Novel Mechanism of Voltage-Gated N-type (Ca(v)2.2) Calcium Channel Inhibition Revealed through alpha-Conotoxin Vc1.1 Activation of the GABA(B) Receptor

Neuronal voltage-gated N-type (Ca(v)2.2) calcium channels are expressed throughout the nervous system and regulate neurotransmitter release and hence synaptic transmission. They are predominantly modulated via G protein-coupled receptor activated pathways, and the well characterized G beta gamma subunits inhibit Ca(v)2.2 currents. Analgesic alpha-conotoxin Vc1.1, a peptide from predatory marine cone snail venom, inhibits Cav2.2 channels by activating pertussis toxin-sensitive G(i/o) proteins via the GABA(B) receptor (GABA(B)R) and potently suppresses pain in rat models. Using a heterologous GABA(B)R expression system, electrophysiology, and mutagenesis, we showed alpha-conotoxin Vc1.1 modulates Ca(v)2.2 via a different pathway from that of the GABA(B)R agonists GABA and baclofen. In contrast to GABA and baclofen, Vc1.1 changes Ca(v)2.2 channel kinetics by increasing the rate of activation and shifting its half-maximum inactivation to a more hyperpolarized potential. We then systematically truncated the GABA(B)1a C terminus and discovered that removing the proximal carboxyl terminus of the GABA(B)1a subunit significantly reduced Vc1.1 inhibition of Ca(v)2.2 currents. We propose a novel mechanismby which Vc1.1 activates GABA(B)R and requires the GABA(B)1a proximal carboxyl terminus domain to inhibit Ca(v)2.2 channels. These findings provide important insights into how GABA(B)Rs mediate Ca(v)2.2 channel inhibition and alter nociceptive transmission.