Acute and long-term effects of the 5-HT2 receptor antagonist ritanserin on EEG power spectra, motor activity, and sleep:: changes at the light-dark phase shift

Parallel effects of a single injection of the 5-HT2 receptor antagonist ritanserin on EEG power spectra, sleep and motor activity were measured for a 20-h period in freely moving Sprague-Dawley rats. Ritanserin (0.3 mg/kg, i.p.), administered at light onset (passive phase), caused an immediate transient increase in the EEG power density in the low frequency range (0.25-6 Hz, mainly delta activity) and a depression in the high frequency range (27-30 Hz) accompanied by a decrease in vigilance and light slow wave sleep (SWS-1), intermediate stage of sleep and increase in deep slow wave sleep (SWS-2) compared to control treatment. All these effects were over 8 It after the injection. Twelve hours after the injection, at dark onset (active phase), there was a marked increase in vigilance and motor activity and decrease in SWS-1 and spindle frequency activity in the control animals, but all these changes were diminished by ritanserin treatment. These effects resulted in a significant relative increase in the intermediate band (peak: 12-15 Hz) of the EEG power spectra and thus, a relative increase in thalamo-cortical synchronization caused by ritanserin at dark onset. Because ritanserin is a selective 5-HT2 receptor antagonist, we conclude that under physiological conditions serotonin increases EEG desynchronization and produces an increase in vigilance level and motor activity by tonic activation of 5-HT2 receptors. This regulatory mechanism plays an important role in the waking process, and the appearances of its effects in the light and dark phase are markedly different. (C) 2002 Elsevier Science B.V. All rights reserved.