Long-term changes of response in the inferior colliculus of senescence accelerated mice after early sound exposure

Influences of un-modulated acoustic inputs on functional maturation and critical-period plasticity of the primary auditory cortex

Sensory experiences contribute to the development and specialization of signal processing capacities in the mammalian auditory system during a "critical period" of postnatal development. Earlier studies have shown that passive exposure to tonal stimuli during this postnatal epoch induces a large-scale expansion of the representations of those stimuli within the primary auditory cortex (A1) [Zhang LI, Bao S, Merzenich MM (2001) Persistent and specific influences of early acoustic environments on primary auditory cortex. Nat Neurosci 4:1123-1130]. Here, we show that exposing rat pups through the normal critical period epoch and beyond to continuous, un-modulated, moderate-level tones induces no such representational distortion, and in fact disrupts the normal development of frequency response selectivity and tonotopicity all across area A1. The area of cortex responding selectively to continuously exposed sound frequencies was actually reduced, when compared with rats reared in normal environments. Strong exposure-driven plasticity characteristic of the critical period could be induced well beyond the normal end of the critical period, by simply modulating the tonal stimulus. Thus, continuous tone exposure, like continuous noise exposure [Chang EF, Merzenich MM (2003) Environmental noise retards auditory cortical development. Science 300:498-502], ineffectively induces critical period plasticity, and indefinitely blocks the closure of a normally-brief critical period window. These findings again demonstrate the crucial role of temporally structured inputs for inducing the progressive cortical maturational changes that result in the closure of the critical period window.

Contributions of mouse models to understanding of age- and noise-related hearing loss

Once an oddity, mice have become the most widely used hearing research model. Their value for research in noise-induced hearing loss (NIHL) stems from their high vulnerability to noise and reduced variance of results, made possible by genetic standardization. To research in age-related hearing loss (ARHL), they offer economies of small size and a short lifespan, both of which reduce housing costs. Inbred mouse strains show a wide range of noise sensitivities and rates of hearing loss with age. These can be studied using classical genetic analysis, as well as hypothesis-driven experiments utilizing genetic engineering. Through such investigations, presently 3 loci have been identified to date that contribute to NIHL, 10 that promote ARHL, and at least 6 loci that promote both. The types of genes involved implicate homeostatic and protective mechanisms as key to the appearance of either type of pathology and support a causal link between injury and some apparent ARHL. While the majority of mouse ARHL models examined most closely resemble sensory ARHL, recent work has identified mice possessing the essential characteristics of neural and strial ARHL. Using these models, it should be possible to identify genes and alleles that promote the major forms of ARHL and their combinations.

Early postnatal sound exposure induces lasting neuronal changes in the inferior colliculus of senescence accelerated mice (SAMP8): a morphometric study on GABAergic neurons and NMDA expression

Senescence-acceleration-prone mice (SAMP8) provide a model to study the influence of early postnatal sound exposure upon the aging auditory midbrain. SAMP8 were exposed to a 9-kHz monotone of either 53- or 65-dB sound pressure level during the first 30 postnatal days, the neurons in the auditory midbrain responding selectively to 9 kHz were localized by c-fos immunohistochemistry and the following parameters were compared to control SAMP8 not exposed to sound: mortality after sound exposure, dendritic spine density, and quantitative neurochemical alterations in this 9-kHz isofrequency lamina. For morphometric analysis, animals were examined at 1, 4, and 8 months of age. Serial sections of the inferior colliculus were Golgi impregnated or stained immunohistochemically for the expression of epsilon1 subunit of NMDA receptor or GABA. Mortality after exposure to 53 dB was the same as in controls, but was markedly increased from 7 months of age onward after postnatal exposure to 65 dB. No gross morphological alterations were observed in the auditory midbrain after sound exposure. However, sound exposure to 53 or 65 dB significantly reduced dendritic spine density by 11% at 4 months or by 11-17% both at 1 and 4 months of age, respectively. The effect of sound exposure upon neurons expressing the NMDAepsilon1 subunit was dose-dependent. Increasing with age until 4 months in control mice and remaining essentially stable thereafter, the percentage of NMDAepsilon1-immunoreactive neurons was significantly elevated by 40-66% in 1- and 8-month-old SAMP8 exposed to 53 dB, whereas no significant effect of 65 dB was apparent. The proportion of GABAergic cells declined with age in controls. It was significantly decreased at 1 month after 53 and 65 dB sound exposure. In contrast, it was elevated at later stages, being significantly increased at 4 months after exposure to 53 dB and at 8 months after exposure to 65 dB. The total cell number in the 9-kHz isofrequency lamina of SAMP8 decreased with age, but was not affected by exposure to either 53 or 65 dB. The present results indicate that early postnatal exposure to a monotone of mild intensity has long-term effects upon the aging auditory brain stem. Some of the changes induced by sound exposure, e.g., decline in spine density, are interpreted as accelerations of the normal aging process, whereas other effects, e.g., increased NMDAepsilon1 expression after 53 dB and elevated GABA expression after both 53 and 65 dB, are not merely explicable by accelerated aging.