The organization of the forelimb representation of the C57BL/6 mouse motor cortex as defined by intracortical microstimulation and cytoarchitecture

Activation of adult endogenous neurogenesis by a hyaluronic acid collagen gel containing basic fibroblast growth factor promotes remodeling and functional recovery of the injured cerebral cortex

JOURNAL/nrgr/04.03/01300535-202510000-00024/figure1/v/2024-11-26T163120Z/r/image-tiff The presence of endogenous neural stem/progenitor cells in the adult mammalian brain suggests that the central nervous system can be repaired and regenerated after injury. However, whether it is possible to stimulate neurogenesis and reconstruct cortical layers II to VI in non-neurogenic regions, such as the cortex, remains unknown. In this study, we implanted a hyaluronic acid collagen gel loaded with basic fibroblast growth factor into the motor cortex immediately following traumatic injury. Our findings reveal that this gel effectively stimulated the proliferation and migration of endogenous neural stem/progenitor cells, as well as their differentiation into mature and functionally integrated neurons. Importantly, these new neurons reconstructed the architecture of cortical layers II to VI, integrated into the existing neural circuitry, and ultimately led to improved brain function. These findings offer novel insight into potential clinical treatments for traumatic cerebral cortex injuries.

Post-stroke spontaneous motor recovery in mice can be predicted from acute-phase local field potential using machine learning

Stroke remains a leading cause of long-term disability, underscoring the urgent need for effective predictors of motor recovery. Understanding the electrophysiological changes underlying spontaneous recovery could offer critical insight into recovery mechanisms and aid in predicting individual rehabilitation trajectories. In this study, we investigated the predictive power of local field potentials recorded 2 days post-stroke to forecast 1 month motor recovery in a mouse model of ischemic stroke. By employing a comprehensive machine learning approach, we identified key electrophysiological features that significantly enhanced prediction accuracy. Through nested leave-one-animal-out cross-validation, we achieved high prediction accuracy, correctly identifying motor recovery status in 15 out of 16 mice. Our findings also revealed that pre-stroke brain activity did not contribute to prediction accuracy, suggesting that post-stroke dynamics are the primary determinants of recovery. Notably, we found that features from the contralesional hemisphere were particularly influential in predicting recovery outcomes, underscoring the critical role of the non-lesioned hemisphere in motor recovery. Our data-driven methodology underscores the importance of balancing feature selection to optimize predictive performance, particularly in the context of spontaneous recovery, where insight into natural recovery processes can guide the development of targeted rehabilitation strategies. Ultimately, our findings advocate for a deeper understanding of post-stroke brain dynamics to improve clinical outcomes for stroke patients.

Corticothalamic neurons in motor cortex have a permissive role in motor execution

The primary motor cortex (M1) is a central hub for motor learning and execution. M1 is composed of heterogeneous cell types with varying relationships to movement. Here, we tagged active neurons at different stages of motor task performance in mice and characterized cell type composition. We identified corticothalamic neurons (M1CT) as consistently enriched with training progression. Using two-photon calcium imaging, we found that M1CT activity is largely suppressed during movement, and this negative correlation augments with training. Increasing M1CT activity through closed-loop optogenetic manipulations during forelimb movement significantly hinders execution, an effect that became stronger with training. Similar manipulations, however, had little effect on locomotion. In contrast, M1 corticospinal neurons positively correlate with movement, with an increase during training. We uncovered that M1CT neurons suppress corticospinal activity via feedforward inhibition, also scaling with training. These results identify a permissive role of corticothalamic neurons in movement execution through disinhibition of corticospinal neurons.

Theta Frequency Electromagnetic Stimulation Enhances Functional Recovery After Stroke

Extremely low-frequency, low-intensity electromagnetic field (ELF-EMF) therapy is a non-invasive brain stimulation method that can modulate neuroprotection and neuroplasticity. ELF-EMF was recently shown to enhance recovery in human stroke in a small pilot clinical trial (NCT04039178). ELF-EMFs encompass a wide range of frequencies, typically ranging from 1 to 100 Hz, and their effects can vary depending on the specific frequency employed. However, whether and to what extent the effectiveness of ELF-EMFs depends on the frequency remains unclear. In the present study, we aimed to assess the efficacy of different frequency-intensity protocols of ELF-EMF in promoting functional recovery in a mouse cortical stroke model with treatment initiated 4 days after the stroke, employing a series of motor behavior tests. Our findings demonstrate that a theta-frequency ELF-EMF (5 Hz) effectively enhances functional recovery in a reach-to-grasp task, whereas neither gamma-frequency (40 Hz) nor combination frequency (5-16-40 Hz) ELF-EMFs induce a significant effect. Importantly, our histological analysis reveals that none of the ELF-EMF protocols employed in our study affect infarct volume, inflammatory, or glial activation, suggesting that the observed beneficial effects may be mediated through non-neuroprotective mechanisms. Our data indicate that ELF-EMFs have an influence on functional recovery after stroke, and this effect is contingent upon the specific frequency used. These findings underscore the critical importance of optimizing the protocol parameters to maximize the beneficial effects of ELF-EMF. Further research is warranted to elucidate the underlying mechanisms and refine the protocol parameters for optimal therapeutic outcomes in stroke rehabilitation.

Layer-Specific Connectivity and Functional Interference of Chrna2+ Layer 5 Martinotti Cells in the Primary Motor Cortex

The cortical somatostatin interneuron population includes several diverse cell types, among them the Martinotti cells. Layer-specific differences in connectivity and function between different subtypes of Martinotti cells are becoming apparent, which require contemporary studies to investigate cortical interneurons in a layer and subtype-specific manner. In this study, we investigate the connectivity of a subtype of Chrna2+ layer 5 Martinotti cells in the primary motor cortex, using a monosynaptic retrograde rabies viral tracer. We found direct input from pyramidal cells and local parvalbumin interneurons. In addition, we found long-range direct inputs from the motor thalamus, substantia innominata of the basal forebrain, and globus pallidus. Based on the observed input pattern, we tested and found an increased number of falls in the hanging wire test upon temporary overexcitation of Chrna2+ layer 5 Martinotti cells, suggesting that Chrna2+ Martinotti cells in the motor cortex can interfere with sensorimotor integration. In summary, our study provides novel insights into the connectivity and functional role of Mα2 cells in the M1 forelimb area, highlighting their unique integration of local and long-range inputs critical for sensorimotor processing, which lay the groundwork for further exploration of their role in cortical plasticity and motor learning.

Selective direct motor cortical influence during naturalistic climbing in mice

It remains poorly resolved when and how motor cortical output directly influences limb muscle activity through descending projections, which impedes mechanistic understanding of motor control. Here we addressed this in mice performing an ethologically inspired climbing behavior. We quantified the direct influence of forelimb primary motor cortex (caudal forelimb area, CFA) on muscles across the muscle activity states expressed during climbing. We found that CFA instructs muscle activity pattern by selectively activating certain muscles, while less frequently activating or suppressing their antagonists. From Neuropixels recordings, we identified linear combinations (components) of motor cortical activity that covary with these effects. These components differ partially from those that covary with muscle activity and differ almost completely from those that covary with kinematics. Collectively, our results reveal an instructive direct motor cortical influence on limb muscles that is selective within a motor behavior and reliant on a distinct neural activity subspace.

