Interhemispheric plasticity is mediated by maximal potentiation of callosal inputs

Bilateral integration in somatosensory cortex is controlled by behavioral relevance

Sensory perception requires the processing of stimuli from both sides of the body. Yet, how neurons bind stimulus information across the hemispheres to create a unified percept remains unknown. Here we perform large-scale recordings from neurons in the left and right primary somatosensory cortex (S1) in mice performing a task requiring active whisker touch to coordinate stimulus features across hemispheres. When mice touched reward-associated stimuli, their whiskers moved with greater bilateral symmetry, and synchronous spiking and enhanced spike-field coupling emerged between the hemispheres. This coordinated activity was absent in stimulus-matched naive animals, indicating that interhemispheric coupling involves a goal-directed, internal process. In S1 neurons, the addition of ipsilateral touch primarily facilitated the contralateral principal whisker response. This facilitation primarily emerged for reward-associated stimuli and was lost on trials where mice failed to respond. Silencing of callosal S1 signaling reduced bilateral facilitation and interhemispheric synchrony. These results reveal a state-dependent logic that augments the flow of tactile information through the corpus callosum.

Targeting microglial NAAA-regulated PEA signaling counters inflammatory damage and symptom progression of post-stroke anxiety

Post-stroke anxiety (PSA) manifests as anxiety symptoms after stroke, with unclear mechanisms and limited treatment strategies. Endocannabinoids, reported to mitigate fear, anxiety, and stress, undergo dynamic alterations after stroke linked to prognosis intricately. However, endocannabinoid metabolism in ischemic microenvironment and their associations with post-stroke anxiety-like behavior remain largely uncovered. Our findings indicated that endocannabinoid metabolism was dysregulated after stroke, characterized by elevated N-palmitoylethanolamide (PEA) hydrolase N-acylethanolamine-acid amidase (NAAA) in activated microglia from ischemic area, accompanied by rapid PEA exhaustion. Microglial PEA metabolite exhaustion is directly associated with more severe pathological damage, anxiety symptoms and pain sensitivity. Naaa knockout or pharmacological supplementation to boost PEA pool content can effectively promote stroke recovery and alleviate anxiety-like behaviors. In addition, maintaining PEA pool content in ischemic area reduces overactivated microglia by confronting against mitochondria dysfunction and inflammasome cascade triggered IL-18 release and diffusion to contralateral hemisphere. Meanwhile, maintenance of microglial PEA pool content in ischemic-damaged lesion can preserve contralateral vCA1 synaptic integrity, enhancing anxiolytic pBLA-vCA1Calb1+ circuit activity by alleviating microglial phagocytosis-mediated synaptic loss. Thus, we conclude that microglial NAAA-regulated lipid signaling in the ischemic focus remodels contralateral anxiolytic circuit to participate in post-stroke anxiety progression. Blocking PEA signaling breakdown promotes stroke recovery and mitigates anxiety-like symptoms.

Intermittent Fasting Enhances Motor Coordination Through Myelin Preservation in Aged Mice

Integrating dietary interventions have been extensively studied for their health benefits, such as Alzheimer's disease, Huntington's disease, and aging. However, it is necessary to fully understand the mechanisms of long-term effects and practical applications of these dietary interventions for health. A 10-week intermittent fasting (IMF) regimen was implemented on the aging animals in the current study. The variations of cerebral functions were analyzed employing a comprehensive experimental design that includes behavioral tests, neuroimaging, and ultrastructural analysis, such as resting-state functional MRI (rsfMRI), EEG/EMG recordings, transmission electron microscopy, and immunohistochemistry. Over a 10-week regimen, IMF significantly improved locomotor activity, motor coordination, and muscle strength compared to controls (p < 0.01). Resting-state fMRI (rsfMRI) demonstrated that IMF modulates brain-wide functional connectivity, enhancing communication between key brain regions. Advanced imaging techniques revealed increased expression of myelin-related proteins, including myelin basic protein (MBP), and myelin-associated glycoprotein (MAG), indicating enhanced myelin integrity and repair, particularly in axons with diameters < 400 nm (p < 0.01). These findings suggest that IMF may mitigate age-related declines by promoting better neuronal signaling. This study highlights the potential function of IMF as a non-pharmacological intervention to promote brain health and mitigate cognitive decline in aging populations.

