Melatonin modulates the hypothalamic-pituitary neuroendocrine axis to regulate physiological color change in teleost fish

Melatonin (MT) is a crucial neuroendocrine regulator of various physiological activities in vertebrates, especially in circadian or seasonal rhythm control. In the present study, the large yellow croaker (Larimichthys crocea), a marine bony fish with circadian body color change behavior, is chosen for functional investigation on teleost MT signaling systems that remain uncharacterized. All five melatonin receptors (LcMtnr1a1, LcMtnr1a2, LcMtnr1b1, LcMtnr1b2, and LcMtnr1c) were significantly activated by MT, triggering extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation through different G protein coupling signaling pathways, with exclusive G(αi)-dependency for LcMtnr1a2 and LcMtnr1c, and G(αq)-dependency for two LcMtnr1b paralogs, whereas LcMtnr1a1 activated G(αi) and G(αs) dual-dependent signaling pathways. A comprehensive model of the MT signaling system in the hypothalamic-pituitary neuroendocrine axis was further constructed based on ligand-receptor interaction analysis using single-cell RNA-seq data, as well as spatial expression patterns of Mtnrs and related neuropeptides in central neuroendocrine tissues. A novel regulatory pathway of MT/melanin-concentrating hormone (MCH) and MT/(tachykinin precursor 1 (TAC1)+corticotropin-releasing hormone (CRH))/melanocyte-stimulating hormone (MSH) was discovered that functions in chromatophore mobilization and physiological color change and was further validated by pharmacological experiments. Together, our findings define multiple intracellular signaling pathways mediated by L. crocea melatonin receptors and provide the first in-depth evidence that uncover the upstream modulating roles of the MT signaling system in the hypothalamic-pituitary neuroendocrine axis of a marine teleost species, particularly in chromatophore mobilization and physiological color change.