Fos protein induction in the medullary dorsal horn and first segment of the spinal cord by tooth-pulp stimulation in cats

The Evolution of Neuroscience as a Research Field Relevant to Dentistry

The field of neuroscience did not exist as such when the Journal of Dental Research was founded 100 y ago. It has emerged as an important scientific field relevant to dentistry in view of the many neurally based functions manifested in the orofacial area (e.g., pain, taste, chewing, swallowing, salivation). This article reviews many of the novel insights that have been gained through neuroscience research into the neural basis of these functions and their clinical relevance to the diagnosis and management of pain and sensorimotor disorders. These include the neural pathways and brain circuitry underlying each of these functions and the role of nonneural as well as neural processes and their "plasticity" in modulating these functions and allowing for adaptation to tissue injury and pain and for learning or rehabilitation of orofacial functions.

Peripheral and Central Mechanisms of Persistent Orofacial Pain

Neuroplastic changes in the neuronal networks involving the trigeminal ganglion (TG), trigeminal spinal subnucleus caudalis (Vc), and upper cervical spinal cord (C1/C2) are considered the mechanisms underlying the ectopic orofacial hypersensitivity associated with trigeminal nerve injury or orofacial inflammation. It has been reported that peripheral nerve injury causes injury discharges in the TG neurons, and a barrage of action potentials is generated in TG neurons and conveyed to the Vc and C1/C2 after trigeminal nerve injury. Long after trigeminal nerve injury, various molecules are produced in the TG neurons, and these molecules are released from the soma of TG neurons and are transported to the central and peripheral terminals of TG neurons. These changes within the TG cause neuroplastic changes in TG neurons and they become sensitized. The neuronal activity of TG neurons is further accelerated, and Vc and C1/C2 neurons are also sensitized. In addition to this cascade, non-neuronal glial cells are also involved in the enhancement of the neuronal activity of TG, Vc, and C1/C2 neurons. Satellite glial cells and macrophages are activated in the TG after trigeminal nerve injury and orofacial inflammation. Microglial cells and astrocytes are also activated in the Vc and C1/C2 regions. It is considered that functional interaction between non-neuronal cells and neurons in the TG, Vc, and C1/C2 regions is a key mechanism involved in the enhancement of neuronal excitability after nerve injury or inflammation. In this article, the detailed mechanisms underlying ectopic orofacial hyperalgesia associated with trigeminal nerve injury and orofacial inflammation are addressed.

Morphological and functional changes in regenerated primary afferent fibres following mental and inferior alveolar nerve transection

BACKGROUND

It is important to know the mechanisms underlying pain abnormalities associated with inferior alveolar nerve (IAN) regeneration in order to develop the appropriate treatment for orofacial neuropathic pain patients. However, peripheral mechanisms underlying orofacial pain abnormalities following IAN regeneration are not fully understood.

METHODS

Head withdrawal threshold (HWT), jaw opening reflex (JOR) thresholds, single-fibre recordings of the regenerated mental nerve (MN) fibres, calcitonin gene-related peptide (CGRP), isolectin B4 (IB4), peripherin, neurofilament-200 (NF-200) and transient receptor potential vanilloid 1 (TRPV1) expression in trigeminal ganglion (TG) cells, and electron microscopic (EM) observations of the regenerated MN fibres were studied in MN- and IAN-transected (M-IANX) rats.

RESULTS

HWT to mechanical or heat stimulation of the mental skin was significantly lower in M-IANX rats compared with sham rats. Mean conduction velocity of action potentials recorded from MN fibres (n = 124) was significantly slower in M-IANX rats compared with sham rats. The percentage of Fluoro-Gold (FG)-labelled CGRP-, peripherin- or TRPV1-immunoreactive (IR) cells was significantly larger in M-IANX rats compared with that of sham rats, whereas that of FG-labelled IB4- and NF-200-IR cells was significantly smaller in M-IANX rats compared with sham rats. Large-sized myelinated nerve fibres were rarely observed in M-IANX rats, whereas large-sized unmyelinated nerve fibres were frequently observed and were aggregated in the bundles at the distal portion of regenerated axons.

CONCLUSIONS

These findings suggest that the demyelination of MN fibres following regeneration may be involved in peripheral sensitization, resulting in the orofacial neuropathic pain associated with trigeminal nerve injury.

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.

Involvement of trigeminal transition zone and laminated subnucleus caudalis in masseter muscle hypersensitivity associated with tooth inflammation

