We recently demonstrated that the microinjection of arginine vasopressin suppressed nociceptive jaw opening reflex in freely moving rats. These results suggest that the amygdala seems to be involved in the pain modulation of the orofacial area. This study was designed to investigate the antinociceptive effect of the central nucleus of amygdala after microinjcetion of mu opioid agonist. We also investigated the mechanisms of antinociception of the central nucleus of amygdala. Male rats were anesthetized with pentobarbital sodium (40 mg/kg, ip). The guide cannulae were implanted into the amygdala and the lateral ventricle respectively. Stimulating and recording electrodes were implanted into the incisor pulp and anterior digastric muscle. Electrodes were led subcutaneously to the miniature cranial connector sealed on the top of the skull with acrylic resin. Jaw opening reflex was used as a pain assessment method. After 48 hours recovery from surgery, the electromyogram of anterior digastric muscle (dEMG) was recorded in freely moving rat to eliminate influences of anesthetic agents in pain perception. Electrical shocks (200 sec duration, 0.5-2 mA intensity) were delivered at 0.5Hz to the dental pulp every 2 minute. The mu opioid agonist, [D-Ala2, N-Me-Phe4, Gly5-ol]-Enkephalin (DAMGO; 1.5㎍ or 3.0 ㎍), was administered into the central amygdaloid nucleus. DAMGO suppressed jaw opening reflex in freely moving rats with a dose dependent manner. dEMG were suppressed to 74±8 % or 46±9% of the control respectively. aCSF had no effects on the dEMG. To investigate the mechanism of DAMGO-induced antinociception, we observed dEMG activity after intracerebroventricular injection of naloxone, an opioid receptor antagonist, methysergide, a serotonin receptor antagonist, phentolamine, a nonselective α-adrenergic receptor antagonist. All drugs did not affect the resting dEMG activity at our dose(30 ㎍/6 ㎕). Naloxone blocked the suppressive effect of DAMGO. Phentolamine did not affect the suppression of dEMG. These results indicate that DAMGO in the central amygdaloid nucleus has analgesic effects in the orofacial area and DAMGO-induced antinociception is mediated by opioid pathways.
Source: Korean J Physiol Pharmacol.2001 Dec;5(Suppl II):S95-S96
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