Effect of acute exposure to arsenic on formalin-induced nociception and tramadol-mediated antinociception in mice.

In vitro studies have suggested that arsenic can modify the activity of macrophages in the mouse producing an over-regulation of the COX-2 and increased concentrations of PGE2 in endothelial cells. These effects may lead in vivo to enhancement of inflammatory and painful responses. In this study we studied the effect of an acute intoxication with sodium arsenite (1, 5, 10, 36 and 100 nmol/kg s.c.) on the nociceptive response of mice in the formalin test. On the other hand, the effect of arsenic on the antinociceptive response mediated by tramadol was evaluated in mice administered with a single dose of the analgesic agent (10 mg/kg s.c.). Arsenic levels in the liver were measured as a marker of the intoxication degree. Our results indicated that the arsenic acute exposure increases the nociceptive behavior in mice in a dose-dependent manner. Accordingly, the exposure to arsenic partially blocked the analgesic effect of tramadol although no statistical differences were reached. These results support the previous in vitro evidences regarding the alterations in the inflammatory-painful processes produced by the acute exposure to arsenic. Moreover, our results suggest that the intoxication with arsenic might exacerbate the pathological state in inflammatory diseases.