Fig. 1.

Serotonergic transmission in neurons. Conversion of tryptophan (TRP) to serotonin (5HT) is catalysed by TPH and DDC. Serotonin is then, by transportation via VMAT, stored into vesicles before it can be released into the synaptic cleft. There, serotonin signalling resulting from binding to serotonin receptors (5HTR) can be terminated by the reuptake of serotonin via SERT transporter. Once serotonin is taken up into the presynaptic neuron it is degraded by MAO and ALDH to 5-HIAA, which is considered a marker for serotonergic activity and measurable for a longer period compared to levels of secreted serotonin. TRP tryptophan, TPH tryptophan hydroxylase, 5-HT serotonin, 5HTR serotonin receptor, 5-HIAA 5 hydroxyindolacetic acid, DDC dopadecarboxylase, VMAT vesicular monoamine transporter, SERT serotonin reuptake transporter, MAO monoamine oxidase, ALDH aldehyde dehydrogenase