Connectivity pattern suggests that incerto-hypothalamic area belongs to the medial hypothalamic system

The incerto-hypothalamic area (IHy) is a poorly defined diencephalic region located at the junction of the medial hypothalamus and zona incerta (ZI). This region is characterized by the presence of the A13 dopaminergic group and also cells expressing melanin-concentrating hormone (MCH) and cocaine- and amphetamine-regulated transcript (CART). The dopaminergic neurons appear to influence luteinizing hormone secretion, but the role of the MCH/CART-expressing cells is unclear. Even though IHy presents a singular neurochemistry, it has long been assumed that it is also part of the zona incerta. By injecting biotinylated dextran amine into the IHy and ZI of adult male Wistar rats, we analyzed the efferent projections from the IHy in comparison to the ZI. We have found that ZI projects mainly to laterally located brain stem structures, whereas the main efferents from the IHy are the reuniens thalamic nucleus, precommissural nucleus, posterior hypothalamic area and dorsolateral periaqueductal gray matter. The IHy projection pattern is quite similar to that of the anterior hypothalamic area and our hodological results suggest that IHy belongs to the medial hypothalamic system and might be part of the defensive behavior system. The IHy could be an integrative area associated with the regulation of neuroendocrine functions related to motivated behaviors, which are mediated by the medial hypothalamus.