The role of local factors in dentin and pulp in intradental pain mechanisms

Orofacial Pain Management: An Overview of the Potential Benefits of Palmitoylethanolamide and Other Natural Agents

Pain is the most common symptom that dentists are confronted with, whether acute (pulpitis, acute periodontitis, post-surgery, etc.) or chronic diseases, such as periodontitis, muscle pain, temporomandibular joint (TMJ) disorders, burning mouth syndrome (BMS), oral lichen planus (OLP) and others. The success of therapy depends on the reduction in and management of pain through specific drugs, hence the need to analyze new pain medications with specific activity, which are suitable for long-term use, with a low risk of side effects and interactions with other drugs, and capable of leading to a reduction in orofacial pain. Palmitoylethanolamide (PEA) is a bioactive lipid mediator, which is synthesized in all tissues of the body as a protective pro-homeostatic response to tissue damage and has aroused considerable interest in the dental field due to its anti-inflammatory, analgesic, antimicrobial, antipyretic, antiepileptic, immunomodulatory and neuroprotective activities. It has been observed that PEA could play a role in the management of the pain of orofacial origin, including BMS, OLP, periodontal disease, tongue a la carte and temporomandibular disorders (TMDs), as well as in the treatment of postoperative pain. However, actual clinical data on the use of PEA in the clinical management of patients with orofacial pain are still lacking. Therefore, the main objective of the present study is to provide an overview of orofacial pain in its many manifestations and an updated analysis of the molecular pain-relieving and anti-inflammatory properties of PEA to understand its beneficial effects in the management of patients with orofacial pain, both neuropathic and nociceptive in nature. The aim is also to direct research toward the testing and use of other natural agents that have already been shown to have anti-inflammatory, antioxidant and pain-relieving actions and could offer important support in the treatment of orofacial pain.

Conservative pulp therapy in the management of reversible and irreversible pulpitis

Conservative pulp therapy is an alternative treatment option to tooth removal and root canal treatment in the management of deep caries, traumatic pulp exposures and developmental anomalies. Pulp tissue preservation can extend the long-term survival of teeth through relatively simple restorative procedures. This article aims to update clinicians on the current state of research in materials, techniques and outcomes of vital pulp therapies and provide practical guidelines for their implementation into daily practice.

Applications of sensory and physiological measurement in oral-facial dental pain

Dentists regularly employ a variety of self‐report and sensory techniques to aid in the diagnosis and treatment of tooth‐related disease. Many of these techniques leverage principles borrowed from psychophysics, the quantitative measurement of the relationship between stimuli and evoked sensations, which falls under the larger umbrella of quantitative sensory testing (QST). However, most clinicians fail to meet the bar for what could be considered quantitative sensory testing, and instead focus on qualitative and dichotomous “yes/no” aspects of sensory experience. With our current subjective measurements for pain assessments, diagnosis and treatment of dental pain in young children and individuals (any age) with severe cognitive impairment rely extensively on third‐party observations. Consequently, the limitation of inadequate pain diagnosis can lead to poor pain management. In this review, it discusses mechanisms that underlie acute and chronic dental pain. It details the measurement of somatosensory responses and pulpal blood flow as objective measures of tooth health and pain. It proposes that bridging these varied methodologies will significantly improve diagnosis and treatment of orofacial pain and pathology. It concludes that improving the precision of sensory measurements could yield important improvements in diagnostic challenges in pulpal pathology for noncommunicative and cognitively impaired individuals.

MATHEMATICAL MODELING FOR THE PREDICTION AND IMPROVEMENT OF TOOTH THERMAL PAIN: A REVIEW

Tooth pain, especially tooth thermal pain, is one of the most important symptoms and signs in dental clinic and daily life. As a special sensation, pain has been studied extensively in both clinic and experimental research aimed at reducing or eliminating the possible negative effects of pain. Unfortunately, the full underlying mechanism of pain is still unclear, because the pain could be influenced by many factors, including physiological, psychological, physical, chemical, and biological factors and so on. Besides, most studies on pain mechanisms in the literature are based on skin pain sensation and only few are based on tooth pain. In this paper, we present a comprehensive review on both neurophysiology of tooth pain mechanism, and corresponding thermal, mechanical, and thermomechanical behaviors of teeth. We also describe a multiscale modeling approach for quantifying tooth thermal pain by integrating the mathematic methods of engineering into the neuroscience. The mathematical model of tooth thermal pain will enable better understanding of thermal pain mechanism and optimization of existing diagnosis and treatment in dental clinic.

