Age-related changes in calbindin D-28k and calretinin immunoreactivity in the inferior colliculus of CBA/CaJ and C57Bl/6 mice

Decreased expression of calretinin and calbindin in the labyrinth of old gerbils

Calretinin and calbindin staining were compared in the vestibular periphery of old (35-48 months) and young (4-12 months) animals. Both stain calyx-only afferents; calbindin stains additional terminals in the apex [Brain Res. 928 (2002) 8-17]. In six of seven pairs of animals, calretinin and calbindin staining was diminished or absent in the old animals. These changes suggest that a reduction in certain calcium-binding proteins may be a characteristic of aging animals.

Temporal processing speed in the inferior colliculus of young and aged rats

A common problem among the elderly is a difficulty in discriminating speech. One factor that may contribute to this is a deterioration in the ability to process dynamic aspects of speech such as formant transitions. Recently, Mendelson and Ricketts [Mendelson, J.R., Ricketts, C., Hear. Res. 158 (2001) 84-94] showed that cells recorded from the auditory cortex of aged animals exhibited a decrease in temporal processing speed compared to young animals. In the present study, we examined whether this age-related effect was exclusive to the auditory cortex or whether it was apparent subcortically. To this end, single units were recorded from the inferior colliculus (IC) of young and aged rats in response to frequency modulated (FM) sweeps. Results showed that there was no age-related difference in speed or direction selectivity of FM sweep responses in the IC. The present results suggest that the effect of aging on temporal processing speed occurs in the cortex, but not subcortically.

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

Neuropathologic research strategies in holoprosencephaly

Hypotheses are presented to explain the pathogenesis of several clinical features of holoprosencephaly, and neuropathologic approaches to testing these hypotheses are suggested. The traditional morphologic classification of holoprosencephaly into alobar, semilobar, and lobar forms is grades of severity, and each occurs in all of the genetic mutations known. Of the four defective genes identified as primary in human holoprosencephaly, three exhibit a ventrodorsal gradient of expression (SHH, SIX3, and TGIF) and one a dorsoventral gradient (ZIC2). But, in addition to the vertical axis, genes expressed in the neural tube also may have rostrocaudal and mediolateral gradients in the other axes. These other gradients may be equally as important as the vertical. If the rostrocaudal gradient extends as far as the mesencephalic neuromere, it may interfere with the formation, migration, or apoptosis of the mesencephalic neural crest, which forms membranous bones of the face, orbits, nose, and parts of the eyes, and may explain the midfacial hypoplasia seen in many, but not all, children with holoprosencephaly. This rostrocaudal gradient also causes noncleavage of the caudate nucleus, thalamus, and hypothalamus and contributes to the formation of the dorsal cyst of holoprosencephaly, which is probably derived from an expanded suprapineal recess of the 3rd ventricle with secondary dilation of the telencephalic monoventricle and at times may produce a unique transfontanellar encephalocele. The extent of the mediolateral gradient may explain the severe disorganization of cerebral cortical architecture in medial parts of the forebrain and normal cortex in lateral parts, including the radial glial fibers. This preserved lateral cortex may explain why some children with holoprosencephaly have better intellectual function than expected and may also be important in the pathogenesis of epilepsy, by contrast with malformations such as lissencephaly, in which the entire cerebral cortex is involved. Epilepsy in some, but not all, cases also may be related to the sequential maturation of axonal terminals in relation to the neurons they innervate. Diabetes insipidus is a complication in a majority of patients; other neuroendocrinopathies occur less frequently. Secondary down-regulation of the OTP gene or of downstream genes such as BRN2 or SIM1 may result in failure of terminal differentiation of magnocellular neurons of the supraoptic and paraventricular hypothalamic nuclei. Disoriented radial glial fibers or abnormal ependyma may allow aberrant migration of neuroepithelial cells into the ventricle. A new classification of holoprosencephaly is needed to integrate morphologic and genetic criteria.

