Conduction velocities of single pulp nerve fibre units in the cat

Painful Tooth Stimulation Elevates Matrix Metalloproteinase-8 Levels Locally in Human Gingival Crevicular Fluid

Recent studies have demonstrated that pulpal pain can induce neurogenic inflammatory reactions in gingiva and the expression of pro-inflammatory neuropeptides in gingival crevicular fluid (GCF). Neuropeptides co-ordinate the activity of immuno-effector cells and may influence the secretion of matrix metalloproteinase (MMP)-8, the major tissue-destructive protease in GCF. With this background, we studied whether experimental pulpal pain can trigger changes in GCF MMP-8 levels. The molecular forms of MMP-8 in the GCF of stimulated and non-stimulated teeth were analyzed by Western immunoblot, and MMP-8 levels by quantitative immunofluorometric assay. Painful stimulation of the upper incisor provoked significant elevations in GCF MMP-8 levels of the stimulated tooth. Western immunoblot revealed elevations in both neutrophil- and mesenchymal-type MMP-8 isoforms. At the same time, the GCF MMP-8 levels of the non-stimulated teeth were not changed. Analysis of these data indicated that pulpal pain can induce local elevations in MMP-8 levels in GCF.

Effect of pulp inflammation on nerve impulse quality with or without anesthesia

Absence of complete anesthesia in teeth with acutely inflamed pulp is a well-known clinical symptom, but the mechanism behind this symptom is poorly understood. In the current electrophysiologic study, afferent nerve fiber responses of the inflamed tooth with and without anesthesia were compared with normal teeth. Seventeen cats were used in this experimental study. After inducing inflammation in one canine tooth, the responses of afferent nerves from the inflamed and control contralateral canine tooth neurons were compared before and after application of anesthesia. In most cases, 2 distinct responses in both normal and inflamed sites were recorded. The conduction velocity was significantly lower in the nerve to the inflamed side compared with the control side. In contrast, the response intensity was significantly higher in the inflamed side compared with the control side. It was also found that it was markedly more difficult to inhibit impulse transmission by using anesthesia on the inflamed side, whereas there was no trouble inhibiting impulses on the control side.

Experimental tooth pain elevates substance P and matrix metalloproteinase-8 levels in human gingival crevice fluid

OBJECTIVE

Tooth pain can induce a neurogenic inflammatory reaction in gingiva in association with local elevations of matrix metalloproteinase (MMP)-8, which is considered the major tissue destructive protease in gingival crevice fluid (GCF). The pro-inflammatory neuropeptides released by sensory nerves coordinate the activities of the immuno-effector cells and may influence the secretion of MMP-8. With this background, we studied whether experimental tooth pain can trigger changes in GCF levels of the neuropeptide substance P (SP) and MMP-8.

MATERIAL AND METHODS

The GCF SP levels of stimulated and non-stimulated teeth were analyzed for SP using a competitive enzyme immunoassay (EIA). The GCF MMP-8 levels were determined by quantitative immunofluorometric assay (IFMA).

RESULTS

Painful stimulation of the upper central incisor caused significant elevations in GCF SP and MMP-8 levels of the stimulated tooth. At the same time, the GCF SP and MMP-8 levels of non-stimulated control teeth were unchanged.

CONCLUSIONS

These data indicate that experimental tooth pain can induce local elevations of SP and MMP-8 levels in GCF simultaneously. This supports the possibility of a local neurogenic spread of inflammatory reactions from intrapulpal to surrounding periodontal tissues.

Cortical representation of experimental tooth pain in humans

Cortical processing of electrically induced pain from the tooth pulp was studied in healthy volunteers with fMRI. In a first experiment, cortical representation of tooth pain was compared with that of painful mechanical stimulation to the hand. The contralateral S1 cortex was activated during painful mechanical stimulation of the hand, whereas tooth pain lead to bilateral activation of S1. The S2 and insular region were bilaterally activated by both stimuli. In S2, the center of gravity of the activation during painful mechanical stimulation was more medial/posterior compared to tooth pain. In the insular region, tooth pain induced a stronger activation of the anterior and medial parts. The posterior part of the anterior cingulate gyrus was more strongly activated by painful stimulation of the hand. Differential activations were also found in motor and frontal areas including the orbital frontal cortex where tooth pain lead to greater activations. In a second experiment, we compared the effect of weak with strong tooth pain. A significantly greater activation by more painful tooth stimuli was found in most of those areas in which tooth pain had induced more activation than hand pain. In the medial frontal and right superior frontal gyri, we found an inverse relationship between pain intensity and BOLD contrast. We concluded that tooth pain activates a cortical network which is in several respects different from that activated by painful mechanical stimulation of the hand, not only in the somatotopically organized somatosensory areas but also in parts of the 'medial' pain projection system.

The importance of stimulus site and intensity in differences of pain-induced vascular reflexes in human orofacial regions

