Nuclear size of c-Fos expression at the auditory brainstem is related to the time-varying nature of the acoustic stimuli

Acoustic stimulation is known to induce c-Fos expression in the auditory system but how the expression might be related to the time-variance of the sound (e.g., steady or frequency-varying) is unclear. Here we measured morphometrically Fos-immunohistochemical stains at the auditory brainstem after exposing rats to a pure tone or a narrow-range frequency modulated (FM) sound for various durations (10, 30 or 90 min). Nuclear sizes of Fos-stains at the cochlear nucleus (CN) and inferior colliculus (IC) were estimated under microscope. Tone stimulation at the given frequency (9 kHz) induced Fos-expression at locations consistent with the tonotopic maps, and no clear difference in the spatial distribution of the Fos-stains was observed across stimuli. In general, more Fos-stains appeared after longer stimulations and most notably cell nuclei labeled with Fos-immunoreactivity were statistically larger in size under longer pure tone and FM tone stimulations. Findings suggest that Fos-expression mechanisms are related to the selective response of different subpopulations of neurons to sounds of different time-varying properties. Results support that the time-variance of acoustic stimulation should be considered in the interpretation of Fos-expression findings.