Differential Ca(v)2.1 and Ca(v)2.3 channel inhibition by baclofen and α-conotoxin Vc1.1 via GABA(B) receptor activation

Neuronal Ca(v)2.1 (P/Q-type), Ca(v)2.2 (N-type), and Ca(v)2.3 (R-type) calcium channels contribute to synaptic transmission and are modulated through G protein–coupled receptor pathways. The analgesic α-conotoxin Vc1.1 acts through γ-aminobutyric acid type B (GABA(B)) receptors (GABA(B)Rs) to inhibit Ca(v)2.2 channels. We investigated GABA(B)R-mediated modulation by Vc1.1, a cyclized form of Vc1.1 (c-Vc1.1), and the GABA(B)R agonist baclofen of human Ca(v)2.1 or Ca(v)2.3 channels heterologously expressed in human embryonic kidney cells. 50 µM baclofen inhibited Ca(v)2.1 and Ca(v)2.3 channel Ba(2+) currents by ∼40%, whereas c-Vc1.1 did not affect Ca(v)2.1 but potently inhibited Ca(v)2.3, with a half-maximal inhibitory concentration of ∼300 pM. Depolarizing paired pulses revealed that ∼75% of the baclofen inhibition of Ca(v)2.1 was voltage dependent and could be relieved by strong depolarization. In contrast, baclofen or Vc1.1 inhibition of Ca(v)2.3 channels was solely mediated through voltage-independent pathways that could be disrupted by pertussis toxin, guanosine 5′-[β-thio]diphosphate trilithium salt, or the GABA(B)R antagonist CGP55845. Overexpression of the kinase c-Src significantly increased inhibition of Ca(v)2.3 by c-Vc1.1. Conversely, coexpression of a catalytically inactive double mutant form of c-Src or pretreatment with a phosphorylated pp60c-Src peptide abolished the effect of c-Vc1.1. Site-directed mutational analyses of Ca(v)2.3 demonstrated that tyrosines 1761 and 1765 within exon 37 are critical for inhibition of Ca(v)2.3 by c-Vc1.1 and are involved in baclofen inhibition of these channels. Remarkably, point mutations introducing specific c-Src phosphorylation sites into human Ca(v)2.1 channels conferred c-Vc1.1 sensitivity. Our findings show that Vc1.1 inhibition of Ca(v)2.3, which defines Ca(v)2.3 channels as potential targets for analgesic α-conotoxins, is caused by specific c-Src phosphorylation sites in the C terminus.