Studies in anaesthetized animals have indicated that noxious stimulation may produce marked blood flow changes in various orofacial structures, but the influence of painful stimulation on the blood flow regulation of the orofacial area of humans has been studied only to a limited extent. The purpose of this investigation was to study whether there are differences in temporal and spatial patterns of pain-induced vasoactive reflexes between various orofacial regions and hand in healthy human volunteers. Dynamic changes in blood flow in various orofacial regions elicited by painful stimulation of the tooth and finger were measured by means of Laser Doppler imaging (LDI) and computer-assisted infrared thermography (IRT). Blood flow of the finger was recorded by laser Doppler flowmetry (LDF) and plethysmography (PLET). During both stimulus paradigms there was a transient elevation in heart rate (HR) and blood pressure (BP). At the same time there was a significant blood flow decrease in the finger (LDF, PLET) and in the nose (LDI, IRT). In contrast to tooth stimulation, finger stimulation caused a more marked blood flow reduction in the finger. Only high intensity tooth stimulation, but not finger stimulation, caused a long-lasting vasodilatation both in lower and upper lip. The blood flow changes in the lips were not correlated with changes in systemic blood pressure or heart rate. In the cheek, there were no marked flow changes during either finger or tooth stimulation. These data indicate that painful tooth (regional) stimulation, but not finger (remote) stimulation, can induce a long-lasting vasodilatation in parts of orofacial tissues which cannot be explained by changes in central cardiovascular parameters. This tooth-stimulation-induced blood flow increase supports the hypothesis of a special vasodilator reflex mechanism in the orofacial area. Furthermore, tooth-stimulation-induced vasoconstriction in the nose and dilatation in the lips indicate that separate vasoactive reflex mechanisms may exist for different orofacial regions.

Changes in tooth pulpal detection and pain thresholds in relation to jaw movement in man

The effect of jaw movements on pulpal sensory thresholds to electrical stimulation was studied in healthy humans. The movements consisted of repeated jaw opening and closing at two different frequencies (1 and 3 s(-1)). The detection/perception and pain thresholds of an upper or lower central incisor were determined by stimulation with monopolar constant current pulses at two different durations (0.5 and 5.0 ms). In the absence of jaw movement, the control (baseline) pain threshold was significantly higher than the detection threshold, and both thresholds were significantly decreased with an increase of the stimulus pulse duration. During jaw movement, pulpal detection and pain thresholds were significantly elevated, independent of the duration of the stimulus pulse. The jaw movement-related increase in detection thresholds was significantly dependent on the rate of cyclical jaw movements and on the site of stimulation. An increase in pulpal sensory thresholds was observed with stimulation of the lower incisor only; there was no change in thresholds for the upper incisor. Pulpal detection thresholds were significantly more elevated during jaw movement than pulpal pain thresholds. The results indicate that the reduction in pulpal sensitivity is related to the jaw movements. The effect of jaw movement on pulpal detection thresholds was segmentally restricted. In contrast, modulation of the pulpal pain thresholds was considerably weaker. The jaw movement-related suppression of pulpal sensitivity may be explained by activation of segmental afferent-induced inhibition, corollary efferent barrage from motor to sensory areas, or a combination of both.

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.

Differential effects of noxious conditioning stimulation of the cheek by capsaicin on human sensory and inhibitory masseter reflex responses evoked by tooth pulp stimulation

In this study, we investigated whether selective activation of nociceptive primary afferent fibers by capsaicin would induce modulations on tooth-pulp-evoked sensory or inhibitory masseter reflex responses in healthy human subjects. The contribution of central N-methyl-D-aspartate (NMDA) receptor mechanisms in capsaicin-induced effects on sensory or reflex responses was evaluated by dextromethorphan, an NMDA-receptor antagonist. The inhibitory masseter reflex was evoked by electrical stimulation (constant current, single pulses) of the upper incisor while the subject was biting at 10% of his maximal force. The sensation of the tooth pulp stimulation was evaluated by visual analogue scale (VAS). The magnitude, duration, and the the latency of the reflex were determined by bite force measurements. The inhibitor masseter reflex could be induced by non-painful tooth pulp stimulation, and the inhibition was enhanced as a function of increasing stimulus intensity. Capsaicin (1%) applied topically to the skin of the cheek produced a spontaneous burning pain sensation. During capsaicin treatment, the VAS ratings for the sensation induced by tooth pulp stimulation were significantly reduced, whereas no significant changes were found in the tooth-pulp-induced masseter reflex responses. Double-blind treatment with dextromethorphan at a dose of 100 mg (= the highest does without side-effects) had no effect on sensory or reflex responses. These data indicate that noxious stimulation of the facial skin by capsaicin induces differential effects on tooth-pulp-evoked sensory and inhibitory masseter reflex responses: Sensory responses are strongly attenuated, while masseter reflex responses are not significantly changed. Dextromethorphan at a clinically applicable dose does not influence tooth-pulp-evoked sensory or reflex responses or their modulation by capsaicin. Furthermore, the lack of modulation of the masseter reflex response by capsaicin differs from the capsaicin-induced enhancement of a nocifensive limb flexion reflex described earlier.

The enigma of potassium ion in the management of dentine hypersensitivity: is nitric oxide the elusive second messenger?

We report the development of a 'second-messenger' model in an attempt to re-evaluate the role of K+ as a desensitising agent. Despite unequivocal validation of the effectiveness of potassium-based dentifrices in the management of dentine hypersensitivity, the mechanism(s) of action of K+ remains unclear. Although experimental paradigms of the Nernst equation demonstrate a direct inhibitory effect of K+ ion upon nerve conduction, in vivo considerable constraints can be argued to preclude this mechanism of action. Indeed, measurements of solution velocity within individual dentinal tubules obtained by scanning electrochemical microscopy indicate that outward movement of tubular fluid may represent a far greater barrier to the inward diffusion of K+ ions than previously estimated from measurements of hydraulic conductance across bulk dentine. Despite such probable limited penetration of dentine tubules, K+ ions may desensitise deeply-located nerve terminals through activation of a second-messenger transduction pathway that is capable of controlling the gain of K+-evoked effects which remain physically restricted to the more superficial aspects of the tubule. In addition to a direct effect upon transmembrane potential K+ can also indirectly attenuate neural activity through effects upon levels of the endogenously-synthesised free radical, nitric oxide (NO). Stimulation of the release of NO by K+ has been observed using a variety of cell preparations, which include endothelium, smooth muscle, adrenal medulla, hypothalamus and cerebellum. Importantly, a growing number of studies now report that an increase in the production of NO is associated with analgesia through a modulation of nociceptive input and a downregulation of sensitised nociceptors, most likely achieved through an increase in intraneural content of cGMP. The clinical role of a K+-evoked liberation of NO as a principal mechanism in the management of dentine hypersensitivity is supported by recent findings which include: (1) the localisation of NADPH-diaphorase activity and inducible nitric oxide synthase (iNOS) immunoreactivity within odontoblasts, their processes in dentine, and the subodontoblast layer of the pulp; (2) iNOS causes a sustained release of large (nanomolar) amounts of NO; (3) NO is freely diffusible and capable of displaying remarkably potent effector actions at distant target cells; (4) the actions of NO may be enhanced by endogenous carrier molecules such as S-nitrosothiols; (5) the synthesis of NO can be evoked by concentrations of K+ ion far less (i.e. <1 mM) than those required for direct inhibitory effects upon neural activity.

