Interneuron diversity and function in the zebrafish locomotor circuits

One of the most essential features of an animal’s behavior is the ability to navigate and interact with its environment. The generation of the locomotor rhythm is carried out by networks of neurons in the spinal cord, so called central pattern generators (CPGs). Deficiencies in these neuronal networks can have detrimental effects on an animal’s well-being and survival. The overall aim of this thesis is to understand how motor neurons and commissural V0 interneurons, influence the operation of the spinal locomotor CPGs. Traditionally, the CPG networks in the spinal cord are thought to be composed of different interneuron classes with homogeneous properties, providing the output to the motor neurons, which are ‘passive recipients’ of the final motor program. In the first study of this thesis, we show that the motor neurons in adult zebrafish can influence the CPG output by retrograde signaling to the rhythm-generating V2a interneurons via gap junctions. This finding suggests that the motor neurons are an integral part of the CPG network instead of a passive component. The two latter studies included in this thesis focus on one of the interneuron classes of the CPG, the V0 interneurons. This interneuron class comprises both excitatory (V0v) interneurons and inhibitory (V0d) interneurons. In the third study of this thesis, we show that the majority of glycinergic V0d interneurons in larval zebrafish are active only at fast frequency locomotion, and that they are homogenous also in terms of their morphologies. By contrast, previous studies in mice suggest that the V0d interneurons are responsible for left-right alternation during slow- frequency locomotion, whereas the V0v interneurons take over this task at high-frequencies. We further show, in this thesis, that the adult zebrafish V0v and V0d sub-populations are heterogeneous and comprise neurons which become active at slow, intermediate, or fast locomotor frequencies. Interestingly, the V0v interneurons had a predominance of fast neurons, whereas the V0d interneurons had a larger proportion of slow neurons. Hence, we show that both the V0v and the V0d sub-populations of the V0 class of interneurons in adult zebrafish are more diverse in their properties than has previously been assumed for the interneuron classes in the spinal cord. Furthermore, the V0d interneurons undergo changes to their morphological characteristics and their activity pattern during locomotion as the fish develops into adulthood. Overall, the work comprising this thesis shows that one of the interneuron classes in the CPG networks, the V0 interneurons, is more diverse than has previously been shown; it can be sub- divided into groups of neurons active at different speeds of locomotion, thereby carrying out different functions from each other in the network. In addition, these interneurons are organized differently at earlier stages of development, indicating that the neurons undergo functional changes during the maturation of the networks. Furthermore, this work has shown that the motor neurons are not merely passive conveyers of upstream generated motor programs, but actively participate, and thus are an integrated component of the CPG networks