Inducible nitric oxide synthase inhibition by 1400W limits pain hypersensitivity in a neuropathic pain rat model

Peripheral neuropathic pain (PNP), resulting from injury to or dysfunction of a peripheral nerve, is a major health problem that affects 7-8% of the population. It is inadequately controlled by current drugs and is characterized by pain hypersensitivity, which is believed to be attributable to sensitization of peripheral and CNS neurons by various inflammatory mediators. Here we examined, in a rat model of PNP: (i) whether reducing levels of nitric oxide (NO) with 1400W, a highly selective inhibitor of inducible NO synthase (iNOS), would prevent or attenuate pain hypersensitivity; and (ii) the effects of 1400W on plasma concentrations of several cytokines that are secreted after iNOS upregulation during chronic pain states. The L5 spinal nerve axotomy (SNA) model of PNP was used, and 1400W (20mgkg(-1)) was administered i.p. at 8h intervals for 3days starting at 18h post-SNA. Changes in plasma concentrations of 12 cytokines in SNA rats treated with 1400W were examined using multiplex enzyme-linked immunosorbent assay. The SNA rats developed behavioural signs of mechanical and heat hypersensitivity. Compared with the vehicle/control, 1400W significantly: (i) limited development of mechanical hypersensitivity at 66h post-SNA and of heat hypersensitivity at 42h and at several time points tested thereafter; and (ii) increased the plasma concentrations of interleukin (IL)-1, IL-1 and IL-10 in the SNA rats. The findings suggest that 1400W might exert its analgesic effects by reducing iNOS and altering the balance between the pro-inflammatory (IL-1 and IL-1) and anti-inflammatory (IL-10) cytokines and that therapies targeting NO or its enzymes might be effective for the treatment of PNP.