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The loudness dependence (LD) of the auditory-evoked N1/P2 component has been shown to be related to the central serotonergic neurotransmission. Allelic variants in the promoter region of the 5-hydroxytryptamine transporter (5-HTT) gene were shown to modulate serotonergic activity. It was hypothesized that the three genotypes (l/l, s/l, s/s) differ with respect to LD. Allelic variants of the 5-HTT promoter region and LD at the Cz electrode were determined in 185 healthy subjects prospectively. A significant association was found between LD and genotype (ANOVA: F ¼ 4.172, p ¼ 0.017). Individuals homozygous for the l allele exhibited a weaker LD compared to heterozygous subjects. The results are consistent with the reported association between 5-HTT genotype and serotonin transport capacity in lymphoblasts, and indicate that auditory stimulus processing is associated with genetic variants of the brain serotonergic system. The LD may serve as endophenotype in human serotonin research.

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Neuronal differentiation has evolved as an essential process even in the adult brain, once disturbed being associated with the pathogenesis of several psychiatric disorders. To study the effects of Raf kinase inhibitor protein (RKIP) on neuronal differentiation, we generated neuroblastoma cell lines overexpressing RKIP (RKIP⁺) and expressing RKIP-directed short hairpin RNA for downregulation of RKIP (RKIP^–). During a 4-week time course of continuous differentiation by retinoic acid (RA), expression of neuronal and glial markers, intracellular cyclic adenosine monophosphate (cAMP) levels, protein kinase C (PKC) signal transduction to extracellular signal-regulated kinase 1/2 (ERK-1/2) and cellular morphology were investigated in relation to RKIP levels. RKIP⁺ cells showed accelerated neurite outgrowth, formation of elaborated neuronal networks and increased neuronal marker expression both in RA-induced differentiation and to some extent even in non-RA-treated cells. RKIP^– cells showed glial-like cell bodies and increased glial fibrillary acidic protein, suggesting a shift from neuronal to glial phenotype. With respect to differentiation-inducing signal pathways, PKC-mediated ERK-1/2 activation significantly correlated with RKIP levels. Furthermore, basal and forskolin-stimulated intracellular cAMP was potently increased in RKIP⁺ cells versus controls. In conclusion, the conserved signal transduction modulator RKIP was shown to enhance several aspects of neuronal differentiation via enhanced crosstalk from PKC to ERK-1/2 and enhancement of G-protein-coupled receptor signaling.