Axon reflex vasodilatation in cat dental pulp elicited by noxious stimulation of the gingiva

Antidromic stimulation of sensory nerves has been shown to increase blood flow in the tissue they innervate. This study was designed to determine if antidromic vasomotor responses occur in feline dental pulp and if they are mediated by branched axons supplying both tooth pulp and gingiva. Dynamic changes in pulpal blood flow (PBF) elicited by electrical stimulation, pinching, heating, and capsaicin application to the gingivae were investigated in cat mandibular canine teeth by means of Laser Doppler Velocimetry. All inferior alveolar nerve bundles and the cervical sympathetic trunk had been previously sectioned to avoid the occurrence of brainstem reflexes, e.g., somato-autonomic vasomotor reflexes. Increases in PBF were observed in seven out of 12 cats when a restricted gingival area adjacent to the canine teeth was stimulated as described, but the increases were abolished after the sensitive gingival area was painted with lidocaine jelly, a surface anesthetic. These vasodilator responses, remarkably reduced following repeated application of 30 mM of capsaicin, are considered to be induced via antidromic activation of capsaicin-sensitive nociceptive nerve fibers, presumably by axon reflex mechanisms, suggesting that nerve terminals supplying the gingiva originate from parent axons which have collaterals that innervate the canine tooth pulp.

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.

Blood flow increase in the orofacial area of humans induced by painful stimulation

The purpose of this study was to investigate if painful stimulation produces blood flow changes in the tooth pulp and the facial skin in humans. Also, we attempted to find out if the possible blood flow changes induced by painful stimulation could be explained by central sympathetic and parasympathetic reflex mechanisms, by an antidromic activation of nociceptive axons (axon reflex), or by a change in central cardiovascular parameters. Laser Doppler flowmeter was used to assess the blood flow changes. Electrical tooth pulp stimulation at painful intensities induced a blood flow increase in the ipsilateral lip adjacent to the stimulus site, and vice versa. Nonpainful stimulation had no effects. Painful thermal stimulation of the upper lip also produced an increase in the blood flow of the ipsilateral upper incisor. The blood flow changes in the lip produced by dental stimulation were not correlated with changes in systemic blood pressure or heart rate. Painful electrical stimulation of the hand did not induce any changes in the pulpal blood flow, whereas painful dental stimulation produced a blood flow decrease in the finger but no change in the contralateral lip or cheek. In monkey experiments a regional block of the central conduction of the inferior alveolar nerve at the level of the mandibular foramen produced varying results: the blood flow increase in the lower incisor produced by noxious thermal stimulation of the ipsilateral lower lip was not abolished in two experiments but was abolished in other two experiments. It is concluded that painful stimulation can induce significant increases in the blood flow of the orofacial regions in humans. This increase is predominantly restricted to the region adjacent to the stimulus site and cannot be explained by changes in the central cardiovascular parameters. Central neuronal reflex mechanisms and an axon reflex may both underlie these blood flow increases.

Influences of central gray matter stimulation on thalamic neuron responses to high- and low-threshold stimulation of trigeminal nerve structures

Responses of the thalamic non-specific medial nuclei and relay ventral posteromedial nucleus neurons evoked by stimulation of the teeth pulp of A alpha and A sigma fibers of infraorbital nerve and caudal nucleus of the spinal trigeminal tract were studied in cats under thiopental-chloralose anesthesia. Three groups of neurons were revealed: low-threshold, high-threshold and convergent. In medial nuclei, 29% of neurons formed the low-threshold group, 12% made up the high-threshold group and 59% were convergent. In the ventral posteromedial nucleus 47% of neurons formed the low-threshold group, 4% made up the high-threshold group and 49% were convergent. Ninety per cent of the medial nuclei neurons and 79% of the ventral posteromedial nucleus neurons responded to stimulation of the caudal nucleus spinal trigeminal tract. The conditioning central gray matter stimulation could suppress responses of low-threshold, high-threshold and convergent neuron groups. In medial nuclei 100% of responses induced by stimulation of teeth pulp and A sigma fibers of the infraorbital nerve were completely suppressed and 86% of responses induced by stimulation of the A alpha fibers of the infraorbital nerve stimulation were also completely suppressed. In the ventral posteromedial nucleus 40% of responses induced by stimulation of teeth pulp and A sigma fibers of the infraorbital nerve were completely suppressed and 26.4% of responses induced by stimulation of the A alpha fibers of the infraorbital nerve were completely suppressed. Conditioning stimulation of the central gray matter suppressed responses evoked by stimulation of the caudal nucleus spinal trigeminal tract and in the most part neurons of the medial nuclei and neuron responses of the ventral posteromedial nucleus. The inhibitory influence of central gray matter stimulation on neuronal responses evoked by stimulation of the caudal nucleus spinal trigeminal tract shows that the central gray matter had a direct influence on the activity of thalamic neurons.

