Membrane currents induced by pentylenetetrazol in identified neurons of Helix pomatia.

After systemic application of pentylenetetrazol (PTZ), mammalian as well as molluscan neurons generate epileptic paroxysmal depolarization shifts. For a further analysis of these potential oscillations the membrane currents induced by local application of PTZ onto identified neurons of Helix pomatia were investigated. Different types of responses were obtained at membrane potentials negative and positive to ca. -30 mV. At holding potentials more negative than -30 mV, PTZ as a rule evoked an inward current, sometimes preceded by a brief outward current. In a few experiments only a solitary outward current was found. The amplitudes of the inward and outward currents increased towards more negative potentials. The inward current was associated with a decrease and the outward current with an increase in membrane resistance. Besides these findings pharmacological and ion substitution experiments indicate that the inward current represents an unspecific current. At holding potentials more positive than -30 mV, PTZ evoked a sequence of currents which was the same in all neurons. This stereotyped current sequence consisted of (i) an early inward current, (ii) an intermediate outward current, and (iii) a late long-lasting inward current. The amplitudes of all these components increased towards more positive potentials with the outward current being particularly enhanced. The early inward current and the following outward current were associated with a decrease and the late inward current with an increase of the membrane resistance. Besides these pharmacological and ion substitution experiments suggest that the early inward current represents a mixed sodium and calcium current, the intermediate outward current a calcium activated potassium current. The late inward current is assumed to be due to a decreased potassium conductance. On the basis of the present results, it may be concluded that the unspecific inward current in the negative potential range is involved in the initiation and the calcium dependent potassium current in the termination of spontaneously occurring paroxysmal depolarization shifts.