Reorganization of Motor Cortex by Vagus Nerve Stimulation Requires Cholinergic Innervation

Abstract

Background: Vagus nerve stimulation (VNS) paired with forelimb training drives robust, specific reorganization of movement representations in the motor cortex. The mechanisms that underlie VNS-dependent enhancement of map plasticity are largely unknown. The cholinergic nucleus basalis (NB) is a critical substrate in cortical plasticity, and several studies suggest that VNS activates cholinergic circuitry.

Objective: We examined whether the NB is required for VNS-dependent enhancement of map plasticity in the motor cortex.

Methods: Rats were trained to perform a lever pressing task and then received injections of the immunotoxin 192-IgG-saporin to selectively lesion cholinergic neurons of the NB. After lesion, rats underwent five days of motor training during which VNS was paired with successful trials. At the conclusion of behavioral training, intracortical microstimulation was used to document movement representations in motor cortex.

Results: VNS paired with forelimb training resulted in a substantial increase in the representation of proximal forelimb in rats with an intact NB compared to untrained controls. NB lesions prevent this VNS-dependent increase in proximal forelimb area and result in representations similar to untrained controls. Motor performance was similar between groups, suggesting that differences in forelimb function cannot account for the difference in proximal forelimb representation.

Conclusions: Together, these findings indicate that the NB is required for VNS-dependent enhancement of plasticity in the motor cortex and may provide insight into the mechanisms that underlie the benefits of VNS therapy.

Keywords: Acetylcholine; Cortical plasticity; Cortical reorganization; Immunotoxin; Motor cortex; Motor training; Nucleus basalis; Vagal nerve stimulation; Vagus nerve stimulation.

Figure 1. Experimental design

(A) Image of a rat performing the lever pressing task. (B, C) Representative data of lever pressing performance depicting a successful and unsuccessful trial. (D) Timeline of the experimental design.

Figure 2. 192-IgG-saporin lesions deplete cortical cholinergic innervation

Representative images of AChE fiber staining in layer V motor cortex of an NB+ control lesioned subject (A) and an NB- 192-IgG-saporin lesioned subject (B). The right-most panel shows a further magnification with white arrowheads marking fiber crossings. Calibrations are 1 mm for main image, and the spacing between gridlines is 50 μm for inset.

Figure 3. Representative ICMS maps

(A,B) Example motor cortex maps from an untrained control rat. (C,D) Example maps depicting the substantial increase in proximal forelimb representation in rats with an intact NB that received VNS paired with motor training. (E,F) Example maps from rats with an NB lesion that received VNS paired with motor training. Note the similarity to the untrained control map. Each square represents a 0.25 mm² (0.5 × 0.5 mm) area. Electrode penetrations occurred in the middle of each square. Raw maps from all subjects can be found in the supplementary data.

Figure 4. NB lesions prevent VNS-dependent motor cortex map reorganization

Total area of multiple movement representations in motor cortex. VNS paired with motor training in rats with an intact NB results in significantly greater proximal forelimb and jaw representations. NB lesions prevent VNS-dependent expansion of movement representations. Other movement representations are unchanged. * indicates p < 0.05 compared to NB+ group for each movement representation.

Figure 5. NB lesions do not change forelimb performance

No differences in forelimb performance measures, including hit rate (A), total number of trials per day (B), or speed of lever presses (C), were observed between groups before or after NB lesion. Additionally, both groups received a similar number of VNS stimulations (D).