A Preclinical Study of Laryngeal Motor-Evoked Potentials as a Marker Vagus Nerve Activation

Vagus nerve stimulation enhances remyelination and decreases innate neuroinflammation in lysolecithin-induced demyelination

BACKGROUND

Current treatments for Multiple Sclerosis (MS) poorly address chronic innate neuroinflammation nor do they offer effective remyelination. The vagus nerve has a strong regulatory role in inflammation and Vagus Nerve Stimulation (VNS) has potential to affect both neuroinflammation and remyelination in MS.

OBJECTIVE

This study investigated the effects of VNS on demyelination and innate neuroinflammation in a validated MS rodent model.

METHODS

Lysolecithin (LPC) was injected in the corpus callosum (CC) of 46 Lewis rats, inducing a demyelinated lesion. 33/46 rats received continuously-cycled VNS (cVNS) or one-minute per day VNS (1minVNS) or sham VNS from 2 days before LPC-injection until perfusion at 3 days post-injection (dpi) (corresponding with a demyelinated lesion with peak inflammation). 13/46 rats received cVNS or sham from 2 days before LPC-injection until perfusion at 11 dpi (corresponding with a partial remyelinated lesion). Immunohistochemistry and proteomics analyses were performed to investigate the extend of demyelination and inflammation.

RESULTS

Immunohistochemistry showed that cVNS significantly reduced microglial and astrocytic activation in the lesion and lesion border, and significantly reduced the Olig2+ cell count at 3 dpi. Furthermore, cVNS significantly improved remyelination with 57.4 % versus sham at 11 dpi. Proteomic gene set enrichment analyses showed increased activation of (glutamatergic) synapse pathways in cVNS versus sham, most pronounced at 3 dpi.

CONCLUSION

cVNS improved remyelination of an LPC-induced lesion. Possible mechanisms might include modulation of microglia and astrocyte activity, increased (glutamatergic) synapses and enhanced oligodendrocyte clearance after initial injury.

Identification of vagus nerve stimulation parameters affecting rat hippocampal electrophysiology without temperature effects

BACKGROUND

Recent experiments in rats have demonstrated significant effects of VNS on hippocampal excitability but were partially attributed to hypothermia, induced by the applied VNS parameters.

OBJECTIVE

To allow meaningful preclinical research on the mechanisms of VNS and translation of rodent results to clinical VNS trials, we aimed to identify non-hypothermia inducing VNS parameters that significantly affect hippocampal excitability.

METHODS

VNS was administered in cycles of 30 s including either 0.1, 0.16, 0.25, 0.5, 1.5, 3 or 7 s of VNS ON time (biphasic pulses, 250μs/phase, 1 mA, 30 Hz) and the effect of different VNS ON times on brain temperature was evaluated. VNS paradigms with and without hypothermia were compared for their effects on hippocampal neurophysiology in freely moving rats.

RESULTS

Using VNS parameters with an ON time/OFF time of up to 0.5 s/30 s did not cause hypothermia, while clear hypothermia was detected with ON times of 1.5, 3 and 7 s/30 s. Relative to SHAM VNS, the normothermic 0.5 s VNS condition significantly decreased hippocampal EEG power and changed dentate gyrus evoked potentials with an increased field excitatory postsynaptic potential slope and a decreased population spike amplitude.

CONCLUSION

VNS can be administered in freely moving rats without causing hypothermia, while profoundly affecting hippocampal neurophysiology suggestive of reduced excitability of hippocampal neurons despite increased synaptic transmission efficiency.

