A colourless gas, NO, that functions as a gaseous second messenger in mammals and other vertebrates, especially in the cardiovascular and nervous systems, and as a neurotransmitter. It is produced in tissues from molecular oxygen and the amino acid arginine by the enzyme nitric oxide synthase and diffuses to neighbouring cells, where it stimulates formation of the intracellular messenger cyclic GMP. The activity of NO synthase, and hence production of NO, is regulated by a signal pathway involving binding of the neurotransmitter acetylcholine to its receptor on the cell surface, the production of the second messenger IP₃ (see inositol), and release of calcium ions from the endoplasmic reticulum. The effects of nitric oxide include relaxation of smooth muscle and dilation of blood vessels. For example, NO induces the local smooth muscle relaxation and vasodilation that enhance blood flow into the erectile tissues of the penis during sexual arousal. It also inhibits platelet aggregation and adhesion, acts as a neurotransmitter in the central nervous system, and may influence neuronal development. Certain cells of the immune system (macrophages and neutrophils) also produce nitric oxide, which is converted to the cytotoxic peroxynitrite anion (⁻O-O–N=O). This has nonspecific activity against pathogens, including protist and metazoan parasites. (see inducible nitric oxide synthase). Plants also produce NO as part of their immune response to attack by pathogens.