Skilled-learning-induced potentiation in rat sensorimotor cortex: a transient form of behavioural long-term potentiation

Site Dependency of Anodal Transcranial Direct-Current Stimulation on Reaction Time and Transfer of Learning during a Sequential Visual Isometric Pinch Task

Considering the advantages of brain stimulation techniques in detecting the role of different areas of the brain in human sensorimotor behaviors, we used anodal transcranial direct-current stimulation (a-tDCS) over three different brain sites of the frontoparietal cortex (FPC) in healthy participants to elucidate the role of these three brain areas of the FPC on reaction time (RT) during a sequential visual isometric pinch task (SVIPT). We also aimed to assess if the stimulation of these cortical sites affects the transfer of learning during SVIPT. A total of 48 right-handed healthy participants were randomly assigned to one of the four a-tDCS groups: (1) left primary motor cortex (M1), (2) left dorsolateral prefrontal cortex (DLPFC), (3) left posterior parietal cortex (PPC), and (4) sham. A-tDCS (0.3 mA, 20 min) was applied concurrently with the SVIPT, in which the participants precisely controlled their forces to reach seven different target forces from 10 to 40% of the maximum voluntary contraction (MVC) presented on a computer screen with the right dominant hand. Four test blocks were randomly performed at the baseline and 15 min after the intervention, including sequence and random blocks with either hand. Our results showed significant elongations in the ratio of RTs between the M1 and sham groups in the sequence blocks of both the right-trained and left-untrained hands. No significant differences were found between the DLPFC and sham groups and the PPC and sham groups in RT measurements within the SVIPT. Our findings suggest that RT improvement within implicit learning of an SVIPT is not mediated by single-session a-tDCS over M1, DLPFC, or PPC. Further research is needed to understand the optimal characteristics of tDCS and stimulation sites to modulate reaction time in a precision control task such as an SVIPT.

Long-lasting forms of plasticity through patterned ultrasound-induced brainwave entrainment

Achieving long-lasting neuronal modulation with low-intensity, low-frequency ultrasound is challenging. Here, we devised theta burst ultrasound stimulation (TBUS) with gamma bursts for brain entrainment and modulation of neuronal plasticity in the mouse motor cortex. We demonstrate that two types of TBUS, intermittent and continuous TBUS, induce bidirectional long-term potentiation or depression-like plasticity, respectively, as evidenced by changes in motor-evoked potentials. These effects depended on molecular pathways associated with long-term plasticity, including N-methyl-d-aspartate receptor and brain-derived neurotrophic factor/tropomyosin receptor kinase B activation, as well as de novo protein synthesis. Notably, bestrophin-1 and transient receptor potential ankyrin 1 play important roles in these enduring effects. Moreover, pretraining TBUS enhances the acquisition of previously unidentified motor skills. Our study unveils a promising protocol for ultrasound neuromodulation, enabling noninvasive and sustained modulation of brain function.

Brain Plasticity in Patients with Spinal Cord Injuries: A Systematic Review

A spinal cord injury (SCI) causes changes in brain structure and brain function due to the direct effects of nerve damage, secondary mechanisms, and long-term effects of the injury, such as paralysis and neuropathic pain (NP). Recovery takes place over weeks to months, which is a time frame well beyond the duration of spinal shock and is the phase in which the spinal cord remains unstimulated below the level of injury and is associated with adaptations occurring throughout the nervous system, often referred to as neuronal plasticity. Such changes occur at different anatomical sites and also at different physiological and molecular biological levels. This review aims to investigate brain plasticity in patients with SCIs and its influence on the rehabilitation process. Studies were identified from an online search of the PubMed, Web of Science, and Scopus databases. Studies published between 2013 and 2023 were selected. This review has been registered on OSF under (n) 9QP45. We found that neuroplasticity can affect the sensory-motor network, and different protocols or rehabilitation interventions can activate this process in different ways. Exercise rehabilitation training in humans with SCIs can elicit white matter plasticity in the form of increased myelin water content. This review has demonstrated that SCI patients may experience plastic changes either spontaneously or as a result of specific neurorehabilitation training, which may lead to positive outcomes in functional recovery. Clinical and experimental evidence convincingly displays that plasticity occurs in the adult CNS through a variety of events following traumatic or non-traumatic SCI. Furthermore, efficacy-based, pharmacological, and genetic approaches, alone or in combination, are increasingly effective in promoting plasticity.

Cx3cr1 deficiency interferes with learning- and direct current stimulation-mediated neuroplasticity of the motor cortex

Microglia are essential contributors to synaptic transmission and stability and communicate with neurons via the fractalkine pathway. Transcranial direct current stimulation [(t)DCS], a form of non-invasive electrical brain stimulation, modulates cortical excitability and promotes neuroplasticity, which has been extensively demonstrated in the motor cortex and for motor learning. The role of microglia and their fractalkine receptor CX3CR1 in motor cortical neuroplasticity mediated by DCS or motor learning requires further elucidation. We demonstrate the effects of pharmacological microglial depletion and genetic Cx3cr1 deficiency on the induction of DCS-induced long-term potentiation (DCS-LTP) ex vivo. The relevance of microglia-neuron communication for DCS response and structural neuroplasticity underlying motor learning are assessed via 2-photon in vivo imaging. The behavioural consequences of impaired CX3CR1 signalling are investigated for both gross and fine motor learning. We show that DCS-mediated neuroplasticity in the motor cortex depends on the presence of microglia and is driven in part by CX3CR1 signalling ex vivo and provide the first evidence of microglia interacting with neurons during DCS in vivo. Furthermore, CX3CR1 signalling is required for motor learning and underlying structural neuroplasticity in concert with microglia interaction. Although we have recently demonstrated the microglial response to DCS in vivo, we now provide a link between microglial integrity and neuronal activity for the expression of DCS-dependent neuroplasticity. In addition, we extend the knowledge on the relevance of CX3CR1 signalling for motor learning and structural neuroplasticity. The underlying molecular mechanisms and the potential impact of DCS in rescuing CX3CR1 deficits remain to be addressed in the future.

Unraveling the Thread of Aphasia Rehabilitation: A Translational Cognitive Perspective

Translational neuroscience is a multidisciplinary field that aims to bridge the gap between basic science and clinical practice. Regarding aphasia rehabilitation, there are still several unresolved issues related to the neural mechanisms that optimize language treatment. Although there are studies providing indications toward a translational approach to the remediation of acquired language disorders, the incorporation of fundamental neuroplasticity principles into this field is still in progress. From that aspect, in this narrative review, we discuss some key neuroplasticity principles, which have been elucidated through animal studies and which could eventually be applied in the context of aphasia treatment. This translational approach could be further strengthened by the implementation of intervention strategies that incorporate the idea that language is supported by domain-general mechanisms, which highlights the impact of non-linguistic factors in post-stroke language recovery. Here, we highlight that translational research in aphasia has the potential to advance our knowledge of brain-language relationships. We further argue that advances in this field could lead to improvement in the remediation of acquired language disturbances by remodeling the rationale of aphasia-therapy approaches. Arguably, the complex anatomy and phenomenology of aphasia dictate the need for a multidisciplinary approach with one of its main pillars being translational research.