Dissociable roles of distinct thalamic circuits in learning reaches to spatial targets

Reaching movements are critical for survival, and are learned and controlled by distributed motor networks. Even though the thalamus is a highly interconnected node in these networks, its role in learning and controlling reaches remains underexplored. We report dissociable roles of two thalamic forelimb circuits coursing through parafascicular (Pf) and ventroanterior/ventrolateral (VAL) nuclei in refining reaches to a spatial target. Using 2-photon calcium imaging as mice learn directional reaches, we observe high reach-related activity from both circuits early in learning, which decreases with learning. Pf activity encodes reach direction early in learning, more so than VAL. Consistently, bilateral lesions of Pf before training impairs refinement of reach direction. Pre-training lesions of VAL does not affect reach direction, but increases reach speed and target overshoot. Lesions of either nucleus after training does not affect the execution of learned reaches. These findings reveal different thalamic circuits governing distinct aspects of learned reaches.

The diversity and plasticity of descending motor pathways rewired after stroke and trauma in rodents

Descending neural pathways to the spinal cord plays vital roles in motor control. They are often damaged by brain injuries such as stroke and trauma, which lead to severe motor impairments. Due to the limited capacity for regeneration of neural circuits in the adult central nervous system, currently no essential treatments are available for complete recovery. Notably, accumulating evidence shows that residual circuits of the descending pathways are dynamically reorganized after injury and contribute to motor recovery. Furthermore, recent technological advances in cell-type classification and manipulation have highlighted the structural and functional diversity of these pathways. Here, we focus on three major descending pathways, namely, the corticospinal tract from the cerebral cortex, the rubrospinal tract from the red nucleus, and the reticulospinal tract from the reticular formation, and summarize the current knowledge of their structures and functions, especially in rodent models (mice and rats). We then review and discuss the process and patterns of reorganization induced in these pathways following injury, which compensate for lost connections for recovery. Understanding the basic structural and functional properties of each descending pathway and the principles of the induction and outcome of the rewired circuits will provide therapeutic insights to enhance interactive rewiring of the multiple descending pathways for motor recovery.

Parvalbumin interneurons regulate rehabilitation-induced functional recovery after stroke and identify a rehabilitation drug

Motor disability is a critical impairment in stroke patients. Rehabilitation has a limited effect on recovery; but there is no medical therapy for post-stroke recovery. The biological mechanisms of rehabilitation in the brain remain unknown. Here, using a photothrombotic stroke model in male mice, we demonstrate that rehabilitation after stroke selectively enhances synapse formation in presynaptic parvalbumin interneurons and postsynaptic neurons in the rostral forelimb motor area with axonal projections to the caudal forelimb motor area where stroke was induced (stroke-projecting neuron). Rehabilitation improves motor performance and neuronal functional connectivity, while inhibition of stroke-projecting neurons diminishes motor recovery. Stroke-projecting neurons show decreased dendritic spine density, reduced external synaptic inputs, and a lower proportion of parvalbumin synapse in the total GABAergic input. Parvalbumin interneurons regulate neuronal functional connectivity, and their activation during training is necessary for recovery. Furthermore, gamma oscillation, a parvalbumin-regulated rhythm, is increased with rehabilitation-induced recovery in animals after stroke and stroke patients. Pharmacological enhancement of parvalbumin interneuron function improves motor recovery after stroke, reproducing rehabilitation recovery. These findings identify brain circuits that mediate rehabilitation-recovery and the possibility for rational selection of pharmacological agents to deliver the first molecular-rehabilitation therapeutic.

3-HKA Promotes Vascular Remodeling After Stroke by Modulating the Activation of A1/A2 Reactive Astrocytes

Ischemic stroke is the most common cerebrovascular disease and the leading cause of permanent disability worldwide. Recent studies have shown that stroke development and prognosis are closely related to abnormal tryptophan metabolism. Here, significant downregulation of 3-hydroxy-kynurenamine (3-HKA) in stroke patients and animal models is identified. Supplementation with 3-HKA improved long-term neurological recovery, reduced infarct volume, and increased ipsilateral cerebral blood flow after distal middle cerebral artery occlusion (MCAO). 3-HKA promoted angiogenesis, functional blood vessel formation, and blood-brain barrier (BBB) repair. Moreover, 3-HKA inhibited A1-like (neurotoxic) astrocyte activation but promoted A2-like (neuroprotective) astrocyte polarization. Proteomic analysis revealed that 3-HKA inhibited AIM2 inflammasome activation after stroke, and co-labeling studies indicated that AIM2 expression typically increased in astrocytes at 7 and 14 days after stroke. Consistently, in co-cultures of primary mouse brain microvascular endothelial cells and astrocytes, 3-HKA promoted angiogenesis after oxygen-glucose deprivation (OGD). AIM2 overexpression in astrocytes abrogated 3-HKA-driven vascular remodeling in vitro and in vivo, suggesting that 3-HKA may regulate astrocyte-mediated vascular remodeling by impeding AIM2 inflammasome activation. In conclusion, 3-HKA may promote post-stroke vascular remodeling by regulating A1/A2 astrocyte activation, thereby improving long-term neurological recovery, suggesting that supplementation with 3-HKA may be an efficient therapy for stroke.

A cell type-specific mechanism driving the rapid antidepressant effects of transcranial magnetic stimulation

Repetitive transcranial magnetic stimulation (rTMS) is an emerging treatment for brain disorders, but its therapeutic mechanism is unknown. We developed a novel mouse model of rTMS with superior clinical face validity and investigated the neural mechanism by which accelerated intermittent theta burst stimulation (aiTBS) - the first rapid-acting rTMS antidepressant protocol - reversed chronic stress-induced behavioral deficits. Using fiber photometry, we showed that aiTBS drives distinct patterns of neural activity in intratelencephalic (IT) and pyramidal tract (PT) projecting neurons in dorsomedial prefrontal cortex (dmPFC). However, only IT neurons exhibited persistently increased activity during both aiTBS and subsequent depression-related behaviors. Similarly, aiTBS reversed stress-related loss of dendritic spines on IT, but not PT neurons, further demonstrating cell type-specific effects of stimulation. Finally, chemogenetic inhibition of dmPFC IT neurons during rTMS blocked the antidepressant-like behavioral effects of aiTBS. Thus, we demonstrate a prefrontal mechanism linking rapid aiTBS-driven therapeutic effects to cell type-specific circuit plasticity.

The Dose-Dependent Effects of Fluorocitrate on the Metabolism and Activity of Astrocytes and Neurons

BACKGROUND

Fluorocitrate (FC) ranging from 5 μM to 5 mM is often used as a specific metabolic inhibitor of the astrocytes to study astrocytic functions. Whether FC at such concentrations may affect neuronal metabolism and function in vivo remains unclear.

METHODS

We examined the effects of FC on the ATP levels and Ca2+ activity of the astrocytes and neurons in the motor cortices of living mice using two-photon microscopy.

RESULTS

We found that 25 μM and 250 μM of FC decreased the intracellular ATP levels and Ca2+ activity in the astrocytes in the motor cortex. Equally, 250 μM of FC, but not 25 μM of FC, reduced the intracellular ATP levels in the dendritic processes of the layer 5 pyramidal neurons. However, 25 μM of FC increased the neuronal Ca2+ activity, whereas ≥250 μM of FC decreased it. To test whether the differential effects of FC on neuronal Ca2+ activity reflect the direct effect of FC on the neurons or its indirect effect on the astrocytes, we used the CNO-hM3Dq chemogenetic approach to block astrocytic Ca2+ activity and examined the effect of FC. In the absence of astrocytic Ca2+ activity, 25 μM of FC still increased and ≥250 μM of FC reduced the dendritic Ca2+ activity of the neurons, respectively, suggesting a direct effect of 250 μM of FC on inhibiting neuronal Ca2+ activity. Further, 250 μM, but not 25 μM, of FC increased the size of the dendritic spines over 2 h.

