Glycine immunoreactivity and receptor binding in the cochlear nucleus of C57BL/6J and CBA/CaJ mice: Effects of cochlear impairment and aging

Aging alters across-hemisphere cortical dynamics during binaural temporal processing

Differences in the timing and intensity of sounds arriving at the two ears provide fundamental binaural cues that help us localize and segregate sounds in the environment. Neural encoding of these cues is commonly represented asymmetrically in the cortex with stronger activation in the hemisphere contralateral to the perceived spatial location. Although advancing age is known to degrade the perception of binaural cues, less is known about how the neural representation of such cues is impacted by age. Here, we use electroencephalography (EEG) to investigate age-related changes in the hemispheric distribution of interaural time difference (ITD) encoding based on cortical auditory evoked potentials (CAEPs) and derived binaural interaction component (BIC) measures in ten younger and ten older normal-hearing adults. Sensor-level analyses of the CAEP and BIC showed age-related differences in global field power, where older listeners had significantly larger responses than younger for both binaural metrics. Source-level analyses showed hemispheric differences in auditory cortex activity for left and right lateralized stimuli in younger adults, consistent with a contralateral activation model for processing ITDs. Older adults, however, showed reduced hemispheric asymmetry across ITDs, despite having overall larger responses than younger adults. Further, when averaged across ITD condition to evaluate changes in cortical asymmetry over time, there was a significant shift in laterality corresponding to the peak components (P1, N1, P2) in the source waveform that also was affected by age. These novel results demonstrate across-hemisphere cortical dynamics during binaural temporal processing that are altered with advancing age.

Objective evidence of temporal processing deficits in older adults

The older listener's ability to understand speech in challenging environments may be affected by impaired temporal processing. This review summarizes objective evidence of degraded temporal processing from studies that have used the auditory brainstem response, auditory steady-state response, the envelope- or frequency-following response, cortical auditory-evoked potentials, and neural tracking of continuous speech. Studies have revealed delayed latencies and reduced amplitudes/phase locking in subcortical responses in older vs. younger listeners, in contrast to enhanced amplitudes of cortical responses in older listeners. Reconstruction accuracy of responses to continuous speech (e.g., cortical envelope tracking) shows over-representation in older listeners. Hearing loss is a factor in many of these studies, even though the listeners would be considered to have clinically normal hearing thresholds. Overall, the ability to draw definitive conclusions regarding these studies is limited by the use of multiple stimulus conditions, small sample sizes, and lack of replication. Nevertheless, these objective measures suggest a need to incorporate new clinical measures to provide a more comprehensive assessment of the listener's speech understanding ability, but more work is needed to determine the most efficacious measure for clinical use.

Subcortical pathways: Towards a better understanding of auditory disorders

Hearing loss is a significant problem that affects at least 15% of the population. This percentage, however, is likely significantly higher because of a variety of auditory disorders that are not identifiable through traditional tests of peripheral hearing ability. In these disorders, individuals have difficulty understanding speech, particularly in noisy environments, even though the sounds are loud enough to hear. The underlying mechanisms leading to such deficits are not well understood. To enable the development of suitable treatments to alleviate or prevent such disorders, the affected processing pathways must be identified. Historically, mechanisms underlying speech processing have been thought to be a property of the auditory cortex and thus the study of auditory disorders has largely focused on cortical impairments and/or cognitive processes. As we review here, however, there is strong evidence to suggest that, in fact, deficits in subcortical pathways play a significant role in auditory disorders. In this review, we highlight the role of the auditory brainstem and midbrain in processing complex sounds and discuss how deficits in these regions may contribute to auditory dysfunction. We discuss current research with animal models of human hearing and then consider human studies that implicate impairments in subcortical processing that may contribute to auditory disorders.

The FBN rat model of aging: investigation of ABR waveforms and ribbon synapse changes

Age-related hearing loss is experienced by one-third of individuals aged 65 years and older and can be socially debilitating. Historically, there has been poor correlation between age-related threshold changes, loss of speech understanding, and loss of cochlear hair cells. We examined changes in ribbon synapse number at four different ages in Fisher Brown Norway rats, an extensively studied rat model of aging. In contrast to previous work in mice/Wistar rats, we found minimal ribbon synapse loss before 20 months, with significant differences in 24- and 28-month-old rats at 4 kHz. Significant outer HC loss was observed at 24 and 28 months in low- to mid-frequency regions. Age-related reductions in auditory brainstem response wave I amplitude and increases in threshold were strongly correlated with ribbon synapse loss. Wave V/I ratios increased across age for click, 2, 4, and 24 kHz. Together, we find that ribbon synapses in the Fisher Brown Norway rat cochlea show resistance to aging until ∼60% of their life span, suggesting species/strain differences may underpin decreased peripheral input into the aging central processor.

Proposed comprehensive ototoxicity monitoring program for VA healthcare (COMP-VA)

Prevention and rehabilitation of hearing loss and tinnitus, the two most commonly awarded service-connected disabilities, are high priority initiatives in the Department of Veterans Affairs (VA). At least 4,000 Veterans, most with significant hearing loss, will receive cisplatin this year, with more than half sustaining permanent hearing shift and nearly 40% developing new tinnitus. With improved survivability following cancer treatment, Veterans treated with cisplatin are approached with the dual goals of effective treatment and preserved quality of life. This article describes COMP-VA, a comprehensive ototoxicity monitoring program developed for VA patients receiving cisplatin. The program includes an individualized pretreatment prediction model that identifies the likelihood of hearing shift given cisplatin dose and patient factors. It supports both manual and automated hearing testing with a newly developed portable audiometer capable of performing the recommended procedures on the chemotherapy unit during treatment. It also includes objective methods for identifying outer hair cell changes and predicting audiogram changes using distortion-product otoacoustic emissions. We describe this program of evidence-based ototoxicity monitoring protocols using a case example to give the reader an understanding of how this program would be applied, along with a plan for future work to accomplish the final stages of program development.

