Characterization of calcium currents and their contributions to firing behaviour of primary motoneurons in developing zebrafish

Calcium currents in vertebrate motoneurons undergo developmental changes as motoneurons mature and develop specialized functions. The rapid development of zebrafish presents a unique opportunity to link changes in calcium currents of motoneurons to changes in the rapid and stereotyped motor maturation of zebrafish. As swimming matures from crude, large amplitude body bends to refined, low amplitude tailbeats, the involvement of the primary motoneurons that innervate fast muscle becomes less prevalent. During this time, primary motoneuron innervation, dendritic arborization, and firing properties become refined. In this study, we aimed to characterize the presence and influence on firing behaviour of low-voltage and high-voltage activated calcium currents in primary motoneurons during early development, when primary motoneurons undergo functional maturation. Our whole-cell patch-clamp electrophysiology data in zebrafish aged 2 to 5 days post-fertilization (dpf) reveal unique characteristics of calcium currents in zebrafish primary motoneurons, such as the influence of L-type and N-type calcium currents on the regulation of repetitive firing at 3 and 5 dpf, the emergence of P/Q-type calcium currents that regulate repetitive firing as of 4 dpf, and the absence of post-inhibitory rebounds characteristic of T-type calcium currents. These findings highlight how precise changes in properties of ion currents can shape neuronal function during development.