Transient action of the endothelial constitutive nitric oxide synthase (ecNOS) mediates the development of thermal hypersensitivity following peripheral nerve injury

Neuropathic pain is a disabling feature of peripheral nerve injury. Following injury, local inflammation and the release of mediators may contribute to ectopic mechanosensitivity of the nerve-trunk and pain hypersensitivity. In the present study we investigated whether nitric oxide (NO) action and local nitric oxide synthase (NOS) expression play a role in pain hypersensitivity and A fibre-mediated ectopic hyperexcitability following a chronic constriction injury to a rat sciatic nerve. Using immunohistochemical methods we provide evidence for a unique endothelial constitutive nitric oxide synthase (ecNOS) immunoreactivity localized in early axonal endbulb-like structures of injured peripheral nerve axons. Moreover, we show that following nerve injury there is increased ecNOS-mRNA expression within the lumbar sympathetic ganglia, and that axoplasmic transport in sympathetic and other axons rather than local non-neural synthesis accounts for its accumulation in nerve fibres. We also demonstrate here that local inhibition of NOS action with the broad-spectrum inhibitor NG-nitro-L-arginine-methyl ester (L-NAME), but not more specific inhibitors of other NOS isoforms, has stereospecific, dose- and time-dependent analgesic effects that were reversed by local administration of L-arginine, the natural precursor of NO. In further work, using a teased fibre preparation, we show that administration of L-NAME, but not D-NAME, to the injury site also blocks ectopic mechanosensitivity of injured A-fibres. Our results indicate that an early and transient local ecNOS expression within early axonal endbulb-like structures, some arising from sympathetic axons, plays a critical role in the development of neuropathic pain.