EXTRACELLULAR LEVELS OF SEROTONIN IN THE MEDIAL PONTINE RETICULAR-FORMATION IN RELATION TO SLEEP-WAKE CYCLE IN CATS - A MICRODIALYSIS STUDY

Extracellular levels of endogenous serotonin (5-hydroxytryptamine; 5-H T) and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA) were measured in the medial pontine reticular formation (medial PRF) of int act cats. A microdialysis probe was inserted through a guide cannula i nto the medial PRF. At least 12 h after the probe insertion, in vivo b rain microdialysis was initiated. The perfusion rate was 1 mu l/min, a nd perfusate fractions at regular intervals of 20 min were collected. Changes in extracellular 5-HT levels were compared across sleep-wake s tates of the animals, such as waking (W), slow wave sleep (SWS) and ra pid eye movement (REM) sleep. To assess sleep-wake states, EEG, EMG, E OG and PGO waves were simultaneously recorded in parallel with microdi alysis of the medial PRF. Extracellular 5-HT levels were highest (20-2 8 fmol/20 mu l) during W. As the animals entered SWS, 5-HT levels decr eased to about 90% of those during W. The state of REM sleep usually i nterrupted SWS for 3-8 min. During the longer periods of REM sleep, du ring the 20 min periods in which the perfusates were collected, we obs erved the lowest 5-HT levels (60-50%).