Serotonergic and noradrenergic interactions in pupil-linked arousal

Changes in central arousal state shape cortical computations underlying perception, thought, and action. Variations in arousal are accompanied by fluctuations in pupil size. In turn, pupil dynamics are often used as a marker of noradrenaline release from neurons of the locus coeruleus. The serotonergic system of the dorsal raphe also contributes to the brainstem control of waking states. However, little is known about its relationship to noradrenergic activity and arousal dynamics in awake animals. Here, we simultaneously accessed both systems in awake mice and unraveled their unique and joint contributions to pupil-linked arousal. Serotonergic and noradrenergic systems co-fluctuated, and serotonergic dorsal raphe neurons affected pupil size partly via noradrenergic populations in the locus coeruleus. Yet, part of the serotonergic control of pupil dynamics was independent of the locus coeruleus. Our findings challenge common assumptions about the neuromodulatory control of pupil dynamics and illuminate the interplay between distinct neurochemical systems within the arousal network of the brainstem.