Role of hydrogen sulfide in the formalin-induced orofacial pain in rats

Hydrogen sulfide (H2S) is a gasotransmitter synthesized in peripheral tissues by the enzyme cystathionine gamma-lyase (CSE). This gas has been documented to be involved in a wide variety of processes including inflammation and nociception. The aim of the present study was to investigate the role of the peripheral H2S pathway in nociceptive response to the orofacial formalin experimental model of pain. Orofacial pain was induced by subcutaneous injection of formalin (1.5%, 50 µl) into the upper lip of rats, and the time spent rubbing the face was measured at 3-min intervals for 45 min. Formalin induced a marked biphasic pain (first phase: 0-3 min; second phase: 15-33 min). Pretreatment with H2S donor (Na2S; 90 µmol/kg), CSE inhibitor (propargylglycine; 26.5 and 88.4 µmol/kg), or a preferential blocker of T-type Ca2+ channels (mibefradil; 0.28 and 2.81 µmol/kg) attenuated the second phase of face rubbing when injected locally as well as systemically. Pretreatment with a selective blocker of K+ATP channels (glybenclamide; 2.81 µmol/kg) suppressed the Na2S-mediated attenuation of the formalin-induced pain second phase. Taken together these results suggest that endogenously produced H2S plays a pronociceptive role probably via T-type Ca2+ channels, whereas exogenous H2S exerts antinociceptive effects mediated by K+ATP channels.