Increased 5-HT(3)-mediated signalling in pelvic afferent neurons from mice deficient in P2X(2) and/or P2X (3) receptor subunits

Extracellular ATP and 5-hydroxytryptamine (5-HT) are both involved in visceral sensory pathways by interacting with P2X and 5-HT(3) receptors, respectively. We have investigated the changes in P2X and 5-HT(3)-mediated signalling in pelvic afferent neurons in mice deficient in P2X(2) and/or P2X(3) subunits by whole-cell recording of L(6)-S(2) dorsal root ganglion (DRG) neurons and by multi-unit recording of pelvic afferents of the colorectum. In wildtype DRG neurons, ATP evoked transient, sustained or mixed (biphasic) inward currents. Transient currents were absent in P2X(3) (-/-) neurons, whereas sustained currents were absent in P2X(2) (-/-) DRG neurons. Neither transient nor sustained currents were observed following application of ATP or alpha,beta-methylene ATP (alpha,beta-meATP) in P2X(2)/P2X(3) (Dbl-/-) DRG neurons. 5-HT was found to induce a fast inward current in 63% of DRG neurons from wildtype mice, which was blocked by tropisetron, a 5-HT(3) receptor antagonist. The percentage of DRG neurons responding to 5-HT was significantly increased in P2X (2) (-/-), P2X(3) (-/-) and P2X(2)/P2X(3) (Dbl-/-) mice, and the amplitude of 5-HT response was significantly increased in P2X(2)/P2X(3) (Dbl-/-) mice. The pelvic afferent response to colorectal distension was attenuated in P2X(2)/P2X(3) (Dbl-/-) mice, but the response to serosal application of 5-HT was enhanced. Furthermore, tropisetron resulted in a greater reduction in pelvic afferent responses to colorectal distension in the P2X(2)/P2X(3) (Dbl-/-) preparations. These data suggest that P2X receptors containing the P2X(2) and/or P2X(3) subunits mediate purinergic activation of colorectal afferents and that 5-HT signalling in pelvic afferent neurons is up-regulated in mice lacking P2X(2) or P2X(3) receptor genes. This effect is more pronounced when both subunits are absent.