Tryptophan is prescribed as an antidepressant in some countries. Studies have shown a reappearance of depressive symptoms after acute tryptophan depletion and a mood-lowering effect in healthy, non-depressed participants. However, there is research that indicates that it might just work as an adjunct to other antidepressant treatments, and not as a primary treatment.

There is evidence that suggests that tryptophan is involved in emotional biases in the perception of socially relevant stimuli. This points to an induction of a negative perceptual bias in the procession of emotional material.

This research

Previous research has shown that serotonergic antidepressants have the opposite effect on information processing and induce a positive bias on several emotion-related tasks. What is not known however, is whether increasing serotonin synthesis has the same effects on emotional processing to those seen after the inhibition of serotonin reuptake. This will be researched here. The hypothesis is that tryptophan will induce cognitive changes and emotional biases opposite to those found in depression and characteristics of those induced by serotonergic antidepressants. They will also be investigating a gender difference, since previous research has indicated that women are more effected by acute tryptophan depletion than men.

Discussion

The results show that repeated administration of tryptophan gives a positive bias in the processing of emotional material in women, but not in men. In women, the tryptophan increased recognition of positive facial expressions and decreased recognition of negative facial expressions. It also lowered attentional vigilance towards negative words, and decreased baseline startle responsivity. The results suggest that just like other serotonergic antidepressants, tryptophan can directly modulate the processing of emotional material. However, the results also indicate that it may be a milder manipulation of the serotonergic system than selective serotonin reuptake inhibitors.

The effects of tryptophan were only seen in females. Also the mood-lowering effects were more consistent in women than in men. This may be because acute tryptophan depletion has a greater biochemical effect in women than in men, or because women have a better clinical response to serotonin reuptake inhibitors than men.

Results indicate that tryptophan supplementation has bigger effects on emotional processing in women than men. This should play a role in the therapeutic consideration when tryptophan is being used.