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.

A comparison of pulpal and tactile detection threshold levels in young adults

Pulpal and tactile sensory detection threshold (SDT) values of the maxillary and mandibular incisor and canine teeth were determined and recorded for young adult subjects at three test sessions. A commercially available monopolar pulp-testing device was used to determine pulpal SDT values, and von Frey hairs were used to determine the tactile SDT values. Statistical analysis of the data indicated that the pulpal and tactile test procedures were sufficiently reliable in identifying what is defined as the true SDT value for both parameters. The study confirmed the constancy of these SDT values over days and the independence of the values for jaw, side, and sex. SDT values were influenced, however, by tooth type, with canine teeth displaying higher tactile and pulpal values than the central and lateral incisor teeth. These data should provide a suitable baseline for a longitudinal study to identify the SDT fluctuations known to occur in tooth pulp and dental supporting tissues in a growing human population.

The intensive aspect of information processing in the intradental A-delta system in man--a psychophysiological analysis of sharp dental pain

The tooth pulp has many attractive features for the study of peripheral pain mechanisms because of its rich innervation, its unique distribution of nerve fibers and its general disposition to give rise to pain upon stimulation. An experimental model has been developed for simultaneous recordings of intradental multi-unit A-delta nerve activity and the subjective intensity and quality of pain evoked by tooth pulp stimulation in conscious, alert humans. The only teeth to be considered for this kind of investigations are those having such a periodontal condition that they have to be extracted. The nerve activity was recorded from two electrodes placed in the dentin on the labial tooth surface, one at the level of the most incisal part of the pulp, and the other as far apically as possible. Brief cold stimulation was produced by using evaporating ethyl chloride administered between the recording electrodes. The magnitude of perceived pain was estimated by means of an intermodal matching technique (finger span) in combination with verbal descriptors. Of three response criteria selected--average response amplitude, peak amplitude and area under the response curve (integral)--for describing the relationship between intradental nerve activity and sharp, shooting pain, the integral yielded the highest mean correlation coefficient.

Afferent C fibre innervation of cat tooth pulp: confirmation by electrophysiological methods

1. The presence of afferent C fibres innervating the lower canine tooth was investigated in Nembutal-anaesthetized cats. 2. Twenty-five single fibres with conduction velocities (CVp) of less than 2.5 m/s, as calculated from the shortest response latency using monopolar electrical stimulation of the tooth, were recorded from the inferior alveolar nerve. In addition, the extradental conduction velocity (CVn) of the fibres was determined by using bipolar electrical stimulation of the trunk of the inferior alveolar nerve. 3. The mean CVp was 1.4 +/- 0.4 m/s (n = 25; range, 0.6-2.4 m/s); the mean CVn was higher, 1.7 +/- 0.9 m/s (n = 25; range, 0.6-4.0 m/s). For 20% of the fibres CVn exceeded 2.5 m/s; these were slowly conducting A delta fibres. For 80% of the fibres, however, the extradental conduction velocity was in the C fibre range. 4. The relationship between CVp (y) and CVn (x) was y = 0.66 + 0.40x, the correlation coefficient being r = 0.85. According to the present results this implies that for a reliable classification of pulpal C fibres (CVn less than or equal to 2.5 m/s) by monopolar tooth stimulation alone, CVp should be less than 1.7 m/s. 5. For twenty-three of the twenty-five fibres, one to three discrete shortenings of the response latency occurred when the intensity of the tooth stimulation was increased. When the nerve trunk itself was stimulated, a constant response latency was measured at all stimulus intensities applied. 6. For twelve fibres tested, the mean rate of electrical stimulation of the tooth, which the response followed with a constant latency, was 4.1 +/- 2.3 Hz (range, 1.5-10.0 Hz). With higher rates of stimulation the response latency increased until the fibres failed to follow each stimulus pulse. 7. Fifteen of the nineteen fibres tested responded to radiant heat stimulation of the tooth they were innervating. The mean temperature threshold was 41.4 +/- 2.7 degrees C (n = 11; range, 37.4 +/- 46.4 degrees C). 8. For eight heat-sensitive pulpal C fibres the receptive field was determined by mechanical stimulation of the exposed pulp tissue. Four C fibres developed a long-lasting on-going discharge after intense mechanical stimulation of the receptive field. 9. The discharge evoked by heat and mechanical stimulation of the tooth occluded the response evoked by simultaneously applied electrical current pulses to the nerve trunk, indicating that the same fibres were activated by both tooth and nerve stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Response characteristics of tooth pulp-driven postsynaptic neurons in the spinal trigeminal subnucleus interpolaris of the cat: comparison with primary afferent fiber, subnucleus caudalis, reflex, and sensory responses

