Changes in P2X receptor responses of sensory neurons from P2X3-deficient mice

Dorsal root ganglion (DRG) neurons respond to ATP with transient, persistent or biphasic inward currents. In contrast, the ATP responses in nodose neurons are persistent. These sustained currents are also heterogeneous, with one component being accounted for by P2X(2/3) receptors, and the residual response probably mediated by P2X(2) receptors, although the direct evidence for this has been lacking. In the present study, we examined the P2X receptors on DRG and nodose neurons from P2X(3)-deficient (P2X(3)(-/-)) mice, using whole cell voltage-clamp recording and immunohistochemistry. We found that all P2X(3)(-/-) DRG neurons lacked rapidly desensitizing response to ATP, and both DRG and nodose neurons from P2X(3)-null mutant mice no longer responded to alpha,beta -methylene ATP (alpha beta meATP). In contrast, ATP evoked persistent inward current in 12% of DRG neurons and 84% of nodose neurons from P2X(3)(-/-) mice. This retained persistent response to ATP on nodose neurons had an EC50 for ATP of 77 muM, was antagonized by Cibacron blue and pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid, potentiated by Zn2+ and acidification, but not enhanced by ivermectin or diinosine pentaphosphate. 2',3'-O Trinitrophenyl-ATP antagonized this response with an IC50 of 8 muM. All these properties are consistent with those of recombinant P2X(2) homomeric receptors. Furthermore, specific P2X(2) receptor immunoreactivity detected in wild-type sensory neurons was unaltered in null mutant mice. Therefore, the alpha beta meATP-insensitive persistent responses on nodose neurons are likely to be mediated by P2X(2) homomers, which contribute to 60% of currents evoked by 100 muM ATP in the wild type.