A rat model of pulpitis/periapical periodontitis was used to study mechanisms underlying extraterritorial enhancement of masseter response associated with tooth inflammation. Periapical bone loss gradually increased and peaked at 6 weeks after complete Freund's adjuvant (CFA) application to the upper molar tooth pulp (M1). On day 3, the number of Fos-immunoreactive (IR) cells was significantly larger in M1 CFA rats compared with M1 vehicle (veh) rats in the trigeminal subnucleus interpolaris/caudalis transition zone (Vi/Vc). The number of Fos-IR cells was significantly larger in M1 CFA and masseter (Mass) capsaicin applied (M1 CFA/Mass cap) rats compared with M1 veh/Mass veh rats in the contralateral Vc and Vi/Vc. The number of phosphorylated extracellular signal-regulated kinase (pERK)-IR cells was significantly larger in M1 CFA/Mass cap and M1 veh/Mass cap rats compared to Mass-vehicle applied rats with M1 vehicle or CFA in the Vi/Vc. Pulpal CFA application caused significant increase in the number of Fos-IR cells in the Vi/Vc but not Vc on week 6. The number of pERK-IR cells was significantly lager in the rats with capsaicin application to the Mass compared to Mass-vehicle treated rats after pulpal CFA- or vehicle-application. However, capsaicin application to the Mass did not further affect the number of Fos-IR cells in the Vi/Vc in pulpal CFA-applied rats. The digastric electromyographic (d-EMG) activity after Mass-capsaicin application was significantly increased on day 3 and lasted longer at 6 weeks after pulpal CFA application, and these increase and duration were significantly attenuated by i.t. PD98059, a MEK1 inhibitor. These findings suggest that Vi/Vc and Vc neuronal excitation is involved in the facilitation of extraterritorial hyperalgesia for Mass primed with periapical periodontitis or acute pulpal-inflammation. Furthermore, phosphorylation of ERK in the Vi/Vc and Vc play pivotal roles in masseter hyperalgesia after pulpitis or periapical periodontitis.

Fish oil concentrate delays sensitivity to thermal nociception in mice

Fish oil has been used to alleviate pain associated with inflammatory conditions such as rheumatoid arthritis. The anti-inflammatory property of fish oil is attributed to the n-3 fatty acids docosahexaenoic acid and eicosapentaenoic acid. Contrarily, vegetable oils such as safflower oil are rich in n-6 fatty acids which are considered to be mediators of inflammation. This study investigates the effect of n-3 and n-6 fatty acids rich oils as dietary supplements on the thermally induced pain sensitivity in healthy mice. C57Bl/6J mice were fed diet containing regular fish oil, concentrated fish oil formulation (CFO) and safflower oil (SO) for 6 months. Pain sensitivity was measured by Plantar test and was correlated to the expression of acid sensing ion channels (ASICs), transient receptor potential vanilloid 1 (TRPV1) and c-fos in dorsal root ganglion cells. Significant delay in sensitivity to thermal nociception was observed in mice fed CFO compared to mice fed SO (p<0.05). A significant diminution in expression of ion channels such as ASIC1a (64%), ASIC13 (37%) and TRPV1 (56%) coupled with reduced expression of c-fos, a marker of neuronal activation, was observed in the dorsal root ganglion cells of mice fed CFO compared to that fed SO. In conclusion, we describe here the potential of fish oil supplement in reducing sensitivity to thermal nociception in normal mice.

NMDA receptor, PKC and ERK prevent fos expression induced by the activation of group I metabotropic glutamate receptors in the spinal trigeminal subnucleus oralis

Fos, a protein product of immediate early gene c-fos, has been used as a marker for activation of nociceptive neurons in central nervous system including spinal trigeminal nucleus (Vsp). By noxious stimulation applied to orofacial area, the expression of Fos occurred in the Vsp pars oralis (Vo), the subnucleus receiving inputs from trigeminal primary afferents that predominantly innervate intraoral receptive fields. The present study demonstrates that the in vitro activation of group I metabotropic glutamate receptors (mGluRs; mGluR1 and 5) by bath-application of their well-known agonist (S)-3,5-dihydroxyphenylglycine (DHPG) increased the number of Fos-expressing neurons in the Vo area. In addition, bath application of DHPG caused inward currents, a parameter of neuronal excitation, in the Vo neurons held at -70 mV in voltage-clamp mode of whole-cell recordings. In further experiments characterizing two phenomena, the increased Fos expression in the Vo was mediated by an additive activation of both mGluR1 and mGluR5, which required the activation of N-methyl-D-aspartate (NMDA) receptors, protein kinase C (PKC) and extracellular signal-regulated kinase (ERK). In contrast, the inward currents were mediated only by mGluR1, but not by others. The data resulting from this in vitro study indicate that the DHPG-induced membrane depolarisation or neuronal excitation may be upstream to, or skip, the NMDA receptor, PKC and ERK pathways for the DHPG-induced Fos expression.

Activation of GABAergic neurons following tooth pulp stimulation

The functional impact of GABA (gamma-aminobutyric acid)ergic neurons in nociceptive transmission of the spinal trigeminal nucleus is not fully established. Using both the glutamic acid decarboxylase (GAD)(67)-green fluorescence protein (GFP) knock-in mouse and the tooth pulp stimulation model, we performed double-immunofluorescent histochemistry to determine the characteristics of GABAergic neuron activation in the spinal trigeminal nucleus. The number of Fos-positive GABAergic neuronal profiles was significantly increased 2 hrs after tooth pulp stimulation. The Fos/GFP double-labeled neurons were mainly present in superficial laminae of the spinal trigeminal subnucleus interpolaris-caudalis transition (Vi/Vc) and subnucleus caudalis (Vc) on the side ipsilateral to the stimulation. Subsequently, the number of double-labeled neurons decreased gradually and became comparable with that of the controls by 48 hrs. Our results provide direct morphological evidence that a subset of GABAergic neurons in the spinal trigeminal system was activated during tooth pulp stimulation.