Behavioral adaptations after unilateral whisker denervation

The rodent whisker system provides an excellent model to study experience dependent plasticity in neural morphology, circuitry, and behavior. Rodents use bilateral whisker sensation to gather information about their environment. Unilateral whisker denervation disrupts whisker circuitry but its impact on task specific behavior is largely unknown. Adult mice with unilateral whisker denervation display a preference to using the intact whisker set to inspect objects, but do not have altered open field navigation. An object localization task requiring only the intact whisker set did not detect any change in performance, but gap crossing was impaired after unilateral whisker denervation. Finally, chronic whisker denervation led to increased anxiety-like behavior which was rescued by training on the gap cross task. These findings indicate that mice use behavioral strategies to adapt to life with only one set of intact whiskers.

Cortical HFS-Induced Neo-Hebbian Local Plasticity Enhances Efferent Output Signal and Strengthens Afferent Input Connectivity

High-frequency stimulation (HFS)-induced long-term potentiation (LTP) is generally regarded as a homosynaptic Hebbian-type LTP, where synaptic changes are thought to occur at the synapses that project from the stimulation site and terminate onto the neurons at the recording site. In this study, we first investigated HFS-induced LTP on urethane-anesthetized rats and found that cortical HFS enhances neural responses at the recording site through the strengthening of local connectivity with nearby neurons at the stimulation site rather than through synaptic strengthening at the recording site. This enhanced local connectivity at the stimulation site leads to increased output propagation, resulting in signal potentiation at the recording site. Additionally, we discovered that HFS can also nonspecifically strengthen distant afferent synapses at the HFS site, thereby expanding its impact beyond local neural connections. This form of plasticity exhibits a neo-Hebbian characteristic as it exclusively manifests in the presence of cholecystokinin release, induced by HFS. The cortical HFS-induced local LTP was further supported by a behavioral task, providing additional evidence. Our results unveil a previously overlooked mechanism underlying cortical plasticity: synaptic plasticity is more likely to occur around the soma site of strongly activated cortical neurons rather than solely at their projection terminals.

Reduced interhemispheric connectivity in traumatic brachial plexopathies after bionic reconstruction

Traumatic brachial plexus lesions (TBPL) can lead to permanent impairment of hand function despite timely brachial plexus surgical treatment. In selected cases with no recovery of hand function, the affected forearm can be amputated and replaced by a bionic hand to regain prehensile function. This cross-sectional study aimed to assess (sub)cortical motor activity and functional connectivity changes after TBPL and bionic reconstruction. Cortical activity was measured with functional MRI (fMRI) during execution, and imagery of hand closing movements with the affected and healthy arm and single subject analysis was performed on the fMRI data. An electromyography training session was performed before fMRI to ensure correct task performance. Additionally, functional connectivity, diffusion tensor imaging (DTI), and cortical thickness were analyzed. Six healthy controls (4 men, median age 27, range 22-54), three TBPL patients without prosthetic reconstruction (3 men, median age 50, range 19-58), and two TBPL patients with a prosthetic reconstruction (2 men, median age 41, range 40-41) were included. In patients, cortical activity in the premotor gyrus and supplementary motor cortex (SMC) was higher and more widespread during both actual and imagery movements of the affected as well as the unaffected arm. Moreover, patients showed increased interhemispheric functional connectivity from the most active voxel in the precentral gyrus and SMC in the actual movement task. Subcortical activation of the thalamus and pallidum was observed only in non-prosthesis patients during actual movements. Corticothalamic functional connectivity was increased mainly in patients without prosthesis during actual and imagery movements. There were no differences in cortical thickness between participants. TBPL patients showed fewer structural DTI-based interhemispheric connections between the left and right precentral gyrus and superior frontal gyrus than controls. Patients without prosthesis also exhibited fewer structural connections between the left and right thalamus and pallidum, whereas those with prosthesis demonstrated increased structural connectivity compared to controls. The increased and more widespread cortical activity and functional connectivity after TBPL may be due to increased central effort necessary for motor execution and planning, representing a compensation mechanism for the decrease of interhemispheric and subcortical connectivity. The initial loss of white matter may be counteracted by increased function and grey matter recruitment, which seems necessary even after white matter recovery later with prosthesis use. The clinical implication of our findings is that a selected group of TBPL patients may benefit from an earlier timing of bionic restoration of hand function.

