Melanin-concentrating hormone axons, but not orexin or tyrosine hydroxylase axons, innervate the claustrum in the rat: An immunohistochemical study

The claustrum is a small, elongated nucleus close to the external capsule and deep in the insular cortex. In rodents, this nucleus is characterized by a dense cluster of parvalbumin labeling. The claustrum is connected with the cerebral cortex. It does not project to the brainstem, but brainstem structures can influence this nucleus. To identify some specific projections from the lateral hypothalamus and midbrain, we analyzed the distribution of projections labeled with antibodies against tyrosine hydroxylase (TH), melanin-concentrating hormone (MCH), and hypocretin (Hcrt) in the region of the claustrum. The claustrum contains a significant projection by MCH axons, whereas it is devoid of TH projections. Unlike TH and MCH axons, Hcrt axons are scattered throughout the region. This observation is discussed mainly with regard to the role of the claustrum in cognitive functions and that of MCH in REM sleep. J. Comp. Neurol. 525:1489–1498, 2017.