Combined retrograde tracing and enzyme/immunohistochemistry of trigeminal ganglion cell bodies innervating tooth pulps in the rat

The anatomy, neurophysiology, and cellular mechanisms of intradental sensation

Somatosensory innervation of the oral cavity enables the detection of a range of environmental stimuli including minute and noxious mechanical forces. The trigeminal sensory neurons underlie sensation originating from the tooth. Prior work has provided important physiological and molecular characterization of dental pulp sensory innervation. Clinical dental experiences have informed our conception of the consequence of activating these neurons. However, the biological role of sensory innervation within the tooth is yet to be defined. Recent transcriptomic data, combined with mouse genetic tools, have the capacity to provide important cell-type resolution for the physiological and behavioral function of pulp-innervating sensory neurons. Importantly, these tools can be applied to determine the neuronal origin of acute dental pain that coincides with tooth damage as well as pain stemming from tissue inflammation (i.e., pulpitis) toward developing treatment strategies aimed at relieving these distinct forms of pain.

Current Concepts of Dentinal Hypersensitivity

INTRODUCTION

Although many clinical studies have reported on the prevalence of dental pain, far fewer studies have focused on the mechanisms of dental pain. This is an important gap because increased understanding of dental pain mechanisms may lead to improved diagnostic tests or therapeutic interventions. The aim of this study was to comprehensively review the literature on the mechanisms of dentinal sensitivity.

METHODS

PubMed and Ovid were searched for articles that addressed dentinal pain and or pulpal sensitivity. Because of the breadth of research ranging from cellular/molecular studies to clinical trials, a narrative review on the mechanisms of dentinal sensitivity was constructed based on the literature.

RESULTS

Five various mechanisms for dentinal sensitivity have been proposed: (1) the classic hydrodynamic theory, (2) direct innervation of dentinal tubules, (3) neuroplasticity and sensitization of nociceptors, (4) odontoblasts serving as sensory receptors, and (5) algoneurons.

CONCLUSIONS

These theories are not mutually exclusive, and it is possible that several of them contribute to dentinal sensitivity. Moreover, pulpal responses to tissue injury may alter the relative contribution of these mechanisms. For example, pulpal inflammation may lead to neuronal sprouting and peripheral sensitization. Knowledge of these mechanisms may prompt the development of therapeutic drugs that aim to disrupt these mechanisms, leading to more effective treatments for pulpal pain.

Transcriptomic Classification of Neurons Innervating Teeth

Toothache is a common painful consequence of damage to the teeth, particularly when coupled to infection. Clinical restoration of tooth integrity, sometimes involving physical and chemical sterilization of the tooth with nerve fiber ablation (i.e., endodontic therapy), generally alleviates pain and allows long-lasting dental function. These observations raise questions regarding the biological role of tooth-innervating fibers. Here, we determined the transcriptomic diversity of the sensory neurons that can be retrogradely labeled from mouse molar teeth. Our results demonstrate that individual molars are each targeted by a dedicated population of about 50 specialized trigeminal neurons. Transcriptomic profiling identifies the majority of these as expressing markers of fast-conducting neurons, with about two-thirds containing nociceptive markers. Our data provide a new view of dental innervation, extending previous reports that used candidate gene approaches. Importantly, almost all retrogradely labeled neurons, including nociceptors, express the recently characterized mechanosensor Piezo2, an ion channel that endows cells with sensitivity to gentle touch. Intriguingly, about a quarter of the labeled neurons do not appear to be nociceptors, perhaps insinuating a role for them in discriminative touch. We hypothesize that dental neurons are capable of providing mechanosensitive information to drive rapid behavioral responses and protect teeth from damage. Damage to the teeth and exposure of the large population of molar nociceptors may trigger prolonged or abnormal activation driving toothache. Future studies examining the responses of these transcriptomically defined classes of neurons will help define their significance in oral sensation.

TRPM8 Mediates Hyperosmotic Stimuli-Induced Nociception in Dental Afferents

Hyperosmolar sweet foods onto exposed tooth dentin evoke sudden and intense dental pain, called dentin hypersensitivity. However, it remains unclear how hyperosmolar stimuli excite dental primary afferent (DPA) neurons and thereby lead to dentin hypersensitivity. This study elucidated whether TRPM8, which is well known as a cold temperature- or menthol-activated receptor, additionally mediates nociception in response to hyperosmolar stimuli in adult mouse DPA neurons, which are identified by a fluorescent retrograde tracer: DiI. Single-cell reverse transcription polymerase chain reaction revealed that TRPM8 was expressed in subsets of DPA neurons and that TRPM8 was highly colocalized with TRPV1 and Piezo2. Immunohistochemical analysis also confirmed TRPM8 expression in DPA neurons. By using Fura-2-based calcium imaging, application of hyperosmolar sucrose solutions elicited calcium transients in subsets of the trigeminal ganglion neurons, which was significantly abolished by a selective TRPM8 antagonist: N-(3-Aminopropyl)-2-[(3-methylphenyl)methoxy]-N-(2-thienylmethyl)benzamide (AMTB) hydrochloride. When we further examined changes of c-fos expression (a neuronal activation marker) in the spinal trigeminal nucleus after hyperosmolar stimulation onto exposed tooth dentin, c-fos mRNA and protein expression were increased and were also significantly reduced by AMTB, especially in the spinal trigeminal interpolaris-caudalis transition zone (Vi/Vc). Taken together, our results provide strong evidence that TRPM8 expressed in DPA neurons might mediate dental pain as a hyperosmosensor in adult mice.

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.

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.

TRPM8 and TRPA1 do not contribute to dental pulp sensitivity to cold

Sensory neurons innervating the dental pulp have unique morphological and functional characteristics compared to neurons innervating other tissues. Stimulation of dental pulp afferents whatever the modality or intensity of the stimulus, even light mechanical stimulation that would not activate nociceptors in other tissues, produces an intense pain. These specific sensory characteristics could involve receptors of the Transient Receptor Potential channels (TRP) family. In this study, we compared the expression of the cold sensitive receptors TRPM8 and TRPA1 in trigeminal ganglion neurons innervating the dental pulp, the skin of the cheek or the buccal mucosa and we evaluated the involvement of these receptors in dental pulp sensitivity to cold. We showed a similar expression of TRPM8, TRPA1 and CGRP in sensory neurons innervating the dental pulp, the skin or the mucosa. Moreover, we demonstrated that noxious cold stimulation of the tooth induced an overexpression of cFos in the trigeminal nucleus that was not prevented by the genetic deletion of TRPM8 or the administration of the TRPA1 antagonist HC030031. These data suggest that the unique sensory characteristics of the dental pulp are independent to TRPM8 and TRPA1 receptors expression and functionality.

