Extracellular serotonin variations during vigilance states in the preoptic area of rats: a microdialysis study

Numerous studies have shown that serotonergic transmission decreases from waking (W) to slow wave sleep (SWS) to paradoxical sleep (PS), suggesting an active role of serotonin (5-HT) in W but not in sleep. Conversely, the inhibition of 5-HT activity produces insomnia. This insomnia can be reversed by injections of 5-hydroxytryptophan in the preoptic area (POA), suggesting that 5-HT is necessary in this cerebral structure for sleep. Using microdialysis, we studied, 5-HT variations in the POA of rats in relation to vigilance states. 5-HT levels were higher during W than during during SWS and PS. 5-HT increased just before the rats fell asleep and then decreased during sleep. A decreased 5-HT transmission was also observed from SWS to PS. These data document a positive correlation between 5-HT levels in POA and wakefulness. Moreover, these observations are in favour of a permissive role of 5-HT in the POA during PS. A comparison between the POA and the prefrontal cortex in the sleep-wake cycle is discussed.