Effects of Cannabinoid and Trpv1 in Formalin-induced Orofacial Pain

Abstract

This study investigated that the modulation of cannabinoid (CB) or TRPV1 (transient receptor potential vanilloid) in inflammatory orofacial pain. Experiments were carried out using adult male Sprague- Dawley rats weighing 240 to 280 g. Formalin (5%, 30 μl) were applied subcutaneously to the vibrissa pad. For each animal, the number of noxious behavioral responses, including rubbing or scratching of the facial region proximal to the injection site, was recorded for 9 sequential 5 minute intervals. Anandamide (AEA), a CB1 agonist, was injected subcutaneously on 20 minute prior to formalin injection. The subcutaneous injection from 0.03 to 0.3 μg of AEA suppressed the number of scratches in the second phase of the behavioral responses to formalin. Low doses of AEA were more significantly suppressed the number of scratches in the second phase. The subcutaneous injection of 10 μg of AM251, a CB1 antagonist, and 0.4 μg of IRTX, a TRPV1 antagonist, were injected at 10 minute prior to AEA injection. Pretreatment with AM251 blocked the antinociceptive effects of AEA. But, pretreatment with 0.4 μg of IRTX partially modulated the antinociceptive effects of AEA. These findings suggest that blockade of CB1 receptor and TRPV1 receptor play an important role in the peripheral processing of inflammatory pain in the orofacial area.