Effect of nitric oxide-cyclic GMP-K channel pathway blockers, naloxone and metformin, on the antinociception induced by the diuretic pamabrom.

Pamabrom is a diuretic that is effective in treating premenstrual syndrome and primary dysmenorrhea. The aim of this study was to examine the effect of metformin and modulators of the opioid receptor-nitric oxide (NO)-cyclic guanosine monophosphate (cGMP)-K channel pathway on the local antinociception induced by pamabrom. The rat paw 1% formalin test was used to assess the effects. Rats were treated with local administration of pamabrom (200-800 µg/paw) or indomethacin (200-800 µg/paw). The antinociception of pamabrom or indomethacin was evaluated with and without the local pretreatment of the blockers. Local administration of pamabrom and indomethacin produced dose-dependent antinociception during the second phase of the test. Local pretreatment of the paws with naloxone (50 µg/paw), l-nitro-arginine methyl ester (10-100 µg/paw), or 1-(1,2,4)-oxadiazolo[4,2-]quinoxalin-1-one (10-100 µg/paw) reverted the antinociception induced by local pamabrom, but not of indomethacin. Similarly, the K channel blockers glibenclamide, glipizide, 4-aminopyridine, tetraethylammonium, charybdotoxin, or apamin reverted the pamabrom-induced antinociception, but not of indomethacin. Metformin significantly blocked the antinociception of pamabrom and indomethacin. Our data suggest that pamabrom could activate the opioid receptor-NO-cGMP-K channel pathway to produce its peripheral antinociception in the formalin test. Likewise, a biguanide-dependent mechanism could be activated by pamabrom and indomethacin to generate antinociception.