EFFECT OF ACUTE ADMINISTRATION OF L-TRYPTOPHAN ON THE RELEASE OF 5-HT IN RAT HIPPOCAMPUS IN RELATION TO SEROTONINERGIC NEURONAL-ACTIVITY - AN INVIVO MICRODIALYSIS STUDY

Here we have used the brain microdialysis method to test the effect of the 5-HT precursor L-tryptophan on 5-HT release. The release ot endogenous 5-HT was measured in ventral hippocampus of the anaesthetized rat both under basal conditions and when serotoninergic neuronal activity was raised by electrical stimulation of the dorsal raphe nucleus (DRN). Low frequency electrical stimulation of the DRN evoked a frequency-dependent (2-10 Hz) release of hippocampal 5-HT. The electrically evoked release of 5-HT was markedly enhanced by pretreatment with L-tryptophan (50 and 100 mg/kg i.p.). The effect of L-tryptophan on evoked release of 5-HT was dose-related, detectable at low (2 Hz) stimulation frequencies, and became stronger as the stimulation frequency increased. L-Tryptophan (10, 50 and 100 mg/kg i.p.) had no effect on basal output of 5-HT. We conclude from these findings that elevation of 5-HT precursor availability increases 5-HT release in hippocampus in vivo under conditions of increased serotoninergic neuronal activity.