TNP-ATP, a potent P2X 3 receptor antagonist, blocks acetic acid-induced abdominal constriction in mice: comparison with reference analgesics

Abstract

Exogenous ATP has been shown to be algogenic in both animal and humans. Research has focused on the P2X 3 ligand-gated ion channel, as it is preferentially expressed on nociceptive C-fibers. In addition, P2X 3 receptor gene disrupted mice show decreased responses to somatic painful stimuli. However, the potential role of P2X receptor activation in visceral pain has not yet been evaluated. In the present study, the systemic administration of suramin, and pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid, PPADS, both non-selective P2X receptor antagonists, dose-dependently reduced acetic acid-induced abdominal constrictions in mice ( ED 50=34.5 μmol/kg and ED 50=70 μmol/kg, respectively). Furthermore, 2′-(or-3′)- O-(trinitrophenyl)adenosine 5′- tri-phosphate (TNP-ATP) potently ( IC 50=10 nM) blocked the functional activation of P2X 3 receptors in vitro and attenuated acetic acid-induced visceral pain. In the abdominal constriction assay, TNP-ATP ( ED 50=6.35 μmol/kg, i.p.) was 6–10 fold more potent than suramin and PPADS to reduce nociceptive behavior. In addition, TNP-ATP was 10 fold more potent than TNP-AMP (2′-(or-3′)- O-(trinitrophenyl)adenosine 5′-mono-phosphate) (ED 50=63.5 μmol/kg, i.p.) at reducing acetic acid-induced nociception. At the highest dose, TNP-ATP completely abolished nociceptive behavior, as did morphine ( ED 50=3 μmol/kg, i.p.). While TNP-ATP is also a potent antagonist of P2X 1 receptors, P2X 1 receptor mediated responses have not been shown in dorsal root ganglia and diinosine pentaphosphate, IP5I, a potent and selective P2X 1 receptor antagonist, was ineffective at reducing abdominal constrictions. Thus, the antinociceptive effects of TNP-ATP appear to be mediated through activation of homomeric P2X 3 and/or heteromeric P2X 2/3 receptors. Together, these results show that activation of P2X 3 containing receptors plays a role in the transmission of inflammatory visceral pain.