Stimulus manipulations permit activation of fiber subpopulations in the mouse and rat vagus

Abstract

Nerve fibers of different types converge in the cervical vagus, providing sensory and motor innervation to visceral organs and mediating therapeutic and off-target effects of vagus nerve stimulation (VNS). Selectively activating fiber subpopulations, especially small, unmyelinated fibers, which constitute the majority of vagal fibers, with cervical VNS remains a challenge, hindering the study of vagal regulation of organ homeostasis and limiting the safety and efficacy of VNS therapies. We sought to select and optimize parameters that preferentially activate large, intermediate or small vagal fibers in 2 popular experimental animal species, rats and mice. VNS trains with different waveforms, pulsing frequencies and intensities were tested. Resulting engagement of fibers was quantified over 3 time scales: in the millisecond range, using stimulus-elicited compound action potentials, in the second to minute range, using cardiorespiratory, vagus-mediated physiological responses and in the minute to hour range, using c-Fos expression in vagal neuronal populations in the brainstem. From those measurements, selectivity indices were compiled for different fiber types and optimal stimulus parameters were determined. Large and intermediate-size fibers are activated by trains of short-square and long-square/quasi-trapezoidal pulses, respectively, at different optimal intensities in different animals. Small fibers are selectively activated by trains of >8KHz frequency and relatively high stimulus intensities, that at the same time block larger fibers. Results from these manipulations were replicated in rats and mice, suggesting that they translate across species. In a computational model of vagal fibers, fiber-selectivity of KES could be explained by how fibers with the same ionic channels but different morphologies respond to KHz stimuli of different intensities and frequencies. This study provides a robust design and optimization framework for targeting vagal fibers of different sizes in physiological and preclinical studies of VNS in rodents.