Expression of metabotropic glutamate receptor mGluR5 in human dental pulp. J Endod. 2009;35:690-694. [PMID: 19410084 DOI: 10.1016/j.joen.2009.02.005]

Peripheral role of glutamate in orofacial pain

Glutamate is the principal excitatory neurotransmitter in the central nervous system. In the periphery, glutamate acts as a transmitter and involves in the signaling and processing of sensory input. Glutamate acts at several types of receptors and also interacts with other transmitters/mediators under various physiological and pathophysiological conditions including chronic pain. The increasing amount of evidence suggests that glutamate may play a role through multiple mechanisms in orofacial pain processing. In this study, we reviewed the current understanding of how peripheral glutamate mediates orofacial pain, how glutamate is regulated in the periphery, and how these findings are translated into therapies for pain conditions.

Transient Receptor Potential (TRP) Ion Channels in Orofacial Pain

Orofacial pain, including temporomandibular joint disorders pain, trigeminal neuralgia, dental pain, and debilitating headaches, affects millions of Americans each year with significant population health impact. Despite the existence of a large body of information on the subject, the molecular underpinnings of orofacial pain remain elusive. Two decades of research has identified that transient receptor potential (TRP) ion channels play a crucial role in pathological pain. A number of TRP ion channels are clearly expressed in the trigeminal sensory system and have critical functions in the transduction and pathogenesis of orofacial pain. Although there are many similarities, the orofacial sensory system shows some distinct peripheral and central pain processing and different sensitivities from the spinal sensory system. Relative to the extensive review on TRPs in spinally-mediated pain, the summary of TRPs in trigeminally-mediated pain has not been well-documented. This review focuses on the current experimental evidence involving TRP ion channels, particularly TRPV1, TRPA1, TRPV4, and TRPM8 in orofacial pain, and discusses their possible cellular and molecular mechanisms.

Parvalbumin-, substance P- and calcitonin gene-related peptide-immunopositive axons in the human dental pulp differ in their distribution of varicosities

Information on the frequency and spatial distribution of axonal varicosities associated with release of neurotransmitters in the dental pulp is important to help elucidate the peripheral mechanisms of dental pain, mediated by myelinated versus unmyelinated fibers. For this, we investigated the distribution of axonal varicosities in the human dental pulp using light- and electron-microscopic immunohistochemistry for the vesicular glutamate transporter 2 (VGLUT2), which is involved in the glutamatergic transmission, and syntaxin-1 and synaptosomal nerve-associated protein 25 (SNAP-25), combined with parvalbumin (PV), which is expressed mostly in myelinated axons, and substance P (SP) and calcitonin gene-related peptide (CGRP), which are expressed mostly in unmyelinated axons. We found that the varicosities of the SP- and CGRP-immunopositive (+) axons were uniformly distributed throughout the dental pulp, whereas those of PV+ axons were only dense in the peripheral pulp, and that the expression of PV, VGLUT2, syntaxin-1, SNAP-25, SP and CGRP was significantly higher in the varicosities than in the axonal segments between them. These findings are consistent with the release of glutamate and neuropeptides by axonal varicosities of SP+ and CGRP+ unmyelinated fibers, involved in pulpal pain throughout the human dental pulp, and by varicosities of PV+ fibers, arising from parent myelinated fibers, and involved in dentin sensitivity primarily in the peripheral pulp.

Distinct and Overlapping Expression Patterns of the Homer Family of Scaffolding Proteins and Their Encoding Genes in Developing Murine Cephalic Tissues

In mammals Homer1, Homer2 and Homer3 constitute a family of scaffolding proteins with key roles in Ca²⁺ signaling and Ca²⁺ transport. In rodents, Homer proteins and mRNAs have been shown to be expressed in various postnatal tissues and to be enriched in brain. However, whether the Homers are expressed in developing tissues is hitherto largely unknown. In this work, we used immunohistochemistry and in situ hybridization to analyze the expression patterns of Homer1, Homer2 and Homer3 in developing cephalic structures. Our study revealed that the three Homer proteins and their encoding genes are expressed in a wide range of developing tissues and organs, including the brain, tooth, eye, cochlea, salivary glands, olfactory and respiratory mucosae, bone and taste buds. We show that although overall the three Homers exhibit overlapping distribution patterns, the proteins localize at distinct subcellular domains in several cell types, that in both undifferentiated and differentiated cells Homer proteins are concentrated in puncta and that the vascular endothelium is enriched with Homer3 mRNA and protein. Our findings suggest that Homer proteins may have differential and overlapping functions and are expected to be of value for future research aiming at deciphering the roles of Homer proteins during embryonic development.