CONCLUSIONS

Our findings suggest that FC at high concentrations (≥250 μM) is not a specific inhibitor of astrocytic functions, as it directly affects neuronal metabolism and synaptic plasticity in vivo.

Selective Targeting of a Defined Subpopulation of Corticospinal Neurons using a Novel Klhl14-Cre Mouse Line Enables Molecular and Anatomical Investigations through Development into Maturity

The corticospinal tract (CST) facilitates skilled, precise movements, which necessitates that subcerebral projection neurons (SCPN) establish segmentally specific connectivity with brainstem and spinal circuits. Developmental molecular delineation enables prospective identification of corticospinal neurons (CSN) projecting to thoraco-lumbar spinal segments; however, it remains unclear whether other SCPN subpopulations in developing sensorimotor cortex can be prospectively identified in this manner. Such molecular tools could enable investigations of SCPN circuitry with precision and specificity. During development, Kelch-like 14 (Klhl14) is specifically expressed by a specific SCPN subpopulation, CSNBC-lat, that reside in lateral sensorimotor cortex with axonal projections exclusively to bulbar-cervical targets. In this study, we generated Klhl14-T2A-Cre knock-in mice to investigate SCPN that are Klhl14+ during development into maturity. Using conditional anterograde and retrograde labeling, we find that Klhl14-Cre is specifically expressed by CSNBC-lat only at specific developmental time points. We establish conditional viral labeling in Klhl14-T2A-Cre mice as a new approach to reliably investigate CSNBC-lat axon targeting and confirm that this identifies known molecular regulators of CSN axon targeting. Therefore, Klhl14-T2A-Cre mice can be used as a novel tool for identifying molecular regulators of CST axon guidance in a relatively high-throughput manner in vivo. Finally, we demonstrate that intersectional viral labeling enables precise targeting of only Klhl14-Cre+ CSNBC-lat in the adult central nervous system. Together, our results establish that developmental molecular delineation of SCPN subpopulations can be used to selectively and specifically investigate their development, as well as anatomical and functional organization into maturity.

Edonerpic maleate prevents epileptic seizure during recovery from brain damage by balancing excitatory and inhibitory inputs

Functional recovery from brain damage, such as stroke, is a plastic process in the brain. The excitatory glutamate α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) plays a crucial role in neuronal functions, and the synaptic trafficking of AMPAR is a fundamental mechanism underlying synaptic plasticity. We recently identified a collapsin response mediator protein 2 (CRMP2)-binding compound, edonerpic maleate, which augments rehabilitative training-dependent functional recovery from brain damage by facilitating experience-driven synaptic delivery of AMPARs. In animals recovering from cryogenic brain injury, a potential compensatory area adjacent to the injured region was observed, where the injection of CNQX, an AMPAR antagonist, significantly attenuated functional recovery. In the compensatory brain area of animals recovering from cryogenic injury, the administration of edonerpic maleate enhanced both excitatory and inhibitory synaptic inputs at pyramidal neurons. In contrast, recovered animals that did not receive the drug exhibited augmentation of only excitatory synaptic input. The threshold of picrotoxin-induced epileptic seizure in recovered animals without edonerpic maleate treatment was lower than in intact animals and recovered animals with edonerpic maleate. Thus, edonerpic maleate enhances motor function recovery from brain damage by balancing excitatory and inhibitory synaptic inputs, which helps prevent epileptic seizures during recovery.

Dynamic lateralization in contralateral-projecting corticospinal neurons during motor learning

Understanding the adaptability of the motor cortex in response to bilateral motor tasks is crucial for advancing our knowledge of neural plasticity and motor learning. Here we aim to investigate the dynamic lateralization of contralateral-projecting corticospinal neurons (cpCSNs) during such tasks. Utilizing in vivo two-photon calcium imaging, we observe cpCSNs in mice performing a "left-right" lever-press task. Our findings reveal heterogeneous populational dynamics in cpCSNs: a marked decrease in activity during ipsilateral motor learning, in contrast to maintained activity during contralateral motor learning. Notably, individual cpCSNs show dynamic shifts in engagement with ipsilateral and contralateral movements, displaying an evolving pattern of activation over successive days. It suggests that cpCSNs exhibit adaptive changes in activation patterns in response to ipsilateral and contralateral movements, highlighting a flexible reorganization during motor learning This reconfiguration underscores the dynamic nature of cortical lateralization in motor learning and offers insights for neuromotor rehabilitation.

Concurrent optogenetic motor mapping of multiple limbs in awake mice reveals cortical organization of coordinated movements

BACKGROUND

Motor mapping allows for determining the macroscopic organization of motor circuits and corresponding motor movement representations on the cortex. Techniques such as intracortical microstimulation (ICMS) are robust, but can be time consuming and invasive, making them non-ideal for cortex-wide mapping or longitudinal studies. In contrast, optogenetic motor mapping offers a rapid and minimally invasive technique, enabling mapping with high spatiotemporal resolution. However, motor mapping has seen limited use in tracking 3-dimensonal, multi-limb movements in awake animals. This gap has left open questions regarding the underlying organizational principles of motor control of coordinated, ethologically-relevant movements involving multiple limbs.

OBJECTIVE

Our first objective was to develop Multi-limb Optogenetic Motor Mapping (MOMM) to concurrently map motor movement representations of multiple limbs with high fidelity in awake mice. Having established MOMM, our next objective was determine whether maps of coordinated and ethologically-relevant motor output were topographically organized on the cortex.

METHODS

We combine optogenetic stimulation with a deep learning driven pose-estimation toolbox, DeepLabCut (DLC), and 3-dimensional triangulation to concurrently map motor movements of multiple limbs in awake mice.

RESULTS

MOMM consistently revealed cortical topographies for all mapped features within and across mice. Many motor maps overlapped and were topographically similar. Several motor movement representations extended beyond cytoarchitecturally defined somatomotor cortex. Finer articulations of the forepaw resided within gross motor movement representations of the forelimb. Moreover, many cortical sites exhibited concurrent limb coactivation when photostimulated, prompting the identification of several cortical regions harboring coordinated and ethologically-relevant movements.

CONCLUSIONS

The cortex appears to be topographically organized by motor programs, which are responsible for coordinated, multi-limbed, and behavior-like movements.

Cortico-thalamic communication for action coordination in a skilled motor sequence

The coordination of forelimb and orofacial movements to compose an ethological reach-to-consume behavior likely involves neural communication across brain regions. Leveraging wide-field imaging and photo-inhibition to survey across the cortex, we identified a cortical network and a high-order motor area (MOs-c), which coordinate action progression in a mouse reach-and-withdraw-to-drink (RWD) behavior. Electrophysiology and photo-inhibition across multiple projection neuron types within the MOs-c revealed differential contributions of pyramidal tract and corticothalamic (CTMOs) output channels to action progression and hand-mouth coordination. Notably, CTMOs display sustained firing throughout RWD sequence and selectively enhance RWD-relevant activity in postsynaptic thalamus neurons, which also contribute to action coordination. CTMOs receive converging monosynaptic inputs from forelimb and orofacial sensorimotor areas and are reciprocally connected to thalamic neurons, which project back to the cortical network. Therefore, motor cortex corticothalamic channel may selectively amplify the thalamic integration of cortical and subcortical sensorimotor streams to coordinate a skilled motor sequence.

