Intensity dependence of auditory evoked potentials (AEPs) as biological marker for cerebral serotonin levels: effects of tryptophan depletion in healthy subjects. 
The intensity dependence of the auditory evoked potentials (AEP) has been suggested to be a specific biological marker of central serotonergic activity.  While previous studies used circumstantial evidence to support this hypothesis, we manipulated (decreased) cerebral levels of serotonin directly by using tryptophan depletion.  Twelve healthy young subjects were investigated using placebo and two different amino acid mixtures in a double blind cross over design on three different occasions.  AEPs recorded during tryptophan depletion were analyzed by dipole analysis and regional sources using methods published in the literature.  For none of the mixtures a significant effect of tryptophan depletion was found.  There was a trend towards reduced intensity dependency after tryptophan depletion, especially in the right hemisphere.  This reduction correlated with the amount of reduced tryptophan in plasma.  The results indicate, in contrast to earlier indirect studies, that the intensity dependence of AEPs is not a specific marker of central serotonergic activity.