The Role of Transient Receptor Potential (TRP) Channels in the Transduction of Dental Pain

Dental pain is a common health problem that negatively impacts the activities of daily living. Dentine hypersensitivity and pulpitis-associated pain are among the most common types of dental pain. Patients with these conditions feel pain upon exposure of the affected tooth to various external stimuli. However, the molecular mechanisms underlying dental pain, especially the transduction of external stimuli to electrical signals in the nerve, remain unclear. Numerous ion channels and receptors localized in the dental primary afferent neurons (DPAs) and odontoblasts have been implicated in the transduction of dental pain, and functional expression of various polymodal transient receptor potential (TRP) channels has been detected in DPAs and odontoblasts. External stimuli-induced dentinal tubular fluid movement can activate TRP channels on DPAs and odontoblasts. The odontoblasts can in turn activate the DPAs by paracrine signaling through ATP and glutamate release. In pulpitis, inflammatory mediators may sensitize the DPAs. They could also induce post-translational modifications of TRP channels, increase trafficking of these channels to nerve terminals, and increase the sensitivity of these channels to stimuli. Additionally, in caries-induced pulpitis, bacterial products can directly activate TRP channels on DPAs. In this review, we provide an overview of the TRP channels expressed in the various tooth structures, and we discuss their involvement in the development of dental pain.

High pH-Sensitive Store-Operated Ca2+ Entry Mediated by Ca2+ Release-Activated Ca2+ Channels in Rat Odontoblasts

Odontoblasts play a crucial role in dentin formation and sensory transduction following the application of stimuli to the dentin surface. Various exogenous and endogenous stimuli elicit an increase in the intracellular free calcium concentration ([Ca²⁺]_i) in odontoblasts, which is mediated by Ca²⁺ release from intracellular Ca²⁺ stores and/or Ca²⁺ influx from the extracellular medium. In a previous study, we demonstrated that the depletion of Ca²⁺ stores in odontoblasts activated store-operated Ca²⁺ entry (SOCE), a Ca²⁺ influx pathway. However, the precise biophysical and pharmacological properties of SOCE in odontoblasts have remained unclear. In the present study, we examined the functional expression and pharmacological properties of Ca²⁺ release-activated Ca²⁺ (CRAC) channels that mediate SOCE and evaluated the alkali sensitivity of SOCE in rat odontoblasts. In the absence of extracellular Ca²⁺, treatment with thapsigargin (TG), a sarco/endoplasmic reticulum Ca²⁺-ATPase inhibitor, induced an increase in [Ca²⁺]_i. After [Ca²⁺]_i returned to near-resting levels, the subsequent application of 2.5 mM extracellular Ca²⁺ resulted in an increase in [Ca²⁺]_i which is a typical of SOCE activation. Additionally, application of 2-methylthioadenosine diphosphate trisodium salt (2-MeSADP), a P2Y₁,₁₂,₁₃ receptor agonist, or carbachol (CCh), a muscarinic cholinergic receptor agonist, in the absence of extracellular Ca²⁺, induced a transient increase in [Ca²⁺]_i. The subsequent addition of extracellular Ca²⁺ resulted in significantly higher [Ca²⁺]_i in 2-MeSADP- or CCh-treated odontoblasts than in untreated cells. SOCE, that is activated by addition of extracellular Ca²⁺ in the TG pretreated odontoblasts was then suppressed by Synta66, BTP2, or lanthanum, which are CRAC channel inhibitors. Treatment with an alkaline solution enhanced SOCE, while treatment with HC030031, a TRPA1 channel antagonist, inhibited it. The amplitude of SOCE at pH 9 in the presence of HC030031 was higher than that at pH 7.4 in the absence of HC030031. These findings indicate that CRAC channel-mediated alkali-sensitive SOCE occurs in odontoblasts. SOCE is mediated by P2Y and muscarinic-cholinergic receptors, which are activated by endogenous ligands in odontoblasts.

