Postnatal exposure to an acoustically enriched environment alters the morphology of neurons in the adult rat auditory system

The structure of neurons in the central auditory system is vulnerable to various kinds of acoustic exposures during the critical postnatal developmental period. Here we explored long-term effects of exposure to an acoustically enriched environment (AEE) during the third and fourth weeks of the postnatal period in rat pups. AEE consisted of a spectrally and temporally modulated sound of moderate intensity, reinforced by a behavioral paradigm. At the age of 3-6 months, a Golgi-Cox staining was used to evaluate the morphology of neurons in the inferior colliculus (IC), the medial geniculate body (MGB), and the auditory cortex (AC). Compared to controls, rats exposed to AEE showed an increased mean dendritic length and volume and the soma surface in the external cortex and the central nucleus of the IC. The spine density increased in both the ventral and dorsal divisions of the MGB. In the AC, the total length and volume of the basal dendritic segments of pyramidal neurons and the number and density of spines on these dendrites increased significantly. No differences were found on apical dendrites. We also found an elevated number of spines and spine density in non-pyramidal neurons. These results show that exposure to AEE during the critical developmental period can induce permanent changes in the structure of neurons in the central auditory system. These changes represent morphological correlates of the functional plasticity, such as an improvement in frequency tuning and synchronization with temporal parameters of acoustical stimuli.

Saturation of long-term potentiation in the dorsal cochlear nucleus and its pharmacological reversal in an experimental model of tinnitus

Animal models have demonstrated that tinnitus is a pathology of dysfunctional excitability in the central auditory system, in particular in the dorsal cochlear nucleus (DCN) of the brainstem. We used a murine model and studied whether acoustic over-exposure leading to hearing loss and tinnitus, affects long-term potentiation (LTP) at DCN multisensory synapses. Whole cell and field potential recordings were used to study the effects on release probability and synaptic plasticity, respectively in brainstem slices. Shifts in hearing threshold were quantified by auditory brainstem recordings, and gap-induced prepulse inhibition of the acoustic startle reflex was used as an index for tinnitus. An increased release probability that saturated LTP and thereby induced metaplasticity at DCN multisensory synapses, was observed 4-5days following acoustic over-exposure. Perfusion of an NMDA receptor antagonist or decreasing extracellular calcium concentration, decreased the release probability and restored LTP following acoustic over-exposure. In vivo administration of magnesium-threonate following acoustic over-exposure restored LTP at DCN multisensory synapses, and reduced gap detection deficits observed four months following acoustic over-exposure. These observations suggest that consequences of noise-induced metaplasticity could underlie the gap detection deficits that follow acoustic over-exposure, and that early therapeutic intervention could target metaplasticity and alleviate tinnitus.

Aging and Speech Understanding

As people age, structural as well as neural degeneration occurs throughout the auditory system. Many older adults experience difficulty in understanding speech especially in adverse listening conditions although they can hear speech sounds. According to a report of the Committee on Hearing and Bioacoustics and Biomechanics of the National Research Council, peripheral, central-auditory, and cognitive systems have long been considered major factors affecting the understanding of speech. The present study aims to review 1) age-related changes in the peripheral, central-auditory, and cognitive systems, 2) the resulting decline in the understanding of speech, and 3) the clinical implication for audiologic rehabilitation of older adults. Once the factors affecting the understanding of speech in older adults are identified and the characteristics of age-related speech understanding difficulties are examined, clinical management could be developed for prevention and treatment. Future research about problems related to the understanding of speech in older adults will help to improve the quality of life in the elderly.

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.