α2-Adrenoceptor agonist induces peripheral antinociception via the endocannabinoid system.

BACKGROUND:

Xylazine is an α₂ adrenoceptor agonist that is extensively used in veterinary medicine and animal experimentation procedures to produce analgesia, sedation and muscle relaxation without causing general anesthesia. Considering the lack of knowledge of the mechanisms involved in peripheral antinociception induced by xylazine and the potential interactions between the adrenergic and endocannabinoid systems, the present study investigated the contribution of the latter system in the mechanism of xylazine.

METHODS:

The rat paw pressure test, in which hyperalgesia was induced by the intraplantar injection of prostaglandin E₂, was performed.

RESULTS:

Xylazine administered via an intraplantar injection (25, 50 and 100 μg) induced a peripheral antinociceptive effect against prostaglandin E₂ (2 μg)-induced hyperalgesia. This effect was blocked by treatment with the selective CB₁ cannabinoid antagonist AM251 (20, 40 and 80 μg) but not by the selective CB₂ cannabinoidantagonist AM630 (100 μg). The anandamide reuptake inhibitor VDM11 (2.5 μg) intensified the peripheral antinociceptive effect of a submaximal dose of xylazine (25 μg), and the inhibitor of endocannabinoid enzymatic hydrolysis, MAFP (0.5 μg), showed a tendency towards this same effect. In addition, liquid-chromatography mass spectrometric analysis indicated that xylazine (100 μg) treatment was associated with an increase in anandamide levels in the rat paws treated with PGE₂.

CONCLUSIONS:

The present results provides evidence that the peripheral antinociceptive effect of the α₂adrenoceptor agonist xylazine probably results from anandamide release and subsequent CB₁ cannabinoid receptor activation.