Parvalbumin and calbindin are differentially distributed within primary and secondary subregions of the mouse auditory forebrain

The calcium binding proteins parvalbumin and calbindin are thought to differentially regulate physiological functions and often show complementary distributions in the CNS. Our goal was to determine parvalbumin and calbindin distributions in the different subdivisions of mouse auditory thalamus and auditory cortex. Following fixation, FVB mouse brains (postnatal days 38-80) were sectioned along coronal and horizontal planes, then processed for parvalbumin and calbindin immunohistochemistry (antibodies: parvalbumin pa-235, calbindin-d-28k cl-300). Strong complementary differences in calcium binding protein distributions were found in mouse auditory thalamus. The ventral division of the medial geniculate, which is the principal relay to primary auditory cortex, exhibited dense parvalbumin but weak calbindin immunoreactivity. In contrast, most of the 'secondary' auditory thalamic regions surrounding the ventral division showed strong calbindin and lighter parvalbumin levels. Thus, the mouse auditory thalamus is composed of a parvalbumin positive 'core' surrounded by a calbindin positive 'shell'. Parvalbumin immunoreactivity was also more prominent in the primary auditory cortex than in the secondary belt auditory cortex. Calbindin immunoreactivity in auditory cortex was less clearly divided along primary/secondary lines, especially in supragranular layers. However, within infragranular layers, there was heavier staining in belt areas than in primary auditory cortex. In auditory thalamus, parvalbumin labeling was largely confined to the neuropil, whereas calbindin labeling involved somata and neuropil. In auditory cortex, somata and neuropil were positive for both proteins.In summary, the calcium binding proteins parvalbumin and calbindin were found to be differentially distributed within the primary and non-primary regions of mouse auditory forebrain. These differences in protein distribution may contribute to the distinct types of physiological responses that occur in the primary vs. non-primary areas.

Early bilateral deafening prevents calretinin up-regulation in the dorsal cortex of the inferior colliculus of aged CBA/CaJ mice

This study was conducted to test the hypothesis that age-related calretinin (CR) up-regulation seen in the dorsal cortex of the inferior colliculus (ICdc) of old hearing CBA mice is dependent upon neural activity within the auditory pathway. We tested this hypothesis by bilaterally deafening young CBA/CaJ mice with kanamycin, and then aging them until 24 months. This manipulation mimics the lack of sound-evoked auditory activity experienced by old C57BL/6J mice, who are deaf and do not show CR up-regulation with age. Cell counts revealed that the density of CR+ cells in the ICdc of old hearing CBA mice was statistically different from old deafened CBA mice raised under identical conditions. Old hearing CBAs possessed an average of 27.54 more CR+ cells/100 microm2 than old deafened CBAs. When old deafened CBAs were compared to young hearing CBAs, young hearing C57s, and old deaf C57s, there was no significant difference in mean CR+ cell density in ICdc. Thus, only the old normal hearing CBAs showed an increase in CR+ cells with age, supporting the hypothesis that CR up-regulation depends upon sound-evoked activity. Moreover, these results demonstrate that up-regulation of CR expression was not simply due to a mouse strain difference.

Subcortical neural coding mechanisms for auditory temporal processing

Biologically relevant sounds such as speech, animal vocalizations and music have distinguishing temporal features that are utilized for effective auditory perception. Common temporal features include sound envelope fluctuations, often modeled in the laboratory by amplitude modulation (AM), and starts and stops in ongoing sounds, which are frequently approximated by hearing researchers as gaps between two sounds or are investigated in forward masking experiments. The auditory system has evolved many neural processing mechanisms for encoding important temporal features of sound. Due to rapid progress made in the field of auditory neuroscience in the past three decades, it is not possible to review all progress in this field in a single article. The goal of the present report is to focus on single-unit mechanisms in the mammalian brainstem auditory system for encoding AM and gaps as illustrative examples of how the system encodes key temporal features of sound. This report, following a systems analysis approach, starts with findings in the auditory nerve and proceeds centrally through the cochlear nucleus, superior olivary complex and inferior colliculus. Some general principles can be seen when reviewing this entire field. For example, as one ascends the central auditory system, a neural encoding shift occurs. An emphasis on synchronous responses for temporal coding exists in the auditory periphery, and more reliance on rate coding occurs as one moves centrally. In addition, for AM, modulation transfer functions become more bandpass as the sound level of the signal is raised, but become more lowpass in shape as background noise is added. In many cases, AM coding can actually increase in the presence of background noise. For gap processing or forward masking, coding for gaps changes from a decrease in spike firing rate for neurons of the peripheral auditory system that have sustained response patterns, to an increase in firing rate for more central neurons with transient responses. Lastly, for gaps and forward masking, as one ascends the auditory system, some suppression effects become quite long (echo suppression), and in some stimulus configurations enhancement to a second sound can take place.

