Lack of effect of tryptophan depletion on the loudness dependency of auditory event related potentials in healthy volunteers.

It has previously been suggested that auditory event related potentials (AEPs) are a potential marker of central serotonergic (5-HT) activity in man, with the slope of the AEP amplitude stimulus intensity function (ASF-slope) inversely correlating with 5-HT activity. However, two recent studies investigating this hypothesis in healthy subjects by rapidly lowering central 5-HT concentrations using the acute tryptophan depletion paradigm have found no effect on ASF-slope [Biological Psychology, 59 (2002) 121; Psychopharmacology (Berl), 146 (1999) 101]. These studies employed a 50g tryptophan depletion drink, which has been argued may not lower central 5-HT concentrations sufficiently. We here report the effect of tryptophan depletion on the AEP ASF-slope using 100g amino acid drinks with and without tryptophan in 14 healthy male subjects, employing a within subject, double blind, random, balanced order, cross-over design. No significant effect of tryptophan depletion was found on ASF-slope. These negative findings cast further doubt on the hypothesis that the ASF-slope is an indicator of central 5-HT function.