Distribution of MCH-containing fibers in the feline brainstem: Relevance for REM sleep regulation

Neurons that utilize melanin-concentrating hormone (MCH) as a neuromodulator are localized in the postero-lateral hypothalamus and incerto-hypothalamic area. These neurons project diffusely throughout the central nervous system and have been implicated in critical physiological processes, such as sleep. Unlike rodents, in the order carnivora as well as in humans, MCH exerts its biological functions through two receptors: MCHR-1 and MCHR-2. Hence, the cat is an optimal animal to model MCHergic functions in humans. In the present study, we examined the distribution of MCH-positive fibers in the brainstem of the cat. MCHergic axons with distinctive varicosities and boutons were heterogeneously distributed, exhibiting different densities in distinct regions of the brainstem. High density of MCHergic fibers was found in the dorsal raphe nucleus, the laterodorsal tegmental nucleus, the periaqueductal gray, the pendunculopontine tegmental nucleus, the locus coeruleus and the prepositus hypoglossi. Because these areas are involved in the control of REM sleep, the present anatomical data support the role of this neuropeptidergic system in the control of this behavioral state.