In vivo holographic photo-stimulation and two photon GCaMP6 imaging of vagus nerve axons using a GRIN lens integrated nerve cuff

Vagus nerve interfacing is of interest due to its central role in parasympathetic regulation of the visceral organs, as well as its modulatory effects on the brain, which have been shown to influence epilepsy, depression and migraines. Electrical vagus nerve stimulation (VNS) has shown therapeutic effect in humans, yet it lacks the specificity for controlling and studying targeted pathways. In contrast, optical techniques may enable axon-specific neuromodulation using genetically targeted opsin expression and spatial patterning of the photo-stimulus. In addition to light-activated stimulation, calcium-sensitive fluorescent reporters such as GCaMP6 present a pathway for axon-specific optical recording of activity. We demonstrate in vivo photo-stimulation and two-photon GCaMP6 fluorescence imaging in the vagus nerve using a novel GRIN lens-coupled nerve cuff in the anesthetized mouse. A pulsed near-IR laser (1040 nm, 300 fs) was modified by a spatial light modulator (SLM) in the Fourier plane and focused by the microscope objective through a GRIN relay lens to the cervical vagus nerve. By actuating the SLM, spatially selected regions of axons could be differentially stimulated within the nerve. Mouse vitals were monitored with a MouseOx suite and used to detect physiological changes in response to photo-stimulation. We were able to induce differential modulations in heart rate, respiratory rate, and blood-oxygen saturation upon photo-stimulation of selective spatial regions of the nerve. Additionally, we recorded two-photon GCaMP6 Ca2+ transients in vagal axons in response to both photo-stimulation and electrical stimulation.