In autonomic and enteric nerves, nitric oxide (NO) is produced from arginine by the tetrahydrobiopterin-dependent nitric oxide synthases. NO has a number of attributed physiological functions including relaxation of both vascular and intestinal smooth muscle and the possible regulation of mitochondrial energy production. Furthermore, within the brain, NO may have a role in memory formation. However, excessive NO formation has been implicated in the neurodegenerative process associated with Parkinson's and Alzheimer's disease. Whilst the exact mechanism whereby excessive NO causes neuronal death is not known, a growing body of evidence suggests that irreversible damage to the mitochondrial electron transport chain may be an important factor.
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| NO is a gas it is not stored in vesicles, but released directly into the extracellular space. Consequently, NO does not, in the strictest sense, meet all the current criteria to be labeled as a neurotransmitter. NO itself diffuses comparatively easily between cells and binds directly to heme groups in the enzyme guanylate cyclase, stimulating the production of cyclic guanosine monophosphate. (See also Chapter 17, p. 236.)
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