Slow outward tail currents in molluscan bursting pacemaker neurons: two components differing in temperature sensitivity.

Bursting pacemaker neurons of Tritonia and Aplysia were studied in voltage-clamp experiments to determine the ionic requirements of the slow outward tail current. This current is important for terminating bursts and hyperpolarizing the neuron during the interval between bursts and therefore contributes to the timing of pacemaker activity. A method for rapid extracellular perfusion of the neuron soma with solutions of different ionic composition was used to study the ionic dependence of the tail current. Measurements of the current-voltage relationship were made at different times during the decay of the tail current to determine the reversal potential in different ionic solutions. These experiments showed that 2 or more slow outward currents contribute to the tail current. Temperature had a large effect on these currents. At cold temperatures (10-15 degrees C), the tail current was predominantly a K current. At warm temperatures (20-23 degrees C), both a K current and a K-insensitive outward current were seen. Bursting pacemaker activity occurred throughout this temperature range. Both the K current and the K-insensitive current required Ca influx for activation. These findings help to reconcile conflicting reports in the literature concerning the ionic dependence of the slow outward tail current and suggest a mechanism for temperature compensation of bursting activity in these pacemaker cells.