A critical period during postnatal auditory development of mice

Increases in GFAP immunoreactive astrocytes in the cerebellar molecular layer of young adult CBA/J mice

Background

CBA/J mice are standard experimental animals in auditory studies, and age-related changes in auditory pathways are well documented. However, changes in locomotion-related brain regions have not been systematically explored.

Results

We showed an increase in immunoreactivity for glial fibrillary acidic protein (GFAP) in the cerebellar molecular layer associated with Purkinje cells in mice at 24 weeks of age but not in the younger mice. Increased GFAP immunoreactivity appeared in the form of clusters and distributed multifocally consistent with hyperplasia of astrocytes that were occasionally associated with Purkinje cell degeneration. Three out of 12 animals at 16 and 24 weeks of age exhibited pre-convulsive clinical signs. Two of these 3 animals also showed increased GFAP immunoreactivity in the cerebellum. Rotarod behavioral assessments indicated decreased performance at 24 weeks of age.

Conclusions

These results suggest minimal to mild reactive astrocytosis likely associated with Purkinje cell degeneration in the cerebellum at 24 weeks of age in CBA/J mice. These findings should be taken into consideration prior to using this mouse strain for studying neuroinflammation or aging.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42826-021-00100-5.

Extrinsic sound stimulations and development of periphery auditory synapses

The development of auditory synapses is a key process for the maturation of hearing function. However, it is still on debate regarding whether the development of auditory synapses is dominated by acquired sound stimulations. In this review, we summarize relevant publications in recent decades to address this issue. Most reported data suggest that extrinsic sound stimulations do affect, but not govern the development of periphery auditory synapses. Overall, periphery auditory synapses develop and mature according to its intrinsic mechanism to build up the synaptic connections between sensory neurons and/or interneurons.

A delicate balance: role of MMP-9 in brain development and pathophysiology of neurodevelopmental disorders

The extracellular matrix (ECM) is a critical regulator of neural network development and plasticity. As neuronal circuits develop, the ECM stabilizes synaptic contacts, while its cleavage has both permissive and active roles in the regulation of plasticity. Matrix metalloproteinase 9 (MMP-9) is a member of a large family of zinc-dependent endopeptidases that can cleave ECM and several cell surface receptors allowing for synaptic and circuit level reorganization. It is becoming increasingly clear that the regulated activity of MMP-9 is critical for central nervous system (CNS) development. In particular, MMP-9 has a role in the development of sensory circuits during early postnatal periods, called ‘critical periods.’ MMP-9 can regulate sensory-mediated, local circuit reorganization through its ability to control synaptogenesis, axonal pathfinding and myelination. Although activity-dependent activation of MMP-9 at specific synapses plays an important role in multiple plasticity mechanisms throughout the CNS, misregulated activation of the enzyme is implicated in a number of neurodegenerative disorders, including traumatic brain injury, multiple sclerosis, and Alzheimer’s disease. Growing evidence also suggests a role for MMP-9 in the pathophysiology of neurodevelopmental disorders including Fragile X Syndrome. This review outlines the various actions of MMP-9 during postnatal brain development, critical for future studies exploring novel therapeutic strategies for neurodevelopmental disorders.

The Structural Development of the Mouse Dorsal Cochlear Nucleus

The dorsal cochlear nucleus (DCN) is a major subdivision of the mammalian cochlear nucleus (CN) that is thought to be involved in sound localization in the vertical plane and in feature extraction of sound stimuli. The main principal cell type (pyramidal cells) integrates auditory and non-auditory inputs, which are considered to be important in performing sound localization tasks. This study aimed to investigate the histological development of the CD-1 mouse DCN, focussing on the postnatal period spanning the onset of hearing (P12). Fluorescent Nissl staining revealed that the three layers of the DCN were identifiable as early as P6 with subsequent expansion of all layers with age. Significant increases in the size of pyramidal and cartwheel cells were observed between birth and P12. Immunohistochemistry showed substantial changes in synaptic distribution during the first two postnatal weeks with subsequent maturation of the presumed mossy fibre terminals. In addition, GFAP immunolabelling identified several glial cell types in the DCN including the observation of putative tanycytes for the first time. Each glial cell type had specific spatial and temporal patterns of maturation with apparent rapid development during the first two postnatal weeks but little change thereafter. The rapid maturation of the structural organization and DCN components prior to the onset of hearing possibly reflects an influence from spontaneous activity originating in the cochlea/auditory nerve. Further refinement of these connections and development of the non-auditory connections may result from the arrival of acoustic input and experience dependent mechanisms.

