The growing understanding of the nature of brain plasticity raises optimism that this knowledge can be capitalized upon to improve rehabilitation efforts and to optimize functional outcome.

Novel motor skills are learned through repetitive practice and, once acquired, persist long after training stops 1,2 . Earlier studies have shown that such learning induces an increase in the efficacy of synapses in the primary motor cortex, the persistence of which is associated with retention of the task [3][4][5] . However, how motor learning affects neuronal circuitry at the level of individual synapses and how long-lasting memory is structurally encoded in the intact brain remain unknown. Here we show that synaptic connections in the living mouse brain rapidly respond to motor-skill learning and permanently rewire. Training in a forelimb reaching task leads to rapid (within an hour) formation of postsynaptic dendritic spines on the output pyramidal neurons in the contralateral motor cortex. Although selective elimination of spines that existed before training gradually returns the overall spine density back to the original level, the new spines induced during learning are preferentially stabilized during subsequent training and endure long after training stops. Furthermore, we show that different motor skills are encoded by different sets of synapses. Practice of novel, but not previously learned, tasks further promotes dendritic spine formation in adulthood. Our findings reveal that rapid, but long-lasting, synaptic reorganization is closely associated with motor learning. The data also suggest that stabilized neuronal connections are the foundation of durable motor memory.Fine motor movements require accurate muscle synergies that rely on coordinated recruitment of intracortical synapses onto corticospinal neurons 6,7 . Obtaining new motor skills has been shown to strengthen the horizontal cortical connections in the primary motor cortex 4,5 . In this study, we taught mice a single-seed reaching task (Supplementary Movie 1). The majority of 1-month-old mice that underwent training gradually increased their reaching success rates during the initial 4 days, and then levelled off (n = 42, Fig. 1a, b). There were a few mice (n = 5) that engaged in extensive reaching, but continually failed to grasp the seeds. These mice normally gave up reaching after 4-8 days (Fig. 1b) investigate the process of learning-induced synaptic remodelling in the intact motor cortex, we repeatedly imaged the same apical dendrites of layer V pyramidal neurons marked by the transgenic expression of yellow fluorescent protein (YFP-H line) in various cortical regions during and after motor learning, using transcranial two-photon microscopy 8 ( Supplementary Fig. 1). Dendritic spines are the postsynaptic sites of most excitatory synapses in the brain and changes in spine morphology and dynamism serve as good indicators of synaptic plasticity 9,10 . Spines that were formed and eliminated were identified by comparing images from two time points, and then normalized to the initial images. Imaged regions were guided by stereotaxic measurements, ensuring the imaged neurons resided in the primary motor cortex. In several experiments,...

The organization of forelimb representation areas of the monkey, cat, and rat motor cortices has been studied in depth, but its characterization in the mouse lags far behind. We used intracortical microstimulation (ICMS) and cytoarchitectonics to characterize the general organization of the C57BL/6 mouse motor cortex, and the forelimb representation in more detail. We found that the forelimb region spans a large area of frontal cortex, bordered primarily by vibrissa, neck, shoulder, and hindlimb representations. It included a large caudal forelimb area, dominated by digit representation, and a small rostral forelimb area, containing elbow and wrist representations. When the entire motor cortex was mapped, the forelimb was found to be the largest movement representation, followed by head and hindlimb representations. The ICMS-defined motor cortex spanned cytoarchitecturally identified lateral agranular cortex (AGl) and also extended into medial agranular cortex. Forelimb and hindlimb representations extended into granular cortex in a region that also had cytoarchitectural characteristics of AGl, consistent with the primary motor-somatosensory overlap zone (OL) characterized in rats. Thus, the mouse motor cortex has homologies with the rat in having 2 forelimb representations and an OL but is distinct in the predominance of digit representations.