Quantification of neuropeptides (calcitonin gene-related peptide, substance P, neurokinin A, neuropeptide Y and vasoactive intestinal polypeptide) expressed in healthy and inflamed human dental pulp

AIM

To quantify the expression of calcitonin gene-related peptide (CGRP), substance P (SP), neurokinin A (NKA), neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP) in healthy and inflamed human dental pulp tissue.

METHODOLOGY

Six pulp samples were obtained from teeth having a clinical diagnosis of acute irreversible pulpitis. Another 12 pulp samples were obtained from premolars where extraction was indicated for orthodontic purposes. In six of these premolar teeth inflammation was induced by mechanical pulp exposure prior to sample collection. All samples were processed and 125I-labelled; neuropeptides were quantified by competition assays. ANOVA and Mann-Whitney's (post hoc) tests were used to establish statistically significant differences between the groups.

RESULTS

Expression of five neuropeptides was found in all human pulp samples. Statistical analysis revealed a significantly higher (P < 0.05) expression of CGRP, SP, NKA and NPY in both inflammatory conditions compared with healthy pulp control values. VIP expression remained stable during the inflammatory conditions.

CONCLUSION

Expression of CGRP, SP and NKA released from C-fibres and NPY released from sympathetic fibres is significantly higher in the inflamed human pulp compared with healthy pulp. Expression of VIP released from parasympathetic fibres is not increased during the inflammatory conditions of human dental pulp.

The application of tissue engineering to regeneration of pulp and dentin in endodontics

Caries, pulpitis, and apical periodontitis increase health care costs and attendant loss of economic productivity. They ultimately result in premature tooth loss and therefore diminishing the quality of life. Advances in vital pulp therapy with pulp stem/progenitor cells might give impetus to regenerate dentin-pulp complex without the removal of the whole pulp. Tissue engineering is the science of design and manufacture of new tissues to replace lost parts because of diseases including cancer and trauma. The three key ingredients for tissue engineering are signals for morphogenesis, stem cells for responding to morphogens and the scaffold of extracellular matrix. In preclinical studies cell therapy and gene therapy have been developed for many tissues and organs such as bone, heart, liver, and kidney as a means of delivering growth factors, cytokines, or morphogens with stem/progenitor cells in a scaffold to the sites of tissue injury to accelerate and/or induce a natural biological regeneration. The pulp tissue contains stem/progenitor cells that potentially differentiate into odontoblasts in response to bone morphogenetic proteins (BMPs). There are two strategies to regenerate dentin. First, is in vivo therapy, where BMP proteins or BMP genes are directly applied to the exposed or amputated pulp. Second is ex vivo therapy and consists of isolation of stem/progenitor cells from pulp tissue, differentiation into odontoblasts with recombinant BMPs or BMP genes and finally transplanted autogenously to regenerate dentin. This review is focused on the recent progress in this area and discusses the barriers and challenges for clinical utility in endodontics.

Delta (delta) opioid receptors in small and medium-sized trigeminal neurons supporting the dental pulp of rats

