Diminishing accelerated long-term forgetting in mild cognitive impairment: Study protocol for a prospective, double-blind, placebo-controlled, randomized controlled trial.

Background: Harnessing the lifelong potential of the human brain for neuroplasticity may serve to maintain the viability of neural structures and postpone the onset of cognitive decline. The absence of effective pharmacological interventions to counter memory decline has encouraged scientists to test the possibility that noninvasive electrical stimulation may serve as an additional tool to improve memory abilities.Previous research showed that electrical stimulation of the greater occipital nerve enhances memory recall performance in young and older healthy subjects. This study aims to extend these findings to determine the effect of transcutaneous electrical stimulation of the greater occipital nerve on the improvement of episodic memory in individuals with amnestic Mild Cognitive Impairment (aMCI). Methods/design: This study is a prospective, double-blind, placebo-controlled, randomized parallel-group study. A total of 100 individuals with a diagnosis of aMCI according to NIA/AA will be recruited. Participants will be randomly assigned to one of four groups. One group will receive active non-invasive transcutaneous electrical stimulation of greater occipital nerve (NITESGON), while three groups will serve as controls (i.e., sham NITESGON, active NITESGON with local anesthesia, and active NITESGON on the C5/C6 nerve). The primary outcome, i.e., memory recall, will be determined by a word association task, and will be recorded at baseline, 7 days after NITESGON, and 28 days after NITESGON. The secondary outcome is neurophysiological changes determined by resting state EEG and will be assessed immediately before and after NITESGON. Discussion: The results will add new insights into improving episodic memory in individuals with aMCI. Trial registration: #NCT05289804 (clinicaltrial.gov). Protocol approval id: #SPREC102021-23 (Ethics Committee at Trinity College Dublin, School of Psychology).

Vagus nerve stimulation does not improve recovery of forelimb motor or somatosensory function in a model of neuropathic pain.

Nerve injury affecting the upper limb is a leading cause of lifelong disability. Damage to the nerves in the arm often causes weakness and somatosensory dysfunction ranging from numbness to pain. Previous studies show that combining brief bursts of electrical vagus nerve stimulation (VNS) with motor or tactile rehabilitation can restore forelimb function after median and ulnar nerve injury, which causes hyposensitivity of the ventral forelimb. Here, we sought to determine whether this approach would be similarly effective in a model of radial nerve injury that produces allodynia in the ventral forelimb. To test this, rats underwent complete transection of the radial nerve proximal to the elbow followed by tubular repair. In the first experiment, beginning ten weeks after injury, rats received six weeks of tactile rehabilitation, consisting of mechanical stimulation of either the dorsal or ventral region of the forepaw in the injured limb, with or without concurrent VNS. In a second experiment, a separate cohort of rats underwent six weeks of forelimb motor rehabilitative training with or without paired VNS. Contrary to findings in previous models of hyposensitivity, VNS therapy fails to improve recovery of either somatosensory or motor function in the forelimb after radial nerve injury. These findings describe initial evidence that pain may limit the efficacy of VNS therapy and thus highlight a characteristic that should be considered in future studies that seek to develop this intervention.

Common Cholinergic, Noradrenergic, and Serotonergic Drugs Do Not Block VNS-Mediated Plasticity.

Vagus nerve stimulation (VNS) delivered during motor rehabilitation enhances recovery from a wide array of neurological injuries and was recently approved by the U.S. FDA for chronic stroke. The benefits of VNS result from precisely timed engagement of neuromodulatory networks during rehabilitative training, which promotes synaptic plasticity in networks activated by rehabilitation. Previous studies demonstrate that lesions that deplete these neuromodulatory networks block VNS-mediated plasticity and accompanying enhancement of recovery. There is a great deal of interest in determining whether commonly prescribed pharmacological interventions that influence these neuromodulatory networks would similarly impair VNS effects. Here, we sought to directly test the effects of three common pharmaceuticals at clinically relevant doses that target neuromodulatory pathways on VNS-mediated plasticity in rats. To do so, rats were trained on a behavioral task in which jaw movement during chewing was paired with VNS and received daily injections of either oxybutynin, a cholinergic antagonist, prazosin, an adrenergic antagonist, duloxetine, a serotonin-norepinephrine reuptake inhibitor, or saline. After the final behavioral session, intracortical microstimulation (ICMS) was used to evaluate reorganization of motor cortex representations, with area of cortex eliciting jaw movement as the primary outcome. In animals that received control saline injections, VNS paired with training significantly increased the movement representation of the jaw compared to naïve animals, consistent with previous studies. Similarly, none of the drugs tested blocked this VNS-dependent reorganization of motor cortex. The present results provide direct evidence that these common pharmaceuticals, when used at clinically relevant doses, are unlikely to adversely impact the efficacy of VNS therapy.

