The effects of several potassium (K(+)) channel blockers were studied to determine which K(+) channels are involved in peripheral antinociception induced by the cannabinoid receptor agonist, anandamide.
Hyperalgesia was induced by subcutaneous injection of 250 μg carrageenan into the plantar surface of the hind paw of rats. The extent of hyperalgesia was measured using a paw pressure test 3 h following carrageenan injection. The weight in grams (g) that elicited a nociceptive response, paw flexion, during the paw pressure test was used as the nociceptive response threshold.
Doses of 50, 75, and 100 ng of anandamide elicited a dose-dependent antinociceptive effect. Following a 100 ng dose of anandamide no antinociception was observed in the paw that was contralateral to the anandamide injection site, which shows that anandamide has a peripheral site of action. Pretreatment with 20, 40 and 80 μg AM251, a CB(1) receptor antagonist, caused a dose-dependent decrease in anandamide-induced antinociception, suggesting that the CB(1) receptor is directly involved in anandamide effect. Treatment with 40, 80 and 160 μg glibenclamide, an ATP-sensitive K(+) channel blocker, caused a dose-dependent reversal of anandamide-induced peripheral antinociception. Treatment with other K(+) channel antagonists, tetraethylammonium (30 μg), paxilline (10 μg) and dequalinium (50 μg), had no effect on the induction of peripheral antinociception by anandamide.
This study provides evidence that the peripheral antinociceptive effect of the cannabinoid receptor agonist, anandamide, is primarily caused by activation of ATP-sensitive K(+) channels and does not involve other potassium channels.