Control of locomotion by the nervous system is essential to adapt behaviour to environmental contexts. Claire Wyart’s team at the ICM, in collaboration with Japanese and Norwegian teams, has used botulinum toxin to make zebra fish neurons selectively silent. Researchers demonstrate that a special population of neurons is required to initiate movement and control swimming speed to explore or escape.
In all animals, locomotion is essential for survival, from searching for food to escaping a predator. The precise control of locomotion is essential to adapt to different environmental contexts.
A crucial question concerns the identification of neuron populations involved in this subtle locomotion control, depending on different movement speeds.
To investigate the role of specific neurons in the locomotor circuit, Claire Wyart’s team has developed a technique to silence in a specific manner large neuron populations in vivo in zebrafish. This fish is a vertebrate sharing 70 % of its genetic heritage with Human. Because of its transparency, this small animal is perfectly adapted to the implementation of all imaging techniques and, in particular, to study its spinal cord without having to extract it.
ICM researchers, in collaboration with a Japanese team, have developed a genetic tool to express, through a transgene, botulinum toxin specifically in V2a interneurons. Researchers can thus remove, without affecting other body cells, these interneurons localised in the brain and spinal cord, and projecting on motor neurons providing muscle contraction during movement. It has allowed them to observe transgenic larvae locomotion behaviour in which V2a neurons do not work, either in fast swimming escape response or during spontaneous swimming.
Escape response, following an acoustic or luminous stimulus, is normally characterised by an increase of swimming speed. In transgenic fish, researchers observe a decrease of this speed compared to control fish. V2A neurons thus have a modulator role which can establish adapted escape speed. Spontaneous behaviour experiences consist in observing the larvae without outside element intervention. The larvae expressing the toxin do not move any more, in contrast to controls. V2A neurons are therefore essential to movement initiation.
These results lead to the conclusion that V2a neurons play a key role in locomotion, since they not only control swimming speed during escape but they are also necessary to movement initiation.
Accurate identification of these neuron populations could have a tremendous impact on the understanding of human motor system functioning because these neurons located within the spinal cord and hindbrain are highly conserved between species.
Reference : Optimization of a Neurotoxin to Investigate the Contribution of Excitatory Interneurons to Speed Modulation In Vivo. Sternberg JR, Severi KE, Fidelin K, Gomez J, Ihara H, Alcheikh Y, Hubbard JM, Kawakami K, Suster M, Wyart C. Curr Biol. 2016 Aug 10.