ATP-induced inward current in neurons freshly dissociated from the tuberomammillary nucleus

1. Neurons in the tuberomammillary nucleus (TMN), which are considered to be histaminergic, were dissociated and their response to extracellularly applied ATP was investigated in the nystatin-perforated patch recording mode under voltage-clamp condition. 2. ATP induced a sustained inward current that was slowly desensitized at a holding potential of -60 mV. 3. The ATP response increased in a concentration-dependent manner. The half-maximum concentration (EC₅₀) was 44 μM and the Hill coefficient was 1.8. 4. The potency of ATP analogues was in the order of ATP ≥ 2-methylthio-ATP >> α,β-methylene ATP ≥ ADP. Neither adenosine nor AMP induced any response. The results suggest that the purinergic receptor in TMN neurons is P(2y). 5. The current-voltage relationship for the 100 μM ATP showed a significant inward rectification at a potential more positive than -20 mV in an external solution with 150 mM Na⁺, but a significant rectification current was not observed in an external solution with 150 mM Cs⁺. The change in the reversal potential of the ATP response (E(ATP)) to a 10-fold change of extracellular Na⁺ concentration was 56 mV, indicating that the ATP-induced current is highly selective for Na⁺ over Cl^-. 6. The permeability ratio for cations was Na⁺:Li⁺:K⁺:Rb⁺:Cs⁺:Ca²⁺ = 2.16:1.36:1.68:1.54:1:2.55, indicating that the ATP-induced current is passing through the ligand-gated nonselective cation channel. 7. These results suggest that ATP has an excitatory effect on the TMN neurons by opening nonselective cation channels.