Effects of conditioned running on plasma, liver and brain tryptophan and on brain 5‐hydroxytryptamine metabolism of the rat

Abstract

1 An investigation was made into the effects of conditioned running (1 h and 2 h at 20 m min⁻¹), which accelerates lipolysis, on the concentrations of tryptophan (Trp) in plasma, liver and brain and on 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels in brain. 2 Running caused time-dependent increases in plasma free Trp and brain Trp of the rat, leading to increased brain 5-HT turnover as revealed by higher amounts of its metabolite, 5-HIAA. The ratio of brain Trp to plasma free Trp was decreased after 2 h of running. 3 Liver Trp content rose only after 3 h of running, while liver unesterified fatty acid (UFA) concentrations remained unmodified. 4 A comparison between food deprivation and running (both of which promote lipolysis) was performed. Running for 2 h affected to the same extent plasma Trp disposition when compared with 24 h food deprivation. Nevertheless, the ratio of brain Trp to plasma free Trp was decreased in the food-deprived rats, when compared to the runners. 5 Valine, an inhibitor of entry of Trp into the brain decreased its level there to the same extent in both controls and 1 h runners. 6 Nicotinic acid, which inhibits fat catabolism, completely abolished the plasma UFA increase induced by 1 h of running. The drug did not affect plasma free Trp, brain Trp, 5-HT or 5-HIAA but enhanced plasma total Trp level. 7 Naloxone, an opiate antagonist, which decreased running-induced lipolysis, did not alter plasma Trp disposition. 8 Desipramine, an antidepressant compound, affected only peripheral Trp concentrations of the runners. Plasma free and total Trp concentrations were increased in desipramine-treated runners, compared with saline-treated runners. In addition, desipramine increased the ratio of brain Trp to plasma free Trp of the runners. Brain 5-HT and 5-HIAA were increased in both desipramine-treated controls and runners. 9 The results suggest that running, which like food deprivation accelerates lipolysis, increases brain Trp content and then 5-HT turnover. Comparison of these two physiological situations suggests that effectiveness of brain Trp entry is much more altered by fasting.