Glycinergic synaptic transmission in the cochlear nucleus of mice with normal hearing and age-related hearing loss

The principal inhibitory neurotransmitter in the mammalian cochlear nucleus (CN) is glycine. During age-related hearing loss (AHL), glycinergic inhibition becomes weaker in CN. However, it is unclear what aspects of glycinergic transmission are responsible for weaker inhibition with AHL. We examined glycinergic transmission onto bushy cells of the anteroventral CN in normal-hearing CBA/CaJ mice and in DBA/2J mice, a strain that exhibits an early onset AHL. Glycinergic synaptic transmission was examined in brain slices of mice at 10-15 postnatal days old, 20-35 days old, and at 6-7 mo old. Spontaneous inhibitory postsynaptic current (sIPSC) event frequency and amplitude were the same among all three ages in both strains of mice. However, the amplitudes of IPSCs evoked (eIPSC) from stimulating the dorsal CN were smaller, and the failure rate was higher, with increasing age due to decreased quantal content in both mouse strains, independent of hearing status. The coefficient of variation of the eIPSC amplitude also increased with age. The decay time constant (τ) of sIPSCs and eIPSCs were constant in CBA/CaJ mice at all ages, but were significantly slower in DBA/2J mice at postnatal days 20-35, following the onset of AHL, and not at earlier or later ages. Our results suggest that glycinergic inhibition at the synapses onto bushy cells becomes weaker and less reliable with age through changes in release. However, the hearing loss in DBA/2J mice is accompanied by a transiently enhanced inhibition, which could disrupt the balance of excitation and inhibition.

Inhibitory neurotransmission in animal models of tinnitus: maladaptive plasticity

Tinnitus is a phantom auditory sensation experienced by up to 14% of the United States population with a smaller percentage experiencing decreased quality of life. A compelling hypothesis is that tinnitus results from a maladaptive plastic net down-regulation of inhibitory amino acid neurotransmission in the central auditory pathway. This loss of inhibition may be a compensatory response to loss of afferent input such as that caused by acoustic insult and/or age-related hearing loss, the most common causes of tinnitus in people. Compensatory plastic changes may result in pathologic neural activity that underpins tinnitus. The neural correlates include increased spontaneous spiking, increased bursting and decreased variance of inter-spike intervals. This review will examine evidence for chronic plastic neuropathic changes in the central auditory system of animals with psychophysically-defined tinnitus. Neurochemical studies will focus on plastic tinnitus-related changes of inhibitory glycinergic neurotransmission in the adult dorsal cochlear nucleus (DCN). Electrophysiological studies will focus on functional changes in the DCN and inferior colliculus (IC). Tinnitus was associated with increased spontaneous activity and altered response properties of fusiform cells, the major output neurons of DCN. Coincident with these physiologic alterations were changes in glycine receptor (GlyR) subunit composition, its anchoring/trafficking protein, gephyrin and the number and affinity of membrane GlyRs revealed by receptor binding. In the IC, the primary afferent target of DCN fusiform cells, multi-dimensional alterations in unit-spontaneous activity (rate, burst rate, bursting pattern) were found in animals with behavioral evidence of chronic tinnitus more than 9 months following the acoustic/cochlear insult. In contrast, immediately following an intense sound exposure, acute alterations in IC spontaneous activity resembled chronic tinnitus-related changes but were not identical. This suggests that long-term neuroplastic changes responsible for chronic tinnitus are likely to be responsible for its persistence. A clear understanding of tinnitus-related plasticity in the central auditory system and its associated neurochemistry may help define unique targets for therapeutic drug development.

Impact of sound exposure and aging on brain-derived neurotrophic factor and tyrosine kinase B receptors levels in dorsal cochlear nucleus 80 days following sound exposure

Recent studies suggested that acute sound exposure resulting in a temporary threshold shift in young adult animals within a series of maladaptive plasticity changes in central auditory structures. Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, is involved in post-trauma peripheral hair cell and spiral ganglion cell survival and has been shown to modulate synaptic strength in cochlear nucleus following sound exposure. The present study evaluated levels of BDNF and its receptor (tyrosine kinase B, [TrkB]) in the dorsal cochlear nucleus (DCN) following a unilateral moderate sound exposure in young (7-8 months) and aged (28-29 months) Fischer Brown Norway (FBN) rats. Eighty days post-exposure, auditory brainstem response (ABR) thresholds for young exposed rats approached control values while aged exposed rats showed residual permanent threshold shifts (PTS) relative to aged controls. BDNF protein levels were significantly up-regulated by 9% in young exposed fusiform cells ipsilateral to the exposure. BDNF levels in aged sound-exposed fusiform cells increased 31% ipsilateral to the exposure. Protein levels of the BDNF receptor, TrkB, were also significantly increased in aged but not in young sound-exposed DCN fusiform cells. The present findings suggest a relationship between the up-regulation of BDNF/TrkB and the increase in spontaneous and driven activity previously observed for aged and sound-exposed fusiform cells. This might be due to a selective maladaptive compensatory down-regulation of glycinergic inhibition in DCN fusiform cells.

Plasticity at glycinergic synapses in dorsal cochlear nucleus of rats with behavioral evidence of tinnitus

Fifteen percent to 35% of the United States population experiences tinnitus, a subjective "ringing in the ears". Up to 10% of those afflicted report severe and disabling symptoms. Tinnitus was induced in rats using unilateral, 1 h, 17 kHz-centered octave-band noise (116 dB SPL) and assessed using a gap-startle method. The dorsal cochlear nucleus (DCN) is thought to undergo plastic changes suggestive of altered inhibitory function during tinnitus development. Exposed rats showed near pre-exposure auditory brainstem response (ABR) thresholds for clicks and all tested frequencies 16 weeks post-exposure. Sound-exposed rats showed significantly worse gap detection at 24 and 32 kHz 16 weeks following sound exposure, suggesting the development of chronic, high frequency tinnitus. Message and protein levels of alpha(1-3,) and beta glycine receptor subunits (GlyRs), and the anchoring protein, gephyrin, were measured in DCN fusiform cells 4 months following sound exposure. Rats with evidence of tinnitus showed significant GlyR alpha(1) protein decreases in the middle and high frequency regions of the DCN while alpha(1) message levels were paradoxically increased. Gephyrin levels showed significant tinnitus-related increases in sound-exposed rats suggesting intracellular receptor trafficking changes following sound exposure. Consistent with decreased alpha(1) subunit protein levels, strychnine binding studies showed significant tinnitus-related decreases in the number of GlyR binding sites, supporting tinnitus-related changes in the number and/or composition of GlyRs. Collectively, these findings suggest the development of tinnitus is likely associated with functional GlyR changes in DCN fusiform cells consistent with previously described behavioral and neurophysiologic changes. Tinnitus related GlyR changes could provide a unique receptor target for tinnitus pharmacotherapy or blockade of tinnitus initiation.