Tooth pulp-evoked single neuron responses were recorded in the spinal trigeminal subnucleus interpolaris of the cat. The thresholds to monopolar electric pulses of varying duration (0.2-20 ms) were determined using a constant current stimulator. The thresholds were comparable with those of primary afferent A-fibers, although the most sensitive primary afferent fibers have lower thresholds. The thresholds and latencies showed that none of the interpolaris neurons received their input solely from intradental C-fibers. The most sensitive subnucleus interpolaris neurons had lower thresholds than the respective subnucleus caudalis neurons studied in our previous work. The thresholds and strength-duration curves of the most sensitive interpolaris neurons and of the tooth pulp-elicited jaw-opening reflex are nearly similar, although the jaw reflex can be elicited at an intensity which is slightly lower than that needed to activate the most sensitive interpolaris neurons of the present sample. The most sensitive interpolaris neurons were activated at current intensities that were below the intensity needed to produce liminal dental pain in man, and the strength-duration curves of these neurons were flatter than the curve depicting liminal dental pain sensation in man. The relationship between stimulus intensity and response magnitude could be well described by power functions, the median exponent of which was 1.251. A conditioning stimulation of the tooth pulp at low intensity produced a short (less than 25 ms) enhancement of the response to the following test stimulus, whereas a high intensity conditioning stimulus produced a longer (greater than 40 ms) suppression of the response to the following stimulus. The threshold of 33% of the neurons was elevated during a noxious tail pinch, and this elevation was not reversed by naloxone, an opioid antagonist. The results indicate that in the trigeminal subnucleus interpolaris there are tooth pulp-driven neurons with an input from intradental A-fibers and that a considerable temporal summation of impulses from primary afferent fibers is needed to activate most of them. Human dental pain thresholds cannot be explained by the liminal response properties of the most sensitive interpolaris neurons, but they may be important in the mediation of near-threshold reflex events. It is possible, however, that the high-threshold interpolaris neurons may have a role in the mediation of sensory responses.

The nociceptive jaw-opening reflex: evidence for alpha 2 adrenoceptor involvement

The ability of alpha adrenoceptor agonists to modulate the tooth pulp stimulation evoked (TPS) jaw-opening reflex (JOR) was investigated in rats and rabbits. Low doses of clonidine (6.25-50 micrograms/kg, IV) significantly increased dEMG thresholds. These effects were antagonized by alpha 2 adrenoceptor antagonists (e.g., yohimbine), but not by alpha 1 adrenoceptor antagonists (e.g., prazosin) or mu receptor antagonists (e.g., naloxone). Polar alpha 2 adrenoceptor agonists (e.g., ST-91 and 4-hydroxyclonidine) that cross the blood brain barrier (BBB) poorly and lipophilic alpha 1 adrenoceptor agonists (e.g., ST-587) that cross the BBB easily were without affect on the TPS-JOR. Structures of the peripheral efferent neurocircuitry of the JOR (e.g., the digastric muscle and the neuromuscular junction of the digastric muscle and its motor nerve, the mylohyoid) were shown not to be active sites of clonidine's effect on the TPS-JOR. Treatment with phentolamine (an alpha adrenoceptor antagonist that poorly crosses the BBB) completely poorly crosses the BBB) completely antagonized clonidine's initial transient cardiovascular (pressor) effect without altering its TPS-JOR effects. Pretreatment with reserpine (a catecholamine depleting agent) failed to alter clonidine's affects on the TPS-JOR. Our studies suggest that alpha 2 adrenoceptors potently modulate the TPS-JOR and such modulation may be important in understanding trigeminal neuronal circuitries that partake in pain processing.

Tooth pulp-evoked activity in the spinal trigeminal nucleus caudalis of cat: comparison to primary afferent fiber, reflex, and sensory responses

Tooth pulp-evoked single-neuron responses were recorded in the spinal trigeminal nucleus caudalis of the cat. The thresholds to monopolar electric pulses of various durations (0.2 to 20 ms) were determined using a constant current stimulator. With stimulus pulse durations of 10 to 20 ms, the thresholds were comparable with those of primary afferent A-fibers, although the most sensitive primary afferent fibers had lower thresholds. Primary afferent C-fibers had higher thresholds than the postsynaptic neurons studied. The threshold for the tooth pulp-elicited jaw-opening response was obtained at a lower stimulus intensity than the liminal response in most postsynaptic neurons of this study. The threshold rise of the postsynaptic trigeminal neurons with decreasing stimulus pulse duration (from 5 to 0.2 ms) was much steeper than that of primary afferent A-fibers or jaw-opening response. The strength-duration curves for tooth pulp-elicited pain sensations in man resemble those of spinal trigeminal neurons. Sixty-two percent of the units had a threshold elevation during a noxious pinch of the tail. The results indicate that the activation of postsynaptic trigeminal neurons requires a considerable temporal summation of primary afferent impulses. The jaw reflex thresholds cannot be explained by the properties of the neurons in the subnucleus caudalis of the trigeminal tract. The results support the concept that dental pain is based on the activation of spinal trigeminal nucleus caudalis neurons receiving their input from intradental A-fibers.

The response of dog intradental nerves to hypertonic solutions of CaCl2 and NaCl, and other stimuli, applied to exposed dentine

Responses of single nerve units from the canine and incisor pulps of anaesthetized beagles to CaCl2 (3.5 M, 4.9 M and saturated) and NaCl (2.5 M and 4.0 M) were recorded. The sensitivity of these nerve units to drilling and probing of dentine, and to drying with air blasts, was also studied. Twenty-one out of 22 units responded immediately, either with a few spikes or, sometimes, with a 2-5 s train of impulses, to hypertonic CaCl2 when applied to superficial dentine. Deep in dentine, CaCl2 induced immediately responses in 15 out of 21 units. There were responses to hypertonic NaCl in 15 out of 19 units, but only when applied deep in dentine. This firing had a latency of 15-300 s (mean 94 s) and continued until the solution was washed away. Units sensitive to CaCl2 also responded to drilling, probing and drying. When applied to the exposed pulp, CaCl2 never induced nerve activity, but hypertonic NaCl induced responses in all units tested (n = 19); the latencies were 0-300 s (mean 34 s). The mechanism of nerve activation in response to hypertonic CaCl2 is probably hydrodynamic, and common to several other stimuli as in drilling, probing and air drying. Responses to hypertonic NaCl may have been induced by a direct excitatory effect of Na+-ions on the nerve endings or axons in the pulp-dentine border.

