Abstract
1. Whole-cell recordings were made from guinea-pig coeliac, rat nodose and rat superior cervical (SCG) neurones in culture, and currents in response to fast-flow (concentration clamp) application of ATP and other ATP analogues were measured. 2. At a holding potential of -70 mV, ATP evoked inward currents in all neurons. ATP-induced currents reversed at approximately 0 mV and showed strong inward rectification. Halving the external sodium concentration shifted the reversal potential by -15 to -17 mV, while increasing external potassium from 2 to 20 mM produced a 6-10 mV shift in reversal potential. Latency to onset of ATP current was < 1 ms; rise time was concentration dependent with maximum time to peak of 5-20 ms in nodose and coeliac neurones but 80 ms in superior cervical neurones. 3. Threshold concentrations of ATP were 0.1 microM for nodose and coeliac neurones but 10 microM for superior cervical neurones; EC50 values were approximately 3 microM for both nodose and coeliac neurones and 43 microM for superior cervical ganglia. Hill slopes for ATP concentration-response curves were not significantly different from unity in nodose and coeliac neurones whereas the Hill slope in superior cervical neurones was two. 4. 2-MethylthioATP (2-MeSATP) acted as a full agonist in all three neuronal preparations; EC50 values were 0.4, 2.8 and 46 microM for nodose, coeliac and superior cervical neurones, respectively. alpha,beta-Methylene ATP (alpha,beta-MeATP) was a full agonist in nodose and coeliac neurones with EC50 values of 9 and 13 microM, respectively. 5. In superior cervical neurones alpha,beta-MeATP had little or no agonist action but produced a concentration-dependent attenuation of the ATP current. Thus, alpha, beta-MeATP appears to behave as a partial agonist at P2X-purinoceptors in superior cervical neurones. 6. The non-selective purinoceptor antagonists suramin (1-100 microM) and pyridoxal-5'-phosphate (30 microM), as well as the putative P2Y-selective antagonist, Cibacron Blue (30 microM), inhibited all agonist-evoked responses to a similar degree in all three neuronal populations. 7. This study demonstrates that an agonist potency profile of 2-MeSATP > or = ATP > or = alpha,beta-MeATP is characteristic of ligand-gated P2X-purinoceptors in isolated peripheral neurones. We also suggest that the P2X-receptor in superior cervical neurones may represent a distinct subtype of P2X-purinoceptor from that present in nodose and coeliac neurones.