The control of pain perception is a challenge in clinical dentistry, most prominent during tooth pulp inflammation. The tooth pulp is a well-defined target, and is densely supplied by a sensory trigeminal innervation. Opioids are signaling molecules that are suggested to participate in pain perception. Here we analysed the presence of delta opioid receptor (DOR) in trigeminal neurons innervating the tooth pulp of rat molars. Immunohistochemical and ultrastructural analysis revealed that DOR was identified in peripheral nerves in the molar dental pulp, both in the root and the coronal pulpal parts, with branching in the highly innervated subodontoblast layer. DOR was localised in about one third of all the trigeminal dental neurons, identified by means of retrograde neuronal transport of fluorogold (FG) from the dental pulp. Of the DOR-labeled neurons, nearly all were small and medium-sized (147.5-1,810.2 microm(2), mean 749.1 +/- 327.3 microm(2)). Confocal microscopy confirmed that DOR-immunoreactivity was distributed as granules in the neuronal cytoplasm. Approximately 70% of the DOR-immunoreactive neurons were also immunopositive for vanilloid receptor 1 (TRPV1). Ultrastructural analysis demonstrated DOR-immunoreactivity in the unmyelinated and in some of the myelinated nerve fibers in the dental pulp. These results indicate that DOR may influence the function in a subset of small and medium-sized trigeminal sensory neurons supporting the tooth, which are mainly known for their ability to mediate nociceptive stimuli. Agonists, acting on DOR, may thus have an influence on a subpopulation of nociceptive neurons supporting the rat tooth.

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.

Dental neuroplasticity, neuro-pulpal interactions, and nerve regeneration

This review covers current information about the ability of dental nerves to regenerate and the role of tooth pulp in recruitment of regenerating nerve fibers. In addition, the participation of dental nerves in pulpal injury responses and healing is discussed, especially concerning pulp regeneration and reinnervation after tooth replantation. The complex innervation of teeth is highly asymmetric and guided towards specific microenvironments along blood vessels or in the crown pulp and dentin. Pulpal products such as nerve growth factor are distributed in the same asymmetric gradients as the dentinal sensory innervation, suggesting regulation and recruitment of those nerve fibers by those specific factors. The nerve fibers have important effects on pulpal blood flow and inflammation, while their sprouting and cytochemical changes after tooth injury are in response to altered pulpal cytochemistry. Thus, their pattern and neuropeptide intensity are indicators of pulp status, while their local actions continually affect that status. When denervated teeth are injured, either by pulp exposure on the occlusal surface or by replantation, they have more pulpal necrosis than occurs for innervated teeth. However, small pulp exposures on the side of denervated crowns or larger lesions in germ-free animals can heal well, showing the value of postoperative protection from occlusal trauma or from infection. Current ideas about dental neuroplasticity, neuro-pulpal interactions, and nerve regeneration are related to the overall topics of tooth biomimetics and pulp/dentin regeneration.

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.

A comparison of four extraction methods for substance P, neurokinin A and calcitonin gene-related peptide from human dental pulp tissue

Measuring neuropeptides in biological tissues by radioimmunoassay requires efficient extraction that maintains their immunoreactivity. Many different methods for extraction have been described, but there is little information on optimal extraction methods for individual neuropeptides from human dental pulp tissue. The aim was therefore to identify an effective extraction procedure for three pulpal neuropeptides; substance P, neurokinin A and calcitonin gene-related peptide. Tissue was obtained from 20 pulps taken from teeth freshly extracted for orthodontic reasons. The pulp samples were divided into four equal groups and different extraction methods were used for each group. Boiling whole pulp in acetic acid gave the highest overall yield and, in addition, offered an easy and rapid means of pulp tissue processing. The use of protease inhibitors did not increase the recovery of the immunoreactive neuropeptides but did provide the best combination of maximal recoveries and minimal variability. These results should be useful for planning the extraction of these neuropeptides from human pulp tissue in future studies.

Chronic dexamethasone treatment and its effects on sensory neuropeptides, pulpal injury reactions and reparative dentin

