NO, citrulline and NADP+ are formed by nitric oxide synthase (NOS) from arginine and NADPH+H+. Citrulline can be recycled to arginine by ASS and ASL. These are the components of the citrulline-NO cycle which is also expressed in the brain. Cationic amino acid transporters (e.g. CAT1) mediate the transport of arginine across the blood-brain barrier and between brain cells.
NO is an important signalling molecule which activates soluble cyclic guanylate cyclase resulting in increased formation of the second messenger cyclic guanosine monophosphate (cGMP) by conversion of guanosine triphosphate (GTP). cGMP has various functions. It acts as a common regulator of ion channel conductance, and causes relaxation of smooth muscles, and apoptosis. In blood vessels, relaxation of smooth muscles leads to vasodilatation and increased blood flow.
In neurons, NO synthesis is stimulated by calcium influx via activated NMDA receptors. Neuronal NOS (nNOS) is localized at the C-terminal end of NMDA receptors and thus is perfectly placed to act as a sensor of NMDA receptor activation and neuronal calcium influx. nNOS requires calcium to become activated. This mechanism plays a key role in the coupling of brain metabolism and local energy demand with cerebral blood flow described in 1890 by Roy and Sherrington. As described above, glutamate reuptake from the synaptic cleft and recycling via the glutamate-glutamine cycle is energy dependent. Energy costs can only be covered by increased glucose uptake via GLUT1 transporters that are expressed in brain capillary endothelial cells and astrocyte end-feet. Glucose influx to the active brain regions is increased by local vasodilatation and thus increased flood flow which is mediated by NO. By this mechanism, the auto regulation of cerebral blood flow is maintained and is directly linked to energy-consuming glutamatergic neurotransmission. In astrocytes, NO synthesis is calcium-independent and is controlled by the cytokine- inducible expression of inducible NO synthase (iNOS) and by the availability of arginine.
If NO is produced at a high rate it can form peroxynitrite with superoxide anion. Peroxynitrite is an important radical and highly toxic for cells. Furthermore, NO has some direct toxic effects. It has been shown that NO can inhibit glutamine synthetase and thus may interfere with the major detoxification mechanism of ammonia in the brain. Therefore, NO production needs to be reliably controlled to prevent the induction of cell damage.
In combination with cerebral edema due to glutamine-induced cell swelling, disturbed NO metabolism significantly aggravates pathologic effects of hyperammonaemia on cerebral blood flow and brain energy metabolism. However, the effects of hyperammonaemia on NO metabolism are not easy to predict since they vary in different brain cell types, during acute or chronic hyperammonaemia, and depend on a normal supply (e.g. by treatment) with arginine.
