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.

Structural changes in the adult rat auditory system induced by brief postnatal noise exposure.

In previous studies (Grécová et al., Eur J Neurosci 29:1921-1930, 2009; Bures et al., Eur J Neurosci 32:155-164, 2010), we demonstrated that after an early postnatal short noise exposure (8 min 125 dB, day 14) changes in the frequency tuning curves as well as changes in the coding of sound intensity are present in the inferior colliculus (IC) of adult rats. In this study, we analyze on the basis of the Golgi-Cox method the morphology of neurons in the IC, the medial geniculate body (MGB) and the auditory cortex (AC) of 3-month-old Long-Evans rats exposed to identical noise at postnatal day 14 and compare the results to littermate controls. In rats exposed to noise as pups, the mean total length of the neuronal tree was found to be larger in the external cortex and the central nucleus of the IC and in the ventral division of the MGB. In addition, the numerical density of dendritic spines was decreased on the branches of neurons in the ventral division of the MGB in noise-exposed animals. In the AC, the mean total length of the apical dendritic segments of pyramidal neurons was significantly shorter in noise-exposed rats, however, only slight differences with respect to controls were observed in the length of basal dendrites of pyramidal cells as well as in the neuronal trees of AC non-pyramidal neurons. The numerical density of dendritic spines on the branches of pyramidal AC neurons was lower in exposed rats than in controls. These findings demonstrate that early postnatal short noise exposure can induce permanent changes in the development of neurons in the central auditory system, which apparently represent morphological correlates of functional plasticity.

Evaluation of microvasculature at the auditory midbrain--the benefits of sectioning at a tangential angle.

Vascular remodeling in the brain occurs as a plastic change following neural over-activity. The auditory midbrain (or inferior colliculus, IC) is an ideal place to study sound-induced vascular changes because it is the brain's most vascularized structure and it is tonotopically organized. However, its micro-vascular pattern remains poorly understood. Since the IC is a sphere-like structure, the histological assessment of vasculature could depend on the angle of sectioning. Here, we studied the effects of cutting the IC at different angles on microvascular assessment, specifically: micro-vascular density and the shape of microvascular lumen. Photomicrographs were taken from 5 µm toluidine blue-stained histological sections obtained at two angles of sectioning: (a) the conventional coronal sectioning, and (b) a novel "tangential" sectioning (tangential to the dorso-medial surface of the IC). Results showed that the tangential sections, in comparison with the coronal sections, yielded (a) a higher count of micro-vascular density and (b) a higher proportion of round-shaped micro-vascular lumens. This discrepancy in results between two cut angles is likely related to the spatial pattern of blood vessels supplying the IC. We propose that the tangential sectioning should be adopted as standard for the accurate study of microvasculature in the IC.

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 microm-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.