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

Abstract

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 3Dbl−/−), 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 3Dbl−/− 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 3Dbl−/− mice had reduced pain‐related behaviours in response to intraplantar injection of formalin. Significantly, P2X 3−/−, P2X 2−/−, and P2X 2/P2X 3Dbl−/− 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.