Age-related temporal processing speed deterioration in auditory cortex

A common problem among the elderly is a difficulty in discriminating speech sounds. One factor that may contribute to this is a deterioration in the ability to process dynamic aspects of speech such as formant transitions. For the aging auditory system, this deterioration in temporal processing speed may be manifest as a deficit in encoding time-varying sounds that contain rapidly changing frequencies such as formant transitions. The primary goal of this study was to explore the neural basis of the effects of aging on temporal processing speed. To this end, single units were recorded from the auditory cortex of young and aged rats in response to frequency-modulated (FM) sweeps that changed from trial to trial in both direction and speed. Results showed that the majority of cells recorded from young rats responded most vigorously to fast and medium speeds. By contrast, the majority of units recorded from aged animals responded best to slow speeds. For preferred direction of FM sweep, similar results were observed for both age groups, namely, approximately half of the units exhibited a direction-selective response. The results of the present study demonstrate an age-related decrease in the rate of change of frequency that can be processed by the auditory cortex.

The amplitude-modulation following response in young and aged human subjects

The amplitude-modulation following response (AMFR) is a steady-state auditory response which may be an objective measure of intensity discrimination. Aged subjects with normal hearing have poorer intensity discrimination for low-frequency tones measured behaviorally, which would predict poorer AMFRs for low-frequency carriers. Experiment 1 was designed to assess age-related differences in AMFR characteristics. Response amplitudes were not significantly different among the young and aged groups for either carrier frequency (520 or 4000 Hz) or modulation depth (0--100%). Response phase did not vary systematically among groups. These results suggest that the AMFR may not be directly comparable to behavioral measures of intensity discrimination in aged subjects with normal hearing. To assess the contribution of high-frequency hearing loss on the AMFR in aged subjects, Experiment 2 compared AMFR amplitudes in aged subjects and in young subjects under the condition of high-pass masking. The amplitude of the AMFR was reduced at 520 Hz for both aged subjects and masked young subjects compared to unmasked young subjects, suggesting that reduced amplitudes in aged subjects with high-frequency hearing loss were associated with threshold elevations. Furthermore, the results suggest that the base of the cochlea contributes to the AMFR for low carrier frequencies.

Long-lasting inhibitory synaptic depression is age- and calcium-dependent

The developmental refinement of excitatory synapses is often influenced by neuronal activity, and underlying synaptic mechanisms have been suggested. In contrast, few studies have asked whether inhibitory synapses are reorganized during development and whether this is accompanied by use-dependent changes of inhibitory synaptic strength. The topographic inhibitory projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) undergoes synapse elimination during development (Sanes and Takács, 1993). To determine whether there is an associated period of synaptic plasticity, whole-cell recordings were obtained from developing LSO neurons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude by 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibited a long-lasting depression of MNTB-evoked inhibitory synaptic currents (34%) after low-frequency stimulation. When LSO neurons were depolarized, low-frequency stimulation of the MNTB produced a significantly larger inhibitory synaptic depression (59%). This synaptic plasticity declined dramatically by postnatal days 17-19. Similar to well studied forms of excitatory synaptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the developmental rearrangement of inhibitory terminals as they compete with neighboring excitatory and/or inhibitory inputs.

