Voltage-gated sodium channels (VGSCs) consist of a pore-forming α-subunit and regulatory β-subunits. Several families of neuroactive peptides of Conus snails target VGSCs, including μO-conotoxins and μ-conotoxins. Unlike μ-conotoxins and the guanidinium alkaloid saxitoxin (STX), which are pore blockers, μO-conotoxins MrVIA and MrVIB inhibit VGSCs by modifying channel gating. μO-MrVIA/B can block NaV1.8 (a tetrodotoxin-resistant isoform of VGSCs) and have analgesic properties. The effect of NaVβ-subunit co-expression on susceptibility to block by μO-MrVIA/B and STX has, until now, not been reported. Here, we show that β1-, β2-, β3- and β4-subunits, when individually co-expressed with NaV1.8 in Xenopus oocytes, increased the kon of the block produced by μO-MrVIB (by 3, 32, 2 and 7-fold, respectively) and modestly decreased the apparent koff. Strong depolarizing pre-pulses markedly accelerated MrVIB washout with rates dependent on β-subunit co-expression. Thus, co-expression of β-subunits with NaV1.8 can strongly influence the affinity of the conopeptide for the channel. This observation is of particular interest because β-subunit expression can be dynamic; e.g., β2-expression is upregulated following nerve injury (Pertin et al., 2005 J Neurosci, 25: 10970); therefore, the effectiveness of a μO-conotoxin as a channel-blocker could be enhanced by the conditions that may call for its use therapeutically. In contrast to MrVIB's action, the STX-induced block of NaV1.8 was only marginally, if at all, affected by co-expression of any of the β-subunits. Our results raise the possibility that μO-conotoxins and perhaps other gating modifiers may provide a means to functionally assess the β-subunit composition of VGSC complexes in neurons.
- Received December 29, 2010.
- Revision received April 27, 2011.
- Accepted May 16, 2011.
- The American Society for Pharmacology and Experimental Therapeutics