Abstract
BACKGROUND: Vagus nerve stimulation (VNS) paired with forelimb motor training enhances reorganization of movement representations in the motor cortex. Previous studies have shown an inverted-U relationship between VNS intensity and plasticity in other brain areas, such that moderate intensity VNS yields greater cortical plasticity than low or high intensity VNS. However, the relationship between VNS intensity and plasticity in the motor cortex is unknown. OBJECTIVE: In this study we sought to test the hypothesis that VNS intensity exhibits an inverted-U relationship with the degree of motor cortex plasticity in rats. METHODS: Rats were taught to perform a lever pressing task emphasizing use of the proximal forelimb musculature. Once proficient, rats underwent five additional days of behavioral training in which low intensity VNS (0.4 mA), moderate intensity VNS (0.8 mA), high intensity VNS (1.6 mA), or sham stimulation was paired with forelimb movement. 24 h after the completion of behavioral training, intracortical microstimulation (ICMS) was used to document movement representations in the motor cortex. RESULTS: VNS delivered at 0.8 mA caused a significant increase in motor cortex proximal forelimb representation compared to training alone. VNS delivered at 0.4 mA and 1.6 mA failed to cause a significant expansion of proximal forelimb representation. CONCLUSION: Moderate intensity 0.8 mA VNS optimally enhances motor cortex plasticity while low intensity 0.4 mA and high intensity 1.6 mA VNS fail to enhance plasticity. Plasticity in the motor cortex exhibits an inverted-U function of VNS intensity similar to previous findings in auditory cortex.