Inhibitory and excitatory subtypes of cochlear nucleus neurons are defined by distinct bHLH transcription factors, Ptf1a and Atoh1

The cochlear nucleus (CN), which consists of dorsal and ventral cochlear nuclei (DCN and VCN), plays pivotal roles in processing and relaying auditory information to the brain. Although it contains various types of neurons, the origins of the distinct subtypes and their developmental molecular machinery are still elusive. Here we reveal that two basic helix-loop-helix transcription factors play crucial roles in specifying neuron subtypes in the CN. Pancreatic transcription factor 1a (Ptf1a) and atonal homolog 1 (Atoh1) were found to be expressed in discrete dorsolateral regions of the embryonic neuroepithelia of the middle hindbrain (rhombomeres 2-5). Genetic lineage tracing using mice that express Cre recombinase from the Ptf1a locus or under the control of the Atoh1 promoter revealed that inhibitory (GABAergic and glycinergic) or excitatory (glutamatergic) neurons of both DCN and VCN are derived from the Ptf1a- and Atoh1-expressing neuroepithelial regions, respectively. In the Ptf1a or Atoh1 null embryos, production of inhibitory or excitatory neurons, respectively, was severely inhibited in the CN. These findings suggest that inhibitory and excitatory subtypes of CN neurons are defined by Ptf1a and Atoh1, respectively and, furthermore, provide important insights into understanding the machinery of neuron subtype specification in the dorsal hindbrain.

Developmental hearing loss disrupts synaptic inhibition: implications for auditory processing

Hearing loss during development leads to central deficits that persist even after the restoration of peripheral function. One key class of deficits is due to changes in central inhibitory synapses, which play a fundamental role in all aspects of auditory processing. This review focuses on the anatomical and physiological alterations of inhibitory connections at several regions within the central auditory pathway following hearing loss. Such aberrant inhibitory synaptic function may be linked to deficits in encoding binaural and spectral cues. Understanding the cellular changes that occur at inhibitory synapses following hearing loss may provide specific loci that can be targeted to improve function.

Age-related changes in glycine receptor subunit composition and binding in dorsal cochlear nucleus

Age-related hearing loss, presbycusis, can be thought of, in part, as a slow progressive peripheral deafferentation. Previous studies suggest that certain deficits seen in presbycusis may partially result from functional loss of the inhibitory neurotransmitter glycine in dorsal cochlear nucleus (DCN). The present study assessed age-related behavioral gap detection changes and neurochemical changes of postsynaptic glycine receptor (GlyRs) subunits and their anchoring protein gephyrin in fusiform cells of young (7-11 months) and aged (28-33 months) Fischer brown Norway (FBN) rats. Aged rats showed significantly (20-30 dB) elevated auditory brainstem-evoked response thresholds across all tested frequencies and worse gap detection ability compared to young FBN rats. In situ hybridization and quantitative immunocytochemistry were used to measure GlyR subunit message and protein levels. There were significant age-related increases in the alpha(1) subunit message with significant age-related decreases in alpha(1) subunit protein. Gephyrin message and protein showed significant increases in aged DCN fusiform cells. The pharmacologic consequences of these age-related subunit changes were assessed using [3H] strychnine binding. In support of the age-related decrease of alpha(1) subunit protein levels in DCN, there was a significant age-related decrease in the total number of GlyR binding sites with no significant change in affinity. These age-related changes may reflect an effort to reestablish a homeostatic balance between excitation and inhibition impacting on DCN fusiform cells by downregulation of inhibitory function in the face of an age-related loss of peripheral input. Age-related decrease in presynaptic glycine release results in altered subunit composition and this may correlate with loss of temporal coding of the aged fusiform cell in DCN. The previously reported role for gephyrin in retrograde intracellular receptor subunit trafficking could contribute to the alpha(1) decrease in the face of increased message.

Loudness perception in the domestic cat: reaction time estimates of equal loudness contours and recruitment effects

The domestic cat is the primary physiological model of loudness coding and recruitment. At present, there are no published descriptions of loudness perception in this species. This study used a reaction time task to characterize loudness perception in six behaviorally trained cats. The psychophysical approach was based on the assumption that sounds of equal loudness elicit responses of equal latency. The resulting equal latency contours reproduced well-known features of human equal loudness contours. At the completion of normal baseline measures, the cats were exposed to intense sound to investigate the behavioral correlates of loudness recruitment, the abnormally rapid growth of loudness that is commonly associated with hearing loss. Observed recruitment effects were similar in magnitude to those that have been reported in hearing-impaired humans. Linear hearing aid amplification is known to improve speech intelligibility but also exacerbate recruitment in impaired listeners. The effects of speech spectra and amplification on recruitment were explored by measuring the growth of loudness for natural and amplified vowels before and after sound exposure. Vowels produced more recruitment than tones, and the effect was exacerbated by the selective amplification of formant structure. These findings support the adequacy of the domestic cat as a model system for future investigations of the auditory processes that underlie loudness perception, recruitment, and hearing aid design.

Encoding intensity in ventral cochlear nucleus following acoustic trauma: implications for loudness recruitment

Loudness recruitment, an abnormally rapid growth of perceived loudness with sound level, is a common symptom of sensorineural hearing loss. Following acoustic trauma, auditory-nerve rate responses are reduced, and rate grows more slowly with sound level, which seems inconsistent with recruitment (Heinz et al., J. Assoc. Res. Otolaryngol. 6:91-105, 2005). However, rate-level functions (RLFs) in the central nervous system may increase in either slope or saturation value following trauma (e.g., Salvi et al., Hear. Res. 147:261-274, 2000), suggesting that recruitment may arise from central changes. In this paper, we studied RLFs of neurons in ventral cochlear nucleus (VCN) of the cat after acoustic trauma. Trauma did not change the general properties of VCN neurons, and the usual VCN functional classifications remained valid (chopper, primary-like, onset, etc.). After trauma, non-primary-like neurons, most noticeably choppers, exhibited elevated maximum discharge rates and steeper RLFs for frequencies at and near best frequency (BF). Primary-like neurons showed the opposite changes. To relate the neurons' responses to recruitment, rate-balance functions were computed; these show the sound level required to give equal rates in a normal and a traumatized ear and are analogous to loudness balance functions that show the sound levels giving equal perceptual loudness in the two ears of a monaurally hearing-impaired person. The rate-balance functions showed recruitment-like steepening of their slopes in non-primary-like neurons in all conditions. However, primary-like neurons showed recruitment-like behavior only when rates were summated across neurons of all BFs. These results suggest that the non-primary-like, especially chopper, neurons may be the most peripheral site of the physiological changes in the brain that underlie recruitment.

