Electrical stimulation of the tooth pulp in the study of pain

A novel computerized system for thermal stimulation of tooth in ferrets

A dual thermal and electrical stimulator was developed to examine the central pathways that transmit noxious stimuli for intact dentition. This system allows computer-controlled stimulation of the canines of ferrets with either noxious heat or electrical stimuli. A set of in vitro studies demonstrated that the application of thermal stimuli to an intact tooth can produce pulpal temperatures above 43 °C, which is perceived as a painful stimulus in humans. In a set of in vivo studies, it was demonstrated that heating an intact tooth at temperatures of at least 40 °C, excited trigeminal brainstem neurons. Only 15% of the neurons activated by electrical stimulation responded to noxious heat applied to the canine. Eight of the 23 neurons were classified as nociceptive specific neurons and responded only to noxious stimulation of their cutaneous receptive fields. Fifteen of the 23 neurons were classified as wide dynamic range neurons and responded to both noxious and non-noxious stimulation applied to their cutaneous receptive fields. This new device can accurately deliver both thermal and electrical stimuli to an intact tooth, which allows an evaluation of the central neural circuits that respond to noxious stimulation of the dentition.

Functional properties of tooth pulp neurons responding to thermal stimulation

The response properties of tooth pulp neurons that respond to noxious thermal stimulation of the dental pulp have been not well-studied. The present study was designed to characterize the response properties of tooth pulp neurons to noxious thermal stimulation of the dental pulp. Experiments were conducted on 25 male ferrets, and heat stimulation was applied by a computer-controlled thermode. Only 15% of tooth pulp neurons (n = 39) responded to noxious thermal stimulation of the teeth. Tooth pulp neurons were found in both the superficial and deep nuclear regions of the subnucleus caudalis (Vc) and in the interface between the nucleus caudalis and interpolaris (Vc/Vi). Thirty-seven neurons had cutaneous receptive fields and were classified as either NS (16) or WDR (21) neurons. Repeated heat stimulation of the dental pulp sensitized and increased the number of electrically evoked potentials of tooth pulp neurons. These results provide evidence that both the Vc and Vc/Vi regions contain neurons that respond to noxious thermal stimulation of the dental pulp, and that these cells may contribute to the sensitization process associated with symptomatic pulpitis.

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

OBJECTIVE

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

ANIMALS

6 male Beagles that were 2 to 6 years old.

PROCEDURE

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

RESULTS

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

CONCLUSIONS AND CLINICAL RELEVANCE

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

Effects of alternating and direct electrical current application on the odontoblastic layer in human teeth: an in vitro study

AIM

The aim of this study was to investigate the influence of a low intensity alternating current on the odontoblasts and odontoblast layer and compare this with the effects of a direct current.

METHODOLOGY

Teeth extracted for orthodontic reasons were immersed in physiological saline stabilized with thymol crystals. Within 1 h of extraction, an alternating or direct current was applied on the crown in the direction of the apex of the tooth for 120-360 s. The current doses were 12, 30, 60, 600, 1800, 3600, 7200, 24,000 and 144,000 microC. The teeth were fixed in Bouin or Baker fluids, the pulps removed, dehydrated and immersed in paraffin, then sectioned, stained with haematoxylin and eosin, and studied under a light microscope.

RESULTS

Neither direct nor alternating current, similar to that applied in electrical caries diagnosis caused histological changes in the odontoblasts.

CONCLUSIONS

There was no difference between direct and low intensity alternating current in the response of the odontoblast.

Noxious stimuli do not determine reflex cardiorespiratory effects in anesthetized rabbits

