Knockdown of the Zebrafish OPRK1 Orthologue Results in Hyperactivity during Development. (A) Activity heatmap illustrating sponta- neous movement of 28 hpf embryos inside the chorion during a 10 seconds period. Higher levels of activity are evident in embryos injected with oprk1 splice-blocking AMO (spOPRK1) or ATG-targeting AMO (OPRK1 KD) when compared to mismatch AMO-injected embryos (Mismatch control). Expression of human OPRK1 transcript (hOPRK1) rescues the hyperactivity phenotype. (B) RT-PCR product of amplification of a region spanning exons 2 and 3 of the zebrafish oprk1 transcript. Injection of the splice-blocking AMO (spOPRK1) results in a defective splicing in this region. (C) Quantification of the total activity of 28 hpf zebrafish embryos. (D) Quantification of the frequency of movement bursts in the 28 hpf embryos.