L-tryptophan injection fails to alter nutrient selection by rats

Area of Science:

• Neuroscience
• Nutritional Science
• Behavioral Pharmacology

Background:

• Previous studies suggested brain serotonin (5-HT) influences dietary choices, specifically protein and carbohydrate intake in rats.
• The proposed role of 5-HT in regulating nutrient selection necessitates further investigation into its physiological significance.

Purpose of the Study:

• To investigate the effect of experimentally elevated brain serotonin (5-HT) on short-term food intake and macronutrient selection in rats.
• To determine if increased 5-HT levels alter the choice between diets varying in protein and carbohydrate content.

Main Methods:

• Rats were administered tryptophan (Trp) at 100 mg/kg to increase brain 5-HT concentrations.
• Food intake and selection between two diets (differing in protein and carbohydrate) were monitored.
• Brain concentrations of 5-HT, 5-HIAA, norepinephrine (NE), dopamine (DA), and DOPAC were measured.

Main Results:

• Tryptophan administration significantly increased brain 5-HT and 5-HIAA levels by approximately 50% without affecting NE, DA, or DOPAC.
• Despite elevated brain 5-HT, no significant changes were observed in total food intake or the selection of protein versus carbohydrate diets.
• The observed increase in serotonin metabolism was selective, indicating no broad impact on catecholamine systems.

Conclusions:

• Elevating brain 5-HT levels via tryptophan injection does not influence short-term macronutrient selection in rats under dietary self-selection conditions.
• These findings challenge the hypothesis that brain 5-HT plays a critical physiological role in regulating food intake and nutrient selection.
• Post-meal alterations in brain 5-HT may not be a significant factor in controlling dietary choices.