Zebrafish were treated with 4-HT as previously described (Kikuchi et al., 2011). Briefly, at 24 hpf, the embryos were placed in embryo medium supplemented with 5 μM 4-HT produced from a 1 mM 4-HT stock solution in 100% ethanol. After 24 hr, the embryos were transferred to fresh 4-HT-containing medium for an additional 24 hr.