Effects on Sleep of Melanin-Concentrating Hormone (MCH) Microinjections into the Dorsal Raphe Nucleus of the Cat

The melanin-concentrating hormone (MCH) is a neuromodulatory peptide present in neurons located in the postero-lateral hypothalamus and incerto-hypothalamic area. These neurons project widely throughout the central nervous system, including the serotonergic dorsal raphe nucleus (DR), a waking-promoting and REM sleep permissive region. Although it is presently accepted that the MCHergic system is involved in the generation of sleep, most studies have been done in rodents, were only the MCH receptor type 1 (MCHR-1) is functionally active. In humans, as well as in carnivores, the MCHR-2 is also a functional receptor. A previous study performed in rats showed that MCH microinjections into the DR induce a significant increase of REM sleep and slow wave sleep (SWS). However, there are no studies on the effect on sleep of MCH applied into the DR of species where both types of MCH receptors are functional.The aim of this work is to confirm the hypothesis that MCH administration into the DR of the cat also promotes sleep. To this purpose, we performed control or MCH (1 µg in 0.2 µl) microinjections into the DR of animals prepared for chronic polysomnographic recordings. Compared to control, MCH produced a significant increase of total sleep time during the first recording hour (43.2 ± 9.5 Vs. 36.6 ± 11.8 min; p < 0.05). In addition, the number of REM sleep episodes was significantly augmented during the first hour and the total recording time (5 h), while SWS latency was decreased (3.5 ± 2.3 Vs. 6.3 ± 4.3 min; p < 0.05). We conclude that the administration of MCH into the DR of the cat promotes sleep.