Rationale: Intensity dependence of the N1/P2 components may be regulated by serotonergic neurons in the primary auditory cortex, where low activity leads to a high intensity dependence and vice versa. Depletion of tryptophan (TRP), a precursor for serotonin has been described to reduce serotonin content in brain of animals and humans.
Objective: We investigated the intensity dependence of magnetic and electric N1/P2 components in ten subjects in a double-blind, controlled, cross-over design study after oral mixture of amino-acids leading to acute tryptophan depletion (ATD) and control.
Methods: Auditory evoked magnetic fields (AEF) and potentials (AEP) were recorded with 122-channel magnetoencephalography simultaneously with 64-channel EEG 5 h after ingestion of mixtures. The AEF sources and strength were estimated by a least-squares fit of a single equivalent current dipole. The amplitudes and latencies of N1 and P2 recorded with EEG were analyzed at frontal electrode site.
Results: TRP depletion decreased the total and free TRP levels by 76 and 45% and control mixture increased it by 48 and 28%. ANOVA showed that ATD had a significant main effect on the N1m/P2m dipole moments at the contralateral ( P=0.02), but failed significantly to influence the ipsilateral responses. A significant mixture ingestion-by-stimulus intensity interaction was observed on the N1m/P2m dipole moments at the contralateral hemisphere ( P=0.01). The N1/P2 slope for intensity dependence function was decreased following ATD compared with the control experiment ( P=0.01) at the contralateral hemisphere. For EEG, a significant mixture ingestion-by-stimulus intensity interaction on the N1 latencies at the Fz electrode position was observed ( P=0.01).
Conclusion: ATD decreased the intensity dependence of N1m/P2m source dipole moments in the primary auditory cortex at the hemisphere contralateral to the ear stimulated. These results suggest that serotonin participates in the regulation of intensity of auditory stimulation.