Initial sensory nerve reactions to dental injuries include terminal sprouting and intensified immunoreactivity for calcitonin gene-related peptide (CGRP) and substance P (SP); those reactions are reduced at 4 days after injury when rats are treated daily with dexamethasone (DEX) [17]. Here we have analyzed long-term effects of DEX (daily, 0.2 mg/kg) on wound healing, sensory nerve sprouting, and CGRP/SP intensity at 7-14 days after cavity preparation. All DEX treated rats had loss of appetite and stopped growing during the postoperative periods while controls had normal postoperative growth. After 7-14 days, CGRP immunoreactivity (IR) was decreased to one-third of normal (P < 0.05) compared to vehicle in both the intact and injured molar pulp, and SP also decreased, but the neuropeptide intensity in adjacent periodontal innervation was not changed. Pulpal injury and inflammation were reduced by DEX treatment, but reparative dentin was formed just as well in the DEX rats as in the vehicle group. When the injured teeth formed fibrous dentin, there was sprouting of nerves towards that matrix, and DEX did not inhibit that reaction. The sprouts could contain intense neuropeptide immunoreactivity in DEX rats even though the CGRP/SP intensity in uninjured pulp was reduced. We conclude that (1) chronic DEX treatment causes a generalized decrease in CGRP and SP neuropeptides in pulpal nerves but not in periodontal ligament; (2) it reduces abscess formation in injured teeth; (3) it does not block reparative dentin formation; and (4) it does not block sprouting of pulpal nerves towards fibrous dentin. The selective loss of pulpal neuropeptides CGRP and SP during dexamethasone treatment may be caused by reduced dental function since there was substantial loss of appetite and chronic weight loss during the 1-2 week treatment periods.

Effects of experimental and clinical noxious counterirritants on pain perception

It is commonly accepted that application of a sustained noxious stimulus frequently suppresses the perception of pain. In this investigation, we have determined whether painful forearm ischemia suppresses tooth pain resulting from an acute irreversible pulpitis. We have also determined whether the physiological responses to toothache alter the perception of pain evoked by experimental procedures. Ten male subjects experiencing a painful toothache (group TA) and 7 age-matched pain-free male subjects (group PF) participated in these studies. During session 1, heat pain threshold and tolerance values were determined for both groups. The times to ischemic pain onset and ischemic pain tolerance were determined for both groups using the submaximal effort tourniquet procedure. The effect of the tourniquet procedure on the intensity, unpleasantness, and spatial distribution of toothache was also assessed. Session 2 was conducted on 7 TA and 7 PF subjects 1 week later and was conducted like session 1 with the exception that group TA was not experiencing tooth pain during this session. Measures of thermal pain perception and forearm ischemic pain perception were not altered by the occurrence of toothache. In contrast, sustained noxious forearm ischemia produced a marked reduction in the intensity, unpleasantness and spatial distribution of pulpal pain. These effects on pulpal pain remained for at least 5 min after removal of the tourniquet while the arm was pain free. These findings suggest that a noxious conditioning stimulus does not universally inhibit pain perception but instead depends on unidentified interactions between the noxious test and conditioning stimuli.

The neurophysiological basis and the role of inflammatory reactions in dentine hypersensitivity

Recent studies indicate that intradental A-type nerve fibres are responsible for the sensitivity of dentine and are activated by fluid movements in dentinal tubules (hydrodynamic mechanism). The patency of the tubules affects dentine sensitivity to a great extent. Both A delta- and A beta-type nerve fibres respond to dentinal (hydrodynamic) stimulation in a similar way. Only a few studies have been made on the regional sensitivity of dentine or the receptive areas of intradental nerve fibres. The results indicate that the fibres innervating different parts of coronal dentine are equally sensitive to dentinal stimulation but those in the cervical area may be less responsive. Inflammation in the pulp can considerably alter dentine sensitivity. In dog teeth with chronically exposed dentine, nerve responses to hydrodynamic stimulation were reduced although other functional changes indicated nerve sensitization. This may be due to spontaneously occurring changes in the exposed dentine that block the tubules. In acute experiments on cat and dog teeth with open dentinal tubules, certain inflammatory mediators increase the sensitivity of the responding nerve fibres. It seems that intradental C-fibres do not respond to hydrodynamic stimulation of dentine. They are polymodal and activated when external stimuli reach the pulp proper. They could perhaps mediate the dull pain connected with pulpitis. However, they might also have an important modifying effect on dentine sensitivity because they can release neuropeptides, which function in the inflammatory reactions.

