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Thermal sensitivity is not changed by acute pain or afferent stimulation.

The effect of conditioning stimulation on thermal sensitivity and clinical pain was studied in 40 patients and six healthy subjects. Thresholds regarding cold, warm and heat pain perception did not differ significantly between the painful and non-painful skin areas in patients or between patients and healthy subjects before stimulation. The patients received either 100 Hz TENS, 2 Hz TENS, 100 Hz vibration, or placebo. No significant changes in thermal sensitivity were observed during and after conditioning stimulation in any of the test groups, although 24/40 (60%) of the patients reported reduction of their clinical pain intensity. The results indicate that (a) thermal sensitivity is not influenced by the presence of clinical pain, (b) the effects of stimulation on thermal sensitivity (thresholds) and clinical pain are not closely related, (c) central inhibitory effects of TENS and vibration are crucial for their pain relieving capacity.

Adult↗

Pain intensity measurements in patients with acute pain receiving afferent stimulation.

Six different pain rating scales, including a "pain relief scale", were compared in 80 patients suffering acute orofacial pain. Pain intensity measurements were made before and after a 30 min period of afferent stimulation (TENS/vibration and placebo). A good correlation was found between pain scores derived from the pain relief scale, visual analogue-, numerical- and graphic rating scales. The verbal rating scale did not perform well. The pain relief scale and the numerical rating scale are interesting alternatives to the established visual analogue scale.

Adolescent↗

On acupuncture analgesia and the mechanism of pain.

The effect on the experimental tooth pain threshold of conditioning electrical stimulation via needles or surface electrodes applied to the hands and cheeks was studied in 34 dental students. Conditioning stimulation with 2/sec. gave a slowly increasing pain threshold followed by a slow return to the control level in the post-conditioning period. In each individual the amplitude of the threshold increase was reproduceable. It was concluded that these effects are not due to motivational but to more basic neurophysiological mechanisms. The pain threshold was increased mainly by segmental conditioning stimulation; segmentally unrelated stimulation gave usually only small effects. Conditioning stimulation with 100/sec. produced only a strict segmental short-lasting effect. Effects with characteristics of both 2/sec. and 100/sec. were obtained by conditioning at 10/sec. It is suggested that the transmission of impulses from the pain afferents to ascending pathways is controlled at the segmental level by (a) presynaptic inhibition within the group of afferents giving rise to the flexion reflex of which the pain afferents are assumed to be a part; (b) postsynaptic inhibition between alternate pathway excited by flexion reflex afferents; and (c) descending control from supraspinal systems which may utilize similar segmental mechanisms as the primary afferents.

Acupuncture Therapy↗

Effect of acupuncture on pain threshold measurement of tooth pulp in the monkey.

The effect of acupuncture on pain threshold from tooth pulp stimulation was studied in alert, awake monkeys. Manual manipulation of acupuncture needles in specific acupuncture points in the hands and legs caused a consistent and significant rise in threshold. Some points, such as Ho-Ku (LI-4) and Tsu-San-Li (St-36), produced more prominent analgesic effects than others. Increase of pain threshold was observed also after application of finger pressure on acupuncture points. Acupuncture and finger pressure applied to certain muscle points exhibited similar analgesic effects.

Acupuncture Therapy↗

Electroacupuncture increases ipsilaterally tooth pain threshold in man.

The effect of electroacupuncture stimulation applied to a unilateral Ho-ku point on the human tooth pain thresholds was studied by electrically stimulating bilateral canines. In six of ten subjects tested, the tooth pain threshold was elevated ipsilaterally but was unchanged contralaterally during and after the electroacupuncture. In the remaining four, neither the ipsilateral nor the contralateral canine showed any change in the pain threshold by the electroacupuncture. The ipsilateral increase of the tooth pain threshold may be explained by a mechanism by which electroacupuncture stimulation of one Ho-ku point causes various pain suppression centers to release ipsilaterally endogenous opioids.

