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 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.

An update and systematic review on drug therapies for the treatment of refractory chronic cough

“…Clinically the P2X7 receptor antagonists CE-224535 and AZD9056 have not demonstrated efficacy in rheumatoid arthritis and it is unknown whether they may be useful in pain indications [45,46]. P2X3 receptor expression changes in animal pain models of P2X3 knock-out mice have shown the development of reduced mechanical allodynia [47] and neuronal P2X3 receptor activation predisposing afferent neurons to inflammatory hyperalgesia [48]. The antitussive actions of the P2X3 antagonist AF-219 occurs in the absence of any effect on capsaicin-induced cough [49], which is consistent with preclinical cough studies in guinea pigs showing that ATP and capsaicin have independent mechanisms of action [50].…”

Properties of ATP-gated ion channels assembled from P2X2 subunits in mouse cochlear Reissner’s membrane epithelial cells

“…Electrophysiological studies in P2X knockout mice have helped to assign the molecular composition of P2X receptors and their functional characterisation in native tissue [21]. In Fig.…”

“…P2rX2 null mice encoding the ATP-gated ion channel P2X 2 subunit (Roche Palo Alto, CA, USA; [21]) provided a control for background membrane conductance. The P2rX2 null mouse colony founders were all genotyped using a DNeasy blood and tissue kit (QIAGEN, New Zealand) and previously described PCR primers [21] to confirm the absence of the P2rX2 gene (data not shown). All experiments were approved by the animal ethics committees of the University of Auckland (New Zealand) and the University of Cambridge (UK).…”

The agonists for nociceptors are ubiquitous, but the modulators are specific: P2X receptors in the sensory neurons are modulated by cannabinoids

“…Both P2X2−/− and P2X2/P2X3(Dbl−/−) mice had reduced pain-related behaviors in response to intraplantar injection of formalin. P2X3−/−, P2X2−/−, and P2X2/P2X3 (Dbl−/−) mice had reduced urinary bladder reflexes and decreased pelvic afferent nerve activity in response to bladder distension [9].…”

“…ATP released as a result of tissue injury, visceral distension, or sympathetic activation [3,6] excites nociceptive primary sensory afferents by activating homomeric and heteromeric combinations of P2X3 and P2X2 receptors [14,19]. Double knockout of these subunits leads to a total disappearance of ATP-activated conductance in the sensory neurons [9].…”