Expression of vesicular glutamate transporters in transient receptor potential melastatin 8 (TRPM8)-positive dental afferents in the mouse

Transient receptor potential melastatin 8 (TRPM8) is activated by innocuous cool and noxious cold and plays a crucial role in cold-induced acute pain and pain hypersensitivity. To help understand the mechanism of TRPM8-mediated cold perception under normal and pathologic conditions, we used light microscopic immunohistochemistry and Western blot analysis in mice expressing a genetically encoded axonal tracer in TRPM8-positive (+) neurons. We investigated the coexpression of TRPM8 and vesicular glutamate transporter 1 (VGLUT1) and VGLUT2 in the trigeminal ganglion (TG) and the dental pulp before and after inducing pulpal inflammation. Many TRPM8+ neurons in the TG and axons in the dental pulp expressed VGLUT2, while none expressed VGLUT1. TRPM8+ axons were dense in the pulp horn and peripheral pulp and also frequently observed in the dentinal tubules. Following pulpal inflammation, the proportion of VGLUT2+ and of VGLUT2+/TRPM8+ neurons increased significantly, whereas that of TRPM8+ neurons remained unchanged. Our findings suggest the existence of VGLUT2 (but not VGLUT1)-mediated glutamate signaling in TRPM8+ neurons possibly underlying the cold-induced acute pain and hypersensitivity to cold following pulpal inflammation.

Expression of vesicular glutamate transporters VGLUT1 and VGLUT2 in the rat dental pulp and trigeminal ganglion following inflammation

Background

There is increasing evidence that peripheral glutamate signaling mechanism is involved in the nociceptive transmission during pathological conditions. However, little is known about the glutamate signaling mechanism and related specific type of vesicular glutamate transporter (VGLUT) in the dental pulp following inflammation. To address this issue, we investigated expression and protein levels of VGLUT1 and VGLUT2 in the dental pulp and trigeminal ganglion (TG) following complete Freund’s adjuvant (CFA) application to the rat dental pulp by light microscopic immunohistochemistry and Western blot analysis.

Results

The density of VGLUT2− immunopositive (+) axons in the dental pulp and the number of VGLUT2+ soma in the TG increased significantly in the CFA-treated group, compared to control group. The protein levels of VGLUT2 in the dental pulp and TG were also significantly higher in the CFA-treated group than control group by Western blot analysis. The density of VGLUT1+ axons in the dental pulp and soma in the TG remained unchanged in the CFA-treated group.

Conclusions

These findings suggest that glutamate signaling that is mediated by VGLUT2 in the pulpal axons may be enhanced in the inflamed dental pulp, which may contribute to pulpal axon sensitization leading to hyperalgesia following inflammation.

Paradoxical surrogate markers of dental injury-induced pain in the mouse

Dental pain, including toothache, is one of the most prevalent types of orofacial pain, causing severe, persistent pain that has a significant negative effect on quality of life, including eating disturbances, mood changes, and sleep disruption. As the primary cause of toothache pain is injury to the uniquely innervated dental pulp, rodent models of this injury provide the opportunity to study neurobiological mechanisms of tissue injury-induced persistent pain. Here we evaluated behavioral changes in mice with a dental pulp injury (DPI) produced by mechanically exposing the pulp to the oral environment. We monitored the daily life behaviors of mice with DPI, including measures of eating, drinking, and movement. During the first 48 hours, the only parameter affected by DPI was locomotion, which was reduced. There was also a significant short-term decrease in the amount of weight gained by DPI animals that was not related to food consumption. As cold allodynia is frequently observed in individuals experiencing toothache pain, we tested whether mice with DPI demonstrate an aversion to drinking cold liquids using a cold-sucrose consumption test. Surprisingly, mice with DPI increased their consumption of sucrose solution, to over 150% of baseline, regardless of temperature. Both the weight loss and increased sucrose intake in the first 2 days of injury were reversed by administration of indomethacin. These findings indicate that enhanced sucrose consumption may be a reliable measure of orofacial pain in rodents, and suggest that alterations in energy expenditure and motivational behaviors are under-recognized outcomes of tooth injury.

Dental pulp and gingivomucosa in rats are innervated by two morphologically and neurochemically different populations of nociceptors

OBJECTIVES

Difference in phenotypes of sensory neurons innervating dental pulp or gingivomucosa may be responsible for intense pain sensations in pulpitis in contrast to relatively painless chronic periodontitis. Therefore, we classified these neurons according to their size and two neurochemical characteristics of nociceptors, their TrkA expression and isolectin IB4 binding.

DESIGN

In rats (n=6) fluorescent tracers Fluorogold and TrueBlue were simultaneously applied into the standard-sized tooth cavity and nearby gingival sulcus, respectively. After the fluorescence on paraffin trigeminal ganglia (TG) sections was identified and photographed, immunohistochemistry for TrkA expression and IB4 binding was performed on the same sections.

RESULTS

The average sizes of TG neurons projecting to the gingivomucosa and dental pulp were 894±441μm(2) and 1012±381μm(2), respectively. The proportions of small-sized gingival and pulpal neurons were 14% and 5%, respectively (p<0.05). The proportions of TrkA-positive neurons among all gingival or pulpal neurons were 76% and 86%, respectively (p<0.05). Among all gingival or pulpal neurons the proportions of IB4-positive neurons were 46% and 3% (p<0.001), respectively, and the majority of them were small-medium sized.

CONCLUSIONS

Dental pulp and gingivomucosa are richly innervated by nociceptive TrkA-expressing neurons. However, while great majority of pulpal neurons are larger NGF-dependent A-fibre nociceptors without affinity to bind IB4, almost half of the gingival neurons are smaller IB4 binding C-fibre nociceptors. The difference in phenotype of sensory neurons might partially explain the different sensitivity of both tissues during normal and pathological conditions.

Projection of non-peptidergic afferents to mouse tooth pulp

A large proportion of pulpal nociceptors are known to contain neuropeptides such as CGRP. However, the projection of non-peptidergic nociceptors to tooth pulp is controversial. Recently, the non- peptidergic subset of nociceptors has been implicated in mechanical pain in the skin. Since mechanical irritation of pulpal nociceptors is critical for evoking tooth pain under pathophysiological conditions, we investigated whether the non-peptidergic afferents project to tooth pulp as potential mechanotransducing afferents. For clear visualization of the non-peptidergic afferents, we took advantage of a recently generated knock-in mouse model in which an axonal tracer, farnesylated green fluorescence protein (GFP), is expressed from the locus of a sensory neuron-specific gene, Mrgprd. In the trigeminal ganglia (TG), we demonstrated that GFP is exclusively expressed in afferents binding to isolectin B4 (IB4), a neurochemical marker of non-peptidergic nociceptors, but is rarely co-localized with CGRP. Retrograde labeling of pulpal afferents demonstrated that a low proportion of pulpal afferents was co-localized with GFP. Immunohistochemical detection of the axonal tracer revealed that GFP-positive afferent terminals were densely projected into the tooth pulp. These results provide convincing evidence that non-peptidergic nociceptors are projected into the tooth pulp and suggest a potential role for these afferents in tooth pain.

