P2X receptor-mediated ionic currents in dorsal root ganglion neurons

Nociceptive neurons in the dorsal root ganglia (DRG) are activated by extracellular ATP, implicating P2X receptors as potential mediators of painful stimuli. However, the P2X receptor subtype(s) underlying this activity remain in question. Using electrophysiological techniques, the effects of P2X receptor agonists and antagonists were examined on acutely dissociated adult rat lumbar DRG neurons. Putative P2X-expressing nociceptors were identified by labeling neurons with the lectin IB4. These neurons could be grouped into three categories based on response kinetics to extracellularly applied ATP. Some DRG responses (slow DRG) were relatively slowly activating, nondesensitizing, and activated by the ATP analogue alpha,beta-meATP. These responses resembled those recorded from 1321N1 cells expressing recombinant heteromultimeric rat P2X2/3 receptors. Other responses (fast DRG) were rapidly activating and desensitized almost completely during agonist application. These responses had properties similar to those recorded from 1321N1 cells expressing recombinant rat P2X3 receptors. A third group (mixed DRG) activated and desensitized rapidly (P2X3-like), but also had a slow, nondesensitizing component that functionally prolonged the current. Like the fast component, the slow component was activated by both ATP and alpha, beta-meATP and was blocked by the P2X antagonist TNP-ATP. But unlike the fast component, the slow component could follow high-frequency activation by agonist, and its amplitude was potentiated under acidic conditions. These characteristics most closely resemble those of rat P2X2/3 receptors. These data suggest that there are at least two populations of P2X receptors present on adult DRG nociceptive neurons, P2X3 and P2X2/3. These receptors are expressed either separately or together on individual neurons and may play a role in the processing of nociceptive information from the periphery to the spinal cord.