Fig. 8.

Regulatory circuits of serotonin signaling in the developing embryo are modulated by MAO-A activity. The activity of tryptophan hydroxylase (TPH) is crucial for the biosynthesis of serotonin (5-hydroxytyptamine, 5-HT), which in turn is degraded by monoamine oxidase A (MAO-A). 5-HT produced in the cytoplasm is packed into vesicles via the vesicular monoamine transporter (VMAT), which facilitates serotonin release into the extracellular space. Extracellular serotonin then either binds to specific serotonin receptors (5-Htr) to initiate cellular signaling cascades or is cleared from extracellular space via the serotonin transporter (5-HTT) to subject serotonin to degradation by MAO-A. Both activation of 5-Htr as well as the by-product of MAO-A activity hydrogen peroxide (H₂O₂) control cellular signaling cascades that affect cell proliferation, differentiation or apoptotic cell death