The Development And Organization Of Dendrites Of Motoneurons In Larval Zebrafish

Abstract

The work described here focuses on novel patterns of motoneuron dendritic organization and dendritic development in relation to the recruitment patterns of spinal motoneurons that drive swimming in larval zebrafish. I first looked at the dendritic organization of motoneurons in freely swimming fish and tracked its emergence in relation to the maturation of locomotor behavior. I used transient expression of fluorescent proteins to visualize the dendritic structure of motoneurons in zebrafish larvae at a stage when they have been swimming freely for a few days. My work showed that there is a dendritic topography related to the recruitment of motoneurons at different locomotor speeds that emerges by the time fish first begin to swim, and is maintained even as dendrites grow after the onset of spontaneous swimming. Since neuronal activity is thought to influence dendritic structure, I then studied the structural dynamics of dendritic arbors of individual motoneurons in larval zebrafish soon after they begin swimming. I found a systematic relationship between the location of a spinal motoneuron and the dynamics of its dendritic arbor - youngest, ventral motoneurons are least dynamic whereas increasingly older and more dorsal motoneurons are more dynamic. This is contrary to the idea that dendrites of younger neurons are more dynamic than dendrites of older neurons because younger ones are growing more. I then asked if this pattern of dendritic dynamics is related to the systematic variation of excitability of motoneurons described recently. I tested this possibility genetically by expressing Kir2.1 to suppress excitability of individual motoneurons. This led to a dramatic increase in the dynamics of ventral motoneurons, which became more dynamic than more dorsal ones. My results suggest that a naturally occurring dorsoventral gradient of excitability may contribute to the variation in dendritic dynamics. The patterns of dendritic organization and development I describe may also be applicable to other interneuron types in the spinal cord and hindbrain.