Role of Heme Oxigenase-Carbon Monoxide in Orofacial Nociception in Rats

Objective: The aim of the present study was to investigate the role of the heme oxygenase (HO)-carbon monoxide (CO)-soluble guanylate cyclase (sGC)-cGMP pathway on orofacial nociceptive response of rats to the formalin experimental orofacial nociceptive model. Methods: For the orofacial formalin test 50 µl of a 1.5% formalin solution was injected into the right upper lip of rats. Fifty three rats were used. Following injections, animals were observed for 45 minutes and the face rubbing activity was measured as the nociceptive response. Animals were pretreated with injections with the HO inhibitor, zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG) (400nmol/50µl), another group with heme-lysinate (152nmol/50µl), which is known to induce the HO pathway and the last group with the sGC inhibitor, methylene blue (150µg/50µl), injected prior to the heme-lysinate (152nmol/50µl). After 5 minutes animals of these experimental groups were submitted to the formalin test and observed for 45 minutes. Control animals were treated with their vehicles. Results: The incidence of rubbing behavior was significantly increased after injection of formalin (1.5%) into the upper lip [F(15,165)= 9.65, P < 0.001] compared with control animals. We observed a significant increase in the nociceptive response of rats treated with ZnDPBG [F(15,135)= 3.21, P < 0.001], caused by the reduction of the concentration of CO. Besides it we observed a drastic reduction of face rubbing activity in the heme-lysinate treated animals [F(15,120)= 8.72, P < 0.001], caused by an increase in the concentration of CO. Furthermore, the HO pathway seems to act via cGMP, since methylene blue (a sGC inhibitor) prevented the reduction of face rubbing activity caused by heme-lysinate [F(15,180)= 0.86, P < 0.001]. Conclusion: These findings strongly indicate that the HO pathway plays an orofacial antinociceptive role during the formalin test, acting via cGMP. Financial support: FAPESP.