Ω-conotoxin GVIA potently inhibits the currents mediated by P2X receptors in rat DRG neurons

We examined effects of ω-conotoxin previously known as a selective blocker of N-type calcium channels, on the adenosine triphosphate (ATP)-induced currents in the rat dorsal root ganglion neurons. These neurons express at least two types of ionotropic purinoreceptors: P2X3 receptors that have very rapid desensitization kinetics and P2X2/X3 heterooligomeric receptor, which exhibits slow desensitization. We have found that ω-conotoxin GVIA potently inhibits the inward currents mediated by both receptor types. This effect was specific for the receptor subtypes: the IC 50 value for responses evoked by 10 μM ATP was 21.2 ± 1.7 nM for the P2X3 receptor-mediated responses and 3.84 ± 0.43 μM for slower responses mediated by P2X2/X3 heteropolymers. The efficacy of another type of ω-conotoxin, MVIIC, is much lower: at 10 μM the latter toxin inhibited the rapidly desensitizing response by 65% and the slowly desensitizing response by 18%. The effects of both toxins were reversible and independent on the membrane potential. Ω-Conotoxin GVIA shifted the dose dependence for the agonistic action of ATP on P2X3 receptors to higher concentrations without producing any effect on the kinetics of the response. It is suggested that ω-conotoxin allosterically modulates the receptor properties, rather than competes for the agonist binding site.