Transient receptor potential vanilloid 4 (TRPV4) expression on the nerve fibers of human dental pulp is upregulated under inflammatory condition

OBJECTIVE

Transient receptor potential vanilloid 4 (TRPV4) has been considered as a mechano-, thermo- and osmo-receptor. Under inflammatory conditions in dental pulp, teeth can become sensitive upon exposure to a variety of innocuous stimuli. The objective of the present study was to investigate the expression of the TRPV4 channel on nerve fibers in human dental pulp of non-symptomatic and symptomatic teeth associated with inflammatory conditions.

DESIGN

Dental pulp from extracted human permanent teeth was processed for fluorescence immunohistochemistry. Ten asymptomatic (normal) and 10 symptomatic (symptoms associated with pulpitis) teeth were used in this study. Nerve fibers were identified by immunostaining for a marker, protein gene product 9.5, and the cells were counterstained with 4',6-diamidino-2-phenylindole. An anti-TRPV4 antibody was used to trace TRPV4 expression.

RESULTS

TRPV4 expression was co-localized with the nerve fiber marker. Immunoreactivity for TRPV4 was more intense (p < 0.05) in the nerves of symptomatic teeth than those of normal teeth. The number of co-localization spots was increased significantly (p < 0.05) in the dental pulp of symptomatic teeth compared with that of asymptomatic (normal) teeth.

CONCLUSIONS

There is expression of TRPV4 channels on the nerve fibers of human dental pulp. Our findings suggest upregulation of TRPV4 expression under inflammatory conditions in the pulp. The upregulation of TRPV4 channels may be associated with the exaggerated response of dental pulp to innocuous mechanical, thermal and osmotic stimuli under inflammatory conditions.

Localization and role of metabotropic glutamate receptors subtype 5 in the gastrointestinal tract

Metabotropic glutamate receptor subtype 5 (mGluR5) is a Group I mGlu subfamily of receptors coupled to the inositol trisphosphate/diacylglycerol pathway. Like other mGluR subtypes, mGluR5s contain a phylogenetically conserved, extracellular orthosteric binding site and a more variable allosteric binding site, located on the heptahelical transmembrane domain. The mGluR5 receptor has proved to be a key pharmacological target in conditions affecting the central nervous system (CNS) but its presence outside the CNS underscores its potential role in pathologies affecting peripheral organs such as the gastrointestinal (GI) tract and accessory digestive organs such as the tongue, liver and pancreas. Following identification of mGluR5s in the mouth, various studies have subsequently demonstrated its involvement in mechanical allodynia, inflammation, pain and oral cancer. mGluR5 expression has also been identified in gastroesophageal vagal pathways. Indeed, experimental and human studies have demonstrated that mGluR5 blockade reduces transient lower sphincter relaxation and reflux episodes. In the intestine, mGluR5s have been shown to be involved in the control of intestinal inflammation, visceral pain and the epithelial barrier function. In the liver, mGluR5s have a permissive role in the onset of ischemic injury in rat and mice hepatocytes. Conversely, livers from mice treated with selective negative allosteric modulators and mGluR5 knockout mice are protected against ischemic injury. Similar results have been observed in experimental models of free-radical injury and in vivo mouse models of acetaminophen intoxication. Finally, mGluR5s in the pancreas are associated with insulin secretion control. The picture is, however, far from complete as the review attempts to establish in particular as regards identifying specific targets and innovative therapeutic approaches for the treatment of GI disorders.

Rat odontoblasts may use glutamate to signal dentin injury

Accumulating evidence indicates that odontoblasts act as sensor cells, capable of triggering action potentials in adjacent pulpal nociceptive axons, suggesting a paracrine signaling via a currently unknown mediator. Since glutamate can mediate signaling by non-neuronal cells, and peripheral axons may express glutamate receptors (GluR), we hypothesized that the expression of high levels of glutamate, and of sensory receptors in odontoblasts, combined with an expression of GluR in adjacent pulpal axons, is the morphological basis for odontoblastic sensory signaling. To test this hypothesis, we investigated the expression of glutamate, the thermo- and mechanosensitive ion channels transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and TWIK-1-related K+channel (TREK-1), and the glutamate receptor mGluR5, in a normal rat dental pulp, and following dentin injury. We also examined the glutamate release from odontoblast in cell culture. Odontoblasts were enriched with glutamate, at the level as high as in adjacent pulpal axons, and showed immunoreactivity for TRPV1, TRPA1, and TREK-1. Pulpal sensory axons adjacent to odontoblasts expressed mGluR5. Both the levels of glutamate in odontoblasts, and the expression of mGluR5 in nearby axons, were upregulated following dentin injury. The extracellular glutamate concentration was increased significantly after treating of odontoblast cell line with calcium permeable ionophore, suggesting glutamate release from odontoblasts. These findings lend morphological support to the hypothesis that odontoblasts contain glutamate as a potential neuroactive substance that may activate adjacent pulpal axons, and thus contribute to dental pain and hypersensitivity.

