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Melanin-concentrating hormone (MCH)-containing neurons directly innervate the adenohypophysis in the teleost pituitary. We examined immunohistochemically the relationship between MCH-containing nerve fibres or endings and somatolactin (SL)-producing cells in the goldfish pituitary. Nerve fibres or endings with MCH-like immunoreactivity were identified in the neurohypophysis in close proximity to the adenohypophysial cells showing SL-like immunoreactivity. We also examined the effect of MCH on SL release from cultured goldfish pituitary cells and SL synthesis using a cell immunoblot and a real-time PCR method. Treatment of individually dispersed pituitary cells with MCH 10 K7 M for 3 h decreased the area of SL-like immunoreactivity on immunoblots, and MCH-induced reductions in SL release were blocked by treatment with the mammalian MCH receptor (MCHR) antagonist, compound-30, at a concentration of 10 K5 M.Treatment with 10 K7 M MCH for 3 h did not affect sl-a and -b (smtla and -b as given in the Zfin Database) mRNA expression levels. These led us to explore the signal transduction mechanism leading to the inhibition of SL release, for which we examined whether MCH-induced reductions in SL release are mediated by the G i or G q proteincoupled signalling pathway. The MCH-induced reductions in SL release were abolished by treatment with the G i/o protein inhibitors, NF023 (10 K5 M) or pertussis toxin (260 ng/ml), but not by the phospholipase C inhibitor, U-73122 (3!10 K6 M). These results indicate that MCH can potentially function as a hypothalamic factor suppressing SL release via the MCHR, and subsequently through the G i protein to inhibit the adenylate cyclase/cAMP/protein kinase A-signalling pathway in goldfish pituitary cells.

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