Melanin-concentrating hormone (MCH) is colocalized with α-melanocyte-stimulating hormone (α-MSH) in the rat but not in the human hypothalamus

Abstract Melanin-concentrating hormone (MCH)-containing neurons have recently been localized in the dorsolateral region of the rat hypothalamus, an area where the second α-MSH system is found which contains only α-MSH and none of the pro-opiomelanocortin (POMC)-related peptides. In order to study the morphological relationships between the MCH and α-MSH neuronal systems, we have studied the immunocytochemical localization of both MCH and α-MSH in the rat hypothalamus. The same study was also performed in the human hypothalamus where there is only one α-MSH system which contains α-MSH as well as the other POMC-related peptides (first α-MSH system). In the rat dorsolateral hypothalamus, we could demonstrate that most neuronal cell bodies stained for MCH also contained immunoreactive α-MSH. In the human hypothalamus, neuronal cell bodies stained for MCH were observed only in the periventricular area whereas cell bodies containing α-MSH were exclusively located in the infundibular (arcuate) nucleus. In the rat, immunoelectron microscopy showed labelling for MCH in the dense core vesicles of positive neurons and double-staining techniques clearly demonstrated that both immunoreactive MCH and α-MSH could be consistently detected in the same dense core vesicles. These ultrastructural studies then suggest that these two peptides should be released simultaneously from neurons located in the rat dorsolateral hypothalamus.