Parametric characterization of neural activity in the locus coeruleus in response to vagus nerve stimulation

Abstract

Vagus nerve stimulation (VNS) has emerged as a therapy to treat a wide range of neurological disorders, including epilepsy, depression, stroke, and tinnitus. Activation of neurons in the locus coeruleus (LC) is believed to mediate many of the effects of VNS in the central nervous system. Despite the importance of the LC, there is a dearth of direct evidence characterizing neural activity in response to VNS. A detailed understanding of the brain activity evoked by VNS across a range of stimulation parameters may guide selection of stimulation regimens for therapeutic use. In this study, we recorded neural activity in the LC and the mesencephalic trigeminal nucleus (Me5) in response to VNS over a broad range of current amplitudes, pulse frequencies, train durations, inter-train intervals, and pulse widths. Brief 0.5s trains of VNS drive rapid, phasic firing of LC neurons at 0.1mA. Higher current intensities and longer pulse widths drive greater increases in LC firing rate. Varying the pulse frequency substantially affects the timing, but not the total amount, of phasic LC activity. VNS drives pulse-locked neural activity in the Me5 at current levels above 1.2mA. These results provide insight into VNS-evoked phasic neural activity in multiple neural structures and may be useful in guiding the selection of VNS parameters to enhance clinical efficacy.

Keywords: Frequency; Locus coeruleus; Mesencephalic trigeminal nucleus; Pulse width; Stimulation intensity; Stimulation parameters; Vagus nerve stimulation.

Fig. 1. Identification of neurons in locus coeruleus

(A) Recording sites in the LC were characterized by a brief increase in firing rate followed by a suppression in response to a hindpaw pinch (denoted by line below panel). (B) Brief trains of VNS elicited driven activity in LC neurons. (C) Characteristic wide spike shape was observed in well-isolated LC units. (D) Example histological verification of LC recording site. White * marks TH-positive neurons in LC contralateral to the recording site. Red diamond marks the electrolytic lesion location. Scale bar is 250 μV × 500 ms in panels A&B; 500 μV × 1 ms in C; 1 mm in D.

Fig. 2. VNS drives rapid, phasic neural activity in LC

(A) Example raster plot showing representative neural activity at one recording location in LC in response to 4, 16, and 64 pulse trains of VNS at 0.8 mA, 100 μs at 30 Hz. Yellow background denotes stimulation period. (B) Population PSTH of neural responses to 4 (blue), 16 (green), and 64 (red) pulses of VNS at 30 Hz. Colored lines above the PSTH represent significant positive driven response duration. VNS pulse timing is illustrated below PSTH. (C) Longer VNS train durations result in linear increases in number of driven spikes. *** p < 0.001; all statistical comparisons versus spontaneous rate.

Fig. 3. Increasing stimulation intensities drive greater phasic neural activity in LC

(A) Example neural activity from a well-isolated single unit across a range of current intensities. The yellow shaded region denotes stimulation period. (B) Average driven spikes for a single unit across 20 recording sweeps between 1–750 ms response period. (C) Example multiunit recording showing phasic driven activity (1–750 ms response period) across a range of current intensities. (D) Analysis of group data of the phasic driven response (1–750 ms response period) demonstrates significant increases in driven activity at 0.1 mA stimulation intensity. Stronger current intensities drive greater increases in firing rate. Bold black line represents group average across 23 sites. Thin gray lines represent data from individual sites. (E) PSTH illustrates monotonic increases in phasic response across stimulation intensities. Colored lines above the PSTH represents significant positive driven response duration. VNS pulse timing represented below. Inset highlights offset response from 751 – 1500 ms. (F) Offset responses (751 – 1500 ms) demonstrate a modest suppression of neural activity compared to spontaneous at intensities from 0.2 – 0.8 mA and an modest increase compared to spontaneous at 1.6 mA and 2.5 mA. * p < 0.05, ** p < 0.01, *** p < 0.001; all statistical comparisons versus 0 mA (spontaneous rate).

Fig. 4. Frequency changes the timing, but not total amount of VNS-driven neural activity in LC

(A) Example raster plot from a single recording site across a range of frequencies. The yellow shaded region denotes stimulation period. (B) PSTH of population data illustrates that the timing and maximal rate of driven activity is influenced by pulse frequency. Colored lines above the PSTH represents significant positive driven response duration. VNS pulse timing represented below. Note that the number of pulses was matched across conditions. Higher frequencies drive stronger, shorter neural activity for a fixed number of pulses. (C) At all frequencies tested, VNS drives significant increases in neural activity. *** p < 0.001; all statistical comparisons versus 0 Hz (spontaneous rate).

Fig. 5. Increasing pulse widths drive greater neural activity in LC

(A) At each current intensity, increasing pulse widths drive greater neural activity in LC neurons. (B) Driven spikes in the LC increase approximately linearly as a function of total charge per pulse (pulse width × current) up to 160 nC. After this point, additional charge results in diminishing increases in neural activity. Line colors in legend apply to both panels. * p < 0.001; all statistical comparisons in panel A versus 0 μs (spontaneous rate).

Fig. 6. VNS drives pulse-locked activity in Me5 neurons

(A) Example Me5 activity demonstrating characteristic increase in firing rate to movement of the jaw (denoted by line). (B & C) Raster plots of representative neural activity from two Me5 recording sites illustrate the strongly pulse-locked response to VNS. The yellow shaded region denotes stimulation period. Pulse timing represented below in (B). (D) Group data demonstrates a significant driven response in Me5 at 1.2 mA and above. Increasing current intensities drives stronger increases in firing rates. Bold black line represents group average across sites. Thin gray lines represent data from individual sites. *** p < 0.001; all statistical comparisons versus 0 mA (spontaneous rate).

Fig. 7. Comparison of LC and Me5 response to VNS at all recording sites

Evaluation of minimum stimulation needed to evoke driven activity and degree of pulse-locking highlights distinctive LC and Me5 response to VNS. LC neurons respond at significantly lower stimulation intensities compared to Me5 neurons. Vector strength at 1.6 mA is significantly greater in Me5 neurons, representative of the strongly pulse-locked responses to individual pulses within a VNS train. Group distributions are plotted on the top and right edge.