P2X2 knockout mice and P2X2/P2X3 double knockout mice reveal a role for the P2X2 receptor subunit in mediating multiple sensory effects of ATP

Extracellular ATP plays a role in nociceptive signaling and sensory regulation of visceral function through ionotropic receptors variably composed of P2X2 and P2X3 subunits. P2X2 and P2X3 subunits can form homomultimeric P2X2, homomultimeric P2X3, or heteromultimeric P2X2/3 receptors. However, the relative contribution of these receptor subtypes to afferent functions of ATP in vivo is poorly understood. Here we describe null mutant mice lacking the P2X2 receptor subunit (P2X2(-/-)) and double mutant mice lacking both P2X2 and P2X3 subunits (P2X2/P2X3(Dbl-/-)), and compare these with previously characterized P2X3(-/-) mice. In electrophysiological studies, nodose, coeliac and superior cervical ganglia (SCG) neurons from wild-type mice responded to ATP with sustained inward currents, while dorsal root ganglia (DRG) neurons gave predominantly transient currents. Sensory neurons from P2X2(-/-) mice responded to ATP with only transient inward currents, while sympathetic neurons had barely detectable responses. Neurons from P2X2/P2X3(Dbl-/-) mice had minimal to no response to ATP. These data indicate that P2X receptors on sensory and sympathetic ganglion neurons involve almost exclusively P2X2 and P2X3 subunits. P2X2(-/-) and P2X2/P2X3(Dbl-/-) mice had reduced pain-related behaviors in response to intraplantar injection of formalin. Significantly, P2X3(-/-), P2X2(-/-), and P2X2/P2X3(Dbl-/-) mice had reduced urinary bladder reflexes and decreased pelvic afferent nerve activity in response to bladder distension. No deficits in a wide variety of CNS behavioral tests were observed in P2X2(-/-) mice. Taken together, these data extend our findings for P2X3(-/-) mice, and reveal an important contribution of heteromeric P2X2/3 receptors to nociceptive responses and mechanosensory transmission within the urinary bladder.