The effect of dentinal stimulation on pulp nerve function and pulp morphology in the dog

The effect of dentinal stimulation on pulpal nerve responses and pulp morphology has been studied in the dog. Canine tooth (n = 25) dentin was stimulated by drilling, probing, and air-blasting for from two to five hours. Acid-etching was used to open dentinal tubules. All test teeth showed disruption of the odontoblast layer and its separation from the predentin; also, dislocation of odontoblast nuclei into dentinal tubules was found in most cases. Single-fiber (n = 14, conduction velocity = 24.3 +/- 7.4 (SD) m/s) recordings of the responses of canine tooth pulpal nerves to dentinal stimulation were made in ten of the stimulated teeth. No changes in the sensitivity of the nerves to dentinal stimulation could be detected. It is concluded that pulpal nerve function and morphological changes of the pulp are not clearly correlated. The condition of the dentin surface seems to be the important factor.

Effects of repeated bupivacaine administration on sciatic nerve and surrounding muscle tissue in rats

The effects of repeated administration of 0.5% bupivacaine or saline into the sciatic notch of rats were studied by light microscopy, electron microscopy and a neurophysiological technique. Very severe myositis, including local necrosis, developed in six of 12 rats treated twice daily with 1 ml bupivacaine for either 3 or 7 days. A 3-h infusion of 1.5 ml 0.5% bupivacaine resulted in minor injury to muscle tissue. A marked degree of disruption and vacuolization of myelin sheaths was evident in nerves exposed to bupivacaine for 3 days. Lymphocyte accumulation was confined to the area surrounding the nervous tissue in 7 of 10 of the preparations from rats treated for 3 days or by a 3-h infusion. No histological changes were detected in nerve and muscle tissue from the opposite extremity exposed to saline. After a recovery period of 3 weeks, no differences in the nerve or muscle histology were seen between samples from bupivacaine- or saline-treated animals. The amplitude of the compound action potential of sciatic nerves was, however, significantly lower after bupivacaine treatment (7 days, 1 ml twice daily). Thus, impaired function may continue despite the lack of histological intraneural injury.

The relation between intradental nerve activity and pulpal pain after heat stimulation

The effects of tooth surface stimulation on intradental nerve activity (INA) and subsequent pain perception were studied in human lower incisors later to be extracted for periodontal reasons. The INA elicited by hot gutta-percha briefly applied to the tooth surface was monitored by means of labial electrodes deeply implanted in the dentin and perceived pain was continuously rated using a finger-span technique. After each stimulation the subject was also requested to select a sensory descriptor that was appropriate to describe the maximal pain intensity. The very first application of heat induced a typical pattern of nerve activity consisting of three phases. An initial burst of 3-5 s duration, phase I, was followed by a depression relative to the baseline lasting for 20-30 s, phase II, that gradually turned into phase III constituting a slowly increasing firing rate. The spontaneously emerging activity (phase III) in the absence of a physical stimulus passed unnoticed by all the subjects despite an average increase in firing rate of 67% relative to the prestimulus noise level. Repeated heat applications at short intervals led to a decrease and finally to abolishment of the whole nerve response. The lack of pain during phase III may be explained in two ways: the rate of increase in firing frequency may be too slow to trigger those perceptual pathways involved in processing of pain; the slow development of the increased sensory nerve activity may lead to central habituation.(ABSTRACT TRUNCATED AT 250 WORDS)

Tooth pulp-evoked jaw-opening reflex in the cat: evidence for central facilitation induced by noxious discharge in the intradental nerve fibers

The tooth pulp-evoked jaw-opening reflex was studied in the barbiturate-anesthetized cat. At liminal intensity of the stimulus, a stable short-latency response was obtained in the digastricus and in the tongue. At a higher stimulus intensity, there occasionally appeared to be a prolonged discharge of variable duration in the digastricus, and a second period of activity in the tongue after a silent period. The threshold intensity for these late discharges was supraliminal for the intradental A-fibers and subliminal for intradental C-fibers. Noxious conditioning stimulation of a tooth led to a temporary decrease of the threshold for the jaw-opening reflex elicited from a contralateral or adjacent tooth; only conditioning stimulation at an intensity producing a marked arousal reaction was effective in this respect. Infiltration of the tooth apex with epinephrine produced a local elevation of the threshold for the tooth pulp-evoked jaw-opening reflex. Distant noxious conditioning stimulation (tail pinch) did not influence the jaw-opening threshold. The results indicated that based on some central mechanisms, conditioning noxious stimulation of a tooth can produce a facilitation of the jaw-opening reflex.

Modification of dental pain and cutaneous thermal sensitivity by physical exercise in man

The effect of physical exercise on dental pain thresholds, the release of pituitary stress hormones and thermal sensitivity of skin was tested in healthy human subjects. Different levels of exercise (100-300 W) at different pedal frequencies were produced by a cycle ergometer. Thermal limen (the interval between warm and cool thresholds) determined from glabrous hand, hairy forearm and leg was used as a parameter of thermal sensitivity. In all subjects the heart rate and blood pressure were increased with increasing work load. Dental pain thresholds were elevated at high work loads with a concomitant activation of pituitary stress hormone (especially growth hormone) release. Thermal limens at all 3 sites were increased work load, too, independent of the pedal frequency. The increase of thermal limen was most marked in the leg and least in the glabrous hand. The results indicate that physical exercise produces a non-segmental, load-dependent decrease of pain and thermal sensitivity with a concomitant activation of pituitary stress mechanisms. The magnitude of modification varies with skin region. Activation of inhibitory mechanisms at spinal levels via muscle and proprioceptive afferents, in a way suggested by the gate control theory of pain mechanisms, seems to have only a minor, if any, contribution to the present findings, since a higher pedal frequency did not produce a more marked decrease of sensitivity.

