A B S T R A C T Introduction: Mercury is the second most metal pollutant in the world and has the potential to induce many pathologic conditions, especially in nervous system, such as depression. Here we tried to find out if nitric oxide has any possible role in the pathophysiology of depression induced by this metal. Although the role of nitric oxide has been shown in mood control, here we use specific doses of nitric oxide inducer and/or inhibitors which had no effect on normal rats. Methods: 120 male wistar rats weighting 200-250 gram were divided into two main groups: control and methyl mercury(MM) treated. Each main group was divided into four different sub-goups: Saline, L-Arginine, L-Name or 7-nitroindazole (7-NI) respectively. The duration of taking MM or saline was daily for 15 days for both. After the 15th injection a forced swimming test was done. This test shows behavioral immobility (BI) or latency of attempt to escape (LAE), as a depression indicator. Results: Our study showed that low dose L-arginine is protective against MM induced depression as it could turn behavioral immobility (BI) to normal levels in groups taking MM plus L-Arginine, while in group taking just MM, BI was much longer showing the intensity of depression. L-Name and 7-NI did aggravated depression in MM groups but not control ones, on the other hand just in the case of 7-NI the result was significant. Discussion: Our results showed 1) MM could induce depression in rat 2) L-Arginine could improve depression to normal situation in MM group, while in control group has no effec 3) 7-NI, a selective nNOS inhibitor can aggravate mental depression in intoxicated rats. These results showed the important role of nNOS in protection against MM induced depression
