Functional MRI detection of bilateral cortical reorganization in the rodent brain following peripheral nerve deafferentation

Bilateral In Vivo Confocal Microscopic Changes of the Corneal Subbasal Nerve Plexus in Patients with Acute Herpes Zoster Ophthalmicus

INTRODUCTION

Unilateral herpes zoster ophthalmicus (HZO) results in bilateral corneal denervation in patients with corneal involvement, which correlates with corneal sensation loss. The study aimed to analyze bilateral corneal nerve changes in patients with acute unilateral HZO and no keratitis compared with healthy controls.

METHODS

This was a prospective, single-center study. Using in vivo confocal microscopy (IVCM) and an automatized single image analysis software (ACCmetrics, University of Manchester, UK), seven corneal nerve parameters, including corneal nerve fiber density (CNFD; no/mm2), corneal nerve branch density (CNBD; no/mm2), corneal nerve fiber length (CNFL; mm/mm2), corneal nerve total branch density (CTBD; no/mm2), corneal nerve fiber area (CNFA; mm2/mm2), corneal nerve fiber width (CNFW; mm/mm2), and corneal nerve fiber fractal dimension (CFracDim) were analyzed. Additionally, central corneal sensitivity was measured.

RESULTS

Forty-six patients with HZO and 49 controls were recruited and compared. In the HZO group, ipsilateral and contralateral eyes presented a significant decrease (p < 0.001) in all seven IVCM parameters compared with controls: CNFD (13.25 ± 5.23 and 15.24 ± 4.70 vs. 23.54 ± 6.54), CNBD (14.67 ± 9.03 and 16.59 ± 7.98 vs. 31.72 ± 17.89), CNFL (8.42 ± 2.83 and 9.06 ± 2.69 vs. 13.08 ± 4.02), CTBD (27.11 ± 13.71 and 23.58 ± 12.69 vs. 46.88 ± 24.90), CNFA (0.0044 ± 0.002 and 0.0042 ± 0.001 vs. 0.0056 ± 0.002), CNFW (0.0213 ± 0.003 and 0.0221 ± 0.003 vs. 0.0222 ± 0.001) and CFracDim (1.39 ± 0.06 and 1.38 ± 0.06 vs. 1.45 ± 0.05). In the ipsilateral HZO eye group, a positive Hutchinson sign or a reduced corneal sensitivity was associated with more extensive corneal denervation. A significant negative correlation was found between patient age and CNFD (rho = - 0.312, p < 0.002), CNFL (rho = - 0.295, p = 0.004), and CFracDim (rho = - 0.284, p = 0.005).

CONCLUSIONS

Unilateral HZO in patients without apparent keratitis leads to bilateral subbasal nerve plexus alteration in the early days after disease onset, especially in those with a positive Hutchinson sign. Early follow-up of patients with HZO and bilateral application of preservative-free artificial tears during the initial months of symptom onset may help reduce the risk of developing neurotrophic keratopathy (NTK).

Reactivated thalamocortical plasticity alters neural activity in sensory-motor cortex during post-critical period

Neuroplasticity in sensory brain areas supports adaptation after nerve injury and fundamentally impacts sensation and movement. However, limited neuroplasticity in somatosensory areas due to the early critical period makes determining the role of thalamocortical (TC) inputs in sensorimotor signal processing challenging. Here, we demonstrated that reactivation of TC neuroplasticity was associated with an increase in the number of neurons in layer IV (L4) of the whisker primary somatosensory cortex (wS1) with a stable excitation-inhibition ratio. Highly synchronized neural activity in L4 propagated throughout the wS1 column and to the downstream areas, including whisker secondary somatosensory, primary motor cortices, and contralateral wS1. These results provide crucial evidence that TC inputs can alter the neural activity of sensory-motor pathways even after the critical period. Altogether, these enormous changes in sensorimotor circuit activity are important for adaptation following an injury such as limb loss, stroke, or other forms of neural injury.

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.

Reorganization of sensorimotor representations of the intact limb after upper but not lower limb traumatic amputation

It is becoming increasingly clear that limb loss induces wider spread reorganization of representations of the body that are nonadjacent to the affected cortical territory. Data from upper extremity amputees reveal intrusion of the representation of the ipsilateral intact limb into the former hand territory. Here we test for the first time whether this reorganization of the intact limb into the deprived cortex is specific to the neurological organization of the upper limbs or reflects large scale adaptation that is triggered by any unilateral amputation. BOLD activity was measured as human subjects with upper limb and lower limb traumatic amputation and their controls moved the toes on each foot, open and closed each hand and pursed their lips. Subjects with amputation were asked to imagine moving the missing limb while remaining still. Bayesian pattern component modeling of fMRI data showed that intact ipsilateral movements and contralateral movements of the hand and foot were distinctly represented in the deprived sensorimotor cortex years after upper limb amputation. In contrast, there was evidence reminiscent of contralateral specificity for hand and foot movements following lower limb amputation, like that seen in controls. We propose the cortical reorganization of the intact limb to be a function of use-dependent plasticity that is more specific to the consequence of upper limb loss of forcing an asymmetric reliance on the intact hand and arm. The contribution of this reorganization to phantom pain or a heightened risk of overuse and resultant maladaptive plasticity needs investigating before targeting such reorganization in intervention.

Low-intensity ultrasound stimulation modulates time-frequency patterns of cerebral blood oxygenation and neurovascular coupling of mouse under peripheral sensory stimulation state

Previous studies have demonstrated that transcranial ultrasound stimulation (TUS) not only modulates cerebral hemodynamics, neural activity, and neurovascular coupling characteristics in resting samples but also exerts a significant inhibitory effect on the neural activity in task samples. However, the effect of TUS on cerebral blood oxygenation and neurovascular coupling in task samples remains to be elucidated. To answer this question, we first used forepaw electrical stimulation of the mice to elicit the corresponding cortical excitation, and then stimulated this cortical region using different modes of TUS, and simultaneously recorded the local field potential using electrophysiological acquisition and hemodynamics using optical intrinsic signal imaging. The results indicate that for the mice under peripheral sensory stimulation state, TUS with a duty cycle of 50% can (1) enhance the amplitude of cerebral blood oxygenation signal, (2) reduce the time-frequency characteristics of evoked potential, (3) reduce the strength of neurovascular coupling in time domain, (4) enhance the strength of neurovascular coupling in frequency domain, and (5) reduce the time-frequency cross-coupling of neurovasculature. The results of this study indicate that TUS can modulate the cerebral blood oxygenation and neurovascular coupling in peripheral sensory stimulation state mice under specific parameters. This study opens up a new area of investigation for potential applicability of TUS in brain diseases related to cerebral blood oxygenation and neurovascular coupling.

Effect of aging on the cerebral metabolic mechanism of electroacupuncture treatment in rats with traumatic brain injury

Objective

Aging has great influence on the clinical treatment effect of cerebrovascular diseases, and evidence suggests that the effect may be associated with age-related brain plasticity. Electroacupuncture is an effective alternative treatment for traumatic brain injury (TBI). In the present study, we aimed to explore the effect of aging on the cerebral metabolic mechanism of electroacupuncture to provide new evidence for developing age-specific rehabilitation strategies.

