Effects of tryptophan depletion on anxiety and on panic provoked by carbon dioxide challenge

Results of several studies suggest that anxiety is under serotonergic control. We studied the influence of tryptophan depletion on generalized anxiety and on the response to a 35% carbon dioxide (CO2) challenge. Fifteen healthy male volunteers received both a mixture of amino acids without tryptophan and a placebo mixture under double-blind conditions. The tryptophan-free mixture led to an 80% decrease in blood tryptophan levels compared to the placebo. There was a significant increase in anxiety on the Spielberger Anxiety Inventory and a trend towards more tension on the Profile of Mood States-'tension' items, both of which were interpreted as increased nervousness. Furthermore, a significant increase in neurovegetative panic symptoms occurred after the CO2 challenge. We conclude that the serotonergic system is causally involved in anxiety-related mechanisms and that it may be worth pursuing the role of tryptophan depletion on CO2-induced panic in patients with anxiety disorders.