Agonist-dependence of recovery from desensitization of P2X 3 receptors provides a novel and sensitive approach for their rapid up or downregulation

Abstract

1. Fast-desensitizing P2X 3 receptors of nociceptive dorsol root ganglion (DRG) neurons are thought to mediate pain sensation. Since P2X 3 receptor efficiency is powerfully modulated by desensitization, its underlying properties were studied with patch-clamp recording. 2. On rat cultured DRG neurons, 2 s application of ATP (EC 50 = 1.52 μM), ADP (EC 50 = 1.1 μM) or α,β-meATP (EC 50 = 1.78 μM) produced similar inward currents that fully desensitized, at the same rate, back to baseline. Recovery from desensitization was much slower after ATP and ADP than after α,β-meATP and, in all cases, it had sigmoidal time course. 3. By alternating the application of ATP and α,β-meATP, we observed complete cross-desensitization indicating that these agonists activated the same receptors. This notion was confirmed by the similar antagonism induced by 2′, 3′-O-(2,4,6,trinitrophenyl)-adenosine triphosphate (TNP-ATP). 4. Recovery from desensitization elicited by ATP was unexpectedly shaped by transient application of α,β-methylene-adenosine triphosphate (α,β-meATP), and vice versa. Thus, short-lasting, full desensitization produced by α,β-meATP protected receptors from long-lasting desensitization induced by subsequent ATP applications. ATP and ADP had similar properties of recovery from desensitization. 5. Low nM concentrations of α,β-meATP (unable to evoke membrane currents) could speed up recovery from ATP-induced desensitization, while low nM concentrations of ATP enhanced it. Ambient ATP levels were found to be in the pM range (52 ± 3 pM). 6. The phenomenon of cross-desensitization and protection was reproduced by rP2X 3 receptors expressed by rat osteoblastic cell 17/2.8 or human embryonic kidney cell 293 cells, indicating P2X 3 receptor specificity. 7. It is suggested that transient application of an agonist that generates rapid recovery from desensitization, is a novel, powerful tool to modulate P2X 3 receptor responsiveness to the natural agonist ATP.