The reception of sensory information is an active process in which sensory input and motor behaviour influence each other. Mice explore the environment by rhythmically moving their whiskers. As a result, their representations in the somatosensory cortex are modulated. In the present study, we analyzed daily structural changes in the barrel cortex associated with locomotor activity level of the C57/BL mouse strain. The locomotor activity was monitored using running wheels. In a day/night regime (LD 12:12), maximum of the activity of animals was at the beginning of the night and minimum in the first half of the day. Using serial EM sections of the barrel cortex of mice, sacrificed in the period of their high or low locomotor activity, we found an increase in numerical density of synapses during the low activity period. In particular, we observed up-regulation of the density of excitatory synapses located on dendritic spines (mostly single-synapse spines). We also detected a slight increase in the density of inhibitory synapses, located on both dendritic shafts and spines, during the high activity period. It seems that in LD 12:12 conditions high locomotor activity of animals leads to the elimination of singlesynapse spines. In conclusion, in LD 12:12 conditions there are cyclical, daily changes in the density of synapses and dendritic spines in mouse barrel cortex, which are associated with the daily locomotor activity pattern of the animal.
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