Age-related changes in the central auditory system.

Aging is accompanied by the deterioration of hearing that complicates our understanding of speech, especially in noisy environments. This deficit is partially caused by the loss of hair cells as well as by the dysfunction of the stria vascularis. However, the central part of the auditory system is also affected by processes accompanying aging that may run independently of those affecting peripheral receptors. Here, we review major changes occurring in the central part of the auditory system during aging. Most of the information that is focused on age-related changes in the central auditory system of experimental animals arises from experiments using immunocytochemical targeting on changes in the glutamic-acid-decarboxylase, parvalbumin, calbindin and calretinin. These data are accompanied by information about age-related changes in the number of neurons as well as about changes in the behavior of experimental animals. Aging is in principle accompanied by atrophy of the gray as well as white matter, resulting in the enlargement of the cerebrospinal fluid space. The human auditory cortex suffers not only from atrophy but also from changes in the content of some metabolites in the aged brain, as shown by magnetic resonance spectroscopy. In addition to this, functional magnetic resonance imaging reveals differences between activation of the central auditory system in the young and old brain. Altogether, the information reviewed in this article speaks in favor of specific age-related changes in the central auditory system that occur mostly independently of the changes in the inner ear and that form the basis of the central presbycusis.

Changes in parvalbumin immunoreactivity with aging in the central auditory system of the rat.

Changes in the levels of calcium binding proteins are known to occur in different parts of the brain during aging. In our study we attempted to define the effect that aging has on the parvalbumin-expressing system of neurons in the higher parts of the central auditory system. Age-related changes in parvalbumin immunoreactivity were investigated in the inferior colliculus (IC), medial geniculate body (MGB) and auditory cortex (AC) in two rat strains, normally aging Long-Evans (LE) and fast aging Fischer 344 (F344). The results demonstrate that the changes in PV-immunoreactivity are strain-dependent with an increase in the number of PV-immunoreactive (PV-ir) neurons occurring in the inferior colliculus of old LE rats and a pronounced decline in the number of PV-ir neurons appearing in the auditory cortex of aged F344 animals. In some parts of the AC of old F344 animals no PV-ir neurons were present at all. The number of PV-ir neurons in the MGB in all examined animals was very low independent of the strain and age. The loss of PV-ir neurons in the auditory cortex of Fischer 344 rats with aging may contribute to the substantial deterioration of hearing function in this strain.

Atorvastatin slows down the deterioration of inner ear function with age in mice.

Statins have revolutionized the treatment of hypercholesterolemia due to their ability to inhibit cholesterol biosynthesis. Their immunomodulatory and anti-inflammatory effects and positive effects on the treatment of atherosclerosis and its complications are well known. Here, we describe the effects of statins on the treatment of presbycusis in C57BL/6J mice. In this strain with accelerated aging, we demonstrate that animals treated with atorvastatin (10mg/kg per day in chow diet) for 2 months showed larger amplitudes of distortion product otoacoustic emissions (DPOAE) than did the non-treated control group. This finding indicates a better survival of outer hair cell function in the inner ear of C57BL/6J mice. The observed decreased expression of intercellular and vascular adhesion molecules in the aortic wall of atorvastatin-treated animals suggests that reducing endothelial inflammatory effects may contribute to the positive effect of atorvastatin on the amplitudes of DPOAE by influencing the blood supply to the inner ear. No such beneficial effect of statins was found in apoE(-/-) mice treated with atorvastatin under the same conditions. Our results suggest that statins could also slow down the age-related deterioration of hearing in man.

Expression of c-Fos in rat auditory and limbic systems following 22-kHz calls.

In the present study, adult Long-Evans rats were exposed either to natural conspecific aversive 22-kHz vocalizations or to artificial call-like stimuli with comparable frequency-temporal features, followed by c-Fos immunohistochemistry. The natural 22-kHz vocalizations was either played from a recording or produced by a foot-shocked animal located nearby (live vocalizations). In comparison with controls (non-exposed animals), c-Fos immunoreactivity was significantly increased in the inferior colliculus (IC), auditory cortex (AC), periaqueductal grey (PAG), basolateral amygdala (BA), and hippocampus (Hip) of rats exposed to either live or recorded 22-kHz natural vocalizations. Exposure to live natural vocalizations of the foot-shocked animal resulted in a similar pattern of c-Fos activity, as did exposure to the playback of the natural vocalizations. In contrast to this, foot-shocked rats (emitting the 22-kHz vocalizations) had the c-Fos positivity increased markedly in the PAG and only slightly in the AC. The expression of c-Fos also increased in the IC, AC, and in the PAG in animals exposed to the artificial call-like stimuli, when compared to controls; however, the increase was much less pronounced. In this case, c-Fos expression was not increased in the hippocampus or basolateral amygdala. Interestingly, almost no c-Fos expression was found in the medial nucleus of the geniculate body in any of the experimental groups. These findings suggest that differences exist between the processing of important natural conspecific vocalizations and artificial call-like stimuli with similar frequency-temporal features, and moreover they suggest the specific role of individual brain structures in the processing of such calls.

