DAILY VARIATIONS IN IN-VIVO TRYPTOPHAN HYDROXYLATION AND IN THE CONTENTS OF SEROTONIN AND 5-HYDROXYINDOLEACETIC ACID IN DISCRETE BRAIN-AREAS OF THE RAT

The in vivo rate of brain tryptophan hydroxylation was determined thro ugh 5-hydroxytryptophan accumulation (5-HTPacc) following the administ ration of NSD 1015, a L-aromatic amino-acid decarboxylase inhibitor. T his measurement was performed every 4h throughout a 24h hour period in 10 discrete brain areas of rats maintained on a regular 12h/12h light -dark cycle. The concentrations of 5-hydroxytryptamine (5-HT) and 5-hy droxyindoleacetic acid (5-HIAA) were also determined in untreated rats . Daily variations in 5-HTPacc were found in all the areas studied, th e 5-HTPacc being higher during the dark period in most structures. The se results strongly suggest that tryptophan hydroxylation is involved in the control of the 5-HT biosynthesis circadian rhythm. However, var ious patterns of 5-HT and 5-HIAA daily variations were observed, sugge sting that the circadian factors affecting serotonin metabolism can be different among brain areas.