• Ho Jeong Lee, Hyo Soon Choi, Chang Young Jung, Jin Sook Ju, Sung Kyo Kim, Yong Chul Bae, and Dong Kuk Ahn.
• Department of Oral Physiology and Neurobiology, School of Dentistry, Kyungpook National University, 188-1, Samduck 2 ga, Chung-gu, Daegu (700-412), South Korea.
• Prog. Neuropsychopharmacol. Biol. Psychiatry. 2004 May 1;28(3):497-503.

AbstractThe present study was performed to investigate the antinociceptive response to the intracisternal administration of NMDA in the orofacial area. To achieve this purpose, the effects of NMDA injected intracisternally on the orofacial formalin test were monitored in freely moving rats. We also investigated underlying the mechanisms of NMDA-induced antinociceptive response. Experiments were carried out on 80 male SD rats and surgical procedures were performed under pentobarbital sodium (40 mg/kg, i.p.). Fifty microliters of 5% formalin was applied subcutaneously to the vibrissa pad without any restraining of the animals. For each animal, the number of noxious behavioral responses and the time spent grooming, rubbing, and/or scratching the facial region proximal to the injection site were recorded for nine successive 5-min intervals. The orofacial formalin responses showed two distinct phases separated by a time of relative inactivity. Intracisternal administration of NMDA produced intense scratching behavioral responses with dose related manner. NMDA injected intracisternally 30 min prior to formalin injection, however, inhibited noxious behavioral responses produced by a formalin injection significantly. Pretreatment with naloxone 20 min prior to NMDA injection abolished the inhibition of number of scratches and the duration of scratching produced by the intracisternal injection of NMDA in the late phase. Pretreatment with L-NAME, NO synthesis inhibitor, however, did not affect the antinociceptive response produced by NMDA injected intracisternally. These results suggest that NMDA injected intracisternally produces brief pain behavioral responses and also produces delayed antinociceptive effects in the orofacial formalin test. The opioid pathway seems to be involved in the NMDA-induced antinociception in the orofacial area.

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