Neuronal Plasticity and Age-Related Functional Decline in the Motor Cortex

Physiological aging causes a decline of motor function due to impairment of motor cortex function, losses of motor neurons and neuromuscular junctions, sarcopenia, and frailty. There is increasing evidence suggesting that the changes in motor function start earlier in the middle-aged stage. The mechanism underlining the middle-aged decline in motor function seems to relate to the central nervous system rather than the peripheral neuromuscular system. The motor cortex is one of the responsible central nervous systems for coordinating and learning motor functions. The neuronal circuits in the motor cortex show plasticity in response to motor learning, including LTP. This motor cortex plasticity seems important for the intervention method mechanisms that revert the age-related decline of motor function. This review will focus on recent findings on the role of plasticity in the motor cortex for motor function and age-related changes. The review will also introduce our recent identification of an age-related decline of neuronal activity in the primary motor cortex of middle-aged mice using electrophysiological recordings of brain slices.

Robust enhancement of motor sequence learning with 4 mA transcranial electric stimulation

BACKGROUND AND OBJECTIVES

Motor learning experiments with transcranial direct current stimulation (tDCS) at 2 mA have produced mixed results. We hypothesize that tDCS boosts motor learning provided sufficiently high field intensity on the motor cortex.

METHODS

In a single-blinded design, 108 healthy participants received either anodal (N = 36) or cathodal (N = 36) tDCS at 4 mA total, or no stimulation (N = 36) while they practiced a 12-min sequence learning task. Anodal stimulation was delivered across four electrode pairs (1 mA each), with anodes above the right parietal lobe and cathodes above the right frontal lobe. Cathodal stimulation, with reversed polarities, served as an active control for sensation, while the no-stimulation condition established baseline performance. fMRI-localized targets on the primary motor cortex in 10 subjects were used in current flow models to optimize electrode placement for maximal field intensity. A single electrode montage was then selected for all participants.

RESULTS

We found a significant difference in performance with anodal vs. cathodal stimulation (Cohen's d = 0.71) and vs. no stimulation (d = 0.56). This effect persisted for at least 1 h, and subsequent learning for a new sequence and the opposite hand also improved. Sensation ratings were comparable in the active groups and did not exceed moderate levels. Current flow models suggest the new electrode montage can achieve stronger motor cortex polarization than alternative montages.

CONCLUSION

The present paradigm shows a medium to large effect size and is well-tolerated. It may serve as a go-to experiment for future studies on motor learning and tDCS.

Corticospinal circuit neuroplasticity may involve silent synapses: Implications for functional recovery facilitated by neuromodulation after spinal cord injury

Spinal cord injury (SCI) leads to devastating physical consequences, such as severe sensorimotor dysfunction even lifetime disability, by damaging the corticospinal system. The conventional opinion that SCI is intractable due to the poor regeneration of neurons in the adult central nervous system (CNS) needs to be revisited as the CNS is capable of considerable plasticity, which underlie recovery from neural injury. Substantial spontaneous neuroplasticity has been demonstrated in the corticospinal motor circuitry following SCI. Some of these plastic changes appear to be beneficial while others are detrimental toward locomotor function recovery after SCI. The beneficial corticospinal plasticity in the spared corticospinal circuits can be harnessed therapeutically by multiple contemporary neuromodulatory approaches, especially the electrical stimulation-based modalities, in an activity-dependent manner to improve functional outcomes in post-SCI rehabilitation. Silent synapse generation and unsilencing contribute to profound neuroplasticity that is implicated in a variety of neurological disorders, thus they may be involved in the corticospinal motor circuit neuroplasticity following SCI. Exploring the underlying mechanisms of silent synapse-mediated neuroplasticity in the corticospinal motor circuitry that may be exploited by neuromodulation will inform a novel direction for optimizing therapeutic repair strategies and rehabilitative interventions in SCI patients.

Closed-Loop, Cervical, Epidural Stimulation Elicits Respiratory Neuroplasticity after Spinal Cord Injury in Freely Behaving Rats

Over half of all spinal cord injuries (SCIs) are cervical, which can lead to paralysis and respiratory compromise, causing significant morbidity and mortality. Effective treatments to restore breathing after severe upper cervical injury are lacking; thus, it is imperative to develop therapies to address this. Epidural stimulation has successfully restored motor function after SCI for stepping, standing, reaching, grasping, and postural control. We hypothesized that closed-loop stimulation triggered via healthy hemidiaphragm EMG activity has the potential to elicit functional neuroplasticity in spinal respiratory pathways after cervical SCI (cSCI). To test this, we delivered closed-loop, electrical, epidural stimulation (CLES) at the level of the phrenic motor nucleus (C4) for 3 d after C2 hemisection (C2HS) in freely behaving rats. A 2 × 2 Latin Square experimental design incorporated two treatments, C2HS injury and CLES therapy resulting in four groups of adult, female Sprague Dawley rats: C2HS + CLES (n = 8), C2HS (n = 6), intact + CLES (n = 6), intact (n = 6). In stimulated groups, CLES was delivered for 12-20 h/d for 3 d. After C2HS, 3 d of CLES robustly facilitated the slope of stimulus-response curves of ipsilesional spinal motor evoked potentials (sMEPs) versus nonstimulated controls. To our knowledge, this is the first demonstration of CLES eliciting respiratory neuroplasticity after C2HS in freely behaving animals. These findings suggest CLES as a promising future therapy to address respiratory deficiency associated with cSCI.

Changes in reach-to-grasp behaviour over the course of training in rats

One complex task involving sequence of movements and movement refinement in the rat is the single-pellet reaching task, comprising orientation, transport and withdrawal in sequence. In turn, orientation comprises front wall detection, slot localization and nose poke until reach start. Video recordings of a rat in the reaching box highlighted three stages of temporal training: start of training (ST), forepaw dominance appearance (D) and fully trained (T). Regarding orientation, ST versus D and T presented a significant smaller frequency of approach to the front wall and a significant higher number of whisker cycles and nose touches during slot localization, involving a significant longer Orientation. At the ST stage, 44% of the trials were interrupted after nose poke, and poke took place at significant higher level from the shelf. The shelf was identified only when short whiskers contacted it, but the tongue and both forepaws were used without distinction to reach and grasp the pellet until a forepaw emerged as dominant at D stage. Regarding the temporal features of transport and withdrawal, comparing the D versus T stage revealed a significant longer duration. Finally, successes were significantly higher in T respect to D, meaning that after dominance emergence, more training was still necessary to improve reaching/grasping performance. This study provides evidence that, during training, the rats develop a strategy to obtain the pellets and then refine their movement pattern.

Hue discrimination in jewellery appraisers exceeds normal performance

PURPOSE

The FM-100 Hue test is used for vocational purposes where hue discrimination is of the upmost importance, such as in jewellery appraisals where small errors in hue discrimination can lead to significant differences in gemstone valuation. The purpose of this study was to determine if the cone contrast test (CCT) could predict performance on the FM-100 Hue, providing a potential alternative test for screening of jewellery appraisers.