Neurochemical properties of dental primary afferent neurons

The long belief that dental primary afferent (DPA) neurons are entirely composed of nociceptive neurons has been challenged by several anatomical and functional investigations. In order to characterize non-nociceptivepopulation among DPA neurons, retrograde transport fluorescent dye was placed in upper molars of rats and immunohistochemical detection of peripherin and neurofilament 200 in the labeled trigeminal ganglia was performed. As the results, majority ofDPA neurons were peripherin-expressing small-sized neurons, showing characteristic ofnociceptive C-fibers. However, 25.7% of DPA were stained with antibody against neurofilament 200, indicating significant portion of DPA neurons are related to large myelinated Aβ fibers. There were a small number of neurons thatexpressed both peripherin and neurofilament 200, suggestive of Aδ fibers. The possible transition of neurochemical properties by neuronal injury induced by retrograde labeling technique was ruled out by detection of minimal expression of neuronal injury marker, ATF-3. These results suggest that in addition to the large population of C-fiber-related nociceptive neurons, a subset of DPA neurons is myelinated large neurons, which is related to low-threshold mechanosensitive Aβ fibers. We suggest that these Aβ fiber-related neurons might play a role as mechanotransducers of fluid movement within dentinal tubules.

Unmyelinated nerve fibers in the human dental pulp express markers for myelinated fibers and show sodium channel accumulations

BACKGROUND

The dental pulp is a common source of pain and is used to study peripheral inflammatory pain mechanisms. Results show most fibers are unmyelinated, yet recent findings in experimental animals suggest many pulpal afferents originate from fibers that are myelinated at more proximal locations. Here we use the human dental pulp and confocal microscopy to examine the staining relationships of neurofilament heavy (NFH), a protein commonly expressed in myelinated afferents, with other markers to test the possibility that unmyelinated pulpal afferents originate from myelinated axons. Other staining relationships studied included myelin basic protein (MBP), protein gene product (PGP) 9.5 to identify all nerve fibers, tyrosine hydroxylase (TH) to identify sympathetic fibers, contactin-associated protein (caspr) to identify nodal sites, S-100 to identify Schwann cells and sodium channels (NaChs).

RESULTS

Results show NFH expression in most PGP9.5 fibers except those with TH and include the broad expression of NFH in axons lacking MBP. Fibers with NFH and MBP show NaCh clusters at nodal sites as expected, but surprisingly, NaCh accumulations are also seen in unmyelinated fibers with NFH, and in fibers with NFH that lack Schwann cell associations.

CONCLUSIONS

The expression of NFH in most axons suggests a myelinated origin for many pulpal afferents, while the presence of NaCh clusters in unmyelinated fibers suggests an inherent capacity for the unmyelinated segments of myelinated fibers to form NaCh accumulations. These findings have broad implications on the use of dental pulp to study pain mechanisms and suggest possible novel mechanisms responsible for NaCh cluster formation and neuronal excitability.

Sensory purinergic receptor P2X3 is elevated in burning mouth syndrome

Recent studies show that P2X(3) may play a role in neuropathic pain, including orofacial pain. Burning mouth syndrome (BMS) is a chronic neuropathic pain condition affecting 0.6-12% of post-menopausal women in the Western world. This study evaluates, for the first time, P2X(3) immunoreactivity levels in lingual mucosa in BMS patients. Patients diagnosed with BMS (n=9) in accordance with International Association for the Study of Pain criteria and patients attending for wisdom tooth removal (n=10, controls), were involved in this study. A pain history and score was recorded on a visual analogue scale (VAS) prior to obtaining a lingual biopsy. Immunohistochemistry and image analysis were used to quantify submucosal nerve fibres expressing P2X(3) and the structural marker neurofilaments. P2X(3) positive fibres were significantly increased in BMS compared with controls (p=0.024). In contrast, neurofilament-staining fibres were reduced in BMS, and when expressed as a ratio of the neurofilament percentage area, there was a trend for an increase of P2X(3) positive fibres in the BMS group. Increased P2X(3) immunoreactivity in the trigeminal sensory system may play a role in the symptoms observed in BMS. P2X(3) may therefore be a therapeutic target for treating BMS and trigeminal neuropathic pain.

Calcitonin gene-related peptide immunoreactive neurons innervating the soft palate, the root of tongue, and the pharynx in the superior glossopharyngeal ganglion of the rat

We have examined whether calcitonin gene-related peptide immunoreactive (CGRP-ir) neurons in the glossopharyngeal ganglia innervate the soft palate, the root of tongue, and the pharynx of the rat. Immunohistochemical observations revealed that numerous CGRP-ir neurons are located in the superior glossopharyngeal ganglion located ventrolateral to the medulla oblongata in the cranial cavity, and that CGRP-ir neurons are also located in the inferior glossopharyngeal ganglion at the jugular foramen. When Fluorogold was injected into the soft palate, the root of tongue, or the pharyngeal constrictor muscles, many retrogradely Fluorogold-labeled neurons were found in the superior glossopharyngeal ganglion and the nodose ganglion, and several Fluorogold-labeled neurons were found in the inferior glossopharyngeal ganglion. Double labeling with immunohistochemistry for CGRP and Fluorogold showed that in every case of injections of Fluorogold into the soft palate, the root of tongue, or the pharynx, about 30% of the Fluorogold-labeled neurons in the superior glossopharyngeal ganglion expressed CGRP-like immunoreactivity, while no double-labeled neurons were found in the inferior glossopharyngeal ganglion or the nodose ganglion. These results indicate that nociceptive sensory information from the soft palate, the root of tongue, and the pharynx might be conveyed by the neurons in the superior glossopharyngeal ganglion to the nucleus tractus solitarii.

A microinjection technique for targeting regions of embryonic and neonatal mouse brain in vivo

A simple pressure injection technique was developed to deliver substances into specific regions of the embryonic and neonatal mouse brain in vivo. The retrograde tracers Fluorogold and cholera toxin B subunit were used to test the validity of the technique. Injected animals survived the duration of transport (24-48 h) and then were sacrificed and perfused with fixative. Small injections (<or=50 nL) were contained within targeted structures of the perinatal brain and labeled distant cells of origin in several model neural pathways. Traced neural pathways in the perinatal mouse were further examined with immunohistochemical methods to test the feasibility of double labeling experiments during development. Several experimental situations in which this technique would be useful are discussed, for example, to label projection neurons in slice or culture preparations of mouse embryos and neonates. The administration of pharmacological or genetic vectors directly into specific neural targets during development should also be feasible. An examination of the form of neural pathways during early stages of life may lead to insights regarding the functional changes that occur during critical periods of development and provide an anatomic basis for some neurodevelopmental disorders.