Motor cortex is responsible for motoric dynamics in striatum and the execution of both skilled and unskilled actions

Striatum and its predominant input, motor cortex, are responsible for the selection and performance of purposive movement, but how their interaction guides these processes is not understood. To establish its neural and behavioral contributions, we bilaterally lesioned motor cortex and recorded striatal activity and reaching performance daily, capturing the lesion's direct ramifications within hours of the intervention. We observed reaching impairment and an absence of striatal motoric activity following lesion of motor cortex, but not parietal cortex control lesions. Although some aspects of performance began to recover after 8-10 days, striatal projection and interneuronal dynamics did not-eventually entering a non-motor encoding state that aligned with persisting kinematic control deficits. Lesioned mice also exhibited a profound inability to switch motor plans while locomoting, reminiscent of clinical freezing of gait (FOG). Our results demonstrate the necessity of motor cortex in generating trained and untrained actions as well as striatal motoric dynamics.

From Individual to Population: Circuit Organization of Pyramidal Tract and Intratelencephalic Neurons in Mouse Sensorimotor Cortex

The sensorimotor cortex participates in diverse functions with different reciprocally connected subregions and projection-defined pyramidal neuron types therein, while the fundamental organizational logic of its circuit elements at the single-cell level is still largely unclear. Here, using mouse Cre driver lines and high-resolution whole-brain imaging to selectively trace the axons and dendrites of cortical pyramidal tract (PT) and intratelencephalic (IT) neurons, we reconstructed the complete morphology of 1,023 pyramidal neurons and generated a projectome of 6 subregions within the sensorimotor cortex. Our morphological data revealed substantial hierarchical and layer differences in the axonal innervation patterns of pyramidal neurons. We found that neurons located in the medial motor cortex had more diverse projection patterns than those in the lateral motor and sensory cortices. The morphological characteristics of IT neurons in layer 5 were more complex than those in layer 2/3. Furthermore, the soma location and morphological characteristics of individual neurons exhibited topographic correspondence. Different subregions and layers were composed of different proportions of projection subtypes that innervate downstream areas differentially. While the axonal terminals of PT neuronal population in each cortical subregion were distributed in specific subdomains of the superior colliculus (SC) and zona incerta (ZI), single neurons selectively innervated a combination of these projection targets. Overall, our data provide a comprehensive list of projection types of pyramidal neurons in the sensorimotor cortex and begin to unveil the organizational principle of these projection types in different subregions and layers.

Differences in motor learning-related structural plasticity of layer 2/3 parvalbumin-positive interneurons of the young and aged motor cortex

Changes to neuronal connectivity are believed to be a key factor in cognitive impairments associated with normal aging. Because of its effect on activities of daily living, deficient motor control is a critical type of cognitive decline to understand. Diminished inhibitory networks in the cortex are implicated in such motor control deficits, pointing to the connectivity of inhibitory cortical interneurons as an important area for study. Here, we used chronic two-photon microscopy to track the structural plasticity of en passant boutons (EPBs) of parvalbumin-positive interneurons in the mouse motor cortex in the first longitudinal, in vivo study of inhibitory interneuron synapses in the context of aging. Young (3-5 months) and aged (23-28 months) mice underwent training on the accelerating rotarod to evoke motor learning-induced structural plasticity. Our analysis reveals that, in comparison with axons from young mice, those from aged mice have fewer EPBs at baseline that also tend to be larger in size. Aged axons also express learning-related structural plasticity-like new bouton stabilization and bouton enlargement-that is less persistent than that of young axons. This study reveals striking baseline differences in young and aged axon morphology as well as differences in the deployment of learning-related structural plasticity across axons.

Spontaneously regenerative corticospinal neurons in mice

The spinal cord receives inputs from the cortex via corticospinal neurons (CSNs). While predominantly a contralateral projection, a less-investigated minority of its axons terminate in the ipsilateral spinal cord. We analyzed the spatial and molecular properties of these ipsilateral axons and their post-synaptic targets in mice and found they project primarily to the ventral horn, including directly to motor neurons. Barcode-based reconstruction of the ipsilateral axons revealed a class of primarily bilaterally-projecting CSNs with a distinct cortical distribution. The molecular properties of these ipsilaterally-projecting CSNs (IP-CSNs) are strikingly similar to the previously described molecular signature of embryonic-like regenerating CSNs. Finally, we show that IP-CSNs are spontaneously regenerative after spinal cord injury. The discovery of a class of spontaneously regenerative CSNs may prove valuable to the study of spinal cord injury. Additionally, this work suggests that the retention of juvenile-like characteristics may be a widespread phenomenon in adult nervous systems.

Motor cortical inactivation impairs corrective submovements in mice performing a hold-still center-out reach task

Holding still and aiming reaches to spatial targets may depend on distinct neural circuits. Using automated homecage training and a sensitive joystick, we trained freely moving mice to contact a joystick, hold their forelimb still, and then reach to rewarded target locations. Mice learned the task by initiating forelimb sequences with clearly resolved submillimeter-scale micromovements followed by millimeter-scale reaches to learned spatial targets. Hundreds of thousands of trajectories were decomposed into millions of kinematic submovements, while photoinhibition was used to test roles of motor cortical areas. Inactivation of both caudal and rostral forelimb areas preserved the ability to produce aimed reaches, but reduced reach speed. Inactivation specifically of contralateral caudal forelimb area (CFA) additionally impaired the ability to aim corrective submovements to remembered locations following target undershoots. Our findings show that motor cortical inactivations reduce the gain of forelimb movements but that inactivation specifically of contralateral CFA impairs corrective movements important for reaching a target location.NEW & NOTEWORTHY To test the role of different cortical areas in holding still and reaching to targets, this study combined home-cage training with optogenetic silencing as mice engaged in a learned center-out-reach task. Inactivation specifically of contralateral caudal forelimb area (CFA) impaired corrective movements necessary to reach spatial targets to earn reward.

Symmetry in Frontal But Not Motor and Somatosensory Cortical Projections

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

Dissociable roles of thalamic nuclei in the refinement of reaches to spatial targets

Reaches are complex movements that are critical for survival, and encompass the control of different aspects such as direction, speed, and endpoint precision. Complex movements have been postulated to be learned and controlled through distributed motor networks, of which the thalamus is a highly connected node. Still, the role of different thalamic circuits in learning and controlling specific aspects of reaches has not been investigated. We report dissociable roles of two distinct thalamic nuclei - the parafascicular (Pf) and ventroanterior/ventrolateral (VAL) nuclei - in the refinement of spatial target reaches in mice. Using 2-photon calcium imaging in a head-fixed joystick task where mice learned to reach to a target in space, we found that glutamatergic neurons in both areas were most active during reaches early in learning. Reach-related activity in both areas decreased late in learning, as movement direction was refined and reaches increased in accuracy. Furthermore, the population dynamics of Pf, but not VAL, covaried in different subspaces in early and late learning, but eventually stabilized in late learning. The neural activity in Pf, but not VAL, encoded the direction of reaches in early but not late learning. Accordingly, bilateral lesions of Pf before, but not after learning, strongly and specifically impaired the refinement of reach direction. VAL lesions did not impact direction refinement, but instead resulted in increased speed and target overshoot. Our findings provide new evidence that the thalamus is a critical motor node in the learning and control of reaching movements, with specific subnuclei controlling distinct aspects of the reach early in learning.

Concurrent optogenetic motor mapping of multiple limbs in awake mice reveals cortical organization of coordinated movements

Background

Motor mapping allows for determining the macroscopic organization of motor circuits and corresponding motor movement representations on the cortex. Techniques such as intracortical microstimulation (ICMS) are robust, but can be time consuming and invasive, making them non-ideal for cortex-wide mapping or longitudinal studies. In contrast, optogenetic motor mapping offers a rapid and minimally invasive technique, enabling mapping with high spatiotemporal resolution. However, motor mapping has seen limited use in tracking 3-dimensonal, multi-limb movements in awake animals. This gap has left open questions regarding the underlying organizational principles of motor control of coordinated, ethologically relevant movements involving multiple limbs.

