Additive antinociceptive effect of the combination of diazoxide, an activator of ATP-sensitive K + channels, and sodium nitroprusside and dibutyryl-cGMP

Abstract

Using the rat paw pressure test, in which increased sensitivity is induced by intraplantar injection of prostaglandin E 2, we assessed the antinociceptive effect of the ATP-sensitive K + channel opener diazoxide and the large-conductance Ca 2+-activated K + channel opener NS-1619 (1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl) phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one) on the peripheral hyperalgesia induced by prostaglandin E 2. Diazoxide, administered locally into the right hindpaw (20, 38, 75, 150, 300 and 600 μg), elicited a dose-dependent antinociceptive effect on prostaglandin E 2-induced hyperalgesia (2 μg/paw). The effect of diazoxide at the dose of 300 μg/paw was shown to be local since it did not produce any effect when administered in the contralateral paw. The action of diazoxide (300 μg/paw) as an ATP-sensitive K + channel opener seems to be specific, since its effect was antagonized in a dose-dependent manner by glibenclamide (40, 80 and 160 μg/paw), a specific blocker of these channels, while tetraethylammonium (7.5, 15 and 30 μg/paw), dequalinium (12.5, 25 and 50 μg/paw) or charybdotoxin (0.5, 1 and 2 μg/paw), blockers of voltage-dependent K + channels and of small- and large-conductance Ca 2+-activated K + channels, respectively, were not able to abolish the antinociception induced by diazoxide. The peripheral antinociceptive effect of diazoxide was not prevented by prior administration of naloxone (12.5, 25 and 50 μg/paw), an opioid receptor antagonist, or methylene blue (75, 125 and 300 μg/paw), an agent that inhibits the activation of guanylate cyclase by nitric oxide. A low dose of diazoxide (20 μg/paw) administered together with a low dose of sodium nitroprusside (125 μg/paw) or dibutyryl cGMP (db-cGMP, 50 μg/paw) induced a marked antinociceptive effect similar to that observed when each drug was administered alone. NS1619 (75, 150 and 300 μg/paw), a specific opener of large-conductance Ca 2+-activated K + channels, had no antinociceptive action on prostaglandin E 2-induced hyperalgesia. This series of experiments provides evidence for a peripheral antinociceptive action of diazoxide and supports the suggestion that the activation of ATP-sensitive K + channels could be the mechanism by which sodium nitroprusside and db-cGMP induce peripheral antinociception, excluding the involvement of large-contuctance Ca 2+-activated K + channels in the process.