To causally test the pupil dilation responses elicited by orexin cell activation, we selectively opto-stimulated LH orexin neurons, while tracking pupil diameter in anesthetized mice (Fig. In particular, the related question of the interplay of arousal and reward representations in individual orexin neurons 9 has not been answered experimentally. However, orexinergic modulation of moment-to-moment arousal, as indexed by pupil dilation, has not been explored, and previous studies contain arguments for both constriction 17, 18 and dilation 19, 20, 21, 22. Pupil dilation has also been correlated to the activity of cholinergic 14 and serotonergic 16 neuromodulatory systems. As such, and with its strong tracking of the locus coeruleus (LC) noradrenergic system 13, 14, pupil size measurements have been used to support the adaptive gain theory of arousal 12, 15. Pupil dilation is routinely used in human experiments as a measure of arousal and autonomic function 10, and for predicting key aspects of cognition, such as the exploration–exploitation balance 11, 12. Through these connections, the orexin network regulates sleep–wake switching and autonomic function, as well as feeding and exploratory behaviors 4, 5, 6, 7, 8, 9. The orexin (hypocretin) system of the lateral hypothalamus (LH) projects brain-wide, with particularly strong connections to arousal and reward centers 1, 2, 3.
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