Pharmacological activation of heme oxygenase (HO)-1/carbon monoxide pathway prevents the development of peripheral neuropathic pain in Wistar rats

Recent studies have emphasized the contribution of neuroinflammation and oxido-nitrosative stress to neuropathic pain. Both, heme oxygenase (HO)-1 and carbon monoxide (CO) play an important role in regulating free radical generation and inflammation. Herein, we investigated the role of HO-1/CO pathway, by using hemin, a selective HO activator, and CO-releasing molecule (CORM)-2, a CO-releasing agent, in rat sciatic nerve chronic constriction injury (CCI)-induced neuropathic pain. CCI rats exhibited full development of behavioral hypersensitivity symptoms, including cold allodynia, mechanical and thermal hyperalgesia and also exhibit of a significant increase in spinal cord pro-inflammatory cytokines (TNF-alpha and IL-1 beta) and oxido-nitrosative stress markers, both in spinal cord and ipsilateral sciatic nerve homogenate. Spinal (10 and 30 mu g/rat, intrathecal (i.t.)), but not systemic (5 and 10 mg/kg, subcutaneous (s.c.)), administration of hemin for 14 days significantly prevented the development of behavioral hypersensitivity. Further, simultaneous administration of hemin via spinal (10 mu g/rat, i.t.) and systemic (5 mg/kg, s.c.) routes led to a more pronounced inhibition of the development of behavioral hypersensitivity. Further, administration of CORM-2 (1 and 5 mg/kg, s.c.), dose-dependently and most effectively, prevented the development of behavioral hypersensitivity. Both hemin and CORM-2 produced ameliorative beneficial effects that paralleled with the extent of reduction of oxido-nitrosative stress and pro-inflammatory cytokines. Also, hemin and CORM-2 significantly improved the levels of HO-1 and activity of anti-oxidant enzymes such as superoxide dismutase and catalase. Thus, it may be concluded that chronic pharmacological activation of HO-1/CO pathway may prevent the development of behavioral symptoms of neuropathic pain, through an activation of anti-inflammatory and anti-oxidant mechanisms.