Unilateral ligature-induced periodontitis influences the expression of neuropeptides in the ipsilateral and contralateral trigeminal ganglion in rats

OBJECTIVES

Expression of neuronal neuropeptides in inflammatory conditions is altered. The changes in expression of substance P (SP) and calcitonin gene-related peptide (CGRP) in ipsilateral and contralateral trigeminal ganglion (TG) neurons were investigated by immunohistochemistry one week after unilateral ligature-induced periodontitis in rats.

DESIGN

A retrograde nerve tracer Fluorogold (FG) was applied into the gingival sulcus of the second maxillary molar to identify the neurons in TG that specifically innervate the inflamed gingivomucosa. In addition, neurons from the corresponding maxillary and the adjacent mandibular-ophthalmic regions in TG were analysed.

RESULTS

Statistically significantly higher frequencies of CGRP-positive neurons, regardless of their size, were found in TG ipsilateral to the periodontitis (83% and 73% in FG-labelled and maxillary regions, respectively) than in the control group without periodontitis (52% and 42% in FG-labelled and maxillary regions, respectively). The frequency of small FG-labelled SP-positive neurons in the ipsilateral TG (60%) was significantly higher than in the control TG (25%). In the contralateral TG the frequency of CGRP-positive neurons in maxillary region (66%) was significantly higher than in the control group. Surprisingly, the number of SP-positive neurons in all regions of contralateral TG decreased when compared to control and ipsilateral TGs.

CONCLUSIONS

Taken together, these results implicate a role of neurogenic component in the pathogenesis of periodontitis. The contralateral response in the TG could be mediated through the transmedian neurological pathways crossing in the trigeminal nuclear complex or through the systemic inflammatory reaction and the activation of the so called "neuro-immune axis".

Peripheral mechanisms of odontogenic pain

In this article, we review the key basic mechanisms associated with this phenomena and more recently identified mechanisms that are current areas of interest. Although many of these pain mechanisms apply throughout the body, we attempt to describe these mechanisms in the context of trigeminal pain.

Identification and neuropeptide content of trigeminal neurons innervating the rat gingivomucosal tissue

OBJECTIVES

The purpose of this study was to identify and characterise the neuropeptide content and the size of trigeminal ganglion (TG) neurons innervating the rat gingivomucosal tissue.

DESIGN

Retrograde nerve tracer Fluorogold (FG) was injected into the gingiva (group 1, n=5) or applied into the gingival sulcus (group 2, n=5) of the first right maxillary molar. After 10 days, the ganglia were dissected and FG fluorescence was observed under UV light microscope. Expression of calcitonin gene-related peptide (CGRP) and substance P (SP) in FG-labelled neurons was investigated by immunohistochemistry. Cross-sectional areas of neuron cell bodies containing FG were determined. As a control group, approximately 1000 neuron cell bodies representing the entire TG neuron population was evaluated in five trigeminal ganglia.

RESULTS

In group 1, the percentages of neurons containing CGRP (median 63%, range 48-72%) and SP (median 64%, range 54-64%) were significantly greater than in the control group (CGRP: median 43%, range 42-47% and SP: median 23%, range 21-27%). In group 2, only the percentage of neurons containing SP (median 50%, range 40-56%) was significantly greater than in the control group. FG-labelled neurons were predominantly small or medium sized (less than 1200 microm2). The neurons in the group 1 were significantly smaller than in group 2. In both experimental groups, immunopositive neurons were significantly smaller than immunonegative neurons.

CONCLUSIONS

The majority of neurons in TG that innervate the rat gingivomucosa are small or medium sized and contain CGRP and SP.

Immunocytochemical evidence that most sensory neurons of the rat molar pulp express receptors for both glial cell line-derived neurotrophic factor and nerve growth factor

Most pulpal afferent neurons have cytochemical features in common with the class of nociceptors that express neuropeptides and respond to NGF, while very few bind the plant lectin IB4, a widely used marker for the class of nociceptors that respond to the GDNF family of neurotrophic factors. The present study was undertaken to determine whether the GDNF receptor, GFRalpha-1, is expressed by pulpal afferents, and, further, to determine whether tooth injury evokes changes in expression of the GDNF and NGF receptors among pulpal afferents. The tracer, fluoro-gold (FG), was applied to shallow cavities in dentin of first and second maxillary molars. After 4 weeks, the molars of one side received a test injury consisting of a deeper cavity that exposed pulp horns. Animals were perfusion fixed 2 days later, and sections of the trigeminal ganglia were double immunostained with combinations of antibodies against GFRalpha-1, and TrkA. Under control conditions, GFRalpha-1 immunostaining was observed in 72% of neurons that projected to the molar pulp, TrkA in 78%, and immunostaining for both receptors was observed in 65% of pulpal afferents. Tooth injury evoked up-regulation of GFRalpha-1 expression (to 93%) and a slight down-regulation of TrkA expression (67%) among tooth afferents, while there was no discernable change in the proportion of pulpal afferents that expressed both TrkA and GFRalpha-1 (to 61%).

Mechanisms of orofacial pain control in the central nervous system

Recent advances in the study of pain have revealed somatotopic- and modality-dependent processing and the integration of nociceptive signals in the brain and spinal cord. This review summarizes the uniqueness of the trigeminal sensory nucleus (TSN) in structure and function as it relates to orofacial pain control. The oral nociceptive signal is primarily processed in the rostral TSN above the obex, the nucleus principalis (Vp), and the subnuclei oralis (SpVo) and interpolaris (SpVi), while secondarily processed in the subnucleus caudalis (SpVc). In contrast, the facial nociceptive signal is primarily processed in the SpVc. The neurons projecting to the thalamus are localized mostly in the Vp, moderately in the SpVi, and modestly in the ventrolateral SpVo and the SpVc. Orofacial sensory inputs are modulated in many different ways: by interneurons in the TSN proper, through reciprocal connection between the TSN and rostral ventromedial medulla, and by the cerebral cortex. A wide variety of neuroactive substances, including substance P, gamma-aminobutyric acid, serotonin and nitric oxide (NO) could be involved in the modulatory functions of these curcuits. The earliest expression of NO synthase (NOS) in the developing rat brain is observed in a discrete neuronal population in the SpVo at embryonic day 15. NOS expression in the SpVc is late at postnatal day 10. The neurons receiving intraoral signals are intimately related with the sensorimotor reflexive function through the SpVo. In summary, a better understanding of the trigeminal sensory system--which differs from the spinal system--will help to find potential therapeutic targets and lend to developing new analgesics for orofacial-specific pain with high efficacy and fewer side effects.