Objective

Our first objective was to develop Multi-limb Optogenetic Motor Mapping (MOMM) to concurrently map motor movement representations of multiple limbs with high fidelity in awake mice. Having established MOMM, our next objective was determine whether maps of coordinated and ethologically relevant motor output were topographically organized on the cortex.

Methods

We combine optogenetic stimulation with a deep learning driven pose-estimation toolbox, DeepLabCut (DLC), and 3-dimentional triangulation to concurrently map motor movements of multiple limbs in awake mice.

Results

MOMM consistently revealed cortical topographies for all mapped features within and across mice. Many motor maps overlapped and were topographically similar. Several motor movement representations extended beyond cytoarchitecturally defined somatomotor cortex. Finer articulations of the forepaw resided within gross motor movement representations of the forelimb. Moreover, many cortical sites exhibited concurrent limb coactivation when photostimulated, prompting the identification of several cortical regions harboring coordinated and ethologically relevant movements.

Conclusions

The cortex appears to be topographically organized by motor programs, which are responsible for coordinated, multi-limbed, and behavioral-like movements.

Decoding Continuous Forelimb Kinematics in Mice Using Single-Photon Calcium Imaging

In the natural environment, most spontaneous behaviors involve compound movements consisting of tightly coupled sequences of sub-movements. Motor commands are issued from motor cortex to downstream areas and effectors, suggesting that the neural representations in the primary motor cortex for different sub-movements should be separable. Previous research has limited insights into how neural system regulates the interplay between the sequential nature and separability of such representations. In this study, we categorized forelimb behaviors during the mouse water-reaching task, employing single-photon calcium signals to classify forelimb postures. We discovered distinct neural patterns associated with different actions during the mouse water-reaching task. The frame-by-frame prediction of water-reaching trajectories revealed the overall continuity of neural changes during the grasping action. By utilizing different time windows for neural decoding of forepaws states, we speculated on the potential temporal overlap of neural patterns during continuous movements. This overlap may underlie the rapid and smooth transitions between sub-movements.

Representation of rhythmic chunking in striatum of mice executing complex continuous movement sequences

We used a step-wheel system to examine the activity of striatal projection neurons as mice practiced stepping on complexly arranged foothold pegs in this Ferris-wheel-like device to receive reward. Sets of dorsolateral striatal projection neurons were sensitive to specific parameters of repetitive motor coordination during the runs. They responded to combinations of the parameters of continuous movements (interval, phase, and repetition), forming "chunking responses"-some for combinations of these parameters across multiple body parts. Recordings in sensorimotor cortical areas exhibited notably fewer such responses but were documented for smaller neuron sets whose heterogeneity was significant. Striatal movement encoding via chunking responsivity could provide insight into neural strategies governing effective motor control by the striatum. It is possible that the striking need for external rhythmic cuing to allow movement sequences by Parkinson's patients could, at least in part, reflect dysfunction in such striatal coding.

Activity-Dependent Remodeling of Corticostriatal Axonal Boutons During Motor Learning

Motor skill learning induces long-lasting synaptic plasticity at not only the inputs, such as dendritic spines1-4, but also at the outputs to the striatum of motor cortical neurons5,6. However, very little is known about the activity and structural plasticity of corticostriatal axons during learning in the adult brain. Here, we used longitudinal in vivo two-photon imaging to monitor the activity and structure of thousands of corticostriatal axonal boutons in the dorsolateral striatum in awake mice. We found that learning a new motor skill induces dynamic regulation of axonal boutons. The activities of motor corticostriatal axonal boutons exhibited selectivity for rewarded movements (RM) and un-rewarded movements (UM). Strikingly, boutons on the same axonal branches showed diverse responses during behavior. Motor learning significantly increased the fraction of RM boutons and reduced the heterogeneity of bouton activities. Moreover, motor learning-induced profound structural dynamism in boutons. By combining structural and functional imaging, we identified that newly formed axonal boutons are more likely to exhibit selectivity for RM and are stabilized during motor learning, while UM boutons are selectively eliminated. Our results highlight a novel form of plasticity at corticostriatal axons induced by motor learning, indicating that motor corticostriatal axonal boutons undergo dynamic reorganization that facilitates the acquisition and execution of motor skills.

Symmetry in frontal but not motor and somatosensory cortical projections

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

Topographical and cell type-specific connectivity of rostral and caudal forelimb corticospinal neuron populations

Corticospinal neurons (CSNs) synapse directly on spinal neurons, a diverse assortment of cells with unique structural and functional properties necessary for body movements. CSNs modulating forelimb behavior fractionate into caudal forelimb area (CFA) and rostral forelimb area (RFA) motor cortical populations. Despite their prominence, the full diversity of spinal neurons targeted by CFA and RFA CSNs is uncharted. Here, we use anatomical and RNA sequencing methods to show that CSNs synapse onto a remarkably selective group of spinal cell types, favoring inhibitory populations that regulate motoneuron activity and gate sensory feedback. CFA and RFA CSNs target similar spinal neuron types, with notable exceptions that suggest that these populations differ in how they influence behavior. Finally, axon collaterals of CFA and RFA CSNs target similar brain regions yet receive highly divergent inputs. These results detail the rules of CSN connectivity throughout the brain and spinal cord for two regions critical for forelimb behavior.

Characterization of Ultrasonic Vocalization-Modulated Neurons in Rat Motor Cortex Based on Their Activity Modulation and Axonal Projection to the Periaqueductal Gray

Vocalization, a means of social communication, is prevalent among many species, including humans. Both rats and mice use ultrasonic vocalizations (USVs) in various social contexts and affective states. The motor cortex is hypothesized to be involved in precisely controlling USVs through connections with critical regions of the brain for vocalization, such as the periaqueductal gray matter (PAG). However, it is unclear how neurons in the motor cortex are modulated during USVs. Moreover, the relationship between USV modulation of neurons and anatomical connections from the motor cortex to PAG is also not clearly understood. In this study, we first characterized the activity patterns of neurons in the primary and secondary motor cortices during emission of USVs in rats using large-scale electrophysiological recordings. We also examined the axonal projection of the motor cortex to PAG using retrograde labeling and identified two clusters of PAG-projecting neurons in the anterior and posterior parts of the motor cortex. The neural activity patterns around the emission of USVs differed between the anterior and posterior regions, which were divided based on the distribution of PAG-projecting neurons in the motor cortex. Furthermore, using optogenetic tagging, we recorded the USV modulation of PAG-projecting neurons in the posterior part of the motor cortex and found that they showed predominantly sustained excitatory responses during USVs. These results contribute to our understanding of the involvement of the motor cortex in the generation of USV at the neuronal and circuit levels.