Is the jaw-opening reflex a valid model of pain?

Tooth pulp shock does not produce only pain; low intensity stimulation results in a non-painful sensation that is termed pre-pain. In animals low intensity tooth pulp shock does not evoke escape behavior; the similarity of the animal escape/detection threshold ratio with the human pain/pre-pain threshold ratio is evidence that pre-pain and pain may be present in animals as in humans. Both pre-pain and pain may arise from the activation of a common afferent modality. The TP-JOR does not correlate with the degree of pain experienced under all conditions. The TP-JOR threshold is at or near the sensory detection threshold, at stimulation intensities which evoke pre-pain. Under normal conditions both the magnitude of the TP-JOR response and the degree of pain experienced increase with increasing stimulation intensity. The TP-JOR and the tooth pulp-evoked pain are affected in parallel by sensory habituation and both appear to relay in the rostral trigeminal complex. There are no cases where the TP-JOR is suppressed and pain is still experienced from tooth pulp shock; the suppression of the TP-JOR may therefore be an accurate index of analgesia. However, in humans treatments that produce analgesia have not been shown to produce suppression of the TP-JOR. Thus, the TP-JOR that persists following analgesic treatments is not a reliable index of either analgesia or pain.

Peripheral and central aspects of trigeminal nociceptive systems

Three aspects of trigeminal pain are considered: the peripheral mechanisms of pain from teeth and from the cornea, and the role of the trigeminal brainstem nuclei in pain. Pain is probably the only sensation that can be evoked by stimulation of dentin or dental pulp in man. Five nerve-endings enter dentinal tubules from the pulp but do not extend into the outer dentine, which is nevertheless sensitive. In teeth of limited growth in experimental animals, the dental pulp is supplied by A beta, A delta and C fibres and these are associated with two categories of receptor: one responds to cooling and to other stimuli that cause displacement of the contents of the dentinal tubules such as probing and drying the dentine, and the other group responds most vigorously to heating. Some cold sensitive units have A beta fibres and the evidence suggests that stimulation of these is capable of evoking both muscle reflexes and pain and, near threshold, 'pre-pain' sensations. Thermal stimulation of the cornea produces sensations of pain and, with less intense stimuli, irritation, Mechanical stimulation also produces pain but it is not clear whether, below the pain threshold, such stimuli produce touch sensation or some other sensation related to pain. Histologically, the nerve-endings in the corneal epithelium consist of fine, bare processes closely associated with the surface of the epithelial cells. Recordings in experimental animals have shown that many of the receptors respond to several different forms of stimulus and their properties correlate well with those predicted from psychophysical experiments in man. The results of trigeminal tractotomy in man and recordings from the trigeminal brainstem nuclei in anaesthetized animals, have generally indicated that nucleus caudalis is the main relay in the pain pathway from the face and associated structures. Recent observations have, however, shown that tractotomy does not produce complete analgesia of this region and responses to thermal stimulation of teeth and noxious stimulation of other oro-facial tissues have been recorded from the more rostral parts of the brainstem nuclear complex. The surgical procedures employed to set up an animal for stereotaxic recording may induce long-lasting depression in the excitability of neurons in these nuclei, which masks some of their properties. The mechanism of this depression has not been established.

Differential inhibition of A, B and C fibres in the rat vagus nerve by lidocaine, eugenol and formaldehyde

Evoked compound-action potentials (cAP) in all three fibre types of the isolated nerve were reversibly depressed by exposure to the drugs. After 20 min exposure, lidocaine (0.5 mM) depressed the amplitude of the A and B fibre potentials significantly more than the C fibre potential. With eugenol (0.8 mM), there was no significant difference in depression of A, B and C fibre activity. With formaldehyde (4.5 mM), the latency was longer, and the C fibres were initially significantly more depressed than the A and B fibres. As dull pain from a chronically-inflamed pulp is believed to be mediated through C fibres, these results may explain the difficulties in obtaining complete local analgesia with lidocaine. They indicate that application of low concentrations of eugenol and formaldehyde to the pulp-dentine organ may have an analgesic effect on pain mediated through both A and C fibres.

Spatial summation of pre-pain and pain in human teeth

The purpose of this work was to investigate the relation between the sensations of pain and 'pre-pain' evoked by stimulation of teeth in human subjects. Electrical pulses of progressively increasing amplitude, generated by a computer-controlled stimulator, were applied to 1 or 2 teeth, and the subjects responded by indicating the nature of the resulting sensation. Pre-pain and pain could be readily and rapidly distinguished by all 11 subjects (response latency about 0.4 sec). Both sensations had stable thresholds with relatively small variance (S.D. 10-15% of threshold value) for a given subject. Subjects characterized the stimuli as indifferent or unpleasant, localized, and brief. By using special stimulation strategies (termed 'optimal trajectories') for exciting 2 teeth simultaneously, spatial summation for pre-pain was demonstrated in most subjects and for pain in almost all subjects. Spatial summation of pre-pain resulted in pain rather than in more intense pre-pain. These results are consistent with both the dual modality (separate afferent fibers for pre-pain and pain) and the single modality hypotheses (single type of afferent fibers) of tooth pulp sensibility, but favor single modality innervation.