Methods

Both aged (18 months) and young (8 weeks) rats with TBI were analyzed. Thirty-two aged rats were randomly divided into four groups: aged model, aged electroacupuncture, aged sham electroacupuncture, and aged control group. Similarly, 32 young rats were also divided into four groups: young model, young electroacupuncture, young sham electroacupuncture, and young control group. Electroacupuncture was applied to "Bai hui" (GV20) and "Qu chi" (LI11) for 8 weeks. CatWalk gait analysis was then performed at 3 days pre- and post-TBI, and at 1, 2, 4, and 8 weeks after intervention to observe motor function recovery. Positron emission computed tomography (PET/CT) was performed at 3 days pre- and post-TBI, and at 2, 4, and 8 weeks after intervention to detect cerebral metabolism.

Results

Gait analysis showed that electroacupuncture improved the forepaw mean intensity in aged rats after 8 weeks of intervention, but after 4 weeks of intervention in young rats. PET/CT revealed increased metabolism in the left (the injured ipsilateral hemisphere) sensorimotor brain areas of aged rats during the electroacupuncture intervention, and increased metabolism in the right (contralateral to injury hemisphere) sensorimotor brain areas of young rats.

Results

This study demonstrated that aged rats required a longer electroacupuncture intervention duration to improve motor function than that of young rats. The influence of aging on the cerebral metabolism of electroacupuncture treatment was mainly focused on a particular hemisphere.

Neural regeneration therapy after spinal cord injury induces unique brain functional reorganizations in rhesus monkeys

PURPOSE

Spinal cord injury (SCI) destroys the sensorimotor pathway and induces brain plasticity. However, the effect of treatment-induced spinal cord tissue regeneration on brain functional reorganization remains unclear. This study was designed to investigate the large-scale functional interactions in the brains of adult female Rhesus monkeys with injured and regenerated thoracic spinal cord.

MATERIALS AND METHODS

Resting-state functional magnetic resonance imaging (fMRI) combined with Granger Causality analysis (GCA) and motor behaviour analysis were used to assess the causal interaction between sensorimotor cortices, and calculate the relationship between causal interaction and hindlimb stepping in nine Rhesus monkeys undergoing lesion-induced spontaneous recovery (injured, n = 4) and neurotrophin-3/chitosan transplantation-induced regeneration (NT3-chitosan, n = 5) after SCI.

RESULTS

The results showed that the injured and NT3-chitosan-treated animals had distinct spatiotemporal features of brain functional reorganization. The spontaneous recovery followed the model of "early intra-hemispheric reorganization dominant, late inter-hemispheric reorganization dominant", whereas regenerative therapy animals showed the opposite trend. Although the variation degree of information flow intensity was consistent, the tendency and the relationship between local neuronal activity properties and coupling strength were different between the two groups. In addition, the injured and NT3-chitosan-treated animals had similar motor adjustments but various relationship modes between motor performance and information flow intensity.

CONCLUSIONS

Our findings show that brain functional reorganization induced by regeneration therapy differed from spontaneous recovery after SCI. The influence of unique changes in brain plasticity on the therapeutic effects of future regeneration therapy strategies should be considered. Key messagesNeural regeneration elicited a unique spatiotemporal mode of brain functional reorganization in the spinal cord injured monkeys, and that regeneration does not simply reverse the process of brain plasticity induced by spinal cord injury (SCI).Independent "properties of local activity - intensity of information flow" relationships between the injured and treated animals indicating that spontaneous recovery and regenerative therapy exerted different effects on the reorganization of the motor network after SCI.A specific information flow from the left thalamus to the right insular can serve as an indicator to reflect a heterogeneous "information flow - motor performance" relationship between injured and treated animals at similar motor adjustments.

Changes in Primary Somatosensory Cortex Following Allogeneic Hand Transplantation or Autogenic Hand Replantation

Former amputees who undergo allogeneic hand transplantation or autogenic hand replantation (jointly, "hand restoration") present a unique opportunity to measure the range of post-deafferentation plastic changes in the nervous system, especially primary somatosensory cortex (S1). However, few such patients exist, and previous studies compared single cases to small groups of typical adults. Here, we studied 5 individuals (n = 8 sessions: a transplant with 2 sessions, a transplant with 3 sessions, and three replants with 1 session each). We used functional magnetic resonance imaging (fMRI) to measure S1 responsiveness to controlled pneumatic tactile stimulation delivered to each patient's left and right fingertips and lower face. These data were compared with responses acquired from typical adults (n = 29) and current unilateral amputees (n = 19). During stimulation of the affected hand, patients' affected S1 (contralateral to affected hand) responded to stimulation in a manner similar both to amputees and to typical adults. The presence of contralateral responses indicated grossly typical S1 function, but responses were universally at the low end of the range of typical variability. Patients' affected S1 showed substantial individual variability in responses to stimulation of the intact hand: while all patients fell within the range of typical adults, some patient sessions (4/8) had substantial ipsilateral responses similar to those exhibited by current amputees. Unlike hand restoration patients, current amputees exhibited substantial S1 reorganization compared to typical adults, including bilateral S1 responses to stimulation of the intact hand. In all three participant groups, we assessed tactile localization by measuring individuals' ability to identify the location of touch on the palm and fingers. Curiously, while transplant patients improved their tactile sensory localization over time, this was uncorrelated with changes in S1 responses to tactile stimuli. Overall, our results provide the first description of cortical responses to well-controlled tactile stimulation after hand restoration. Our case studies indicate that hand restoration patients show S1 function within the range of both typical adults and amputees, but with low-amplitude and individual-specific responses that indicate a wide range of potential cortical neurological changes following de-afferentation and re-afferentation.

Pain-related reorganization in the primary somatosensory cortex of patients with postherpetic neuralgia

Studies on functional and structural changes in the primary somatosensory cortex (S1) have provided important insights into neural mechanisms underlying several chronic pain conditions. However, the role of S1 plasticity in postherpetic neuralgia (PHN) remains elusive. Combining psychophysics and magnetic resonance imaging (MRI), we investigated whether pain in PHN patients is linked to S1 reorganization as compared with healthy controls. Results from voxel-based morphometry showed no structural differences between groups. To characterize functional plasticity, we compared S1 responses to noxious laser stimuli of a fixed intensity between both groups and assessed the relationship between S1 activation and spontaneous pain in PHN patients. Although the intensity of evoked pain was comparable in both groups, PHN patients exhibited greater activation in S1 ipsilateral to the stimulated hand. Pain-related activity was identified in contralateral superior S1 (SS1) in controls as expected, but in bilateral inferior S1 (IS1) in PHN patients with no overlap between SS1 and IS1. Contralateral SS1 engaged during evoked pain in controls encoded spontaneous pain in patients, suggesting functional S1 reorganization in PHN. Resting-state fMRI data showed decreased functional connectivity between left and right SS1 in PHN patients, which scaled with the intensity of spontaneous pain. Finally, multivariate pattern analyses (MVPA) demonstrated that BOLD activity and resting-state functional connectivity of S1 predicted within-subject variations of evoked and spontaneous pain intensities across groups. In summary, functional reorganization in S1 might play a key role in chronic pain related to PHN and could be a potential treatment target in this patient group.

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.

Multi-session delivery of synchronous rTMS and sensory stimulation induces long-term plasticity

BACKGROUND

Combining training or sensory stimulation with non-invasive brain stimulation has shown to improve performance in healthy subjects and improve brain function in patients after brain injury. However, the plasticity mechanisms and the optimal parameters to induce long-term and sustainable enhanced performance remain unknown.

OBJECTIVE

This work was designed to identify the protocols of which combining sensory stimulation with repetitive transcranial magnetic stimulation (rTMS) will facilitate the greatest changes in fMRI activation maps in the rat's primary somatosensory cortex (S1).