Dexamethasone treatment reduces sensory neuropeptides and nerve sprouting reactions in injured teeth

Dental injuries have been shown to generate extensive structural and cytochemical changes in sensory fibers that contain neuropeptides such as calcitonin gene-related peptide (CGRP) or substance P (SP). The present study was designed to test whether the anti-inflammatory drug dexamethasone (DEX) can alter neural responses to dental injuries. DEX (20 micrograms/100 g body weight) was given to adult rats (n = 10) prior to dental surgery and daily thereafter for 4 days. Control animals received sterile saline vehicle (n = 6) or no injection (n = 1). Each rat was then anesthetized for dental surgery and a cavity was drilled partway through dentin on the anterior side of the right maxillary first molar. Pulp exposure injuries were also made on two right mandibular molars in 14 of 17 rats. After 4 days of daily drug treatment, the rats were anesthetized and fixed by perfusion with formaldehyde-picric acid, and their jaws were prepared for immunocytochemistry. Neural CGRP immunoreactivity near the maxillary cavity injury site of DEX-treated rats was reduced more than 50% compared to controls, as determined both qualitatively and by digital analysis. The SP immunoreactive (IR) fibers in molar pulp also had extensive inhibition of neural reactions to cavity injury. DEX also reduced the immunoreactivity for CGRP and SP in normal contralateral rat molars of all treated rats, and it caused a postoperative loss of weight. Pretreatment for 1-5 days prior to the 4 day injury gave the same results as pretreatment for 1 h. The mandibular pulp exposure injuries induced a chronic abscess and advancing pulpal necrosis but did not show differences in nerve reactions between DEX-treated rats and the controls. In conclusion, the synthetic steroid dexamethasone suppressed the CGRP and SP neuropeptide immunoreactivity in normal dental nerves and it reduced nerve-sprouting responses to dentin cavity injuries; however, sensory nerve reactions to pulpal exposure injuries were not affected by DEX in these experiments.

Potassium currents in cells derived from human dental pulp

Patch-clamp recording methods were used to monitor ion currents in tissue-cultured cells derived from human dental pulp. Recordings were made in excised, outside-in or whole-cell patches. In single-channel experiments, the majority of patches contained a high-conductance (approx. 140-180 pS) K(+)-selective ion channel. The probability of the channel being in an open state was dependent on membrane potential, internal calcium and negative pressure applied to the cell membrane. Whole-cell recordings were consistent with these findings; in response to step-wise depolarizations of the cell membrane, most displayed a family of outwardly rectifying, barium-sensitive currents. In addition, a number of patches contained a second class of potassium channel of intermediate (approx. 85-100 pS) conductance, which was largely voltage insensitive and independent of calcium concentration. These results suggest that pulp cells contain a high-conductance potassium channel which probably underlies the outwardly rectifying current found at the whole-cell level. Further, the existence of mechanosensitive channels in these cells raises the possibility that the response to mechanical perturbation of dental pulp may be mediated, in part, by direct effects on odontogenic cells.

Effect of sensory denervation on the response of rat molar pulp to exposure injury

Sensory nerve fibers that contain calcitonin gene-related peptide (CGRP) have been shown previously to sprout into inflamed tissue surrounding sites of pulpal injury. The sprouting fibers contain increased CGRP immunoreactivity (IR), and neuropeptide levels increase in the surrounding pulp. We compared denervated and innervated first mandibular molars of rats to determine whether the absence of sensory nerve fibers affected tissue survival and healing after pulp exposure. Significant differences were seen between innervated and denervated teeth six days after occlusal exposure, with more extensive necrosis in the denervated teeth, and less survival of vascular pulp. When exposures were on the side of the crown, there was no significant difference between the innervated and denervated teeth. Both the innervated and denervated teeth had begun to make reparative dentin and osteodentin by six days after tooth injury. This study shows that teeth with sensory denervation had an accelerated loss of pulp tissue following occlusal exposure compared with innervated teeth with similar injury.

