Hindbrain V2a neurons in the excitation of spinal locomotor circuits during zebrafish swimming

Background: During locomotion in vertebrates, reticulospinal neurons in the hindbrain play critical roles in providing descending excitation to the spinal cord locomotor systems. However, despite the fact that many genes that are used to classify the neuronal identities of neurons in the hindbrain have been identified, the molecular identity of the reticulospinal neurons that are critically involved in locomotor drive is not well understood. Chx10-expressing neurons (V2a neurons) are ipsilaterally projecting glutamatergic neurons in the spinal cord and the hindbrain. Many of the V2a neurons in the hindbrain are known to project to the spinal cord in zebrafish, making hindbrain V2a neurons a prime candidate in descending locomotor drive. Results: We investigated the roles of hindbrain V2a neurons using optogenetic and electrophysiological approaches. The forced activation of hindbrain V2a neurons using channelrhodopsin efficiently evoked swimming, whereas the forced inactivation of them using Archearhodopsin3 or Halorhodpsin reliably stopped ongoing swimming. Electrophysiological recordings of two populations of hindbrain reticulospinal V2a neurons showed that they were active during swimming. One population of neurons, small V2a neurons in the caudal hindbrain, fired with low rhythmicity, whereas the other population of neurons, large reticulospinal V2a neurons, called MiV1 neurons, fired more rhythmically. Conclusions: These results indicated that hindbrain reticulospinal V2a neurons play critical roles in providing excitation to the spinal locomotor circuits during swimming by providing both tonic and phasic inputs to the circuits.