No abstract

No abstract

Bilateral alternation of muscle contractions requires reciprocal inhibition between the two sides of the hindbrain and spinal cord, and disruption of this inhibition should lead to simultaneous activation of bilateral muscles. At 1 day after fertilization, wildtype zebrafish respond to mechanosensory stimulation with multiple fast alternating trunk contractions, whereas bandoneon (beo) mutants contract trunk muscles on both sides simultaneously. Similar simultaneous contractions are observed in wild-type embryos treated with strychnine, a blocker of the inhibitory glycine receptor (GlyR). This result suggests that glycinergic synaptic transmission is defective in beo mutants. Muscle voltage recordings confirmed that muscles on both sides of the trunk in beo are likely to receive simultaneous synaptic input from the CNS. Recordings from motor neurons revealed that glycinergic synaptic transmission was missing in beo mutants. Furthermore, immunostaining with an antibody against GlyR showed clusters in wild-type neurons but not in beo neurons. These data suggest that the failure of GlyRs to aggregate at synaptic sites causes impairment of glycinergic transmission and abnormal behavior in beo mutants. Indeed, mutations in the GlyR ␤-subunit, which are thought to be required for proper localization of GlyRs, were identified as the basis for the beo mutation. These data demonstrate that GlyR␤ is essential for physiologically relevant clustering of GlyRs in vivo. Because GlyR mutations in humans lead to hyperekplexia, a motor disorder characterized by startle responses, the zebrafish beo mutant should be a useful animal model for this condition.channel ͉ synapse ͉ hyperekplexia ͉ strychnine Z ebrafish embryos display three stereotyped behaviors by 36 h postfertilization (hpf) (1, 2). The earliest behavior consists of spontaneous, alternating coiling of the tail. This slow coiling behavior is independent of sensory stimulation and starts at 17 hpf and declines by 26 hpf. After 21 hpf embryos start to respond to mechanosensory stimulation with the two or three rapid trunk contractions that constitute the escape response. After 26 hpf, mechanosensory stimulation starts to initiate swimming episodes. The frequency of muscle contractions during swimming increases from 7 Hz at 26 hpf to 30 Hz at 36 hpf, the latter being comparable with the frequency of swimming in adult zebrafish (3).The large-scale Tübingen mutagenesis screen isolated 63 zebrafish mutants with abnormal touch responses (4). Amongst them, mutations in seven genes, including accordion, zieharmonika͞ache, and bandoneon (beo) were classified as accordion-type mutants. All mutations in this class displayed apparent simultaneous muscle contractions in both sides of the trunk, resulting in the shortening of the trunk in response to touch. Because this class of mutants was phenocopied by exposing wild-type animals to strychnine, a glycine receptor (GlyR) blocker, accordion-type mutants were predicted to have defects in inhibitory synaptic transmission within the CNS...

The anatomy of the developing zebrafish spinal cord is relatively simple but, despite this simplicity, it generates a sequence of three patterns of locomotive behaviors. The first behavior exhibited is spontaneous movement, then touch-evoked coiling, and finally swimming. Previous studies in zebrafish have suggested that spontaneous movements occur independent of supraspinal input and do not require chemical neurotransmission, while touch-evoked coiling and swimming depend on glycinergic neurotransmission as well as supraspinal input. In contrast, studies in other vertebrate preparations have shown that spontaneous movement requires glycine and other neurotransmitters and that later behaviors do not require supraspinal input. Here, we use lesion analysis combined with high-speed kinematic analysis to re-examine the role of glycine and supraspinal input in each of the three behaviors. We find that, similar to other vertebrate preparations, supraspinal input is not essential for spontaneous movement, touch-evoked coiling, or swimming behavior. Moreover, we find that blockade of glycinergic neurotransmission decreases the rate of spontaneous movement and impairs touch-evoked coiling and swimming, suggesting that glycinergic neurotransmission plays critical yet distinct roles for individual patterns of locomotive behaviors.