Effect of ageing on responses of nerve fibres to pulpal inflammation in rat molars analysed by quantitative immunocytochemistry

The response of sensory nerve fibres to inflammation in young adult rat molars has recently been shown to include increases in nerve sprouting and neuropeptide content. The objective was to evaluate neural responses to class V dental preparations in molars of old (1-2 yr) as compared with young adult rats (3-4 months). Tissues were investigated immunocytochemically 4 days post-injury for the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P. Quantitative image analysis of the material demonstrated that more immunoreactivity was present for CGRP than for substance P in intact control teeth for each age group. Four days after injury, both immunoreactivities were increased in pulp adjacent to the injury in both young and old teeth. The increase depended on at least three factors: (1) enhanced immunoreactivity of the nerve fibres; (2) increased terminal nerve sprouts near the injury and (3) elevated peptide content of the pulp tissue. Although the incidence of CGRP- and substance P-immunoreactive nerve fibres had decreased in older teeth, the proportional increases in both neuropeptides near the injury were greater in old than in young teeth, owing to a reduction in pulpal volume during ageing. Pulpal tissue was also immunostained for the low-affinity nerve growth factor receptor (p75-NGFR) as an index of pulpal ageing; and an extensive decrease was found in the old adult as compared to young adult rats. These results indicate that old rats maintain the capacity for nerve sprouting despite the decreases in p75-NGFR labelling of pulp cells, pulp volume and nerve fibre numbers that occur as part of dental ageing.

Biologic properties of eugenol and zinc oxide-eugenol. A clinically oriented review

Eugenol-containing dental materials are frequently used in clinical dentistry. When zinc oxide-eugenol (ZOE) is applied to a dentinal cavity, small quantities of eugenol diffuse through the dentin to the pulp. Low concentrations of eugenol exert anti-inflammatory and local anesthetic effects on the dental pulp. Thus use of ZOE temporary filling may facilitate pulpal healing; on the other hand, high eugenol concentrations are cytotoxic. Direct application of eugenol to pulp tissue may result in extensive tissue damage. The ability of ZOE-based endodontic sealers to influence periapical tissue healing is considered in view of eugenol's anti-inflammatory and toxic properties.

Altered expression of NGF and P75 NGF-receptor by fibroblasts of injured teeth precedes sensory nerve sprouting

Profuse sprouting of sensory nerve fibers occurs in tooth pulp by 1-4 days following dentin injury. A possible role for nerve growth factor (NGF) in that neural response is suggested here by the demonstration that NGF mRNA and protein are increased 6 hr after injury to adult rat molars. The enhanced expression of NGF mRNA was localized to fibroblasts underlying the injury. A concomitant depletion of mRNA encoding the 75 Kd NGF receptor (NGFR) was observed in those fibroblasts. The increase in NGF mRNA was transitory and mRNA levels fell below normal levels by 2 days after injury. Both NGF and NGFR mRNA remained low thereafter in injured pulp. The inverse shifts in fibroblastic mRNA encoding NGF and NGFR were not affected by prior denervation of the tissue, or by pretreatment with dexamethasone. The regulatory mechanisms therefore must involve endogenous, non-neuronal, non-inflammatory factors that are released in response to injury.

Immunocytochemical evidence for the presence of somatostatin-like immunoreactive nerves in human dental pulp

Specific antiserum to somatostatin was used for the immunocytochemical detection of this neuropeptide in human dental pulp. Immunoreactive axon varicosities were observed in the radicular as well as coronal pulp. Fibers displaying somatostatin-like immunoreactivity were detectable within radicular nerve trunks and were found to be associated mainly with blood vessels. Nevertheless, positive fibers with no apparent relation to blood vessels were also observed. No pulp cell was found to be immunoreactive. Previous physiological studies demonstrated that somatostatin may function as a regulatory peptide in feline dental pulp via a pre-synaptic inhibition of substance P release from sensory nerve terminals. It is tempting to speculate that such a mechanism may also be effective in human teeth and may be of value in the regulation of pulpal blood flow and in situations when sensory nerve fibers are activated, e.g., during pulpal inflammation.

