The contribution of the locus coeruleus – norepinephrine system to the coupling between pupil-linked arousal and cortical state

Understanding how pupil-linked arousal couples with cortical state is crucial for uncovering the neural mechanisms underlying brain state-dependent cognitive and sensory processing. Pupil size fluctuations reflect rapid changes of the pupil-linked arousal system, indexing brain states as well as the activity of neuromodulatory systems, including the locus coeruleus-norepinephrine (LC-NE) system. We investigated the relationship among phasic pupil dilation, cortical state, and neuromodulation by combining optogenetic LC stimulation with EEG recordings and pupillometry in awake mice of both sexes. A comparison between EEG signals during spontaneous phasic pupil dilation and those during phasic pupil dilation evoked by LC stimulation revealed distinct cortical states. Using machine learning techniques, we trained a convolutional neural network classifier to distinguish between types of pupil dilation based on the power dynamics of individual EEG frequency bands. The results confirmed that all EEG bands, but most significantly Gamma, differ markedly between spontaneous phasic arousal and LC stimulation-evoked arousal. Moreover, pharmacological manipulations to either block α or β adrenergic receptors or agonize α-2 adrenergic receptors were employed to explore how adrenergic receptors could influence the coupling between phasic pupil dilation and cortical state. With each manipulation uniquely modulating EEG power and pupil size, our results highlight the differentiated role of adrenergic receptors in moderating the coupling between pupil-linked arousal and cortical state. This study provides new insights into the complex relationship between pupil-linked arousal and cortical arousal state, underscoring the significant role of the LC-NE system in influencing these arousal states. Significance Statement This study reveals the role of the locus coeruleus in pupil-linked arousal coupling with cortical arousal state by uncovering the different relationships between LC stimulation-evoked versus spontaneous phasic pupil dilations and cortical EEG. By integrating machine learning and noradrenergic pharmacological manipulation, our findings highlight the distinct cortical state associated with spontaneous phasic arousal and LC stimulation-evoked arousal, as well as the crucial role of different subtypes of adrenergic receptors in moderating the coupling between pupil size and cortical state.