METHODS

Members of the National Association of Jewelry Appraisers (NAJA; n = 18, ages 34 to 76 years) requiring colour vision certification with the FM-100 Hue were invited to participate in a study to assess performance on the Ishihara test, FM-100 Hue, Lanthony Desaturated D-15 and the CCT. The FM-100 Hue test was administered to award or renew NAJA certification, while the CCT was included as a possible alternative certification test.

RESULTS

Average CCT M and S cone scores were predictive of FM-100 Hue total error score (TES: F_1,16  = 7.77, p < 0.02; r²  = 0.33). The regression equation indicates that a CCT score of <60 is 3SD below the mean TES in our cohort of jewellery appraisers. Seventeen of 18 jewellery appraisers had FM-100 Hue TES scores significantly below the lower limit for age-matched normal values, indicating exceptional performance (p < 0.001).

CONCLUSIONS

Results indicate the CCT may be an effective substitute for the FM-100 Hue to provide certification of jewellery appraisers, but the small sample size warrants additional comparative validation to support sole utilisation of the CCT. It is of interest that this sample of jewellery appraisers showed lower (i.e., better) than normal TES scores despite mitigating senescence factors. It is conceivable that the enhanced jewellery appraisers' hue discrimination reflects perceptual learning, wherein reward-based repetition on specific tasks can improve performance, even in adulthood beyond critical periods.

Corticospinal Motor Circuit Plasticity After Spinal Cord Injury: Harnessing Neuroplasticity to Improve Functional Outcomes

Spinal cord injury (SCI) is a devastating condition that affects approximately 294,000 people in the USA and several millions worldwide. The corticospinal motor circuitry plays a major role in controlling skilled movements and in planning and coordinating movements in mammals and can be damaged by SCI. While axonal regeneration of injured fibers over long distances is scarce in the adult CNS, substantial spontaneous neural reorganization and plasticity in the spared corticospinal motor circuitry has been shown in experimental SCI models, associated with functional recovery. Beneficially harnessing this neuroplasticity of the corticospinal motor circuitry represents a highly promising therapeutic approach for improving locomotor outcomes after SCI. Several different strategies have been used to date for this purpose including neuromodulation (spinal cord/brain stimulation strategies and brain-machine interfaces), rehabilitative training (targeting activity-dependent plasticity), stem cells and biological scaffolds, neuroregenerative/neuroprotective pharmacotherapies, and light-based therapies like photodynamic therapy (PDT) and photobiomodulation (PMBT). This review provides an overview of the spontaneous reorganization and neuroplasticity in the corticospinal motor circuitry after SCI and summarizes the various therapeutic approaches used to beneficially harness this neuroplasticity for functional recovery after SCI in preclinical animal model and clinical human patients' studies.

Age-related changes in motor cortex plasticity assessed with non-invasive brain stimulation: an update and new perspectives

It is commonly accepted that the brains capacity to change, known as plasticity, declines into old age. Recent studies have used a variety of non-invasive brain stimulation (NIBS) techniques to examine this age-related decline in plasticity in the primary motor cortex (M1), but the effects seem inconsistent and difficult to unravel. The purpose of this review is to provide an update on studies that have used different NIBS techniques to assess M1 plasticity with advancing age and offer some new perspective on NIBS strategies to boost plasticity in the ageing brain. We find that early studies show clear differences in M1 plasticity between young and older adults, but many recent studies with motor training show no decline in use-dependent M1 plasticity with age. For NIBS-induced plasticity in M1, some protocols show more convincing differences with advancing age than others. Therefore, our view from the NIBS literature is that it should not be automatically assumed that M1 plasticity declines with age. Instead, the effects of age are likely to depend on how M1 plasticity is measured, and the characteristics of the elderly population tested. We also suggest that NIBS performed concurrently with motor training is likely to be most effective at producing improvements in M1 plasticity and motor skill learning in older adults. Proposed NIBS techniques for future studies include combining multiple NIBS protocols in a co-stimulation approach, or NIBS strategies to modulate intracortical inhibitory mechanisms, in an effort to more effectively boost M1 plasticity and improve motor skill learning in older adults.

Multidimensional Phase I Dose Ranging Trials for Stroke Recovery Interventions: Key Challenges and How to Address Them

Despite an increase in the amount of published stroke recovery research, interventions have failed to markedly affect the trajectory of recovery poststroke. We argue that early-phase research to systematically investigate dose is an important contributor to advance the science underpinning stroke recovery. In this article, we aim to (a) define the problem of insufficient use of a systematic approach to early-phase, multidimensional dose articulation research and (b) propose a solution that applies this approach to design a multidimensional phase I trial to identify the maximum tolerated dose (MTD). We put forward a design template as a decision support tool to increase knowledge of how to develop a phase I dose-ranging trial for nonpharmaceutical stroke recovery interventions. This solution has the potential to advance the development of efficacious stroke recovery interventions, which include activity-based rehabilitation interventions.

Net decrease in spine-surface GluA1-containing AMPA receptors after post-learning sleep in the adult mouse cortex

The mechanisms by which sleep benefits learning and memory remain unclear. Sleep may further strengthen the synapses potentiated by learning or promote broad synaptic weakening while protecting the newly potentiated synapses. We tested these ideas by combining a motor task whose consolidation is sleep-dependent, a marker of synaptic AMPA receptor plasticity, and repeated two-photon imaging to track hundreds of spines in vivo with single spine resolution. In mouse motor cortex, sleep leads to an overall net decrease in spine-surface GluA1-containing AMPA receptors, both before and after learning. Molecular changes in single spines during post-learning sleep are correlated with changes in performance after sleep. The spines in which learning leads to the largest increase in GluA1 expression have a relative advantage after post-learning sleep compared to sleep deprivation, because sleep weakens all remaining spines. These results are obtained in adult mice, showing that sleep-dependent synaptic down-selection also benefits the mature brain.

Neural fatigue due to intensive learning is reversed by a nap but not by quiet waking

Do brain circuits become fatigued due to intensive neural activity or plasticity? Is sleep necessary for recovery? Well-rested subjects trained extensively in a visuo-motor rotation learning task (ROT) or a visuo-motor task without rotation learning (MOT), followed by sleep or quiet wake. High-density electroencephalography showed that ROT training led to broad increases in EEG power over a frontal cluster of electrodes, with peaks in the theta (mean ± SE: 24% ± 6%, p = 0.0013) and beta ranges (10% ± 3%, p = 0.01). These traces persisted in the spontaneous EEG (sEEG) between sessions (theta: 42% ± 8%, p = 0.0001; beta: 35% ± 7%, p = 0.002) and were accompanied by increased errors in a motor test with kinematic characteristics and neural substrates similar to ROT (81.8% ± 0.8% vs. 68.2% ± 2.3%; two-tailed paired t-test: p = 0.00001; Cohen's d = 1.58), as well as by score increases of subjective task-specific fatigue (4.00 ± 0.39 vs. 5.36 ± 0.39; p = 0.0007; Cohen's d = 0.60). Intensive practice with MOT did not affect theta sEEG or the motor test. A nap, but not quiet wake, induced a local sEEG decrease of theta power by 33% (SE: 8%, p = 0.02), renormalized test performance (70.9% ± 2.9% vs 79.1% ± 2.7%, p = 0.018, Cohen's d = 0.85), and improved learning ability in ROT (adaptation rate: 71.2 ± 1.2 vs. 73.4 ± 0.9, p = 0.024; Cohen's d = 0.60). Thus, sleep is necessary to restore plasticity-induced fatigue and performance.

