Graded recruitment of pupil-linked neuromodulation by parametric stimulation of the vagus nerve

Vagus nerve stimulation (VNS) is thought to affect neural activity by recruiting brain-wide release of neuromodulators. VNS is used in treatment-resistant epilepsy, and is increasingly being explored for other disorders, such as depression, and as a cognitive enhancer. However, the promise of VNS is only partially fulfilled due to a lack of mechanistic understanding of the transfer function between stimulation parameters and neuromodulatory response, together with a lack of biosensors for assaying stimulation efficacy in real time. We here develop an approach to VNS in head-fixed mice on a treadmill and show that pupil dilation is a reliable and convenient biosensor for VNS-evoked cortical neuromodulation. In an ‘optimal’ zone of stimulation parameters, current leakage and off-target effects are minimized and the extent of pupil dilation tracks VNS-evoked basal-forebrain cholinergic axon activity in neocortex. Thus, pupil dilation is a sensitive readout of the moment-by-moment, titratable effects of VNS on brain state. Despite its wide and growing use, the mechanisms by which in vivo vagus nerve stimulation (VNS) exerts its therapeutic benefits are still largely unknown. Here, the authors show in mice that pupil dilation is a reliable and noninvasive biosensor for titratable VNS-evoked cortical neuromodulation by acetylcholine.