Analgesic alpha-conotoxins Vc1.1 and RgIA inhibit N-type calcium channels in sensory neurons of alpha 9 nicotinic receptor knockout mice

Alpha-conotoxins Vc1.1 and RgIA are peptides from the venom of marine Conus snails that are currently in development as a treatment for neuropathic pain. We have reported previously that the alpha 9 alpha 10 nicotinic acetylcholine receptor (nAChR) selective-conotoxins Vc1.1 and RgIA potently and selectively inhibit high voltage-activated (HVA) N-type calcium channel currents in dissociated neurons from rat dorsal root ganglia (DRG). Our data indicated that Vc1.1 does not interact directly with N-type Ca2+ channels but inhibits them via GABA(B) receptor activation. The present study investigated Vc1.1 and RgIA inhibition of N-type Ca2+ channels currents in DRG neurons of wild-type and alpha 9 knockout (KO) mice to determine if the alpha 9 nAChR was necessary for inhibition of the Ca2+ channel current. Application of Vc1.1 (100 nM) inhibited N-type Ca2+ channel currents to 69.2 +/- 3.5% of control in DRG neurons isolated from wild-type mice. In >70% of DRG neurons isolated from the alpha 9 KO mice, both Vc1.1 and RgIA selectively inhibited N-type Ca2+ channel currents with an IC50 of 24.6 nM and 22.4 nM, respectively. The GABA(B) receptor antagonist CGP55845 (1 mu M) antagonized the effect of Vc1.1 and RgIA on the N-type calcium channels in alpha 9 KO mice. RT-PCR and western blot analysis confirmed the absence of the alpha 9 nAChR in mice carrying a null mutation for the nAChR alpha 9 gene. These results demonstrate that the inhibition of N-type Ca2+ channel channels by Vc1.1 and RgIA is not mediated by the expression of alpha 9 alpha 10 nAChRs in DRG neurons.