Partial sciatic nerve ligation leads to an upregulation of Ni2+-resistant T-type Ca2+ currents in capsaicin-responsive nociceptive dorsal root ganglion neurons

Background: Neuropathic pain resulting from peripheral nerve lesions is a common medical condition, but current analgesics arc often insufficient. The identification of key molecules involved in pathological pain processing is a prerequisite for the development of new analgesic drugs. Hyperexcitability of nociceptive DRG-neurons due to regulation of voltage-gated ion-channels is generally assumed to contribute strongly to neuropathic pain. There is increasing evidence, that T-type Ca2+-currents and in particular the Ca(v)3.2 T-type-channel isoform play an important role in neuropathic pain, but experimental results are contradicting. Purpose: To clarify the role of T-type Ca2+-channels and in particular the Ca(v)3.2 T-type-channel isoform in neuropathic pain. Methods: The effect of partial sciatic nerve ligation (PNL) on pain behavior and the properties ofT-type-currents in nociceptive DRG-neurons was tested in wild-type and Ca(v)3 .2-deficient mice. Results: In wild-type mice, PNL of the sciatic nerve caused neuropathic pain and an increase of T-typc Ca2+-currents in capsaicin-responsive neurons, while capsaicin-unresponsive neurons were unaffected. Pharmacological experiments revealed that this upregulation was due to an increase of a Ni2+-resistant Ca2+-current component, inconsistent with Ca(v)3.2 up-regulation. Moreover, following PNL Ca(v)3.2-deficient mice showed neuropathic pain behavior and an increase of T-Type Ca2+-currents indistinguishable to that of PNL treated wild-type mice. Conclusion: These data suggest that PNL induces an upregulation of T-Type Ca2+-currents in capsaicin-responsive DRG-neurons mediated by an increase of a Ni2+-insensitive current component (possibly Ca(v)3.1 or Ca(v)3.3). These findings provide relevance for the development of target specific analgesic drugs.