Fig 1

(A-B) Confocal projection of immunolabelling with MCH in the adult zebrafish brain (A) and pituitary gland (B). The outline area represents the hypothalamic MCH projection at immediate vicinity of the adult zebrafish pituitary, lost during the tissue preparation. (C, D) Confocal projection of immunolabelling with EGFP in Tg(mch2:egfp) hypothalamus (C) or with the MCH2 antibody (D) at 5 dpf, reveals that MCH2 neurons in the hypothalamus have axonal projections on the pituitary gland. (E, E’) Confocal projection of double immunolabelling with MCH2 and α-MSH at 7 dpf, showing MCH cellular projections on the α-MSH+ domain (E). Schematic representation of the ventral part of the zebrafish brain showing the hypothalamic neurons expressing MCH2 targeting the posterior part of the pituitary gland expressing α-MSH (neurohypophysis) (E’). (F) Confocal projection of double immunolabelling with Oxtl and EGFP in Tg(mch2:egfp) hypothalamus at 7 dpf, demonstrating that MCH neurons target the neurohypophysis. (G, G’) Confocal projection of double immunolabelling with MCH2 and blood vessels at 5 dpf, suggesting that MCH peptide can be released in the bloodstream (G). Schematic representation of the ventral part of the zebrafish brain showing the hypothalamic MCH2 terminal projections targeting the neurohypophysis surrounded by blood vessels (Hypothalamo-Hypophyseal system) (G’). Ventral (A-C) or (D-F) dorsal view with anterior up. Scale bars: 50 μm (A) or 10 μm (B-G).