Intercellular signal communication among odontoblasts and trigeminal ganglion neurons via glutamate

Various stimuli to the exposed surface of dentin induce changes in the hydrodynamic force inside the dentinal tubules resulting in dentinal pain. Recent evidences indicate that mechano-sensor channels, such as the transient receptor potential channels, in odontoblasts receive these hydrodynamic forces and trigger the release of ATP to the pulpal neurons, to generate dentinal pain. A recent study, however, has shown that odontoblasts also express glutamate receptors (GluRs). This implies that cells in the dental pulp tissue have the ability to release glutamate, which acts as a functional intercellular mediator to establish inter-odontoblast and odontoblast-trigeminal ganglion (TG) neuron signal communication. To investigate the intercellular signal communication, we applied mechanical stimulation to odontoblasts and measured the intracellular free Ca²⁺ concentration ([Ca²⁺]_i). During mechanical stimulation in the presence of extracellular Ca²⁺, we observed a transient [Ca²⁺]_i increase not only in single stimulated odontoblasts, but also in adjacent odontoblasts. We could not observe these responses in the absence of extracellular Ca²⁺. [Ca²⁺]_i increases in the neighboring odontoblasts during mechanical stimulation of single odontoblasts were inhibited by antagonists of metabotropic glutamate receptors (mGluRs) as well as glutamate-permeable anion channels. In the odontoblast-TG neuron coculture, we observed an increase in [Ca²⁺]_i in the stimulated odontoblasts and TG neurons, in response to direct mechanical stimulation of single odontoblasts. These [Ca²⁺]_i increases in the neighboring TG neurons were inhibited by antagonists for mGluRs. The [Ca²⁺]_i increases in the stimulated odontoblasts were also inhibited by mGluRs antagonists. We further confirmed that the odontoblasts express group I, II, and III mGluRs. However, we could not record any currents evoked from odontoblasts near the mechanically stimulated odontoblast, with or without extracellular Mg²⁺, indicating that N-methyl-d-aspartic acid receptor does not contribute to inter-odontoblast signal communication. The results suggest that a mechanically stimulated odontoblast is capable of releasing glutamate into the extracellular space via glutamate-permeable anion channels. The released glutamate activates mGluRs on the odontoblasts in an autocrine/paracrine manner, forming an inter-odontoblasts communication, which drives dentin formation via odontoblast-odontoblast signal communication. Glutamate and mGluRs also mediate neurotransmission between the odontoblasts and neurons in the dental pulp to modulate sensory signal transmission for dentinal sensitivity.

mGluR5 stimulating Homer-PIKE formation initiates icariin induced cardiomyogenesis of mouse embryonic stem cells by activating reactive oxygen species

Icariin (ICA) has been reported to facilitate cardiac differentiation of mouse embryonic stem (ES) cells; however, the mechanism by which ICA induced cardiomyogenesis has not been fully elucidated yet. Here, an underlying signaling network including metabotropic glutamate receptor 5 (mGluR5), Homer, phosphatidylinositol 3-Kinase Enhancer (PIKE), phosphatidylinositol 3-Kinase (PI3K), reactive oxygen species (ROS) and nuclear factor-kappaB (NF-κB) was investigated in ICA induced cardiomyogenesis. Our results showed that the co-expression of mGluR5 together with α-actinin or Troponin T in embryoid bodies (EBs) treated with ICA was elevated to 10.86% and 9.62%, compared with the case in the control (4.04% and 3.45%, respectively). Exposure of EBs to ICA for 2 h remarkably increased the dimeric form of mGluR5, which was inhibited by small interfering RNA targeting mGluR5 (si-mGluR5). Moreover, the extracellular glutamate concentration in ICA treatment medium was elevated to 28.9±3.5 μM. Furthermore, the activation of mGluR5 by ICA triggered the formation of Homer-PIKE complex and activated PI3K, stimulating ROS generation and NF-κB nuclear translocation. Knockdown of mGluR5 or inhibition of PI3K by LY294002 blocked ICA induced cardiomyogenesis via repressing mGluR5 pathway, reducing ROS and NF-κB activation. These results revealed that the inducible mechanisms of ICA were related to activate mGluR5 pathway.

