The Effect of Vagus Nerve Stimulation on Heart Rate and Respiration Rate and their Impact on Seizure Susceptibility in Anaesthetized Rats under Pentylenetetrazol

Javier Chávez Cerda ^1* Elena Acedo Reina ² Ayse S. Dereli ² Louis Vande Perre ¹ Romain Raffoul ¹ Maxime Verstraeten ¹ Michel-Antony Ngan Yamb ¹ Enrique Germany Morrison ² Elise Collard ² Auriane Apaire ² Pascal Doguet ¹ Jérôme Garnier ³ Jean Delbeke ² Riëm El Tahry ² Antoine Nonclercq ¹

• ¹ Bio-,Electro-and Mechanical-Systems (BEAMS), Université Libre de Brussels, Brussels, Belgium
• ² Institute of Neurosciences (IoNS), Université Catholique de Louvain, Brussels, Belgium
• ³ Synergia Medical, Mont -Saint-Guibert, Belgium

Despite the proven efficacy of vagus nerve stimulation (VNS) in seizure control, its precise mechanism remains unclear. VNS affects the cardiorespiratory system. This study explored the effects of standard and breathing-synchronized VNS on heart and respiratory rates in anesthetized epileptic rats and their impact on seizure susceptibility. Seizures were induced using intravenous pentylenetetrazol (PTZ). Three groups (n=4) received Sham VNS, Standard VNS, or Breathing-Synchronized VNS. Measurements included respiration, electrocardiogram, electroencephalogram, and vagal electroneurogram.Each experiment began with a 5-minute baseline, followed by PTZ infusion until tonic-clonic seizure onset. Stimulation significantly decreased heart rate below baseline for standard VNS (-120.0 ± 69.1 bpm) and breathing-synchronized VNS (-84.9 ± 61.0 bpm), overcoming the PTZ-induced increase in the sham VNS (+79.2 ± 35.5 bpm), with no recovery during OFF periods. Regarding breathing rate, the sham VNS group showed a slight increase (+13.6 ± 1.8 bpm). Stimulation slightly increased the breathing rate for standard VNS (+13.0 ± 14.6 bpm) and breathing-synchronized VNS (+13.7 ± 10.4 bpm), with enlarged standard deviation, suggesting respiratory hypoxia under stimulation.Heart rate and breathing rate modulation were similar between standard and breathing-synchronized VNS, suggesting cumulative effects. Unexpectedly, the sham VNS group required a higher PTZ dose (79.7 ± 13.4 mg/kg) to reach seizures compared to standard VNS (57.9 ± 9.8 mg/kg) and breathing-synchronized VNS (60.0 ± 8.7 mg/kg), pointing to increased seizure susceptibility in stimulated groups. Seizure latency was longer in sham VNS (291.5 ± 84.4 s) than in standard VNS (200.5 ± 59.5 s) and breathing-synchronized VNS (206.9 ± 66.0 s), meaning seizures occurred earlier under stimulation. A significant linear relationship was found between heart rate, respiratory rate changes, and seizure susceptibility (R² = 0.62, p = 0.012).We hypothesize that the drop in heart rate and altered respiration, including apneas and breathing rate changes caused by VNS, relate to hypoxia and hypotension, increasing PTZ susceptibility. Future studies with larger samples, blood pressure, and oxygen monitoring are needed to clarify hypoxia and hypotension as potential factors in VNS-induced seizure susceptibility, emphasizing the need for precise VNS adjustments.