Effects of 5-HT1cR activation in zebrafish embryos. (A) Injection of Xwnt-5A RNA and then Xwnt-8 RNA blocks the ability of Xwnt-8 to induce ectopic goosecoid, and embryos develop with relatively normal single domains of goosecoid expression (single arrowheads). (B) Injection of globin RNA and then Xwnt-8 does not block the activity of Xwnt-8, and embryos develop with expanded and radialized domains of goosecoid expression (double arrowheads per embryo). (C) Injection of 5-HT1cR RNA and then Xwnt-8, in the presence of the agonist 5-HT, blocks goosecoid induction like Xwnt-5A, and most embryos develop with single domains of goosecoid (single arrowheads). (D) Injections as in C but treatment with the antagonist mianserin to prevent activation of the serotonin receptor reveals that Xwnt-8 is now able to induce ectopic domains of goosecoid (double arrowheads). (E,F) A single control embryo is shown stained for Krox-20 (E, arrowhead), while the other three embryos were injected with 5HT1cR RNA and then treated with the agonist 5- HT (E, asterisks). The resulting Krox-20 patterns (E) resemble those obtained by injection of Xwnt-5A RNA (F).