EFFECTS OF TRYPTOPHAN AND OR ACUTE RUNNING ON EXTRACELLULAR 5-HT AND 5-HIAA LEVELS IN THE HIPPOCAMPUS OF FOOD-DEPRIVED RATS/

The present microdialysis study has examined whether exercise-elicited increases in brain tryptophan availability (and in turn 5-HT synthesi s) alter 5-HT release in the hippocampus of food-deprived rats. To thi s end, we compared the respective effects of acute exercise, administr ation of tryptophan, and the combination of both treatments, upon extr acellular 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels. All rat s were trained to run on a treadmill before implantation of the microd ialysis probe and 24 h of food deprivation. Acute exercise (12 m/min f or 1 h) increased in a time-dependent manner extracellular 5-HT levels (maximal increase: 47%), these levels returning to their baseline lev els within the first hour of the recovery period. Besides, exercise-in duced increases in extracellular 5-HIAA levels did not reach significa nce. Acute administration of a tryptophan dose (50 mg/kg i.p.) that in creased extracellular 5-HIAA (but not 5-HT) levels in fed rats, increa sed within 60 min extracellular 5-HT levels (maximal increase: 55%) in food-deprived rats. Whereas 5-HT levels returned toward their baselin e levels within the 160 min that followed tryptophan administration, e xtracellular 5-HIAA levels rose throughout the experiment (maximal inc rease: 75%). Lastly, treatment with tryptophan (60 min beforehand) bef ore acute exercise led to marked increases in extracellular 5-HT and 5 -HIAA levels (maximal increases: 100% and 83%, respectively) throughou t the 240 min that followed tryptophan administration. This study indi cates that exercise stimulates 5-HT release in the hippocampus of fast ed rats, and that a pretreatment with tryptophan (at a dose increasing extracellular 5-HT levels) amplifies exercise-induced 5-HT release.