␣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...

Chronic pain is a vexing worldwide problem that causes substantial disability and consumes significant medical resources. Although there are numerous analgesic medications, these work through a small set of molecular mechanisms. Even when these medications are used in combination, substantial amounts of pain often remain. It is therefore highly desirable to develop treatments that work through distinct mechanisms of action. While agonists of nicotinic acetylcholine receptors (nAChRs) have been intensively studied, new data suggest a role for selective antagonists of nAChRs. α-Conotoxins are small peptides used offensively by carnivorous marine snails known as Conus. A subset of these peptides known as α-conotoxins RgIA and Vc1.1 produces both acute and long lasting analgesia. In addition, these peptides appear to accelerate the recovery of function after nerve injury, possibly through immune mediated mechanisms. Pharmacological analysis indicates that RgIA and Vc1.1 are selective antagonists of α9α10 nAChRs. A recent study also reported that these α9α10 antagonists are also potent GABA-B agonists. In the current study, we were unable to detect RgIA or Vc1.1 binding to or action on cloned GABA-B receptors expressed in HEK cells or Xenopus oocytes. We review the background, findings and implications of use of compounds that act on α9* nAChRs.

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Conus peptides that are selectively targeted to different molecular isoforms of nicotinic acetylcholine receptors (nAChRs) have been identified and characterized; several have recently been shown to have significant biomedical potential. An emerging strategy for the discovery from animal biodiversity of subtype-specific ligands for ion channel families is described in this review. Characterization of the gene family encoding a set of related ligands is required for discovery using a molecular genetics approach; when discovery is guided by a knowledge of the phylogeny of the biodiverse animal lineage being used as a source of ligands, a rational, efficient scan of the library of putative ligands becomes feasible. Together, these constitute an approach to uncover subtype-specific ligands, called "concerted discovery"; this was applied to the α-conotoxins, a family of Conus peptides generally targeted to nAChRs.Subtype-specific α-conotoxins were developed that target two groups of nAChRs, α 6 * and α 9 *. α-conotoxin MII has become the defining ligand for identifying the α 6 * nAChR subtype. A synthetic analog, MII [E11A], further subdivides α 6 * nAChRs into those that contain an α 4 subunit and those that do not. Importantly, these two subtypes are differentially affected by nigrostriatal damage, findings of likely relevance to the pathopysiology of Parkinson's disease. In contrast, α-conotoxins that target α 9 nAChR subtypes have potential as analgesics for the treatment of neuropathic pain that develops after nerve injury. The discovery of α-conotoxin RgIA enabled the identification of a novel role for α 9 * nAChRs. Use of α 9 * nAChR antagonists is associated with reversal of inflammation caused by the nerve injury. Thus, subtype-specific α-conotoxins targeted to particular nAChR isoforms are not only useful for understanding the physiological role of these receptors, but can have important diagnostic and therapeutic applications as well. Note: *Denotes the possible presence of additional subunits in the nAChR complex.