The excitation of nociceptive sensory neurons by ATP released in injured tissue is believed to be mediated partly by P2X₃receptors. Although an analysis of P2X₃ knock-out mice has revealed some deficits in nociceptive signaling, detailed analysis of the role of these receptors is hampered by the lack of potent specific pharmacological tools. Here we have used antisense oligonucleotides (ASOs) to downregulate P2X₃ receptors to examine their role in models of chronic pain in the rat.

ASOs and control missense oligonucleotides (180 μg/d) were administered intrathecally to naive rats for up to 7 d via a lumbar indwelling cannula attached to an osmotic minipump. Functional downregulation of the receptors was confirmed by αβ-methylene ATP injection into the hindpaw, which evoked significantly less mechanical hyperalgesia as early as 2 d after treatment with ASOs relative to controls. At this time point, P2X₃ protein levels were significantly downregulated in lumbar L4 and L5 dorsal root ganglia. After 7 d of ASO treatment, P2X₃ protein levels were reduced in the primary afferent terminals in the lumbar dorsal horn of the spinal cord.

In models of neuropathic (partial sciatic ligation) and inflammatory (complete Freund's adjuvant) pain, inhibition of the development of mechanical hyperalgesia as well as significant reversal of established hyperalgesia were observed within 2 d of ASO treatment. The time course of the reversal of hyperalgesia is consistent with downregulation of P2X₃ receptor protein and function.

This study demonstrates the utility of ASO approaches for validating gene targets in in vivo pain models and provides evidence for a role of P2X₃ receptors in the pathophysiology of chronic pain.