Effects of melatonin and the melatonin receptor agonist S-20098 on the vigilance states, EEG spectra, and cortical temperature in the rat.

The effects of melatonin (3 mg/kg i.p.) and the melatonin receptor agonist S-20098 (3 mg/kg i.p.) on the vigilance states, electroencephalogram power spectra (0.25-25.0 Hz), and cortical temperature were determined in eight rats in the first 6-hr interval of the 12-hr light period. Compared to the vehicle injection both compounds reduced the power density in non-rapid eye movement sleep in the low frequency range (1-8 Hz) but did not affect the vigilance states and brain temperature. The present findings do not indicate that the stimulation of the melatonin receptor exerts a hypnotic effect at doses that had been shown to affect the circadian rest-activity rhythm.