Research progress in transient receptor potential vanilloid 1 of sensory nervous system

The transient receptor potential vanilloid subfamily member 1 (TRPV1) is a protein mainly expressed in sensory neurons and fibers, such as in trigeminal ganglion and dorsal root ganglion, and has been indicated to be involved in several physiological and pathological processes. Studies on thermal activation have revealed that phosphorylation is involved in TRPV1 activation and 2 putative phosphorylation sites, Ser residues 502 (Ser-502) and Ser residues 800 (Ser-800), have been recently confirmed to possess the capability of resensitizing TRPV1. In addition to acidification, alkalization has also been proved to be a highly effective stimulator for TRPV1. TRPV1 could be regulated by various physical and chemical modulators, as well as the chronic pain. TRPV1 plays a crucial role in the transmission of pain signals, especially under inflammation and the neoplasm conditions, and it can also modulate nociceptive afferents by reinforcing morphine tolerance. The present review mainly focused on the structural and functional complexities of TRPV1, together with its activation and modulation by a wide variety of physical and chemical stimuli. Its pharmacological manipulation (sensitization/desensitization) and therapeutical targets were also discussed.

Experimental occlusal interference induces long-term masticatory muscle hyperalgesia in rats

Temporomandibular joint or related masticatory muscle pain represents the most common chronic orofacial pain condition. Patients frequently report this kind of pain after dental alterations in occlusion. However, lack of understanding of the mechanisms of occlusion-related temporomandibular joint and muscle pain prevents treating this problem successfully. To explore the relationship between improper occlusion (occlusal interference) and masticatory muscle pain, we created an occlusal interference animal model by directly bonding a crown to a maxillary molar to raise the masticating surface of the tooth in rats. We raised the occlusal surface to three different heights (0.2, 0.4, and 0.6mm), and for one month we quantitatively measured mechanical nociceptive thresholds of the temporal and masseter muscles on both sides. Results showed a stimulus-response relationship between the height of occlusal interference and muscle hyperalgesia. Removal of the crown 6 days after occlusal interference showed that the removal at this time could not terminate the 1 month duration of mechanical hyperalgesia in the masticatory muscles. Lastly, we systemically administered NMDA antagonist MK801 (0.2, 0.1, and 0.05 mg/kg) to the treated rats and found that MK801 dose dependently attenuated the occlusal interference-induced hyperalgesia. These findings suggest that occlusal interference is directly related to masticatory muscle pain, and that central sensitization mechanisms are involved in the maintenance of the occlusal interference-induced mechanical hyperalgesia.

Organization of pERK-immunoreactive cells in trigeminal spinal nucleus caudalis and upper cervical cord following capsaicin injection into oral and craniofacial regions in rats

To define the somatotopic arrangement of neurons in the trigeminal spinal subnucleus caudalis and upper cervical cord activated by acute noxious stimulation of various orofacial sites, pERK expression was analyzed in these neurons. After capsaicin injection into the tongue, lower gum, upper and lower lips, or mental region, pERK-like immunoreactive (pERK-LI) cells were distributed mainly in the dorsal half of the trigeminal spinal nucleus interporalis (Vi) and caudalis (Vc) transition zone (Vi/Vc zone), middle Vc, and Vc and upper cervical cord transition zone (Vc/C2 zone). pERK-LI cells were distributed throughout the dorsal to ventral portion of the Vi/Vc zone, middle Vc, and Vc/C2 zone following capsaicin injection into the anterior hard palate, upper gum, buccal mucosa, or vibrissal pad and in the ventral portion of the Vi/Vc zone, middle Vc, and Vc/C2 zone following snout, ophthalmic, or ocular injection of capsaicin. The rostrocaudal distribution area of pERK-LI cells was more extensive from the Vi/Vc zone to the Vc/C2 zone after intraoral injection than that after facial injection, and the rostrocaudal distribution of pERK-LI cells from the Vi/Vc zone to the Vc/C2 zone had a somatotopic arrangement, with the snout being represented most rostrally and ophthalmic, ocular, or mental regions represented most caudally. These findings suggest that the pERK-LI cells expressed from the Vi/Vc zone to the Vc/C2 zone following injection of capsaicin in facial and intraoral structures may be differentially involved in pain perception in facial and intraoral sites.

Acidic microenvironment created by osteoclasts causes bone pain associated with tumor colonization

Bone pain is one of the most common complications in cancer patients with bone metastases. Previous findings that inhibitors of osteoclastic bone resorption such as bisphosphonates (BPs) reduce bone pain suggest a critical role of osteoclasts. Osteoclasts destroy bone by secreting protons, thereby making adjacent microenvironment acidic. Because acidosis is a well-known cause of pain, it is plausible that an osteoclasts-created acidic microenvironment may cause bone pain associated with cancer colonization in bone. To test this notion, we studied an animal model in which inoculation of MRMT-1 rat breast cancer cells into the tibiae in female rats induced hyperalgesia. Radiographic and histological analyses demonstrated that MRMT-1 cells caused aggressive bone destruction with an increased number of osteoclasts. Behavioral analyses showed that rats exhibited hyperalgesia in the tumor-inoculated legs. The BP zoledronic acid (ZOL) significantly reduced the hyperalgesia. In addition, immunohistochemical examinations revealed that c-Fos expression in the ipsilateral spinal cord neurons was increased. ZOL decreased these c-Fos-positive neurons. To investigate the role of acidosis, mRNA expression of acid-sensing receptors including acid-sensing channels (ASICs) and transient receptor potential channel-vanilloid subfamily member 1 (TRPV1) in the dorsal root ganglions (DRGs) was determined. The expression of ASIC1a and ASIC1b was increased in the ipsilateral DRGs, whereas the ASIC3 and TRPV1 expression was not changed. Of note, ZOL reduced the expression of ASIC1a and ASIC1b. In conclusion, our data suggest that an acidic microenvironment created by osteoclasts, at least in part, contributes to the induction of hyperalgesia through upregulating ASICs expression.

