Jaw opening reflex affected by electroacupuncture in rat

Analgesic action of acupuncture and moxibustion: a review of unique approaches in Japan

The mechanism of acupuncture analgesia (AA) is one of the most widely researched topics in complementary and alternative medicine (CAM) based on modern medical methodology. Endogenous opioid-mediated mechanisms of acupuncture have been well established since the 1970s. In this review, we have covered the progress of AA research by Japanese investigators. In particular, we have reviewed the physiological basis of analgesic effects induced by acupuncture and moxibustion, including the actions of endogenous opioid and diffuse noxious inhibitory controls (DNICs), and the afferent fibers participating in acupuncture and moxibustion stimuli are discussed.

Traditional and Evidence‐Based Acupuncture in Headache Management: Theory, Mechanism, and Practice

Acupuncture, traditional Chinese needle therapy, has become widely used for the relief of headache. The history of the practice of acupuncture in the United States and the theoretical framework for acupuncture in Chinese medicine are reviewed. The basic scientific background and clinical application of acupuncture in the headache management are discussed.

Unilateral application of an inflammatory irritant to the rat temporomandibular joint region produces bilateral modulation of the jaw-opening reflex

The aim of this study was to determine the effect of unilateral acute inflammation of craniofacial deep tissues on the ipsilateral and contralateral jaw-opening reflex (JOR). The effects of mustard oil (MO), injected into the temporomandibular joint region, were tested on the JOR recorded in the digastric muscle and evoked by low-intensity electrical stimulation of the ipsilateral and contralateral inferior alveolar nerve in anesthetized rats. The MO injection induced a long-lasting suppression of the amplitude of both ipsilaterally and contralaterally evoked JOR, although the latency and duration of the JOR were unaffected. The suppressive effect was more prominent for the contralaterally evoked JOR, and observed even when background activity in the digastric muscle was increased by the MO injection. The results indicate that changes in the JOR amplitude following MO injection do not simply reflect alterations in motoneuronal excitability, and suggest that inflammation of deep craniofacial tissues modulates low-threshold sensory transmission to the motoneurons.

Characteristics of nociceptors in the periodontium—an in vitro study in rats

Recent studies have indicated that nociceptors can be classified into various types according to their physiological properties. These studies have clarified that the frequency distribution of various nociceptor types is different among body sites and animal species. In the present study, we investigated the physiological properties of rat's periodontal nociceptors in an in vitro jaw-nerve preparation. Responses were recorded from functional single filaments in the inferior alveolar nerve. To determine the nociceptor type, calibrated von Frey filaments, heat, and bradykinin (BK) stimuli were used. We found five subtypes of nociceptors in the periodontal ligaments of the lower incisor: Adelta-high threshold mechanonociceptors (Adelta-HTM, n=28), Adelta-mechanoheat nociceptors (Adelta-MH, n=6), Adelta-polymodal nociceptors (Adelta-POLY, n=26), C-high threshold mechanonociceptors (C-HTM, n=3) and C-polymodal nociceptors (C-POLY, n=4). Most nociceptors were Adelta-innervated, while only a small number of C-innervated nociceptors were found. The present results suggest that periodontal nociceptors transmit mainly fast pain, and may thus play a role in rapid detection of injure-related stimuli during mastication.

Modulation of temporalis muscle exteroceptive suppression by limb stimuli in normal man

The effects of noxious and non-noxious limb stimulations on the second exteroceptive suppression of voluntary temporalis muscle activity (ES2) were studied in healthy human volunteers. Duration of temporalis ES2 was measured on averaged rectified responses obtained after stimulating the labial commissure at an intensity of 25 mA. Single peripheral electrical stimuli applied over nerve trunks or over the skin before the labial stimulus decreased ES2 duration. This effect was most pronounced after cutaneous stimuli, especially of the index finger, and it was not observed when the conditioning stimulus was a 10 second, high frequency train. For stimulation at the index finger, temporalis ES2 inhibition progressively increased with intensity from 10 mA to 40 mA; it was maximal for an interstimulus interval between 50 and 140 ms. After naloxone (0.4 mg or 4 mg, i.v.) there was a partial reversal of the index-induced ES2 depression, but this effect was not significant. Immersion of one hand in water heated at 47 degrees C produced a short-lasting ES2 reduction. These results are comparable, though not similar, to the inhibition of the digastric reflex (or jaw opening reflex) observed in animals after limb stimuli and to the depression of the spinal flexion reflex reported in man after heterotopic peripheral stimuli. Although peripheral stimuli were able by themselves to suppress temporalis EMG activity in some subjects, it is likely that they reduce labial-induced ES2 via activation of brainstem structures, such as periaqueductal gray matter or raphe magnus nucleus, which are thought to inhibit the medullary inhibitory interneurons mediating ES2.