Post-Critical Period Transcriptional and Physiological Adaptations of Thalamocortical Connections after Sensory Loss

Unilateral whisker denervation activates plasticity mechanisms and circuit adaptations in adults. Single nucleus RNA sequencing and multiplex fluorescence in situ hybridization revealed differentially expressed genes related to altered glutamate receptor distributions and synaptogenesis in thalamocortical (TC) recipient layer 4 (L4) neurons of the sensory cortex, specifically those receiving input from the intact whiskers after whisker denervation. Electrophysiology detected increased spontaneous excitatory events at L4 neurons, confirming an increase in synaptic connections. Elevated expression levels of Gria2 mRNA and functional GluA2 subunit of AMPA receptors at the TC synapse indicate the presence of stabilized and potentiated TC synapses to L4 excitatory neurons along the intact pathway after unilateral whisker denervation. These adaptations likely underlie the increased cortical activity observed in rodents during intact whisker sensation after unilateral whisker denervation. Our findings provide new insights into the mechanisms by which the adult brain supports recovery after unilateral sensory loss.

The presence of cognitive impairments in the acute phase of traumatic upper limb injuries: A cross-sectional observational study

OBJECTIVES

The purpose of this study was to investigate the association between cognitive impairments and traumatic upper limb injuries of the acute phase.

MATERIAL AND METHODS

A cross-sectional observational study was conducted with three groups: a nerve-injury group, a without nerve injury group, and a control group (uninjured participants). Demographic characteristics (e.g. age, sex, body mass index, and education) and traumatic characteristics (duration since injury, injury side, pain, light touch sensation, hand motor function) were recorded. Short-term memory and executive functions were assessed using Rey Auditory and Verbal Learning Test (RAVLT) and Stroop Color and Word Test (SCWT, including SIECT and SIECN), respectively.

RESULTS

The study comprised 43 participants in the nerve-injury group, 30 participants in the group without nerve injury, and 104 participants in the control group. Generalized linear model was applied to explore the difference of cognitive functions among three groups with impactors. Significantly poorer performance on the RAVLT was observed in the nerve-injury group compared to the other two groups, and lower score of SIECT in nerve-injury group was lower compared to the control group. However, there was no significant difference of SIECN among three groups. In addition, traumatic characteristics did not significantly impact RAVLT and SIECT (p > 0.05) in all injured participants.

CONCLUSION

Traumatic nerve injury to the upper limb appears to be associated with both short-term memory and executive function impairment, whereas musculoskeletal injuries without nerve damage showed no cognitive impairment. Therefore, it is important to monitor cognitive function following upper limb nerve injuries.

Resting-State Functional Magnetic Resonance Imaging Reveals the Effects of rTMS on Neural Activity and Brain Connectivity After Experimental Stroke

AIMS

Limited understanding of neurobiological mechanisms of repetitive transcranial magnetic stimulation (rTMS) prevents us from choosing optimal therapeutic regimen for patients to improve therapeutic efficiency. Resting-state functional magnetic resonance imaging (rs-fMRI) has been demonstrated to obtain comparable functional readouts across species.

METHODS

Intermittent and continuous theta burst stimulation were used to stimulate ipsilesional and contralesional hemisphere, respectively, during the subacute phase after stroke. We used a rat middle cerebral artery occlusion stroke model. The amplitude of low-frequency fluctuations and functional connectivity analyses of rs-fMRI were chosen to detect neuron activity and functional connectivity. The expression of neuron activation marker c-Fos and axonal plasticity marker GAP43 was examined by an immunochemistry method to corroborate the results of rs-fMRI.

RESULTS

iTBS altered the long-term neuronal activity in bilateral sensorimotor cortex, whereas cTBS influenced immediate neuronal activity of bilateral sensorimotor cortex. In addition, cTBS enhanced interhemispheric and intrahemisheric functional connectivity in contralesional hemisphere, accompanied by axonal and dendritic remodeling in the perilesional cortical areas and contralesional homologous areas after large stroke.

CONCLUSION

rTMS exerted complex effects on brain structural and functional connectivity in addition to affecting cortical excitability. cTBS promoted the compensatory effect of contralesional hemisphere after stroke with large lesions.

