Immunocytochemical localization and ontogenic development of melanin-concentrating hormone in the brain of a pleuronectiform fish, the barfin flounder

Melanin-concentrating hormone (MCH) was first discovered in the pituitary of chum salmon because of its role in the regulation of skin pallor. Later, it was found that MCH could also play a role as a central neurotransmitter or neuromodulator in the brain. However, knowledge of the function of MCH in fish has been restricted to certain fish species. Therefore, in the present study, the immunocytochemical localization and ontogenic development of MCH in the brain of a pleuronectiform fish, the barfin flounder Verasper moseri, were examined to obtain a better understanding of this hormone. In adult barfin flounder, MCH-immunoreactive (ir) neuronal somata were most prevalent in the magnocellular neurons of the nucleus tuberis lateralis (NLT), which project to the pituitary. In the pituitary, MCH-ir fibers were distributed in the neurohypophysial tissues within the pars intermedia and, to a lesser extent, into the pars distalis. MCH-ir neuronal somata were also present in dorsally projecting parvocellular neurons, located more posteriorly in the area above the lateral ventricular recess (LVR). LVR-MCH neurons did not seem to project to the pituitary. In the brain, MCH-ir fibers were detected not only in the hypothalamus but also in areas such as the optic tectum and thalamus. MCH-ir neuronal somata and fibers were not detected on the day of hatching. MCH-ir neuronal somata and fibers were first detected in the hypothalamus and the pituitary, respectively, 7 days after hatching. Subsequently, MCH-ir neuronal somata were observed in the NLT and in the area above the LVR 14 days after hatching. The distribution of MCH-ir neuronal somata and fibers showed a pattern similar to that in the adult fish 35-42 days after hatching. These results indicate that MCH neurons were located in the NLT and in the area above the LVR and that NLT-MCH neurons project to the pituitary. MCH neurons were first detected 7 days after hatching, suggesting that MCH plays some physiological role in the early development of barfin flounder.