Inhibitory neurotransmission, plasticity and aging in the mammalian central auditory system

Aging and acoustic trauma may result in partial peripheral deafferentation in the central auditory pathway of the mammalian brain. In accord with homeostatic plasticity, loss of sensory input results in a change in pre- and postsynaptic GABAergic and glycinergic inhibitory neurotransmission. As seen in development, age-related changes may be activity dependent. Age-related presynaptic changes in the cochlear nucleus include reduced glycine levels, while in the auditory midbrain and cortex, GABA synthesis and release are altered. Presumably, in response to age-related decreases in presynaptic release of inhibitory neurotransmitters, there are age-related postsynaptic subunit changes in the composition of the glycine (GlyR) and GABA(A) (GABA(A)R) receptors. Age-related changes in the subunit makeup of inhibitory pentameric receptor constructs result in altered pharmacological and physiological responses consistent with a net down-regulation of functional inhibition. Age-related functional changes associated with glycine neurotransmission in dorsal cochlear nucleus (DCN) include altered intensity and temporal coding by DCN projection neurons. Loss of synaptic inhibition in the superior olivary complex (SOC) and the inferior colliculus (IC) likely affect the ability of aged animals to localize sounds in their natural environment. Age-related postsynaptic GABA(A)R changes in IC and primary auditory cortex (A1) involve changes in the subunit makeup of GABA(A)Rs. In turn, these changes cause age-related changes in the pharmacology and response properties of neurons in IC and A1 circuits, which collectively may affect temporal processing and response reliability. Findings of age-related inhibitory changes within mammalian auditory circuits are similar to age and deafferentation plasticity changes observed in other sensory systems. Although few studies have examined sensory aging in the wild, these age-related changes would likely compromise an animal's ability to avoid predation or to be a successful predator in their natural environment.

Aged-related loss of temporal processing: altered responses to amplitude modulated tones in rat dorsal cochlear nucleus

Loss of temporal processing is characteristic of age-related loss of speech understanding observed in the elderly. Inhibitory glycinergic circuits provide input onto dorsal cochlear nucleus (DCN) projection neurons which likely serve to modulate excitatory responses to time-varying complex acoustic signals. The present study sought to test the hypothesis that age-related loss of inhibition would compromise the ability of output neurons to encode sinusoidally amplitude modulated (SAM) tones. Extracellular recordings were obtained from young and aged FBN rat DCN putative fusiform cells. Stimuli were SAM tones at three modulation depths (100, 50, and 20%) at 30 dB hearing level with the carrier frequency set to the unit's characteristic frequency. Discharge rate and synchrony were calculated to describe SAM responses. There were significant age-related changes in the shape and peak vector strength [best modulation frequency (BMF)] of temporal modulation transfer functions (tMTFs), with no significant age-related changes in rate modulation transfer functions (rMTFs) at BMF. Young neurons exhibited band-pass tMTFs for most SAM conditions while aged fusiform cells exhibited significantly more low-pass or double-peaked tMTFs. There were significant differences in tMTFs between buildup, pauser-buildup, and wide-chopper temporal response types. Young and aged wide-choppers displayed significantly lower vector strength values than the other two temporal DCN response types. Age-related decreases in the number of pauser-buildup response types and increases in wide-chopper types reported previously, could account, in part, for the observed loss of temporal coding of the aged fusiform cell. Age-related changes in SAM coding were similar to changes observed with receptor blockade of glycinergic inhibition onto fusiform cells and consistent with previously observed age-related loss of endogenous glycine levels and changes in normal adult glycine receptor function. DCN changes in SAM coding could, in part, underpin temporal processing deficits observed in the elderly.

Pathophysiology of tinnitus

Guided by findings from neural imaging and population responses in humans, where tinnitus is well characterized, several morphological and physiological substrates of tinnitus in animal studies are reviewed. These include changes in ion channels, receptor systems, single unit firing rate, and population responses. Most findings in humans can be interpreted as resulting from increased neural synchrony.

Sensorineural hearing loss and neural correlates of temporal acuity in the inferior colliculus of the C57BL/6 mouse

Perception of complex sounds depends on the encoding of the dynamic and static structures within the ongoing stimulus by the auditory system. Aging has been associated with deficits in both areas, thus, the difficulty that the elderly have in speech comprehension could due to hearing loss, or to a loss of temporal sensitivity, or some combination of both. We investigated the effects of sensorineural hearing loss (SNHL) on neural correlates of temporal resolution by recording the responses of inferior colliculus neurons to a gap detection paradigm. We used C57BL/6 (C57) strain of laboratory mouse, which carries the Ahl deafness gene that initiates a progressive high frequency SNHL beginning at about 2 months of age and rapidly progresses to total deafness by 18 months. We compared gap encoding from inferior collicular neurons from young, normal-hearing C57 mice and middle-aged, hearing-impaired, C57 mice, quantifying minimal gap threshold, and recovery functions. The proportion of unit types, spontaneous rates and degree of monotonicity were comparable between young and middle-aged C57 mice. As expected, single unit thresholds were elevated by 30-40 dB in middle-aged C57 mice. However, no significant differences in mean minimal gap thresholds or in the slopes of the gap recovery functions were found between the two age groups. Thus, the results suggest that moderate high frequency SNHL does not affect temporal processing as measured by the gap detection paradigm.

