Limbed vertebrates exhibit the basic form of locomotion, consisting of alternating activities of the flexor and extensor muscles within each limb coupled with left/right limb alternation. Zebrafish pectoral fin movements show the fundamental aspects of this basic movement: abductor/adductor alternation (corresponding to flexor/extensor alternation) and left/right fin alternation. Thus, zebrafish can serve as a good model to identify the neuronal networks of the central pattern generator (CPG) that controls rhythmic appendage movements. Here, we investigate neuronal circuits underlying rhythmic pectoral fin movements in larval zebrafish, using transgenic fish that specifically express GFP in abductor or adductor motor neurons (MNs) and candidate CPG neurons. First, we showed that spiking activities of abductor and adductor MNs are essentially alternating. Second, both abductor and adductor MNs receive rhythmic excitatory and inhibitory synaptic inputs in their active and inactive phases, respectively, indicating that the MN activities are controlled in a push-pull manner. Further, we obtained the evidence that dmrt3a-expressing commissural inhibitory neurons are involved in regulating the activities of abductor MNs: (1) strong inhibitory synaptic connections were found from dmrt3a neurons to abductor MNs; and (2) ablation of dmrt3a neurons shifted the spike timing of abductor MNs. Thus, in this simple system of abductor/adductor alternation, the last-order inhibitory inputs originating from the contralaterally located neurons play an important role in controlling the firing timings of MNs.

This study was conducted as collaboration with Professor Shin-ichi Higashijima at National Institute for Basic Biology. This study was partially supported by NBRP and NBRP/Fundamental Technologies Upgrading Program from the Japan Agency for Medical Research and Development.

Figure: Rhythmic pectoral fin movements in 3 dpf larvae.

Figure: A1, A2 Transgenic fish that labeled abductor motor neurons.
B1, B2 Transgenic fish that labeled abductor motor neurons.