Enlargement of neuronal size in rat auditory cortex after prolonged sound exposure

Prolonged sound exposure produces functional changes in the auditory neurons. It remains unclear whether such changes are detectable with morphometric measures like cell size. Here, after exposing juvenile rats (starting on week-4) to a monotone for 7 days, we measured the size of their cortical neurons. Neuronal profiles (nuclei and perikarya) in deep layers of the primary auditory cortex were digitized and measured on photomicrographs taken from 7 μm-thick histological sections stained with toluidine blue. To facilitate digitizing cell profiles, we used an image-analysis software that contains a confocal-like image-merging function to sharpen the edges. After sound exposure, both nuclei and perikarya expanded by about 1/3 in volume compared with controls (p < 0.0001, Student's t-test). Such changes were not found in the visual cortex. Results showed that prolonged sound exposure increased the size of auditory neurons. Such activity-driven cell enlargement can be used as a simple measure to find other plastic changes in the brain.