Excitatory, inhibitory and facilitatory frequency response areas in the inferior colliculus of hearing impaired mice

Individuals with age-related hearing loss often have difficulty understanding complex sounds such as basic speech. The C57BL/6 mouse suffers from progressive sensorineural hearing loss and thus is an effective tool for dissecting the neural mechanisms underlying changes in complex sound processing observed in humans. Neural mechanisms important for processing complex sounds include multiple tuning and combination sensitivity, and these responses are common in the inferior colliculus (IC) of normal hearing mice. We examined neural responses in the IC of C57Bl/6 mice to single and combinations of tones to examine the extent of spectral integration in the IC after age-related high frequency hearing loss. Ten percent of the neurons were tuned to multiple frequency bands and an additional 10% displayed non-linear facilitation to the combination of two different tones (combination sensitivity). No combination-sensitive inhibition was observed. By comparing these findings to spectral integration properties in the IC of normal hearing CBA/CaJ mice, we suggest that high frequency hearing loss affects some of the neural mechanisms in the IC that underlie the processing of complex sounds. The loss of spectral integration properties in the IC during aging likely impairs the central auditory system's ability to process complex sounds such as speech.

Age-related changes in the response properties of cartwheel cells in rat dorsal cochlear nucleus

The fusiform cell and deep layers of the dorsal cochlear nucleus (DCN) show neurotransmitter and functional age-related changes suggestive of a downregulation of inhibitory efficacy onto DCN output neurons. Inhibitory circuits implicated in these changes include vertical and D-multipolar cells. Cartwheel cells comprise a large additional population of DCN inhibitory neurons. Cartwheel cells receive excitatory inputs from granule cell parallel fibers and provide a source of glycinergic inhibitory input onto apical dendrites of DCN fusiform cells. The present study compared the response properties from young and aged units meeting cartwheel-cell criteria in anesthetized rats. Single unit recordings from aged cartwheel cells revealed significantly higher thresholds, increased spontaneous activity and significantly altered rate-level functions characterized by hyperexcitability at higher intensities. Aged cartwheel cells showed a significant reduction in off-set suppression. Collectively, these findings suggest a loss of tonic and perhaps response inhibition onto aged DCN cartwheel neurons. These changes likely reflect a compensatory downregulation of synaptic inhibition in response to a loss of excitatory drive from auditory and non-auditory excitatory inputs via granule cells. The impact of increased excitability of cartwheel cells on DCN output neurons is likely to be complex, influenced by loss of glycinergic release and/or subunit receptor changes which would only partially off-set age-related loss of inhibition onto the somata and basal dendrites of fusiform cells.

Dorsal cochlear nucleus response properties following acoustic trauma: response maps and spontaneous activity

Recordings from single neurons in the dorsal cochlear nucleus (DCN) of unanesthetized (decerebrate) cats were done to characterize the effects of acoustic trauma. Trauma was produced by a 250 Hz band of noise centered at 10 kHz, presented at 105-120 dB SPL for 4h. After a one-month recovery period, neurons were recorded in the DCN. The threshold shift, determined from compound action-potential audiograms, showed a sharp threshold elevation of about 60 dB at BFs above an edge frequency of 5-10 kHz. The response maps of neurons with best frequencies (BFs) above the edge did not show the typical organization of excitatory and inhibitory areas seen in the DCN of unexposed animals. Instead, neurons showed no response to sound, weak responses that were hard to tune and characterize, or "tail" responses, consisting of broadly-tuned, predominantly excitatory responses, with a roughly low-pass shape similar to the tuning curves of auditory nerve fibers with similar threshold shifts. In some tail responses whose BFs were near the edge of the threshold elevation, a second weak high-frequency response was seen that suggests convergence of auditory nerve inputs with widely separated BFs on these cells. Spontaneous rates among neurons with elevated thresholds were not increased over those in populations of principal neurons in unexposed animals.

Age-related structural and functional changes in the cochlear nucleus

Presbycusis - age-related hearing loss - is a key communication disorder and chronic medical condition of our aged population. The cochlear nucleus is the major site of projections from the auditory portion of the inner ear. Relative to other levels of the peripheral and central auditory systems, relatively few studies have been conducted examining age-related changes in the cochlear nucleus. The neurophysiological investigations suggest declines in glycine-mediated inhibition, reflected in increased firing rates in cochlear nucleus neurons from old animals relative to young adults. Biochemical investigations of glycine inhibition in the cochlear nucleus are consistent with the functional aging declines of this inhibitory neurotransmitter system that affect complex sound processing. Anatomical reductions in neurons of the cochlear nucleus and their output pathways can occur due to aging changes in the brain, as well as due to age-dependent plasticity of the cochlear nucleus in response to the age-related loss of inputs from the cochlea, particularly from the basal, high-frequency regions. Novel preventative and curative biomedical interventions in the future aimed at alleviating the hearing loss that comes with age, will likely emanate from increasing our knowledge and understanding of its neural and molecular bases. To the extent that this sensory deficit resides in the central auditory system, including the cochlear nucleus, future neural therapies will be able to improve hearing in the elderly.

Auditory processing disorder (APD)-potential contribution of mouse research

APD is a common, heterogeneous and poorly understood listening impairment that is receiving increasing recognition, especially in children and the elderly. The primary symptom in humans is poor speech perception despite normal pure tone audiometry. Diagnostic practice is patchy, but current proposals are to distinguish APD by reduced ability to detect, discriminate, localize and order non-speech sounds. APD appears to result predominantly from core malfunctioning of the central auditory system, but with strong cognitive influences and possible additional influences from peripheral auditory and crossmodal sources. APD may be acquired (e.g. through middle ear disease), but it is likely that a more common etiology would involve both environmental and inherited contributions. Mice are increasingly being used for studies of central auditory processing. Most work to date has documented the basic response properties of auditory neurons, or used single gene mutations to investigate specific functions. For high throughput, primary screening, reflex and/or event-related potential measures are required. Pre-pulse inhibition (PPI), the regulation of an acoustic startle reflex by a preceding sound having distinct spatial, temporal or spectral properties, is one way in which auditory processing may be screened. For secondary assessment, standard psychoacoustic tests for mice have been developed.

Tinnitus as a plastic phenomenon and its possible neural underpinnings in the dorsal cochlear nucleus

Tinnitus displays many features suggestive of plastic changes in the nervous system. These can be categorized based on the types of manipulations that induce them. We have categorized the various forms of plasticity that characterize tinnitus and searched for their neural underpinnings in the dorsal cochlear nucleus (DCN). This structure has been implicated as a possible site for the generation of tinnitus-producing signals owing to its tendency to become hyperactive following exposure to tinnitus inducing agents such as intense sound and cisplatin. In this paper, we review the many forms of plasticity that have been uncovered in anatomical, physiological and neurochemical studies of the DCN. Some of these plastic changes have been observed as consequences of peripheral injury or as fluctuations in the behavior and chemical activities of DCN neurons, while others can be induced by stimulation of auditory or even non-auditory structures. We show that many parallels can be drawn between the various forms of plasticity displayed by tinnitus and the various forms of neural plasticity which have been defined in the DCN. These parallels lend further support to the hypothesis that the DCN is an important site for the generation and modulation of tinnitus-producing signals.

