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 signalling and sensory regulation of visceral function through ionotropic receptors variably composed of P2X 2 and P2X 3 subunits. P2X 2 and P2X 3 subunits can form homomultimeric P2X 2 , homomultimeric P2X 3 , or heteromultimeric P2X 2/3 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 P2X 2 receptor subunit (P2X 2 −/− ) and double mutant mice lacking both P2X 2 and P2X 3 subunits (P2X 2 /P2X 3 Dbl−/− ), and compare these with previously characterized P2X 3 −/− mice. In patch-clamp studies, nodose, coeliac and superior cervical ganglia (SCG) neurones from wild-type mice responded to ATP with sustained inward currents, while dorsal root ganglia (DRG) neurones gave predominantly transient currents. Sensory neurones from P2X 2 −/− mice responded to ATP with only transient inward currents, while sympathetic neurones had barely detectable responses. Neurones from P2X 2 /P2X 3 Dbl−/− mice had minimal to no response to ATP. These data indicate that P2X receptors on sensory and sympathetic ganglion neurones involve almost exclusively P2X 2 and P2X 3 subunits. P2X 2 −/− and P2X 2 /P2X 3 Dbl−/− mice had reduced pain-related behaviours in response to intraplantar injection of formalin. Significantly, P2X 3 −/− , P2X 2 −/− , and P2X 2 /P2X 3 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 behavioural tests were observed in P2X 2 −/− mice. Taken together, these data extend our findings for P2X 3 −/− mice, and reveal an important contribution of heteromeric P2X 2/3 receptors to nociceptive responses and mechanosensory transduction within the urinary bladder.