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.

The influence of aging on the number of neurons and levels of non-phosporylated neurofilament proteins in the central auditory system of rats.

In the present study, an unbiased stereological method was used to determine the number of all neurons in Nissl stained sections of the inferior colliculus (IC), medial geniculate body (MGB), and auditory cortex (AC) in rats (strains Long Evans and Fischer 344) and their changes with aging. In addition, using the optical fractionator and western blot technique, we also evaluated the number of SMI-32-immunoreactive (-ir) neurons and levels of non-phosphorylated neurofilament proteins in the IC, MGB, AC, and visual cortex of young and old rats of the two strains. The SMI-32 positive neuronal population comprises about 10% of all neurons in the rat IC, MGB, and AC and represents a prevalent population of large neurons with highly myelinated and projecting processes. In both Long Evans and Fischer 344 rats, the total number of neurons in the IC was roughly similar to that in the AC. With aging, we found a rather mild and statistically non-significant decline in the total number of neurons in all three analyzed auditory regions in both rat strains. In contrast to this, the absolute number of SMI-32-ir neurons in both Long Evans and Fischer 344 rats significantly decreased with aging in all the examined structures. The western blot technique also revealed a significant age-related decline in the levels of non-phosphorylated neurofilaments in the auditory brain structures, 30-35%. Our results demonstrate that presbycusis in rats is not likely to be primarily associated with changes in the total number of neurons. On the other hand, the pronounced age-related decline in the number of neurons containing non-phosphorylated neurofilaments as well as their protein levels in the central auditory system may contribute to age-related deterioration of hearing function.

Immunocytochemical profiles of inferior colliculus neurons in the rat and their changes with aging.

The inferior colliculus (IC) plays a strategic role in the central auditory system in relaying and processing acoustical information, and therefore its age-related changes may significantly influence the quality of the auditory function. A very complex processing of acoustical stimuli occurs in the IC, as supported also by the fact that the rat IC contains more neurons than all other subcortical auditory structures combined. GABAergic neurons, which predominantly co-express parvalbumin (PV), are present in the central nucleus of the IC in large numbers and to a lesser extent in the dorsal and external/lateral cortices of the IC. On the other hand, calbindin (CB) and calretinin (CR) are prevalent in the dorsal and external cortices of the IC, with only a few positive neurons in the central nucleus. The relationship between CB and CR expression in the IC and any neurotransmitter system has not yet been well established, but the distribution and morphology of the immunoreactive neurons suggest that they are at least partially non-GABAergic cells. The expression of glutamate decarboxylase (GAD) (a key enzyme for GABA synthesis) and calcium binding proteins (CBPs) in the IC of rats undergoes pronounced changes with aging that involve mostly a decline in protein expression and a decline in the number of immunoreactive neurons. Similar age-related changes in GAD, CB, and CR expression are present in the IC of two rat strains with differently preserved inner ear function up to late senescence (Long-Evans and Fischer 344), which suggests that these changes do not depend exclusively on peripheral deafferentation but are, at least partially, of central origin. These changes may be associated with the age-related deterioration in the processing of the temporal parameters of acoustical stimuli, which is not correlated with hearing threshold shifts, and therefore may contribute to central presbycusis.

Distribution of SMI-32-immunoreactive neurons in the central auditory system of the rat.

SMI-32 antibody recognizes a non-phosphorylated epitope of neurofilament proteins, which are thought to be necessary for the maintenance of large neurons with highly myelinated processes. We investigated the distribution and quantity of SMI-32-immunoreactive(-ir) neurons in individual parts of the rat auditory system. SMI-32-ir neurons were present in all auditory structures; however, in most regions they constituted only a minority of all neurons (10-30%). In the cochlear nuclei, a higher occurrence of SMI-32-ir neurons was found in the ventral cochlear nucleus. Within the superior olivary complex, SMI-32-ir cells were particularly abundant in the medial nucleus of the trapezoid body (MNTB), the only auditory region where SMI-32-ir neurons constituted an absolute majority of all neurons. In the inferior colliculus, a region with the highest total number of neurons among the rat auditory subcortical structures, the percentage of SMI-32-ir cells was, in contrast to the MNTB, very low. In the medial geniculate body, SMI-32-ir neurons were prevalent in the ventral division. At the cortical level, SMI-32-ir neurons were found mainly in layers III, V and VI. Within the auditory cortex, it was possible to distinguish the Te1, Te2 and Te3 areas on the basis of the variable numerical density and volumes of SMI-32-ir neurons, especially when the pyramidal cells of layer V were taken into account. SMI-32-ir neurons apparently form a representative subpopulation of neurons in all parts of the rat central auditory system and may belong to both the inhibitory and excitatory systems, depending on the particular brain region.