Localization of P2X and P2Y Receptors in Dorsal Root Ganglia of the Cat

The distribution of P2X and P2Y receptor subtypes in upper lumbosacral cat dorsal root ganglia (DRG) has been investigated using immunohistochemistry. Intensity of immunoreactivity for six P2X receptors (P2X5 receptors were immuno-negative) and the three P2Y receptors examined in cat DRG was in the order of P2Y2 = P2Y4>P2X3>P2X2 = P2X7>P2X6>P2X1 = P2X4>P2Y1. P2X3, P2Y2, and P2Y4 receptor polyclonal antibodies stained 33.8%, 35.3%, and 47.6% of DRG neurons, respectively. Most P2Y2, P2X1, P2X3, P2X4, and P2X6 receptor staining was detected in small- and medium-diameter neurons. However, P2Y4, P2X2, and P2X7 staining was present in large- and small-diameter neurons. Double-labeling immunohistochemistry showed that 90.8%, 32.1%, and 2.4% of P2X3 receptor-positive neurons coexpressed IB4, CGRP, and NF200, respectively; whereas 67.4%, 41.3%, and 39.1% of P2Y4 receptor-positive neurons coexpressed IB4, CGRP, and NF200, respectively. A total of 18.8%, 16.6%, and 63.5% of P2Y2 receptor-positive neurons also stained for IB4, CGRP, and NF200, respectively. Only 30% of DRG neurons in cat were P2X3-immunoreactive compared with 90% in rat and in mouse. A further difference was the low expression of P2Y1 receptors in cat DRG neurons compared with more than 80% of the neurons in rat. Many small-diameter neurons were NF200-positive in cat, again differing from rat and mouse.

Histochemical localisation of a galactose-containing glycoconjugate expressed by sensory neurones innervating different peripheral tissues in the rat

The plant lectin Bandeiraea simplicifolia I-isolectin B4 (BSI-B4) identifies a galactose-containing, membrane-associated glycoconjugate expressed by a discrete subpopulation of unmyelinated primary sensory neurones in the rat. We have previously suggested that BSI-B4 selectively binds to primary sensory neurones that innervate the skin. However, in that study, the tracer diamidino yellow was applied to the cut ends of peripheral nerves to identify neurones innervating particular target tissues. In this study, we have avoided axotomy by retrogradely labelling primary sensory neurones from peripheral tissues using the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbacyanine perchlorate (DiI). DiI was injected into the plantar skin, gastrocnemius muscle, and pyloric region of the stomach in rats. Corresponding ganglia were sectioned, incubated in BSI-B4 conjugated to fluorescein isothiocyanate, and examined with a fluorescence microscope. DiI-labelled cells were identified by red fluorescence within the cytoplasm, whereas cells binding BSI-B4 displayed green fluorescence associated with the plasma membrane and Golgi apparatus. Quantitative analysis revealed that 36.2% of cutaneous neurones, 7.6% of muscle neurones, and 6.8% of visceral neurones expressed the BSI-B4-binding site, indicating that a small but significant proportion of small-diameter primary sensory neurones innervating muscle and viscera also express BSI-B4-binding sites.

Changes in P2X3 purinoceptors in sensory ganglia of the mouse during embryonic and postnatal development

The expression of the P2X(3) nucleotide receptor in embryonic day 14-18, postnatal day 1-14 and adult mouse sensory ganglia was examined using immunohistochemistry. Nearly all sensory neurons in dorsal root ganglia, trigeminal ganglia and nodose ganglia in embryos at embryonic day 14 expressed P2X(3) receptors, but after birth there was a gradual decline to about 50% of neurons showing positive immunostaining for P2X(3). In embryos there were only small neurons, while from postnatal day 7 both large and small neurons were present. Isolectin B(4) (IB(4))-positive neurons in dorsal, trigeminal and nodose ganglia did not appear until birth, but the numbers increased to about 50% by postnatal day 14 when a high proportion of IB(4)-positive neurons were also positively labelled for the P2X(3) receptor. About 10% of neurons in dorsal, trigeminal and nodose ganglia were positive for calcitonin gene-related peptide in embryos, nearly all of which stained for P2X(3) receptors. This increased postnatally to about 35-40% in adults, although only a few colocalised with P2X(3) receptors. Neurofilament 200 was expressed in about 50% of neurons in trigeminal ganglia in the embryo, and this level persisted postnatally. All neurofilament 200-positive neurons stained for P2X(3) in embryonic dorsal root ganglia, trigeminal ganglia and nodose ganglia, but by adulthood this was significantly reduced. The neurons that were positive for calbindin in embryonic dorsal, trigeminal and nodose ganglia showed colocalisation with P2X(3) receptors, but few showed colocalisation postnatally.

Glial cell line-derived neurotrophic factor (GDNF) from adult rat tooth serves a distinct population of large-sized trigeminal neurons

Glial cell line-derived neurotrophic factor (GDNF) mediates trophic effects for specific classes of sensory neurons. The adult tooth pulp is a well-defined target of sensory trigeminal innervation. Here we investigated potential roles of GDNF in the regulation of adult trigeminal neurons and the dental pulp nerve supply of the rat maxillary first molar. Western blot analysis and radioactive 35S-UTP in situ hybridization revealed that GDNF in the dental pulp and its mRNAs were localized with Ngf in the coronal pulp periphery, in particular in the highly innervated subodontoblast layer. Retrograde neuronal transport of iodinated GDNF and Fluorogold (FG) from the dental pulp indicated that GDNF was transported in about one third of all the trigeminal dental neurons. Of the GDNF-labelled neurons, nearly all (97%) were large-sized (> or =35 microm in diameter). Analysis of FG-labelled neurons revealed that, of the trigeminal neurons supporting the adult dental pulp, approximately 20% were small-sized, lacked isolectin B4 binding and did not transport GDNF. Of the large-sized dental trigeminal neurons approximately 40% transported GDNF. About 90% of the GDNF-accumulating neurons were negative for the high-temperature nociceptive marker VRL-1. Our results show that a subclass of large adult trigeminal neurons are potentially dependent on dental pulp-derived GDNF while small dental trigeminal neurons seems not to require GDNF. This suggests that GDNF may function as a neurotrophic factor for subsets of nerves in the tooth, which apparently mediate mechanosensitive stimuli. As in dorsal root ganglia both small- and large-sized neurons are known to be GDNF-dependent; these data provide molecular evidence that the sensory supply in the adult tooth differs, in some aspects, from the cutaneous sensory system.