Perception of pulpal pain as a function of intradental nerve activity

The purpose of the present investigation was to find neurophysiological correlates of pain perception. The magnitude and time course of perceived pain was successfully related to the neural discharge evoked by rapid cooling of the tooth surface in 6 dental patients whose lower incisors were to be extracted for prosthodontic reasons. Two cavities were prepared on the facial surface of human lower incisors. The cavities were deepened using hand driven instruments until the pulp was visible through a thin layer of dentin. A metal tube was placed in contact with amalgam on each cavity bottom and fixed in place by composite filling material. The tubes were connected to standard equipment for electrophysiological recordings by a flexible circuit. The magnitude of perceived pain was assessed by a cross-modality matching to finger span in combination with sensory verbal pain descriptors and magnitude estimation. The striking agreement between the integrated nerve activity, probably of the A delta type and pain perception, is of great importance from the methodological point of view since it strongly argues in favor of the appropriateness of the techniques applied here to elucidate the neural substrate of some types of nociception and also to evaluate various means of relieving such pain.

An electrophysiological study of canine, premolar and molar tooth pulp afferents and their convergence on medullary trigeminal neurons

The two main aims of this study were (1) to compare the conduction velocities of tooth pulp (TP) afferents innervating the cat canine, premolar and molar teeth and (2) to determine the degree of convergence of afferent input originating in these different teeth on medullary dorsal horn (MDH) neurons. Experiments were conducted on 10 cats anesthetized with chloralose. Single unit extracellular recordings were obtained from the tooth pulp or the MDH. The distribution of conduction velocities of afferents originating in the mandibular and maxillary canines, premolars and mandibular molar were all found to be similar except that the mean conduction velocity of canine afferents was slightly higher than the means for the other teeth. A total of 48 MDH neurons excited by TP stimulation was studied. Most MDH neurons activated by electrical stimulation of one of the TPs could also be activated by stimulation of one or more of the other TPs. In addition to the marked convergence from different teeth, most of the TP-activated neurons also had convergent inputs from facial skin and/or intraoral mucosa.

Thresholds of intradental A- and C-nerve fibres in the cat to electrical current pulses of different duration

Electrical current pulses of quite variable duration have been used in activation of intradental nerves both in human subjects and experimental animals. It seems, however, that little information is available about the effect of pulse duration on the responses of single pulp nerve units. The aim of the present study was to investigate the effect of pulse duration on excitation thresholds of intradental A- and C-fibres in the cat. In 12 anesthetized cats 61 C- and 53 A-nerve units were identified and recorded. Electrical thresholds were determined with current pulses of different duration from 0.2 to 50.0 ms. The maximal stimulus intensity was 200 microA. Conduction velocities of all recorded units and absolute refractory periods of 20 A- and 20 C-units were determined. Intradental A- and C-fibres had different strength-duration properties. with all pulse durations A-fibres had the lowest thresholds. Part of the C-fibres did not respond to the shortest current pulses even with the maximum stimulus intensity (200 microA). with 0.2 ms pulses only 31.1% of the recorded C-fibres could be activated. In some A-fibres a single current pulse of long duration was capable of inducing several action potentials, when the stimulus intensity was increased suprathreshold. Refractory periods of A-units were less than 2.0 ms and those of C-units 5.0-9.0 ms. It is concluded that in electrical stimulation of teeth duration of current pulses strongly affects responses of single intradental fibre units.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of electrical thresholds of intradental nerves and jaw-opening reflex in the cat

In experimental animals the jaw-opening reflex in response to stimulation of pulp nerves has been used as a nociceptive reflex. However, there seems to be only scanty information about the amount and types of pulp nerve fibres that mediate the reflex. In the present work on 8 anesthetized cats electrical thresholds of single functional pulp nerve units were compared to the thresholds of jaw-opening reflex. Monopolar cathodal current pulses were applied to each canine tooth. Reflex responses of the digastric muscle were recorded. The inferior alveolar nerve of the left side in 3 cats was exposed for nerve dissection and responses of pulp nerve units coming from the lower left canine tooth were recorded. The mean threshold of the jaw-opening reflex with 10 ms pulses was 5.9 +/- 3.0 (SD) microA. Below or at the level only part of the fast conducting pulp nerve units could be activated. Thresholds of A- (n = 32) and C- (n = 24) fibres were 9.9 +/- 5.7 and 37.4 +/- 14.5 (SD) microA respectively. Nerves of the periodontal tissues (20 units recorded) were not activated with current pulses of up to 200 microA applied to the tooth. Consequently, at threshold level the jaw-opening reflex in response to the present type of stimulation is mediated by the fast conducting intradental nerve units.

Intradental nerve activity induced by reduced pressure applied to exposed dentine in the cat

In order to study the excitation of intradental nerves by fluid flow in dentinal tubules the following experiments were performed. The fluid flow was caused by reduced hydrostatic pressure applied to the exposed dentine surface of the canine teeth in the anesthetized cat. In one series of 7 cats the intradental nerve activity was recorded by means of electrodes inserted in dentinal cavities. Provided that the pressure was sufficiently reduced and applied to acid-etched dentine in preparations of sufficient depth, intradental nerve activity of different impulse amplitudes was recorded. The responding pressure sensitive units were found to be sensitized by a brief local application of veratrine and desensitized by potassium chloride. In the second series 26 single functional pulp nerve fibre units were dissected from the inferior alveolar nerve in 7 cats. Nerve impulses were recorded by means of platinum-iridium wire electrodes. The conduction velocities of the 9 fibres responding to reduced pressure varied from 8.3 to 43.0 m/s. Five of these fibres also responded to elevated pressure. None of the 9 fibres conducting impulses with a velocity below 2 m/s responded to a reduction in pressure. thus, the present data strongly suggest that intradental nerve endings with myelinated axons are activated by fluid flow in dentinal tubules. Our results support the hydrodynamic mechanism of dentine sensitivity.