An Implantable System For Chronic In Vivo Electromyography

Electromyography (EMG) measures the muscle response to electrical stimulation or spontaneous activity of motor units and plays an important role in assessing neuromuscular function. Chronic recording of EMG activity reflecting a muscle's reinnervation status after nerve injury has been limited, due to the invasive nature of traditional EMG recording techniques. In this regard, an implantable system is designed for long-term, in vivo EMG recording and nerve stimulation. It has been applied and tested in a study on reinnervation of laryngeal muscles. This system consists of 1) two bipolar electrode nerve cuffs and leads for stimulating each of two nerves: the recurrent laryngeal nerve (RLN) and internal branch of the superior laryngeal nerve (SLN); 2) two EMG recording electrodes and leads for each of the two laryngeal muscles: posterior cricoarytenoid (PCA) muscle and thyroarytenoid-lateral cricoarytenoid (TA-LCA) muscle complex; and 3) a skin receptacle interfacing all implanted lead terminals to an external recording preamplifier and stimulator using a connection cable. The wire leads are Teflon-coated, multi-filament, type 316 stainless steel. They are coiled and can stretch during body movement of the awake animal to prevent lead breakage and electrode migration. This system is implanted during an aseptic surgery. Afterwards, baseline EMG recordings are performed before the RLN is transected in the second surgery to study muscle reinnervation. Throughout the study, multiple physiological sessions are conducted in the anesthetized animal to obtain evoked and spontaneous EMG activity that reflects the reinnervation status of laryngeal muscles. The system is compact, free of infection over the course of the study, and highly durable. This implantable system can provide a reliable platform for research in which long-term recording or nerve stimulation is required in an anesthetized or freely moving animal.

Vagus Nerve Stimulation-Induced Laryngeal Motor Evoked Potentials: A Possible Biomarker of Effective Nerve Activation

Vagus nerve stimulation (VNS) therapy is associated with laryngeal muscle activation and induces voice modifications, well-known side effects of the therapy resulting from co-activation of the recurrent laryngeal nerve. In this study, we describe the non-invasive transcutaneous recording of laryngeal motor evoked potentials (LMEPs), which could serve as a biomarker of effective nerve activation and individual titration in patients with drug-resistant epilepsy. We recruited drug-resistant epileptic patients treated for at least 6 months with a VNS. Trains of 600-1200 VNS pulses were delivered with increasing current outputs. We placed six skin electrodes on the ventral surface of the neck, in order to record LMEPs whenever the laryngeal muscular threshold was reached. We studied the internal consistency and the variability of LMEP recordings, and compared different methods for amplitude calculation. Recruitment curves were built based on the stimulus-response relationship. We also determined the electrical axis of the LMEPs dipole in order to define the optimal electrode placement for LMEPs recording in a clinical setting. LMEPs were successfully recorded in 11/11 patients. The LMEPs threshold ranged from 0.25 to 1 mA (median 0.50 mA), and onset latency was between 5.37 and 8.77 ms. The signal-to-noise ratio was outstanding in 10/11 patients. In these cases, excellent reliability (Intraclass correlation coefficient, ICC > 0.90 across three different amplitude measurements) was achieved with 10 sample averages. Moreover, our recordings showed very good internal consistency (Cronbach's alpha > 0.95 for 10 epochs). Area-under-the-curve and peak-to-peak measurement proved to be complementary methods for amplitude calculation. Finally, we determined that an optimal derivation requires only two recording electrodes, aligned on a horizontal axis around the laryngeal prominence. In conclusion, we describe here an optimal methodology for the recording of VNS-induced motor evoked responses from the larynx. Although further clinical validation is still necessary, LMEPs might be useful as a non-invasive marker of effective nerve activation, and as an aid for the clinician to perform a more rational titration of VNS parameters.

Hypothermia Masks Most of the Effects of Rapid Cycling VNS on Rat Hippocampal Electrophysiology

AIM. Vagus nerve stimulation (VNS) modulates hippocampal dentate gyrus (DG) electrophysiology and induces hypothermia in freely moving rats. This study evaluated whether hippocampal (CA1) electrophysiology is similarly modulated and to what extent this is associated with VNS-induced hypothermia. METHODS. Six freely moving rats received a first 4h session of rapid cycling VNS (7s on/18s off), while CA1 evoked potentials, EEG and core temperature were recorded. In a second 4h session, external heating was applied during the 3rd and 4thh of VNS counteracting VNS-induced hypothermia. RESULTS. VNS decreased the slope of the field excitatory postsynaptic potential (fEPSP), increased the population spike (PS) amplitude and latency, decreased theta (4-12Hz) and gamma (30-100Hz) band power and theta peak frequency. Normalizing body temperature during VNS through external heating abolished the effects completely for fEPSP slope, PS latency and gamma band power, partially for theta band power and theta peak frequency and inverted the effect on PS amplitude. CONCLUSIONS. Rapid cycle VNS modulates CA1 electrophysiology similarly to DG, suggesting a wide-spread VNS-induced effect on hippocampal electrophysiology. Normalizing core temperature elucidated that VNS-induced hypothermia directly influences several electrophysiological parameters but also masks a VNS-induced reduction in neuronal excitability.

