Prevalence of sympathetic fibers within the rat cervical vagus, and functional consequence on physiological effects mediated by vagus nerve stimulation (VNS).

Electrical stimulation of the vagus nerve (VNS) is an FDA approved therapy for epilepsy, depression and rehabilitation after stroke, with recent clinical trials to treat heart failure and inflammation. VNS is often assumed to activate either parasympathetic efferents projecting to visceral organs, and/or sensory afferents projecting from these organs, for its therapeutic effects. Recent studies in humans, swine and dogs have shown that sympathetic nerve fibers from the sympathetic trunk (ST) can frequently be found within the cervical vagus nerve (VN). However, the prevalence and functional consequence of sympathetic fibers on VNS have yet to be elucidated in the most common high throughput animal model to study disease, the rodent. Methods: We carefully traced ST from sympathetic cervical ganglion (SCG) to find its location in the carotid sheath with reference to the VN in a cohort of Long Evans rats. We then assessed the prevalence of ST fibers with the cervical VN across the cohort using microCT and immunohistochemistry. Finally, we stimulated the VN and the ST in isolation, and where they were conjoined, to evaluate the ST contribution to changes in heart rate. VNS induced heart rate changes are a commonly used surrogate for changes in sympathetic/parasympathetic tone. Results: The ST frequently runs in very close proximity to the VN in rats when traced caudally from the SCG. The ST is even conjoined with the VN for stretches within the carotid sheathe at the most common location to place an epineural cuff. Cross-connecting branches were found between the ST and the VN. VNS performed at locations where there was minimal ST crossover induced dose-dependent bradycardia (decrease in heart rate) across the cohort, with detectable bradycardia across the cohort beginning at 50 μA (n=8 right, n=3 left). Conversely, stimulation of the isolated ST induced tachycardia (increase in heart rate) across the cohort beginning at ~200 μA (n=7 right, n=3 left). Conclusion: These data suggest that studies of VNS in the rodent model may also be stimulating sympathetic fibers from the ST in addition to canonical VN pathways. Concurrent sympathetic activation has profound implications for dissecting mechanisms of VNS for a host of diseases/disorders. As such, careful post-mortem assessment of the presence of 'hitchhiking' sympathetic fibers within the VN is critical for understanding sources of variability in VNS outcomes.