Neurotransmitters, Cell Types, and Circuit Mechanisms of Motor Skill Learning and Clinical Applications

Animals acquire motor skills to better survive and adapt to a changing environment. The ability to learn novel motor actions without disturbing learned ones is essential to maintaining a broad motor repertoire. During motor learning, the brain makes a series of adjustments to build novel sensory–motor relationships that are stored within specific circuits for long-term retention. The neural mechanism of learning novel motor actions and transforming them into long-term memory still remains unclear. Here we review the latest findings with regard to the contributions of various brain subregions, cell types, and neurotransmitters to motor learning. Aiming to seek therapeutic strategies to restore the motor memory in relative neurodegenerative disorders, we also briefly describe the common experimental tests and manipulations for motor memory in rodents.

Development and plasticity of complex movement representations

The mammalian motor cortex is topographically organized into representations of discrete body parts (motor maps). Studies in adult rats using long-duration intracortical microstimulation (LD-ICMS) reveal that forelimb motor cortex is functionally organized into several spatially distinct areas encoding complex, multijoint movement sequences: elevate, advance, grasp, and retract. The topographical arrangement of complex movements during development and the influence of skilled learning are unknown. Here, we determined the emergence and topography of complex forelimb movement representations in rats between postnatal days (PND) 13 and 60. We further investigated the expression of the maps for complex movements under conditions of reduced cortical inhibition and whether skilled forelimb motor training could alter their developing topography. We report that simple forelimb movements are first evoked at PND 25 and are confined to the caudal forelimb area (CFA), whereas complex movements first reliably appear at PND 30 and are observed in both the caudal and rostral forelimb areas (RFA). During development, the topography of complex movement representations undergoes reorganization with "grasp" and "elevate" movements predominantly observed in the RFA and all four complex movements observed in CFA. Under reduced cortical inhibition, simple and complex movements were first observed in the CFA on PND 15 and 20, respectively, and the topography is altered relative to a saline control. Further, skilled motor learning was associated with increases in "grasp" and "retract" representations specific to the trained limb. Our results demonstrate that early-life motor experience during development can modify the topography of complex forelimb movement representations.NEW & NOTEWORTHY The motor cortex is topographically organized into maps of different body parts. We used to think that the function of motor cortex was to drive individual muscles, but more recently we have learned that it is also organized to make complex movements. However, the development and plasticity of those complex movements is completely unknown. In this paper, the emergence and topography of complex movement representation, as well as their plasticity during development, is detailed.

The history of long-term potentiation as a memory mechanism: Controversies, confirmation, and some lessons to remember

The discovery of long-term potentiation (LTP) provided the first, direct evidence for long-lasting synaptic plasticity in the living brain. Consequently, LTP was proposed to serve as a mechanism for information storage among neurons, thus providing the basis for the behavioral and psychological phenomena of learning and long-term memory formation. However, for several decades, the LTP-memory hypothesis remained highly controversial, with inconsistent and contradictory evidence providing a barrier to its general acceptance. This review summarizes the history of these early debates, challenges, and experimental strategies (successful and unsuccessful) to establish a link between LTP and memory. Together, the empirical evidence, gathered over a period of about four decades, strongly suggests that LTP serves as one of the mechanisms affording learning and memory storage in neuronal circuits. Notably, this body of work also offers some important lessons that apply to the broader fields of behavioral and cognitive neuroscience. As such, the history of LTP as a learning mechanism provides valuable insights to neuroscientists exploring the relations between brain and psychological states.

Constraint-induced movement therapy improves functional recovery after ischemic stroke and its impacts on synaptic plasticity in sensorimotor cortex and hippocampus

Constraint-induced movement therapy (CIMT) has proven to be an effective way to restore functional deficits following stroke in human and animal studies, but its underlying neural plasticity mechanism remains unknown. Accumulating evidence indicates that rehabilitation after stroke is closely associated with synaptic plasticity. We therefore investigated the impact of CIMT on synaptic plasticity in ipsilateral and contralateral brain of rats following stroke. Rats were subjected to 90 minutes of transient middle cerebral artery occlusion (MCAO). CIMT was performed from 7 days after stroke and lasted for two weeks. Modified Neurology Severity Score (mNSS) and the ladder rung walking task tests were conducted at 7,14 and 21 days after stroke. Golgi-Cox staining was used to observe the plasticity changes of dendrites and dendritic spines. The expression of glutamate receptors (GluR1, GluR2 and NR1) were examined by western blot. Our data suggest that the dendrites and dendritic spines are damaged to varying degrees in bilateral sensorimotor cortex and hippocampus after acute stroke. CIMT treatment enhances the plasticity of dendrites and dendritic spines in the ipsilateral and contralateral sensorimotor cortex, increases the expression of synaptic GluR2 in ipsilateral sensorimotor cortex, which may be mechanisms for CIMT to improve functional recovery after ischemic stroke.

Neural ensemble reactivation in rapid eye movement and slow-wave sleep coordinate with muscle activity to promote rapid motor skill learning

Neural activity patterns of recent experiences are reactivated during sleep in structures critical for memory storage, including hippocampus and neocortex. This reactivation process is thought to aid memory consolidation. Although synaptic rearrangement dynamics following learning involve an interplay between slow-wave sleep (SWS) and rapid eye movement (REM) sleep, most physiological evidence implicates SWS directly following experience as a preferred window for reactivation. Here, we show that reactivation occurs in both REM and SWS and that coordination of REM and SWS activation on the same day is associated with rapid learning of a motor skill. We performed 6 h recordings from cells in rats' motor cortex as they were trained daily on a skilled reaching task. In addition to SWS following training, reactivation occurred in REM, primarily during the pre-task rest period, and REM and SWS reactivation occurred on the same day in rats that acquired the skill rapidly. Both pre-task REM and post-task SWS activation were coordinated with muscle activity during sleep, suggesting a functional role for reactivation in skill learning. Our results provide the first demonstration that reactivation in REM sleep occurs during motor skill learning and that coordinated reactivation in both sleep states on the same day, although at different times, is beneficial for skill learning. This article is part of the Theo Murphy meeting issue 'Memory reactivation: replaying events past, present and future'.