A model experimental system for monitoring changes in sensory neuron phenotype evoked by tooth injury

The dental pulp is a favorable model for studies of interactions between nociceptive sensory neurons and their peripheral target tissues. In the present study, we retrogradely labeled pulpal afferent neurons with an improved method that permits monitoring of changes in neuronal phenotype in response to controlled tooth injuries. The capacity of retrograde neuronal tracers to diffuse through dentinal tubules was exploited, thereby avoiding the severe injury to the pulp associated with previous tracer application methods. The strategy was to apply the durable fluorescent tracer, Fluoro-gold (FG), to exposed dentin in the floor of shallow cavities in molars, in order to pre-label pulpal neurons in trigeminal ganglia of young adult Sprague-Dawley rats. A high percentage of pupal afferent neurons were retrogradely labeled by application of FG to exposed dentin and the FG fluorescent signal persisted in most labeled neurons for at least 8 weeks. Following tracer application to dentin, the pulp tissue appeared normal histologically, with the exception that a layer of reactive dentin was deposited at the pulp-dentin border beneath the shallow cavities. Assessment of expression of calcitonin gene-related peptide (CGRP) and brain derived neurotrophic factor (BDNF) indicated that pulpal neurons remained in a quiescent, baseline condition cytochemically following application of tracer to cavities in dentin and upregulation of these markers could be detected in neurons that projected to teeth that received a test injury subsequent to tracer application. Thus, labeling of trigeminal neurons via dentinal tubules provides the basis for a useful model for precisely assessing properties of pulpal afferents in both quiescent and activated states.

Kv1.2-immunoreactive primary sensory neurons in the rat trigeminal ganglion

Immunohistochemistry for Kv1.2, a subunit of voltage-gated K(+) channels, was performed on the trigeminal ganglion (TG). Immunoreactivity (ir) was detected in half (48%) the TG neurons. These neurons were mostly medium-sized to large (range 137.6-2664.8 microm(2), mean+/-S.D. 892.6+/-413.3 microm(2)). A double immunofluorescence method also revealed co-expression of Kv1.2 and parvalbumin. Half (54%) the Kv1.2-immunoreactive (ir) neurons exhibited parvalbumin-ir, and parvalbumin-ir neurons mostly showed Kv1.2-ir (95%). Kv1.2-ir neurons which co-expressed CGRP-ir were rare in this ganglion. Some 40% of TG neurons retrogradely labeled from the facial skin exhibited Kv1.2-ir, whereas ir was detected in 16% of those labeled from the tooth pulp. The present study indicates that Kv1.2-ir TG neurons include low-threshold mechanoreceptors and nociceptors which innervate the facial skin and tooth pulp, respectively.

Patterns of fluoro-gold entry into rat molar enamel, dentin, and pulp

Permeabilities of enamel and dentin are not fully understood despite their importance for caries, restorative materials, and pulp-dentin-enamel interactions. We have found that Fluoro-Gold is useful for examining tooth permeability, and we designed studies to test the effects of aging, injury, neural function, and dentinal repair on its influx into vital rat teeth. We used fluorescence microscopy and immunocytochemistry to show that Fluoro-Gold rapidly penetrates enamel, the dentin-enamel junction, and outer dentinal acellular tubules, and then concentrates in odontoblasts, where it remains for weeks. As predicted, influx was greatest in immature teeth, and formation of reparative dentin impeded it. We expected that denervation would disrupt influx, because of neural regulation of dentinal fluid movement, but it did not. Damage to odontoblasts under injured dentin caused increased influx and efflux of Fluoro-Gold. Analysis of our data suggests that permeabilities of enamel and dentin to Fluoro-Gold are age-related, inter-dependent, and regulated by odontoblasts.

Changes in TrkB-like immunoreactivity in rat trigeminal ganglion after tooth injury

The purpose of this study was to characterize the impact of tooth injury on the distribution of tyrosine receptor kinase B (TrkB) among trigeminal ganglion neurons and assess the time course for tooth injury-induced TrkB distribution changes. In addition, we sought to further characterize the subpopulation of the afferents expressing TrkB receptors. Fifteen adult male Sprague-Dawley rats were studied. Pulpal inflammation was induced and ganglia were subsequently harvested and processed at different time points. Standard immunohistochemical fluorescence techniques were used to visualize TrkB-like immunoreactivity and isolectin B4 binding. Results indicate that full-length TrkB receptors are present in 36.6% of trigeminal ganglion neurons. This percentage decreases for the first 48 h and then increases to 41% by 7 days after tooth injury. Finally, TrkB appears to be present in a large percentage (54%) of isolectin B4+ neurons, suggesting that it is present in nociceptive afferents. These data highlight the fact that even mild injury results in sustained changes in nociceptive circuitry and raise the possibility that the brain-derived neurotrophic factor/TrkB system may contribute to persistent pain after tooth repair.

trkA modulation of developing somatosensory neurons in oro-facial tissues: tooth pulp fibers are absent in trkA knockout mice

To investigate the nerve growth factor requirement of developing oro-facial somatosensory afferents, we have studied the survival of sensory fibers subserving nociception, mechanoreception or proprioception in receptor tyrosine kinase (trkA) knockout mice using immunohistochemistry. trkA receptor null mutant mice lack nerve fibers in tooth pulp, including sympathetic fibers, and showed only sparse innervation of the periodontal ligament. Ruffini endings were formed definitively in the periodontal ligament of the trkA knockout mice, although calcitonin gene-related peptide- and substance P-immunoreactive fibers were reduced in number or had disappeared completely. trkA gene deletion had also no obvious effect on the formation of Meissner corpuscles in the palate. In the vibrissal follicle, however, some mechanoreceptive afferents were sensitive for trkA gene deletion, confirming a previous report [Fundin et al. (1997) Dev. Biol. 190, 94-116]. Moreover, calretinin-positive fibers innervating longitudinal lanceolate endings were completely lost in trkA knockout mice, as were the calretinin-containing parent cells in the trigeminal ganglion.These results indicate that trkA is indispensable for developing nociceptive neurons innervating oral tissues, but not for developing mechanoreceptive neurons innervating oral tissues (Ruffini endings and Meissner corpuscles), and that calretinin-containing, trkA dependent neurons in the trigeminal ganglion normally participate in mechanoreception through longitudinal lanceolate endings of the vibrissal follicle.

Immunohistochemical evidence for ATP receptors in human dental pulp

Evidence is accumulating which supports a role for ATP in the initiation of pain by acting on P2X receptors, in particular P2X3, expressed on nociceptive afferent nerve terminals. To investigate whether this receptor plays a role in dental pain, we studied the presence and distribution of P2X3 receptors in human dental pulp, and their co-localization with other neural markers. Pulps were removed from extracted third molars and immunohistochemically stained with an antibody against P2X3 receptors. P2X3 immunoreactive (-ir) nerve fibers were detected in the main body of the pulp, in the sub-odontoblastic plexus of Raschkow, and within the odontoblastic area. Co-localization of the P2X3-ir neurons with neurofilament protein (NF) showed that the majority of the fibers were positive for both NF and P2X3. Double labeling with isolectin B4 (IB4) showed that all P2X3-ir neurons also bind IB4. We conclude that P2X3 receptors are present on both myelinated and unmyelinated nerve fibers in human dental pulp and may play a role in dental pain mechanisms.