Osteoclasts play a part in pain due to the inflammation adjacent to bone

Bone disorders with increased osteoclastic bone resorption are frequently associated with bone pain and inhibitors of osteoclasts reduce bone pain. Osteoclasts degrade bone minerals by secreting protons through the vacuolar H+-ATPase, creating acidic microenvironments. Because acidosis is a well-known cause of pain, we reasoned that osteoclasts cause pain through proton secretion. We explored this using an animal model in which a single subcutaneous injection of the complete Freund's adjuvant (CFA) in the hind-paw caused inflammatory hyperalgesia (hyper-responsiveness to noxious stimuli). Osteoclastic bone resorption was increased in the metatarsal bones in the CFA-injected hind-paws. CFA-induced hyperalgesia was significantly suppressed by the bisphosphonates, zoledronic acid (ZOL) and alendronate and osteoprotegerin. c-src-deficient mice in which osteoclasts are inherently dysfunctional exhibited reduced CFA-induced hyperalgesia. Repeated subcutaneous injections of parathyroid hormone-related protein into the hind-paw also induced hyperalgesia with increased osteoclastic bone resorption. The hyperalgesia was associated with increased mRNA expression of acid-sensing ion channel (ASIC) 1a, 1b and 3 in the ipsi-lateral dorsal root ganglions (DRGs) by RT-PCR and c-Fos in the ipsi-lateral spinal dorsal horn by immunohistochemistry. Of note, ZOL decreased the ASIC1a mRNA expression and c-Fos. Treatment of the DRG cell line F-11 with acid (pH5.5) increased ASIC1a, 1b and 3 mRNA expression and nuclear c-Fos expression. The ASIC blocker amiloride inhibited acid-induced c-Fos expression in F-11 cells. Moreover, F-11 cells transfected with the transient receptor potential channel vanilloid subfamily member 1 (TRPV1) showed increased acid-induced nuclear c-Fos expression compared with parental F-11 cells. Finally, bafilomycin A1, an inhibitor of the vacuolar H+-ATPase, reversed the hyperalgesia and down-regulated ASIC1a mRNA expression in the DRGs. These results led us to propose that osteoclasts play a part in CFA-induced inflammatory pain through an activation of the acid-sensing receptors including ASICs and TRPV1 by creating acidosis.

Bee venom injection significantly reduces nociceptive behavior in the mouse formalin test via capsaicin-insensitive afferents

Peripheral bee venom (BV) administration produces 2 contrasting effects, nociception and antinociception. This study was designed to evaluate whether the initial nociceptive effect induced by BV injection into the Zusanli acupoint is involved in producing the more prolonged antinociceptive effect observed in the mouse formalin test, and whether capsaicin-sensitive primary afferents are involved in these effects. BV injection into the Zusanli point increased spinal Fos expression but not spontaneous nociceptive behavior. BV pretreatment 10 minutes before intraplantar formalin injection dose-dependently attenuated nociceptive behavior associated with the second phase of the formalin test. The destruction of capsaicin-sensitive primary afferents by resiniferatoxin (RTX) pretreatment selectively decreased BV-induced spinal Fos expression but did not affect BV-induced antinociception. Furthermore, BV injection increased Fos expression in tyrosine hydroxylase immunoreactive neurons in the locus caeruleus, and this expression was unaltered by RTX pretreatment. Finally, BV's antinociception was blocked by intrathecal injection of 10 microg idazoxan, and this effect was not modified by RTX pretreatment. These findings suggest that subcutaneous BV stimulation of the Zusanli point activates central catecholaminergic neurons via capsaicin-insensitive afferent fibers without induction of nociceptive behavior. This in turn leads to the activation of spinal alpha2-adrenoceptors, which ultimately reduces formalin-evoked nociceptive behaviors.

PERSPECTIVE

This study demonstrates that BV acupuncture produces a significant antinociception without nociceptive behavior in rodents, which is mediated by capsaicin-insensitive afferents and involves activation of central adrenergic circuits. These results further suggest that BV stimulation into this acupuncture point might be a valuable alternative to traditional electrical or mechanical acupoint stimulation.

