Pacemaker and Plateau Potentials Shape Output of a Developing Locomotor Network

BACKGROUND During development, spinal networks undergo an intense period of maturation in which immature forms of motor behavior are observed. Such behaviors are transient, giving way to more mature activity as development proceeds. The processes governing age-specific transitions in motor behavior are not fully understood. RESULTS Using in vivo patch clamp electrophysiology, we have characterized ionic conductances and firing patterns of developing zebrafish spinal neurons. We find that a kernel of spinal interneurons, the ipsilateral caudal (IC) cells, generate inherent bursting activity that depends upon a persistent sodium current (I(NaP)). We further show that developmental transitions in motor behavior are accompanied by changes in IC cell bursting: during early life, these cells generate low frequency membrane oscillations that likely drive "coiling," an immature form of motor output. As fish mature to swimming stages, IC cells switch to a sustained mode of bursting that permits generation of high-frequency oscillations during locomotion. Finally, we find that perturbation of IC cell bursting disrupts motor output at both coiling and swimming stages. CONCLUSIONS Our results suggest that neurons with unique bursting characteristics are a fundamental component of developing motor networks. During development, these may shape network output and promote stage-specific reconfigurations in motor behavior.