Effects of deprivation of neonatal nerve growth factor on the expression of neurotrophin receptors and brain-derived neurotrophic factor by dental pulp afferents of the adult rat

The dental pulp is richly innervated by peptidergic nociceptive neurones that are of special interest because of their central role in dental pain and because they have some features that are not typical of other somatic nociceptors. Here, (35)S-riboprobes were used to determine whether pulpal afferents of adult (2-month-old) rats express the nerve growth-factor (NGF) receptors, p75(NTR) and trkA, which are characteristic of peptidergic nociceptors, and additionally, whether these cells express receptors (trkB and trkC) for other members of the neurotrophin family. In order to begin characterizing the postnatal role of NGF in regulating these neurones, the susceptibility of pulpal afferents to antiserum-mediated early postnatal NGF depletion spanning the period of pulpal innervation development was also examined. In control animals, about 200 trigeminal ganglion cells were labelled after application of the retrograde tracer Fluoro-gold to the first maxillary molar. Among the labelled cells, 79% had positive hybridization signals for p75(NTR), 72% for trkA, 34% for trkB, 1% for trkC, and 77% for BDNF. Neonatal NGF depletion reduced the number of retrogradely labelled pulpal afferents by 33%, with numbers of smaller neurones being most strikingly subnormal. This reduction could be attributed to a partial depletion of the neurone population that expressed p75(NTR) and trkA. Consistent with reports that NGF-responsive neurones also express BDNF, NGF deprivation resulted in a reduction in the number of pulpal afferents that expressed BDNF to an extent similar to that seen for trkA. In contrast, anti-NGF exposure had little effect on the number of pulpal afferents that expressed trkB. These findings indicate that most pulpal afferents in the adult express the NGF receptors p75(NTR) and trkA, and thus have a continuing potential susceptibility to NGF-mediated regulation of functions such as neuropeptide and BDNF synthesis. However, only a subpopulation of this group of neurones requires NGF in order to develop connections to the pulp during the neonatal period. Few, if any, pulpal afferents express the high-affinity neurotrophin-3 (NT3) receptor trkC, although many have large cell bodies typical of NT3-responsive sensory neurones. A small subpopulation of pulpal afferents seems to express no neurotrophin receptors, yet it is unlikely that these cells belong to the class of small sensory cells known to bind isolectin IB4.

Dental injury models: experimental tools for understanding neuroinflammatory interactions and polymodal nociceptor functions

Recent research has shown that peripheral mechanisms of pain are much more complex than previously thought, and they differ for acutely injured normal tissues compared with chronic inflammation or neuropathic (nerve injury) pain. The purpose of the present review is to describe uses of dental injury models as experimental tools for understanding the normal functions of polymodal nociceptive nerves in healthy tissues, their neuroinflammatory interactions, and their roles in healing. A brief review of normal dental innervation and its interactions with healthy pulp tissue will be presented first, as a framework for understanding the changes that occur after injury. Then, the different types of dental injury that allow gradation of the extent of tissue damage will be described, along with the degree and duration of inflammation, the types of reactions in the trigeminal ganglion and brainstem, and the type of healing. The dental injury models have some unique features compared with neuroinflammation paradigms that affect other peripheral tissues such as skin, viscera, and joints. Peripheral inflammation models can all be contrasted to nerve injury studies that produce a different kind of neuroplasticity and neuropathic pain. Each of these models provides different insights about the normal and pathologic functions of peripheral nerve fibers and their effects on tissue homeostasis, inflammation, and wound healing. The physical confinement of dental pulp and its innervation within the tooth, the high incidence of polymodal A-delta and C-fibers in pulp and dentin, and the somatotopic organization of the trigeminal ganglion provide some special advantages for experimental design when dental injury models are used for the study of neuroinflammatory interactions.

Course and composition of the nerves that supply the mandibular teeth of the rat

The rodent dentition has become an important model for investigations of interactions between dental tissues and peripheral neurons. Although experimental nerve injury has been widely used for such studies, there is uncertainty about the courses of nerve fibers supplying the mandibular teeth. In order to clarify this, we used a mixture of monoclonal antibodies against neurofilament proteins to enhance demonstration of nerve fibers so that small nerves could be readily traced in serial frozen sections of mandibles of Sprague Dawley rats ranging in age from embryonic day (E) 18 to postnatal day (P) 90. The 1st molar and anterior portion of the 2nd molar were innervated by small nerves that emerged as distinct branches of the IAN trunk at or near the mandibular foramen. In contrast, the nerve supply to the 3rd molar and posterior part of the 2nd molar was a branch of the lingual nerve that bypassed the mandibular canal altogether. The IAN trunk split into the mental nerve and a large branch to the incisor about 2 mm anterior to the mandibular foramen. Thick branches of the incisor nerve descended into the incisor socket to form a dense plexus of nerve fiber bundles extending along the length of the incisor periodontium. The sparse pulpal innervation of the incisor was provided by a few thin fascicles that emerged from the caudal portion of the periodontal plexus to enter the incisor apex. The dental branches of the IAN and lingual nerve seen in the adult were well established and readily identifiable at age E18 even though their targets were limited to the follicles of the developing teeth. These studies show that the trigeminal branches that supply the mandibular teeth can be identified at a wide range of ages as distinct nerves at a considerable distance proximal to their targets. This detailed information on the courses taken by the dental nerves can provide an anatomical basis for increased precision in characterization and perturbation of neural pathways from the molars and incisor.

P2X3 is expressed by DRG neurons that terminate in inner lamina II

The P2X3 receptor subunit, a member of the P2X family of ATP-gated ion channels, is almost exclusively localized in sensory neurons. In the present study, we sought to gain insight into the role of P2X3 and P2X3-containing neurons in sensory transmission, using immunohistochemical approaches. In rat dorsal root ganglia (DRG), P2X3-immunoreactivity (-ir) was observed in small- and medium-sized neurons. Approximately 40% of DRG neuronal profiles in normal rats contained P2X3-ir. In rats that had received neonatal capsaicin treatment, the number of P2X3-positive neurons was decreased by approximately 70%. Analysis of the colocalization of P2X3-ir with cytochemical markers of DRG neurons indicated that approximately 94% of the P2X3-positive neuronal profiles were labelled by isolectin B4 from Bandeiraea simplicifolia, while only 3% contained substance P-ir, and 7% contained somatostatin-ir. In dorsal horn of rat spinal cord, P2X3-ir was observed in the inner portion of lamina II and was reduced subsequent to dorsal rhizotomy, as well as subsequent to neonatal capsaicin treatment. Finally, P2X3-ir accumulated proximal to the site of sciatic nerve ligation, and was seen in nerve fibres in skin and corneal epithelium. In summary, our results suggest that P2X3 is expressed by a functionally heterogeneous population of BSI-B4-binding sensory neurons, and is transported into both central and peripheral processes of these neurons.

