Evidence for increased nitric oxide production in multiple sclerosis.

Within the CNS two isoforms of nitric oxide synthase (NOS) exist-the constitutive calcium dependent neuronal form and the inducible calcium independent form associated with glial cells. Stimulation of microglia or astrocytes by cytokines in vitro leads to increased nitric oxide (NO) formation as a result ofNOS induction and stimulation of the biosynthesis of tetrahydrobiopterin, an essential cofactor for NOS.' Excessive NO formation has been implicated in the pathogenesis of multiple sclerosis as oligodendrocytes, by contrast with other glial cells, are killed as a result of the induction of microglial NOS in vitro.2 Activation of the immune response is apparent in multiple sclerosis and the demonstration that such patients have increased concentrations of neopterin,3 a precursor of tetrahydrobiopterin, in their CSF supports the notion that increased NO production probably occurs in multiple sclerosis. Nitric oxide is unstable and is readily converted to nitrate (NO3-) and nitrite (NO2-) and recent work has suggested that the concentration of NO3and NO2in the CSF is a useful indicator ofNO production within the CNS. In view of this we have measured4 the total concentration of NO3and NO2in the CSF from 10 patients with multiple sclerosis and from an appropriate control group. All patients had a clinical picture consistent with multiple sclerosis and had oligoclonal bands in the CSF. The control group comprised 10 patients with non-inflammatory diseases and they were negative for oligoclonal bands. Analysis of our data showed a highly significant increase in NO3and NO2in the CSF of patients with multiple sclerosis (table). In part this is surprising as demyelination leads to the specific release of dimethylarginine,5 a known inhibitor of NOS.6 We have shown with tandem mass spectrometry that dimethylarginine does not, however, seem to accumulate in the CSF of patients with multiple sclerosis. These findings provide, for the first time, evidence for increased NO production in multiple sclerosis. Recently, we showed that induction of NOS in astrocytes leads to mitochondrial damage.' Such NO mediated damage may therefore contribute to plaque formation and to the cell death associated with multiple sclerosis. These results suggest a possible therapeutic role for NOS inhibitors in the management of multiple sclerosis.