METHODS

Several protocols of combining forepaw electrical stimulation with rTMS were tested, including a single stimulation session compared to multiple, daily stimulation sessions, as well as synchronous and asynchronous delivery of both modalities. High-resolution fMRI was used to determine how pairing sensory stimulation with rTMS induced short and long-term plasticity in the rat S1.

RESULTS

All groups that received a single session of rTMS showed short-term increases in S1 activity, but these increases did not last three days after the session. The group that received a stimulation protocol of 10 Hz forepaw stimulation that was delivered simultaneously with 10 Hz rTMS for five consecutive days demonstrated the greatest increases in the extent of the evoked fMRI responses compared to groups that received other stimulation protocols.

CONCLUSIONS

Our results provide direct indication that pairing peripheral stimulation with rTMS induces long-term plasticity, and this phenomenon appears to follow a time-dependent plasticity mechanism. These results will be important to lead the design of new training and rehabilitation paradigms and training towards achieving maximal performance in healthy subjects.

Non-invasive neuromodulation using rTMS and the electromagnetic-perceptive gene (EPG) facilitates plasticity after nerve injury

BACKGROUND

Twenty million Americans suffer from peripheral nerve injury. These patients often develop chronic pain and sensory dysfunctions. In the past decade, neuroimaging studies showed that these changes are associated with altered cortical excitation-inhibition balance and maladaptive plasticity. We tested if neuromodulation of the deprived sensory cortex could restore the cortical balance, and whether it would be effective in alleviating sensory complications.

OBJECTIVE

We tested if non-invasive repetitive transcranial magnetic stimulation (rTMS) which induces neuronal excitability, and cell-specific magnetic activation via the Electromagnetic-perceptive gene (EPG) which is a novel gene that was identified and cloned from glass catfish and demonstrated to evoke neural responses when magnetically stimulated, can restore cortical excitability.

METHODS

A rat model of forepaw denervation was used. rTMS was delivered every other day for 30 days, starting at the acute or at the chronic post-injury phase. A minimally-invasive neuromodulation via EPG was performed every day for 30 days starting at the chronic phase. A battery of behavioral tests was performed in the days and weeks following limb denervation in EPG-treated rats, and behavioral tests, fMRI and immunochemistry were performed in rTMS-treated rats.

RESULTS

The results demonstrate that neuromodulation significantly improved long-term mobility, decreased anxiety and enhanced neuroplasticity. The results identify that both acute and delayed rTMS intervention facilitated rehabilitation. Moreover, the results implicate EPG as an effective cell-specific neuromodulation approach.

CONCLUSION

Together, these results reinforce the growing amount of evidence from human and animal studies that are establishing neuromodulation as an effective strategy to promote plasticity and rehabilitation.

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.

Caffeine enhances BOLD responses to electrical whisker pad stimulation in rats during alpha-chloralose anaesthesia

By reducing the cerebral blood flow and thereby increasing the resting deoxyhaemoglobin concentration, many human studies have shown that caffeine has a beneficial effect on enhancing the magnitude of blood-oxygenation-level-dependent (BOLD) responses. However, the effect of caffeine on BOLD responses in animals under anaesthesia has not been demonstrated. In this study, we aimed to determine the effect of systemic caffeine administration on BOLD responses in rats under alpha-chloralose. By applying electric whisker pad stimulation to male Sprague-Dawley rats, we performed fMRI measurements before and after the caffeine injection (40 mg/kg, n = 7) or an equivalent volume of saline (n = 6) at 7T. To understand the potential perturbation of animal physiology during stimulation, arterial blood pressure was measured in a separate group of animals (n = 3) outside the scanner. Caffeine significantly decreased baseline BOLD signals (p = .05) due to the increased deoxyhaemoglobin level. Both BOLD responses and t-values in the primary somatosensory cortex were significantly increased (both p < .05). The blood pressure changed insignificantly (p > .05). No significant differences in BOLD responses and t-values were observed in the control condition of saline injection (both p > .05). These findings suggested that, although the cerebral activity was lower under alpha-chloralose anaesthesia, the higher level of deoxygemoglobin at the baseline under the caffeinated condition can benefit the magnitude of BOLD responses in rats. These findings suggest that animal models might serve as potential platforms for further caffeine-related fMRI research studies.

Terminal Arbors of Callosal Axons Undergo Plastic Changes in Early-Amputated Rats

Sensory information is processed in specific brain regions, and shared between the cerebral hemispheres by axons that cross the midline through the corpus callosum. However, sensory deprivation usually causes sensory losses and/or functional changes. This is the case of people who suffered limb amputation and show changes of body map organization within the somatosensory cortex (S1) of the deafferented cerebral hemisphere (contralateral to the amputated limb), as well as in the afferented hemisphere (ipsilateral to the amputated limb). Although several studies have approached these functional changes, the possible finer morphological alterations, such as those occurring in callosal axons, still remain unknown. The present work combined histochemistry, single-axon tracing and 3D microscopy to analyze the fine morphological changes that occur in callosal axons of the forepaw representation in early amputated rats. We showed that the forepaw representation in S1 was reduced in the deafferented hemisphere and expanded in the afferented side. Accordingly, after amputation, callosal axons originating from the deafferented cortex undergo an expansion of their terminal arbors with increased number of terminal boutons within the homotopic representation at the afferented cerebral hemisphere. Similar microscale structural changes may underpin the macroscale morphological and functional phenomena that characterize limb amputation in humans.

Interhemispheric transfer of post-amputation cortical plasticity within the human somatosensory cortex

Animal models reveal that deafferenting forelimb injuries precipitate reorganization in both contralateral and ipsilateral somatosensory cortices. The functional significance and duration of these effects are unknown, and it is unclear whether they also occur in injured humans. We delivered cutaneous stimulation during functional magnetic resonance imaging (fMRI) to map the sensory cortical representation of the intact hand and lower face in a group of chronic, unilateral, upper extremity amputees (N = 19) and healthy matched controls (N = 29). Amputees exhibited greater activity than controls within the deafferented former sensory hand territory (S1f) during stimulation of the intact hand, but not of the lower face. Despite this cortical reorganization, amputees did not differ from controls in tactile acuity on their intact hands. S1f responses during hand stimulation were unrelated to tactile acuity, pain, prosthesis usage, or time since amputation. These effects appeared specific to the deafferented somatosensory modality, as fMRI visual mapping paradigm failed to detect any differences between groups. We conclude that S1f becomes responsive to cutaneous stimulation of the intact hand of amputees, and that this modality-specific reorganizational change persists for many years, if not indefinitely. The functional relevance of these changes, if any, remains unknown.

Effects of Hemodynamic Response Function Selection on Rat fMRI Statistical Analyses

The selection of the appropriate hemodynamic response function (HRF) for signal modeling in functional magnetic resonance imaging (fMRI) is important. Although the use of the boxcar-shaped hemodynamic response function (BHRF) and canonical hemodynamic response (CHRF) has gained increasing popularity in rodent fMRI studies, whether the selected HRF affects the results of rodent fMRI has not been fully elucidated. Here we investigated the signal change and t-statistic sensitivities of BHRF, CHRF, and impulse response function (IRF). The effect of HRF selection on different tasks was analyzed by using data collected from two groups of rats receiving either 3 mA whisker pad or 3 mA forepaw electrical stimulations (n = 10 for each group). Under whisker pad stimulation with large blood-oxygen-level dependent (BOLD) signal change (4.31 ± 0.42%), BHRF significantly underestimated signal changes (P < 0.001) and t-statistics (P < 0.001) compared with CHRF or IRF. CHRF and IRF did not provide significantly different t-statistics (P > 0.05). Under forepaw stimulation with small BOLD signal change (1.71 ± 0.34%), different HRFs provided insignificantly different t-statistics (P > 0.05). Therefore, the selected HRF can influence data analysis in rodent fMRI experiments with large BOLD responses but not in those with small BOLD responses.