Symmetry in Frontal But Not Motor and Somatosensory Cortical Projections

The neocortex and striatum are topographically organized for sensory and motor functions. While sensory and motor areas are lateralized for touch and motor control, respectively, frontal areas are involved in decision-making, where lateralization of function may be less important. This study contrasted the topographic precision of cell-type-specific ipsilateral and contralateral cortical projections while varying the injection site location in transgenic mice of both sexes. While sensory cortical areas had strongly topographic outputs to the ipsilateral cortex and striatum, they were weaker and not as topographically precise to contralateral targets. The motor cortex had somewhat stronger projections but still relatively weak contralateral topography. In contrast, frontal cortical areas had high degrees of topographic similarity for both ipsilateral and contralateral projections to the cortex and striatum. Corticothalamic organization is mainly ipsilateral, with weaker, more medial contralateral projections. Corticostriatal computations might integrate input outside closed basal ganglia loops using contralateral projections, enabling the two hemispheres to act as a unit to converge on one result in motor planning and decision-making.

Corpus Callosum-Mediated Interhemispheric Interactions in Cervical Spondylotic Myelopathy

PURPOSE

The corpus callosum is crucial for interhemispheric interactions in the motor control of limb functions. Human and animal studies suggested spinal cord pathologies may induce cortical reorganization in sensorimotor areas. We investigate participation of the corpus callosum in executions of a simple motor task in patients with cervical spondylotic myelopathy (CSM) using transcranial magnetic stimulation.

METHODS

Twenty patients with CSM with various MRI grades of severity of cord compression were compared with 19 normal controls. Ipsilateral silent period, contralateral silent period, central motor conduction time, and transcallosal conduction time (TCT) were determined.

RESULTS

In both upper and lower limbs, TCTs were significantly increased for patients with CSM than normal controls ( p < 0.001 for all), without side-to-side differences. Ipsilateral silent period and contralateral silent period durations were significantly increased bilaterally for upper limbs in comparison to controls ( p < 0.01 for all), without side-to-side differences. There were no significant correlations of TCT with central motor conduction time nor severity of CSM for both upper and lower limbs ( p > 0.05 for all) bilaterally.

CONCLUSIONS

Previous transcranial magnetic stimulation studies show increased motor cortex excitability in CSM; hence, increased TCTs observed bilaterally may be a compensatory mechanism for effective unidirectional and uniplanar execution of muscle activation in the distal limb muscles. Lack of correlation of TCTs with severity of CSM or central motor conduction time may be in keeping with a preexistent role of the corpus callosum as a predominantly inhibitory pathway for counteracting redundant movements resulting from increased motor cortex excitability occurring after spinal cord lesions.

Symmetry in frontal but not motor and somatosensory cortical projections

Neocortex and striatum are topographically organized for sensory and motor functions. While sensory and motor areas are lateralized for touch and motor control, respectively, frontal areas are involved in decision making, where lateralization of function may be less important. This study contrasted the topographic precision of cell type-specific ipsilateral and contralateral cortical projections while varying the injection site location in transgenic mice of both sexes. While sensory cortical areas had strongly topographic outputs to ipsilateral cortex and striatum, they were weaker and not as topographically precise to contralateral targets. Motor cortex had somewhat stronger projections, but still relatively weak contralateral topography. In contrast, frontal cortical areas had high degrees of topographic similarity for both ipsilateral and contralateral projections to cortex and striatum. Corticothalamic organization is mainly ipsilateral, with weaker, more medial contralateral projections. Corticostriatal computations might integrate input outside closed basal ganglia loops using contralateral projections, enabling the two hemispheres to act as a unit to converge on one result in motor planning and decision making.

Increased Interhemispheric Connectivity of a Distinct Type of Hippocampal Pyramidal Cells

Neurons typically generate action potentials at their axon initial segment based on the integration of synaptic inputs. In many neurons, the axon extends from the soma, equally weighting dendritic inputs. A notable exception is found in a subset of hippocampal pyramidal cells where the axon emerges from a basal dendrite. This structure allows these axon-carrying dendrites (AcDs) a privileged input route. We found that in male mice, such cells in the CA1 region receive stronger excitatory input from the contralateral CA3, compared with those with somatic axon origins. This is supported by a higher count of putative synapses from contralateral CA3 on the AcD. These findings, combined with prior observations of their distinct role in sharp-wave ripple firing, suggest a key role of this neuron subset in coordinating bi-hemispheric hippocampal activity during memory-centric oscillations.

