Impact of anesthesia on micromagnetic stimulation (MS) of the vagus nerve

To treat diseases associated with vagal nerve control of peripheral organs, it is necessary to selectively activate efferent and afferent fibers in the vagus. As a result of the nerve's complex anatomy, fiber-specific activation proves challenging. Spatially selective neuromodulation using micromagnetic stimulation(mu MS) is showing incredible promise. This neuromodulation technique uses microcoils(mu coils) to generate magnetic fields by powering them with a time-varying current. Following the principles of Faraday's law of induction, a highly directional electric field is induced in the nerve from the magnetic field. In this study on rodent cervical vagus, a solenoidal mu coil was oriented at an angle to left and right branches of the nerve. The aim of this study was to measure changes in the mean arterial pressure (MAP) and heart rate (HR) following mu MS of the vagus. The mu coils were powered by a single-cycle sinusoidal current varying in pulse widths(PW = 100, 500, and 1000 mu sec) at a frequency of 20 Hz. Under the influence of isoflurane, mu MS of the left vagus at 1000 mu sec PW led to an average drop in MAP of 16.75 mmHg(n = 7). In contrast, mu MS of the right vagus under isoflurane resulted in an average drop of 11.93 mmHg in the MAP(n = 7). Surprisingly, there were no changes in HR to either right or left vagal mu MS suggesting the drop in MAP associated with vagus mu MS was the result of stimulation of afferent, but not efferent fibers. In urethane anesthetized rats, no changes in either MAP or HR were observed upon mu MS of the right or left vagus(n = 3). These findings suggest the choice of anesthesia plays a key role in determining the efficacy of mu MS on the vagal nerve. Absence of HR modulation upon mu MS could offer alternative treatment options using VNS with fewer heart-related side-effects.