Neuroplasticity of Language Networks in Aphasia: Advances, Updates, and Future Challenges

Researchers have sought to understand how language is processed in the brain, how brain damage affects language abilities, and what can be expected during the recovery period since the early 19th century. In this review, we first discuss mechanisms of damage and plasticity in the post-stroke brain, both in the acute and the chronic phase of recovery. We then review factors that are associated with recovery. First, we review organism intrinsic variables such as age, lesion volume and location and structural integrity that influence language recovery. Next, we review organism extrinsic factors such as treatment that influence language recovery. Here, we discuss recent advances in our understanding of language recovery and highlight recent work that emphasizes a network perspective of language recovery. Finally, we propose our interpretation of the principles of neuroplasticity, originally proposed by Kleim and Jones (1) in the context of extant literature in aphasia recovery and rehabilitation. Ultimately, we encourage researchers to propose sophisticated intervention studies that bring us closer to the goal of providing precision treatment for patients with aphasia and a better understanding of the neural mechanisms that underlie successful neuroplasticity.

From cortex to cord: motor circuit plasticity after spinal cord injury

Spinal cord injury is associated with chronic sensorimotor deficits due to the interruption of ascending and descending tracts between the brain and spinal cord. Functional recovery after anatomically complete spinal cord injury is limited due to the lack of long-distance axonal regeneration of severed fibers in the adult central nervous system. Most spinal cord injuries in humans, however, are anatomically incomplete. Although restorative treatment options for spinal cord injury remain currently limited, research from experimental models of spinal cord injury have revealed a tremendous capability for both spontaneous and treatment-induced plasticity of the corticospinal system that supports functional recovery. We review recent advances in the understanding of corticospinal circuit plasticity after spinal cord injury and concentrate mainly on the hindlimb motor cortex, its corticospinal projections, and the role of spinal mechanisms that support locomotor recovery. First, we discuss plasticity that occurs at the level of motor cortex and the reorganization of cortical movement representations. Next, we explore downstream plasticity in corticospinal projections. We then review the role of spinal mechanisms in locomotor recovery. We conclude with a perspective on harnessing neuroplasticity with therapeutic interventions to promote functional recovery.

Persistent Increases of PKMζ in Sensorimotor Cortex Maintain Procedural Long-Term Memory Storage

Procedural motor learning and memory are accompanied by changes in synaptic plasticity, neural dynamics, and synaptogenesis. Missing is information on the spatiotemporal dynamics of the molecular machinery maintaining these changes. Here we examine whether persistent increases in PKMζ, an atypical protein kinase C (PKC) isoform, store long-term memory for a reaching task in rat sensorimotor cortex that could reveal the sites of procedural memory storage. Specifically, perturbing PKMζ synthesis (via antisense oligodeoxynucleotides) and blocking atypical PKC activity (via zeta inhibitory peptide [ZIP]) in S1/M1 disrupts and erases long-term motor memory maintenance, indicating atypical PKCs and specifically PKMζ store consolidated long-term procedural memories. Immunostaining reveals that PKMζ increases in S1/M1 layers II/III and V as performance improved to an asymptote. After storage for 1 month without reinforcement, the increase in M1 layer V persists without decrement. Thus, the persistent increases in PKMζ that store long-term procedural memory are localized to the descending output layer of the primary motor cortex.

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Perturbing PKMζ synthesis in S1/M1 slows the formation of skilled motor memory

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Blocking PKMζ activity specifically erases memories maintained without reinforcement

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Skilled motor learning induces the increase of PKMζ in S1/M1 layers II/III and V

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PKMζ sustains the engram for procedural motor memory in M1 layer V

Human Depotentiation following Induction of Spike Timing Dependent Plasticity

Depotentiation (DP) is a crucial mechanism for the tuning of memory traces once LTP (Long Term Potentiation) has been induced via learning, artificial procedures, or other activities. Putative unuseful LTP might be abolished via this process. Its deficiency is thought to play a role in pathologies, such as drug induced dyskinesia. However, since it is thought that it represents a mechanism that is linked to the susceptibility to interference during consolidation of a memory trace, it is an important process to consider when therapeutic interventions, such as psychotherapy, are administered. Perhaps a person with an abnormal depotentiation is prone to lose learned effects very easily or on the other end of the spectrum is prone to overload with previously generated unuseful LTP. Perhaps this process partly explains why some disorders and patients are extremely resistant to therapy. The present study seeks to quantify the relationship between LTP and depotentiation in the human brain by using transcranial magnetic stimulation (TMS) over the cortex of healthy participants. The results provide further evidence that depotentiation can be quantified in humans by use of noninvasive brain stimulation techniques. They provide evidence that a nonfocal rhythmic on its own inefficient stimulation, such as a modified thetaburst stimulation, can depotentiate an associative, focal spike timing-dependent PAS (paired associative stimulation)-induced LTP. Therefore, the depotentiation-like process does not seem to be restricted to specific subgroups of synapses that have undergone LTP before. Most importantly, the induced LTP seems highly correlated with the amount of generated depotentiation in healthy individuals. This might be a phenomenon typical of health and might be distorted in brain pathologies, such as dystonia, or dyskinesias. The ratio of LTP/DP might be a valuable marker for potential distortions of persistence versus deletion of memory traces represented by LTP-like plasticity.

Applications of Acupuncture Therapy in Modulating Plasticity of Central Nervous System

OBJECTIVE

Acupuncture is widely applied for treatment of various neurological disorders. This manuscript will review the preclinical evidence of acupuncture in mediating neural plasticity, the mechanisms involved.

MATERIALS AND METHODS

We searched acupuncture, plasticity, and other potential related words at the following sites: PubMed, EMBASE, Cochrane Library, Chinese National Knowledge Infrastructure (CNKI), and VIP information data base. The following keywords were used: acupuncture, electroacupuncture, plasticity, neural plasticity, neuroplasticity, neurogenesis, neuroblast, stem cell, progenitor cell, BrdU, synapse, synapse structure, synaptogenesis, axon, axon regeneration, synaptic plasticity, LTP, LTD, neurotrophin, neurotrophic factor, BDNF, GDNF, VEGF, bFGF, EGF, NT-3, NT-4, NT-5, p75NTR, neurotransmitter, acetylcholine, norepinephrine, noradrenaline, dopamine, monamine. We assessed the effects of acupuncture on plasticity under pathological conditions in this review.

RESULTS

Relevant references were reviewed and presented to reflect the effects of acupuncture on neural plasticity. The acquired literatures mainly focused on neurogenesis, alterations of synapses, neurotrophins (NTs), and neurotranimitters. Acupuncture methods mentioned in this article include manual acupuncture and electroacupuncture.

CONCLUSIONS

The cumulative evidences demonstrated that acupuncture could induce neural plasticity in rodents exposed to cerebral ischemia. Neural plasticity mediated by acupuncture in other neural disorders, such as Alzheimer's disease, Parkinson's disease, and depression, were also investigated and there is evidence of positive role of acupuncture induced plasticity in these disorders as well. Mediation of neural plasticity by acupuncture is likely associated with its modulation on NTs and neurotransmitters. The exact mechanisms underlying acupuncture's effects on neural plasticity remain to be elucidated. Neural plasticity may be the potential bridge between acupuncture and the treatment of various neurological diseases.

