Motor control in zebrafish : excitatory drive and developmental changes

Abstract: An essential characteristic of human and animal life is the ability to move from one place to another, in order to survive in a complex environment. All the different forms of locomotion, like walking, swimming, crawling and flying, have one common feature: rhythmic and alternating movements of the body. These movements are generated by neuronal networks in the spinal cord. The overall aim of this thesis is to investigate the mechanisms underlying locomotion in zebrafish, with particular focus on excitatory drive and developmental changes. Excitatory interneurons are believed to represent the core components for the generation of the locomotor rhythm, since they drive both inhibitory interneurons and motoneurons. By ablating one specific group of interneurons, the V2a interneurons, we show that they represent an intrinsic source of excitation necessary for the normal expression of the locomotor rhythm. Ablation of V2a interneurons results in an increase in the threshold to induce swimming and a decrease in swimming frequency and episode duration. To demonstrate that the excitatory drive from ipsilateral premotor V2a interneurons is also sufficient to drive swimming, we used optogenetics to activate the V2a interneurons specifically. Upon illumination, V2a interneurons displayed rhythmic oscillations that resembled the typical beat-and-glide swimming. Peripheral nerve recordings confirmed that the bursting activity in single neurons corresponds to swimming activity, which is characterized by left-right-alternation and rostrocaudal delay. This indicates that swimming activity emerges from the activity of an underlying V2a interneuron network. The third aim of this thesis is to reveal the developmental changes of the swimming pattern and the motoneuron properties. By systematically recording peripheral nerve activity and primary motoneuron properties during different developmental stages, we were able to define the time frame of the switch in swimming behavior from larval episodic to adult continuous swimming to 4-5 weeks post fertilization. Primary motoneurons stop participating in swimming within the same time window and are from around 6 weeks onward only active during escape behavior. In conclusion, we show that the excitatory V2a interneurons in zebrafish are necessary and sufficient for the rhythm generating network to generate a coordinated swimming motor pattern and that there is a major switch in the locomotor pattern and primary motoneuron recruitment around 4-5 weeks of development.