Changes of the brainstem auditory neurons after neonatal tone exposure

• Main Authors: Tzai-Wen Chiu, 曲在雯
• Other Authors: Wai-Fung Poon
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/67641343471830419865

博士 === 國立成功大學 === 基礎醫學研究所 === 92 === 　　Response characteristics of central auditory neurons can be altered by early acoustic experience but details are not clear. Previous studies in young rats showed that the tonotopic representation and response properties of midbrain neurons can be shaped following sound exposure during the first three weeks after birth. Aims of this following-up study are to determine (a) the longer-term effects of neonatal sound exposure; (b) details of time-dependent functional changes following sound exposure at different stages and (c) changes of GABAergic system following sound exposure at the critical period. 　　The first part of experiment is to determine if such experience-dependent changes at the inferior colliculus (IC) could last into old ages. A mutant strain of senescence accelerated mice (SAM) was for this study. At the old age of 15 months, an apparent cell clustering of best frequencies of cells towards the frequency of the exposing tone was still found at the IC after sound exposure during the first month. In addition, there was an increase of spontaneous activity and loss of high frequency sensitivity. Results suggested that the early sound exposure has a long-lasting effect on IC neurons and that the early acoustic over- stimulation also accelerates the senescence of the auditory system. 　　The second part of experiment adopted two exposure protocols (weekly: from week-1 to week-4 or monthly: week-1to -4 and week-2 to-5). Extracellular single unit recordings and activity labeling with Fos-immunohistochemistry were used to characterize functional changes at the IC and cochlear nucleus (CN) respectively. After weekly exposures, only the week-2 exposed group showed a significant over-representation of best frequencies at the exposing frequency. In addition, only the week-2 exposed rats showed more Fos-positive labels when re-exposed to a tone with frequency modulated across the exposing tone. The fact that the aforementioned results were not found at the CN suggested that the changes occurred locally at the IC. After monthly exposures, both the week-1 to -4 and week-2 to -5 rats showed over-representation at the exposing frequency. These cells showed a narrowed frequency tuning in the week-1 to -4 exposed groups. But those in either the week-2 to -5 or week-2 exposed groups showed broadened frequency tuning. Results suggested that both tuning properties and tonotopic representation of collicular neurons are sensitively altered by acoustic over-stimulation during the first two weeks, which likely represent the critical period. 　　The third part of experiment is to determine changes in the GABAergic system in the IC following the neonatal sound exposure using immunohistochemistry. In control animals, GABA-immunoreactivity increased in the IC after brief acoustic stimulation in the week-2 but not week-3 old rats. Following weekly sound exposures, more GABA-containing neurons were found in the whole IC of rats put in a silent room before sacrifice. The increased GABA-containing neurons did not express Fos protein simultaneously in their nuclei. These GABA-containing neurons disappeared after a brief acoustic stimulation. On the other hand, neonatal sound exposures down regulated the expression of GABAA receptor under a silent condition and they can be elevated by acoustic re-stimulation. Results suggested the neonatal sound exposures can shape the GABAergic inhibition in the IC. 　　Findings suggested that IC is particularly plastic during the first two weeks after birth. Rats exposed to tones during this critical period have time-dependent changes on tonotopic representations and tuning properties. Such plastic changes last into old ages. In addition, the neonatal sound exposure also had effects on altering the GABAergic system in the IC.