Monoaminergic Modulation of Motor Cortex Function

Elaboration of appropriate responses to behavioral situations rests on the ability of selecting appropriate motor outcomes in accordance to specific environmental inputs. To this end, the primary motor cortex (M1) is a key structure for the control of voluntary movements and motor skills learning. Subcortical loops regulate the activity of the motor cortex and thus contribute to the selection of appropriate motor plans. Monoamines are key mediators of arousal, attention and motivation. Their firing pattern enables a direct encoding of different states thus promoting or repressing the selection of actions adapted to the behavioral context. Monoaminergic modulation of motor systems has been extensively studied in subcortical circuits. Despite evidence of converging projections of multiple neurotransmitters systems in the motor cortex pointing to a direct modulation of local circuits, their contribution to the execution and learning of motor skills is still poorly understood. Monoaminergic dysregulation leads to impaired plasticity and motor function in several neurological and psychiatric conditions, thus it is critical to better understand how monoamines modulate neural activity in the motor cortex. This review aims to provide an update of our current understanding on the monoaminergic modulation of the motor cortex with an emphasis on motor skill learning and execution under physiological conditions.

The malleable brain: plasticity of neural circuits and behavior - a review from students to students

One of the most intriguing features of the brain is its ability to be malleable, allowing it to adapt continually to changes in the environment. Specific neuronal activity patterns drive long-lasting increases or decreases in the strength of synaptic connections, referred to as long-term potentiation and long-term depression, respectively. Such phenomena have been described in a variety of model organisms, which are used to study molecular, structural, and functional aspects of synaptic plasticity. This review originated from the first International Society for Neurochemistry (ISN) and Journal of Neurochemistry (JNC) Flagship School held in Alpbach, Austria (Sep 2016), and will use its curriculum and discussions as a framework to review some of the current knowledge in the field of synaptic plasticity. First, we describe the role of plasticity during development and the persistent changes of neural circuitry occurring when sensory input is altered during critical developmental stages. We then outline the signaling cascades resulting in the synthesis of new plasticity-related proteins, which ultimately enable sustained changes in synaptic strength. Going beyond the traditional understanding of synaptic plasticity conceptualized by long-term potentiation and long-term depression, we discuss system-wide modifications and recently unveiled homeostatic mechanisms, such as synaptic scaling. Finally, we describe the neural circuits and synaptic plasticity mechanisms driving associative memory and motor learning. Evidence summarized in this review provides a current view of synaptic plasticity in its various forms, offers new insights into the underlying mechanisms and behavioral relevance, and provides directions for future research in the field of synaptic plasticity. Read the Editorial Highlight for this article on page 788. Cover Image for this issue: doi: 10.1111/jnc.13815.

Motor training and the combination of action observation and peripheral nerve stimulation reciprocally interfere with the plastic changes induced in primary motor cortex excitability

AO-PNS is a stimulation protocol combining action observation (AO) and peripheral nerve stimulation (PNS) to induce plasticity in the primary motor cortex (M1) (increased excitability). Another method to increase M1 excitability is motor training. The combination of two protocols, which individually induce long-term potentiation (LTP)-like plasticity in overlapping neural circuits, results in a transitory occlusion or reverse of this phenomenon. This study aimed to understand the neurophysiological mechanisms underlying AO-PNS by testing whether AO-PNS and motor training induced LTP-like plasticity in, at least partially, overlapping neural networks. One group of participants practiced a motor training (finger opposition movements) followed by AO-PNS, whereas another group performed the two protocols in reverse order. Motor performance was evaluated by means of a sensor-engineered glove and transcranial magnetic stimulation was used to assess M1 excitability before and after each conditioning protocol. Motor training increased movement frequency, suggesting the occurrence of motor learning in both groups. When applied on first, both motor training and AO-PNS significantly increased the motor-evoked potential (MEP), but occluded the increase of cortical excitability expected after the following protocol, leading to a significant decrease of MEP amplitude. These results suggest that motor training and AO-PNS act on partially overlapping neuronal networks, which include M1, and that AO-PNS might be able to induce LTP-like plasticity in a similar way to overt movement execution. This candidates AO-PNS as methodology potentially useful when planning rehabilitative interventions on patients who cannot voluntarily move.

Mapping the dynamics of cortical neuroplasticity of skilled motor learning using micro X-ray fluorescence and histofluorescence imaging of zinc in the rat

Synchrotron-based X-ray fluorescence imaging (XFI) of zinc (Zn) has been recently implemented to understand the efficiency of various therapeutic interventions targeting post-stroke neuroprotection and neuroplasticity. However, it is uncertain if micro XFI can resolve neuroplasticity-induced changes. Thus, we explored if learning-associated behavioral changes would be accompanied by changes in cortical Zn concentration measured by XFI in healthy adult rats. Proficiency in a skilled reach-to-eat task during early and late stages of motor learning served as a functional measure of neuroplasticity. c-Fos protein and vesicular Zn expression were employed as indirect neuronal measures of brain plasticity. A total Zn map (20×20×30μm³ resolution) generated by micro XFI failed to reflect increases in either c-Fos or vesicular Zn in the motor cortex contralateral to the trained forelimb or improved proficiency in the skilled reaching task. Remarkably, vesicular Zn increased in the late stage of motor learning along with a concurrent decrease in the number of c-fos-ip neurons relative to the early stage of motor learning. This inverse dynamics of c-fos and vesicular Zn level as the motor skill advances suggest that a qualitatively different neural population, comprised of fewer active but more efficiently connected neurons, supports a skilled action in the late versus early stage of motor learning. The lack of sensitivity of the XFI-generated Zn map to visualize the plasticity-associated changes in vesicular Zn suggests that the Zn level measured by micro XFI should not be used as a surrogate marker of neuroplasticity in response to the acquisition of skilled motor actions. Nanoscopic XFI could be explored in future as a means of imaging these subtle physiological changes.

Using senses to encourage head and upper limb voluntary movement in young infants: Implications for early intervention

PRIMARY OBJECTIVE

It has long been suggested that a neonate's movement and responses to external stimuli are the product of reflexive reactions rather than purposeful movements. However, several studies have demonstrated that this is not the case. Rationale of literature included: This study seeks to review reports showing that sensory stimuli resulted in newborns recognising and responding to different stimuli with active head or upper limb movements. We also discuss this in the context of current literature about early training on the advancement of movement and brain development. Results and outcomes: Taken together, it is clear that early active experience shapes learning in newborns.

CONCLUSIONS

The impact of this research is most exciting for applications that would induce infants to make purposeful movements, especially as a means for early intervention and rehabilitation for the treatment of infants with or at high risk for developmental delay.

Forelimb training drives transient map reorganization in ipsilateral motor cortex

Skilled motor training results in reorganization of contralateral motor cortex movement representations. The ipsilateral motor cortex is believed to play a role in skilled motor control, but little is known about how training influences reorganization of ipsilateral motor representations of the trained limb. To determine whether training results in reorganization of ipsilateral motor cortex maps, rats were trained to perform the isometric pull task, an automated motor task that requires skilled forelimb use. After either 3 or 6 months of training, intracortical microstimulation (ICMS) mapping was performed to document motor representations of the trained forelimb in the hemisphere ipsilateral to that limb. Motor training for 3 months resulted in a robust expansion of right forelimb representation in the right motor cortex, demonstrating that skilled motor training drives map plasticity ipsilateral to the trained limb. After 6 months of training, the right forelimb representation in the right motor cortex was significantly smaller than the representation observed in rats trained for 3 months and similar to untrained controls, consistent with a normalization of motor cortex maps. Forelimb map area was not correlated with performance on the trained task, suggesting that task performance is maintained despite normalization of cortical maps. This study provides new insights into how the ipsilateral cortex changes in response to skilled learning and may inform rehabilitative strategies to enhance cortical plasticity to support recovery after brain injury.

