A novel mechanism of inhibition of high-voltage activated calcium channels by α-conotoxins contributes to relief of nerve injury-induced neuropathic pain

Abstract

Abstractα-Conotoxins that are thought to act as antagonists of nicotinic acetylcholine receptors (nAChRs) containing α3-subunits are efficacious in several preclinical models of chronic pain. Potent interactions of Vc1.1 with other targets have suggested that the pain relieving actions of α-conotoxins might be mediated by either α9α10 nAChRs or a novel GABA B receptor-mediated inhibition of N-type calcium channels. Here we establish that three α-conotoxins, Vc1.1, AuIB and MII, have distinct selectivity profiles for these three potential targets. Their potencies after intramuscular administration were then determined for reversal of allodynia produced by partial nerve ligation in rats. Vc1.1, which potently inhibits α9α10 nAChRs and GABA B /Ca 2+ channels but weakly blocks α3β2 and α3β4 nAChRs, produced potent, long-lasting reversal of allodynia that were prevented by pretreatment with the GABA B receptor antagonist, SCH50911. α-Conotoxin AuIB, a weak α3β4 nAChR antagonist, inhibited GABA B /Ca 2+ channels but did not act on α9α10 nAChRs.AuIB also produced reversal of allodynia. These findings suggest that GABA B receptor-dependent inhibition of N-type Ca 2+ channels can mediate the sustained anti-allodynic actions of some α-conotoxins. However, MII, a potent α3β2 nAChR antagonist but inactive on α9α10 and α3β4 nAChRs and GABA B /Ca 2+ channels, was demonstrated to have short-acting anti-allodynic action.This suggests that α3β2 nAChRs may also contribute to reversal of allodynia. Together, these findings suggest that inhibition of α9α10 nAChR is neither necessary nor sufficient for relief of allodynia and establish that α-conotoxins selective for GABA B receptor dependent inhibition of Ntype Ca 2+ channels relieve allodynia, and could therefore be developed to manage chronic pain.

Intrathecal α-conotoxins Vc1.1, AuIB and MII acting on distinct nicotinic receptor subtypes reverse signs of neuropathic pain

“…To measure motor side effects, ambulation was tested by measuring the latency to fail negotiation of a rotarod device (Ugo Basile, Italy), with a maximal cut-off time of 300 s as previously described (Klimis et al 2011). Each animal was tested immediately prior to intrathecal injection, then 60 and 120 min after injection.…”

“…The α-conotoxins represent one such family that specifically target nicotinic acetylcholine receptor (nAChR) subtypes. Owing to the combination of a large variety of nAChR subunit assemblies and subunit-selective α-conotoxins, many potential novel analgesics have recently been identified (Dutton and Craik 2001;Alonso et al 2003;Sandall et al 2003;Lang et al 2005;Satkunanathan et al 2005;Olivera et al 2008;McIntosh et al 2009) α-Conotoxins, including Vc1.1, RgIA, MII and AuIB, have all been reported to potently reverse signs of neuropathic pain, particularly tactile allodynia, in animal models when administered systemically (Satkunanathan et al 2005;Klimis et al 2011).…”

“…Vc1.1 and RgIA are both potent antagonists of α9α10 nAChRs, suggesting this may be the anti-allodynia target (Vincler et al 2006). However, MII and AuIB are both devoid of activity at α9α10 nAChRs (McIntosh et al 1999;Callaghan et al 2008;Azam and McIntosh 2009;Callaghan and Adams 2010;Klimis et al 2011) and other -conotoxin analogues that act on these nAChRs fail to inhibit allodynia (Nevin et al 2007). Moreover, 910 nAChRs show very limited tissue distribution, being expressed predominantly in the olivochochlear system (Vetter et al, 2007) and their role in sensory nerve function is unclear.…”

“…Moreover, 910 nAChRs show very limited tissue distribution, being expressed predominantly in the olivochochlear system (Vetter et al, 2007) and their role in sensory nerve function is unclear. We have recently shown that Vc1.1, AuIB and RgIA inhibit N-type calcium channels in dorsal root ganglion (DRG) neurons through a novel GABA B receptor-dependent mechanism distinct from the well-known modulation of these channels by Gprotein β subunits (Callaghan et al 2008;Callaghan and Adams 2010;Klimis et al 2011). MII is inactive at this target, although it produces partial reversal of allodynia in nerve injured rats (Klimis et al 2011), suggesting this is not the only mechanism.…”

“…We have recently shown that Vc1.1, AuIB and RgIA inhibit N-type calcium channels in dorsal root ganglion (DRG) neurons through a novel GABA B receptor-dependent mechanism distinct from the well-known modulation of these channels by Gprotein β subunits (Callaghan et al 2008;Callaghan and Adams 2010;Klimis et al 2011). MII is inactive at this target, although it produces partial reversal of allodynia in nerve injured rats (Klimis et al 2011), suggesting this is not the only mechanism. Taken together, these findings suggest Ntype calcium channels and possibly 3 subunit-containing nAChR may both be important, but it is unlikely that α9α10 nAChRs are responsible for pain relief after systemic administration.…”