Functional meaning of tryptophan-induced increase of 5-HT metabolism as clarified by in vivo voltammetry.
Differential pulse voltammetry with carbon fiber electrodes was used to study serotonin (5-HT) metabolism in freely moving rats. The electrodes implanted in the striatum recorded the extracellular 5-hydroxyindoleacetic acid (5-HIAA) oxidation peak after oral tryptophan (150 mg/kg). This 5-HT precursor did not modify the 5-HIAA peak in any rat tested, but it raised 5-HIAA levels determined in total tissue by a classical biochemical method (HPLC). The administration of 5-hydroxytryptophan (5-HTP) (25 mg/kg i.p.) induced an increase of 5-HIAA detectable both in the extracellular medium by voltammetry and in tissue samples. As previously shown, dorsal raphe electrical stimulation raises extracellular 5-HIAA in the striatum and this effect is enhanced by pretreatment with tryptophan. The results suggest that tryptophan in 'normal' conditions enhances 5-HT metabolism without affecting 5-HT release unless such release is stimulated. 5-HTP increases 5-HT metabolism and release.
D000818:Animals; D002421:Caudate Nucleus / Q000187:drug effects / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D004558:Electric Stimulation; D004563:Electrochemistry; D010134:Fenclonine / Q000494:pharmacology; D006897:Hydroxyindoleacetic Acid / Q000378:metabolism; D008297:Male; D009434:Neural Pathways / Q000502:physiology; D011903:Raphe Nuclei / Q000502:physiology; D051381:Rats; D012701:Serotonin / Q000378:metabolism*; D014364:Tryptophan / Q000494:pharmacology*
D012701:Serotonin; D006897:Hydroxyindoleacetic Acid; D014364:Tryptophan; D010134:Fenclonine