Effects of halothane and nitrous oxide anaesthesia on 5-HT turn-over in the rat brain

The effects of anaesthesia with halothane and nitrous oxide on the turn-over of 5-HT in the rat brain were analysed. The rate of 5-HT synthesis was estimated on the basis of 5-HTP accumulation following 5-HTP decarboxylase inhibition. Inhalation anaesthetics reduced the rate of 5-HT synthesis in the whole brain, this effect being particularly marked in the forebrain. In addition, the reduction of the rate of 5-HT synthesis was still detected after a large tryptophan loading indicating that halothane and nitrous oxide anaesthesia reduced the activity of tryptophan hydroxylase in vivo. The utilization of 5-HT was also delayed during anaesthesia since the rate of 5-HT disappearance following the inhibition of its synthesis at the decarboxylation step was decreased in the whole brain. In spite of this reduced turnover of 5-HT in the brain of rats exposed to inhalation anaesthetics, the 5-HIAA levels were permanently elevated. This was the result of a partial inhibition of the active elimination of the acidic metabolite from the brain during this kind of anaesthesia.