Modulation of excitatory but not inhibitory synaptic inputs in the mouse primary motor cortex in the late phase of motor learning.

Motor skill training induces functional and structural changes in the primary motor cortex. New dendritic spines are formed with training and the horizontal connections in the layer II/III area of the primary motor cortex are strengthened. Here we investigated the functional synaptic properties of pyramidal neurons following motor skill training. We trained mice on a single forelimb-reaching task for five days and performed whole cell recordings from layer II/III pyramidal neurons in the forelimb representation area of the primary motor cortex in the ipsilateral (untrained) and contralateral (trained) hemispheres in acute brain slices. Success rate in the forelimb-reaching task rapidly improved over the first 3 days and stabilized on subsequent days. After five days of training, a time at which learning has peaked and synaptic strengthening with field potential recordings show enhancement, we observed an increase in mEPSC frequency while increases in mEPSC amplitudes was only observed in 20% of the cells. Increase in excitatory synaptic properties were correlated with improved motor skill. Measurement of miniature IPSC (mIPSC) after five days of training showed no difference in either frequency or amplitude between the trained and untrained hemispheres. Our present results indicate dynamic changes in excitatory but not inhibitory synapses in M1 layer II/III pyramidal neurons at the late stages of motor skill learning.