Effects of the CLIP fragment ACTH 20–24 on the duration of REM sleep episodes

Substances acting upon rapid eye movement (REM) sleep or paradoxical sleep (PS) can affect the number and/or the duration of PS episodes. In the present study, we investigated the effects of the proopiomelanocortin-derived peptide CLIP (corticotropin-like intermediate lobe peptide, ACTH 18–39) and its N-terminal fragments ACTH 18–24 and ACTH 20–24 on the duration of PS episodes in rats. Intracerebroventricular injection of ACTH 20–24 caused a pronounced prolongation of PS episodes (up to 7 min duration, never seen under baseline conditions), whereas ACTH 18–24 acted in a similar way but without reaching statistical significance. We suggest that short N-terminal CLIP fragment(s) may represent endogenous hypnogenic factor(s) involved in the regulation of paradoxical sleep.