Auditory-nerve rate responses are inconsistent with common hypotheses for the neural correlates of loudness recruitment

A number of perceptual phenomena related to normal and impaired level coding can be accounted for by the degree of compression in the basilar-membrane (BM) magnitude response. However, the narrow dynamic ranges of auditory-nerve (AN) fibers complicate these arguments. Because the AN serves as an information bottleneck, an improved understanding of the neural coding of level may clarify some of the limitations of current hearing aids. Here three hypotheses for the neural correlate of loudness recruitment were evaluated based on AN responses from normal-hearing cats and from cats with a noise-induced hearing loss (NIHL). Auditory-nerve fiber rate-level functions for tones were analyzed to test the following hypotheses: Loudness recruitment results from steeper AN rate functions after impairment. This hypothesis was not supported; AN rate functions were not steeper than normal following NIHL, despite steeper estimated BM responses based on the AN data. Loudness is based on the total AN discharge count, and recruitment results from an abnormally rapid spread of excitation after impairment. Whereas abnormal spread of excitation can be observed, steeper growth of total AN rate is not seen over the range of sound levels where recruitment is observed in human listeners. Loudness of a narrowband stimulus is based on AN responses in a narrow BF region, and recruitment results from compression of the AN-fiber threshold distribution after impairment. This hypothesis was not supported because there was no evidence that impaired AN threshold distributions were compressed and the growth of AN activity summed across BFs near the stimulus frequency was shallower than normal.Overall, these results suggest that loudness recruitment cannot be accounted for based on summed AN rate responses and may depend on neural mechanisms involved in the central representation of intensity.

Age-related increases in calcium-binding protein immunoreactivity in the cochlear nucleus of hearing impaired C57BL/6J mice

Aging C57BL/6J (C57) mice (1-30 months old), were used to study calcium-binding protein immunoreactivity (parvalbumin, calbindin and calretinin) in the cochlear nucleus. A quantitative stereological method, the optical fractionator was used to determine the total number of neurons, and the total number of immunostained neurons in the posteroventral- and dorsal cochlear nuclei (PVCN and DCN). A statistically significant age-related decrease of the total number of neurons was found in the PVCN and DCN using Nissl staining. In the DCN, an age-related increase in the total number of parvalbumin-positive neurons was found, while no changes in the total number of calbindin or calretinin positive neurons were demonstrated. In the PVCN, the total number of parvalbumin, calbindin, or calretinin positive neurons remained stable with increasing age. The percentage of parvalbumin, calbindin, and calretinin positive neurons significantly increased in the DCN, and the percentage of parvalbumin and calbindin-positive neurons increased in the PVCN. These findings imply that there is a relative up-regulation of calcium-binding proteins in neurons that had not previously expressed these proteins. This plastic response in the profoundly hearing impaired C57 mouse may be a survival strategy for cochlear nucleus neurons.

Age-dependent changes in the lateral superior olive of the gerbil (Meriones unguiculatus)

Data from humans and animal models provide evidence for an age-dependent impairment in the ability to localize sound. The lateral superior olive (LSO) in the ascending auditory pathway is one important center involved in processing of binaural auditory stimuli. To identify potential age-dependent changes we characterized the LSO in young (< 15 months) and old (> or =3 years) gerbils with a special emphasis on the expression of GABA- and glycine-like immuno-reactivity. The dimensions of the LSO, as well as the number and density of glycine- and GABA-immuno-reactive neurons, were not significantly different between young and old gerbils. The size of glycine- and GABA-immuno-reactive neurons was significantly reduced in the high-frequency (medial) limb of the LSO. Over all, age-dependent changes in the LSO of the gerbil were small.

A diminished rate of "physiological decay" at noise offset contributes to age-related changes in temporal acuity in the CBA mouse model of presbycusis

The persistence of afferent activity at stimulus offset was examined in mice as a function of age (experiment 1) and noise level (experiment 2), using a procedure devised by von Bekesy to study the physiological decay of afferent activity. Noise offset was presented from 1 to 10 ms (the temporal gap) prior to an acoustic startle stimulus, with an abrupt edge or a linear ramp having the duration of the gap. Noise offset inhibited the startle reflex, this effect declining with age. For young (2-3 months old) and "young-old" mice (18-19 months old), the abrupt offset was always more inhibitory than the ramp, even for the one millisecond gap. In "old-old" mice (24-26 months old) abrupt and ramped offsets were not different until the gap exceeded 4 ms. The behavioral difference between abrupt and ramped decay times did not depend on noise level in young mice (4-5 months old), though overall inhibition increased with level. These data indicate that temporal acuity as measured by this gap detection method is limited in very old mice by their reduced ability to follow the abrupt change in noise level at the beginning of the gap.

Effects of age and cochlear damage on the metabolic activity of the avian cochlear nucleus

Most aging commercially raised broiler chickens display a progressive loss of cochlear hair cells in a pattern similar to the cochlear degeneration found in aging humans: basal (high frequency) hair cells are affected first, followed by apical (low frequency) hair cells [Durham et al., Hear. Res. 166 (2002) 82-95]. Here, cochlear anatomy was assessed from scanning electron micrographs. Then, the metabolic activity of cochlear nucleus (nucleus magnocellularis, NM) neurons in 15-19, 30, 39, 40, and 65-66 week old broiler chickens was examined using cytochrome oxidase histochemistry and compared to the degree of cochlear abnormality. Cochleae of 15-19 week old birds are largely normal; therefore the level of NM metabolic activity is considered the baseline. Cochleae of the 30 week old group display mild damage and hair cell regeneration in the base. Metabolic activity in rostral (high frequency) NM is increased relative to the baseline, while activity remains unchanged in caudal (low frequency) NM. The 39 and 65-66 week old groups display severe and total damage extending into the apex of the cochlea. Metabolic activity is decreased in rostral and caudal NM at these ages. These results suggest that auditory central nervous system metabolism (cytochrome oxidase activity) is affected by changes in the aging chicken cochlea.