In Search of the Motor Engram: Motor Map Plasticity as a Mechanism for Encoding Motor Experience

Motor skill acquisition occurs through modification and organization of muscle synergies into effective movement sequences. The learning process is reflected neurophysiologically as a reorganization of movement representations within the primary motor cortex, suggesting that the motor map is a motor engram. However, the specific neural mechanisms underlying map plasticity are unknown. Here the authors review evidence that 1) motor map topography reflects the capacity for skilled movement, 2) motor skill learning induces reorganization of motor maps in a manner that reflects the kinematics of acquired skilled movement, 3) map plasticity is supported by a reorganization of cortical microcircuitry involving changes in synaptic efficacy, and 4) motor map integrity and topography are influenced by various neurochemical signals that coordinate changes in cortical circuitry to encode motor experience. Finally, the role of motor map plasticity in recovery of motor function after brain damage is discussed.

Delta-9-tetrahydrocannabinol (THC) affects forelimb motor map expression but has little effect on skilled and unskilled behavior

It has previously been shown in rats that acute administration of delta-9-tetrahydrocannabinol (THC) exerts a dose-dependent effect on simple locomotor activity, with low doses of THC causing hyper-locomotion and high doses causing hypo-locomotion. However the effect of acute THC administration on cortical movement representations (motor maps) and skilled learned movements is completely unknown. It is important to determine the effects of THC on motor maps and skilled learned behaviors because behaviors like driving place people at a heightened risk. Three doses of THC were used in the current study: 0.2mg/kg, 1.0mg/kg and 2.5mg/kg representing the approximate range of the low to high levels of available THC one would consume from recreational use of cannabis. Acute peripheral administration of THC to drug naïve rats resulted in dose-dependent alterations in motor map expression using high resolution short duration intracortical microstimulation (SD-ICMS). THC at 0.2mg/kg decreased movement thresholds and increased motor map size, while 1.0mg/kg had the opposite effect, and 2.5mg/kg had an even more dramatic effect. Deriving complex movement maps using long duration (LD)-ICMS at 1.0mg/kg resulted in fewer complex movements. Dosages of 1.0mg/kg and 2.5mg/kg THC reduced the number of reach attempts but did not affect percentage of success or the kinetics of reaching on the single pellet skilled reaching task. Rats that received 2.5mg/kg THC did show an increase in latency of forelimb removal on the bar task, while dose-dependent effects of THC on unskilled locomotor activity using the rotorod and horizontal ladder tasks were not observed. Rats may be employing compensatory strategies after receiving THC, which may account for the robust changes in motor map expression but moderate effects on behavior.

A Randomized Trial Comparing Two Tongue-Pressure Resistance Training Protocols for Post-Stroke Dysphagia

The objective of this study was to compare the outcomes of two tongue resistance training protocols. One protocol ("tongue-pressure profile training") emphasized the pressure-timing patterns that are typically seen in healthy swallows by focusing on gradual pressure release and saliva swallowing tasks. The second protocol ("tongue-pressure strength and accuracy training") emphasized strength and accuracy in tongue-palate pressure generation and did not include swallowing tasks. A prospective, randomized, parallel allocation trial was conducted. Of 26 participants who were screened for eligibility, 14 received up to 24 sessions of treatment. Outcome measures of posterior tongue strength, oral bolus control, penetration-aspiration and vallecular residue were made based on videofluoroscopy analysis by blinded raters. Complete data were available for 11 participants. Significant improvements were seen in tongue strength and post-swallow vallecular residue with thin liquids, regardless of treatment condition. Stage transition duration (a measure of the duration of the bolus presence in the pharynx prior to swallow initiation, which had been chosen to capture impairments in oral bolus control) showed no significant differences. Similarly, significant improvements were not seen in median scores on the penetration-aspiration scale. This trial suggests that tongue strength can be improved with resistance training for individuals with tongue weakness following stroke. We conclude that improved penetration-aspiration does not necessarily accompany improvements in tongue strength; however, tongue-pressure resistance training does appear to be effective for reducing thin liquid vallecular residue.

Differences in stress-induced changes in extinction and prefrontal plasticity in postweanling and adult animals

BACKGROUND

Postweaning is a critical developmental stage during which the medial prefrontal cortex (mPFC) undergoes major changes and the brain is vulnerable to the effects of stress. Surprisingly, the engagement of the mPFC in extinction of fear was reported to be identical in postweanling (PW) and adult animals. Here, we examined whether the effect of stress on extinction and mPFC plasticity would be similar in PW and adult animals.

METHODS

PW and adult animals were fear conditioned and exposed to the elevated platform stress paradigm, and extinction and long-term potentiation were examined. The dependency of stress-induced modulation of extinction and plasticity on N-methyl-D-aspartate receptors was examined as well.

RESULTS

We show that exposure to stress is associated with reduction of fear and enhanced induction of long-term potentiation (LTP) in PW pups, in contrast to its effects in adult animals. Furthermore, we report opposite effects in the occlusion of LTP following the enhanced or impaired extinction in the two age groups and that the reversal of the effects of stress is independent of N-methyl-D-aspartate receptor activation in PW animals.

CONCLUSIONS

Our results show that qualitatively different mechanisms control the modulatory effects of stress on extinction and plasticity in postweanling pups compared with adult rats. Our results point to significant differences between young and adult brains, which may have potential implications for the treatment of anxiety and stress disorders across development.

An effective repetitive training schedule to achieve skill proficiency using a novel robotic virtual reality simulator

PURPOSE

A robotic virtual reality simulator (Mimic dV-Trainer) can be a useful training method for the da Vinci surgical system. Herein, we investigate several repetitive training schedules and determine which is the most effective.

METHODS

A total of 30 medical students were enrolled and were divided into 3 groups according to the training schedule. Group 1 performed the task 1 hour daily for 4 consecutive days, group II performed the task on once per week for 1 hour for 4 consecutive weeks, and group III performed the task for 4 consecutive hours in 1 day. The effects of training were investigated by analyzing the number of repetitions and the time required to complete the "Tube 2" simulation task when the learning curve plateau was reached. The point at which participants reached a stable score was evaluated using the cumulative sum control graph.

RESULTS

The average time to complete the task at the learning curve plateau was 150.3 seconds in group I, 171.9 seconds in group II, and 188.5 seconds in group III. The number of task repetitions required to reach the learning curve plateau was 45 repetitions in group I, 36 repetitions in group II, and 39 repetitions in group III. Therefore, there was continuous improvement in the time required to perform the task after 40 repetitions in group I only. There was a significant correlation between improvement in each trial interval and attempt, and the correlation coefficient (0.924) in group I was higher than that in group II (0.899) and group III (0.838).

