Optogenetic dissection of a behavioural module in the vertebrate spinal cord

In the brief period during which we have known of their existence, light-gated ion channels have been used to assess the function of known cell types to which they are genetically targeted. Here Wyart et al. search for unknown cell types that drive the central pattern generator of locomotion. GAL4 lines of zebrafish in which light-gated glutamate receptors were sparsely expressed in diverse, partially overlapping sets of neurons were screened. Common behavioural effects of light could thus be attributed to activity in a specific cell type when it is the only cell shared between the different lines. The photo-stimulation of one specific cell type, the Kolmer–Agduhr cell, was sufficient to induce a symmetrical tail beating sequence that mimics spontaneous slow forward swimming. Genetically silencing Kolmer–Agduhr cells reduced the frequency of spontaneous free swimming, indicating that Kolmer–Agduhr cell activity provides necessary tone for spontaneous forward swimming. Kolmer–Agduhr cells have been known for over 75 years, but their function has been mysterious. This work shows that during early development in low vertebrates these cells provide a positive drive to the spinal central pattern generator for spontaneous locomotion.