Immunohistochemical localization of orexin/hypocretin-like immunoreactive peptides and melanin-concentrating hormone in the brain and pituitary of medaka

Orexin/hypocretin is a neuropeptide that is involved in the regulation of feeding behavior and the sleep–wakefulness cycle in mammals. Melanin-concentrating hormone (MCH) is believed to be another candidate involved in food intake in teleost fish as well. Thus, it is interesting to examine whether neural connections exist between the neurons producing these two hormones. We first examined the localization of orexin-like immunoreactivity (orexin-LI) in the brain of the medaka Oryzias latipes by using immunohistochemistry. We further examined the interaction between the orexin and MCH neurons in the medaka brain by performing double-staining immunohistochemistry. Orexin-LI cell bodies were located in the nucleus posterioris periventricularis (NPPv) of the hypothalamus, and orexin-LI fibers were detected not only in the hypothalamus but also extensively throughout the brain. Some orexin-LI fibers were in close contact with the MCH-immunoreactive (ir) cell bodies in the hypothalamus, as revealed by double-staining immunohistochemistry. Moreover, a few MCH-ir fibers were in close contact with the orexin-LI cell bodies. These results suggest that in the medaka brain, orexin performs various functions, including neuromodulation, and that neural connections exist between the orexin and MCH neurons.