Role of nitric oxide in mitochondrial energetics and stress/ease-responsive neuroendocrine receptor expression in developing zebrafish larvae.

As an essential gaseous neurotransmitter, nitric oxide (NO) regulates various physiological processes in vertebrates, including fish. It is still being determined how NO signals neuroendocrine functions during larval development in zebrafish. Therefore, we investigated the effects of N-omega-nitro-l-arginine methyl ester (l-NAME), a competitive inhibitor of nitric oxide synthase, on developing zebrafish larvae (3, 4, and 5 days post-fertilisation; dpf) concerning mitochondrial energetics, NO and ROS status, and transcript expression of stress and ease-responsive receptor genes to understand the role of NO in zebrafish larval development. A decline in NO content was observed at all larval stages after l-NAME exposure (4.3 and 43 nmol L-1 for 30 min), except at 4.3 nmol L-1 in 3 dpf larvae, where it increased. l-NAME reduced ROS production but increased the activities of cytochrome C oxidase and succinate dehydrogenase in the mitochondria of zebrafish larvae, depending on larval stage. The transcript expression of stress and ease-responsive receptor genes, including thyroid hormone receptors (thraa, thrb), mineralocorticoid receptor (nr3c2), serotonin transporter (slc6a4a), serotonin receptor (htr2b), and melatonin receptor (mtnr1aa), showed activation after l-NAME (4.3 nmol L-1) in 3 dpf larvae. Similarly, the activation of hypoxia-inducible factor 1 alpha (hif1a), hif1a-antisense RNA (hif1an), nnos, and inos transcript expression by l-NAME confirmed a role for NO in developing zebrafish larvae. These data provide novel evidence for the integrative role of NO in enhancing developmental plasticity in zebrafish larvae by modulating mitochondrial energetics and the functionality of stress- and ease-responsive genes within the neuroendocrine receptor system.