Molecular mechanism for analgesia involving specific antagonism of α9α10 nicotinic acetylcholine receptors

Abstract

␣9␣10 nicotinic acetylcholine receptors (nAChRs) have been identified in a variety of tissues including lymphocytes and dorsal root ganglia; except in the case of the auditory system, the function of ␣9␣10 nAChRs is not known. Here we show that selective block (rather than stimulation) of ␣9␣10 nAChRs is analgesic in an animal model of nerve injury pain. In addition, blockade of this nAChR subtype reduces the number of choline acetyltransferase-positive cells, macrophages, and lymphocytes at the site of injury. Chronic neuropathic pain is estimated to affect up to 8% of the world's population; the numerous analgesic compounds currently available are largely ineffective and act through a small number of pharmacological mechanisms. Our findings not only suggest a molecular mechanism for the treatment of neuropathic pain but also demonstrate the involvement of ␣9␣10 nAChRs in the pathophysiology of peripheral nerve injury.N europathic pain is a prolonged, debilitating state characterized by allodynia (pain produced by previously innocuous stimuli), hyperalgesia (an increased or exaggerated response to painful stimuli), and spontaneous pain. Neuropathic pain is often refractory to conventional pain therapeutics such as opioids and nonsteroidal antiinflammatory agents and, therefore, represents a large, unmet clinical need. Neuropathic pain can be triggered in a variety of ways; injury to a peripheral nerve is one of the most common causes.The involvement of nicotinic acetylcholine receptors (nAChRs) in pain has been suggested by a number of experimental observations, and the administration of nAChR agonists reduces pain-related behaviors in several animal models (1-5). nAChRs are pentameric ligand-gated ion channels composed of ␣ (␣1-␣10) and non-␣ (␤1-␤4, , ␥, and ␦) subunits. The ␣2-␣6 and ␤2-␤4 subunits form heteromeric channels consisting of a combination of ␣ and ␤ subunits (6). Homomeric channels can be formed by ␣7 or ␣9 subunits; the ␣10 subunit will only form functional receptors when it is expressed with the ␣9 subunit (6). Many of the nAChRs show widespread patterns of neuronal and nonneuronal distribution; ␣9 and/or ␣10 subunits have been reported within hair cells of the inner ear (7), sperm (8), dorsal root ganglion neurons (9), skin keratinocytes (10), the pars tuberalis of the pituitary (11), and lymphocytes (12). The function of ␣9␣10 nAChRs in the auditory system has been well characterized (13), but little is known regarding the function of ␣9␣10 nAChRs in other tissues. Here we demonstrate that the highly selective antagonist of ␣9␣10 nAChRs, RgIA, is analgesic and reduces migration of macrophages, lymphocytes, and acetylcholine (ACh)-producing cells into the area of nerve injury. ResultsRgIA Is Antinociceptive. Chronic constriction injury (CCI) produced mechanical hypersensitivity within 7 days of sciatic nerve ligation (Fig. 1). Paw withdrawal thresholds (PWTs) were reduced from 122 Ϯ 5 g to 26 Ϯ 5 g 7 days after CCI. The i.m. administration of the ␣9␣10-selective Conus peptide, RgIA, increased...

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

“…Early studies suggested that interaction with 3 subunit-containing nAChRs may mediate these actions (Livett et al 2006), but the affinity of Vc1.1 and AuIB for these subtypes is rather weak (Clark et al 2006;Vincler et al 2006). Vc1.1 and RgIA are both potent antagonists of α9α10 nAChRs, suggesting this may be the anti-allodynia target (Vincler et al 2006).…”

“…Early studies suggested that interaction with 3 subunit-containing nAChRs may mediate these actions (Livett et al 2006), but the affinity of Vc1.1 and AuIB for these subtypes is rather weak (Clark et al 2006;Vincler et al 2006). 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).…”

Hydrophobic residues at position 10 of α-conotoxin PnIA influence subtype selectivity between α7 and α3β2 neuronal nicotinic acetylcholine receptors

“…Neuronal nAChRs are a diverse class of ligand-gated ion channels involved in neurological conditions such as neuropathic pain [52,53] and Alzheimer's disease [54,55]. Therefore, a suite of potent and selective ligands would be an invaluable pharmacological tool to identify the physiological role of nAChR subunit combinations.…”

Developmental Biology of Neoplastic Growth

“…Potentially more interesting are the neuronally selective α-conotoxins that target nAChR subtypes comprising different combinations of α3-α10 and/or β2-β4 subunits expressed as either homomeric or heteromeric receptors depending on the neuronal cell type. Interestingly, α-conotoxin Vc1.1 (Sandall et al 2003) and RgI (Vincler et al 2006) have been identified as having potential analgesic properties following peripheral administration to rats. This result contrasts with the analgesic effects usually attributed to agonists of the nAChR acting centrally, and appears to be mediated by a specific subtype of the receptor (α9α10) possessing a previously unrecognised role in pain (Vincler et al 2006), although the involvement of this target in its analgesic action has been questioned (Nevin et al 2007).…”

“…Interestingly, α-conotoxin Vc1.1 (Sandall et al 2003) and RgI (Vincler et al 2006) have been identified as having potential analgesic properties following peripheral administration to rats. This result contrasts with the analgesic effects usually attributed to agonists of the nAChR acting centrally, and appears to be mediated by a specific subtype of the receptor (α9α10) possessing a previously unrecognised role in pain (Vincler et al 2006), although the involvement of this target in its analgesic action has been questioned (Nevin et al 2007). Metabolic Pharmaceuticals Ltd has discontinued clinical trials of Vc1.1 due to an anticipated lack of efficacy in humans when it was revealed experimentally that Vc1.1 had low affinity for the human form of the α9α10 receptor.…”