Selective stimulation of the ferret abdominal vagus nerve with multi-contact nerve cuff electrodes

Abstract

Dysfunction and diseases of the gastrointestinal (GI) tract are a major driver of medical care. The vagus nerve innervates and controls multiple organs of the GI tract and vagus nerve stimulation (VNS) could provide a means for affecting GI function and treating disease. However, the vagus nerve also innervates many other organs throughout the body, and off-target effects of VNS could cause major side effects such as changes in blood pressure. In this study, we aimed to achieve selective stimulation of populations of vagal afferents using a multi-contact cuff electrode wrapped around the abdominal trunks of the vagus nerve. Four-contact nerve cuff electrodes were implanted around the dorsal (N = 3) or ventral (N = 3) abdominal vagus nerve in six ferrets, and the response to stimulation was measured via a 32-channel microelectrode array (MEA) inserted into the left or right nodose ganglion. Selectivity was characterized by the ability to evoke responses in MEA channels through one bipolar pair of cuff contacts but not through the other bipolar pair. We demonstrated that it was possible to selectively activate subpopulations of vagal neurons using abdominal VNS. Additionally, we quantified the conduction velocity of evoked responses to determine what types of nerve fibers (i.e., Aδ vs. C) responded to stimulation. We also quantified the spatial organization of evoked responses in the nodose MEA to determine if there is somatotopic organization of the neurons in that ganglion. Finally, we demonstrated in a separate set of three ferrets that stimulation of the abdominal vagus via a four-contact cuff could selectively alter gastric myoelectric activity, suggesting that abdominal VNS can potentially be used to control GI function.

Figure 1

Experimental setup. In six ferrets, (a,b) a four-contact cuff electrode was wrapped around either the ventral or dorsal abdominal vagus nerve to deliver biphasic symmetric stimulus pulses, either between contacts 1 and 2 or 3 and 4. (b,c) A 32-channel microelectrode array (MEA) was inserted into the left or right nodose ganglion to record evoked responses to stimulation of either the ventral or dorsal abdominal vagus nerve.

Figure 2

Nodose compound action potentials. (a) 120 repetitions of a single stimulus pulse were delivered at 2 Hz and a stimulus-triggered average was calculated to reduce noise amplitude and improve signal-to-noise ratio. (b) A sliding 1 ms RMS window smoothed the averaged signal to further reduce noise. Detection threshold (red line) was set to 2.4–2.6 times the standard deviation of the smoothed, averaged signal.

Figure 3

Selectivity as a function of stimulation amplitude. Grids of blue traces show 500 ms post-stimulation recordings from 32 MEA channels, with red horizontal lines representing the threshold for detecting an evoked response. Traces with red shading include a response evoked by stimulation from cuff pair 1–2. Traces with blue shading include a response evoked by stimulation from cuff pair 3–4. Grids of red, blue, and purple squares show the pattern of recruitment and overlap in responses at various stimulation amplitudes. Red, blue, and purple squares represent MEA channels with responses evoked by only cuff pair 1–2, only cuff pair 3–4, or both cuff pairs, respectively. In this example from a single animal, as stimulation amplitude increases from 400 to 580 µA for bipolar cuff pair 1–2, the number of selectively responding MEA channels increases from one to fifteen. Similarly, as stimulation amplitude increases from 400 to 460 µA for bipolar cuff pair 3–4, the number of selectively responding MEA channels increases from one to three. Additionally, as stimulation amplitude increases, the number of MEA channels responding to both bipolar cuff pairs increases from one to four. The orientation of channels in the figure is from caudal to rostral, left to right, on the ventral side of the nodose ganglia.

Figure 4

Selectivity of abdominal VNS. Number of responding MEA channels for each animal at (left) the stimulation amplitude that maximized SI and (right) the maximum tested stimulation amplitude (i.e. 3 mA). Pairs of bars represent the two bipolar pairs of cuff contacts in each animal (left is cuff pair 1–2, right is cuff pair 3–4). Blue bars represent MEA channels responding to only one of the two bipolar pairs, orange bars allow for up to 3 overlapping channels (i.e. 10% overlap) responding to stimulation from both cuff pairs, and gray bars include all non-selectively stimulated channels. Stimulation pulse widths include (a) 0.1 ms, (b) 0.5 ms, and (c) 1 ms.

Figure 5

Conduction velocities of evoked responses. Histograms of the conduction velocities of evoked responses at (left) threshold and (right) maximum stimulation amplitude (i.e. 3 mA) for pulse widths of (a) 0.1, (b) 0.5, and (c) 1 ms. Most evoked responses had conduction velocities between 0 and 3 m/s (i.e. c fibers), although a smaller set of responses had conduction velocities between 3 and 30 m/s (i.e. Aδ fibers).

Figure 6

Distance between responding MEA channels. (a,c,e) Distance between each responding MEA channel and the nearest responding MEA channel from the same bipolar cuff pair (purple) or the other bipolar cuff pair (green) for pulse widths of (a) 0.1, (c) 0.5, and (e) 1 ms at the stimulation amplitude that maximized SI. (b,d,f) Distance between centroids of activation on the MEA for stimulation through each of the bipolar cuff pairs at the stimulation amplitude that maximized SI (blue) and the maximum tested stimulation amplitude (i.e. 3 mA; red) for pulse widths of (b) 0.1, (d) 0.5, and (f) 1 ms. Yellow lines show the median distance.

Figure 7

Low amplitude abdominal VNS selectively changes GI myoelectric activity but not breathing or heart rate. (a,e,i) In three animals, stimulation (orange) through one cuff pair caused a substantial change in normogastric power as compared to baseline (blue), while stimulation through the other cuff pair had a smaller or no effect. (b,f,j) Waterfall plots show signal power in frequencies between 6 and 15 CPM over time. Blue lines are pre-stimulation baseline and orange lines are during stimulation. (c,g,k) Stimulation through either cuff pair had little or no effect on breathing rate. (d,h,l) Stimulation through either cuff pair had little or no effect on heart rate.