Cortical astrocytes prime the induction of spine plasticity and mirror image pain

Peripheral nerve injury causes maladaptive plasticity in the central nervous system and induces chronic pain. In addition to the injured limb, abnormal pain sensation can appear in the limb contralateral to the injury, called mirror image pain. Because synaptic remodeling in the primary somatosensory cortex (S1) has critical roles in the induction of chronic pain, cortical reorganization in the S1 ipsilateral to the injured limb may also accompany mirror image pain. To elucidate this, we conducted in vivo 2-photon calcium imaging of neuron and astrocyte activity in the ipsilateral S1 after a peripheral nerve injury. We found that cross-callosal inputs enhanced the activity of both S1 astrocytes and inhibitory neurons, whereas activity of excitatory neurons decreased. When local inhibitory circuits were blocked, astrocyte-dependent spine plasticity and allodynia were revealed. Thus, we propose that cortical astrocytes prime the induction of spine plasticity and mirror image pain after peripheral nerve injury. Moreover, this result suggests that cortical synaptic rewiring could be sufficient to cause allodynia on the uninjured periphery.

Lower limb amputees undergo long-distance plasticity in sensorimotor functional connectivity

Amputation in adults is associated with an extensive remapping of cortical topography in primary and secondary sensorimotor areas. Here, we used tactile residual limb stimulation and 3T functional magnetic resonance imaging in humans to investigate functional connectivity changes in the sensorimotor network of patients with long-term lower limb traumatic amputations with phantom sensation, but without pain. We found a pronounced reduction of inter-hemispheric functional connectivity between homologous sensorimotor cortical regions in amputees, including the primary (S1) and secondary (S2) somatosensory areas, and primary (M1) and secondary (M2) motor areas. We additionally observed an intra-hemispheric increased functional connectivity between primary and secondary somatosensory regions, and between the primary and premotor areas, contralateral to amputation. These functional connectivity changes in specialized small-scale sensory-motor networks improve our understanding of the functional impact of lower limb amputation in the brain. Our findings in a selective group of patients with phantom limb sensations, but without pain suggest that disinhibition of neural inputs following traumatic limb amputation disrupts sensorimotor topology, unbalancing functional brain network organization. These findings step up the description of brain plasticity related with phantom sensations by showing that pain is not critical for sensorimotor network changes after peripheral injury.

Cortical functional reorganization in response to intact forelimb stimulation from acute to chronic stage in rodent amputation model

Brain plasticity after amputation is related to the short-term unmasking of latent synapses as well as the long-term reorganization due to the sprouting new synaptic connections. The cortical functional reorganization has been reported along the intact somatosensory pathway after unilateral deafferentation. Cerebral blood flow (CBF) change serves as an important biomarker of the functional reorganization of brain. Using laser speckle contrast imaging (LSCI) technology, we performed a longitudinal study to unveil the cortical functional reorganization after forelimb amputation in rodent model, particularly along the intact somatosensory pathway. Our results showed that the CBF response to electrical stimulation of the intact forepaw increased significantly at 9 hours after amputation in acute stage. While in chronic stage (>14 days), the CBF response showed a pattern similar to the control group. The results showed the dynamic brain functional response along the intact somatosensory pathway at different stages after amputation and indicated that cortical functional reorganization occurred within the acute stage. Our work provided additional insights in understanding the inter-hemispheric functional changes from acute to chronic stages of amputation.

A guide to using functional magnetic resonance imaging to study Alzheimer's disease in animal models

Alzheimer's disease is a leading healthcare challenge facing our society today. Functional magnetic resonance imaging (fMRI) of the brain has played an important role in our efforts to understand how Alzheimer's disease alters brain function. Using fMRI in animal models of Alzheimer's disease has the potential to provide us with a more comprehensive understanding of the observations made in human clinical fMRI studies. However, using fMRI in animal models of Alzheimer's disease presents some unique challenges. Here, we highlight some of these challenges and discuss potential solutions for researchers interested in performing fMRI in animal models. First, we briefly summarize our current understanding of Alzheimer's disease from a mechanistic standpoint. We then overview the wide array of animal models available for studying this disease and how to choose the most appropriate model to study, depending on which aspects of the condition researchers seek to investigate. Finally, we discuss the contributions of fMRI to our understanding of Alzheimer's disease and the issues to consider when designing fMRI studies for animal models, such as differences in brain activity based on anesthetic choice and ways to interrogate more specific questions in rodents beyond those that can be addressed in humans. The goal of this article is to provide information on the utility of fMRI, and approaches to consider when using fMRI, for studies of Alzheimer's disease in animal models.

Peripheral Sensory Deprivation Restores Critical-Period-like Plasticity to Adult Somatosensory Thalamocortical Inputs

Recent work has shown that thalamocortical (TC) inputs can be plastic after the developmental critical period has closed, but the mechanism that enables re-establishment of plasticity is unclear. Here, we find that long-term potentiation (LTP) at TC inputs is transiently restored in spared barrel cortex following either a unilateral infra-orbital nerve (ION) lesion, unilateral whisker trimming, or unilateral ablation of the rodent barrel cortex. Restoration of LTP is associated with increased potency at TC input and reactivates anatomical map plasticity induced by whisker follicle ablation. The reactivation of TC LTP is accompanied by reappearance of silent synapses. Both LTP and silent synapse formation are preceded by transient re-expression of synaptic GluN2B-containing N-methyl-D-aspartate (NMDA) receptors, which are required for the reappearance of TC plasticity. These results clearly demonstrate that peripheral sensory deprivation reactivates synaptic plasticity in the mature layer 4 barrel cortex with features similar to the developmental critical period.

Magnetic resonance spectroscopy of current hand amputees reveals evidence for neuronal-level changes in former sensorimotor cortex

Deafferentation is accompanied by large-scale functional reorganization of maps in the primary sensory and motor areas of the hemisphere contralateral to injury. Animal models of deafferentation suggest a variety of cellular-level changes including depression of neuronal metabolism and even neuronal death. Whether similar neuronal changes contribute to patterns of reorganization within the contralateral sensorimotor cortex of chronic human amputees is uncertain. We used functional MRI-guided proton magnetic resonance spectroscopy to test the hypothesis that unilateral deafferentation is associated with lower levels of N-acetylaspartate (NAA, a putative marker of neuronal integrity) in the sensorimotor hand territory located contralateral to the missing hand in chronic amputees (n = 19) compared with the analogous hand territory of age- and sex-matched healthy controls (n = 28). We also tested whether former amputees [i.e., recipients of replanted (n = 3) or transplanted (n = 2) hands] exhibit NAA levels that are indistinguishable from controls, possible evidence for reversal of the effects of deafferentation. As predicted, relative to controls, current amputees exhibited lower levels of NAA that were negatively and significantly correlated with the time after amputation. Contrary to our prediction, NAA levels in both replanted and transplanted patients fell within the range of the current amputees. We suggest that lower levels of NAA in current amputees reflects altered neuronal integrity consequent to chronic deafferentation. Thus local changes in NAA levels may provide a means of assessing neuroplastic changes in deafferented cortex. Results from former amputees suggest that these changes may not be readily reversible through reafferentation.NEW & NOTEWORTHY This study is the first to use functional magnetic resonance-guided magnetic resonance spectroscopy to examine neurochemical mechanisms underlying functional reorganization in the primary somatosensory and motor cortices consequent to upper extremity amputation and its potential reversal through hand replantation or transplantation. We provide evidence for selective alteration of cortical neuronal integrity associated with amputation-related deafferentation that may not be reversible.