Developing mouse models of aging: a consideration of strain differences in age-related behavioral and neural parameters

Increased interest is emerging for using mouse models to assess the genetics of brain aging and age-related neurodegenerative diseases. Despite this demand, relatively little information is available on aging in behavioral or neuromorphological parameters in various mouse strains that are being used to create transgenic and null mutant mice. We review several issues regarding selection of appropriate strains as follows: (1) Does the behavioral parameter exhibit a significant age by strain interaction? (2) Do the strains differ in lifespan? (3) Are there potential intervening variables, such as strain-specific performance strategies or disease, in the behavioral task being investigated that would confound the desired conclusions? (4) Does the behavioral difference have an underlying neural correlate? In this context we present a conceptual model pertaining to the selection of mouse strains and behavioral parameters for genetic analyses. We also review the importance of applying stereological techniques for determining age-related structural changes in the mouse brain as well as the potential value of a database that would catalog this information. Thus, our intention is to underscore the growing importance of mouse models of brain aging and the concomitant need for additional information about mouse aging in general.

Sound stimulation increases calcium-binding protein immunoreactivity in the inferior colliculus in mice

The numerical density of calbindin D-28k and parvalbumin immunopositive neurons in the inferior colliculus (IC) in mice was increased after sound stimulation. An increased number of calbindin positive neurons was found in the deep layers of the external cortex (EC) and particularly in the dorsal cortex (DC) and commissural nucleus (NCO). An increase of parvalbumin positive neurons was found in the EC, central nucleus (ICC) and DC, but not in the NCO. The increased immunoreactivity related to sound exposure suggests the appearance of neurons which express these proteins after sound stimulation. The up-regulation of calcium-binding proteins in these neurons may be due to their protective role against overstimulation, their response to a higher auditory metabolic activity, or increasing effect of excitatory inputs after noise-induced hearing loss.

Modulating calbindin and parvalbumin immunoreactivity in the cochlear nucleus by moderate noise exposure in mice. . A quantitative study on the dorsal and posteroventral cochlear nucleus

The number of calbindin D-28k and parvalbumin immunoreactive (IR) neurons were characterized on sections from the cochlear nucleus, dorsal cochlear nucleus (DCN) and posteroventral cochlear nucleus (PVCN) using two-dimensional quantification. After noise exposure (6-12 kHz, 2 h, at either 80 dB SPL or 103 dB SPL), the number of calbindin and parvalbumin immunoreactive neurons increased in CBA/CBA mice. Quantitative analysis of calbindin-IR in the PVCN did not show a statistically significant difference between any of the groups, whereas statistically significant differences in calbindin-IR were found in the DCN for the 103 dB and 80 dB group compared to the control group, and 103 dB compared to the 80 dB group, respectively. A statistically significant increase in the number of parvalbumin-IR neurons in the PVCN and the DCN was evident in the 103 dB and 80 dB group compared to the control group, and in the 103 dB compared to the 80 dB group. The data indicate that increasing sound stimulation causes a graded increase in the expression of calcium-binding protein immunoreactivity in the DCN and PVCN neurons and neuropil. This increase of protein expression is due to increased positive immunoreactivity in 'silent' neurons. These findings implicate that these neurons have the possibility to react against trauma and display calbindin or parvalbumin as a rescue event. The ability to map sound-induced calcium-binding protein changes in auditory neurons may be useful in future studies designed for detecting early patterns of neurodegeneration and neuroprotection in the central auditory pathway.

