Variations in pupil size under constant luminance are closely coupled to changes in arousal state. It is assumed that such fluctuations are primarily controlled by the noradrenergic system. Phasic activity of noradrenergic axons precedes pupil dilations associated with rapid changes in arousal, and is believed to be driven by unexpected uncertainty. However, the role of other modulatory pathways in the control of pupil-linked arousal has not been as thoroughly investigated, but evidence suggests that noradrenaline may not be alone. Administration of serotonergic drugs seems to affect pupil size, but these effects have not been investigated in detail. Here, we show that transient serotonin (5-HT) activation, like noradrenaline, causes pupil-size changes. We used phasic optogenetic activation of 5-HT neurons in the dorsal raphe nucleus (DRN) in head-fixed mice locomoting in a foraging task. 5-HT-driven modulations of pupil size were maintained throughout the photostimulation period and sustained for several seconds after the end of the stimulation. The activation of 5-HT neurons increased pupil size additively with locomotor speed, suggesting that 5-HT transients affect pupil-linked arousal independently from locomotor states. We found that the effect of 5-HT on pupil size depended on the level of environmental uncertainty, consistent with the idea that 5-HT may report a salience or surprise signal . Together, these results challenge the classic view of the neuromodulatory control of pupil-linked arousal, revealing a tight relationship between the activation of 5-HT neurons and changes in pupil size.
bioRxiv Subject Collection: Neuroscience