relaxing smooth muscle in their linings. eNOS is the primary controller of smooth muscle tone. NO activates guanylate cyclase, which induces smooth muscle relaxation by: Increased intracellular cGMP, which inhibits calcium entry into the cell, and decreases intracellular calcium concentrations Activation of K+ channels, which leads to hyperpolarization and relaxation Stimulates a cGMP-dependent protein kinase that activates myosin light chain phosphatase, the enzyme that dephosphorylates myosin light chains, which leads to smooth muscle relaxation. eNOS plays a critical role in embryonic heart development and morphogenesis of coronary arteries and cardiac valves. The neuronal isoform is involved in the development of nervous system. It functions as a retrograde neurotransmitter important in long term potentiation and hence is likely to be important in memory and learning. nNOS has many other physiological functions, including regulation of cardiac function and peristalsis and sexual arousal in males and females. An alternatively spliced form of nNOS is a major muscle protein that produces signals in response to calcium release from the SR. nNOS in the heart protects against cardiac arrhythmia induced by myocardial infarction. The primary receiver for NO produced by eNOS and nNOS is soluble guanylate cyclase, but many secondary targets have been identified. S-nitrosylation appears to be an important mode of action. The inducible isoform iNOS produces large amounts of NO as a defense mechanism. It is synthesized by many cell types in response to cytokines and is an important factor in the response of the body to attack by parasites, bacterial infection, and tumor growth. It is also the cause of septic shock and may play a role in many diseases with an autoimmune etiology. NOS signaling is involved in development and in fertilization in vertebrates. It has been implicated in transitions between vegetative and reproductive states in invertebrates, and in differentiation leading to spore formation in slime molds. NO produced by bacterial NOS is protective against oxidative damage. NOS activity has also been correlated with major depressive episodes (MDEs) in the context of major depressive disorder, in a large case-control treatment study published in mid-2021. 460 patients with a current major depressive episode were compared to 895 healthy patients, and by measuring L-citrulline/L-arginine ratio before and after 3–6 months of antidepressant treatment, results indicate that patients in a major depressive episode have significantly lower NOS activity compared to healthy patients, whilst treatment with antidepressants significantly elevated NOS activity levels in patients in