The main purpose of this study is to examine whether the stimulation of an exclusively pain-sensing receptive field (dental pulp) could determine cardiorespiratory effects in animals in which the cortical integration of the peripheral information is abolished by deep anesthesia. In 15 anesthetized (alpha-chloralose and urethan) rabbits, low (3-Hz)- and high-frequency (100-Hz) electrical dental pulp stimulation was performed. Because this stimulation caused dynamic and static reflex contractions of the digastric muscles leading to jaw opening jaw-opening reflex (JOR); an indirect sign of algoceptive fiber activation], experimentally induced direct dynamic and static contractions of the digastric muscle were also performed. The low- and high-frequency stimulation of the dental pulp determined cardiovascular [systolic arterial pressure (SAP): -21.7 +/- 4.6 and 10.8 +/- 4.7 mmHg, respectively] and respiratory [pulmonary ventilation (VE): 145.1 +/- 44.9 and 109.3 +/- 28.4 ml/min, respectively] reflex responses similar to those observed during experimentally induced dynamic (SAP: -17.5 +/- 4.2 mmHg; VE: 228.0 +/- 58.5 ml/min) and static (SAP: 5.8 +/- 1.5 mmHg; VE: 148.0 +/- 75.3 ml/min) muscular contractions. The elimination of digastric muscular contraction (JOR) obtained by muscular paralysis did away with the cardiovascular changes induced by dental pulp stimulation, the effectiveness of which in stimulating dental pulp receptors has been shown by recording trigeminal-evoked potentials in six additional rabbits. The main conclusion was that, in deeply anesthetized animals, an algesic stimulus is unable to determine cardiorespiratory effects, which appear to be exclusively linked to the stimulation of ergoreceptors induced by muscular contraction.

Intracellular responses of raphe magnus neurons during the jaw-opening reflex evoked by tooth pulp stimulation

Neurons in the nucleus raphe magnus (RM) may play an important role in the modulation of nociception. To determine how RM neurons are activated during a nociceptive reflex, the intracellular responses of raphe neurons were studied during the jaw-opening reflex (JOR) elicited by tooth pulp shock in lightly anesthetized cats. Tooth pulp stimulation produces reflex EMG activation of the digastric muscle at a latency of 7-11 ms, resulting in jaw opening. Tooth pulp shock that elicits the JOR also produces an EPSP in a subset of raphe neurons. This EPSP consists of an early small depolarization that occurs at a latency of 10-15 ms followed by a larger depolarization at a latency of 20-60 ms. In all cases the latency to EPSP is longer than the latency to digastric EMG onset. Electrical stimulation of the 4 paws elicits oligosynaptic EPSPs in the same cells at a latency of 16-20 ms. Electrical train stimulation of the midbrain periaqueductal gray region (PAG) suppresses the JOR. Single shock stimulation at the same PAG sites that suppress the JOR evokes monosynaptic EPSPs in the large majority of raphe neurons recorded. In all cases, the threshold for EPSP is below the threshold for suppression of the JOR. The EPSP amplitude is a direct function of PAG stimulus intensity and there is temporal summation of EPSPs evoked by paired PAG shocks. At condition-test intervals of 40-90 ms, train stimulation of PAG suppresses the tooth pulp-evoked EPSP in raphe neurons. The threshold for EPSP suppression occurs at a PAG stimulation intensity below that required for suppression of the JOR. The present findings provide evidence that RM neurons may play an important role in the modulation of the tooth pulp-evoked JOR, but only after the initial withdrawal reflex has occurred.

Periaqueductal gray inhibition of trigeminal subnucleus caudalis unitary responses evoked by dentine and nonnoxious facial stimulation

The possible pain inhibitory effects of periaqueductal gray (PAG) stimulation were investigated in cats anesthetized with Nembutal and immobilized with Flaxedil. Unitary responses evoked by electrical stimulation of the upper canine dentine and by cutaneous facial noxious and nonnoxious stimuli were recorded extracellularly from the trigeminal subnucleus caudalis. A bipolar electrode was introduced into the PAG to test the effects of PAG excitation on the trigeminal response to dentine (TRED) and cutaneous nonnoxious stimulation. In some experiments, a similar electrode was lowered into the contralateral posterior thalamus to study the antidromic activation of subnucleus caudalis cells and the effects of thalamic stimulation on the TRED. Dentine stimulation evoked brief (6- to 15-ms) bursts of 1 to 10 spikes with 3- to 25-ms latencies. Most units (88%) were also activated by cutaneous facial stimulation. Stimulation of the posterior thalamus had no effect on the TRED or on responses to cutaneous stimulation, but activated antidromically 10% of the units. In 71% of the units PAG stimulation inhibited the TRED. In some of those cases (12%), the inhibitory effect persisted 30- to 60 s. The PAG stimulation could produce paradoxical effects, potentiating the TRED evoked by threshold intensity and inhibiting the TRED elicited by suprathreshold stimulation. About one-half the PAG points evoked detectable effects. Their location had no clear topographical distribution, although ventral sites were more potent than dorsal sites. Responses evoked by nonnoxious facial stimulation were also inhibited by the PAG.

