Analgesic α-Conotoxins Vc1.1 and Rg1A Inhibit N-Type Calcium Channels in Rat Sensory Neurons via GABA_BReceptor Activation

Abstract

does not interact with N-type calcium channels directly but inhibits them via a voltage-independent mechanism involving a PTX-sensitive, G-protein-coupled receptor. Preincubation with a variety of selective receptor antagonists demonstrated that only the GABA B receptor antagonists, [S-(R*,R*)][-3-[[1-(3,4-dichlorophenyl)ethyl]amino]-2-hydroxy propyl]([3,4]-cyclohexylmethyl) phosphinic acid hydrochloride (2S)-3[[(1S)- 1-(3,4-dichlorophenyl

Intrathecal α-conotoxins Vc1.1, AuIB and MII acting on distinct nicotinic receptor subtypes reverse signs of neuropathic pain

“…We have previously shown that Vc1.1 is anti-allodynic in neuropathic pain models when administered intramuscularly, and that sustained reversal of allodynia appears due to GABA B -receptor-dependent inhibition of Ntype Ca 2+ channels because it is reversed by a selective GABA B -receptor antagonist (Klimis et al 2011). Furthermore, we observed no reversal of allodynia with peripheral administration of two analogs of Vc1.1, vc1a and [P60]Vc1.1 that exhibited no activity at GABA B receptor/N-type Ca 2+ channels but full activity at α9α10 nAChRs, suggesting that antagonism of α9α10 is not a requisite for anti-allodynia (Nevin et al 2007;Callaghan et al 2008). However, Vc1.1, AuIB and MII have markedly different (> 1000-fold) potencies for GABA B receptors/N-type calcium channels, with MII being nearly inactive.…”

“…The finding that even very high concentrations of Vc1.1, MII and AuIB had little effect on the amplitude of primary afferent eEPSCs in spinal cord slices suggests that the GABA B receptordependent inhibition of N-type Ca 2+ channels we found for -conotoxins Vc1.1 and AuIB (Callaghan et al 2008;Klimis et al 2011) in rodent DRG neurons is not present at N-type channels in primary afferent nerve terminals. This is not particularly surprising, because the conventional mechanism of GABA B -receptor agonist inhibition of N-channels via G-protein β-subunits in both DRG cell bodies and primary afferent terminals is not responsible for -conotoxin effect on N-type calcium channels in DRG neurons.…”

“…A higher concentration (10 µM) of Vc1.1 was also without effect (97 ± 4% of baseline, N = 9). As Vc1.1 has been shown to interact with somatic GABA B receptors (Callaghan et al 2008;Callaghan and Adams 2010;Klimis et al 2011), in some experiments the GABA B agonist baclofen was superfused following washout of Vc1.1 ( figure 3Ai, 3B). Baclofen (10 µM) markedly reduced eEPSC amplitude by 81% ( Figure 3C, N = 3).…”

“…Vc1.1 and RgIA are both potent antagonists of α9α10 nAChRs, suggesting this may be the anti-allodynia target (Vincler et al 2006). However, MII and AuIB are both devoid of activity at α9α10 nAChRs (McIntosh et al 1999;Callaghan et al 2008;Azam and McIntosh 2009;Callaghan and Adams 2010;Klimis et al 2011) and other -conotoxin analogues that act on these nAChRs fail to inhibit allodynia (Nevin et al 2007). Moreover, 910 nAChRs show very limited tissue distribution, being expressed predominantly in the olivochochlear system (Vetter et al, 2007) and their role in sensory nerve function is unclear.…”

“…Moreover, 910 nAChRs show very limited tissue distribution, being expressed predominantly in the olivochochlear system (Vetter et al, 2007) and their role in sensory nerve function is unclear. We have recently shown that Vc1.1, AuIB and RgIA inhibit N-type calcium channels in dorsal root ganglion (DRG) neurons through a novel GABA B receptor-dependent mechanism distinct from the well-known modulation of these channels by Gprotein β subunits (Callaghan et al 2008;Callaghan and Adams 2010;Klimis et al 2011). MII is inactive at this target, although it produces partial reversal of allodynia in nerve injured rats (Klimis et al 2011), suggesting this is not the only mechanism.…”