Diinosine pentaphosphate: an antagonist which discriminates between recombinant P2X3 and P2X2/3 receptors and between two P2X receptors in rat sensory neurones

We have compared the antagonist activity of trinitrophenyl‐ATP (TNP‐ATP) and diinosine pentaphosphate (Ip5I) on recombinant P2X receptors expressed in Xenopus oocytes with their actions at native P2X receptors in sensory neurones from dorsal root and nodose ganglia. Slowly‐desensitizing responses to α,β‐methylene ATP (α,β‐meATP) recorded from oocytes expressing P2X2/3 receptors were inhibited by TNP‐ATP at sub‐micromolar concentrations. However, Ip5I at concentrations up to 30 μM was without effect. Nodose ganglion neurones responded to α,β‐meATP with slowly‐desensitizing inward currents. These were inhibited by TNP‐ATP (IC50, 20 nM), but not by Ip5I at concentrations up to 30 μM. In DRG neurones that responded to ATP with a rapidly‐desensitizing inward current, the response was inhibited by TNP‐ATP with an IC50 of 0.8 nM. These responses were also inhibited by Ip5I with an IC50 of 0.1 μM. Both antagonists are known to inhibit homomeric P2X3 receptors. Some DRG neurones responded to α,β‐meATP with a biphasic inward current, consisting of transient and sustained components. While the transient current was abolished by 1 μM Ip5I, the sustained component remained unaffected. In conclusion, Ip5I is a potent antagonist at homomeric P2X3 receptors but not at heteromeric P2X2/3 receptors, and therefore should be a useful tool for elucidating the subunit composition of native P2X receptors.