Phosphorylation of Extracellular Signal-Regulated Kinase in medullary and upper cervical cord neurons following noxious tooth pulp stimulation

The phosphorylated Extracellular Signal-regulated Kinase (pERK) and Fos expression and masticatory muscle activity were analyzed in rats with capsaicin-induced acute inflammation of the tooth pulp in order to clarify the role of the spinal trigeminal nucleus and upper cervical spinal cord in tooth pulp pain. Digastric and masseteric muscle activities were significantly increased following capsaicin injection into the molar tooth pulp but not after vehicle treatment. The pERK-like immunoreactive (LI) neurons were observed in the subnuclei interpolaris-caudalis transition (Vi/Vc) zone, the paratrigeminal nucleus (Pa5) and the superficial laminae of the caudal Vc/C2 zone. The pERK expression was detected as early as 2 min and peaked at 5 min after capsaicin or vehicle injection. The pERK expression in the Vi/Vc zone and Pa5 was bilateral, whereas it was predominantly ipsilateral in the caudal Vc/C2 zone. The capsaicin treatment of the whisker pad produced pERK expression in the rostro-caudal middle portion of the ipsilateral Vc, but small number of pERK-LI cells were observed after vehicle treatment. The pERK expression was similar in the Vi/Vc zone following capsaicin injection into the upper or lower molar tooth pulp, whereas the pERK expression was in the lateral portion of the caudal Vc/C2 zone after upper molar injection and restricted to the medial portion of the Vc/C2 zone after the lower molar capsaicin. These data were confirmed with Western blots. There were differences in the distribution of Fos protein-like immunoreactive (LI) cells and pERK-LI cells following tooth pulp stimulation. After capsaicin application into the upper molar tooth pulp, no pERK-LI cells were observed in the ventral part of the Vi/Vc zone, whereas many Fos protein-LI cells were expressed in this region. The difference in the distribution pattern of pERK- and Fos protein-LI cells in the Vi/Vc zone suggests their differential temporal expression profiles after capsaicin. The present findings suggest that tooth-pulp-driven neurons in the spinal trigeminal nucleus are involved in tooth pulp pain through activation of the intracellular signal transduction pathway that involves earlier ERK phosphorylation and subsequent Fos expression.

Nociceptive stimulation activates locus coeruleus neurones projecting to the somatosensory thalamus in the rat

In the thalamus, noradrenergic output from the pontine nucleus locus coeruleus (LC) may actively shape the response properties of various sensory networks en route to the cortex. Little is known, however, about the involvement of ascending noradrenergic innervation of the somatosensory thalamus in the processing of nociceptive information. To address this question, we combined the study of Fos expression upon nociceptive tooth pulp stimulation in the anaesthetized rat, with the detection of retrogradely traced neurones from the somatosensory thalamus. Cell bodies labelled retrogradely from the left thalamus were observed on both sides of the LC, with an ipsilateral predominance (n = 8). Electrical stimulation of the right incisor pulp (n = 4) provoked a significantly stronger Fos expression (around twice) than sham surgery (n = 4), in both the ipsi- and contralateral LC. Significantly larger numbers of double labelled neurones were counted in the LC of tooth-pulp-stimulated animals (representing around 30% of retrogradely labelled cells in LC) than in the LC of sham animals. They were found bilaterally, but with a clear, significant, ipsilateral (i.e. left) predominance. The present data offer an anatomical framework to understand how the LC is involved in the sensory processing of nociceptive information in the thalamus. For the first time, it is shown that nociceptive stimulation activates LC neurones projecting to the somatosensory thalamus. This suggests a new role for LC in modulating nociception within the thalamus.

The role of 5-hydroxytryptamine3 receptors in the vagal afferent activation-induced inhibition of the first cervical dorsal horn spinal neurons projected from tooth pulp in the rat

To test the hypothesis that vagal afferent (VA) stimulation modulates the first cervical dorsal horn (C(1)) neuron activity, which is projected by tooth pulp (TP) afferent inputs through the activation of a local GABAergic mechanism via 5-hydroxytryptamine(3) (5-HT(3)) receptors, we used the technique of microiontophoretic application of drugs. In pentobarbital-anesthetized rats, we recorded C(1) spinal neuron activity responding to TP stimulation. The TP stimulation-evoked C(1) spinal neuron excitation was inhibited by VA stimulation, and this inhibition was significantly attenuated by iontophoretic application of the 5-HT(3) receptor antagonist ICS 205-930 (3-tropanyl-indole-3-carboxylate hydrochloride [endo-8-methyl-8-azabicyclo [3.2.1] oct-3-ol indol-3-yl-carboxylate hydrochloride]) (40 nA) or the GABA(A) receptor antagonist bicuculline (40 nA). In another series of experiments, we determined that 60 nA iontophoretic application of glutamate produced a maximal increase in the C(1) spinal neuron activity at a minimal current. In 53 of 65 neurons (81.5%), VA conditioning stimulation (1.0 mA x 0.1 ms, 50 Hz for 30 s) caused a significant inhibition (35.1%) of the glutamate (60 nA) application-evoked C(1) spinal neuron excitation. Iontophoretic application of ICS 205-930 (40 nA) or bicuculline (40 nA) significantly attenuated the VA stimulation-induced inhibition of glutamate iontophoretic application (60 nA)-evoked C(1) spinal neuron excitation. These results suggest that VA stimulation-induced suppression of C(1) spinal neuron activity, responding to TP stimulation, involve 5-HT(3) receptor activation, possibly originating in the descending serotonergic inhibitory system, and postsynaptic modulation of inhibitory GABAergic neurons.