Novel Neuromodulation Techniques to Assess Interhemispheric Communication in Neural Injury and Neurodegenerative Diseases

Interhemispheric interaction has a major role in various neurobehavioral functions. Its disruption is a major contributor to the pathological changes in the setting of brain injury such as traumatic brain injury, peripheral nerve injury, and stroke, as well as neurodegenerative diseases. Because interhemispheric interaction has a crucial role in functional consequence in these neuropathological states, a review of noninvasive and state-of-the-art molecular based neuromodulation methods that focus on or have the potential to elucidate interhemispheric interaction have been performed. This yielded approximately 170 relevant articles on human subjects or animal models. There has been a recent surge of reports on noninvasive methods such as transcranial magnetic stimulation and transcranial direct current stimulation. Since these are noninvasive techniques with little to no side effects, their widespread use in clinical studies can be easily justified. The overview of novel neuromodulation methods and how they can be applied to study the role of interhemispheric communication in neural injury and neurodegenerative disease is provided. Additionally, the potential of each method in therapeutic use as well as investigating the pathophysiology of interhemispheric interaction in neurodegenerative diseases and brain injury is discussed. New technologies such as transcranial magnetic stimulation or transcranial direct current stimulation could have a great impact in understanding interhemispheric pathophysiology associated with acquired injury and neurodegenerative diseases, as well as designing improved rehabilitation therapies. Also, advances in molecular based neuromodulation techniques such as optogenetics and other chemical, thermal, and magnetic based methods provide new capabilities to stimulate or inhibit a specific brain location and a specific neuronal population.

Tor1a+/- mice develop dystonia-like movements via a striatal dopaminergic dysregulation triggered by peripheral nerve injury

Isolated generalized dystonia is a central motor network disorder characterized by twisted movements or postures. The most frequent genetic cause is a GAG deletion in the Tor1a (DYT1) gene encoding torsinA with a reduced penetrance of 30-40 % suggesting additional genetic or environmental modifiers. Development of dystonia-like movements after a standardized peripheral nerve crush lesion in wild type (wt) and Tor1a+/- mice, that express 50 % torsinA only, was assessed by scoring of hindlimb movements during tail suspension, by rotarod testing and by computer-assisted gait analysis. Western blot analysis was performed for dopamine transporter (DAT), D1 and D2 receptors from striatal and quantitative RT-PCR analysis for DAT from midbrain dissections. Autoradiography was used to assess the functional DAT binding in striatum. Striatal dopamine and its metabolites were analyzed by high performance liquid chromatography. After nerve crush injury, we found abnormal posturing in the lesioned hindlimb of both mutant and wt mice indicating the profound influence of the nerve lesion (15x vs. 12x relative to control) resembling human peripheral pseudodystonia. In mutant mice the phenotypic abnormalities were increased by about 40 % (p < 0.05). This was accompanied by complex alterations of striatal dopamine homeostasis. Pharmacological blockade of dopamine synthesis reduced severity of dystonia-like movements, whereas treatment with L-Dopa aggravated these but only in mutant mice suggesting a DYT1 related central component relevant to the development of abnormal involuntary movements. Our findings suggest that upon peripheral nerve injury reduced torsinA concentration and environmental stressors may act in concert in causing the central motor network dysfunction of DYT1 dystonia.

Longitudinal evaluation of functional connectivity variation in the monkey sensorimotor network induced by spinal cord injury

AIM

Given the unclear pattern of cerebral function reorganization induced by spinal cord injury (SCI), this study aimed to longitudinally evaluate the changes in resting-state functional connectivity (FC) in the sensorimotor network after SCI and explore their relationship with gait performance.

METHODS

Four adult female rhesus monkeys were examined using resting-state functional magnetic resonance imaging during their healthy stage and after hemitransected SCI (4, 8 and 12 weeks after SCI), and the gait characteristics of their hindlimbs were recorded (except 4 weeks after SCI). Twenty sensorimotor-related cortical areas were adopted in the FC analysis to evaluate the functional network reorganization. Correlation analyses were then used to explore the relationship between functional network variations and gait characteristic changes.

RESULTS

Compared with that during the healthy stage, the FC strength during post-SCI period was significantly increased in multiple areas of the motor control network, including the primary sensorimotor cortex, supplementary motor area (SMA) and putamen (Pu). However, the FC strength was remarkably reduced in the thalamus and parieto-occipital association cortex of the sensory network 8 weeks after SCI. Most FC intensities gradually approached the normal level 12 weeks after the SCI. Correlation analyses revealed that the enhanced FC strength between Pu and SMA in the left hemisphere, which regulates motor functions of the right side, was negatively correlated with the gait height of the right hindlimb.

CONCLUSION

The cerebral functional network presents an adjust-recover pattern after SCI, which may help us further understand the cerebral function reorganization after SCI.

Topography of Synchronization of Somatosensory Evoked Potentials Elicited by Stimulation of the Sciatic Nerve in Rat

Purpose: Traditionally, the topography of somatosensory evoked potentials (SEPs) is generated based on amplitude and latency. However, this operation focuses on the physical morphology and field potential-power, so it suffers from difficulties in performing identification in an objective manner. In this study, measurement of the synchronization of SEPs is proposed as a method to explore brain functional networks as well as the plasticity after peripheral nerve injury.

Method: SEPs elicited by unilateral sciatic nerve stimulation in twelve adult male Sprague-Dawley (SD) rats in the normal group were compared with SEPs evoked after unilateral sciatic nerve hemisection in four peripheral nerve injured SD rats. The characterization of synchronized networks from SEPs was conducted using equal-time correlation, correlation matrix analysis, and comparison to randomized surrogate data. Eigenvalues of the correlation matrix were used to identify the clusters of functionally synchronized neuronal activity, and the participation index (PI) was calculated to indicate the involvement of each channel in the cluster. The PI value at the knee point of the PI histogram was used as a threshold to demarcate the cortical boundary.

Results: Ten out of the twelve normal rats showed only one synchronized brain network. The remaining two normal rats showed one strong and one weak network. In the peripheral nerve injured group, only one synchronized brain network was found in each rat. In the normal group, all network shapes appear regular and the network is largely contained in the posterior cortex. In the injured group, the network shapes appear irregular, the network extends anteriorly and posteriorly, and the network area is significantly larger. There are considerable individual variations in the shape and location of the network after peripheral nerve injury.

Conclusion: The proposed method can detect functional brain networks. Compared to the results of the traditional SEP-morphology-based analysis method, the synchronized functional network area is much larger. Furthermore, the proposed method can also characterize the rapid cortical plasticity after a peripheral nerve is acutely injured.

Functional connectivity MRI tracks memory networks after maze learning in rodents

Learning and memory employs a series of cognitive processes which require the coordination of multiple areas across the brain. However in vivo imaging of cognitive function has been challenging in rodents. Since these processes involve synchronous firing among different brain loci we explored functional connectivity imaging with resting-state fMRI. After 5-day training on a hidden platform watermaze task, notable signal correlations were seen between the hippocampal CA3 and other structures, including thalamus, septum and cingulate cortex, compared to swim control or naïve animals. The connectivity sustained 7 days after training and was reorganized toward the cortex, consistent with views of memory trace distribution leading to memory consolidation. These data demonstrates that, after a cognitive task, altered functional connectivity can be detected in the subsequently sedated rodent using in vivo imaging. This approach paves the way to understand dynamics of area-dependent distribution processes in animal models of cognition.

