The most basic form of locomotion in limbed vertebrates consists of alternating activities of the flexor and extensor muscles within each limb coupled with left/right limb alternation. Although larval zebrafish are not limbed, their pectoral fin movements exhibit the following fundamental aspects of this basic movement: abductor/adductor alternation (corresponding to flexor/extensor alternation) and left/right fin alternation. Because of the simplicity of their movements and the compact neural organization of their spinal cords, 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 set out to 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 were essentially alternating. Second, both abductor and adductor MNs received rhythmic excitatory and inhibitory synaptic inputs in their active and inactive phases, respectively, indicating that the MN spiking activities are controlled in a push-pull manner. Further, we obtained the following 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.

SIGNIFICANCE STATEMENT Pectoral fin movements in larval zebrafish exhibit fundamental aspects of basic rhythmic appendage movement: alternation of the abductor and adductor (corresponding to flexor–extensor alternation) coupled with left–right alternation. We set out to investigate the neuronal circuits underlying rhythmic pectoral fin movements in larval zebrafish. We showed that both abductor and adductor MNs received rhythmic excitatory and inhibitory synaptic inputs in their active and inactive phases, respectively. This indicates that MN activities are controlled in a push-pull manner. We further obtained evidence that dmrt3a-expressing commissural inhibitory neurons exert an inhibitory effect on abductor MNs. The current study marks the first step toward the identification of central pattern generator organization for rhythmic fin movements.

在四肢脊椎动物中最基本的运动形式是每个肢体内屈肌和伸肌交替运动以及左/右肢交替。尽管幼体斑马鱼没有肢体，但它们的胸鳍运动表现出该基本运动的以下基本方面：外展肌/内展肌交替（对应于屈肌/伸肌交替）和左/右鳍交替。由于它们的运动简单并且脊髓的神经结构紧凑，因此斑马鱼可以用作识别控制节律性肢体运动的中央模式发生器（CPG）的神经元网络的良好模型。在这里，我们着手研究幼虫斑马鱼有节奏的胸鳍运动的神经元回路，使用在外展肌或内收肌运动神经元（MNs）和候选CPG神经元中特异性表达GFP的转基因鱼。首先，我们证明了外展肌和内收肌的增高活动实质上是交替的。其次，外展肌和外展肌都分别在活跃期和非活跃期接受节律性兴奋性和抑制性突触输入，这表明以加推方式控制了MN的尖峰活动。此外，我们获得了以下证据，指示以加推方式控制MN尖峰活动。此外，我们获得了以下证据，指示以加推方式控制MN尖峰活动。此外，我们获得了以下证据，表达dmrt3a的连合抑制神经元参与调节外展肌MNs的活性：（1）在dmrt3a神经元与外展性MNs之间发现强抑制性突触连接。（2）dmrt3a神经元的切除改变了外展肌MNs的尖峰时间。因此，在这种简单的外展肌/内展肌交替系统中，源自对侧神经元的末级抑制性输入在控制MNs的发射时间中起着重要的作用。

意义声明幼虫斑马鱼的胸鳍运动表现出基本节律性肢体运动的基本方面：外展肌和内收肌的交替（对应于屈伸交替）以及左右交替。我们着手调查幼虫斑马鱼的有节奏的胸鳍运动的神经元回路。我们显示，外展肌和内收肌MN分别在其活跃期和非活跃期接受节律性兴奋性和抑制性突触输入。这表明以推挽方式控制MN活动。我们进一步获得了dmrt3a的证据表达连合抑制神经元对外展肌MNs有抑制作用。当前的研究标志着迈向确定有节奏的鳍运动的中央模式发生器组织的第一步。