Effects of injury and inflammation on pulpal and periapical nerves

Several studies dealing with the reactions of dental nerve fibers to injury and inflammation are reviewed in this article. The subgroup of dental nerve fibers that contains calcitonin gene-related peptide (CGRP) was examined by immunocytochemistry at various times (1 to 35 days) after one of three degrees of injury: (a) Mild: Four days after making shallow cavities into cervical dentin of first molars of anesthetized adult rats, we found that CGRP fibers had sprouted into the subjacent odontoblast layer and dentin, and then returned to normal by 3 wk. (b) Intermediate: If the cervical cavities were acid etched, we found damage to the odontoblast layer, microabscess formation, and sprouting of CGRP fibers near the abscess, with subsequent formation of reparative dentin and healing. (c) Severe: If the pulp was exposed, a variety of reactions could occur, the most prevalent of which was a severe necrosis leading to development of periapical lesions. Analysis of the progressive stages of pulpal abscess and necrosis showed sprouting CGRP nerve fibers (a) at the retreating interface between abscess and vital pulp; (b) in periapical areas during onset of lesions; and (c) around chronic abscesses in granulomatous periodontal tissues. These studies are discussed in relation to various dental clinical problems such as hypersensitive teeth, episodic toothache, early onset of periapical lesions, dental anesthesia, and possible roles for sensory fibers and neuropeptides in tissue defense and healing.

Neurovascular interactions in the dental pulp in health and inflammation

The two key components in pulpal inflammation are microcirculation and sensory nerve activity. With advancement of techniques they can be measured simultaneously in the same tooth. Excitation of A-delta fibers seems to have an insignificant effect on pulpal blood flow (PBF), whereas C fiber activation causes an increase in PBF. This C fiber-induced PBF increase is caused by neurokinins, especially substance P, which is released from the C fiber nerve terminals. Manipulation of PBF has varying effects on sensory nerve activity. An increase in PBF causes excitation of both A-delta and C fibers via an increase in tissue pressure, whereas flow reduction has an inhibitory effect on A-delta fibers, but no discernible effect on C fiber activity. Understanding of this complex neurovascular relationship in the pulp, especially given the fact that the pulp is in a low compliance system, is prerequisite to more comprehensive characterization of pulpal inflammation.

Innervation of the tooth pulp by the mesencephalic trigeminal nucleus in the cat: a retrograde horseradish peroxidase study

HRP was applied to the tooth pulp of 8 cats. Six were subjected to postoperative administration of the anti-inflammatory drug, prednisolone, whereas the remaining two were not. In all prednisolone-treated cats, labeled neurons were found in both the mesencephalic trigeminal nucleus and trigeminal ganglion, ipsilaterally. On the other hand, no labeled neurons were observed in the mesencephalic nucleus in cats receiving no steroid.

Inflammation of rat molar pulp and periodontium causes increased calcitonin gene-related peptide and axonal sprouting

We have studied the response of nerve fibers containing calcitonin gene-related peptide immunoreactivity (CGRP-IR) to inflammation using a rate dental experimental system. Inflammation was induced by drilling tooth cusps to create pulpal exposures; the induced pulpitis and subsequent periapical lesions were studied 1-35 days later using standard CGRP immunohistochemistry and the avidin-biotin peroxidase method. The injury and resulting inflammation caused a disruption of CGRP-IR nerve fiber location and arborization that varied depending on whether the initial injury was limited to the pulp tip or extended throughout the pulp horn. At shorter survival periods (24 hr, 3 days) nerve fibers were either decreased or bundled into the center of the pulp with sprouting along the wound border. At 6 days necrosis and acute inflammation had advanced to varying degrees, and CGRP-IR fibers were extensively sprouted in the surviving pulp; the pulp also stained specifically for CGRP within 1-2 mm of the inflamed tissue at 6 days. At 35 days, we found total pulp necrosis in most teeth and the development of periapical bone loss, granulomatous tissue, and periapical abscesses. There was also an extensive increase in CGRP-IR nerve fibers in the tissues surrounding sites of severe periodontal inflammation and necrosis. In some cases, macrophage-like cells staining specifically for CGRP were near the abscesses. The results show important interactions between peptidergic nerve fibers and inflammatory cells, and are discussed in terms of the role of nerve fibers containing CGRP in neurogenic inflammation, mechanisms for intensification of CGRP immunoreactivity in affected fibers or neighboring cells, and implications for chronic inflammatory conditions, dental pain, and anesthesia.

