γ-Aminobutyric Acid Type B (GABAB) Receptor Expression Is Needed for Inhibition of N-type (Cav2.2) Calcium Channels by Analgesic α-Conotoxins

α-Conotoxins Vc1.1 and RgIA are small peptides isolated from the venom of marine cone snails. They have effective anti-nociceptive actions in rat models of neuropathic pain. Pharmacological studies in rodent dorsal root ganglion (DRG) show their analgesic effect is mediated by inhibition of N-type (Ca v 2.2) calcium channels via a pathway involving γ-aminobutyric acid type B (GABA B ) receptor. However, there is no direct demonstration that functional GABA B receptors are needed for inhibition of the Ca v 2.2 channel by analgesic α-conotoxins. This study examined the effect of the GABA B agonist baclofen and α-conotoxins Vc1.1 and RgIA on calcium channel currents after transient knockdown of the GABA B receptor using RNA interference. Isolated rat DRG neurons were transfected with small interfering RNAs (siRNA) targeting GABA B subunits R1 and R2. Efficient knockdown of GABA B receptor expression at mRNA and protein levels was confirmed by quantitative real time PCR (qRT-PCR) and immunocytochemical analysis, respectively. Whole-cell patch clamp recordings conducted 2–4 days after transfection showed that inhibition of N-type calcium channels in response to baclofen, Vc1.1 and RgIA was significantly reduced in GABA B receptor knockdown DRG neurons. In contrast, neurons transfected with a scrambled nontargeting siRNA were indistinguishable from untransfected neurons. In the HEK 293 cell heterologous expression system, Vc1.1 and RgIA inhibition of Ca v 2.2 channels needed functional expression of both human GABA B receptor subunits. Together, these results confirm that GABA B receptors must be activated for the modulation of N-type (Ca v 2.2) calcium channels by analgesic α-conotoxins Vc1.1 and RgIA. Background: A class of analgesic α-conotoxins potently inhibits N-type calcium channels. Results: The activity of α-conotoxins Vc1.1 and RgIA was reduced following knockdown of GABA B receptor expression in sensory neurons and could be reconstituted in HEK 293 cells expressing human GABA B receptors and Ca v 2.2. Conclusion: GABA B receptors are needed for inhibition of Ca v 2.2 by Vc1.1 and RgIA. Significance: These analgesic α-conotoxins activate human GABA B receptors.