Visualizing Wallerian degeneration in the corticospinal tract after sensorimotor cortex ischemia in mice

Stroke can cause Wallerian degeneration in regions outside of the brain, particularly in the corticospinal tract. To investigate the fate of major glial cells and axons within affected areas of the corticospinal tract following stroke, we induced photochemical infarction of the sensorimotor cortex leading to Wallerian degeneration along the full extent of the corticospinal tract. We first used a routine, sensitive marker of axonal injury, amyloid precursor protein, to examine Wallerian degeneration of the corticospinal tract. An antibody to amyloid precursor protein mapped exclusively to proximal axonal segments within the ischemic cortex, with no positive signal in distal parts of the corticospinal tract, at all time points. To improve visualization of Wallerian degeneration, we next utilized an orthograde virus that expresses green fluorescent protein to label the corticospinal tract and then quantitatively evaluated green fluorescent protein-expressing axons. Using this approach, we found that axonal degeneration began on day 3 post-stroke and was almost complete by 7 days after stroke. In addition, microglia mobilized and activated early, from day 7 after stroke, but did not maintain a phagocytic state over time. Meanwhile, astrocytes showed relatively delayed mobilization and a moderate response to Wallerian degeneration. Moreover, no anterograde degeneration of spinal anterior horn cells was observed in response to Wallerian degeneration of the corticospinal tract. In conclusion, our data provide evidence for dynamic, pathogenic spatiotemporal changes in major cellular components of the corticospinal tract during Wallerian degeneration.

Alternate Day Fasting Leads to Improved Post-Stroke Motor Recovery in Mice

BACKGROUND

Caloric restriction promotes neuroplasticity and recovery after neurological injury. In mice, we tested the hypothesis that caloric restriction can act post-stroke to enhance training-associated motor recovery.

METHODS

Mice were trained to perform a skilled prehension task. We then induced a photothrombotic stroke in the caudal forelimb area, after which we retrained animals on the prehension task following an 8-day delay. Mice underwent either ad libitum feeding or alternate day fasting beginning 1-day after stroke and persisting for either 7 days or the entire post-stroke training period until sacrifice.

RESULTS

Prior studies have shown that post-stroke recovery of prehension can occur if animals receive rehabilitative training during an early sensitive period but is incomplete if rehabilitative training is delayed. In contrast, we show complete recovery of prehension, despite a delay in rehabilitative training, when mice underwent alternate day fasting beginning 1-day post-stroke and persisting for either 7 days or the entire post-stroke training period until sacrifice. Recovery was independent of weight loss. Stroke volumes were similar across groups.

CONCLUSIONS

Post-stroke caloric restriction led to recovery of motor function independent of a protective effect on stroke volume. Prehension recovery improved even after ad libitum feeding was reinstituted suggesting that the observed motor recovery was not merely a motivational response. These data add to the growing evidence that post-stroke caloric restriction can enhance recovery.

Corticocortical connections of the rostral forelimb area in rats: a quantitative tract-tracing study

The rostral forelimb area (RFA) in the rat is a premotor cortical region based on its dense efferent projections to primary motor cortex. This study describes corticocortical connections of RFA and the relative strength of connections with other cortical areas. The goal was to provide a better understanding of the cortical network in which RFA participates, and thus, determine its function in sensorimotor behavior. The RFA of adult male Long-Evans rats (n = 6) was identified using intracortical microstimulation techniques and injected with the tract-tracer, biotinylated dextran amine (BDA). In post-mortem tissue, locations of BDA-labeled terminal boutons and neuronal somata were plotted and superimposed on cortical field boundaries. Quantitative estimates of terminal boutons in each region of interest were based on unbiased stereological methods. The results demonstrate that RFA has dense connections with primary motor cortex and frontal cortex medial and lateral to RFA. Moderate connections were found with insular cortex, primary somatosensory cortex (S1), the M1/S1 overlap zone, and lateral somatosensory areas. Cortical connections of RFA in rat are strikingly similar to cortical connections of the ventral premotor cortex in non-human primates, suggesting that these areas share similar functions and allow greater translation of rodent premotor cortex studies to primates.

Micropopulation mapping of the mouse parafascicular nucleus connections reveals diverse input-output motifs

Introduction

In primates, including humans, the centromedian/parafascicular (CM-Pf) complex is a key thalamic node of the basal ganglia system. Deep brain stimulation in CM-Pf has been applied for the treatment of motor disorders such as Parkinson's disease or Tourette syndrome. Rodents have become widely used models for the study of the cellular and genetic mechanisms of these and other motor disorders. However, the equivalence between the primate CM-Pf and the nucleus regarded as analogous in rodents (Parafascicular, Pf) remains unclear.

Methods

Here, we analyzed the neurochemical architecture and carried out a brain-wide mapping of the input-output motifs in the mouse Pf at micropopulation level using anterograde and retrograde labeling methods. Specifically, we mapped and quantified the sources of cortical and subcortical input to different Pf subregions, and mapped and compared the distribution and terminal structure of their axons.

Results

We found that projections to Pf arise predominantly (>75%) from the cerebral cortex, with an unusually strong (>45%) Layer 5b component, which is, in part, contralateral. The intermediate layers of the superior colliculus are the main subcortical input source to Pf. On its output side, Pf neuron axons predominantly innervate the striatum. In a sparser fashion, they innervate other basal ganglia nuclei, including the subthalamic nucleus (STN), and the cerebral cortex. Differences are evident between the lateral and medial portions of Pf, both in chemoarchitecture and in connectivity. Lateral Pf axons innervate territories of the striatum, STN and cortex involved in the sensorimotor control of different parts of the contralateral hemibody. In contrast, the mediodorsal portion of Pf innervates oculomotor-limbic territories in the above three structures.

Discussion

Our data thus indicate that the mouse Pf consists of several neurochemically and connectively distinct domains whose global organization bears a marked similarity to that described in the primate CM-Pf complex.

Topographical and cell type-specific connectivity of rostral and caudal forelimb corticospinal neuron populations

Corticospinal neurons (CSNs) synapse directly on spinal neurons, a diverse group of neurons with unique structural and functional properties necessary for body movements. CSNs modulating forelimb behavior fractionate into caudal forelimb area (CFA) and rostral forelimb area (RFA) motor cortical populations. Despite their prominence, no studies have mapped the diversity of spinal cell types targeted by CSNs, let alone compare CFA and RFA populations. Here we use anatomical and RNA-sequencing methods to show that CSNs synapse onto a remarkably selective group of spinal cell types, favoring inhibitory populations that regulate motoneuron activity and gate sensory feedback. CFA and RFA CSNs target similar spinal cell types, with notable exceptions that suggest these populations differ in how they influence behavior. Finally, axon collaterals of CFA and RFA CSNs target similar brain regions yet receive surprisingly divergent inputs. These results detail the rules of CSN connectivity throughout the brain and spinal cord for two regions critical for forelimb behavior.

Large-scale recording of neuronal activity in freely-moving mice at cellular resolution

Current methods for recording large-scale neuronal activity from behaving mice at single-cell resolution require either fixing the mouse head under a microscope or attachment of a recording device to the animal's skull. Both of these options significantly affect the animal behavior and hence also the recorded brain activity patterns. Here, we introduce a different method to acquire snapshots of single-cell cortical activity maps from freely-moving mice using a calcium sensor called CaMPARI. CaMPARI has a unique property of irreversibly changing its color from green to red inside active neurons when illuminated with 400 nm light. We capitalize on this property to demonstrate cortex-wide activity recording without any head fixation, tethering, or attachment of a miniaturized device to the mouse's head. Multiple cortical regions were recorded while the mouse was performing a battery of behavioral and cognitive tests. We identified task-dependent activity patterns across motor and somatosensory cortices, with significant differences across sub-regions of the motor cortex and correlations across several activity patterns and task parameters. This CaMPARI-based recording method expands the capabilities of recording neuronal activity from freely-moving and behaving mice under minimally-restrictive experimental conditions and provides large-scale volumetric data that are currently not accessible otherwise.

