NOS2 and depressive disorder: The enzyme NOS, which is reliant on calcium (Ca2+) and calmoduline, converts L-arginine into L-citrulline to produce NO.[20] 7-Nitroindole (7-NI) is a selective neuronal nitric oxide synthase (nNOS) inhibitor that treats depression by competing with L-arginine for the binding site of L-arginine on NOS.[21] 7-NI exerts similar antidepressant effects in the rat experiments.[22] NO affects neurotransmitter release by synthesizing cGMP through sGC.