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Serotonin (5-HT) and the serotonergic system have recently been indicated as modulators of adult hippocampal neurogenesis. In this study, we evaluated the role of 5-HT on the functional features in neurospheres derived from adult neural stem cells (ANSC). We cultured neurospheres derived from mouse hippocampus in serum-free medium containing epidermal (EGF) and type-2 fibroblast growth factor (FGF2). Under these conditions ANSC expressed both isoforms of tryptophane-hydroxylase (TPH) and produced 5-HT. Blocking TPH function by para-chlorophenylalanine (PCPA) reduced ANSC proliferation, which was rescued by exogenous 5-HT. 5-HT action on ANSC was mediated predominantly by the serotonin receptor subtype 5-HT1A and, to a lesser extent, through the 5-HT2C (receptor) subtype, as shown by selectively antagonizing these receptors. Finally, we documented a 5-HT-induced increase of ANSC migration activity. In summary, we demonstrated a powerful serotonergic impact on ANSC functional features, which was mainly mediated by 5-HT1A receptors.

Gilles de la Tourette syndrome (GTS) (MIM 137580) is a complex neuropsychiatric disorder probably originating from a disturbed interplay of several neurotransmitter systems in the prefrontal-limbic-basal ganglia loop. Polygenetic multifactorial inheritance has been postulated; nevertheless, no confirmed susceptible genes have been identified yet. As neuroimaging studies allude to dopaminergic and serotonergic dysfunction in GTS and serotonin as an important factor for dopamine release, genotyping of common polymorphisms in the serotonergic receptor (HTR1A: C-1019G; HTR2A: T102C, His452Tyr, A-1438G; HTR2C: C-759T, G-697C) and transporter genes (SLC6A4) was carried out in 87 patients with GTS, compared with 311 matched controls. We found a nominally significant association between both polymorphisms in the HTR2C and the GTS, which was more pronounced in male patients. Analysis of the further serotonergic polymorphisms did not reveal any significant result. A modified function of these promoter polymorphisms may contribute to the complex interplay of serotonin and dopamine and then to the manifestation of GTS.

Serotonin (5-HT) is a regulator of morphogenetic activities during early brain development and neurogenesis, including cell proliferation, migration, differentiation, and synaptogenesis. The 5-HT transporter (5-HTT, SLC6A4) mediates high-affinity reuptake of 5-HT into presynaptic terminals and thereby fine-tunes serotonergic neurotransmission. Inactivation of the 5-HTT gene in mice reduces 5-HT clearance resulting in persistently increased concentrations of synaptic 5-HT. In the present study, we investigated the effects of elevated 5-HT levels on adult neurogenesis in the hippocampus of 5-HTT deficient mice, including stem cell proliferation, survival, and differentiation. Using an in vivo approach, we showed an increase in proliferative capacity of hippocampal adult neural stem cells in aged 5-HTT knockout mice (approximately 14.5 months) compared to wildtype controls. In contrast, in vivo and additional in vitro analyses of younger adult 5-HTT knockout mice (approximately 7 weeks and approximately 3.0 months) did not reveal significant changes in proliferation of neural stem cells or survival of newborn cells. We showed that the cellular fate of newly generated cells in 5-HTT knockout mice is not different with respect to the total number and percentage of neurons or glial cells from wildtype controls. Our findings indicate that elevated synaptic 5-HT concentration throughout early development and later life of 5-HTT deficient mice does not induce adult neurogenesis in adult mice, but that elevated 5-HT levels in aged mice influence stem cell proliferation.

Mutations of the transcription factor 4 (TCF4) gene cause mental retardation with or without associated facial dysmorphisms and intermittent hyperventilation. Subsequently, a polymorphism of TCF4 was shown in a genome-wide association study to slightly increase the risk of schizophrenia. We have further analysed the impact of this TCF4 variant rs9960767 on early information processing and cognitive functions in schizophrenia patients. We have shown in a sample of 401 schizophrenia patients that TCF4 influences verbal memory in the Rey Auditory Verbal Learning Test. Contrary to expectations, carriers of the schizophrenia-associated allele showed better recognition, thus indicating that while TCF4 influences verbal memory, the TCF4-mediated schizophrenia risk is not determined by the influence of TCF4 on verbal memory. TCF4 does not impact on various other cognitive functions belonging to the domains of attention and executive functions. Moreover, in a pharmacogenetic approach, TCF4 does not modulate the improvement of positive or negative schizophrenia symptoms during treatment with antipsychotics. Finally, we have assessed a key electrophysiological endophenotype of schizophrenia, sensorimotor gating. As measured by prepulse inhibition, the schizophrenia risk allele C of TCF4 rs9960767 reduces sensorimotor gating. This indicates that TCF4 influences key mechanisms of information processing, which may contribute to the pathogenesis of schizophrenia.