Somatodendritic serotonin release and re-uptake in mouse embryonic stem cell-derived serotonergic neurons

Serotonergic neurotransmission plays an important role during neural development. Serotonergic dysfunction is observed in various psychiatric disorders and many psychoactive drugs target proteins on serotonergic neurons. Serotonergic neurons are located in the raphé nuclei and densely innervate the whole brain. The low number and the intricate accessibility of these neurons do not allow to culture them and therefore to date it was impossible to study drug-target interactions on bona fide serotonergic neurons. In order to circumvent such problems we have developed a protocol that allows the rapid and efficient generation of serotonergic neurons from mouse embryonic stem cells. Neuronal precursors were obtained by neuronal stem sphere formation in floating culture in the presence of various mitogens. Differentiation into neurons was induced by withdrawal of the mitogens. About 90% of the resulting neurons exhibited a serotonergic phenotype as judged by immunostaining against serotonin, its synthesising enzyme tryptophan hydroxylase 2, the serotonin transporter as well as 5-HT1(A) and 5-HT1(B) autoreceptors. In addition, we found expression of the vesicular monoamine transporter vMAT2 and the presynaptic protein Bassoon, which is involved in organizing the assembly of the presynaptic active zone. Depolarisation-induced calcium influx was visualised by Fluo-4, and accompanying exocytotic events by FM dye staining. Proteins involved in 5-HT release and re-uptake as well as depolarisation evoked exocytosis were evenly co-distributed on neurites and cell bodies suggesting that ES cell-derived serotonergic neurons also exhibit somatodendritic release comparable to serotonergic neurons in the raphé nuclei.