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 precise annotation and accurate identification of neural structures are prerequisites for studying mammalian brain function. The orientation of neurons and neural circuits is usually determined by mapping brain images to coarse axial-sampling planar reference atlases. However, individual differences at the cellular level likely lead to position errors and an inability to orient neural projections at single-cell resolution. Here, we present a high-throughput precision imaging method that can acquire a co-localized brain-wide data set of both fluorescent-labelled neurons and counterstained cell bodies at a voxel size of 0.32 × 0.32 × 2.0 μm in 3 days for a single mouse brain. We acquire mouse whole-brain imaging data sets of multiple types of neurons and projections with anatomical annotation at single-neuron resolution. The results show that the simultaneous acquisition of labelled neural structures and cytoarchitecture reference in the same brain greatly facilitates precise tracing of long-range projections and accurate locating of nuclei.

No abstract