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.