Occurrence of ghrelin-producing cells, the ghrelin receptor and Na+,K + -ATPase in tissues of Atlantic halibut (Hippoglossus hippoglossus) during early development.

Ghrelin is a pituitary growth hormone (GH)-secretagogue that also has metabolic, reproductive, proliferative, immunological and brain functions in mammals. Far less is known about its role in fish. We have therefore performed an immunohistochemical determination of its tissue distribution in the developing Atlantic halibut (Hippoglossus hippoglossus) to gain insights into its potential function. Ghrelin immunoreactivity was detected in first-feeding halibut larvae in the skin, urinary bladder, gastrointestinal (GI) tract and olfactory lobe of the brain. In subsequent stages up to metamorphosis, ghrelin immunoreactivity declined in the skin and became evident in the gills. When the stomach developed, ghrelin immunoreactivity declined throughout the GI tract with the exception of the stomach, which exhibited an intense signal. Immunoreactive ghrelin cells were also present in the olfactory lobe, nerve and epithelium and in occasional cells of the buccal cavity and oesophagus. Ghrelin immunoreactivity had an overlapping distribution with that for Na(+),K(+)-ATPase, colocalisation also being observed in some ionocytes of the gill. The co-expression of ghrelin and the GH-secretagogue receptor in the same tissue indicates that ghrelin can exert both endocrine and paracrine actions in the developing halibut. The presence of immunoreactive ghrelin in several osmoregulatory tissues, the GI tract and sensory tissue provides strong evidence that ghrelin has multiple functions during development and also suggests targets for future investigations.