Nitric oxide influences injury-induced microglial migration and accumulation in the leech CNS

Damage to the leech or mammalian CNS increases nitric oxide (NO) production and causes accumulation of phagocytic microglial cells at the injury site. The aim of this study was to determine whether NO plays a role in microgli al migration and accumulation at lesions in which NO is generated by a rapi dly appearing endothelial nitric oxide synthase (eNOS) in leeches. Immunohi stochemistry and cytochemistry demonstrated active eNOS before and througho ut the period of microglial accumulation at the lesion. Decreasing NO synth esis by application of the NOS inhibitor N-w-nitro-L-arginine methyl ester (1 mM) significantly reduced microglial accumulation, whereas its inactive enantiomer N-w-nitro-D-arginine methyl ester (1 mM) resulted in microglial accumulation similar to that in crushed controls. Increasing NO with the do nor spermine NONOate (SPNO) (1 mM) also inhibited accumulation, but not in the presence of the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-teramethylimid azolineoxyl-3-oxide (50 mu M). The effect of SPNO was reversed by washout. SPNO application reduced average microglial migratory speeds and even rever sibly arrested cell movement, as measured in living nerve cords. These resu lts suggest that NO produced at a lesion may be a stop signal for microglia to accumulate there and that it can act on microglia early in their migrat ion. Thus, NO may assume a larger role in nerve repair and recovery from in jury by modulating accumulation of microglia, which appear to be important for axonal regeneration.