Neuropeptide glutamic acid-isoleucine (NEI)-induced paradoxical sleep in rats

•Neuropeptide glutamic acid-isoleucine (NEI) and melanin-concentrating-hormone (MCH) are cleaved from prepro-MCH.•The ventrolateral periaqueductal gray (vlPAG) is one of the sites for REM sleep.•Microinjection of NEI and MCH into the vlPAG increase REM episodes.•NEI and MCH induce REM sleep at least in part through acting on the vlPAG.

Neuropeptideglutamic acid-isoleucine (NEI) as well as melanin concentrating hormone (MCH) is cleaved from the 165 amino acid protein, prepro-melanin concentrating hormone (prepro-MCH). Among many physiological roles of MCH, we demonstrated that intracerebroventricular (icv) injection of MCH induced increases in REM sleep episodes as well as in non REM sleep episodes. However, there are no studies on the effect of NEI on the sleep-wake cycle. As for the sites of action of MCH for induction of REM sleep, the ventrolateral periaqueductal gray (vlPAG) has been reported to be one of its site of action. Although MCH neurons contain NEI, GABA, MCH, and other neuropeptides, we do not know which transmitter(s) might induce REM sleep by acting on the vlPAG. Thus, we first examined the effect of icv injection of NEI on the sleep-wake cycle, and investigated how microinjection of either NEI, MCH, or GABA into the vlPAG affected REM sleep in rats. Icv injection of NEI (0.61 μg/5 μl: n = 7) significantly increased the time spent in REM episodes compared to control (saline: 5 μl; n = 6). Microinjection of either NEI (61 ng/0.2 μl: n = 7), MCH (100 ng/0.2 μl: n = 6) or GABA (250 mM/0.2 μl: n = 7) into the vlPAG significantly increased the time spent in REM episodes and the AUC. Precise hourly analysis of REM sleep also revealed that after those microinjections, NEI and MCH increased REM episodes at the latter phase, compared to GABA which increased REM episodes at the earlier phase. This result suggests that NEI and MCH may induce sustained REM sleep, while GABA may initiate REM sleep. In conclusion, our findings demonstrate that NEI, a cleaved peptide from the same precursor, prepro-MCH, as MCH, induce REM sleep at least in part through acting on the vlPAG.