Interaction of orexin/hypocretin-like immunoreactive neurons with melanin-concentrating hormone and α-melanocyte-stimulating hormone neurons in brain of a pleuronectiform fish, barfin flounder

Immunohistochemical localization of orexin/hypocretin in the brain of a pleuronectiform fish, the barfin flounder Verasper moseri was examined as the first step in unraveling the possible function of the hormone in the brain. Orexin-A-like immunoreactive (ir) cell bodies were found to be located in the nucleus posterioris periventricularis (NPPv) of the hypothalamus, and orexin-A-like-ir fibers were detected not only in the hypothalamus but also extensively throughout the brain. The orexin-A-like-ir cell bodies did not project their fibers to the pituitary gland. Since melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) are suggested to regulate food intake in addition to orexin/hypocretin in the teleost fish, it was examined whether neural connections exist between orexin neurons and the MCH and alpha-MSH neurons in the barfin flounder brain by using double-staining immunohistochemistry. Some orexin-A-like-ir fibers were in close contact with the MCH-ir and alpha-MSH-ir cell bodies in the hypothalamus. Moreover, a few MCH-ir and alpha-MSH-ir fibers were in close contact with the orexin-A-like-ir cell bodies in the hypothalamus. These results suggest that reciprocal connections exist between the orexin and MCH neurons and between the orexin and alpha-MSH neurons in the brain of the barfin flounder.