Expression of c-Fos protein in the trigeminal nuclear complex resulting from quantified force application to the rat molar

This study was conducted to investigate the expression and distribution of c-Fos-like immunoreactive neurones (Fos-neurones), in the rat trigeminal sensory nuclear complex, produced by mechanical forces with various magnitudes and durations applied to the left upper first molar. The magnitudes of forces applied to the tooth were 25, 50 and 100 g and the duration was 2 h. A quantified force of 100 g was also applied to the upper molar for varying durations [short-time (1-2 min)], 2, 4, 8 and 12 h. Fos-neurones distributed in the bilateral superficial laminae of the subnucleus caudalis, and the ipsilateral dorsomedial part of subnucleus oralis (Sp5Odm). The number of Fos-neurones increased in the subnucleus caudalis (Sp5C) according to the force magnitude. In the Sp5C, the number of Fos-neurones exhibited maximum level, 2 or 4 h after the application. In the Sp5Odm, however, the number of Fos-neurones reached the maximum level at 8 h. These data suggest that the change in the number of nociceptive neurones in Sp5C reflect changes in encoding the magnitude of force to tooth, and that the nature of pain response to orthodontic forces might have some relation to the delayed expression of c-Fos protein in the Sp5Odm.

Use of noninvasive dental dolorimetry to evaluate analgesic effects of intravenous and intrathecal administration of morphine in anesthetized dogs

OBJECTIVE

To determine whether changes in amplitude of the reflex-evoked muscle action potential (REMP) elicited by noninvasive dental dolorimetry (electrical stimulation of the tooth pulp) in anesthetized dogs may be used to objectively evaluate the effectiveness of IV and intrathecal (IT) administration of morphine.

ANIMALS

6 male Beagles that were 2 to 6 years old.

PROCEDURE

Dogs were used in a crossover design with at least a 5-day washout period between treatments. Each dog received morphine, saline (0.9% NaCl) solution, and oxytocin via the IV and IT routes of administration; however, only results for morphine and saline treatments were reported here. Dogs were anesthetized and prepared for noninvasive dental dolorimetry. After IV or IT administration, electrical stimulation was applied to a tooth, and REMPs of the digastricus muscle were recorded at 5-minute intervals for 60 minutes. To determine differences in REMP amplitude between treatments, a linear regression line was fitted for each dog-treatment combination.

RESULTS

The IV administration of morphine significantly inhibited REMP amplitude, compared with IV administration of saline solution. Intrathecal administration of morphine significantly inhibited REMP amplitude, compared with IT administration of saline solution.

CONCLUSIONS AND CLINICAL RELEVANCE

Noninvasive dental dolorimetry in anesthetized dogs has promise as a technique for use in evaluating the analgesic potential of drugs administered IV and IT through evaluation of their effect on REMP amplitude recorded for the digastricus muscle.

Trigeminal c-Fos expression and behavioral responses to pulpal inflammation in ferrets

Injury to peripheral dental tissues evokes dynamic alternations in central sensory pathways. We have previously reported that transient stimulation of the dental pulp with noxious heat evokes the induction of the immediate early gene product Fos in the transitional region between subnucleus interpolaris and caudalis (Vi/Vc) and subnucleus caudalis (Vc). A question arises as to whether similar changes occur in response to inflammation to the tooth pulp. In this study, the effects of pulpal inflammation produced by bacterial lipopolysaccharide (LPS) on face-grooming behavior and trigeminal Fos expression were examined. Face-grooming behaviors were recorded daily for 3 days pre- and 24, 48 and 72 h post- LPS or saline application. All animals were perfused 72 h post- LPS or saline application. Brainstems were processed for Fos-like immunoreactivity (Fos-LI). Teeth were processed for H&E staining. Histological examination of LPS-treated teeth revealed features of an acute pulpitis. Moreover, LPS-treated animals showed greater face-grooming activity (i.e. tongue protrusions) directed to the injured tooth than the sham-operated group. The number of Fos-positive neurons was greater in the trigeminal subnucleus caudalis (Vc) and the transitional regions (Vi/Vc) in LPS-treated animals compared with sham-operated animals, and greater in the deeper laminae than the superficial laminae of each trigeminal region. LPS treatment did not evoke Fos expression in the rostral trigeminal regions above Vi/Vc. These results demonstrate that LPS-induced pulpal inflammation results in significant alterations in the Vi/Vc and Vc, and such changes may underlie the observed nociceptive behavioral responses and may play an important role in producing a symptomatic pulpitis in humans.

