(NTR)). These results demonstrate the expression of TREK-1 in human odontoblasts in vitro and in vivo.

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

We characterized a voltage-dependent transient K(+) current in dental pulp fibroblasts on dental pulp slice preparations by using a nystatin perforated-patch recording configuration. The mean resting membrane potential of dental pulp fibroblasts was -53 mV. Depolarizing voltage steps to +60 mV from a holding potential of -80 mV evoked transient outward currents that are activated rapidly and subsequently inactivated during pulses. The activation threshold of the transient outward current was -40 mV. The reversal potential of the current closely followed the K(+) equilibrium potential, indicating that the current was selective for K(+). The steady-state inactivation of the peak outward K(+) currents described by a Boltzmann function with half-inactivation occurred at -47 mV. The K(+) current exhibited rapid activation, and the time to peak amplitude of the current was dependent on the membrane potentials. The inactivation process of the current was well fitted with a single exponential function, and the current exhibited slow inactivating kinetics (the time constants of decay ranged from 353 ms at -20 mV to 217 ms at +60 mV). The K(+) current was sensitive to intracellular Cs(+) and to extracellular 4-aminopyridine in a concentration-dependent manner, but it was not sensitive to tetraethylammonium, mast cell degranulating peptide, and dendrotoxin-I. The blood depressing substance-I failed to block the K(+) current. These results indicated that dental pulp fibroblasts expressed a slow-inactivating transient K(+) current.

Characterization and gene expression of high conductance calcium-activated potassium channels displaying mechanosensitivity in human odontoblasts

Odontoblasts form a layer of cells responsible for the dentin formation and possibly mediate early stages of sensory processing in teeth. Several classes of ion channels have previously been identified in the odontoblast or pulp cell membrane, and it is suspected that these channels assist in these events. This study was carried out to characterize the K(Ca) channels on odontoblasts fully differentiated in vitro using the patch clamp technique and to investigate the HSLO gene expression encoding the alpha-subunit of these channels on odontoblasts in vivo. In inside-out patches, K(Ca) channels were identified on the basis of their K(+) selectivity, conductance, voltage, and Ca(2+) dependence. In cell-attached patches, these channels were found to be activated by application of a negative pressure as well as an osmotic shock. By reverse transcription-polymerase chain reaction, a probe complementary to K(Ca) alpha-subunit mRNA was constructed and used for in situ hybridization on human dental pulp samples. Transcripts were expressed in the odontoblast layer. The use of antibodies showed that the K(Ca) channels were preferentially detected at the apical pole of the odontoblasts. These channels could be involved in mineralization processes. Their mechanosensitivity suggests that the fluid displacement within dentinal tubules could be transduced into electrical cell signals.

A small-conductance Ca(2+)-activated K+ current and Cl- current in rat dental pulp cells

We characterized a voltage-dependent ionic current in dental pulp cells on dental pulp slices using a nystatin perforated-patch recording configuration. The outward currents in dental pulp cells were inhibited by the following channel blockers: 1) Ca(2+)-free extracellular solution containing 10 mM Ba2+, 2) extracellular 400 nM apamin and 3) extracellular 300 nM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). On the other hand, 15 mM tetraethylammonium (TEA) did not inhibit the outward currents. The inhibitory effects of Ca(2+)-free extracellular solution, apamin and DIDS had voltage-dependency. These results indicated that dental pulp cells expressed a small-conductance Ca(2+)-activated K+ current (SK current) and a DIDS-sensitive Cl- current. The functional significance of these channels is discussed.

The regulating manner of opioid receptors on distinct types of calcium channels in hamster submandibular ganglion cells

It is well known that opioids produce inhibitory effects on neuronal activity and on synaptic transmission at most synapses. In this study, we have investigated the effects of opioids on the low voltage- and high voltage-activated calcium channels in acutely dissociated submandibular ganglion (SMG) neurons, using the whole-cell configuration of the patch-clamp technique. The kappa-opioid-receptor agonist U-50488H, the delta-opioid-receptor agonist [D-Pen 2,5]-enkephalin and the mu-opioid-receptor agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin inhibited L-, N- and P/Q-type calcium-current components in a dose-dependent manner at 10 nM-1 microM, respectively, but not the T-type calcium current. These inhibitory effects were antagonized by naloxone (1 microM). The results showed that three types of opioid receptors regulate the L-, N- and P/Q-types of calcium channels, respectively, but not the T-type, in SMG neurones.

Potassium and chloride channels in freshly isolated rat odontoblasts

It has been suggested that understanding the physiological properties of odontoblasts may be important in understanding the mechanisms underlying both metabolic and transductive processes in dental pulp. Because ion flux(es) may play a critical role in these events, it is of particular interest to understand ionic mechanisms in odontoblast cells. Thus, the aim of this study was to use patch-clamp recording techniques to examine the properties of resident ion channels in freshly dissociated odontoblasts. In recordings made in potassium-rich solutions, cells displayed at least three distinct channel amplitudes, with conductances of 130 +/- 18 pS, 52 +/- 4 pS, and 25 +/- 2 pS, respectively. Channel activity persisted in the presence of potassium salts of impermeant anions, and could be abolished by barium, a non-specific potassium channel blocker. In addition to the potassium conductances, we saw two separate anion channels in the odontoblast membrane. These channels were predominantly chloride-selective, weakly permeable to both acetate and aspartate, and had conductances of 391 +/- 64 pS and 24 +/- 3 pS. While questions remain regarding the functional role of these and other ion channels that presumably reside in the odontoblast membrane, our results demonstrate that it is possible to study ionic mechanisms of the odontoblast at the level of the single cell.

Properties of Ca(2+)-dependent K+ channels of human gingival fibroblasts

Cells in the oral cavity are normally exposed to different temperatures. Ion transport systems are influenced by temperature in other tissues: In particular, changes in intracellular K+ ion can affect cell growth and synthesis of macromolecules. The purpose of this investigation was to identify K+ channels in human gingival fibroblast cells and analyze the effect of temperature on their K+ conduction properties. Ca(2+)-dependent K+ channels with a large conductance (125 pS in symmetrical K(+)-rich solutions) were identified in human gingival fibroblasts and studied by the patch-clamp technique. The open probability of the channels varied with membrane potential between +40 and -100 mV. When the bath temperature was decreased from 40 to 4 degrees C, channel conductance was reduced, but the mean open time of the channels was increased. The activation energies for the conductance and the reciprocal of the mean open time were estimated to be 9.1 and 22.9 kJ/mol, respectively. These values are lower than those reported for these and other types of channels in cells from other tissues. The open probability of the channels was nearly constant in the temperature range studied. These results suggest that the properties of Ca(2+)-dependent K+ channels of gingival fibroblasts remain relatively unchanged when the cells are exposed to a wide range of temperatures.

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.