Extracellular ATP is implicated in numerous sensory processes ranging from the response to pain to the regulation of motility in visceral organs. The ATP receptor P2X3 is selectively expressed on small diameter sensory neurons, supporting this hypothesis. Here we show that mice deficient in P2X3 lose the rapidly desensitizing ATP-induced currents in dorsal root ganglion neurons. P2X3 deficiency also causes a reduction in the sustained ATP-induced currents in nodose ganglion neurons. P2X3-null mice have reduced pain-related behaviour in response to injection of ATP and formalin. Significantly, P2X3-null mice exhibit a marked urinary bladder hyporeflexia, characterized by decreased voiding frequency and increased bladder capacity, but normal bladder pressures. Immunohistochemical studies localize P2X3 to nerve fibres innervating the urinary bladder of wild-type mice, and show that loss of P2X3 does not alter sensory neuron innervation density. Thus, P2X3 is critical for peripheral pain responses and afferent pathways controlling urinary bladder volume reflexes. Antagonists to P2X3 may therefore have therapeutic potential in the treatment of disorders of urine storage and voiding such as overactive bladder.

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.

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