Serotonin in pre-implantation mouse embryos is localized to the mitochondria and can modulate mitochondrial potential

Serotonin is reported to be present in early embryos of many species and plays an important role in early patterning. Since it is a fluorophore, it can be directly visualized using fluorescence microscopy. Here, we use three-photon microscopy to image serotonin in live pre-implantation mouse embryos. We find that it is present as puncta averaging 1.3 square microns and in concentrations as high as 442 mM. The observed serotonin puncta were found to co-localize with mitochondria. Live embryos pre-incubated with serotonin showed a higher mitochondrial potential, indicating that it can modulate mitochondrial potential. Pre-implantation mouse embryos were also examined at various developmental stages for the presence of transcripts of the peripheral and neuronal forms of tryptophan hydroxylase (Tph1 and Tph2 respectively) and the classical serotonin transporter (Slc6a4). Transcripts of Tph2 were seen in oocytes and in two-cell stages, whereas transcripts of Tph1 were not detected at any stage. Transcripts of the transporter, Slc6a4, were present in all pre-implantation stages investigated. These results suggest that serotonin in embryos can arise from a combination of synthesis and uptake from the surrounding milieu.