Effects of Acute Tryptophan Depletion on the Loudness Intensity Dependence in a Healthy Female Population

Objectives: Animal and human studies suggest an inverse influence of the serotonergic system on the intensity dependence of early event-related potentials for the primary acoustic cortex, namely the N1/P2 component. Meanwhile, procedures by tryptophan depletion tests (TDT) represent an established human challenge tool for a temporary reduction of tryptophan levels and central nervous serotonin. Methods: 16 healthy females volunteered twice to ingest a highly concentrated amino acid mixture with (+) TRP or without (-) TRP in a double-blind cross-over study design: In relation to the individual basic value plasma tryptophan levels decreased extensively to 18.69% (±5.32) in the (-) TRP condition. Sinus tones of 1000Hz of different intensity levels (60–90 dB) were presented binaurally. A 32-multichannel EEG was recorded continuously. Following the averaging procedures we performed dipole source analysis [BESA] applying a model of a tangential and radial dipole per hemisphere. Results: Within their groups, all subjects showed increasing N1/P2 amplitudes of the relevant tangential dipole in series of higher stimulus intensity with significant differences of the individual change rates (p<0.001–0.05). Augmentation of tangential N1/P2 activity was partly more pronounced in the (-) TRP condition while there were some trends for an increase of the overall resulting intensity dependence as opposed to the control (+) TRP condition. There were no significant differences for latencies between the study conditions. Conclusions: Contrary to the hypothesis, our results of a homogeneous female population provide some minor arguments for the hypothesis of an influence of serotonin on the N1/P2 intensity dependence in spite of a strong depletion situation. A low serotonergic neurotransmission may contribute to the intensity effect in AEP only to a lesser extent while interactions with other neurotransmitters of the CNS will have to be considered.