NOS1 and major depressive disorder: Furthermore, it has been demonstrated that inhibitors of NOS elevated extracellular serotonin and dopamine levels, key neurotransmitters associated with depression, in the HIP and subsequently mitigated depression, whereas l‐arginine, a precursor of endogenous NO, produces an opposing effect, indicating a potential negative regulatory role of NO on depression (Dhir & Kulkarni, 2011).