Vagus nerve stimulation (VNS) is a promising neuromodulatory therapy under investigation for a range of disorders, including heart failure, gastric dysmotility, and migraine. Two primary forms of VNS are currently investigated: cervical VNS (cVNS), involving surgically implantation to activate vagal afferents in the cervical branch in the neck and transcutaneous auricular VNS (taVNS) which subcutaneously stimulates the auricular branch in the outer ear. The nucleus of the solitary tract (NTS) serves as a relay-station receiving 90% of vagal afferents, enabling connections with higher-order brain regions and other brainstem nuclei like the spinal trigeminal nucleus (Sp5) and locus coeruleus (LC), facilitating neuromodulation through VNS. Research has established the efficacy of VNS at 20–30 Hz for disorders like depression, but the impact of alternative stimulation parameters on medullary nuclei neuromodulation remains unclear. These studies used anesthetized rats to extracellularly record neuronal activity across varying VNS parameters within NTS, Sp5, and LC. Neuronal responses were classified as positive (increased activity), negative (decreased activity), or non-responders (no response). In LC, cVNS at standard paradigms (≥ 10 Hz) and bursting paradigms with shorter interburst intervals or increased pulses induced more positive responders, while standard 5 Hz generated more negative responders. Additionally, a build-up effect was observed in LC, with increased responders over consecutive VNS cycles. In NTS and Sp5, taVNS evoked comparable activation, with more positive responders at 20 Hz and 100 Hz and stronger responses at higher intensities. However, Sp5 responses were twice as strong compared to NTS. Furthermore, comparative analysis between taVNS and cVNS revealed similar overall activation in NTS, but distinct activation profiles in individual neurons indicate different pathways. Finally, the therapeutic efficacy of VNS therapy was evaluated in heart failure using a pressure-overload rat model. A 60-day cVNS treatment restored adverse cardiac remodeling and dysfunction, mitigated cardiac molecular changes, and prevented neuroinflammatory responses within brainstem nuclei. The findings presented herein demonstrated differential parameter-specific and nuclei-specific responses to taVNS and cVNS, investigated the mechanisms responsible for taVNS modulation, and confirmed that VNS therapy, when initiated early, can mitigate heart failure development and restore multiorgan homeostasis in a PO model.

This thesis describes a novel method to reversibly and safely block nerve conduction using a low frequency alternating current (LFAC) waveform at 1 Hz applied through a bipolar extrafascicular electrode. This work follows up on observations made on excised mammalian peripheral nerves and earthworm nerve cords. An<i> in-situ</i> electrophysiology setup was used to assess the LFAC<br>waveform on propagating action potentials (APs) within the cervical vagus nerve in anaesthetized Sprague-Dawley rats (n = 12). Two sets of bipolar cuff or hook electrodes were applied unilaterally to the cervical vagus nerve, which was crushed rostral to the electrodes to exclude reflex effects<br>on the animal. Pulse stimulation was applied to the rostral electrode, while the LFAC conditioning waveform was applied to the caudal electrode. The efferent volley, if unblocked, elicits acute bradycardia and hypotension. The degree of block of the vagal stimulation induced bradycardia<br>was used as a biomarker. Block was assessed by the ability to reduce the bradycardic drive by monitoring the heart rate (HR) and blood pressure (BP) during LFAC alone, LFAC with vagal stimulation, and vagal stimulation alone. LFAC applied via a hook electrode (n = 7) achieved 86.6 +/- 11% block at current levels 95 +/- 38 uAp (current to peak). When applied via a cuff electrode (n = 5) 85.3 +/- 4.60% block was achieved using current levels of 110 +/- 65 uAp. Furthermore, LFAC was explored on larger vagal afferent fibers in larger human sized nerve bundles projecting to effects mediated by a reflex. The effectiveness of LFAC was assessed in an <i>in-situ</i> electrophysiological setup on the left cervical vagus in anaesthetized domestic swine (n = 5). Two bipolar cuff electrodes were applied unilaterally to the cervical vagus nerve, which was crushed caudal to the electrodes to eliminate cardiac effects. A tripolar extrafascicular cuff electrode was placed most rostral on the nerve for recording of propagating APs induced by<br>electrical stimulation and blocked via the LFAC waveform.<br>Standard pulse stimulation was applied to the left cervical vagus to induce the Hering-Breuer reflex. If unblocked, the activation of the Hering-Breuer reflex would cause breathing to slow down and potentially cease. Block was quantified by the ability to reduce the effect of the Hering-Breuer<br>reflex by monitoring the breathing rate during LFAC alone, LFAC and vagal stimulation, and vagal stimulation alone. LFAC achieved 87.2 +/- 8.8% (n = 5) block at current levels of 0.8 +/- 0.3 mAp. Compound nerve action potentials (CNAP) were monitored directly. They show changes<br>in nerve activity during LFAC, which manifests itself as the slowing and amplitude reduction of components of the CNAPs. Since the waveform is balanced, all forward reactions are reversed, leading to a blocking method that is similar in nature to DC block without the potential issues of<br>toxic byproduct production. These results suggest that LFAC can achieve a high degree of nerve block in both small and large nerve bundles, resulting in the change in behavior of a biomarker, <i>in-vivo </i>in the mammalian nervous system at low amplitudes of electrical stimulation that are within the water window of the electrode.<br>