Limited Efficacy of α-Conopeptides, Vc1.1 and RgIA, To Inhibit Sensory Neuron Ca_VCurrent

Abstract

Chronic pain is very difficult to treat. Thus, novel analgesics are a critical area of research. Strong pre-clinical evidence supports the analgesic effects of α-conopeptides, Vc1.1 and RgIA, which block α9α10 nicotinic acetylcholine receptors (nAChRs). However, the analgesic mechanism is controversial. Some evidence supports the block of α9α10 nAChRs as an analgesic mechanism, while other evidence supports the inhibition of N-type CaV (CaV2.2) current via activation of GABAB receptors. Here we reassess the effect of Vc1.1 and RgIA on CaV current in rat sensory neurons. Unlike the previous findings, we found highly variable effects among individual sensory neurons, but on average only minimal inhibition induced by Vc1.1, and no significant effect on the current by RgIA. We also investigated the potential involvement of GABAB receptors in the Vc1.1 induced inhibition, and found no correlation between the size of CaV current inhibition induced by baclofen (GABAB agonist) vs. that induced by Vc1.1. Thus, GABAB receptors are unlikely to mediate the Vc1.1 induced CaV current inhibition. Based on the present findings, CaV current inhibition in dorsal root ganglia is unlikely to be the predominant mechanism by which either Vc1.1 or RgIA induce analgesia. Significance Statement Better analgesic drugs are desperately needed to help physicians to treat pain. While many pre-clinical studies support the analgesic effects of α-conopeptides, Vc1.1 and RgIA, the mechanism is controversial. The development of improved α-conopeptide analgesics would be greatly facilitated by a complete understanding of the analgesic mechanism. However, we show that we cannot reproduce one of the proposed analgesic mechanisms, which is an irreversible inhibition of CaV current in a majority of sensory neurons.

Cloning, synthesis, and characterization of αO-conotoxin GeXIVA, a potent α9α10 nicotinic acetylcholine receptor antagonist

“…Several studies have shown that some α-conotoxins block VGCCs via stimulation of GABA B receptors, suggesting that antinociceptive effects were mediated through blockade of N-type calcium channels (28)(29)(30)(31)(32)(33). However, some studies have not fully reproduced this effect (34,35), and we show that GeXIVA fails to block VGCCs. Because GeXIVA is active at α9α10 nAChRs, but not VGCCs, our findings provide support for the involvement of α9α10 nAChRs in pain sensation.…”

αO-Conotoxin GeXIVA disulfide bond isomers exhibit differential sensitivity for various nicotinic acetylcholine receptors but retain potency and selectivity for the human α9α10 subtype

“…Subsequent reports indicated that Vc1.1 has potent GABA B agonist activity (Berecki et al, 2014; Callaghan and Adams, 2010; Callaghan et al, 2008; Castro et al, 2016; Cuny et al, 2012; Huynh et al, 2015; Klimis et al, 2011; Nevin et al, 2007; van Lierop et al, 2013). These findings have prompted an ongoing debate as to whether the analgesic mechanism of action of the α-conotoxins is via α9α10 nAChRs and/ or GABA B receptors (Adams and Berecki, 2013; Mohammadi and Christie, 2014; Mohammadi and Christie, 2015; Wright et al, 2015). The pharmacological profile of GeXIVA coupled with its demonstrated analgesia in rat models of neuropathic pain has prompted further interest in α9α10 nAChRs and GeXIVA as a potential therapeutic agent (Luo et al, 2015, Li et al, 2016).…”

Inhibition of α9α10 nicotinic acetylcholine receptors prevents chemotherapy-induced neuropathic pain

“…The pipet solution contained 104 mM NMG•Cl, 14 mM creatine•PO 4 , 6 mM MgCl 2 , 10 mM NMG•Hepes, 5 mM Tris•ATP, 10 mM NMG 2 •EGTA, and 0.3 mM Tris 2 •GTP with pH 7.4 and osmolarity of 300 mOsm. Test solutions were applied, and ionic currents were recorded and analyzed as previously described (24,53). Recordings were carried out at room temperature, and the holding potential was −80 mV.…”