Peripheral antinociceptive effect of 2-arachidonoyl-glycerol and its interaction with endomorphin-1 in arthritic rat ankle joints.

1. Both cannabinoid and opioid receptors are localized at the peripheral level, and drugs acting on these receptors may produce antinociception after topical administration; however, the effect of endogenous ligands at these receptors is poorly understood. Our goal was to determine the antinociceptive potency of the endogenous cannabinoid 2-arachidonoyl-glycerol (2-AG), and its interaction with endomorphin-1 (EM1) at joint level in the rat inflammation model. 2. Mechanical hypersensitivity was produced by injection of carrageenan (300 microg/30 microL) into the tibiotarsal joint of the right hind leg. The mechanical threshold was assessed by von Frey filaments. 2-AG (3-200 microg), EM1 (100-300 microg) and their combinations in a fixed-dose ratio (1 : 10) were given into the inflamed joint, and the threshold was determined repeatedly for 105 min after the drug administrations. 3. Both ligands produced dose-dependent anti-hyperalgesia, and the highest doses caused prolonged effects, but they did not influence the degree of oedema and the withdrawal threshold at the non-inflamed side. EM1 had lower potency compared to 2-AG (ED(25): 233 (CI: 198-268) microg and 126 (CI: 88-162) microg, respectively; P < 0.05). The effects of EM1 and 2-AG were prevented by mu-opioid and cannabinoid 1 receptor antagonists, respectively. The ED(25) value for the combination (98 (CI: 80-112) microg) did not differ significantly from the value of 2-AG; however, the largest dose combination produced a significantly higher effect than the ligands by themselves. 4. Our data showed that 2-AG was an effective antinociceptive ligand at joint level, and its combination with EM1 produced long-lasting, effective antinociception.