Voltage-dependent sodium channels and calcium-activated potassium channels in human odontoblasts in vitro

INTRODUCTION

Transmembrane ionic signaling regulates many cellular processes in both physiological and pathologic settings. In this study, the biophysical properties of voltage-dependent Na(+) channels in odontoblasts derived from human dental pulp (HOB cells) were investigated together with the effect of bradykinin on intracellular Ca(2+) signaling and expression of Ca(2+)-activated K(+) channels.

METHODS

Ionic channel activity was characterized by using whole-cell patch-clamp recording and fura-2 fluorescence.

RESULTS

Mean resting membrane potential in the HOB cells was -38 mV. Depolarizing steps from a holding potential of -80 mV activated transient voltage-dependent inward currents with rapid activation/inactivation properties. At a holding potential of -50 mV, no inward current was recorded. Fast-activation kinetics exhibited dependence on membrane potential, whereas fast-inactivation kinetics did not. Steady-state inactivation was described by a Boltzmann function with a half-maximal inactivation potential of -70 mV, indicating that whereas the channels were completely inactivated at physiological resting membrane potential, they could be activated when the cells were hyperpolarized. Inward currents disappeared in Na(+)-free extracellular solution. Bradykinin activated intracellular Ca(2+)-releasing and influx pathways. When the HOB cells were clamped at a holding potential of -50 mV, outward currents were recorded at positive potentials, indicating sensitivity to inhibitors of intermediate-conductance Ca(2+)-activated K(+) channels.

CONCLUSIONS

Human odontoblasts expressed voltage-dependent Na(+) channels, bradykinin receptors, and Ca(2+)-activated K(+) channels, which play an important role in driving cellular functions by channel-receptor signal interaction and membrane potential regulation.

Lipopolysaccharide-induced pulpitis up-regulates TRPV1 in trigeminal ganglia

Tooth pain often accompanies pulpitis. Accumulation of lipopolysaccharides (LPS), a product of Gram-negative bacteria, is associated with painful clinical symptoms. However, the mechanisms underlying LPS-induced tooth pain are not clearly understood. TRPV1 is a capsaicin- and heat-gated nociceptive ion channel implicated in thermosensation and hyperalgesia under inflammation or injury. Although TRPV1 is expressed in pulpal afferents, it is not known whether the application of LPS to teeth modulates TRPV1 in trigeminal nociceptors. By assessing the levels of protein and transcript of TRPV1 in mouse trigeminal ganglia, we demonstrate that dentinal application of LPS increases the expression of TRPV1. Our results suggest that the up-regulation of TRPV1 in trigeminal nociceptors following bacterial infection could contribute to hyperalgesia under pulpitis conditions.

Insulin-like growth factor 1 and transforming growth factor-β stimulate cystine/glutamate exchange activity in dental pulp cells

INTRODUCTION

The growth factors insulin-like growth factor (IGF-1) and transforming growth factor-β (TGF-β) are protective to dental pulp cells in culture against the toxicity of the composite materials Durafill VS and Flow Line (Henry Schein Inc, New York, NY). Because the toxicity of these materials is mediated by oxidative stress, it seemed possible that the protective effects of IGF-1 and TGF-β were through the enhancement of an endogenous antioxidant mechanism.

METHODS

We used cultured dental pulp cells to determine the mechanism of the protective effects of IGF-1 and TGF-β, focusing on the glutathione system and the role of cystine/glutamate exchange (system xc-).

