Margaret R. Byers

Dental Sensory Receptors

Teeth are innervated by unmyelinated sympathetic axons, and by unmyelinated and small myelinated sensory axons. Some sensory axons in teeth are terminal branches of larger parent axons, so that conduction from teeth to CNS in trigeminal nerves includes C-fiber, A-delta, and A-beta velocities. Sensory dental axons contain acetylcholine or substance P-like immunoreactivity. The sympathetic axons contain noradrenalin. Other neuropeptides may also be present, such as vasoactive intestinal peptide and serotonin. Dental axons of mature teeth of many species (man, monkey, cat, rodents, fish) are essentially the same, but continuously erupting teeth have smaller and fewer axons. Free sensory nerve endings in mature teeth are found in the peripheral plexus of Raschkow, the odontoblastic layer, the predentin, and the dentin. Free nerve endings are most numerous in those regions near the tip of the pulp horn, where more than 40% of the dentinal tubules can be innervated. Many dentinal tubules contain more than one free nerve ending. Intradentinal axons can extend as far as 0.2 mm into dentin but usually end less than 0.1 mm from the pulp. Some sensory endings also occur along pulpal blood vessels. In continuously erupting teeth nerve endings do not enter the dentin but remain within the pulp. Nerve endings in dentin are labeled by axonal transport. They are therefore as viable and active as the nerve endings in pulp. The axoplasm of the free nerve endings contains organelles typical of other somatosensory receptors. These organelles are most common in the successive beaded regions along the free nerve endings and include mitochondria, clear and dense-core vesicles, multivesicular bodies, profiles of smooth endoplasmic reticulum, and relatively few microtubules and neurofilaments. The beads can vary in size from about 0.2 to 2.0 microns and can have varying amounts of receptor organelles. The interbead axonal regions are thin and contain mainly microtubules and neurofilaments. Nerve endings are associated with companion cells after they leave the coronal nerve bundles; these companion cells include Schwann cells, fibroblasts, and odontoblasts. There is no good evidence of gap junctions or synapses between nerve endings and odontoblasts. Instead, the two cell types form appositions that have a 20-40 nm extracellular cleft and parallel apposed plasmalemmas but no unusual membrane-associated material. No special organelles occur in the odontoblastic cytoplasm at these sites.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

GFAP immunoreactivity in trigeminal ganglion satellite cells after tooth injury in rats

Glial fibrillary acidic protein (GFAP) is a member of a heterogeneous group of intermediate filaments in glial cells of both the central and peripheral nervous systems. We demonstrate here that satellite cells in the trigeminal ganglion (TG) increase immunoreactivity (IR) for GFAP in response to dental injury. The satellite cell reaction was most often confined to the somatotopic region of the ganglion that corresponded to the zone of damage in the periphery, although in some cases it was seen to spread out from a focal center in the maxillary region to neighboring zones corresponding to cell bodies that innervate other tissues such as the cornea. We used two methods to demonstrate that the increase in satellite cell GFAP-IR was site specific and injury related. First, by altering the site of the pulp exposure from the maxillary molars to the mandibular molars, we could change the site of satellite cell reaction in the TG. Second, we used combined retrograde transport of DiI from the molar pulp and GFAP immunofluorescence to show direct correspondence between neurons that innervate the molars and neurons that are encircled by GFAP-IR satellite cells. The satellite cell GFAP-IR was seen at 3 and 7 days, the longest time point examined here. This GFAP response in satellite cells around injured sensory neurons will be a useful tool in future studies of mechanisms in trigeminal pain and neuron-support cell interactions. We conclude that a GFAP-IR satellite cell reaction is induced in TG by an injury to the molar pulp in a site specific manner at 3-7 days.

Sprouting of CGRP nerve fibers in response to dentin injury in rat molars.

Abstract Light and electron microscopic immunocytochemistry showed extensive increases in the number of calcitonin gene-related peptide immunoreactive (CGRP-IR) nerve fibers subjacent to injured root dentin of rat molars. Sprouting was greatest at 4 days and returned to normal 10–21 days post injury. Non-CGRP-IR fibers were also increased at 4 days. The result suggest that rapid reversible sprouting of sensory nerve fibers may be an integral part of tissue reactions to injury.

Dynamics of colchicine effects on rapid axonal transport and axonal morphology

Abstract Concentration dependence and reversibility of inhibition of rapid axonal transport by colchicine were correlated with morphological changes in rabbit vagus nerve. Influx of [ 14 C]colchicine into the nerve was delayed by the presence of the nerve sheath, and inhibition of rapid transport took over twice as long as in desheathed nerves. Graded effects on transport of labeled protein were seen with 0.1–30 mM colchicine, associated with a progressive loss of microtubules. With 7.5 and 30 mM the number of neurofilaments increased. Inhibition of rapid transport was not reversed by washing for 3–20 h nor were the morphological effects. Conduction of the compound action potential was essentially unimpaired in all experiments. This contrasts with the effect of lidocaine, which blocks conduction as well as rapid transport reversibly and blocks rapid transport prior to affecting microtubule structure.

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.

Delineation of Somatic Nerve Endings in Rat Teeth by Radioautography of Axon-Transported Protein

We have used radioautographic tracing to show that somatic nerves of rat molars fan out in the coronal pulp to end in an extensive subodontoblast. plexus, on coronal odontoblasts, and along their dentinal processes. Molar response dentin has sparse innervation. Incisors have only a few somatic nerve endings that occur near pulp blood vessels.

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

&NA; 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 &mgr;g/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.

Responses of nerve fibers to pulpal inflammation and periapical lesions in rat molars demonstrated by calcitonin gene-related peptide immunocytochemistry

The response of sensory nerve fibers to inflammation was studied in the pulp and periapex of 64 injured molars from 16 adult rats. Tissues were evaluated at 1, 5, 8, 11, 14, or 21 days after injury using immunocytochemistry for calcitonin gene-related peptide. The extent of surviving pulpal innervation was studied and the size of periapical lesions was measured. Persistence of nerve fibers in the pulp was found up to 14 days after injury, whereas periapical lesions were present as early as 5 days after injury. Extensive sprouting of the nerve fibers was found throughout the surviving pulp by 5 days after injury and persisted in later groups. Innervation of the periapical area had also increased by 5 days with additional sprouting at later times. The active role of calcitonin gene-related peptide fibers in neurogenic inflammation and dental pathosis is discussed as well as the clinical relevance of coexisting pulpal innervation and periapical lesions.

An immunocytochemical study of the morphological reaction of nerves containing calcitonin gene-related peptide to microabscess formation and healing in rat molars

Pulpal inflammation was induced by cutting a class V cavity to within 0.1-0.3 mm of the pulp on the mesial aspect of maxillary and mandibular first molars at the cervical line. The exposed dentine was briefly acid-etched and left open to the mouth until the animals were killed. Histological examination of teeth 4 days after injury showed microabscesses, blood vessel dilation and increased numbers of terminal nerve sprouts around the lesion and in radicular pulp and dentine. Specimens at 7, 11, 21 and 35 days after injury showed progressive healing of the lesions with the formation of reparative dentine and a coincident return to a normal patterns of innervation in the remaining pulp. Thus pulpal nerves are not static structures, but rather are capable of rapid change in response to inflammation. The morphological association of CGRP-immunoreactive nerve fibres with the edges of the healing lesions and with zones of reparative dentine suggests a role for these fibres and for the neuropeptide CGRP in the healing response of pulpal tissue.