Several lines of evidence point to the participation of serotonin (5HT) in anxiety. Its specific role, however, remains obscure. The objective of the present study was to evaluate the effect of reducing 5HT-neurotransmission through an acute tryptophan depletion on anxiety induced by a simulated public speaking (SPS) test. Two groups of 14-15 subjects were submitted to a 24-h diet with a low or normal content of tryptophan and received an amino acid mixture without (TRY-) or with (TRY+) tryptophan under double-blind conditions. Five hours later they were submitted to the SPS test. The state-trait anxiety inventory (STAI) and the visual analogue mood scale (VAMS) were used to measure subjective anxiety. Both scales showed that SPS induced a significant increase in anxiety. Although no overall difference between groups was found, there was a trend (P = 0.078) to an interaction of group x gender x phases of the SPS, and a separate analysis of each gender showed an increase in anxiety measured by the STAI in females of the TRY- group. The results for the female TRY- group also suggested a greater arousing effect of the SPS test. In conclusion, the tryptophan depletion procedure employed in the present study did not induce a significant general change in subjective anxiety, but tended to induce anxiety in females. This suggests a greater sensitivity of the 5HT system to the effects of the procedure in this gender.

Key words: serotonin, anxiety, tryptophan depletion, public speaking, healthy volunteers, gender differences

Introduction

Several lines of evidence point to the participation of serotonin (5HT) in anxiety. Its specific role, however, remains obscure (1,2). Recently, it has been suggested that although the general role of 5HT is to modulate responses to aversive stimuli, different 5HT subsystems have distinct roles. 5HT fibers originating in the dorsal raphe nucleus (DRN) and projecting to 5HT2/5HT3 receptors in the amygdala and frontal cortex would modulate responses to distal danger, facilitating avoidance responses and fear conditioning. On the other hand, activation of 5HT2 or 5HT1A receptors in the dorsal central gray (DCG), a region related to flight/fight reactions to proximal, unconditioned danger in animals, would be anxiolytic. This dual effect of 5HT might help to explain much of the controversy in the literature (1). At the clinical level these two systems have been related to generalized anxiety and panic disorder, respectively (1,2). Some clinical findings favor this interpretation. For example, ritanserin, a 5HT2 antagonist, is anxiolytic in generalized anxiety but anxiogenic in panic (2-4).

We have recently initiated studies on healthy volunteers to test this theory using two different clinical models: aversive fear conditioning to sounds (AFC) and simulated public speaking (SPS). We hypothesized that the first one would be related to the DRN-amygdala/frontal cortex 5HT system whereas the SPS would be related to the DRN-DCG system. The latter involves an almost universal, probably unconditioned fear, and raises anxiety feelings in healthy subjects independently of their trait anxiety (5). We showed that ritanserin, similar to that found in generalized anxiety and panic disorder patients, has opposite effects in the two models (6), being anxiolytic in the AFC model and anxiogenic in the SPS test.

As a continuation of these studies we have recently completed a study on healthy volunteers with d-fenfluramine, a 5HT neuronal releaser and reuptake blocker. As predicted by the theory (1), the drug was anxiolytic in the SPS model, but tended to be anxiogenic in the AFC model (7). Dietary changes were able to decrease up to 87 and 91% of the plasma concentration of total and free tryptophan (TRY), respectively (8). This amino acid is the precursor of 5HT, and the neurotransmitter synthesis seems to depend on tryptophan availability. This procedure reversed the therapeutic effects of antidepressants in symptom-free depressive patients (8). These results, besides implicating 5HT in the therapeutic effects of antidepressant drugs, also indicate that this is an interesting method for manipulating 5HT neurotransmission in humans.

The objective of the present study was to use this method to investigate the role of 5HT in anxiety induced by SPS in healthy volunteers.