Cortical Reorganization after Limb Loss: Bridging the Gap between Basic Science and Clinical Recovery

Despite the increasing incidence and prevalence of amputation across the globe, individuals with acquired limb loss continue to struggle with functional recovery and chronic pain. A more complete understanding of the motor and sensory remodeling of the peripheral and central nervous system that occurs postamputation may help advance clinical interventions to improve the quality of life for individuals with acquired limb loss. The purpose of this article is to first provide background clinical context on individuals with acquired limb loss and then to provide a comprehensive review of the known motor and sensory neural adaptations from both animal models and human clinical trials. Finally, the article bridges the gap between basic science researchers and clinicians that treat individuals with limb loss by explaining how current clinical treatments may restore function and modulate phantom limb pain using the underlying neural adaptations described above. This review should encourage the further development of novel treatments with known neurological targets to improve the recovery of individuals postamputation.Significance Statement In the United States, 1.6 million people live with limb loss; this number is expected to more than double by 2050. Improved surgical procedures enhance recovery, and new prosthetics and neural interfaces can replace missing limbs with those that communicate bidirectionally with the brain. These advances have been fairly successful, but still most patients experience persistent problems like phantom limb pain, and others discontinue prostheses instead of learning to use them daily. These problematic patient outcomes may be due in part to the lack of consensus among basic and clinical researchers regarding the plasticity mechanisms that occur in the brain after amputation injuries. Here we review results from clinical and animal model studies to bridge this clinical-basic science gap.

Intracortical and interhemispheric excitability changes in arm amputees: A TMS study

OBJECTIVE

To evaluate cortical circuits and excitability of the motor cortex in the hemisphere contralateral to the affected (AH) and to the unaffected arm (UH), in upper limb amputees.

METHODS

Motor evoked potentials (MEP) were recorded in 17 subjects who had upper limb amputation: 11 trans-radial (TR) and 6 trans-humeral (TH). Motor thresholds (MT), short interval intracortical inhibition (SICI), and interhemispheric inhibition (IHI) in the available arm muscles of the stump were evaluated.

RESULTS

There was no significant difference in MT between hemispheres. SICI was preserved in TR but not in TH group. Additionally, in the TR group, the MEP amplitudes in AH were higher than in UH. A significant IHI was observed in the whole sample but not in each hemisphere or patient group.

CONCLUSIONS

In our population of TR amputees, we found increased corticospinal excitability in the AH with preserved intracortical inhibition. This finding was not observed in the TH population.

SIGNIFICANCE

Understanding the changes in intracortical excitability in amputees may enhance knowledge of the functional reorganization of the brain in the post-amputation phase, bringing useful information for prosthetic rehabilitation.

Ischemic stroke rehabilitation through optogenetic modulation of parvalbumin neurons in the contralateral motor cortex

BACKGROUND

Based on the theory of interhemispheric inhibition and the bimodal balance-recovery model in stroke, we explored the effects of excitation/inhibition (E/I) of parvalbumin (PV) neurons in the contralateral primary motor cortex (cM1) connecting the ipsilateral M1 (iM1) via the corpus callosum (cM1-CC-iM1) of ischemic stroke rats by optogenetic stimulation.

METHODS

We tested this by injecting anterograde and retrograde virus in rats with middle cerebral artery occlusion (MCAO), and evaluated the neurological scores, motor behavior, volume of cerebral infarction and the E/I balance of the bilateral M1 two weeks after employing optogenetic treatment.

RESULTS

We found that concentrations of Glu and GABA decreased and increased, respectively, in the iM1 of MCAO rats, and that the former increased in the cM1, suggesting E/I imbalance in bilateral M1 after ischemic stroke. Interestingly, optogenetic stimulation improved M1 E/I imbalance, as illustrated by the increase of Glu in the iM1 and the decrease of GABA in both iM1 and cM1, which were accompanied by an improvement in neurological deficit and motor dysfunction. In addition, we observed a reduced infarct volume, an increase in the expression of the NMDAR and AMPAR, and a decrease in GAD67 in the iM1 after intervention.

CONCLUSIONS

Optogenetic modulation of PV neurons of the iM1-CC-cM1 improve E/I balance, leading to reduced neurological deficit and improved motor dysfunction following ischemic stroke in rats.

