ATP-gated current in dissociated rat nucleus solitarii neurons

1. The excitatory response of extracellularly applied ATP was investigated in freshly dissociated rat nucleus tractus solitarii neurons under whole-cell configuration using the "concentration-clamp" technique. 2. At a holding potential of -70 mV, 100 microM ATP evoked inward current that was slowly desensitized in the continuous presence of ATP. The ATP-gated current increased in a concentration-dependent manner over the concentration range between 10 microM and 1 mM. The half-maximum concentration was 31 microM and the Hill coefficient was 1.2. 3. The potency of ATP analogues for the purinergic receptor was in the order of ATP = 2-methylthio-ATP much greater than ADP greater than alpha,beta-methylene ATP. Neither adenosine nor AMP evoked any responses. The order was consistent with a P2y receptor subtype. 4. The current-voltage relationship for the 100 microM ATP response showed a clear inward rectification at positive potentials beyond -50 mV. The reversal potential of the ATP-gated current was +13 mV. 5. The time constants of activation and inactivation of the ATP-gated current solution were dependent on the extracellular ATP concentration, and both kinetics became faster at higher ATP concentrations. 6. The ATP-gated current was also elicited in an external solution containing Ca2+ as a permeable cation. The inactivation kinetics in an external solution containing 75 mM Ca2+ were faster than those in an external solution with 150 mM Na+. 7. Calculated relative permeability ratios were PNa/PCs = 1.64 ([Na+]o = 30-150 mM), PCa/PCs = 2.17 ([Ca2+]o = 2 mM). Anions were not measurably permeable in this preparation.