Developmental changes in the control of primary motoneuron excitability by the M-current in larval zebrafish

Spinal circuits for locomotion undergo maturation during early development. How intrinsic properties of individual spinal neuron populations change throughout motor maturation is not fully understood. Here we identify for the first time the presence of the persistent outward potassium current known as the M-current in primary motoneurons of larval zebrafish. We show that the M-current controls excitability of primary motoneurons and its role in excitability control changes during development such that the magnitude of the M-current in primary motoneurons transiently increases at 3 days post-fertilization. These findings reveal a novel mechanism by which control over excitability of primary motoneurons in larval zebrafish is ensured, underscoring developmental changes in ion current contributions to intrinsic properties. Broadly, these data support the M-current as a conserved means to control motoneuron excitability across vertebrates.