Assessing bilateral ankle proprioceptive acuity in stroke survivors: An exploratory study

Background

Bilateral proprioception deficits were reported in stroke survivors. However, whether bilateral proprioception deficits exist in the ankle joint after stroke was unclear. Ankle proprioception is a significant predictor of balance dysfunction after stroke, and previous studies to date are lacking appropriate evaluation methods.

Objectives

We want to determine whether the active movement extent discrimination apparatus (AMEDA) is a reliable tool for assessing ankle proprioceptive acuity in stroke survivors and the presence of deficits in ankle proprioception on the affected and unaffected sides in patients after stroke.

Methods

Bilateral ankle proprioception was assessed in 20 stroke patients and 20 age-matched healthy controls using AMEDA. Test-retest reliability was assessed using the intraclass correlation coefficient (ICC).

Results

The ICC in the affected and unaffected sides was 0.713 and 0.74, respectively. Analysis of variance revealed significant deficits in ankle proprioception in subacute stroke survivors vs. healthy controls (F = 2.719, p = 0.045). However, there were no significant differences in proprioception acuity scores between the affected and unaffected sides in patients after stroke (F = 1.14, p = 0.331).

Conclusions

Stroke survivors had bilateral deficits in ankle proprioceptive acuity during active movements compared with age-matched healthy controls, underscoring the need to evaluate these deficits on both sides of the body and develop effective sensorimotor rehabilitation methods for this patient population. The AMEDA can reliably determine bilateral ankle proprioceptive acuity in stroke survivors.

Homotopic contralesional excitation suppresses spontaneous circuit repair and global network reconnections following ischemic stroke

Understanding circuit-level manipulations that affect the brain's capacity for plasticity will inform the design of targeted interventions that enhance recovery after stroke. Following stroke, increased contralesional activity (e.g. use of the unaffected limb) can negatively influence recovery, but it is unknown which specific neural connections exert this influence, and to what extent increased contralesional activity affects systems- and molecular-level biomarkers of recovery. Here, we combine optogenetic photostimulation with optical intrinsic signal imaging to examine how contralesional excitatory activity affects cortical remodeling after stroke in mice. Following photothrombosis of left primary somatosensory forepaw (S1FP) cortex, mice either recovered spontaneously or received chronic optogenetic excitation of right S1FP over the course of 4 weeks. Contralesional excitation suppressed perilesional S1FP remapping and was associated with abnormal patterns of stimulus-evoked activity in the unaffected limb. This maneuver also prevented the restoration of resting-state functional connectivity (RSFC) within the S1FP network, RSFC in several networks functionally distinct from somatomotor regions, and resulted in persistent limb-use asymmetry. In stimulated mice, perilesional tissue exhibited transcriptional changes in several genes relevant for recovery. Our results suggest that contralesional excitation impedes local and global circuit reconnection through suppression of cortical activity and several neuroplasticity-related genes after stroke, and highlight the importance of site selection for targeted therapeutic interventions after focal ischemia.

Peripheral Nerve Injury Induces Changes in the Activity of Inhibitory Interneurons as Visualized in Transgenic GAD1-GCaMP6s Rats

Peripheral nerve injury induces cortical remapping that can lead to sensory complications. There is evidence that inhibitory interneurons play a role in this process, but the exact mechanism remains unclear. Glutamate decarboxylase-1 (GAD1) is a protein expressed exclusively in inhibitory interneurons. Transgenic rats encoding GAD1–GCaMP were generated to visualize the activity in GAD1 neurons through genetically encoded calcium indicators (GCaMP6s) in the somatosensory cortex. Forepaw denervation was performed in adult rats, and fluorescent Ca2+ imaging on cortical slices was obtained. Local, intrahemispheric stimulation (cortical layers 2/3 and 5) induced a significantly higher fluorescence change of GAD1-expressing neurons, and a significantly higher number of neurons were responsive to stimulation in the denervated rats compared to control rats. However, remote, interhemispheric stimulation of the corpus callosum induced a significantly lower fluorescence change of GAD1-expressing neurons, and significantly fewer neurons were deemed responsive to stimulation within layer 5 in denervated rats compared to control rats. These results suggest that injury impacts interhemispheric communication, leading to an overall decrease in the activity of inhibitory interneurons in layer 5. Overall, our results provide direct evidence that inhibitory interneuron activity in the deprived S1 is altered after injury, a phenomenon likely to affect sensory processing.

