␣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...

Background Gabapentin reduces acute postoperative and chronic neuropathic pain, but its sites and mechanisms of action are unclear. Based on previous electrophysiologic studies, we tested whether gabapentin reduced γ-Amino butyric acid (GABA) release in the locus coeruleus (LC), a major site of descending inhibition, rather than in the spinal cord. Methods Male Sprague-Dawley rats with or without L5-L6 spinal nerve ligation (SNL) were used. Immunostaining for glutamic acid decarboxylase and GABA release in synaptosomes and microdialysates were examined in the LC and spinal dorsal horn. Results Basal GABA release and expression of glutamic acid decarboxylase increased in the LC but decreased in the spinal dorsal horn following SNL. In microdialysates from the LC, intravenously administered gabapentin decreased extracellular GABA concentration in normal and SNL rats. In synaptosomes prepared from the LC, gabapentin and other α2δ ligands inhibited KCl-evoked GABA release in normal and SNL rats. In microdialysates from the spinal dorsal horn, intravenous gabapentin did not alter GABA concentrations in normal rats but slightly increased them in SNL rats. In synaptosomes from the spinal dorsal horn, neither gabapentin nor other α2δ ligands affected KCl-evoked GABA release in normal and SNL rats. Discussion These results suggest that peripheral nerve injury induces plasticity of GABAergic neurons differently in the LC and spinal dorsal horn, and that gabapentin reduces pre-synaptic GABA release in the LC but spinal dorsal horn. The present study supports the idea that gabapentin activates descending noradrenergic inhibition via disinhibition of LC neurons.

Although gabapentin may relieve neuropathic pain by actions at many sites, these results suggest that its actions in the brain to cause spinal cholinergic activation predominate after oral administration. Side effects, particularly nausea, cannot be accurately determined on rats. Nevertheless, oral donepezil is well tolerated by patients in the treatment of Alzheimer dementia, and the current study provides the rationale for clinical study of combination of gabapentin and donepezil to treat neuropathic pain.

The results of this study suggest that intraplantar CDP-choline has antihypersensitivity and antiinflammatory effects mediated via alpha7nAChRs in the carrageenan-induced inflammatory pain model.

Spinally released dynorphin contributes to hypersensitivity from nerve injury, inflammation, and sustained morphine treatment, but its role in post-operative pain has not been tested. Intrathecal injection of dynorphin activates cyclooxygenase (COX)-1 and -2 to induce hypersensitivity. Spinal COX-1 expression and activity increase following incisional paw surgery in rats, although the stimulus for this increase is not known. In the current study we tested whether spinal dynorphin expression increases after incisional surgery and induces hypersensitivity in this setting, and whether dynorphin stimulates COX-1 activity in spinal cord microglia. Paw incision resulted in increased prodynorphin immunoreactivity in laminae I, IIo, and V in the L4-L6 spinal cord dorsal horn ipsilateral to surgery. Change in prodynorphin expression did not parallel that of mechanical hypersensitivity. Repeated intrathecal dynorphin A antiserum injection failed to alter mechanical hypersensitivity after incisional surgery, although it was effective against mechanical hypersensitivity following spinal nerve ligation. Paw incision increased COX-1 immunoreactivity in the L4-L6 ipsilateral spinal cord, and these cells were confirmed to be microglia by co-localization with OX-42. Spinal cord microglia in culture expressed COX-1 immunoreactivity and released PGE2, but dynorphin A failed to increase release of PGE2 in these cultures. These results suggest that increased COX-1 expression occurs in spinal cord microglia following incisional surgery. Although prodynorphin immunoreactivity also increases, it likely does not drive COX-1 expression or mechanical hypersensitivity in this setting.