title: The P2X3 antagonist P1, P5-di[inosine-5'] pentaphosphate binds to the desensitized state of the receptor in rat dorsal root ganglion neurons.
abstract: P2X3 purinergic receptors are predominantly expressed in dorsal root ganglion (DRG) neurons and play an important role in pain sensation. P2X3-specific antagonists are currently being sought to ameliorate pain in several indications. Understanding how antagonists interact with the P2X3 receptor can aid in the discovery and development of P2X3-specific antagonists. We studied the activity of the noncompetitive antagonist P1, P5-di[inosine-5'] pentaphosphate (IP5I) at the P2X3 receptor, compared with the well studied competitive antagonist TNP-ATP, using a whole-cell voltage-clamp technique in dissociated rat DRG neurons. IP5I blocked alphabeta-methylene ATP (alphabeta-meATP)-evoked P2X3 responses in a concentration-dependent manner (IC50 = 0.6 +/- 0.1 microM). IP5I effectively inhibited P2X3 currents when pre-exposed to desensitized but not unbound receptors. Furthermore, IP5I equally blocked 1 and 10 microM alphabeta-meATP-evoked currents and had no effect on the desensitization rate constant of these currents. This supports the action of IP5I as a noncompetitive antagonist that interacts with the desensitized state of the P2X3 receptor. In contrast, TNP-ATP inhibited the current evoked by 1 microM alphabeta-meATP significantly more than the one evoked by 10 microM alphabeta-meATP. It also significantly slowed down the desensitization rate constant of the current. These results suggest that TNP-ATP acts as a competitive antagonist and competes with alphabeta-meATP at the P2X3 agonist binding site. These findings may help to explain why IP5I acts selectively at the fast-desensitizing P2X1 and P2X3 subtypes of the P2X purinoceptor, while having much less potency at slow-desensitizing P2X2 and P2X(2/3) subtypes that lack the fast desensitized conformational state.
authors: Department of Electrophysiology, Neurogen Corporation, 35 Northeast Industrial Road, Branford, CT 06405, USA.
authors: Kristen K
authors: KK
authors: Ford
authors: Michele
authors: M
authors: Matchett
authors: James E
authors: JE
authors: Krause
authors: Weifeng
authors: W
authors: Yu
mesh_terms: D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive; D015226:Dinucleoside Phosphates / Q000378:metabolism* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005727:Ganglia, Spinal / Q000378:metabolism*; D058919:Purinergic P2 Receptor Antagonists; D051381:Rats; D017207:Rats, Sprague-Dawley; D018048:Receptors, Purinergic P2 / Q000378:metabolism; D058477:Receptors, Purinergic P2X3
chemical_list: D015226:Dinucleoside Phosphates; C546410:P2rx3 protein, rat; D058919:Purinergic P2 Receptor Antagonists; D018048:Receptors, Purinergic P2; D058477:Receptors, Purinergic P2X3; C109479:diinosine pentaphosphate; D000255:Adenosine Triphosphate; C002630:alpha,beta-methyleneadenosine 5'-triphosphate