Characterization of brain-wide somatosensory BOLD fMRI in mice under dexmedetomidine/isoflurane and ketamine/xylazine

Mouse fMRI under anesthesia has become increasingly popular due to improvement in obtaining brain-wide BOLD response. Medetomidine with isoflurane has become well-accepted for resting-state fMRI, but whether this combination allows for stable, expected, and robust brain-wide evoked response in mice has yet to be validated. We thus utilized intravenous infusion of dexmedetomidine with inhaled isoflurane and intravenous infusion of ketamine/xylazine to elucidate whether stable mouse physiology and BOLD response are obtainable in response to simultaneous forepaw and whisker-pad stimulation throughout 8 h. We found both anesthetics result in hypercapnia with depressed heart rate and respiration due to self-breathing, but these values were stable throughout 8 h. Regardless of the mouse condition, brain-wide, robust, and stable BOLD response throughout the somatosensory axis was observed with differences in sensitivity and dynamics. Dexmedetomidine/isoflurane resulted in fast, boxcar-like, BOLD response with consistent hemodynamic shapes throughout the brain. Ketamine/xylazine response showed higher sensitivity, prolonged BOLD response, and evidence for cortical disinhibition as significant bilateral cortical response was observed. In addition, differing hemodynamic shapes were observed between cortical and subcortical areas. Overall, we found both anesthetics are applicable for evoked mouse fMRI studies.

Functional Weight of Somatic and Cognitive Networks and Asymmetry of Compensatory Mechanisms: Collaboration or Divergency among Hemispheres after Cerebrovascular Accident?

The human brain holds highly sophisticated compensatory mechanisms relying on neuroplasticity. Neuronal degeneracy, redundancy, and brain network organization make the human nervous system more robust and evolvable to continuously guarantee an optimal environmental-related homeostasis. Nevertheless, after injury, restitution processes appear dissimilar, depending on the pathology. Following a cerebrovascular accident, asymmetry, within- and across-network compensation and interhemispheric inhibition are key features to functional recovery. In moderate-to-severe stroke, neurological outcome is often poor, and little is known about the paths that enable either an efficient collaboration among hemispheres or, on the contrary, an antagonism of adaptative responses. In this review, we aim to decipher key issues of ipsilesional and contralesional hemispheric functioning allowing the foundations of effective neurorehabilitation strategies.

Cortical plasticity and nerve regeneration after peripheral nerve injury

With the development of neuroscience, substantial advances have been achieved in peripheral nerve regeneration over the past decades. However, peripheral nerve injury remains a critical public health problem because of the subsequent impairment or absence of sensorimotor function. Uncomfortable complications of peripheral nerve injury, such as chronic pain, can also cause problems for families and society. A number of studies have demonstrated that the proper functioning of the nervous system depends not only on a complete connection from the central nervous system to the surrounding targets at an anatomical level, but also on the continuous bilateral communication between the two. After peripheral nerve injury, the interruption of afferent and efferent signals can cause complex pathophysiological changes, including neurochemical alterations, modifications in the adaptability of excitatory and inhibitory neurons, and the reorganization of somatosensory and motor regions. This review discusses the close relationship between the cerebral cortex and peripheral nerves. We also focus on common therapies for peripheral nerve injury and summarize their potential mechanisms in relation to cortical plasticity. It has been suggested that cortical plasticity may be important for improving functional recovery after peripheral nerve damage. Further understanding of the potential common mechanisms between cortical reorganization and nerve injury will help to elucidate the pathophysiological processes of nerve injury, and may allow for the reduction of adverse consequences during peripheral nerve injury recovery. We also review the role that regulating reorganization mechanisms plays in functional recovery, and conclude with a suggestion to target cortical plasticity along with therapeutic interventions to promote peripheral nerve injury recovery.

