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Analgesic conotoxins: modulation of voltage-gated calcium channels in pain pathways

D.J. Adams, Health Innovations Research Institute, RMIT University, Melbourne, VIC 3083, Australia.

The small and highly structured peptides found in the venom of marine cone snails target a wide variety of membrane receptors and ion channels in normal and diseased states. A number of these peptides (conotoxins) have shown efficacy in vivo including inhibitors of voltage-gated sodium (Nav) and calcium (Cav) channels and nicotinic acetylcholine receptors (nAChRs) which are in preclinical development for the treatment of chronic and neuropathic pain. A number of structurally related ω-conotoxins bind directly to and selectively inhibit N-type calcium channels of nociceptive dorsal root ganglion (DRG) neurons. Among these, ω-conotoxin MVIIA (Prialt) still maintains its orphan drug status as a valuable alternative intrathecal analgesic for the management of chronic intractable pain, especially in patients refractory to opioids. Newly discovered ω-conotoxins from Conus catus are more potent and selective for N-type (Cav2.2) calcium channels over other Cavs (Berecki et al., 2010). Furthermore, in spinal cord slices, these peptides reversibly inhibited excitatory monosynaptic transmission between primary afferents and dorsal horn superficial lamina neurons. In the rat partial sciatic nerve ligation model of neuropathic pain, ω-conotoxins CVIE and CVIF significantly reduced allodynic behaviour. Another family of conotoxins, the α-conotoxins, competitively inhibit nAChRs and bind at the interface between specific subunits allowing them to discriminate among different nAChR subtypes. α-Conotoxins Vc1.1 (ACV1) and RgIA are small disulfide bonded peptides currently in development as a treatment for neuropathic pain (Vincler et al., 2006). It was proposed that the primary target of Vc1.1 and RgIA is the α9α10 neuronal nAChRs. Surprisingly, however, we found that Vc1.1 and RgIA more potently inhibit the N-type (Cav2.2) Ca2+ channel currents in rat sensory neurons via a voltage-independent mechanism involving the G protein-coupled GABAB receptor (GABABR) (Callaghan et al., 2008). This was the first demonstration of α-conotoxins acting via the G protein-coupled GABABR modulating native Cav2.2 channels. Recent molecular studies confirm that Vc1.1 and RgIA inhibit N-type Ca2+ channels via GABABR activation. Transient transfection of DRG neurons with small interfering RNAs (siRNAs) to knock-down the GABABR reduced mRNA levels for GABAB subunits by >50% compared to control cells and suppressed GABABR protein expression. Whole-cell patch clamp recording of DRG neurons conducted 1-3 days after transfection demonstrated that knockdown of functional GABABR expression significantly reduced the inhibition of N-type Ca2+ channels in response to both baclofen and Vc1.1. This was in contrast to neurons transfected with a non-targeting siRNA which were indistinguishable from untransfected neurons, confirming that α-conotoxin Vc1.1 modulates N-type Ca2+ channels via activation of GABABR in DRG neurons. Our current findings have the potential to introduce a paradigm shift in thinking about the targets of α-conotoxins. GABABR may play a critical role in pain pathways and are a clear therapeutic target for these and modified conotoxins.

Berecki G, Motin L, Haythornthwaite A, Vink S, Bansal P, Drinkwater R, Wang CI, Moretta M, Lewis RJ, Alewood PF, Christie MJ, Adams DJ. (2010) Molecular Pharmacology, 77: 139-148.

Callaghan B, Haythornthwaite A, Berecki G, Clark RJ, Craik DJ, Adams DJ. (2008) Journal of Neuroscience, 28: 10943-10951.

Vincler M, Wittenauer S, Parker R, Ellison M, Olivera BM, McIntosh JM. (2006) Proceedings of the National Academy of Sciences of the USA, 103: 17880-17884.