5-Hydroxytryptamine (serotonin) causes a reduction in the afterhyperpolarization following the action potential in lamprey motoneurons and premotor interneurons.

The actions of 5-hydroxytryptamine (5-HT) on single neurons in the lamprey spinal cord have been investigated by intracellular recordings in vitro. Administration of 5-HT either by pressure ejection or by perfusion in the bath caused a reversible reduction of the late phase of the afterhyperpolarization (AHP) following the action potentials. This effect was antagonized by methysergide. A reduction of the late AHP was observed in lateral interneurons, motoneurons and some unclassified cells.