Circuit-Specific Plasticity of Callosal Inputs Underlies Cortical Takeover

Injury induces synaptic, circuit, and systems reorganization. After unilateral amputation or stroke, this functional loss disrupts the interhemispheric interaction between intact and deprived somatomotor cortices to recruit deprived cortex in response to intact limb stimulation. This recruitment has been implicated in enhanced intact sensory function. In other patients, maladaptive consequences such as phantom limb pain can occur. We used unilateral whisker denervation in male and female mice to detect circuitry alterations underlying interhemispheric cortical reorganization. Enhanced synaptic strength from the intact cortex via the corpus callosum (CC) onto deep neurons in deprived primary somatosensory barrel cortex (S1BC) has previously been detected. It was hypothesized that specificity in this plasticity may depend on to which area these neurons projected. Increased connectivity to somatomotor areas such as contralateral S1BC, primary motor cortex (M1) and secondary somatosensory cortex (S2) may underlie beneficial adaptations, while increased connectivity to pain areas like anterior cingulate cortex (ACC) might underlie maladaptive pain phenotypes. Neurons from the deprived S1BC that project to intact S1BC were hyperexcitable, had stronger responses and reduced inhibitory input to CC stimulation. M1-projecting neurons also showed increases in excitability and CC input strength that was offset with enhanced inhibition. S2 and ACC-projecting neurons showed no changes in excitability or CC input. These results demonstrate that subgroups of output neurons undergo dramatic and specific plasticity after peripheral injury. The changes in S1BC-projecting neurons likely underlie enhanced reciprocal connectivity of S1BC after unilateral deprivation consistent with the model that interhemispheric takeover supports intact whisker processing.SIGNIFICANCE STATEMENT Amputation, peripheral injury, and stroke patients experience widespread alterations in neural activity after sensory loss. A hallmark of this reorganization is the recruitment of deprived cortical space which likely aids processing and thus enhances performance on intact sensory systems. Conversely, this recruitment of deprived cortical space has been hypothesized to underlie phenotypes like phantom limb pain and hinder recovery. A mouse model of unilateral denervation detected remarkable specificity in alterations in the somatomotor circuit. These changes underlie increased reciprocal connectivity between intact and deprived cortical hemispheres. This increased connectivity may help explain the enhanced intact sensory processing detected in humans.

Mapping the Brain-Wide Network Effects by Optogenetic Activation of the Corpus Callosum

Optogenetically driven manipulation of circuit-specific activity enables causality studies, but its global brain-wide effect is rarely reported. Here, we applied simultaneous functional magnetic resonance imaging (fMRI) and calcium recording with optogenetic activation of the corpus callosum (CC) connecting barrel cortices (BC). Robust positive BOLD was detected in the ipsilateral BC due to antidromic activity, spreading to the ipsilateral motor cortex (MC), and posterior thalamus (PO). In the orthodromic target, positive BOLD was reliably evoked by 2 Hz light pulses, whereas 40 Hz light pulses led to reduced calcium, indicative of CC-mediated inhibition. This presumed optogenetic CC-mediated inhibition was further elucidated by pairing light pulses with whisker stimulation at varied interstimulus intervals. Whisker-induced positive BOLD and calcium signals were reduced at intervals of 50/100 ms. The calcium-amplitude-modulation-based correlation with whole-brain fMRI signal revealed that the inhibitory effects spread to contralateral BC, ipsilateral MC, and PO. This work raises the need for fMRI to elucidate the brain-wide network activation in response to optogenetic stimulation.

Local Perturbations of Cortical Excitability Propagate Differentially Through Large-Scale Functional Networks

Electrophysiological recordings have established that GABAergic interneurons regulate excitability, plasticity, and computational function within local neural circuits. Importantly, GABAergic inhibition is focally disrupted around sites of brain injury. However, it remains unclear whether focal imbalances in inhibition/excitation lead to widespread changes in brain activity. Here, we test the hypothesis that focal perturbations in excitability disrupt large-scale brain network dynamics. We used viral chemogenetics in mice to reversibly manipulate parvalbumin interneuron (PV-IN) activity levels in whisker barrel somatosensory cortex. We then assessed how this imbalance affects cortical network activity in awake mice using wide-field optical neuroimaging of pyramidal neuron GCaMP dynamics as well as local field potential recordings. We report 1) that local changes in excitability can cause remote, network-wide effects, 2) that these effects propagate differentially through intra- and interhemispheric connections, and 3) that chemogenetic constructs can induce plasticity in cortical excitability and functional connectivity. These findings may help to explain how focal activity changes following injury lead to widespread network dysfunction.