Induction of immediate-early genes c-fos and zif268 in the subnucleus oralis by noxious tooth pulp stimulation

c-fos and zif268 expression were assessed by immunocytochemistry for c-Fos and Zif268 proteins in the sensory trigeminal nuclear complex following noxious mechanical stimulation of the mandibular incisor pulp of rats. Marked up-regulation of both immediate early genes was observed in the subnucleus oralis ipsilateral to the stimulation. Cavity preparation of the dentine without reaching the pulp did not cause significant up-regulation detectable by immunocytochemistry. These results provide evidence that noxious dental signals reach the ipsilateral subnucleus oralis and up-regulate the transcription of immediate early genes c-fos and zif268.

Expression of TrkB-like immunoreactivity in non-neural cells of rat periodontal ligament

The Trk family, a group of high-affinity neurotrophin receptors, is divided into three subtypes, TrkA, TrkB and TrkC. These were originally found in neural elements, and are involved in neural development, maintenance and survival. Recent studies have shown that non-neural cells in vitro also express mRNA encoding some neurotrophin receptors. In our preliminary study, TrkB-like immunoreactivity (LI) was found in the various non-neural cells in the rat periodontal ligament. The present study was undertaken to clarify which cell types express Trk-LI, in particular two types of TrkB-LI, in the periodontal ligament of mature rats, using an immunocytochemical technique with polyclonal antibodies. Intense full-length TrkB-LI was clearly recognized in non-neural cells such as fibroblasts, osteoclasts, odontoclasts and cementoblasts as well as in neural elements. Relatively large cells with many cytoplasmic processes were also frequently immunopositive for full-length TrkB. Immunocytochemistry for TrkB[TK-], a truncated type, also demonstrated a similar immunostaining pattern to that of full-length TrkB in non-neural periodontal cells, and intense positive reactions in endothelial cells. Some non-neural cells were positive for TrkA and TrkC. These findings suggest that neurotrophic factors, the ligands of the Trk family, have certain effects on the proliferation and/or differentiation of non-neural cells, as well as on their neurotrophic functions.

Central projection of calcitonin gene-related peptide (CGRP)- and substance P (SP)-immunoreactive trigeminal primary neurons in the rat

Substance P (SP) is implicated in transmission of primary afferent nociceptive signals. In primary neurons, SP is colocalized with calcitonin gene-related peptide (CGRP), which is another neuropeptide marker for small to medium primary neurons. CGRP coreleased with SP augments the postsynaptic effect of SP and thereby modulates the nociceptive transmission. This study demonstrates the distribution of CGRP-like immunoreactivity (-ir) and SP-ir in the lower brainstem of normal rats and after trigeminal rhizotomy or tractotomy at the level of subnucleus interpolaris (Vi). By comparing the results obtained from normal and deafferented rats, we analyzed the central projection of trigeminal primary nociceptors. The CGRP-immunoreactive (-ir) trigeminal primaries projected to the entire rostrocaudal extent of the spinal trigeminal nucleus, the principal nucleus (PrV), the paratrigeminal nucleus (paraV), and the lateral subnucleus of solitary tract nucleus (STN) on the ipsilateral side. The trigeminal primaries projecting to the spinal trigeminal nucleus, paraV and STN also contained SP-ir. The ipsilateral trigeminal primaries were the exclusive source of CGRP-ir terminals in the PrV, the Vi and the dorsomedial nucleus within the subnucleus oralis (Vo). The medullary dorsal horn (MDH) and the lateral edge of Vo received convergent CGRP-ir projection from the ipsilateral trigeminal primaries and other neurons. The glossopharyngeal and vagal primaries are candidates for the source of CGRP-ir projection to the Vo and the MDH, while the dorsal root axons supply the MDH with CGRP-ir terminals. In addition, contralateral primary neurons crossing the midline appear to contain CGRP and to terminate in the MDH.

Effects of neonatal exposure to anti-nerve growth factor on the number and size distribution of trigeminal neurones projecting to the molar dental pulp in rats

The first aim of the present study was to determine whether depletion of endogenous nerve growth factor (NGF) during early postnatal development results in a long-term deficit in the number of trigeminal ganglion cells and axons projecting to the molar pulp. The second aim was to identify selectivity of the effects of NGF deprivation for any specific size group among pulp neurones. Newborn Sprague-Dawley rats were given subcutaneous injections of either rabbit anti-mouse-NGF serum or non-immune (control) rabbit serum for a period of 1 month. At age 4 months, Fluoro-gold (FG) was applied to the pulp chamber of the right maxillary first molar. One week later the animals were perfusion-fixed, and the trigeminal ganglia were removed and serially sectioned with a cryostat. Labelled neurones were seen only in the trigeminal ganglia ipsilateral to the injected teeth. The area of every labelled cell profile was measured, and from these data, estimates of the true number and size distribution of FG-labelled cells were obtained by recursive translation. Ganglia of control animals had a mean of 197 labelled neurones, all in the maxillary division, and most of the somas were of medium or large diameter. NGF-deprived animals had significantly fewer (mean = 145) FG-labelled cells in the trigeminal ganglion ipsilateral to the injected tooth. Neurones with somas of less than 30 microns dia were most strikingly subnormal in anti-NGF treated animals (64% of controls). In accordance with the greater susceptibility of small neurones to anti-NGF exposure, deficits in apical nerve fibres of the mandibular first molar were greater in degree and duration for unmyelinated axons than for myelinated axons. It is concluded that NGF is an important mediator in regulation of postnatal development of the sensory innervation of the dental pulp. The results also indicate that postnatal development of at least one class of larger pulpal afferent neurones is regulated by factors other than NGF.

Neuropeptide Y-immunoreactive primary afferents in the dental pulp and periodontal ligament following nerve injury to the inferior alveolar nerve in the rat

The distribution of neuropeptide Y (NPY)-immunoreactive (IR) primary afferents in the dental pulp and periodontal ligament of the rat mandible were examined following combined chronic constriction injury (CCI) of the inferior alveolar nerve (IAN) and sympathectomy of the superior cervical ganglion (SCG). NPY-IR nerve fibers were observed around the blood vessels in the trigeminal ganglion, dental pulp and periodontal ligament in normal animals. Following combined CCI of the IAN and sympathectomy of SCG (SCGx), perivascular NPY-IR nerve fibers originating from SCG disappeared completely, but many NPY-IR nerve fibers coming from the trigeminal ganglion appeared in the dental pulp and periodontal ligament. In the molar dental pulp, thick NPY-IR nerve fibers were observed within the nerve bundle, and some thin NPY-IR nerve fibers ran towards the odontoblast layer; very few NPY-IR nerve fibers were observed in the incisor pulp. In the periodontal ligament of molar, thick NPY-IR nerve fibers appeared at the alveolar part following combined CCI of IAN and SCGx. In the lingual portion of the periodontal ligament of the incisor, many thick NPY-IR nerve fibers were observed. These occasionally showed a tree-like appearance, resembling immature Ruffini endings; slowly adapting mechanoreceptors. The present results indicate that periodontal mechanoreceptors are among the main targets of injury-evoked NPY following IAN injury.