ELSEVIER Neuroscience Letters 214 (1996) 123-126 lililHSCiiliCl LEII S Hypnotic and hypothermic effects of melatonin on daytime sleep in humans: lack of antagonism by flumazenil Rachel Nave, Paula Herer, Iris Haimov, Arie Shlitner, Peretz Lavie* Sleep Laboratory, Bruce Rappaport Faculty of Medicine, Gutwirth Building, Technion-lsrael Institute of Technology, Haifa, 32000, Israel Received 29 April 1996; revised version received 12 July 1996; accepted 12 July 1996 Abstract In this double-blind, placebo-controlled study we investigated whether 10 mg flumazenil, a pure benzodiazepine antagonist, can block the hypnotic and hypothermic effects of 3 mg melatonin. The design comprised four 7-h (1200-1900 h) testing periods, preceded by a 'no-treatment' adaptation period of the '7/13' sleep-wake paradigm. Six young healthy adult males were paid to participate. During each experimental period, tablets were administered at 1145 h (flumazenil or placebo) and at 1200 h (melatonin or placebo) in a randomized, double-blind, partially repeated Latin square design. Polysomnographic recordings and core body temperature recordings revealed that melatonin, either in combination with placebo or with flumazenil, significantly increased the amounts of sleep, and decreased core body temperature in comparison with placebo alone or the combination of flumazenil plus placebo. These results do not support the hypothesis that melatonin exerts its hypothermic and hypnotic effects via the central benzodiazepine receptors. Keywords: Melatonin; Flumazenil; Hypnotic effects; EEG spectral density Recent studies have shown that melatonin, an indolea- mine hormone synthesized and secreted by the pineal gland primarily at night [13], possesses hypnotic effects when administered at times when endogenous levels of melatonin are low, or :in melatonin-deficient insomnia patients [1,5,8,15,21]. Although the mechanism by which melatonin induces sleep is not fully elucidated, there are at least three possible explanations: phase shift- ing of the underlying sleep-wake oscillator, hypothermia, and direct effects on brain somnogenic structures [3]. Recently, results from our laboratory [10,16] and by Dijk et al. [4] demonstrated that melatonin affected elec- troencephalogram (EEG) spectra during sleep in the direc- tion of increased power in the tr EEG band and decreased power in the O and ~ EEG bands. These results are similar to the effects of benzodiazepine hypnotics on EEG, and have raised the possibilit3, that melatonin may induce sleep by acting through the benzodiazepine sites on the GABAA benzodiazepine receptor complex. The imidazodiazepine derivative flumazenil fRo 15- * Corresponding author. Tel.: +972 4 8226695; fax: +972 4 8323045; e-mail: plavie@tx.technion.ac.il 1788) is considered to be a pure benzodiazepine receptor antagonist which is devoid of major intrinsic effects. The present study aimed to investigate whether flumazenil blocks the daytime hypnotic and hypothermic effects of melatonin in normal young adults. Six healthy males (age 24.5 + 0.9 years) participated in the study. All were students living on campus; none had used any kind of drugs or medications for at least 3 months prior to the beginning of the experiment. All were free of sleep complaints. They were requested to refrain from daytime sleep for 2 weeks before the start of the experi- ment, as well as during its 5-week duration. Actigraphic recordings were collected 3 days before each of the experi- mental periods for behavior control. On the experimental day, subjects were requested to avoid major physical exertion, and to abstain from alco- holic or caffeinated drinks. The study comprised four experimental periods pre- ceded by a no-treatment adaptation period of the '7/13' ultrashort sleep-wake paradigm [9]. In this paradigm, sub- jects are instructed to attempt to sleep for 7 min every 20 min during the duration of the experimental period. The amount of sleep in each 7-min trial is used as a measure of 0304-3940/96]$12.00 © 1996 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(96) 12899-8