Induction of Fos protein-like immunoreactivity in the trigeminal spinal nucleus caudalis and upper cervical cord following noxious and non-noxious mechanical stimulation of the whisker pad of the rat with an inferior alveolar nerve transection

After transection of the inferior alveolar nerve (IAN: the third branch of the trigeminal nerve), the whisker pad area, which is innervated by the second branch of the trigeminal nerve, showed hypersensitivity to mechanical stimulation. Two days after IAN transection, the threshold intensity for escape behavior to mechanical stimulation of the ipsilateral whisker pad area was less than 1.0 g, a sign of allodynia, and returned to the preoperative level (preoperative threshold: 52.0 g) at 32 days after surgery. This decrement of escape threshold lasted for more than 3 weeks. The whisker pad area contralateral to the IAN transection also showed a decrease in escape threshold to non-noxious mechanical stimulation as compared with sham-operated rats. However, the change in threshold intensity for the side contralateral to transection was not as pronounced as that on the ipsilateral side. Fos protein-like immunoreactive (LI) cells were observed in the superficial laminae but not dominant in deeper laminae of the trigeminal spinal nucleus caudalis (Vc) and the first segment of the spinal cord (C1) after non-noxious mechanical stimulation of the whisker pad area in the rats with IAN transection. Fos protein-LI cells were expressed bilaterally in the Vc and C1, but were more numerous on the ipsilateral side to transection than on the contralateral side. The largest number of Fos protein-LI cells was observed at 2400 microm caudal from the trigeminal subnucleus interporalis (Vi)-Vc border both in ipsilateral and contralateral sides. The number of Fos protein-LI cells increased after application of 1, 4, and 16 g stimuli as compared to rats without mechanical stimulation. Furthermore, an extensively greater number of Fos protein-LI cells were expressed both in superficial and deep laminae of the bilateral Vc and C1 of the spinal cord after subcutaneous injection of mustard oil into the whisker pad. Fos protein expression after mustard oil injection was much stronger than that observed after any mechanical stimulation in the rats with IAN transection. These data suggest that the change in the numbers and spatial arrangement of nociceptive neurons in the Vc and C1 after IAN transection reflect the development of mechanical hyperalgesia in the area adjacent to the IAN innervated region.

Tooth extraction-induced internalization of the substance P receptor in trigeminal nucleus and spinal cord neurons: imaging the neurochemistry of dental pain

Although pains arising from the craniofacial complex can be severe and debilitating, relatively little is known about the peripheral and central mechanisms that generate and maintain orofacial pain. To better understand the neurons in the trigeminal complex and spinal cord that are activated following nociceptive stimuli to the orofacial complex, we examined substance P (SP) induced internalization of substance P receptors (SPR) in neurons following dental extraction in the rat. Unilateral gingival reflection or surgical extraction of a rat maxillary incisor or molar was performed and tissues harvested at various time points post-extraction. Immunohistochemical analysis of brainstem and cervical spinal cord sections was performed using an anti-SPR antibody and confocal imaging. Both the number and location of neurons showing SPR internalization was dependent on the location and extent of tissue injury. Whereas extraction of the incisor induced internalization of SPR in neurons bilaterally in nucleus caudalis and the spinal cord, extraction of the molar induced strictly unilateral internalization of SPR-expressing neurons in the same brain structures. Minor tissue injury (retraction of the gingiva) activated SPR neurons located in lamina I whereas more extensive and severe tissue injury (incisor or molar extraction) induced extensive SPR internalization in neurons located in both laminae I and III-V. The rostrocaudal extent of the SPR internalization was also correlated with the extent of tissue injury. Thus, following relatively minor tissue injury (gingival reflection) neurons showing SPR internalization were confined to the nucleus caudalis while procedures which cause greater tissue injury (incisor or molar extraction), neurons showing SPR internalization extended from the interpolaris/caudalis transition zone through the C7 spinal level. Defining the population of neurons activated in orofacial pain and whether analgesics modify the activation of these neurons should provide insight into the mechanisms that generate and maintain acute and chronic orofacial pain.

The effects of noxious dental heating on the jaw-opening reflex and trigeminal Fos expression in the ferret

Previous studies have established that the activation of peripheral nociceptors alters the central processing of nociceptive stimuli. In this study, we examined whether noxious heating of the dental pulp enhances the nociceptive jaw-opening reflex (JOR) and the expression of the immediate early gene c-fos in chloral hydrate/pentobarbital-anesthetized ferrets. We hypothesized that the application of noxious heat to the dental pulp, a procedure that evokes a preferential activation of pulpal C-fibers, will enhance JOR responses to electrical stimulation of the tooth pulp and that this enhanced response will be associated with the expression of Fos protein in discrete regions of the trigeminal nucleus. Consistent with our predictions, we observed that noxious heat conditioning enhanced the JOR as indicated by an increase in the magnitude of the signal averaged digastric electromyogram response evoked by electrical stimuli applied to either a heat-conditioned maxillary canine or the contralateral nonconditioned canine. The enhancement in JOR responses was independent of temporal summation of the electrical stimulus for test stimuli delivered at either 1.0 or 0.1 Hz. Sensitization of the JOR was associated with an increase in the number of immunohistochemically identified Fos-positive nuclei in trigeminal caudalis (Vc) and the transition zone between trigeminal interpolaris and caudalis (Vi/Vc) ipsilateral to the site of stimulation compared with sham stimulated animals. These findings suggest that neuronal populations in Vc and Vi/Vc play a role in the enhanced reflex responses to tooth pulp stimulation and may contribute to the pain and hyperalgesia associated with a symptomatic pulpitis.

