1. Tetrodotoxin (TTX)-sensitive Ca2+ conducting channels which produce a transient inward current were investigated in pyramidal neurones freshly dissociated from the dorsal part of rat hippocampal CA1 region by the use of the suction-pipette technique, which allows for intracellular perfusion under a single-electrode voltage clamp. 2. In all cells superfused with Na+- and K+-free external solution containing 10 mM-Ca2+ and 10(-5) M-La3+, a transient inward Ca2+ current was evoked by a step depolarization to potentials more positive than about -50 mV from a holding potential (V(H)) of -100 mV. This current was inhibited by either removing the extracellular Ca2+ or adding TTX (termed as 'TTX-I(Ca)'). 3. Activation and inactivation processes of the TTX-I(Ca) were highly potential dependent at 20-22-degrees-C, and the latter was fitted by a double exponential function. The time to peak of the current decreased from 5.0 to 2.3 ms at a test potential change from -50 to 0 mV. The time constants of the current decay decreased from 2.8 to 2.2 ms for fast component (tau(if)) and from 16.0 to 8.2 ms for slow component (tau(is)) at a potential change from -35 to -10 mV. 4. The TTX-I(Ca) was activated at threshold potential of about -55 mV and reached full activation at -30 mV. The steady-state inactivation of TTX-I(Ca) could be fitted by a Boltzmann equation with a slope factor of 6.0 mV and a half-inactivation voltage of -72.5 mV. 5. Biphasic recovery (reactivation) from the complete inactivation of TTX-I(Ca) was observed. The time constant of the major component (78.8 to 91.6% of total) of the reactivation was 13.1 ms, and that of the minor one was 120 to 240 ms. Therefore, TTX-I(Ca) remained fairly constant at a train of stimulation up to 3 Hz. However, the inhibition of current amplitude occurred as the repetitive stimulation increased more than 10 Hz, and considerable tonic inhibition occurred with increasing stimulation frequency. 6. When the peak amplitudes in the individual current-voltage (I-V) relationships of TTX-I(Ca) at various extracellular Ca2+ concentrations ([Ca2+]o) were plotted as a function of [Ca2+]o, the current amplitude increased linearly without showing any saturation. 7. The ratio of peak amplitude in the individual I-V relationships of Ca2+, Sr2+ and Ba2+ currents passing through the TTX-sensitive Ca2+ conducting channel was 1:0.33:0.05, although the current kinetics were much the same. 8. Adding Na+ to an external solution containing 5 mM-Ca2+ and 10(-5) M-La3+ elicited the current consisting of two components: one had a rapid inactivation and another a slow inactivation. The inactivation kinetics of fast and slow current components were the same as those of Na+ current (I(Na)) and TTX-I(Ca), respectively. The current amplitude of the slow component increased with increasing the extracellular Na+ concentration ([Na+]o). 9. TTX inhibited the TTX-I(Ca) in a time- and concentration-dependent manner without affecting the current kinetics. The time course for steady-state inhibition shortened in minutes with increasing concentration. Lignocaine inhibited the TTX-I(Ca) within a second in a concentration-dependent manner, with accelerating the inactivation process. The concentrations of half-inhibition (IC50) were 3.5 x 10(-9) M for TTX and 3.6 x 10(-4) M for lignocaine. 10. Scorpion toxin prolonged the inactivation phase of TTX-I(Ca) in a time- and concentration-dependent manner. In the toxin-treated neurones, both the time constant of tau(is) and its functional contribution to the total current increased with increasing the toxin concentration. 11. It was concluded that the TTX-I(Ca) is carried by Ca2+ and that Na+ can pass through this TTX-sensitive Ca2+ conducting channels. The physiological and pharmacological properties of the TTX-sensitive Ca2+ conducting channels were also compared with those of voltage-dependent Na+ channels and low-threshold (T-type) Ca2+ channels.
