Agonist-dependence of recovery from desensitization of P2X3 receptors provides a novel and sensitive approach for their rapid up or downregulation

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 (EC50 = 1.52 muM), ADP (EC50 = 1.1 muM) or alpha,beta-meATP (EC50 = 1.78 muM) 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 alpha,beta-meATP and, in all cases, it had sigmoidal time course. 3 By alternating the application of ATP and alpha,beta-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 alpha,beta-methylene-adenosine triphosphate (alpha,beta-meATP), and vice versa. Thus, short-lasting, full desensitization produced by alpha,beta-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 alpha,beta-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.