P2X3 and P2X2/3 receptors are highly localized on peripheral and central processes of sensory afferent nerves, and activation of these channels contributes to the pronociceptive effects of ATP. A-317491 is a novel non-nucleotide antagonist of P2X3 and P2X2/3 receptor activation. A-317491 potently blocked recombinant human and rat P2X3 and P2X2/3 receptor-mediated calcium flux (Ki ‫؍‬ 22-92 nM) and was highly selective (IC50 >10 M) over other P2 receptors and other neurotransmitter receptors, ion channels, and enzymes. A-317491 also blocked native P2X3 and P2X2/3 receptors in rat dorsal root ganglion neurons. Blockade of P2X3 containing channels was stereospecific because the R-enantiomer (A-317344) of A-317491 was significantly less active at P2X3 and P2X2/3 receptors. A-317491 dosedependently (ED50 ‫؍‬ 30 mol͞kg s.c.) reduced complete Freund's adjuvant-induced thermal hyperalgesia in the rat. A-317491 was most potent (ED50 ‫؍‬ 10 -15 mol͞kg s.c.) in attenuating both thermal hyperalgesia and mechanical allodynia after chronic nerve constriction injury. The R-enantiomer, A-317344, was inactive in these chronic pain models. Although active in chronic pain models, A-317491 was ineffective (ED 50 >100 mol͞kg s.c.) in reducing nociception in animal models of acute pain, postoperative pain, and visceral pain. The present data indicate that a potent and selective antagonist of P2X 3 and P2X2/3 receptors effectively reduces both nerve injury and chronic inflammatory nociception, but P2X 3 and P2X2/3 receptor activation may not be a major mediator of acute, acute inflammatory, or visceral pain.T he cloning and characterization of the P2X 3 receptor, a specific ATP-sensitive ligand-gated ion channel that is selectively localized on peripheral and central processes of sensory afferent neurons (1-3), has generated much interest in the role of this receptor in nociceptive signaling (4). The discovery of the P2X 3 receptor has provided a putative mechanism for previous reports that ATP, released from sensory nerves (5), produces fast excitatory potentials in dorsal root ganglion (DRG) neurons (6). These actions appear to be physiologically relevant because iontophoretic application of ATP to human skin elicits pain (7) and exogenously applied ATP enhances pain sensations in a human blister base model (8).The P2X 3 receptor is natively expressed as a functional homomer and as a heteromultimeric combination with the P2X 2 (P2X 2/3 ) receptor (1, 2, 9). Both P2X 3 -containing channels are expressed on a high proportion of isolectin IB4-positive neurons in DRG (3, 10). These receptors share similar pharmacological profiles (11), but differ in their acute desensitization kinetics (10, 12). Immunohistochemical studies have shown that P2X 3 receptor expression is up-regulated in DRG neurons and ipsilateral spinal cord after chronic constriction injury (CCI) of the sciatic nerve (13). Additionally, CCI results in a specific ectopic sensitivity to ATP that is not observed on contralateral (uninjured) nerves (14).Recently, the phenotyp...