Age-related changes in calbindin and calretinin immunoreactivity in the central auditory system of the rat.

Age-related changes in the levels of major intracellular calcium buffers are known to occur in different parts of the mammalian brain, including the central auditory pathway. In the present study, we evaluate with immunohistochemistry and the western blot technique the effect that aging has on the calbindin- and calretinin-expressing system of neurons in the higher structures of the central auditory pathway, in the inferior colliculus (IC), medial geniculate body (MGB) and auditory cortex (AC) of two rat strains, the slowly aging Long-Evans and the fast aging Fischer 344. Interestingly, the age-related changes demonstrated a similar character regardless of the rat strain. In the IC of young animals, the majority of calbindin and calretinin immuno-reactive (CB and CR-ir) cells were found in the dorsal and external cortices and only sparse positive cells were present in the central nucleus of the IC. With aging, the number of CB-ir and CR-ir neurons decreased significantly in both the dorsal and external cortices. Furthermore, these declines were accompanied by an age-related reduction in the mean volumes of CB- and CR-ir neuronal somas. In the MGB of young rats, CB-ir neurons were present in abundant numbers in both the dorsal and ventral subdivisions, while CR-ir neurons were practically absent in this structure. With aging, the number and mean volume of CB-ir cells in the ventral subdivision of the MGB were significantly decreased. In comparison with the IC and MGB, age-related numerical and volumetric declines of both CB-ir and CR-ir neurons in the AC were less pronounced. Western blot protein analysis revealed a pronounced age-related decline in the levels of calbindin in both strains and in all examined brain regions. In contrast, the decline in calretinin levels with aging was less prominent, with a significant decline only in the IC of both strains. The observed age-related changes in the calbindin- and calretinin-expressing systems may contribute significantly to the deterioration of hearing function known as central presbycusis.

Effects of damage to auditory cortex on the discrimination of speech sounds by rats.

The intensity of a noise-induced startle response can be reduced by the presentation of an otherwise neutral stimulus immediately before the noise ("prepulse inhibition" or PPI). We used a form of PPI to study the effects of damage to auditory cortex on the discrimination of speech sounds by rats. Subjects underwent control surgery or treatment of the auditory cortex with the vasoconstrictor endothelin-1. This treatment caused damage concentrated in primary auditory cortex (A1). Both before and after lesions, subjects were tested on 5 tasks, most presenting a pair of human speech sounds (consonant-vowel syllables) so that the capacity for discrimination would be evident in the extent of PPI. Group comparisons failed to reveal any consistent lesion effect. At the same time, the analysis of individual differences in performance by multiple regression suggests that some of the temporal processing required to discriminate speech sounds is concentrated anteroventrally in the right A1. These results also confirm that PPI can be adapted to studies of the brain mechanisms involved in the processing of speech and other complex sounds.

Age-related changes in GAD levels in the central auditory system of the rat.

Changes in the levels of gamma-aminobutyric acid (GABA) are known to occur in different parts of the brain during aging. In our study we attempted to define the effect that aging has on glutamate decarboxylase (GAD), the key enzyme in the synthesis of GABA, in the central parts of the auditory system. Age-related changes in GAD65 and GAD67 levels were investigated using immunohistochemistry and Western blotting in the inferior colliculus (IC), the auditory cortex (AC) and the visual cortex in Long-Evans rats. The results show that aging is associated with a decrease in the numbers of GAD65- and 67-immunoreactive neurons and the optical density of their somas in both the IC and AC. Western blot analysis revealed a pronounced age-related decline in the levels of GAD65 and 67 proteins in both the IC and AC. For comparison, in the visual cortex the decrease in both proteins was less pronounced than in the IC and AC. A similar pattern of age-related changes was found in Fischer 344 rats, a strain that manifests a rapid loss of hearing function with aging. The observed age-related decline in the levels of GAD65 and 67 may contribute significantly to the deterioration of hearing function that accompanies aging in mammals, including man.