Nitric Oxide-Mediated Mitochondrial Damage in the Brain: Mechanisms and Implications for Neurodegenerative Diseases

Abstract: Within the CNS and under normal conditions, nitric oxide (^•NO) appears to be an important physiological signalling molecule. Its ability to increase cyclic GMP concentration suggests that ^•NO is implicated in the regulation of important metabolic pathways in the brain. Under certain circumstances ^•NO synthesis may be excessive and ^•NO may become neurotoxic. Excessive glutamate-receptor stimulation may lead to neuronal death through a mechanism implicating synthesis of both ^•NO and superoxide (O₂^•−) and hence peroxynitrite (ONOO⁻) formation. In response to lipopolysaccharide and cytokines, glial cells may also be induced to synthesize large amounts of ^•NO, which may be deleterious to the neighbouring neurones and oligodendrocytes. The precise mechanism of ^•NO neurotoxicity is not fully understood. One possibility is that it may involve neuronal energy deficiency. This may occur by ONOO⁻ interfering with key enzymes of the tricarboxylic acid cycle, the mitochondrial respiratory chain, mitochondrial calcium metabolism, or DNA damage with subsequent activation of the energy-consuming pathway involving poly(ADP-ribose) synthetase. Possible mechanisms whereby ONOO⁻ impairs the mitochondrial respiratory chain and the relevance for neurotoxicity are discussed. The intracellular content of reduced glutathione also appears important in determining the sensitivity of cells to ONOO⁻ production. It is concluded that neurotoxicity elicited by excessive ^•NO production may be mediated by mitochondrial dysfunction leading to an energy deficiency state.