Role of pertussis toxin-sensitive G-protein, K+ channels, and voltage-gated Ca2+ channels in the antinociceptive effect of inosine

Inosine is the first metabolite of adenosine. It exerts an antinociceptive effect by activating the adenosine A₁ and A_2A receptors. We have previously demonstrated that inosine exhibits antinociceptive properties in acute and chronic mice models of nociception. The aim of this study was to investigate the involvement of pertussis toxin-sensitive G-protein-coupled receptors, as well as K⁺ and Ca²⁺ channels, in the antinociception promoted by inosine in the formalin test. Mice were pretreated with pertussis toxin (2.5 μg/site, i.t., an inactivator of G_i/0 protein); after 7 days, they received inosine (10 mg/kg, i.p.) or morphine (2.5 mg/kg, s.c., used as positive control) immediately before the formalin test. Another group of animals received tetraethylammonium (TEA) or 4-aminopyridine (4-AP) (1 μg/site, i.t., a non-specific voltage-gated K⁺ channel blockers), apamin (50 ng/site, i.t., a small conductance Ca²⁺-activated K⁺ channel blocker), charybdotoxin (250 pg/site, i.t., a large-conductance Ca²⁺-activated K⁺ channel blocker), glibenclamide (100 μg/site, i.t., an ATP-sensitive K⁺ channel blocker) or CaCl₂ (200 nmol/site, i.t.). Afterwards, the mice received inosine (10 mg/kg, i.p.), diclofenac (10 mg/kg, i.p., a positive control), or morphine (2.5 mg/kg, s.c., a positive control) immediately before the formalin test. The antinociceptive effect of inosine was reversed by the pre-administration of pertussis toxin (2.5 μg/site, i.t.), TEA, 4-aminopyridine, charybdotoxin, glibenclamide, and CaCl₂, but not apamin. Further, all K⁺ channel blockers and CaCl₂ reversed the antinociception induced by diclofenac and morphine, respectively. Taken together, these data suggest that the antinociceptive effect of inosine is mediated, in part, by pertussis toxin-sensitive G-protein coupled receptors and the subsequent activation of voltage gated K⁺ channel, large conductance Ca²⁺-activated and ATP-sensitive K⁺ channels or inactivation of voltage-gated Ca²⁺ channels. Finally, small conductance Ca²⁺-activated K⁺ channels are not involved in the antinociceptive effect of inosine.