The interaction of pulse width and current intensity on the extent of cortical plasticity evoked by vagus nerve stimulation

BACKGROUND

Repeatedly pairing a tone with a brief burst of vagus nerve stimulation (VNS) results in a reorganization of primary auditory cortex (A1). The plasticity-enhancing and memory-enhancing effects of VNS follow an inverted-U response to stimulation intensity, in which moderate intensity currents yield greater effects than low or high intensity currents. It is not known how other stimulation parameters effect the plasticity-enhancing effects of VNS.

OBJECTIVE

We sought to investigate the effect of pulse-width and intensity on VNS efficacy. Here, we used the extent of plasticity induced by VNS-tone pairing to assess VNS efficacy.

METHODS

Rats were exposed to a 9 kHz tone paired to VNS with varying current intensities and pulse widths. Cortical plasticity was measured as changes in the percent of area of primary auditory cortex responding to a range of sounds in VNS-treated rats relative to naïve rats.

RESULTS

We find that a combination of low current intensity (200 μA) and short pulse duration (100 μs) is insufficient to drive cortical plasticity. Increasing the pulse duration to 500 μs results in a reorganization of receptive fields in A1 auditory cortex. The extent of plasticity engaged under these conditions is less than that driven by conditions previously reported to drive robust plasticity (800 μA with 100 μs wide pulses).

CONCLUSION

These results suggest that the plasticity-enhancing and memory-enhancing effects of VNS follow an inverted-U response of stimulation current that is influenced by pulse width. Furthermore, shorter pulse widths may offer a clinical advantage when determining optimal stimulation current. These findings may facilitate determination of optimal VNS parameters for clinical application.

EEG Derived Brain Activity Reflects Treatment Response from Vagus Nerve Stimulation in Patients with Epilepsy

The mechanism of action of vagus nerve stimulation (VNS) is yet to be elucidated. To that end, the effects of VNS on the brain of epileptic patients were studied. Both when VNS was switched "On" and "Off", the brain activity of responders (R, seizure frequency reduction of over 50%) was compared to the brain activity of nonresponders (NR, seizure frequency reduction of less than 50%). Using EEG recordings, a significant increase in P300 amplitude for R and a significant decrease in P300 amplitude for NR were found. We found biomarkers for checking the efficacy of VNS with accuracy up to 94%. The results show that P300 features recorded in nonmidline electrodes are better P300 biomarkers for VNS efficacy than P300 features recorded in midline electrodes. Using source localization and connectivity analyses, the activity of the limbic system, insula and orbitofrontal cortex was found to be dependent on VNS switched "On" versus "Off" or patient group (R versus NR). The results suggest an important role for these areas in the mechanism of action of VNS, although a larger patient study should be done to confirm the findings.

Vagus Nerve Stimulation Applied with a Rapid Cycle Has More Profound Influence on Hippocampal Electrophysiology Than a Standard Cycle

Although vagus nerve stimulation (VNS) is widely used, therapeutic mechanisms and optimal stimulation parameters remain elusive. In the present study, we investigated the effect of VNS on hippocampal field activity and compared the efficiency of different VNS paradigms. Hippocampal electroencephalography (EEG) and perforant path dentate field-evoked potentials were acquired before and during VNS in freely moving rats, using 2 VNS duty cycles: a rapid cycle (7 s on, 18 s off) and standard cycle (30 s on, 300 s off) and various output currents. VNS modulated the evoked potentials, reduced total power of the hippocampal EEG, and slowed the theta rhythm. In the hippocampal EEG, theta (4-8 Hz) and high gamma (75-150 Hz) activity displayed strong phase amplitude coupling that was reduced by VNS. Rapid-cycle VNS had a greater effect than standard-cycle VNS on all outcome measures. Using rapid cycle VNS, a maximal effect on EEG parameters was found at 300 μA, beyond which effects saturated. The findings suggest that rapid-cycle VNS produces a more robust outcome than standard cycle VNS and support already existing preclinical evidence that relatively low output currents are sufficient to produce changes in brain physiology and thus likely also therapeutic efficacy.