In Vivo Two-Photon Microscopy Reveals Sensory-Evoked Serotonin (5-HT) Release in Adult Mammalian Neocortex

The recent development of genetically encoded fluorescent neurotransmitter biosensors has opened the door to recording serotonin (5-hydroxytryptamine, 5-HT) signaling dynamics with high temporal and spatial resolution in vivo. While this represents a significant step forward for serotonin research, the utility of available 5-HT biosensors remains to be fully established under diverse in vivo conditions. Here, we used two-photon microscopy in awake mice to examine the effectiveness of specific 5-HT biosensors for monitoring 5-HT dynamics in somatosensory cortex. Initial experiments found that whisker stimulation evoked a striking change in 5-HT biosensor signal. However, similar changes were observed in controls expressing green fluorescent protein, suggesting a potential hemodynamic artifact. Subsequent use of a second control fluorophore with emission peaks separated from the 5-HT biosensor revealed a reproducible, stimulus-locked increase in 5-HT signal. Our data highlight the promise of 5-HT biosensors for in vivo application, provided measurements are carried out with appropriate optical controls.