CONCLUSION

Daily 1-hour practice sessions performed for 4 consecutive days resulted in the best final score, continuous score improvement, and effective training while minimizing fatigue. This repetition schedule can be used for effectively training novices in future.

Motor Cortex and Motor Cortical Interhemispheric Communication in Walking After Stroke: The Roles of Transcranial Magnetic Stimulation and Animal Models in Our Current and Future Understanding

Despite the plethora of human neurophysiological research, the bilateral involvement of the leg motor cortical areas and their interhemispheric interaction during both normal and impaired human walking is poorly understood. Using transcranial magnetic stimulation (TMS), we have expanded our understanding of the role upper-extremity motor cortical areas play in normal movements and how stroke alters this role, and probed the efficacy of interventions to improve post-stroke arm function. However, similar investigations of the legs have lagged behind, in part, due to the anatomical difficulty in using TMS to stimulate the leg motor cortical areas. Additionally, leg movements are predominately bilaterally controlled and require interlimb coordination that may involve both hemispheres. The sensitive, but invasive, tools used in animal models of locomotion hold great potential for increasing our understanding of the bihemispheric motor cortical control of walking. In this review, we discuss 3 themes associated with the bihemispheric motor cortical control of walking after stroke: (a) what is known about the role of the bihemispheric motor cortical control in healthy and poststroke leg movements, (b) how the neural remodeling of the contralesional hemisphere can affect walking recovery after a stroke, and (c) what is the effect of behavioral rehabilitation training of walking on the neural remodeling of the motor cortical areas bilaterally. For each theme, we discuss how rodent models can enhance the present knowledge on human walking by testing hypotheses that cannot be investigated in humans, and how these findings can then be back-translated into the neurorehabilitation of poststroke walking.

Prenatal stress enhances excitatory synaptic transmission and impairs long-term potentiation in the frontal cortex of adult offspring rats

The effects of prenatal stress procedure were investigated in 3 months old male rats. Prenatally stressed rats showed depressive-like behavior in the forced swim test, including increased immobility, decreased mobility and decreased climbing. In ex vivo frontal cortex slices originating from prenatally stressed animals, the amplitude of extracellular field potentials (FPs) recorded in cortical layer II/III was larger, and the mean amplitude ratio of pharmacologically-isolated NMDA to the AMPA/kainate component of the field potential—smaller than in control preparations. Prenatal stress also resulted in a reduced magnitude of long-term potentiation (LTP). These effects were accompanied by an increase in the mean frequency, but not the mean amplitude, of spontaneous excitatory postsynaptic currents (sEPSCs) in layer II/III pyramidal neurons. These data demonstrate that stress during pregnancy may lead not only to behavioral disturbances, but also impairs the glutamatergic transmission and long-term synaptic plasticity in the frontal cortex of the adult offspring.

Metaplastic up-regulation of LTP in the rat visual cortex by monocular visual training: requirement of task mastery, hemispheric specificity, and NMDA-GluN2B involvement

"Metaplasticity" is defined as an alteration of synaptic plasticity properties or mechanisms by a priming event without actual changes in synaptic strength. For example, visual discrimination training of rats leads to a facilitation of the subsequent induction of long-term potentiation (LTP) between the lateral geniculate nucleus (LGN) and the primary visual cortex (V1). Here, rats received visual discrimination training in a modified water maze, with one eye occluded during training to create monocular viewing conditions; 63% of rats acquired the task under these conditions. Following training, in vivo electrophysiology was used to examine LTP of field postsynaptic potentials (fPSPs) in V1 elicited by LGN stimulation. Rats that had successfully learned the task showed significantly greater LTP in the "trained V1" (contralateral to the open, trained eye) relative to the "untrained" hemisphere. Rats that underwent training but failed to acquire the task did not show this lateralized plasticity enhancement and had similar levels of LTP in both cerebral hemispheres. Cortical application of the NMDA receptor-GluN2B subunit antagonist Ro 25-6981 (2 mM) reversed the training-induced LTP facilitation without affecting LTP in the untrained V1. Whole-cell patch clamp recordings of V1 (layers II/III) pyramidal cells in vitro demonstrated that pharmacologically isolated NMDA currents exhibit a greater sensitivity to GluN2B blockade in the trained relative to the untrained V1. Together, these experiments reveal a surprising degree of anatomical (only in the hemisphere contralateral to the trained eye) and behavioral specificity (only in rats that mastered the task) for the effect of visual training to enhance LTP in V1. Further, cortical GluN2B subunits appear to be directly involved in this metaplastic facilitation of thalamocortical plasticity, suggesting that NMDA subunit composition or functioning is, at least in part, regulated by the exposure to behaviorally significant stimuli in an animal's sensory environment.

Metaplasticity in human cortex

Metaplasticity refers to the modification of plasticity induction (direction, magnitude, duration) by previous activity of the same postsynaptic neuron or neuronal network. In recent years evidence from animal studies has been accumulated that metaplasticity significantly contributes to network function and behavior. Here, we review the evidence for metaplasticity at the system level of the human cortex as investigated by non-invasive brain stimulation. These studies support the notion that metaplasticity is also operative in the human brain and is mostly homeostatic in nature, that is, keeping network activity within a physiological range. However, non-homeostatic metaplasticity has also been described, which can increase non-invasive brain stimulation-induced aftereffects on cortical excitability, or learning. Current evidence further suggests that aberrant metaplasticity may underlie some neurological and psychiatric diseases. Finally, first proof-of-principle studies show that the concept of metaplasticity can be harnessed for treatment of patients suffering from brain diseases.

Chronic fluoxetine treatment suppresses plasticity (long-term potentiation) in the mature rodent primary auditory cortex in vivo

Several recent studies have provided evidence that chronic treatment with the selective serotonin reuptake inhibitor (SSRI) fluoxetine can facilitate synaptic plasticity (e.g., ocular dominance shifts) in the adult central nervous system. Here, we assessed whether fluoxetine enhances long-term potentiation (LTP) in the thalamocortical auditory system of mature rats, a developmentally regulated form of plasticity that shows a characteristic decline during postnatal life. Adult rats were chronically treated with fluoxetine (administered in the drinking water, 0.2 mg/mL, four weeks of treatment). Electrophysiological assessments were conducted using an anesthetized (urethane) in vivo preparation, with LTP of field potentials in the primary auditory cortex (A1) induced by theta-burst stimulation of the medial geniculate nucleus. We find that, compared to water-treated control animals, fluoxetine-treated rats did not express higher levels of LTP and, in fact, exhibited reduced levels of potentiation at presumed intracortical A1 synapses. Bioactivity of fluoxetine was confirmed by a reduction of weight gain and fluid intake during the four-week treatment period. We conclude that chronic fluoxetine treatment fails to enhance LTP in the mature rodent thalamocortical auditory system, results that bring into question the notion that SSRIs act as general facilitators of synaptic plasticity in the mammalian forebrain.