Electron microscopic identification of axons containing melanin-concentrating hormone in the lamprey, Lampetra fluviatilis L.

Melanin-concentrating hormone-like immunoreactivity (MCH-LI) was examined in the hypothalamo-hypophysial region of the lamprey. At the light microscopic level, MCH-LI neuronal cell bodies were shown with the peroxidase anti-peroxidase method; they were seen in the hypothalamic region close to the third ventricle. MCH-LI axons were observed to run toward the proximal neurosecretory contact region (PNCR) and the most proximal part of the neurohypophysis. Electron microscopic analysis with the immunogold method revealed that presumed secretory granules within axonal profiles showed MCH-LI. MCH-LI axonal profiles were most often seen on or near the basement membrane of connective tissue layer separating the PNCR from the hypophysial pars distalis.