CB1 and CB2 contribute to antinociceptive and anti-inflammatory effects of electroacupuncture on experimental arthritis of the rat temporomandibular joint.

Electroacupuncture (EA) and cannabinoids have been reported to have anti-inflammatory and antinociceptive effects in animal models of arthritis. Male Wistar rats were injected with saline or zymosan (2 mg) into the temporomandibular joint (TMJ). EA (10 Hz, 30 min) was performed 2 h after or 1 h before zymosan administration. AM251 or AM630 (3 mg/kg, i.p.)were administered before EA treatment. Mechanical hypernociception was accessed after zymosan administration. Rats were sacrificed 6 h after zymosan administration and the joint was removed for histopathological analysis. The gene expression of CB₁ and CB₂ receptors was assessed after sacrifice of the TMJ arthritic animals. EA inhibited zymosan-induced hypernociception (p < 0.05). AM251 reversed significantly the antinociceptive effect of EA, suggesting that the CB₁ receptor is involved in this effect. AM630 reversed the anti-inflammatory effect of EA. CB₁ and CB₂ receptor gene expression was upregulated 6 h after zymosan-induced arthritis in the EA-treated group. We observed downregulation of CB₂ receptor gene expression in the EA group at the 24th hour compared with the 6th hour. Higher CB₁ receptor gene expression was also found compared with the 6th hour. EA produced antinociceptive and anti-inflammatory effects, and these effects appeared to be mediated through CB₁ and CB₂ receptor activation.