Modulation by central postsynaptic alpha 2-adrenoceptors of the jaw-opening reflex induced by orofacial stimulation in rats

1. The modulation by alpha 2-adrenoceptors of the jaw-opening reflex (digastric electromyographic responses) elicited by orofacial electrical stimulation (OF-JOR) in pentobarbitone anaesthetized rats was investigated. 2. Increasing doses of clonidine (0.1-1000 micrograms kg-1, i.v.) reduced, in a dose-dependent manner until abolition, the amplitude and duration of the OF-JOR and increased the latency to onset. The sum of amplitudes of the reflex was the most sensitive parameter to the inhibitory effects of clonidine (ED50 = 13.9 micrograms kg-1). 3. Pretreatment with the alpha 2-adrenoceptor antagonist, idazoxan (0.03-1 mg kg-1, i.v.), caused a dose-dependent shift (1.5 to 37 fold) to the right of the dose-response curve for clonidine without significant change of maximum inhibitory effect, in a manner compatible with competitive antagonism (ED50B = 29.0 micrograms kg-1). Pretreatment with yohimbine (0.3 mg kg-1, i.v.) also antagonized the inhibitory effect of clonidine on the OF-JOR. In contrast, the alpha 2-adrenoceptor antagonist ARC-239 (0.3 mg kg-1, i.v.) did not antagonize the effect of clonidine on the reflex. 4. In rats pretreated with reserpine (5 mg kg-1, s.c., 18 h) the OF-JOR was not modified, but the potency of clonidine in inhibiting the reflex was potentiated (ED50 value decreased to 6.8 micrograms kg-1) without a significant change of maximum inhibitory effect. 5. Increasing doses of amphetamine (0.1-3000 micrograms kg-1, i.v.) caused a dose-related, but partial, inhibition of the OF-JOR (ED50 = 135 micrograms kg-1; Emax = 67%). Pretreatment with idazoxan (0.1 mg kg-1, i.v.)induced a nine fold shift to the right of the dose-response curve for amphetamine, while treatment with the depleting drug alpha-methyl-p-tyrosine (150mg kg-1 daily, i.p., for 14 days) abolished the inhibitory effect of this indirect adrenoceptor agonist on the OF-JOR.6. Morphine (0.1-3000 microgkg-1, i.v.) also reduced the OF-JOR in a dose-dependent manner (ED50 value about 325 microg kg-1) but, in contrast to clonidine, it failed to inhibit the reflex fully (Emax = 48%).As expected, pretreatment with the opioid antagonist naloxone (1 mg kg-1, i.v.) abolished the inhibitory effect of morphine on the OF-JOR, while it did not alter that of clonidine.7. Chronic, but not acute, pretreatment with idazoxan (3 mg kg-1 daily, i.p. for 14 days) led to a marked potentiation of the inhibitory effect of clonidine on the OF-JOR (ED50 value decreased to 4.2 microg kg-1), without a significant change of maximum inhibitory effect.8. Together the results indicate that clonidine evokes a potent inhibition of the OF-JOR in rats through the activation of postsynaptic alpha2-adrenoceptors. It is suggested that this functional response represents a simple and useful in vivo model for studying various regulatory mechanisms of central alpha2-adrenoceptors.

Localisation of the first-order neurone of the jaw opening reflex elicited by periodontal stimulation

The localisation of the first-order neurone of the jaw-opening reflex (JOR), provoked by periodontal stimulation, was investigated in the rat. A section of the mandibular part of the trigeminal ganglion was carried out without impairing the motor root. It suppressed the reflex triggered by the stimulation of the lower incisor. Bilateral destruction of the mesencephalic nucleus and tract does not modify the JOR. These results suggest that the first-order neurone of the reflex is located in the trigeminal ganglion.

Alpha 2 adrenoceptors modulate the nociceptive jaw-opening reflex in rats

The jaw-opening reflex in lightly anesthetized rats induced by intrapulpal (left maxillary) electrical tooth pulp stimulation and quantified by the electromyograms (threshold values) recorded from the ipsilateral digastric muscle was used as the experimental pain index. The threshold for the jaw-opening reflex was significantly elevated by clonidine (12.5 to 50 micrograms/kg, i.v.) and was inversely correlated with the frequency of stimulation. The analgesia elicited by clonidine was antagonized by pretreatment with the alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg, i.v.). All doses of clonidine produced an initial transient pressor response followed by a sustained hypotension and bradycardia. However, there was no direct correlation between the antinociceptive and cardiovascular effects of clonidine. It is proposed that clonidine modulates jaw-opening reflex analgesia by specifically activating alpha 2-adrenoceptors.

Effects, in the rat, of various stressing procedures on the jaw-opening reflex induced by tooth-pulp stimulation

Jaw-opening reflex (JOR) induced by a repetitive tooth-pulp stimulation delivered at 1 Hz was observed in awake, freely-moving rats. Modifications of the reflex responses of the digastric muscle were produced by foot-shock stimulation, by an oesophageal administration of placebo and a subcutaneous injection of saline. Subcutaneous injection of saline evoked a total disappearance of the reflex which returned within 30 s. Foot shock delivered for 10 s and repeated twice a minute for 30 min of intensity, just below the vocalization threshold, evoked a marked decrease of the reflex to 60 +/- 6 per cent of the control value (n = 10). Administration of arabic gum by oesophageal tube gave either: the same transient disappearance as for the subcutaneous injection in 6 rats; a progressive 28 min +/- 11 (n = 6) return to the initial value; a progressive return to a subcontrol value of 52 +/- 4 per cent (n = 7). The application of two successive stimuli resulted in a more pronounced JOR decrease. The response to electrical foot shock can be explained by a central analgesia induced by stress.

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.

Suppression of tooth pulp evoked responses by activation of raphe-trigeminal neurons in rat

Electrical stimulation (0.1 msec duration, 50 Hz, 1 min) applied to the nucleus raphe magnus markedly suppressed the tooth pulp evoked responses in caudal part of the spinal trigeminal nucleus but only slightly altered those in its rostral area. Raphe stimulation also greatly suppressed jaw opening reflex evoked by tooth pulp stimulation, which lasted longer than suppression of spinal trigeminal nucleus.