Acupuncture Therapy↗

Origins of tooth pulp-evoked far-field and early near-field potentials in the cat.

Electrical stimulation of the mandibular canine tooth pulp in barbiturate-anesthetized cats activated myelinated A-beta-fibers in the inferior alveolar (dental) nerve and evoked a complex pattern of short-latency potentials that were recorded epidurally from the cortex. Three far-field components ( FFPs designated I, IIa,b, and IIIa,b) and two early near-field components ( ENFPs designated P1 and N1) were identified from computer-averaged potentials recorded over the contralateral lateral sigmoid gyrus or ipsilateral presylvian /anterior coronal gyri. Several corroborative lines of evidence indicated that tooth pulp-evoked FFPs and ENFPs originate from the following generator sources: I, inferior alveolar (dental) nerve/semilunar (gasserian) ganglion; IIa,b, trigeminal lemniscal fibers from the principal (main) sensory nucleus; IIIa,b, thalamocortical fibers from the thalamic ventral posteromedial nucleus; P1, N1, cytoarchitectural area 3 on the banks of the coronal or orbital sulcus. The latencies and waveforms of volume-conducted, surface-recorded potentials were not significantly different from those recorded at putative generator sites along the trigeminal lemniscal projection system. The short latencies and distribution of surface- and depth-recorded tooth pulp-evoked potentials suggested bilateral, trisynaptic paths from peripheral nerve to cortical neurons. Amplitude measurements of FFPs and ENFPs recorded at the cortical, epidural surface were compared to calculated values based on a mathematical model of two concentric spheres of inhomogeneous media in which a single equivalent dipole is radially oriented at some distance from the center. The morphology and distribution of FFPs and ENFPs are discussed in relationship to source geometry. Surface- and depth-recorded tooth pulp-evoked potentials had recovery functions that were covariant with rates of stimulation. In addition, stimulation of putative generator sites along the trigeminal lemniscal system reproduced portions of the tooth pulp-evoked surface-recorded potentials. Selective ablation of trigeminal cortical area 3 abolished ENFPs and lesions of lemniscal brain stem sites abolished FFPs . Disruption of extralemniscal brain stem sites had no effect on the surface-recorded evoked potentials examined. These findings and other supporting evidence from previous studies imply that the projection of fast-conducting tooth pulp input to the cortex may subserve a nonnociceptive modality and participate in sensorimotor integration.

Afferent Pathways↗

Comparison of responses of cutaneous nociceptive and nonnociceptive brain stem neurons in trigeminal subnucleus caudalis (medullary dorsal horn) and subnucleus oralis to natural and electrical stimulation of tooth pulp.

The activity of 160 single neurons excited by electrical stimulation of the canine tooth pulp was studied in the subnucleus caudalis (medullary dorsal horn) and the subnucleus oralis of the trigeminal (V) spinal tract nucleus in chloralose-anesthetized cats to test the effects of natural as well as electrical stimulation of the tooth pulp. The neurons were functionally classified on the basis of their cutaneous receptive-field properties as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), or nociceptive specific (NS). The orofacial receptive-field properties and responses evoked by electrical stimulation of the tooth pulp indicated that the oralis and caudalis neurons examined had characteristics typical of those previously documented for oralis LTM neurons and for caudalis LTM, WDR, and NS neurons. Each neuron was also tested with cold and warm stimulation of the canine tooth, and some neurons were also tested for responsiveness to thermal stimulation of the premolar tooth or to mechanical and chemical stimuli delivered to the dentine of the canine tooth. Although all the neurons could be excited by electrical stimulation of the pulp, we found that the only neurons that consistently responded to thermal pulp stimuli were those located in the V subnucleus caudalis. Moreover, only those caudalis neurons that had been functionally classified as nociceptive (4 WDR and 21 NS neurons) showed this responsiveness. Heating of the canine or premolar tooth excited 24 of these 25 nociceptive neurons; cooling activated only 3, and none of the small number of neurons tested with mechanical and chemical stimulation of the dentine was excited. The response of the nociceptive neurons to heating of the tooth contrasted with the responses of the same neurons to pinching and heating of their cutaneous receptive field.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

Neuroplasticity induced by tooth pulp stimulation in trigeminal subnucleus oralis involves NMDA receptor mechanisms.

