Serotonin modulates a specific potassium current in the sensory neurons that show presynaptic facilitation in Aplysia

Serotonin exerts a long-lasting excitatory action on sensory neurons of Aplysia californica by decreasing outward K+ current. The depression of outward current delays repolarization of the action potential and extends the duration of Ca2+ influx into the presynaptic terminals, thereby contributing to the facilitation of transmitter release that underlies behavioral sensitization. We have extended the analysis of serotonin's action and find that it acts on a specific serotonin-sensitive K+ current (S current), which is different from the early K+ current (IA), the delayed K+ current (IK), the Ca2+-dependent K+ current (IC), and the muscarine-sensitive M current. The serotonin-sensitive current in these cells persists when IA and IK are reduced by conditioning depolarization or channel-blocking agents. The S current is not activated by intracellular injection of Ca2+, nor is it affected by substitution of Ba2+ for Ca2+, a treatment that reduces IC. Moreover, intracellular injection of cyclic AMP exerts an effect indistinguishable from that of serotonin. This observation and the insensitivity of the current to Ba2+ distinguishes the S current from M current. S current is activated at the resting potential and does not inactivate with steady-state depolarization. It is active sufficiently early during an action potential to contribute to the repolarization of the action potential and therefore accounts for the physiological effects of serotonin.