  The cerebrospinal fluid (CSF) constitutes an interface through which chemical
cues can reach and modulate the activity of neurons located at the epithelial
boundary within the entire nervous system. Here, we investigate the role and
functional connectivity of a class of GABAergic sensory neurons contacting the
CSF in the vertebrate spinal cord and referred to as CSF-cNs. The remote
activation of CSF-cNs was shown to trigger delayed slow locomotion in the
zebrafish larva, suggesting that these cells modulate components of locomotor
central pattern generators (CPGs). Combining anatomy, electrophysiology, and
optogenetics in vivo, we show that CSF-cNs form active GABAergic synapses onto
V0-v glutamatergic interneurons, an essential component of locomotor CPGs. We
confirmed that activating CSF-cNs at rest induced delayed slow locomotion in
the fictive preparation. In contrast, the activation of CSF-cNs promptly
inhibited ongoing slow locomotion. Moreover, selective activation of rostral
CSF-cNs during ongoing activity disrupted rostrocaudal propagation of
descending excitation along the spinal cord, indicating that CSF-cNs primarily
act at the premotor level. Altogether, our results demonstrate how a spinal
GABAergic sensory neuron can tune the excitability of locomotor CPGs in a
state-dependent manner by projecting onto essential components of the
excitatory premotor pool.