The role of epigenetic-related codes in neurocomputation: dynamic hardware in the brain

This paper presents a review of recent work on the role that two epigenetic-related systems may play in information processing mechanisms in the brain. The first consists of exosomes that transport epigenetic-related molecules between neurons. The second consists of homeoproteins like Otx2 that carry information from sense organs to primary sensory cortex. There is developing evidence that presynaptic neurons may be able to modulate the fine microanatomical structure in the postsynaptic neuron. This may be conducted by three mechanisms, of which the first is well established and the latter two are novel. (i) By the well-established activation of receptors that trigger a chain of signalling molecules (second messengers) that result in the upregulation and/or activation of a transcription factor. The two novel systems are the exosome system and homeoproteins. (ii) Exosomes are small vesicles that are released upon activation of the axon terminal, traverse the synaptic cleft, probably via astrocytes and are taken up by the postsynaptic neuron. They carry a load of signalling proteins and a variety of forms of RNA. These loads may then be transported widely throughout the postsynaptic neuron and engineer modulations in the fine structure of computational machinery by epigenetic-related processes. (iii) Otx2 is a transcription factor that, inter alia, controls the development and survival of PV+ GABAergic interneurons (PV cells) in the primary visual cortex. It is synthesized in the retina and is transported to the cortex by a presently unknown mechanism that probably includes direct cell-to-cell transfer, and may, or may not, include transfer by the dynein and exosome systems in addition. These three mechanisms explain a quantity of data from the field of de- and reafferentation plasticity. These data show that the modality of the presynaptic neuron controls to a large extent the modality of the postsynaptic neuron. However, the mechanism that effects this is currently unknown. The exosome and the homeoprotein hypotheses provide novel explanations to add to the well-established earlier mechanism described above.

Interhemispheric plasticity protects the deafferented somatosensory cortex from functional takeover after nerve injury

Functional changes across brain hemispheres have been reported after unilateral cortical or peripheral nerve injury. Interhemispheric callosal connections usually underlie this cortico-cortical plasticity. However, the effect of the altered callosal inputs on local cortical plasticity in the adult brain is not well studied. Ipsilateral functional magnetic resonance imaging (fMRI) activation has been reliably detected in the deafferented barrel cortex (BC) at 2 weeks after unilateral infraorbital denervation (IO) in adult rats. The ipsilateral fMRI signal relies on callosal-mediated interhemispheric plasticity. This form of interhemispheric plasticity provides a good chronic model to study the interaction between callosal inputs and local cortical plasticity. The receptive field of forepaw in the primary somatosensory cortex (S1), which is adjacent to the BC, was mapped with fMRI. The S1 receptive field expanded to take over a portion of the BC in 2 weeks after both ascending inputs and callosal inputs were removed in IO rats with ablated contralateral BC (IO+ablation). This expansion, estimated specifically by fMRI mapping, is significantly larger than what has been observed in the IO rats with intact callosal connectivity, as well as in the rats with sham surgery. This work indicates that altered callosal inputs prevent the functional takeover of the deafferented BC from adjacent cortices and may help preserve the functional identity of the BC.

Molecular neuroimaging of post-injury plasticity

Nerve injury induces long-term changes in neuronal activity in the primary somatosensory cortex (S1), which has often been implicated as the origin of sensory dysfunction. However, the cellular mechanisms underlying this phenomenon remain unclear. C-fos is an immediate early gene, which has been shown to play an instrumental role in plasticity. By developing a new platform to image real-time changes in gene expression in vivo, we investigated whether injury modulates the levels of c-fos in layer V of S1, since previous studies have suggested that these neurons are particularly susceptible to injury. The yellow fluorescent protein, ZsYellow1, under the regulation of the c-fos promoter, was expressed throughout the rat brain. A fiber-based confocal microscope that enabled deep brain imaging was utilized, and local field potentials were collected simultaneously. In the weeks following limb denervation in adult rats (n=10), sensory stimulation of the intact limb induced significant increases in c-fos gene expression in cells located in S1, both contralateral (affected, 27.6±3 cells) and ipsilateral (8.6±3 cells) to the injury, compared to controls (n=10, 13.4±3 and 1.0±1, respectively, p value<0.05). Thus, we demonstrated that injury activates cellular mechanisms that are involved in reshaping neuronal connections, and this may translate to neurorehabilitative potential.

Repeated BOLD-fMRI imaging of deep brain stimulation responses in rats

Functional magnetic resonance imaging (fMRI) provides a picture of the global spatial activation pattern of the brain. Interest is growing regarding the application of fMRI to rodent models to investigate adult brain plasticity. To date, most rodent studies used an electrical forepaw stimulation model to acquire fMRI data, with α-chloralose as the anesthetic. However, α-chloralose is harmful to animals, and not suitable for longitudinal studies. Moreover, peripheral stimulation models enable only a limited number of brain regions to be studied. Processing between peripheral regions and the brain is multisynaptic, and renders interpretation difficult and uncertain. In the present study, we combined the medetomidine-based fMRI protocol (a noninvasive rodent fMRI protocol) with chronic implantation of an MRI-compatible stimulation electrode in the ventroposterior (VP) thalamus to repetitively sample thalamocortical responses in the rat brain. Using this model, we scanned the forebrain responses evoked by the VP stimulation repeatedly of individual rats over 1 week. Cortical BOLD responses were compared between the 2 profiles obtained at day1 and day8. We discovered reproducible frequency- and amplitude-dependent BOLD responses in the ipsilateral somatosensory cortex (S1). The S1 BOLD responses during the 2 sessions were conserved in maximal response amplitude, area size (size ratio from 0.88 to 0.91), and location (overlap ratio from 0.61 to 0.67). The present study provides a long-term chronic brain stimulation protocol for studying the plasticity of specific neural circuits in the rodent brain by BOLD-fMRI.

Early evaluation of nerve regeneration after nerve injury and repair using functional connectivity MRI

Resting state functional connectivity magnetic resonance imaging studies in rat brain show brain reorganization caused by nerve injury and repair. In this study, distinguishable differences were found in healthy, nerve transection without repair (R-) and nerve transection with repair (R+) groups in the subacute stage (2 weeks after initial injury). Only forepaw on the healthy side was used to determine seed voxel regions in this study. Disturbance of neuronal network in the primary sensory region of cortex occurs within two hours after initial injury, and the network pattern was restored in R+ group in subacute stage, while the disturbed pattern remained in R- group. These are the central findings of the study. This technique provides a novel way of detecting and monitoring the effectiveness of peripheral nerve injury treatment in the early stage and potentially offers a tool for clinicians to avoid poor clinical outcomes.

Re-wiring the brain: increased functional connectivity within primary somatosensory cortex following synchronous co-activation

The primary somatosensory cortex shows precise topographical organisation, but can be quickly modified by alterations to sensory inputs. Temporally correlated sensory inputs to the digits can result in the merging of digit representations on the cortical surface. Underlying mechanisms driving these changes are unclear but the strengthening of intra-cortical synaptic connections via Hebbian mechanisms has been suggested. We use fMRI measures of temporal coherence to infer alterations in the relative strength of neuronal connections between digit regions 2 and 4 following 3 hours of synchronous and asynchronous co-activation. Following synchronous co-activation we find a 20% increase in temporal coherence of the fMRI signal (p=0.0004). No significant change is seen following asynchronous co-activation suggesting that temporal coincidence between the two digit inputs during co-activation is driving this coherence change. In line with previous work we also find a trend towards reduced separation of the digit representations following synchronous co-activation and significantly increased separation for the asynchronous case. Increased coherence is significantly correlated with reduced digit separation for the synchronous case. This study shows that passive synchronous stimulation to the digits strengthens the underlying cortical connections between the digit regions in only a few hours, and that this mechanism may be related to topographical re-organisation.