High intensity VNS disrupts VNS-mediated plasticity in motor cortex.

Vagus nerve stimulation (VNS) paired with motor rehabilitation enhances recovery of function after neurological injury in rats and humans. This effect is ascribed to VNS-dependent facilitation of plasticity in motor networks. Previous studies document an inverted-U relationship between VNS intensity and cortical plasticity, such that moderate intensities increase plasticity, while low or high intensity VNS does not. We tested the interaction of moderate and high intensity VNS trains to probe the mechanisms that may underlie VNS-dependent plasticity. Rats performed a behavioral task where VNS was paired with jaw movement during chewing. For five days, subjects received 100 pairings of moderate intensity VNS (Standard VNS), 100 pairings alternating between moderate and high intensity VNS (Interleaved VNS), or 50 pairings of moderate intensity VNS (Short VNS) approximately every 8 s. After the final behavioral session, intracortical microstimulation (ICMS) was used to evaluate movement representations in motor cortex. 100 pairings of moderate intensity VNS enhanced motor cortex plasticity. Replacing half of moderate intensity stimulation with high intensity VNS blocked this enhancement of plasticity. Removing high intensity stimulation, leaving only 50 pairings of moderate intensity VNS, reinstated plasticity. These results demonstrate that there is a period for at least 8 s after high intensity stimulation in which moderate intensity VNS is not able to engage mechanisms required for synaptic reorganization. More importantly, this study demonstrates that changes in stimulation parameters are a critical determinant of the magnitude of plasticity and likely the efficacy of VNS-enhanced recovery.

Radial nerve injury causes long-lasting forelimb sensory impairment and motor dysfunction in rats.

INTRODUCTION: Peripheral nerve injury is a common cause of lifelong disability in the United States. Although the etiology varies, most traumatic nerve injuries occur in the upper limb and include damage to the radial nerve. In conjunction with the well-described effects of peripheral damage, nerve injuries are accompanied by changes in the central nervous system. A comprehensive understanding of the functional consequences of nerve injury is necessary to develop new therapeutic interventions. OBJECTIVES: We sought to characterize changes in sensory and motor function and central neurophysiology after radial nerve injury in rats. METHODS: To evaluate somatosensory function in the forelimb, we assessed mechanical withdrawal threshold, spontaneous forelimb use, and cold sensitivity in rats 10 and 16 weeks after radial nerve injury. To evaluate motor function, we assessed performance on a forelimb supination task for up to 16 weeks after nerve injury. Physiological changes in the motor and somatosensory cortex were assessed using intracortical microstimulation and multiunit recordings, respectively. RESULTS: Our results indicate that radial nerve injury causes long-lasting sensory and motor dysfunction. These behavioral deficits are accompanied by abnormal cortical activity in the somatosensory and motor cortex. CONCLUSION: Our results provide a novel characterization of functional deficits that are consistent with the clinical phenotype in patients with radial nerve injury and provide a framework for future studies to evaluate potential interventions.

Deficits in skilled motor and auditory learning in a rat model of Rett syndrome.

BACKGROUND: Rett syndrome is an X-linked neurodevelopmental disorder caused by a mutation in the gene MECP2. Individuals with Rett syndrome display developmental regression at an early age, and develop a range of motor, auditory, cognitive, and social impairments. Several studies have successfully modeled some aspects of dysfunction and Rett syndrome-like phenotypes in transgenic mouse and rat models bearing mutations in the MECP2 gene. Here, we sought to extend these findings and characterize skilled learning, a more complex behavior known to be altered in Rett syndrome. METHODS: We evaluated the acquisition and performance of auditory and motor function on two complex tasks in heterozygous female Mecp2 rats. Animals were trained to perform a speech discrimination task or a skilled forelimb reaching task. RESULTS: Our results reveal that Mecp2 rats display slower acquisition and reduced performance on an auditory discrimination task than wild-type (WT) littermates. Similarly, Mecp2 rats exhibit impaired learning rates and worse performance on a skilled forelimb motor task compared to WT. CONCLUSIONS: Together, these findings illustrate novel deficits in skilled learning consistent with clinical manifestation of Rett syndrome and provide a framework for development of therapeutic strategies to improve these complex behaviors.