Subventricular zone cytogenesis provides trophic support for neural repair in a mouse model of stroke

Stroke enhances proliferation of neural precursor cells within the subventricular zone (SVZ) and induces ectopic migration of newborn cells towards the site of injury. Here, we characterize the identity of cells arising from the SVZ after stroke and uncover a mechanism through which they facilitate neural repair and functional recovery. With genetic lineage tracing, we show that SVZ-derived cells that migrate towards cortical photothrombotic stroke in mice are predominantly undifferentiated precursors. We find that ablation of neural precursor cells or conditional knockout of VEGF impairs neuronal and vascular reparative responses and worsens recovery. Replacement of VEGF is sufficient to induce neural repair and recovery. We also provide evidence that CXCL12 from peri-infarct vasculature signals to CXCR4-expressing cells arising from the SVZ to direct their ectopic migration. These results support a model in which vasculature surrounding the site of injury attracts cells from the SVZ, and these cells subsequently provide trophic support that drives neural repair and recovery.

Enhancing Prediction of Forelimb Movement Trajectory through a Calibrating-Feedback Paradigm Incorporating RAT Primary Motor and Agranular Cortical Ensemble Activity in the Goal-Directed Reaching Task

Complete reaching movements involve target sensing, motor planning, and arm movement execution, and this process requires the integration and communication of various brain regions. Previously, reaching movements have been decoded successfully from the motor cortex (M1) and applied to prosthetic control. However, most studies attempted to decode neural activities from a single brain region, resulting in reduced decoding accuracy during visually guided reaching motions. To enhance the decoding accuracy of visually guided forelimb reaching movements, we propose a parallel computing neural network using both M1 and medial agranular cortex (AGm) neural activities of rats to predict forelimb-reaching movements. The proposed network decodes M1 neural activities into the primary components of the forelimb movement and decodes AGm neural activities into internal feedforward information to calibrate the forelimb movement in a goal-reaching movement. We demonstrate that using AGm neural activity to calibrate M1 predicted forelimb movement can improve decoding performance significantly compared to neural decoders without calibration. We also show that the M1 and AGm neural activities contribute to controlling forelimb movement during goal-reaching movements, and we report an increase in the power of the local field potential (LFP) in beta and gamma bands over AGm in response to a change in the target distance, which may involve sensorimotor transformation and communication between the visual cortex and AGm when preparing for an upcoming reaching movement. The proposed parallel computing neural network with the internal feedback model improves prediction accuracy for goal-reaching movements.

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.

Altered Thalamocortical Signaling in a Mouse Model of Parkinson's Disease

Activation of the primary motor cortex (M1) is important for the execution of skilled movements and motor learning, and its dysfunction contributes to the pathophysiology of Parkinson's disease (PD). A well-accepted idea in PD research, albeit not tested experimentally, is that the loss of midbrain dopamine leads to decreased activation of M1 by the motor thalamus. Here, we report that midbrain dopamine loss altered motor thalamus input in a laminar- and cell type-specific fashion and induced laminar-specific changes in intracortical synaptic transmission. Frequency-dependent changes in synaptic dynamics were also observed. Our results demonstrate that loss of midbrain dopaminergic neurons alters thalamocortical activation of M1 in both male and female mice, and provide novel insights into circuit mechanisms for motor cortex dysfunction in a mouse model of PD.SIGNIFICANCE STATEMENT Loss of midbrain dopamine neurons increases inhibition from the basal ganglia to the motor thalamus, suggesting that it may ultimately lead to reduced activation of primary motor cortex (M1). In contrast with this line of thinking, analysis of M1 activity in patients and animal models of Parkinson's disease report hyperactivation of this region. Our results are the first report that midbrain dopamine loss alters the input-output function of M1 through laminar and cell type specific effects. These findings support and expand on the idea that loss of midbrain dopamine reduces motor cortex activation and provide experimental evidence that reconciles reduced thalamocortical input with reports of altered activation of motor cortex in patients with Parkinson's disease.

Anti-Hebbian plasticity in the motor cortex promotes defensive freezing

Regional brain activity often decreases from baseline levels in response to external events, but how neurons develop such negative responses is unclear. To study this, we leveraged the negative response that develops in the primary motor cortex (M1) after classical fear learning. We trained mice with a fear conditioning paradigm while imaging their brains with standard two-photon microscopy. This enabled monitoring changes in neuronal responses to the tone with synaptic resolution through learning. We found that M1 layer 5 pyramidal neurons (L5 PNs) developed negative tone responses within an hour after conditioning, which depended on the weakening of their dendritic spines that were active during training. Blocking this form of anti-Hebbian plasticity using an optogenetic manipulation of CaMKII activity disrupted negative tone responses and freezing. Therefore, reducing the strength of spines active at the time of memory encoding leads to negative responses of L5 PNs. In turn, these negative responses curb M1's capacity for promoting movement, thereby aiding freezing. Collectively, this work provides a mechanistic understanding of how area-specific negative responses to behaviorally relevant cues can be achieved.

Extensive complex neocortical movement topography devolves to simple output following experimental stroke in mice

The neocortex encodes complex and simple motor outputs in all mammalian species that have been tested. Given that changes in neocortical reorganization (and corresponding corticospinal output) have been implicated in long term motor recovery after stroke injury, there remains a need to understand this biology in order to expedite and optimize clinical care. Here, changes in the neocortical topography of complex and simple movement outputs were evaluated in mice following experimental middle cerebral artery occlusion (MCAo). Neocortical motor output was defined using long-duration parameters of intracortical microstimulation (LD-ICMS) based on area and spatial coordinates of separate motor output types to build upon our recent report in uninjured mice. LD-ICMS test sites that elicited complex (multi-joint) movement, simple (single skeletal joint) movement, as well as co-elicited FORELIMB + HINDLIMB responses were detected and recorded. Forelimb reaching behavior was assessed using the single pellet reaching (SPR) task. At 6 weeks post-surgery, behavioral deficits persisted and neocortical territories for separate movements exhibited differences in neocortical area, and spatial location, and differed between MCAo-Injured animals (i.e., the MCAo group) and Sham-Injured animals (i.e., the Control group). MCAo-Injury reduced neocortical area of complex movements while increasing area of simple movements. Limited effects of injury were detected for spatial coordinates of neocortical movements. Significant positive correlations were detected between final SPR performance and either area of complex retract or area of co-occurring FORELIMB + HINDLIMB sites.

Hindlimb muscle representations in mouse motor cortex defined by viral tracing

Introduction

Descending pathways from the cortex to the spinal cord are involved in the control of natural movement. Although mice are widely used to study the neurobiology of movement and as models of neurodegenerative disease, an understanding of motor cortical organization is lacking, particularly for hindlimb muscles.

Methods

In this study, we used the retrograde transneuronal transport of rabies virus to compare the organization of descending cortical projections to fast- and slow-twitch hindlimb muscles surrounding the ankle joint in mice.

Results

Although the initial stage of virus transport from the soleus muscle (predominantly slow-twitch) appeared to be more rapid than that associated with the tibialis anterior muscle (predominantly fast-twitch), the rate of further transport of virus to cortical projection neurons in layer V was equivalent for the two injected muscles. After appropriate survival times, dense concentrations of layer V projection neurons were identified in three cortical areas: the primary motor cortex (M1), secondary motor cortex (M2), and primary somatosensory cortex (S1).

Discussion

The origin of the cortical projections to each of the two injected muscles overlapped almost entirely within these cortical areas. This organization suggests that cortical projection neurons maintain a high degree of specificity; that is, even when cortical projection neurons are closely located, each neuron could have a distinct functional role (controlling fast- versus slow-twitch and/or extensor versus flexor muscles). Our results represent an important addition to the understanding of the mouse motor system and lay the foundation for future studies investigating the mechanisms underlying motor system dysfunction and degeneration in diseases such as amyotrophic lateral sclerosis and spinal muscular atrophy.

