Abstract

Background: Recent studies show that together with serotonin (5-hydroxytryptamine; 5-HT), leptin also plays an important role in the etiology of depression. Tryptophan increases brain 5-HT but the effects of tryptophan on leptin and responses to stress are not known.

Objective: To evaluate the effect of tryptophan load (300mg/kg, orally) on serotonin metabolism, circulating levels of leptin and behavioral responses to single (2hr) and repeated (2hr/day for 6 days) immobilization stress in rats.

Method: Albino-wistar rats were randomly assigned to two groups (n = 24) for single and repeated immobilization with further division in four equal groups; water/tryptophan treated unstressed and stressed groups. Behavioral studies and biochemical estimation were performed.

Results: Acute exposure to 2hr immobilization decreased food intake, body weight and elicited anxiety like behavior. It increased circulating leptin and brain 5-HT metabolism. The stress-induced behavioral deficits and increases of leptin did not occur following repeated immobilization while the levels of brain 5-HT decreased. Immobilization-induced behavioral deficits were smaller in tryptophan than water treated animals. Tryptophan administration also increased serum leptin levels but inhibited stress-induced increases of leptin. Repeated administration of tryptophan did not increase leptin; tryptophan treated animals exposed to repeated immobilization exhibited smaller leptin levels.

Conclusion: Tryptophan induced increases of brain 5-HT reduces stress effects on hormones and behavior but have little effects in unstressed animals suggesting, these increases are available at functional sites only in stressful conditions. While, in control animals these increases occur only intra-neuronally.

Keywords: Tryptophan, serotonin, leptin, adaptation to stress, elevated plus maze, light-dark transition.

Graphical Abstract