Chen, R. C.-H. | Search bioRxiv

bioRxiv2025

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

IntroductionElectrical 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. MethodsWe 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. ResultsThe 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 A (n=8 right, n=3 left). Conversely, stimulation of the isolated ST induced tachycardia (increase in heart rate) across the cohort beginning at [~]200 A (n=7 right, n=3 left). ConclusionThese 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.

neuroscience↗

bioRxiv2025

Combined Central and Peripheral Nerve Stimulation Improves Functional Recovery of Mixed Peripheral Nerve Injury in a Rat Forelimb Model

IntroductionPeripheral nerve reinnervation following nerve injury is often a slow and incomplete process, resulting in significant morbidity and permanent loss of function of the injured extremity in many patients. Prior studies have shown the efficacy of electrical stimulation to synchronize the axonal regeneration of both motor and sensory neurons in peripheral nerve injury models. Moreover, separate investigations have also shown the use of cranial nerve stimulation, principally the vagus nerve, to improve functional outcomes. However, no study has investigated the synergistic effects of both intraoperative electrical stimulation and cranial nerve stimulation for functional improvement within a peripheral nerve injury model. This investigation quantifies the efficacy of combined intraoperative electrical stimulation and trigeminal nerve stimulation on motor and sensory functional recovery in a rat peripheral nerve injury model. MethodsTwelve adult male Lewis rats were trained in a reach and pull task for a food reward using their right forelimb with baseline force thresholds and percent success of the pull task recorded. Baseline sensory data was retrieved using an automated von Frey monofilament test. All rats underwent surgical transection and 2mm gap repair of their median and ulnar nerve of their right forelimb followed by 1 hour of intraoperative electrical stimulation. Trigeminal nerve stimulation throughout the rehabilitation period was completed via supraorbital nerve headcap electrodes. Motor and sensory data were compared to historic cohorts comprised of sham surgery (no nerve injury), brief intraoperative electrical stimulation, trigeminal nerve stimulation, and a sham peripheral and trigeminal nerve stimulation group. Polarization sensitive optical computed tomography (PS-OCT) was used to assess nerve regeneration in fixed tissue samples. ResultsThe combined cohort of rodents were able to recover to their pre-injury motor function by the third week of rehabilitation, faster than either of the singular electrical stimulation cohorts assessed previously. Moreover, functional sensory data of the combined stim cohort demonstrated no change when compared to their pre-injury baseline. ConclusionsPeripheral nerve electrical stimulation and trigeminal nerve stimulation are two separately acting mechanisms of therapy that employ electric waveforms to improve the functional recovery of injured peripheral nerves. The former acts within the periphery to synchronize axonal growth and regeneration of the injured neurons, while the latter acts centrally to augment neuroplasticity. When used simultaneously in a rodent peripheral nerve injury model, these modalities have shown to build upon each other to deliver a faster functional motor recovery, while sensory recovery outcomes remain to be demonstrated.

neuroscience↗