Effects of the 5-HT₆ receptor antagonists SB-399885 and RO-4368554 and of the 5-HT(2A) receptor antagonist EMD 281014 on sleep and wakefulness in the rat during both phases of the light-dark cycle

The effects of the 5-HT₆ receptor antagonists SB-399885 (2.5, 5 and 10 mg/kg) and RO-4368554 (2.5, 5 and 10 mg/kg) and of the 5-HT(2A) receptor antagonist EMD 281014 (2.5, 5 and 10 mg/kg) were studied in rats implanted for chronic sleep procedures. Administration of 10 mg/kg SB-399885, i.p., to rats 2 h after the beginning of the LIGHT phase of the light-dark cycle caused a significant increase of wakefulness (W) and a reduction of slow wave sleep (SWS), REM sleep (REMS) and the number of REM periods during 6-h recording sessions. LIGHT sleep was increased after the whole range of doses. The increase of W and reduction of SWS and REMS occurred predominantly during the first 2-h period whereas LIGHT sleep was augmented over the first and the second 2-h recording periods. Injection of RO-4368554 (10 mg/kg, i.p.) 2 h after the beginning of the LIGHT period significantly increased W and reduced SWS and REMS during the first 2-h recording period. Administration of EMD 281014 (10 mg/kg, i.p.) during the LIGHT phase significantly increased SWS and reduced LIGHT sleep during 6-h sessions. REMS and the number of REM period were reduced with the entire range of doses. The reduction of REMS and LIGHT sleep and the increase of SWS occurred predominantly during the first and the second 2-h of recording, respectively. Injection of SB-399885 (10 mg/kg, i.p.) 2 h after the beginning of the dark period induced a significant reduction of REMS during the first 2-h of recording. In contrast, RO-4368554 did not modify values corresponding to sleep variables during the dark period. Treatment with EMD 281014 (2.5-10 mg/kg, i.p.) during the dark phase significantly increased SWS during the second 2-h period. Our study supports the proposal that blockade of postsynaptic 5-HT₆ receptors with systemic administration of SB-399885 and RO-4368554 increases W and reduces SWS and REMS during the LIGHT phase of the sleep-wake cycle. SB-399885 also induces a suppression of REMS during the dark period. It was confirmed, in addition, that administration of the 5-HT(2A) receptor antagonist EMD 281014 enhances SWS during both phases of the light-dark cycle in the rat.