Vasoactive intestinal polypeptide promotes sleep without effects on brain temperature in rats at night

The possible sleep-promoting activity of vasoactive intestinal polypeptide (VIP) was contrasted with the physiological sleep suppression in the diurnal active period through the i.c.v. injection of 100 ng VIP into rats at dark onset. The sleep-wake activity and brain temperature (Tbr) were recorded for 24 h (dark period and light period, 12 h each), and the effects were evaluated with respect to records obtained after artificial cerbrospinal fluid injection. Without altering the normal course of Tbr, VIP induced a prompt and persistent increase of sleep. Wakefulness was significantly suppressed and non-REM sleep increased for 6 h, while REM sleep increased for 3 h. The obvious sleep-promoting action of VIP, unrelated to thermoregulatory effects, supports the notion that the peptide might be involved in sleep regulation.