Whole-brain mapping of long-range inputs to the VIP-expressing inhibitory neurons in the primary motor cortex

The primary motor cortex (MOp) is an important site for motor skill learning. Interestingly, neurons in MOp possess reward-related activity, presumably to facilitate reward-based motor learning. While pyramidal neurons (PNs) and different subtypes of GABAergic inhibitory interneurons (INs) in MOp all undergo cell-type specific plastic changes during motor learning, the vasoactive intestinal peptide-expressing inhibitory interneurons (VIP-INs) in MOp have been shown to preferentially respond to reward and play a critical role in the early phases of motor learning by triggering local circuit plasticity. To understand how VIP-INs might integrate various streams of information, such as sensory, pre-motor, and reward-related inputs, to regulate local plasticity in MOp, we performed monosynaptic rabies tracing experiments and employed an automated cell counting pipeline to generate a comprehensive map of brain-wide inputs to VIP-INs in MOp. We then compared this input profile to the brain-wide inputs to somatostatin-expressing inhibitory interneurons (SST-INs) and parvalbumin-expressing inhibitory interneurons (PV-INs) in MOp. We found that while all cell types received major inputs from sensory, motor, and prefrontal cortical regions, as well as from various thalamic nuclei, VIP-INs received more inputs from the orbital frontal cortex (ORB) - a region associated with reinforcement learning and value predictions. Our findings provide insight on how the brain leverages microcircuit motifs by both integrating and partitioning different streams of long-range input to modulate local circuit activity and plasticity.

Focused Epicranial Brain Stimulation by Spatial Sculpting of Pulsed Electric Fields Using High Density Electrode Arrays

Transcranial electrical neuromodulation of the central nervous system is used as a non-invasive method to induce neural and behavioral responses, yet targeted non-invasive electrical stimulation of the brain with high spatial resolution remains elusive. This work demonstrates a focused, steerable, high-density epicranial current stimulation (HD-ECS) approach to evoke neural activity. Custom-designed high-density (HD) flexible surface electrode arrays are employed to apply high-resolution pulsed electric currents through skull to achieve localized stimulation of the intact mouse brain. The stimulation pattern is steered in real time without physical movement of the electrodes. Steerability and focality are validated at the behavioral, physiological, and cellular levels using motor evoked potentials (MEPs), intracortical recording, and c-fos immunostaining. Whisker movement is also demonstrated to further corroborate the selectivity and steerability. Safety characterization confirmed no significant tissue damage following repetitive stimulation. This method can be used to design novel therapeutics and implement next-generation brain interfaces.

Differential Effects of Astrocyte Manipulations on Learned Motor Behavior and Neuronal Ensembles in the Motor Cortex

Although motor cortex is crucial for learning precise and reliable movements, whether and how astrocytes contribute to its plasticity and function during motor learning is unknown. Here, we report that astrocyte-specific manipulations in primary motor cortex (M1) during a lever push task alter motor learning and execution, as well as the underlying neuronal population coding. Mice that express decreased levels of the astrocyte glutamate transporter 1 (GLT1) show impaired and variable movement trajectories, whereas mice with increased astrocyte Gq signaling show decreased performance rates, delayed response times, and impaired trajectories. In both groups, which include male and female mice, M1 neurons have altered interneuronal correlations and impaired population representations of task parameters, including response time and movement trajectories. RNA sequencing further supports a role for M1 astrocytes in motor learning and shows changes in astrocytic expression of glutamate transporter genes, GABA transporter genes, and extracellular matrix protein genes in mice that have acquired this learned behavior. Thus, astrocytes coordinate M1 neuronal activity during motor learning, and our results suggest that this contributes to learned movement execution and dexterity through mechanisms that include regulation of neurotransmitter transport and calcium signaling.SIGNIFICANCE STATEMENT We demonstrate for the first time that in the M1 of mice, astrocyte function is critical for coordinating neuronal population activity during motor learning. We demonstrate that knockdown of astrocyte glutamate transporter GLT1 affects specific components of learning, such as smooth trajectory formation. Altering astrocyte calcium signaling by activation of Gq-DREADD upregulates GLT1 and affects other components of learning, such as response rates and reaction times as well as trajectory smoothness. In both manipulations, neuronal activity in motor cortex is dysregulated, but in different ways. Thus, astrocytes have a crucial role in motor learning via their influence on motor cortex neurons, and they do so by mechanisms that include regulation of glutamate transport and calcium signals.

Peripersonal encoding of forelimb proprioception in the mouse somatosensory cortex

Conscious perception of limb movements depends on proprioceptive neural responses in the somatosensory cortex. In contrast to tactile sensations, proprioceptive cortical coding is barely studied in the mammalian brain and practically non-existent in rodent research. To understand the cortical representation of this important sensory modality we developed a passive forelimb displacement paradigm in behaving mice and also trained them to perceptually discriminate where their limb is moved in space. We delineated the rodent proprioceptive cortex with wide-field calcium imaging and optogenetic silencing experiments during behavior. Our results reveal that proprioception is represented in both sensory and motor cortical areas. In addition, behavioral measurements and responses of layer 2/3 neurons imaged with two-photon microscopy reveal that passive limb movements are both perceived and encoded in the mouse cortex as a spatial direction vector that interfaces the limb with the body's peripersonal space.

Wide-field calcium imaging of cortical activation and functional connectivity in externally- and internally-driven locomotion

The neural dynamics underlying self-initiated versus sensory driven movements is central to understanding volitional action. Upstream motor cortices are associated with the generation of internally-driven movements over externally-driven. Here we directly compare cortical dynamics during internally- versus externally-driven locomotion using wide-field Ca2+ imaging. We find that secondary motor cortex (M2) plays a larger role in internally-driven spontaneous locomotion transitions, with increased M2 functional connectivity during starting and stopping than in the externally-driven, motorized treadmill locomotion. This is not the case in steady-state walk. In addition, motorized treadmill and spontaneous locomotion are characterized by markedly different patterns of cortical activation and functional connectivity at the different behavior periods. Furthermore, the patterns of fluorescence activation and connectivity are uncorrelated. These experiments reveal widespread and striking differences in the cortical control of internally- and externally-driven locomotion, with M2 playing a major role in the preparation and execution of the self-initiated state.

Inter-axonal molecular crosstalk via Lumican proteoglycan sculpts murine cervical corticospinal innervation by distinct subpopulations

How CNS circuits sculpt their axonal arbors into spatially and functionally organized domains is not well understood. Segmental specificity of corticospinal connectivity is an exemplar for such regional specificity of many axon projections. Corticospinal neurons (CSN) innervate spinal and brainstem targets with segmental precision, controlling voluntary movement. Multiple molecularly distinct CSN subpopulations innervate the cervical cord for evolutionarily enhanced precision of forelimb movement. Evolutionarily newer CSN_BC-lat exclusively innervate bulbar-cervical targets, while CSN_medial are heterogeneous; distinct subpopulations extend axons to either bulbar-cervical or thoraco-lumbar segments. We identify that Lumican controls balance of cervical innervation between CSN_BC-lat and CSN_medial axons during development, which is maintained into maturity. Lumican, an extracellular proteoglycan expressed by CSN_BC-lat, non-cell-autonomously suppresses cervical collateralization by multiple CSN_medial subpopulations. This inter-axonal molecular crosstalk between CSN subpopulations controls murine corticospinal circuitry refinement and forelimb dexterity. Such crosstalk is generalizable beyond the corticospinal system for evolutionary incorporation of new neuron populations into preexisting circuitry.