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The vanilloid receptor 1 (VR1, TRPV1), which is a member of the transient receptor potential (TRP) superfamily, is highly localized on peripheral and central processes of nociceptive afferent fibers. Activation of TRPV1 contributes to the pronociceptive effects of capsaicin, protons, heat, and various endogenous lipid agonists such as anandamide and N-arachidonoyl-dopamine. A-425619 [1-isoquinolin-5-yl-3-(4-trifluoromethyl-benzyl)urea] is a novel potent and selective antagonist at both human and rat TRPV1 receptors. In vivo, A-425619 dose dependently reduced capsaicininduced mechanical hyperalgesia (ED 50 ϭ 45 mol/kg p.o.). A-425619 was also effective in models of inflammatory pain and postoperative pain. A-425619 potently reduced complete Freund's adjuvant-induced chronic inflammatory pain after oral administration (ED 50 ϭ 40 mol/kg p.o.) and was also effective after either i.t. administration or local injection into the inflamed paw. Furthermore, A-425619 maintained efficacy in the postoperative pain model after twice daily dosing p.o. for 5 days. A-425619 also showed partial efficacy in models of neuropathic pain. A-425619 did not alter motor performance at the highest dose tested (300 mol/kg p.o.). Taken together, the present data indicate that A-425619, a potent and selective antagonist of TRPV1 receptors, effectively relieves acute and chronic inflammatory pain and postoperative pain.The vanilloid receptor VR1 or TRPV1 is a nonselective cation channel that is activated by exogenous vanilloid compounds such as capsaicin (Caterina and Julius, 2001). Anatomical and functional studies have shown that TRPV1 receptors are expressed on peripheral nociceptors (for review, see Cortright and Szallasi, 2004). Recently, the analgesic potential of TRPV1 receptor blockade has been demonstrated by various approaches including gene disruption, neutralizing antibodies, or receptor antagonism (Caterina et al., 2000;Davis et al., 2000;Kamei et al., 2001;Walker et al., 2003). Although TRPV1 gene-disrupted mice showed mostly normal behavioral responses to noxious heat, they did not develop thermal hyperalgesia to mustard oil or complete Freund's adjuvant (CFA;Caterina et al., 2000). These results suggest that TRPV1 receptors are required for responses to noxious thermal stimuli under inflammatory conditions but that other mechanisms are in part responsible for normal sensation of noxious heat. Consistent with this conclusion, Davis et al. (2000) showed that TRPV1 knockout mice did not develop thermal hyperalgesia in response to carrageenan but showed normal responses to noxious heat. However, TRPV1 knockout mice did develop mechanical allodynia in response to CFA and mustard oil, showed normal responses to formalin, and developed both thermal hyperalgesia and mechanical allodynia after partial nerve injury (Caterina et al., 2000). A role for TRPV1 in thermal hypersensitivity has also been described in diabetic mice. Following i.t. administration of a TRPV1-neutralizing antibody, a partial reduction in thermal hypersen...

1 We have recently reported that systemic delivery of A-317491, the first non-nucleotide antagonist that has high affinity and selectivity for blocking P2X 3 homomeric and P2X 2/3 heteromeric channels, is antinociceptive in rat models of chronic inflammatory and neuropathic pain. In an effort to further evaluate the role of P2X 3 /P2X 2/3 receptors in nociceptive transmission, A-317491 was administered either intrathecally or into the hindpaw of a rat in several models of acute and chronic nociception. 2 Intraplantar (ED 50 ¼ 300 nmol) and intrathecal (ED 50 ¼ 30 nmol) injections of A-317491 produced dose-related antinociception in the CFA model of chronic thermal hyperalgesia. Administration of A-317491 by either route was much less effective to reduce thermal hyperalgesia in the carrageenan model of acute inflammatory hyperalgesia. 3 Intrathecal, but not intraplantar, delivery of A-317491 attenuated mechanical allodynia in both the chronic constriction injury and L5-L6 nerve ligation models of neuropathy (ED 50 ¼ 10 nmol for both models). Intrathecal injections of A-317491 did not impede locomotor performance. 4 Both routes of injection were effective in reducing the number of nocifensive events triggered by the injection of formalin into a hindpaw. Nocifensive behaviors were significantly reduced in both the first and second phases of the formalin assay (intrathecal ED 50 ¼ 10 nmol, intraplantar ED 50 4300 nmol). Nocifensive behaviors induced by the P2X receptor agonist a,b-meATP were also significantly reduced by intraplantar injection of A-317491. 5 These data indicate that both spinal and peripheral P2X 3 /P2X 2/3 receptors have significant contributions to nociception in several animal models of nerve or tissue injury. Intrathecal administration of A-317491 appears to be more effective than intraplantar administration to reduce tactile allodynia following peripheral nerve injury.