T-type Ca²⁺ channel lowers the threshold of spike generation in the newt olfactory receptor cell

Mechanisms underlying action potential generation in the newt olfactory receptor cell were investigated by using the whole-cell version of the patch-clamp technique. Isolated olfactory cells had a resting membrane potential of - 70 ± 9 mV. Injection of a depolarizing current step triggered action potentials under current clamp condition. The amplitude of the action potential was reduced by lowering external Na⁺ concentration. After a complete removal of Na⁺, however, cells still showed action potentials which was abolished either by Ca²⁺ removal or by an application of Ca²⁺ channel blocker (Co²⁺ or Ni²⁺), indicating an involvement of Ca²⁺ current in spike generation of newt olfactory receptor cells. Under the voltage clamp condition, depolarization of the cell to -40 mV from the holding voltage of -100 mV induced a fast transient inward current, which consisted of Na⁺ (I_Na) and T-type Ca²⁺ (I_Ca,T) currents. The amplitude of I_Ca,T was about one fourth of that of I_Na. Depolarization to more positive voltages also induced L-type Ca²⁺ current (I_Ca,I.). I_Ca.I. was as small as a few pA in normal Ringer solution. The activating voltage of I_Ca,T was approximately 10 mV more negative than that of I_Na. Under current clamp, action potentials generated by a least effective depolarization was almost completely blocked by 0.1 mM Ni²⁺ (a specific T-type Ca²⁺ channel blocker) even in the presence of Na⁺. These results suggest that I_Ca,T contributes to action potential in the newt olfactory receptor cell and lowers the threshold of spike generation.