Age-related alteration in processing of temporal sound features in the auditory midbrain of the CBA mouse

The perception of complex sounds, such as speech and animal vocalizations, requires the central auditory system to analyze rapid, ongoing fluctuations in sound frequency and intensity. A decline in temporal acuity has been identified as one component of age-related hearing loss. The detection of short, silent gaps is thought to reflect an important fundamental dimension of temporal resolution. In this study we compared the neural response elicited by silent gaps imbedded in noise of single neurons in the inferior colliculus (IC) of young and old CBA mice. IC neurons were classified by their temporal discharge patterns. Phasic units, which accounted for the majority of response types encountered, tended to have the shortest minimal gap thresholds (MGTs), regardless of age. We report three age-related changes in neural processing of silent gaps. First, although the shortest MGTs (1-2 msec) were observed in phasic units from both young and old animals, the number of neurons exhibiting the shortest MGTs was much lower in old mice, regardless of the presentation level. Second, in the majority of phasic units, recovery of response to the stimulus after the silent gap was of a lower magnitude and much slower in units from old mice. Finally, the neuronal map representing response latency versus best frequency was found to be altered in the old IC. These results demonstrate a central auditory system correlate for age-related decline in temporal processing at the level of the auditory midbrain.

Inputs to a physiologically characterized region of the inferior colliculus of the young adult CBA mouse

Presbycusis is a sensory perceptual disorder involving loss of high-pitch hearing and reduced ability to process biologically relevant acoustic signals in noisy environments. The present investigation is part of an ongoing series of studies aimed at discerning the neural bases of presbycusis. The purpose of the present experiment was to delineate the inputs to a functionally characterized region of the dorsomedial inferior colliculus (IC, auditory midbrain) in young, adult CBA mice. Focal, iontophoretic injections of horseradish peroxidase were made in the 18-24 kHz region of dorsomedial IC of the CBA strain following physiological mapping experiments. Serial sections were reacted with diaminobenzidine or tetramethylbenzidine, counterstained and examined for retrogradely labeled cell bodies. Input projections were observed contralaterally from: all three divisions of cochlear nucleus; intermediate and dorsal nuclei of the lateral lemniscus (LL); and the central nucleus, external nucleus and dorsal cortex of the IC. Input projections were observed ipsilaterally from: the medial and lateral superior olivary nuclei; the superior paraolivary nucleus; the dorsolateral and anterolateral periolivary nuclei; the dorsal and ventral divisions of the ventral nucleus of LL; the dorsal and intermediate nuclei of LL; the central nucleus, external nucleus and dorsal cortex of the IC outside the injection site; and small projections from central gray and the medial geniculate body. These findings in young, adult mice with normal hearing can now serve as a baseline for similar experiments being conducted in mice of older ages and with varying degrees of hearing loss to discover neural changes that may cause age-related hearing disorders.

Calbindin D-28k immunoreactivity in the medial nucleus of the trapezoid body declines with age in C57BL/6, but not CBA/CaJ, mice

This study compared calbindin D-28k immunoreactivity in the medial nucleus of the trapezoid body (MNTB) in young (3-4 month old) and old (24-26 month old) CBA/CaJ mice, and young (3-4 month old), middle-aged (6.5-8.5 month old), and old (24-29 month old) C57BL/6 mice. C57BL/6 mice exhibit progressively more severe peripheral (sensorineural) hearing loss between 4 and 12 months of age, whereas CBA/CaJ mice show little change in peripheral sensitivity until very late in life. We obtained auditory brainstem response audiograms on all subject mice. Old CBA mice were selected for study whose audiograms matched those of young CBA and C57 controls. Middle-aged C57 mice showed elevated thresholds indicative of peripheral degeneration. Brain sections were reacted with anti-calbindin D-28k (CB). Staining patterns in Nissl and anti-CB material were characterized and cells were counted. We found no significant change in the number of CB+ cells or the total number of cells in the MNTB of old CBA mice compared to young controls. However, the mean number of CB+ cells decreased by 11% in middle-aged, and by 14.8% in old C57 mice. Since the decline in C57 mice was significant by 6.5-8.5 months of age, the decrease could be the consequence of a loss of input from the cochlear nucleus where cell numbers are known to decline by this age in this strain. The total number of neurons in MNTB assessed from Nissl material showed a more modest 7.1% decline with age in C57 mice, implying that the greater loss of CB immunoreactive cells with age cannot be completely attributed to a reduction in the total number of cells.