We have recently demonstrated that application of the mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant increases in jaw muscle electromyographic (EMG) activity and neuroplastic changes in trigeminal (V) subnucleus caudalis. Since subnucleus oralis (Vo) as well as caudalis receives projections from molar pulp afferents and is also an integral brain stem relay of afferent input from orofacial structures, we tested whether MO application to the exposed pulp induces neuroplastic changes in oralis neurons and whether microinjection of MK-801, a noncompetitive NMDA antagonist, into the Vo influences the pulp/MO-induced neuroplastic changes in chloralose/urethan-anesthetized rats. Single neuronal activity was recorded in Vo, and neurons classified as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), nociceptive-specific (NS), deep (D), or skin/mucosa and deep (S + D). The spontaneous activity, mechanoreceptive field (RF) size, mechanical threshold, and response to suprathreshold mechanical stimuli applied to the neuronal RF were assessed prior to and throughout a 40- to 60-min period after MO application to the maxillary molar pulp. In animals pretreated with saline microinjection (0.3 microl) into the Vo, MO application to the pulp produced a significant increase in spontaneous activity, expansion of the pinch or deep RF, decrease in the mechanical threshold, and increase in response to suprathreshold mechanical stimuli of the nociceptive (WDR, NS, and S + D) neurons except for those nociceptive neurons having their RF only in the intraoral region. The pulpal application of MO did not produce any significant neuroplastic changes in LTM neurons. Furthermore, in animals pretreated with MK-801 microinjection (3 microg/0.3 microl) into the Vo, MO application to the pulp did not produce any significant changes in the RF and response properties of nociceptive neurons. In other animals pretreated with saline (0.3 microl) or MK-801 (3 microg/0.3 microl) microinjected into the Vo, mineral oil application to the pulp did not produce any significant changes in RF and response properties of nociceptive neurons. These findings indicate that the application of MO to the tooth pulp can induce significant neuroplastic changes in oralis nociceptive neurons and that central NMDA receptor mechanisms may be involved in these neuroplastic changes.

Animals↗

Inhibition of nociceptive neurons by internal capsule stimulation.

The mechanism of pain relief by internal capsule (IC) stimulation was investigated in 32 adult cats. Nociceptive neuronal activity of the nucleus ventralis posteromedialis (VPM), responding to contralateral pulp stimulation, was suppressed by IC stimulation to a greater extent than activity in the posterior nuclear group (PO) or centre-median nucleus. On the contrary, suppression of neuronal firing by intraventricular morphine-HCl predominated in PO neurons. These results suggest that pain relief by IC stimulation may be mediated through inhibitory effects on nociceptive neurons of the thalamic sensory relay nuclei.

Analgesia↗

Stimulus-evoked pain in teeth.

When pain occurs in teeth as a result of external stimulation it is more likely to be due to physical factors affecting the enamel and dentine than to pulp disease. The mechanisms are described, clinical conditions are mentioned and a speculative attempt is made to correlate them by introducing the idea of streaming potentials in the dentine and pulp. The pain perception threshold is considered in relation to different teeth, age, sex, lateral dominance, personality, and the area of electrode contact. The latter has proved interesting, for manipulation of it has led to possibly important findings on the nature of convergence in the trigeminal system, two-point discrimination, referred pain and somatopic representation of dental pulps in the medulla oblongata.

Age Factors↗

Immunohistochemical evidence for ATP receptors in human dental pulp.

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

Adenosine Triphosphate↗