Deciphering laminar-specific neural inputs with line-scanning fMRI

Using a line-scanning method during functional magnetic resonance imaging (fMRI), we obtained high temporal (50-ms) and spatial (50-μm) resolution information along the cortical thickness and showed that the laminar position of fMRI onset coincides with distinct neural inputs in rat somatosensory and motor cortices. This laminar-specific fMRI onset allowed us to identify the neural inputs underlying ipsilateral fMRI activation in the barrel cortex due to peripheral denervation-induced plasticity.

Evidence for impaired plasticity after traumatic brain injury in the developing brain

The robustness of plasticity mechanisms during brain development is essential for synaptic formation and has a beneficial outcome after sensory deprivation. However, the role of plasticity in recovery after acute brain injury in children has not been well defined. Traumatic brain injury (TBI) is the leading cause of death and disability among children, and long-term disability from pediatric TBI can be particularly devastating. We investigated the altered cortical plasticity 2-3 weeks after injury in a pediatric rat model of TBI. Significant decreases in neurophysiological responses across the depth of the noninjured, primary somatosensory cortex (S1) in TBI rats, compared to age-matched controls, were detected with electrophysiological measurements of multi-unit activity (86.4% decrease), local field potential (75.3% decrease), and functional magnetic resonance imaging (77.6% decrease). Because the corpus callosum is a clinically important white matter tract that was shown to be consistently involved in post-traumatic axonal injury, we investigated its anatomical and functional characteristics after TBI. Indeed, corpus callosum abnormalities in TBI rats were detected with diffusion tensor imaging (9.3% decrease in fractional anisotropy) and histopathological analysis (14% myelination volume decreases). Whole-cell patch clamp recordings further revealed that TBI results in significant decreases in spontaneous firing rate (57% decrease) and the potential to induce long-term potentiation in neurons located in layer V of the noninjured S1 by stimulation of the corpus callosum (82% decrease). The results suggest that post-TBI plasticity can translate into inappropriate neuronal connections and dramatic changes in the function of neuronal networks.

Genetic tools to manipulate MRI contrast

Advances in molecular biology in the early 1970s revolutionized research strategies for the study of complex biological processes, which, in turn, created a high demand for new means to visualize these dynamic biological changes noninvasively and in real time. In this respect, MRI was a perfect fit, because of the versatile possibility to alter the different contrast mechanisms. Genetic manipulations are now being translated to MRI through the development of reporters and sensors, as well as the imaging of transgenic and knockout mice. In the past few years, a new molecular biology toolset, namely optogenetics, has emerged, which allows for the manipulation of cellular behavior using light. This technology provides a few particularly attractive features for combination with newly developed MRI techniques for the probing of in vivo cellular and, in particular, neural processes, specifically the ability to control focal, genetically defined cellular populations with high temporal resolution using equipment that is magnetically inert and does not interact with radiofrequency pulses. Recent studies have demonstrated that the combination of optogenetics and functional MRI (fMRI) can provide an appropriate platform to investigate in vivo, at the cellular and molecular levels, the neuronal basis of fMRI signals. In addition, this novel combination of optogenetics with fMRI has the potential to resolve pre-synaptic versus post-synaptic changes in neuronal activity and changes in the activity of large neuronal networks in the context of plasticity associated with development, learning and pathophysiology.

Cortical plasticity induced by different degrees of peripheral nerve injuries: a rat functional magnetic resonance imaging study under 9.4 Tesla

BACKGROUND

Major peripheral nerve injuries not only result in local deficits but may also cause distal atrophy of target muscles or permanent loss of sensation. Likewise, these injuries have been shown to instigate long-lasting central cortical reorganization.

METHODS

Cortical plasticity changes induced after various types of major peripheral nerve injury using an electrical stimulation technique to the rat upper extremity and functional magnetic resonance imaging (fMRI) were examined. Studies were completed out immediately after injury (acute stage) and at two weeks (subacute stage) to evaluate time affect on plasticity.

RESULTS

After right-side median nerve transection, cortical representation of activation of the right-side ulnar nerve expanded intra-hemispherically into the cortical region that had been occupied by the median nerve representation After unilateral transection of both median and ulnar nerves, cortical representation of activation of the radial nerve on the same side of the body also demonstrated intra-hemispheric expansion. However, simultaneous electrical stimulation of the contralateral uninjured median and ulnar nerves resulted in a representation that had expanded both intra- and inter-hemispherically into the cortical region previously occupied by the two transected nerve representations.

CONCLUSIONS

After major peripheral nerve injury, an adjacent nerve, with similar function to the injured nerve, may become significantly over-activated in the cortex when stimulated. This results in intra-hemispheric cortical expansion as the only component of cortical plasticity. When all nerves responsible for a certain function are injured, the same nerves on the contralateral side of the body are affected and become significantly over-activated during a task. Both intra- and inter-hemispheric cortical expansion exist, while the latter dominates cortical plasticity.

Peripheral nerve injury induces immediate increases in layer v neuronal activity

BACKGROUND

Peripheral nerve injury leads to changes in neuronal activity in the contralateral and ipsilateral primary somatosensory cortices (S1), which may lead to enduring sensory dysfunction and pain. Plasticity in the barrel and visual cortices has been shown to occur in a layer-specific manner. However, little is known about the layer specific changes associated with limb injury.

OBJECTIVE

To determine the layer-specific changes in neuronal activity associated with short-term plasticity induced by peripheral nerve injury in the rat.

METHODS

In vivo electrophysiology recordings (multiunit activity and local field potential) and high-resolution functional magnetic resonance imaging techniques were applied to characterize neuronal and hemodynamic responses across the depth of S1 contralateral and ipsilateral to the injury.

RESULTS

Within 60 minutes following injury, atypical increases in neuronal and hemodynamic responses in the deprived S1, ipsilateral to the noninjured limb, were observed in response to stimulation of the noninjured limb. The most prominent increases in neuronal activity in the deprived S1 occurred in layer V.

CONCLUSION

Layer V neurons provide the major output of S1 and they send and receive transcallosal input. Thus, the immediate changes in neuronal firing patterns in layer V induced by the injury, can adversely affect the activity of subcortical regions and also interfere with normal cortical processing and interhemispheric communication. Therefore, a rehabilitation strategy that targets layer V neurons activity and starts immediately after the injury may benefit the functional outcome.

Transhemispheric cortical plasticity following contralateral C7 nerve transfer: a rat functional magnetic resonance imaging survival study

PURPOSE

To perform contralateral C7 nerve transfer in a controlled, survival rat functional magnetic resonance imaging model, so as to understand the extent of cortical plasticity after brachial plexus injury and surgical manipulation with this procedure.

METHODS

A total of 24 rats divided into 3 groups underwent surgery followed by functional magnetic resonance imaging in this study. Group I rats served as sham controls. Group II injury rats underwent complete right brachial plexus root avulsion. Group III repair rats underwent complete right brachial plexus root avulsion and then contralateral C7 nerve transfer to the right median nerve. We assessed cortical response to median nerve stimulation at 0, 3, and 5 months after injury using functional magnetic resonance imaging at 9.4 T. We concurrently performed sensory and motor functional testing.

