The molecular mechanisms underlying neuropathic pain are incompletely understood, but recent data suggest that down-regulation of the chloride extruding co-transporter KCC2 in spinal cord sensory neurons is critical: Following peripheral nerve injury, activated microglia in the spinal cord release BDNF, which stimulates neuronal TrkB receptors and ultimately results in the reduction of KCC2 levels. Consequently, neuronal intracellular chloride ion concentration increases, impairing GABAA-receptor mediated inhibition. We have previously described how the receptor sortilin modulates neurotrophin signaling by facilitating anterograde transport of Trk receptors. Unpublished data further link SorCS2, another member of the Sortilins family of sorting receptors (sortilin, SorLA and SorCS1–3) to BDNF signaling by regulating presynaptic TrkB trafficking. The purpose of this study is to explore the involvement of Sortilins in neuropathic pain.
We subjected wild-type (wt), sortilin knockout (Sort1-/-) and SorCS2 knockout (SorCS2-/-) mice to the Spared Nerve Injury (SNI) model of peripheral nerve injury. Mechanical allodynia was measured by von Frey filaments using the up-down-up method and a 3-out-of-5 thresshold.
As previously described by several groups, wt mice developed significant mechanical allodynia following SNI. Interestingly however, mice lacking sortilin or SorCS2 were fully protected from development of allodynia and did not display KCC2 down-regulation following injury. In addition, a single intrathecal injection of antibodies against sortilin or SorCS2 could delay or rescue mechanical allodynia in wt SNI mice for 2-3 days. Finally, neither sortilin nor SorCS2 deficient mice responded to intrathecal injection of BDNF, in contrast to wt mice which developed transient mechanical allodynia.
We hypothesize that sortilin and SorCS2 are involved in neuropathic pain development by regulating TrkB signaling. Alternatively, Sortilins may directly influence the regulation of KCC2 membrane levels following injury. Both hypotheses are currently being investigated by our group.
© 2012 Scandinavian Association for the Study of Pain