Vagus nerve stimulation paired with tones restores auditory processing in a rat model of Rett syndrome.

BACKGROUND: Rett syndrome is a rare neurological disorder associated with a mutation in the X-linked gene MECP2. This disorder mainly affects females, who typically have seemingly normal early development followed by a regression of acquired skills. The rodent Mecp2 model exhibits many of the classic neural abnormalities and behavioral deficits observed in individuals with Rett syndrome. Similar to individuals with Rett syndrome, both auditory discrimination ability and auditory cortical responses are impaired in heterozygous Mecp2 rats. The development of therapies that can enhance plasticity in auditory networks and improve auditory processing has the potential to impact the lives of individuals with Rett syndrome. Evidence suggests that precisely timed vagus nerve stimulation (VNS) paired with sound presentation can drive robust neuroplasticity in auditory networks and enhance the benefits of auditory therapy. OBJECTIVE: The aim of this study was to investigate the ability of VNS paired with tones to restore auditory processing in Mecp2 transgenic rats. METHODS: Seventeen female heterozygous Mecp2 rats and 8 female wild-type (WT) littermates were used in this study. The rats were exposed to multiple tone frequencies paired with VNS 300 times per day for 20 days. Auditory cortex responses were then examined following VNS-tone pairing therapy or no therapy. RESULTS: Our results indicate that Mecp2 mutation alters auditory cortex responses to sounds compared to WT controls. VNS-tone pairing in Mecp2 rats improves the cortical response strength to both tones and speech sounds compared to untreated Mecp2 rats. Additionally, VNS-tone pairing increased the information contained in the neural response that can be used to discriminate between different consonant sounds. CONCLUSION: These results demonstrate that VNS-sound pairing may represent a strategy to enhance auditory function in individuals with Rett syndrome.

A limited range of vagus nerve stimulation intensities produce motor cortex reorganization when delivered during training.

Pairing vagus nerve stimulation (VNS) with rehabilitation has emerged as a potential strategy to improve recovery after neurological injury, an effect ascribed to VNS-dependent enhancement of synaptic plasticity. Previous studies demonstrate that pairing VNS with forelimb training increases forelimb movement representations in motor cortex. However, it is not known whether VNS-dependent enhancement of plasticity is restricted to forelimb training or whether VNS paired with other movements could induce plasticity of other motor representations. We tested the hypothesis that VNS paired with orofacial movements associated with chewing during an unskilled task would drive a specific increase in jaw representation in motor cortex compared to equivalent behavioral experience without VNS. Rats performed a behavioral task in which VNS at a specified intensity between 0 and 1.2 mA was paired with chewing 200 times per day for five days. Intracortical microstimulation (ICMS) was then used to document movement representations in motor cortex. VNS paired with chewing at 0.8 mA significantly increased motor cortex jaw representation compared to equivalent behavioral training without stimulation (Bonferroni-corrected unpaired t-test, p < 0.01). Higher and lower intensities failed to alter cortical plasticity. No changes in other movement representations or total motor cortex area were observed between groups. These results demonstrate that 0.8 mA VNS paired with training drives robust plasticity specific to the paired movement, is not restricted to forelimb representations, and occurs with training on an unskilled task. This suggests that moderate intensity VNS may be a useful adjuvant to enhance plasticity and support benefits of rehabilitative therapies targeting functions beyond upper limb movement.

Vagus nerve stimulation intensity influences motor cortex plasticity.

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.

Closed-loop neuromodulation restores network connectivity and motor control after spinal cord injury.

Recovery from serious neurological injury requires substantial rewiring of neural circuits. Precisely-timed electrical stimulation could be used to restore corrective feedback mechanisms and promote adaptive plasticity after neurological insult, such as spinal cord injury (SCI) or stroke. This study provides the first evidence that closed-loop vagus nerve stimulation (CLV) based on the synaptic eligibility trace leads to dramatic recovery from the most common forms of SCI. The addition of CLV to rehabilitation promoted substantially more recovery of forelimb function compared to rehabilitation alone following chronic unilateral or bilateral cervical SCI in a rat model. Triggering stimulation on the most successful movements is critical to maximize recovery. CLV enhances recovery by strengthening synaptic connectivity from remaining motor networks to the grasping muscles in the forelimb. The benefits of CLV persist long after the end of stimulation because connectivity in critical neural circuits has been restored.