The neurobiology of facial and dental pain: present knowledge, future directions

This review outlines recent research which has identified critical neural elements and mechanisms concerned with the transmission of sensory information related to oral-facial pain, and which has also revealed some of the pathways and processes by which pain transmission can be modulated. The review highlights recent advances in neurobiological research that have contributed to our understanding of pain, how acute and chronic pain conditions can develop, and how pain can be controlled therapeutically. Each section of the review also identifies gaps in knowledge that still exist as well as research approaches that might be taken to clarify even further the mechanisms underlying acute and chronic oral-facial pain. The properties of the sense organs responding to a noxious oral-facial stimulus are first considered. This section is followed by a review of the sensory pathways and mechanisms by which the sensory information is relayed in nociceptive neurones in the brainstem and then transmitted to local reflex centers and to higher brain centers involved in the various aspects of the pain experience--namely, the sensory-discriminative, affective (emotional), cognitive, and motivational dimensions of pain. Reflex and behavioral responses to noxious oral-facial stimuli are also considered. The next section provides an extensive review of how these responses and the activity of the nociceptive neurones are modulated by higher brain center influences and by stimulation of, or alterations (e.g., by trauma) to, other sensory inputs to the brain. The neurochemical processes, involved in these modulatory mechanisms are also considered, with special emphasis on the role of neuropeptides and other neurochemicals recently shown to be involved in pain transmission and its control. The final section deals with recent findings of peripheral and central neural mechanisms underlying pain from the dental pulp.

An interpretation of dental innervation based upon the pattern of calcitonin gene-related peptide (CGRP)-immunoreactive thin sensory axons

Calcitonin gene-related peptide (CGRP) is a recently characterized neuroactive substance that is expressed in a large proportion of small- to medium-diameter sensory ganglion neurons whose central terminals lie in the superficial spinal and medullary dorsal horn. This restricted distribution within the peripheral nervous system suggests a prominent role for the peptide in nociceptive processing. The mammalian tooth pulp, which receives a relatively homogeneous afferent input from thin (putative nociceptive) fibers originating from this subpopulation of trigeminal ganglion cells, thus affords an ideal target zone in which to examine peripheral nociceptive mechanisms. The large percentage of these neurons displaying CGRP-like immunoreactivity (CGRP-LI) furthermore provides a valuable tool to study its thin-fiber afferent innervation. CGRP-LI has been localized within intact, decalcified specimens of rat, cat, monkey, and human teeth and associated dental structures. A remarkably robust CGRP-LI innervation of molar pulp and dentin was revealed in all species, with fibers coursing both in fascicles and individually, in variable relation to blood vessels and pulpal stroma. Our methods enabled tracing of a large number of axons through Raschkow's plexus and odontoblast layer into dentinal tubules. Paralleling anterograde axonal transport studies, a greater share of fibers was found in coronal vis-à-vis radicular dentin. In the rat, this fiber pattern stood in contrast both to incisor dentin, which appeared devoid of CGRP-LI, and to the abundant labeled axons in gingiva and periodontal tissues. Surgical deafferentation of rat mandible resulted in widespread depletion of CGRP-LI, while superior cervical ganglionectomy was without effect, confirming the sensory nature of the CGRP-LI fibers. Neonatal capsaicin treatment greatly attenuated the immunostaining, providing evidence for CGRP-LI localization in chemosensitive unmyelinated afferents. The great density of CGRP-LI axons demonstrated is considered in contrast to the restricted range and extent of sensory stimuli to which teeth are presumably subjected, and in relation to the diverse ongoing trophic, regulatory, and reparative processes in tooth structures. It is therefore suggested that these fibers may be subserving prominent efferent roles in dental pulp not directly related to nociception.