Abstract
The spinal cord contains a diverse array of interneurons that govern motor output. Traditionally, models of spinal circuits have emphasized the role of inhibition in enforcing reciprocal alternation between left and right sides or flexors and extensors. However, recent work has shown that inhibition also increases coincident with excitation during contraction. Here, using larval zebrafish, we investigate the V2b (Gata3+) class of neurons, which contribute to flexor-extensor alternation but are otherwise poorly understood. Using newly generated transgenic lines we define two stable subclasses with distinct neurotransmitter and morphological properties. These V2b subclasses synapse directly onto motor neurons with differential targeting to speed-specific circuits. In vivo, optogenetic manipulation of V2b activity modulates locomotor frequency: suppressing V2b neurons elicits faster locomotion, whereas activating V2b neurons slows locomotion. We conclude that V2b neurons serve as a brake on axial motor circuits. Together, these results indicate a role for ipsilateral inhibition in speed control.
Highlights
Rhythmic, coordinated body movements require selective recruitment of motor neurons by spinal and supraspinal premotor circuits
Most analysis of drive from V2b neurons has focused on the shared contributions of V1s and V2bs to reciprocal inhibition governing flexor/extensor alternation in limbed animals (Britz et al, 2015; Zhang et al, 2014; McCrea and Rybak, 2008). This does not shed light on potential functions of ipsilateral inhibition in gain control for regulation of motor neuron firing during contraction, as opposed to suppression of motor neuron firing during extension
In this study we demonstrate that V2b neurons exert direct control over axial musculature in the larval zebrafish
Summary
Rhythmic, coordinated body movements require selective recruitment of motor neurons by spinal and supraspinal premotor circuits. Most analysis of drive from V2b neurons has focused on the shared contributions of V1s and V2bs to reciprocal inhibition governing flexor/extensor alternation in limbed animals (Britz et al, 2015; Zhang et al, 2014; McCrea and Rybak, 2008) This does not shed light on potential functions of ipsilateral inhibition in gain control for regulation of motor neuron firing during contraction, as opposed to suppression of motor neuron firing during extension. V2b neurons are good candidates for in-phase gain control because they are exclusively inhibitory in mouse and zebrafish (Batista et al, 2008) with ipsilateral, descending axons within the spinal cord (Britz et al, 2015; Lundfald et al, 2007) They arise from a final progenitor division that produces pairs of V2a and V2b neurons (Kimura et al, 2008). Optogenetic suppression of V2b activity elicits faster locomotion whereas optogenetic activation of V2b activity reduces tail frequency, consistent with a role for ipsilateral inhibition in speed control
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