Whole zebrafish at 7 dpf or 4 wpf, or dissected zebrafish eyes at 8 wpf, were fixed in 4% paraformaldehyde (PFA) in PBS for 3 hours at room temperature, or overnight at 4°C. They were cryoprotected in 30% sucrose in PBS, embedded in Tissue-Tek OCT compound, and cryosectioned (12 µm; CM 1860 Cryostat; Leica, Wetzlar, Germany). For wild-type and pdzk1-KO larvae, antibody staining was carried out at room temperature using standard protocols. Antigen retrieval was performed by incubating slides in boiled 10-mM sodium citrate (pH 6) until the solution was cooled to room temperature. Slides were blocked in 5% fetal bovine serum (FBS) for 30 minutes and incubated overnight in rabbit anti-PSD-95 (ab18258, 1:100; Abcam, Cambridge, UK), anti-CRFR1 (ab59023, 1:100; Abcam), anti-GluN1 (to label an NMDA receptor subunit; 114 011, 1:400; Synaptic Systems, Goettingen, Germany), or anti-serotonin (ab66047, 1:500; Abcam) primary antibodies diluted in FBS. Slides were subsequently incubated for 2 hours in secondary antibodies (all 1:500; Thermo Fisher Scientific, Waltham, MA, USA) diluted in 5% FBS. The secondary antibodies used were Goat anti-Rabbit Alexa Fluor 488 (A11008) or Alexa Fluor 546 (A11010), Goat anti-Mouse Alexa Fluor 488 (A11001), and Donkey anti-Goat Alexa Fluor 488 (A11055). Nuclei were counterstained with 4′,6-diamidino-2-phenylindole (DAPI; D9542-10MG, 1:10000; Sigma-Aldrich) in PBS for 20 minutes, and sections were mounted in Mowiol (81381-250G; Sigma-Aldrich). For Tg(ptf1a:GFP) and pdzk1-KO/Tg(ptf1a:GFP) larvae, samples were stained with DAPI only, following cryostat sectioning.