The intensity dependence of the auditory evoked potential (AEP) has been suggested as an indicator of central serotonergic function, a strong intensity dependence presumably reflecting low serotonergic activity. As individual differences in serotonergic neurotransmission can be accounted for in part by genetic variation in genes of the serotonergic pathway, we investigated whether a functional polymorphism in the promoter region of the serotonin transporter gene (5-HTTLPR) is associated with the AEP intensity dependence. Because dopaminergic influences on the intensity dependence have also been reported, we furthermore explored the role of a functional polymorphism in the dopamine D4 receptor gene (DRD4 exon III) in the modulation of the AEP intensity dependence. AEPs to tones of six intensity levels were recorded from 60 healthy young individuals, and N1/P2 linear as well as median slopes at central electrode sites were computed as measures of the AEP intensity dependence. Analyses of variance showed that there was a significant effect of the 5-HTTLPR on the AEP intensity dependence. Individuals with the ll genotype exhibited a stronger intensity dependence compared to individuals with the ls genotype. This effect was even more pronounced when DRD4 exon III was considered in the analyses. In conclusion, these findings provide further evidence for a role of serotonergic neurotransmission in the modulation of the AEP intensity dependence. The results also point to possible dopaminergic influences on the AEP intensity dependence.
Copyright 2003 Wiley-Liss, Inc.