Mechanism of the EEG-synchronizing action of serotonin

Abstract

In cats anesthetized with Dial and urethan, or treated with Flaxedil and sedated with 1/10 surgical dose of Dial and urethan, intracarotid injection of serotonin (0.2-5.0 [mu]g/kg body wt) induced an initial arousal pattern replaced after 30-150 sec by a protracted phase of hypersynchrony which lasted often as long as 15 min. Recruiting responses produced by medial thalamlc stimulation showed a similar biphasic reaction to serotonin characterized by initial depression followed by often marked and long-lasting enhancement. These eeg changes were accompanied by initial widening then narrowing of the pupils. After transection of the brain stem at the midpontine level, serotonin produced only signs of arousal in the eeg. Intravertebral injections of 5-HT [5-Hydroxytryptamine] and injections of 5-HT into the 4th ventricle induced only hypersynchronizing effects. After cauterization of the area postrema or after application of 5-HT blocking agents to the posterior 4th ventricle the hypersynchronizing effects of intracarotid or intravertebral 5-HT were reduced or altogether eliminated. It is concluded that serotonin produces the eeg and ocular signs of synchronized sleep by an action exerted on receptor sites in the area postrema, from which nervous signals travel to the nucleus of the solitary tract and then to more rostrally situated hypnogenic areas.