RESULTS

We noted a progression in cortical activation in the repair rats over 0, 3, and 5 months. Initially, right median nerve stimulation in the repair group showed complete loss of activation in the contralateral somatosensory cortex. Nerve stimulation at 3 months produced primarily ipsilateral cortical activation; at 5 months, 3 patterns of cortical activation emerged: ipsilateral, bilateral, and contralateral activation. After right median nerve stimulation, injury rats maintained a lack of cortical activation and control rats maintained exclusive contralateral activation throughout all time points. Functional testing revealed a degree of return of sensory and motor function over time in the repair group compared with the injured group.

CONCLUSIONS

A high degree of transhemispheric cortical plasticity occurred after contralateral C7 nerve transfer. There appears to be a predilection for the rat brain to restore the preinjury somatotopic representation of the brain.

CLINICAL RELEVANCE

Understanding the cortical changes after nerve injury and repair may lead to specific pharmacologic or behavioral interventions that can improve functional outcomes.

Transsynaptic modality codes in the brain: possible involvement of synchronized spike timing, microRNAs, exosomes and epigenetic processes

This paper surveys two different mechanisms by which a presynaptic cell can modulate the structure and function of the postsynaptic cell. We first present the evidence that this occurs, and then discuss two mechanisms that could bring this about. The first hypothesis relates to the long lasting effects that the spike patterns of presynaptic axons may exert by modulating activity-inducible programs in postsynaptic cells. The second hypothesis is based on recently obtained evidence that, the afferent neuron at the neuromuscular junction buds off exosomes at its synapse and carries a cargo of Wg and Evi, which are large molecular transsynaptic signaling agents (LMTSAs). Further evidence indicates that many types of neurons bud off exosomes containing payloads of various lipids, proteins, and types of RNA. The evidence suggests that they are transmitted across the synapse and are taken up by the postsynaptic structure either by perisynaptic or exosynaptic mechanisms, thus mediating the transfer of information between neurons. To date, the molecular hypothesis has been limited to local interactions within the synapse of concern. In this paper, we explore the possibility that this represents a mechanism for information transfer involving the postsynaptic neuron as a whole. This entails a review of the known functions of these molecules in neuronal physiology, together with an estimate of the possible types of information they could carry and how they might affect neurocomputations.

Cerebral changes after injury to the median nerve: a long-term follow up

Injury to the peripheral nerves in the upper extremity results in changes in the nerve, and at multiple sites throughout the central nervous system (CNS). We studied the long-term effects of an injury to the median nerve in the forearm with a focus on changes in the CNS. Four patients with isolated injuries of the median nerve in their 20s were examined a mean of 14 years after the injury. Cortical activation was monitored during tactile stimulation of the fingers of the injured and healthy hand using functional magnetic resonance imaging at 3 Tesla. The neurophysiological state and clinical outcome were also examined. Activation in the primary somatosensory cortex was substantially larger during tactile stimulation of the injured hand than with stimulation of the uninjured hand. We also saw a redistribution of hemispheric dominance. Stimulation of the injured median nerve resulted in a substantially increased dominance of the contralateral hemisphere. However, stimulation of the healthy ulnar nerve resulted in a decreased dominance of the contralateral hemisphere. Neurophysiology showed low sensory amplitudes, velocity, and increased motor latency in the injured nerve. Clinically there were abnormalities predominately in the sensory domain. However, there was an overall improved mean result compared with a five year follow-up in the same subjects. The cortical changes could be the result of cortical reorganisation after a changed afferent signal pattern from the injured nerve. Even though the clinical function improved over time it did not return to normal, and neither did the cortical response.

Thalamocortical inputs show post-critical-period plasticity

Experience-dependent plasticity in the adult brain has clinical potential for functional rehabilitation following central and peripheral nerve injuries. Here, plasticity induced by unilateral infraorbital (IO) nerve resection in 4-week-old rats was mapped using MRI and synaptic mechanisms were elucidated by slice electrophysiology. Functional MRI demonstrates a cortical potentiation compared to thalamus 2 weeks after IO nerve resection. Tracing thalamocortical (TC) projections with manganese-enhanced MRI revealed circuit changes in the spared layer 4 (L4) barrel cortex. Brain slice electrophysiology revealed TC input strengthening onto L4 stellate cells due to an increase in postsynaptic strength and the number of functional synapses. This work shows that the TC input is a site for robust plasticity after the end of the previously defined critical period for this input. Thus, TC inputs may represent a major site for adult plasticity in contrast to the consensus that adult plasticity mainly occurs at cortico-cortical connections.

Imaging the spatio-temporal dynamics of supragranular activity in the rat somatosensory cortex in response to stimulation of the paws

We employed voltage-sensitive dye (VSD) imaging to investigate the spatio-temporal dynamics of the responses of the supragranular somatosensory cortex to stimulation of the four paws in urethane-anesthetized rats. We obtained the following main results. (1) Stimulation of the contralateral forepaw evoked VSD responses with greater amplitude and smaller latency than stimulation of the contralateral hindpaw, and ipsilateral VSD responses had a lower amplitude and greater latency than contralateral responses. (2) While the contralateral stimulation initially activated only one focus, the ipsilateral stimulation initially activated two foci: one focus was typically medial to the focus activated by contralateral stimulation and was stereotaxically localized in the motor cortex; the other focus was typically posterior to the focus activated by contralateral stimulation and was stereotaxically localized in the somatosensory cortex. (3) Forepaw and hindpaw somatosensory stimuli activated large areas of the sensorimotor cortex, well beyond the forepaw and hindpaw somatosensory areas of classical somatotopic maps, and forepaw stimuli activated larger cortical areas with greater activation velocity than hindpaw stimuli. (4) Stimulation of the forepaw and hindpaw evoked different cortical activation dynamics: forepaw responses displayed a clear medial directionality, whereas hindpaw responses were much more uniform in all directions. In conclusion, this work offers a complete spatio-temporal map of the supragranular VSD cortical activation in response to stimulation of the paws, showing important somatotopic differences between contralateral and ipsilateral maps as well as differences in the spatio-temporal activation dynamics in response to forepaw and hindpaw stimuli.

Interhemispheric functional reorganization after cross nerve transfer: via cortical or subcortical connectivity?

It has been demonstrated that there could be long range interhemispheric reorganization between bilateral hemispheres after peripheral cross nerve transfer. Our previous studies found a striking dynamic process of interhemispheric functional reorganization in adult rats with cross seventh cervical nerve transfer. But it remains a question whether the extensive interhemispheric functional reorganization after cross nerve transfer depends on connectivities at the cortical or subcortical level. In the present study, 18 rats with cross C7 transfer were concurrently treated with corpus callosotomy while the other 18 were not. Intracortical microstimulation was performed in the primary motor cortex (M1) at intervals of 5, 7, and 10 months postoperatively. The neural electrophysiology study showed that the representation of the injured forepaw appeared in the ipsilateral cortex at 5 months after the cross nerve transfer combined with corpus callosotomy, and it shared great overlapping zones with the representation of the health forepaw. And then, at 7-10 months, the cortical representation of the paralyzed forepaw was still located in the ipsilateral motor cortex, although significantly contracted. In contrast, rats with mere cross nerve transfer still presented interhemispheric reorganization. The results indicated that corpus callosotomy in the early stage after cross C7 transfer may had interrupted the interhemispheric functional reorganization. Combined the present study with our previous research findings, we explored the possible pathway and mechanisms of the interhemispheric functional reorganization. Thus we came to the conclusion that interhemispheric connectivity at the cortical level was essential in establishing the new contralateral control of the paralyzed limb at the initial stage after cross nerve transfer.