Effect of knocking down dre-oprm1 on the expression pattern of the opioid precursors. The expression of proenkephalins (dre-penk a and dre-penk b) and proopiomelanocortin (dre-pomc a) was assayed in dre-oprm1 morphants of 24 and 48 hpf. Embryos are oriented anterior toward the left and posterior toward the right. Arrowheads indicate the border between tegmentum and optic tectum; and arrows indicate the most posterior area of the medulla oblongata. A total of 25 embryos were used for each treatment and for each time point, and the experiment was repeated three times. mor MO, morpholino designed against dre-oprm1 gene.