Phasic Activation of Dorsal Raphe Serotonergic Neurons Increases Pupil Size.

Transient variations in pupil size (PS) under constant luminance are coupled to rapid changes in arousal state, which have been interpreted as vigilance, salience, or a surprise signal. Neural control of such fluctuations presumably involves multiple brain regions and neuromodulatory systems, but it is often associated with phasic activity of the noradrenergic system. Serotonin (5-HT), a neuromodulator also implicated in aspects of arousal such as sleep-wake transitions, motivational state regulation, and signaling of unexpected events, seems to affect PS, but these effects have not been investigated in detail. Here we show that phasic 5-HT neuron stimulation causes transient PS changes. We used optogenetic activation of 5-HT neurons in the dorsal raphe nucleus (DRN) of head-fixed mice performing a foraging task. 5-HT-driven modulations of PS were maintained throughout the photostimulation period and sustained for a few seconds after the end of stimulation. We found no evidence that the increase in PS with activation of 5-HT neurons resulted from interactions of photostimulation with behavioral variables, such as locomotion or licking. Furthermore, we observed that the effect of 5-HT on PS depended on the level of environmental uncertainty, consistent with the idea that 5-HT could report a surprise signal. These results advance our understanding of the neuromodulatory control of PS, revealing a tight relationship between phasic activation of 5-HT neurons and changes in PS.