RESULTS

We found that the toxicity of Durafill VS and Flow Line was attenuated by the addition of glutathione monoethylester, suggesting a specific role for the cellular antioxidant glutathione. Supporting this hypothesis, we found that IGF-1 and TGF-β were protective against the toxicity of the glutathione synthesis inhibitor buthionine sulfoximine. Because levels of cellular cystine are the limiting factor in the production of glutathione, we tested the effects of IGF-1 and TGF-β on cystine uptake. Both growth factors stimulated system xc-mediated cystine uptake. Furthermore, they attenuated the glutathione depletion induced by Durafill VS and Flow Line.

CONCLUSIONS

The results suggest that IGF-1 and TGF-β are protective through the stimulation of system xc-mediated cystine uptake, leading to maintenance of cellular glutathione. This novel action of growth factors on dental pulp cells has implications not only for preventing toxicity of dental materials but also for the general function of these cells.

Exciting times beyond the brain: metabotropic glutamate receptors in peripheral and non-neural tissues

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors expressed primarily on neurons and glial cells, where they are located in the proximity of the synaptic cleft. In the central nervous system (CNS), mGlu receptors modulate the effects of l-glutamate neurotransmission in addition to that of a variety of other neurotransmitters. However, mGlu receptors also have a widespread distribution outside the CNS that has been somewhat neglected to date. Based on this expression, diverse roles of mGlu receptors have been suggested in a variety of processes in health and disease including controlling hormone production in the adrenal gland and pancreas, regulating mineralization in the developing cartilage, modulating lymphocyte cytokine production, directing the state of differentiation in embryonic stem cells, and modulating gastrointestinal secretory function. Understanding the role of mGlu receptors in the periphery will also provide a better insight into potential side effects of drugs currently being developed for neurological and psychiatric conditions. This review summarizes the new potential roles of mGlu receptors and raises the possibility of novel pharmacological targets for various disorders.

Quantitative ultrastructural analysis of the neurofilament 200-positive axons in the rat dental pulp

INTRODUCTION

Previous studies have suggested that myelinated axons lose their myelin and become thinner in their peripheral course to the target organ. In this study, we investigated the morphologic changes of pulpal myelinated axons between their root portion (radicular pulp) and their terminal area (peripheral pulp).

METHODS

Sections of pulp of the rat upper molar teeth were immunostained for the marker of myelinated axons neurofilament (NF) 200. The proportion of NF200+ myelinated and unmyelinated fibers and their sizes were analyzed by using quantitative electron microscopy.

RESULTS

The axon area, myelin thickness, and fraction of NF200+ myelinated axons of all NF200+ axons were significantly lower in peripheral than in radicular pulp. In addition, large unmyelinated axons were frequently observed in peripheral pulp.

CONCLUSIONS

These results suggest that pulpal innervation originates predominantly from myelinated axons, and the myelinated axons undergo extensive morphologic changes during their course from the radicular to the peripheral pulp.

Adaptive calcified matrix response of dental pulp to bacterial invasion is associated with establishment of a network of glial fibrillary acidic protein+/glutamine synthetase+ cells

We report evidence for anatomical and functional changes of dental pulp in response to bacterial invasion through dentin that parallel responses to noxious stimuli reported in neural crest-derived sensory tissues. Sections of resin-embedded carious adult molar teeth were prepared for immunohistochemistry, in situ hybridization, ultrastructural analysis, and microdissection to extract mRNA for quantitative analyses. In odontoblasts adjacent to the leading edge of bacterial invasion in carious teeth, expression levels of the gene encoding dentin sialo-protein were 16-fold greater than in odontoblasts of healthy teeth, reducing progressively with distance from this site of the carious lesion. In contrast, gene expression for dentin matrix protein-1 by odontoblasts was completely suppressed in carious teeth relative to healthy teeth. These changes in gene expression were related to a gradient of deposited reactionary dentin that displayed a highly modified structure. In carious teeth, interodontoblastic dentin sialo-protein(-) cells expressing glutamine synthetase (GS) showed up-regulation of glial fibrillary acidic protein (GFAP). These cells extended processes that associated with odontoblasts. Furthermore, connexin 43 established a linkage between adjacent GFAP(+)/GS(+) cells in carious teeth only. These findings indicate an adaptive pulpal response to encroaching caries that includes the deposition of modified, calcified, dentin matrix associated with networks of GFAP(+)/GS(+) interodontoblastic cells. A regulatory role for the networks of GFAP(+)/GS(+) cells is proposed, mediated by the secretion of glutamate to modulate odontoblastic response.