Elevation of pain threshold to tooth shock by brain stimulation in primates

Electrical stimulation of the brain (ESB) for modulation of pain has been previously demonstrated in primates, but many of the sites which yield stimulation-produced analgesia (SPA) also elicit aversive side effects. In order to examine the aversive as well as analgesic effects of brain stimulation, nine rhesus monkeys (Macaca mulatta) were first trained to press a lever to escape or titrate noxious tooth shock. Stimulating electrodes were placed under the frontal cortex in 4 monkeys and were implanted in the diencephalon, brain stem, and cerebellum of five remaining monkeys. Diencephalic stimulation sites resulted in marked elevations of tooth shock threshold at ESB intensities which did not elicit aversive behaviors. The analgesic effects lasted up to 2 h past ESB offset. Moderate elevations of tooth shock threshold were also observed with orbital cortex stimulation. The midbrain central gray and the nucleus raphe magnus, however, did not greatly alter tooth shock level and typically resulted in aversive reactions. The diencephalic sites which elicited SPA also led to self-stimulation behavior, whereas stimulation of the brain stem or cerebellum usually resulted in escape responses. These findings thus indicate that, in primates, more effective relief of pain can be achieved with electrical activation of the medial diencephalon than with brain stem stimulation.

Relative cerebral glucose metabolism evoked by dental-pulp stimulation in the rat

Electrical stimulation of dental pulp is widely acknowledged to produce a sensation that is predominantly or exclusively noxious in character. The authors report the pattern of local cerebral glucose utilization evoked by dental-pulp stimulation in the barbiturate-anesthetized rat, using the [14C]2-deoxyglucose method of Sokoloff. Autoradiographs were prepared from cryostat-cut brain sections of animals given an intravenous pulse of [14C]2-deoxyglucose and sacrificed after 45 minutes of continuous bipolar stimulation of the incisor tooth pulp. Areas of high optical density on the autoradiographs identified brain regions where glucose consumption, and hence functional activity, was maximal. Stimulus-related increases in glucose utilization were seen ipsilaterally in an uninterrupted column from the lower levels of trigeminal nucleus caudalis to the rostral extent of the main sensory nucleus. Mandibular incisor pulp stimulation yielded increased deoxyglucose uptake in relatively restricted dorsal portions of the nuclei, while maxillary pulp stimulation produced a more extensive area of uptake ventrally. Elevated deoxyglucose uptake was also seen in the contralateral ventrobasal thalamus and sensory cortex with maxillary, but not mandibular, pulp stimulation. No changes in metabolic activity were detected in extralemniscal or limbic structures. These initial results suggest that the [14C]2-deoxyglucose method may be a useful means of mapping central structures involved in nociception.

Sensation evoked by bipolar intrapulpal stimulation in man

Bipolar intrapulpal stimulation was applied to human teeth using the same procedure as in animal experiments. The effects of variation of stimulus parameters on the quality of sensation were studied. A prepain sensation exists which cannot be explained by diffusion of the stimulus to periodontal tissues. When the intensity of stimulation is increased, the prepain sensation is gradually replaced by a pinprick sensation. With long, high intensity stimulation, an acute long lasting very painful sensation appears. To evoke a pinprick sensation the best stimulation seems to be a 50 msec train (0.5 msec, 300 Hz, 0.5 mA). Longer train duration and a higher intensity of current are necessary to evoke a long lasting, acute very painful sensation. Since the exclusively Adelta and C nerve fiber content of the dental pulp is well documented and since it is possible to avoid current diffusion outside the dental pulp cavity, the tooth pulp implantation seems to be a good technic for studying pain, as long as the investigator uses adequate stimulation.