Synaptic vesicle protein 2 (SV2) is a membrane glycoprotein common to all synaptic and endocrine vesicles. Unlike many proteins involved in synaptic exocytosis, SV2 has no homolog in yeast, indicating that it performs a function unique to secretion in higher eukaryotes. Although the structure and protein interactions of SV2 suggest multiple possible functions, its role in synaptic events remains unknown. To explore the function of SV2 in an in vivo context, we generated mice that do not express the primary SV2 isoform, SV2A, by using targeted gene disruption. Animals homozygous for the SV2A gene disruption appear normal at birth. However, they fail to grow, experience severe seizures, and die within 3 weeks, suggesting multiple neural and endocrine deficits. Electrophysiological studies of spontaneous inhibitory neurotransmission in the CA3 region of the hippocampus revealed that loss of SV2A leads to a reduction in action potential-dependent ␥-aminobutyric acid (GABA)ergic neurotransmission. In contrast, action potential-independent neurotransmission was normal. Analyses of synapse ultrastructure suggest that altered neurotransmission is not caused by changes in synapse density or morphology. These findings demonstrate that SV2A is an essential protein and implicate it in the control of exocytosis.

Unilateral damage to the forelimb representation area of the sensorimotor cortex in adult rats increases dendritic arborization of layer V pyramidal neurons of the contralateral homotopic cortex. Arbor size was maximum at approximately 18 d postlesion, following which there was a partial elimination, or pruning, of dendritic processes. These neural changes were closely associated with behavioral events. The overgrowth of dendrites was related in time to disuse of the contralateral (to the lesion) forelimb and over-reliance on the ipsilateral forelimb for postural and exploratory movements. The pruning of dendrites was related to a return to more symmetrical use of the forelimbs. To investigate the possibility that lesion-induced asymmetries in motor behavior contributed to dendritic arborization changes, movements of the forelimb ipsilateral to the lesion were restricted during the period of dendritic overgrowth through the use of one-holed vests. This interfered with the increase in dendritic arborization. In contrast, animals that were allowed to use both forelimbs, or only the forelimb ipsilateral to the lesion, showed the expected increases. When sham-operated rats were forced to use only one forelimb, no significant increases in arborization were found. Therefore, neither a lesion nor asymmetrical limb use alone could account for the dendritic overgrowth--it depended on a lesion-behavior interaction. Furthermore, greater sensorimotor impairments were found when the dendritic growth was blocked, suggesting that the neural growth and/or associated limb-use behavior were related to functional recovery from the cortical damage. Finally, in a second experiment, immobilization of the impaired limb during the pruning period did not prevent the elimination of processes. Thus, the pruning of neural processes was not related simply to the recovery of more symmetrical forelimb use. There may be a period early after brain damage during which marked neural structural changes can occur in the presence of adequate behavioral demand.

Stroke instigates a dynamic process of repair and remodelling of remaining neural circuits, and this process is shaped by behavioural experiences. The onset of motor disability simultaneously creates a powerful incentive to develop new, compensatory ways of performing daily activities. Compensatory movement strategies that are developed in response to motor impairments can be a dominant force in shaping post-stroke neural remodelling responses and can have mixed effects on functional outcome. The possibility of selectively harnessing the effects of compensatory behaviour on neural reorganization is still an insufficiently explored route for optimizing functional outcome after stroke.

To assess behavioral experience effects on synaptic plasticity after brain damage, the present study examined the effects of complex motor skills training (the acrobatic task) on synaptic changes in layer V of the motor cortex opposite unilateral damage to the forelimb sensorimotor cortex (FLsmc). Adult male rats were given lesions or sham operations followed by 28 d of training on the acrobatic task [acrobat condition (AC)]. As a motor activity control [motor control (MC)], lesion and sham animals were given simple repetitive exercise. Previously, FLsmc lesions and acrobatic training have independently been found to result in increases in synapse to neuron ratios in the intact motor cortex relative to controls, and both of these effects were replicated in the present study. In addition, acrobat training after lesions significantly increased layer V synapses per neuron relative to sham-AC and lesion-MC rats. Thus, the combination of acrobatic training and lesions resulted in an enhanced synaptogenic response. Synapse subtypes were also differentially affected by the conditions. Lesion-MC and sham-AC primarily had increases in the number of synapses per neuron formed by multiple synaptic boutons in comparison to sham-MC. In contrast, lesion-AC had increases in both multiple and single synapses. Multiple synaptic spines and perforated synapses were also differentially affected by training versus lesions. On tests of coordinated forelimb use, lesion-AC rats performed better than lesion-MC rats. In addition to supporting a link between behavioral experience and structural plasticity after brain damage, these findings suggest that adaptive neural plasticity may be enhanced using behavioral manipulations as "therapy."

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