Temporal and spatial distribution of Fos protein in the parabrachial nucleus neurons during experimental tooth movement of the rat molar

The present study was undertaken to reveal spatio-temporal changes in the distribution of Fos-like immunoreactive (-IR) neurons in the parabrachial nucleus (PBN), one of the important relay nuclei for processing autonomic and somatosensory information from the oro-facial regions, following the induction of experimental tooth movement in rat upper molars. The experimental tooth movement was induced by the insertion of elastic rubber between the first and second upper molars. In normal animals, the PBN contained a smaller number of Fos-IR neurons. Following experimental tooth movement, the Fos-IR neurons increased in number significantly on both the ipsilateral and contralateral PBN, reaching a maximum at 4 h (about 10 times that of normal animals), and then decreased gradually. However, a significant number of Fos-IR neurons remained at 24 h post-operation. Remarkable side-by-side differences in the number of Fos-IR neurons were recognized at 1 to 4 h following the experimental tooth movement. Their number returned to normal (basal) levels at 5 days post. All subnuclei of PBN showed similar temporal changes in the number of Fos-IR neurons, this being particularly apparent in lateral PBN. Administrations of morphine (3 and 10 mg/kg, i.p.) drastically reduced the induction of Fos-IR neurons in all subnuclei of both the ipsilateral and contralateral PBN in a dose-dependent manner, and its effect was antagonized by pretreatment with naloxone (2 mg/kg, i.p.). The reduction of Fos-IR neurons by morphine pretreatment suggests that the appearance of Fos-IR neurons in the PBN may be partly due to the noxious stimulation and/or stress arising from tooth movement. The bilateral expression of Fos-IR neurons in the PBN indicates that the experimental tooth movement causes the activation of PBN neurons for the processing of somatosensory as well as autonomic information. The prolonged expression of Fos-IR neurons in all the subnuclei of bilateral PBN reflects clinical features of the transient discomfort and/or abnormal sensations, which many patients often complain about during orthodontic treatment.

Suppressive effects of Neiting acupuncture on toothache: an experimental analysis on Fos expression evoked by tooth pulp stimulation in the trigeminal subnucleus pars caudalis and the periaqueductal gray of rats

To clarify the antinociceptive mechanism of acupuncture on acute pain, c-fos protein (Fos) expression induced by tooth pulp stimulation was immunohistochemically examined in the spinal trigeminal subnucleus pars caudalis (spVc) and the periaqueductal gray (PAG) of rats with or without Neiting acupuncture. The central projection of trigeminal ganglion neurons innervating in the tooth pulp was examined by tract-tracing method with horseradish peroxidase-conjugated wheat germ agglutinin (WGA-HRP). Central terminals from the first maxillary molar tooth were labeled transganglionically in the dorsomedial part of spVc with WGA-HRP. Numerous numbers of Fos-immunoreactive (Fos-ir) cells were found in the spVc and PAG by stimulation of the tooth pulp with acetic acid or saline. Neiting acupuncture significantly reduced the Fos expression in the spVc induced by tooth pulp stimulation. On the other hand, Neiting acupuncture evoked many Fos-ir cells in the PAG. The present results suggest that Neiting acupuncture activated PAG neurons that sent descending inhibitory fibers to medullo-spinal nociceptive neurons, and reduced the number of Fos-expressed neurons in the trigeminal subnucleus pars caudalis mediating noxious information from teeth to the higher central nervous system.

Chronic tooth pulp inflammation causes transient and persistent expression of Fos in dynorphin-rich regions of rat brainstem

We have analyzed central Fos immunoreactivity (Fos-IR) brainstems of adult rats after three clinically relevant dental injuries: filled dentin (DF) cavities that cause mild pulp injury and heal within 1-2 weeks; open pulp exposures (PX) that cause gradual pulp loss and subsequent periodontal lesions; and filled pulp exposures (PXF). By 1 week after DF cavities, no Fos-IR remained except for sites such as lateral-ventral periolivary nucleus (LVPO) that had Fos-IR in all rats including controls. PX injury induced (1) a delayed transient expression of Fos at 1-2 weeks at three loci (ipsilateral neurons in dorsomedial nucleus oralis, paratrigeminal nucleus, and trigeminal tract), (2) persistent ipsilateral Fos for at least 4 weeks after injury in dynorphin (Dyn)-rich regions (rostral lateral solitary nucleus, periobex dorsal nucleus caudalis), and (3) late Fos-IR at 2-4 weeks (bilateral superficial cervical dorsal horn, contralateral dorsal nucleus caudalis, contralateral rostral lateral solitary nucleus). Rats with PXF injury were examined at 2 weeks, and they had greater numbers and more extensive rostro-caudal distribution of Fos neurons than the PX group. One week after PX injury, Fos-IR neurons were found in regions with strong Dyn-IR central fibers. Co-expression of Dyn and Fos was found in some unusually large neurons of the ipsilateral rostral lateral solitary nucleus, trigeminal tract, and dorsal nucleus caudalis. Immunocytochemistry for the p75 low affinity neurotrophin receptor (p75NTR) or for calcitonin gene-related peptide (CGRP) showed no consistent change in trigeminal central endings in any Fos-reactive brainstem areas, despite the extensive structural and cytochemical reorganization of the peripheral endings of the dental neurons. The Fos responses of central neurons to tooth injury have some unusual temporal and spatial patterns in adult rats compared to other trigeminal injury models.