Analgesic Effects And Underlying Mechanisms Of Recombinant Human Erythropoietin On Rats With Neuropathic Pain

[Background]Injury or dysfunction of nervous system may result in neuropathic pain characterised by increased sensitivity to painful stimuli (hyperalgesia), painful perception of innocuous stimuli (allodynia) and spontaneous pain. It is often severely debilitating and largely resistant to treatments due to the complicated underlying mechanisms. Therefore further understanding the underlying mechanisms of neuropathic pain and exploring new therapeutic drugs is pivotal for future study. It is worth considering how neuroprotective strategies can eliminate or obtund the mechanisms of cellular activation or death mediated by the upregulation of proinflammatory cytokines. Some agents can be administrated immediately after injury or, ideally, before surgical iatrogenic injury to protect neurons and glia from injury or death. Rationale for this form of therapy derives from the trials of neurotrophic factors, such as NGF-βand NT-4, which revealed potential neuroprotective effects. Unfortunately, these factors have limited clinical value because they may also induce hyperalgesia. Erythropoietin (Epo), by contrast, does not induce hyperalgesia and is a neuroprotective agent for primary sensory neurons, spinal neurons and glial cells. Some studies suggested that erythropoietin significantly reduces the neuropathic pain consequences of injury and corresponds with the prevention of apoptosis of primary sensory neurons and reduction the proinflammatory cytokine of glia in the peripheral nerve system.[Objective]The purpose of present study is to determine the effect of intraperitoneally administration of recombinant human erythropoietin (rhEPO) before and after nerve injury on the behavioral manifestation, the activation of NF-κB, the release of cytokines in brain using a rat model of neuropathic pain induced by L5 nerve transection.[Materials and Methods]The male Sprague-Dawley rats (190-220g, aged 8-10 weeks and housed for a minimum of one-week prior to use) were randomly assigned into five groups (n=10): sham-operation with saline; and L5 spinal nerve transection with rhEPO (1000, 3000,5000 U/kg) or saline. rhEPO or saline was given intraperitoneally 1 day before surgery and continued daily to day 14 post-transection. In the following 14 days, rats received the forementioned treatment once a day. The hind paw mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) of all animals were measured before operation days (0 day) and on days 3,7, and 14 after operation with von Frey hairs and Hargreaves tests respectively. The levels of proinflammatory cytokines TNF-α, IL-1β, IL-6, and IL-10 were measured in the brain after surgery using ELSIA (n=8). The activity of NF-κB in the brain 7 days after surgery was assessed by EMSA.[Results]Serum spinal nerve injury caused long-lasting changes of pain thresholds in the hind paw, which reached the lowest level on the third day after operation, suggesting that all rats have successfully developed neuropathic pain syndromes (mechanical allodynia and thermal hyperalgesia). The behavioral values of sham-operated rats were steady during the experimental period and no difference was found between those treated by rhEPO (1000 U/kg i.p.) and saline. rhEPO (3000 or 5000 U/kg i.p.) administration 1 h before surgery could attenuate the development of mechanical and thermal hyperalgesia in L5 nerve transection rat compared with saline treatment on days 1,3,7 and 14 post-operation (P<0.01 for 5000 units/kg and P<0.05 for rhEPO (3000 or 5000 U/kg). The results indicated that rhEPO can alleviate the pain behavior of rats after spinal nerve injury. Compared with those receiving sham operation, the level of TNF-α,IL-1β,IL-6 was significantly increased in nerve-injuried rats, while only those pretreated with rhEPO (3000 or 5000 U/kg i.p.) were found significant elevation of the expression of IL-10. Compared with those pretreated with saline before nerve transection, the level of TNF-α,IL-1βand IL-6 in both tissues was significantly decreased in nerve-injuried rats pretreated with rhEPO (3000 or 5000 U/kg i.p.), furthermore the secretion of IL-10 was significantly enhanced. Compared with those received sham operation, the activity of NF-κB in the brain was significantly increased in nerve-injuried rats. Compared with those pretreated with saline before nerve transection, the activity of NF-κB in the brain was significantly decreased in nerve-injuried rats pretreated with rhEPO (3000 or 5000 U/kg i.p.).[Conclusions]The present study indicated that pre-treatment with rhEPO may prevent the development of neuropathic pain. Furthermore, the different effect of rhEPO intimately correlated with the distinct impact of rhEPO on the activation of NF-κB and the secretion of proinflammatory cytokines. The effect of glia-centered neuroinflammation on the development and maintenance of neuropathic pain was confirmed by the present study. Moreover, the prospect of the use of rhEPO in the treatment of neuropathic pain was warranted by the present study.