The effect of bilateral deafness on excitatory and inhibitory synaptic strength in the inferior colliculus

The consequences of deafness on the central auditory nervous system have been examined at many levels, from molecular to functional. However, there has never been a direct and selective measurement of excitatory synaptic function following total hearing loss. In the present study, gerbils were deafened at postnatal day 9, an age at which there is no deafferentation-induced cell death of ventral cochlear nucleus neurons. One to five days after bilateral cochlear ablation, the amplitude of evoked excitatory postsynaptic currents (EPSC) was measured with whole-cell voltage-clamp recordings in an inferior colliculus (IC) brain slice preparation in response to electrical stimulation of the ipsilateral lateral lemniscus (LL) or the commissure of the inferior colliculus (CIC). Deafness resulted in larger LL- and CIC-evoked EPSC amplitudes and durations. This result was observed at a depolarized holding potential. In addition, deafness caused a decrease in excitatory neurotransmitter release at the LL pathway, as assessed with a paired-pulse stimulation protocol. In contrast to its effect on excitatory synapses, bilateral cochlear ablation reduced inhibitory synaptic strength in IC neurons. The effects included a postsynaptic decrease in IPSC conductance, a 25-mV depolarization in the IPSC equilibrium potential and a decrease of neurotransmitter release. Thus normal innervation differentially affects excitatory and inhibitory synaptic strength in IC neurons, and these changes may contribute to alterations in auditory coding properties following sensory deprivation.

Morphology, GluR1 and GRIP-C localization differ in octopus cells of C57BL6 and B6Cast mice

The C57BL6 mouse (B6) is homozygous for the gene for age-related hearing loss (ahl/ahl) and shows normal adult-like hearing before subtle changes in hearing begin at about 30 days of age. The B6Cast mouse is congenic to B6, having the wild type allele for normal hearing from Castaneous Ei on a B6 background. It has normal hearing throughout most of its lifespan. This study characterized the morphology of octopus cell (OC) somata in the posterior-ventral cochlear nucleus and of synaptic terminals on the OC somata in 8-week-old B6 and B6Cast mice, and the immunolocalization of antibodies to GluR1 (glutamate receptor subunit 1) and GRIP-C (glutamate receptor interacting protein-C terminus). By 8 weeks of age there are significant changes in the morphology of OCs and synaptic terminals around their somata in B6 mice compared to B6Cast mice. The distribution of immunoreactivity for the proteins GluR1 and GRIP is also significantly different in B6 mice from that in B6Cast mice. The modest degenerative changes reported in some B6 outer hair cells of the basal turn at this age do not seem adequate to explain the major changes observed in most OCs at a time when physiological studies show that many measures of the animals' hearing are still near normal. The findings suggest that changes in the alpha-amino-3-hydroxy-5-methyl-4-isoxazole glutamate receptor subunits and/or their binding proteins are part of the phenotype of ahl, and may reflect a role of the glutamate receptor pathway in the mechanism of ahl.

Precise inhibition is essential for microsecond interaural time difference coding

Microsecond differences in the arrival time of a sound at the two ears (interaural time differences, ITDs) are the main cue for localizing low-frequency sounds in space. Traditionally, ITDs are thought to be encoded by an array of coincidence-detector neurons, receiving excitatory inputs from the two ears via axons of variable length ('delay lines'), to create a topographic map of azimuthal auditory space. Compelling evidence for the existence of such a map in the mammalian lTD detector, the medial superior olive (MSO), however, is lacking. Equally puzzling is the role of a--temporally very precise glycine--mediated inhibitory input to MSO neurons. Using in vivo recordings from the MSO of the Mongolian gerbil, we found the responses of ITD-sensitive neurons to be inconsistent with the idea of a topographic map of auditory space. Moreover, local application of glycine and its antagonist strychnine by iontophoresis (through glass pipette electrodes, by means of an electric current) revealed that precisely timed glycine-controlled inhibition is a critical part of the mechanism by which the physiologically relevant range of ITDs is encoded in the MSO. A computer model, simulating the response of a coincidence-detector neuron with bilateral excitatory inputs and a temporally precise contralateral inhibitory input, supports this conclusion.

Age dependent changes in the medial nucleus of the trapezoid body in gerbils

The results of a quantitative light microscopic analysis of serial glycine immunoreacted sections through the medial nucleus of the trapezoid body (MNTB) of young and old gerbils are presented. Spongiform lesions were prominent in the MNTB of gerbils that were 3 years and older, but were virtually absent in animals below 1 year of age. In old animals the prevalence and density of spongiform lesions were most pronounced in the caudal MNTB and decreased towards the rostral MNTB. Total MNTB volume and rostro-caudal extent were independent of age and the cross-sectional area of MNTB varied in an identical fashion along the MNTB in young and old gerbils. Mean MNTB soma size (cross-sectional area) varied with the age of the animal. In young gerbils soma size increased between 1 and 6 months of age reaching a maximum near 160 microm(2). In old gerbils mean soma size was significantly reduced to 130 microm(2). At all three rostro-caudal positions analyzed along MNTB, soma size varied systematically being largest in the ventro-lateral and smallest in the dorso-medial part of MNTB. The reduction of soma size in old animals appeared uniform across MNTB.

Purkinje cell degeneration and control mice: responses of single units in the dorsal cochlear nucleus and the acoustic startle response

The cartwheel cell is the most numerous inhibitory interneuron of the dorsal cochlear nucleus (DCN). It is expected to be an important determinant of DCN function. To assess the contribution of the cartwheel cell, we examined the discharge characteristics of DCN neurons and behavioral measures in the Purkinje cell degeneration (pcd) mice, which lack cartwheel cells, and compared them to those of the control mice. Distortion product otoacoustic emissions and auditory brainstem-evoked response thresholds were similar between the two groups. Extracellularly recorded DCN single units in ketamine/xylazine-anesthetized mice were classified according to post-stimulus time histogram (PSTH) and excitatory-inhibitory response area (EI-area) schemes. PSTHs recorded in mouse DCN included chopper, pauser/buildup, onset, inhibited and nondescript types. EI-areas recorded included Types I, II, III, I/III, IV and V. There were no significant differences in the proportions of various unit types between the pcd and control mice. The pcd units had slightly lower thresholds to characteristic frequency tones; however, they had spontaneous rates, thresholds to noise, and maximum driven rates to noise that were similar to those of the control units. Pcd mice had smaller startle amplitudes, but startle latency, prepulse inhibition/augmentation and facilitation by a background tone were comparable between the two groups. From these results, we conclude that DCN function in response to relatively simple acoustic stimuli is minimally affected by the absence of the cartwheel cells. Future studies employing more complex and/or multimodal stimuli should help assess the role of the cartwheel cells.

