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.