Changes in cochlear electrical stimulation induced Fos expression in the rat inferior colliculus following deafness

Fos immunoreactive (IR) staining was used to examine changes in excitatory neuronal activity in the rat inferior colliculus (IC) between normal hearing and 21 day deaf rats evoked by basal or apical monopolar cochlear electrical stimulation. The location of evoked Fos IR neurons was consistent with expected tonotopic areas. The number of Fos IR cells increased as stimulation intensity increased in both normal and 21 day deaf animals. Stimulation at 1. 5x threshold evoked fewer Fos IR cells in 21 day deafened animals compared to normal hearing animals. At 5x and above, however, significantly increased numbers of Fos IR neurons (in a larger grouping) were evoked in 21 day deafened animals compared to normal hearing animals. Another group of animals had 7 days of deafness followed by 14 days of chronic basal cochlear electrical stimulation. In this group basal monopolar stimulation at 5x evoked not only a greater number of Fos IR neurons, compared to normal hearing animals, but the location of their grouping was slightly shifted to a more dorso-lateral region in the contralateral IC, compared to the normal hearing and 21 day deaf groups. These observations indicate that both deafness and chronic electrical stimulation may alter central auditory processing.

Age-related synaptic changes in sensorimotor cortex of the Brown Norway X fischer 344 rat

Investigations of age-related changes in synapse density have yielded contradictory conclusions. The goal of the present study was to determine whether there is a significant decline in the number of cortical synapses in old age. Therefore, brains from 10-, 15-, and 32-month-old Brown Norway X Fischer 344 rats were prepared for electron microscopy and synapses were counted in a stereotaxically-identified region of sensorimotor cortex. Within this cortical area, synapses were counted in layers 2 and 4 and the data have been presented both as number of synapses per volume of neuropil and as the ratio of synapses per neuron. Results indicated that there was a decline in synapse density between 15 and 32 months in layer 2, but not in layer 4. This decline was significant not only for total synapses, but also for subcategories of synapses when classified by presynaptic features or postsynaptic element. Specifically, there was a significant decline in presumptive inhibitory synaptic terminals, i.e., those containing nonround synaptic vesicles, as well as a significant decline in synapses that contact dendritic spines.

NMDAR1 isoforms in the rat superior olivary complex and changes after unilateral cochlear ablation

Normal expression and deafness related changes in expression of NMDAR1 isoforms were examined in the rat superior olivary complex (SOC) using in situ hybridization with S35 labeled oligoprobes. Expression was assessed in three SOC nuclei, the lateral and medial superior olives (LSO, MSO) and the medial nucleus of the trapezoid body (MNTB). Silver grain labeling over principal cells of each region was assessed using METAMORPH image analysis system. Counts were made in ipsi- and contralateral sides after unilateral cochlear ablation and in treated and untreated animals. In the normal SOC, NMDAR1a expression was higher than 1b and 1-2 expression was followed by 1-4 and 1-1, with 1-3 below the level for detection. The levels and ratio were comparable in LSO, MSO and MNTB. Five days after cochlear ablation 1a, 1-1, 1-2 and 1-4 showed significant decreases in the ipsilateral LSO and 1-a and 1-2 showed significant decreases in the contralateral MNTB, with no significant changes in the MSO. At 20 days after deafening, no significant changes were seen for any isoform in any nucleus. The transient deafness-induced decreases in expression of NMDAR1 isoforms correlate with loss of excitation.

Age-related changes in [3H]strychnine binding in the vestibular nuclei of rats

Glycine plays an important role as a neurotransmitter in the four vestibular nuclei (VN). The objective of this study was to determine if the levels of glycine-receptor binding in the VN change as a function of age. Quantitative receptor autoradiography was performed on brainstem sections from three age groups (3, 18 and 26 months) of Fischer 344 rats to assess binding in the VN. Glycine receptors were localized using [3H]strychnine binding. Strychnine binding declined monotonically with increasing age, such that the level of strychnine binding in each of the VN in the 28-month-old animals was approximately one-half that in the 3-month-olds. The age-related decrease in levels of strychnine binding suggest altered glycinergic function in the VN, which may in turn contribute to disturbances in equilibrium observed in the elderly.

The BALB/c mouse as an animal model for progressive sensorineural hearing loss

To develop the BALB/c mouse strain as an animal model for the study of progressive sensorineural hearing loss, mice ranging in age from young adult through middle age were studied. Auditory brainstem response thresholds, histopathology [cytocochleograms for hair cells, the packing density of spiral ganglion cells (SGCs), the number of neurons and overall size of the anterior ventral cochlear nucleus (AVCN)], and behavioral paradigms (prepulse inhibition, fear-potentiated startle) were compared with previous data from C57BL/6J (C57) and DBA/2J (DBA) mouse strains. Progressive high frequency hearing loss in BALB/c mice was generally more rapid than C57 and slower than DBA (e.g. mean thresholds for 16 kHz: 10-month-old BALB/c mice = 71 dB SPL; 55-day-old DBA mice = 79 dB SPL; 12-month-old C57 mice = 50 dB SPL). Like the other strains, BALB/c exhibited a progressive loss of hair cells and SGCs that was most severe in the cochlear base and least severe in the middle turns; however, BALB/c mice had relatively more SGC loss in the apex. Unlike C57 and DBA, no loss of neurons was observed in the AVCN following cochlear pathology (although AVCN volume was reduced). Like the other strains, successful fear conditioning was obtained with a 12 kHz conditioned stimulus. Prepulse inhibition showed that middle and low frequency tones (4-12 kHz) became more salient as high frequency hearing declined. Similar results had been previously obtained with C57 and DBA mice and were interpreted as reflecting hearing-loss-induced plasticity in the central auditory system.