Murine CMV-induced hearing loss is associated with inner ear inflammation and loss of spiral ganglia neurons

Congenital human cytomegalovirus (HCMV) occurs in 0.5–1% of live births and approximately 10% of infected infants develop hearing loss. The mechanism(s) of hearing loss remain unknown. We developed a murine model of CMV induced hearing loss in which murine cytomegalovirus (MCMV) infection of newborn mice leads to hematogenous spread of virus to the inner ear, induction of inflammatory responses, and hearing loss. Characteristics of the hearing loss described in infants with congenital HCMV infection were observed including, delayed onset, progressive hearing loss, and unilateral hearing loss in this model and, these characteristics were viral inoculum dependent. Viral antigens were present in the inner ear as were CD3+ mononuclear cells in the spiral ganglion and stria vascularis. Spiral ganglion neuron density was decreased after infection, thus providing a mechanism for hearing loss. The lack of significant inner ear histopathology and persistence of inflammation in cochlea of mice with hearing loss raised the possibility that inflammation was a major component of the mechanism(s) of hearing loss in MCMV infected mice.

Congenital infection with human cytomegalovirus (HCMV) is the most common viral infection of the fetus and occurs in 0.5–2.0% of all live births in most regions in the world. Infection of the fetus can result in a spectrum of end-organ disease, including long term damage to the central nervous system (CNS). Although less than 10% of infected infants exhibit clinical evidence of end-organ disease, up to 10% of the total number of infected infants develop hearing loss. Mechanisms of disease leading to hearing loss are poorly understood because of the limited availability of pathological specimens and accessibility of the inner ear. Existing small animal models fail to recapitulate many features of this infection of the inner ear. In this report we describe a mouse model in which newborn animals infected peripherally with murine CMV develop hearing loss following hematogenous spread of virus to the inner ear. Hearing loss occurs in 30–50% of animals and characteristics of hearing loss in infants with congenital HCMV infection, including delayed onset of hearing loss, progressive hearing loss, and unilateral hearing loss were present in infected mice. Our findings suggest that host derived inflammatory responses and not direct virus-mediated cytopathology are responsible for hearing loss. Findings from this study provide insight into potential mechanisms of hearing loss in infants with congenital HCMV infection.

Developmental plasticity of spatial hearing following asymmetric hearing loss: context-dependent cue integration and its clinical implications

Under normal hearing conditions, comparisons of the sounds reaching each ear are critical for accurate sound localization. Asymmetric hearing loss should therefore degrade spatial hearing and has become an important experimental tool for probing the plasticity of the auditory system, both during development and adulthood. In clinical populations, hearing loss affecting one ear more than the other is commonly associated with otitis media with effusion, a disorder experienced by approximately 80% of children before the age of two. Asymmetric hearing may also arise in other clinical situations, such as after unilateral cochlear implantation. Here, we consider the role played by spatial cue integration in sound localization under normal acoustical conditions. We then review evidence for adaptive changes in spatial hearing following a developmental hearing loss in one ear, and show that adaptation may be achieved either by learning a new relationship between the altered cues and directions in space or by changing the way different cues are integrated in the brain. We next consider developmental plasticity as a source of vulnerability, describing maladaptive effects of asymmetric hearing loss that persist even when normal hearing is provided. We also examine the extent to which the consequences of asymmetric hearing loss depend upon its timing and duration. Although much of the experimental literature has focused on the effects of a stable unilateral hearing loss, some of the most common hearing impairments experienced by children tend to fluctuate over time. We therefore propose that there is a need to bridge this gap by investigating the effects of recurring hearing loss during development, and outline recent steps in this direction. We conclude by arguing that this work points toward a more nuanced view of developmental plasticity, in which plasticity may be selectively expressed in response to specific sensory contexts, and consider the clinical implications of this.

Synaptic plasticity after chemical deafening and electrical stimulation of the auditory nerve in cats

The effects of deafness on brain structure and function have been studied using animal models of congenital deafness that include surgical ablation of the organ of Corti, acoustic trauma, ototoxic drugs, and hereditary deafness. This report describes the morphologic plasticity of auditory nerve synapses in response to ototoxic deafening and chronic electrical stimulation of the auditory nerve. Normal kittens were deafened by neonatal administration of neomycin that eliminated auditory receptor cells. Some of these cats were raised deaf, whereas others were chronically implanted with cochlear electrodes at 2 months of age and electrically stimulated for up to 12 months. The large endings of the auditory nerve, endbulbs of Held, were studied because they hold a key position in the timing pathway for sound localization, are readily identifiable, and exhibit deafness-associated abnormalities. Compared with those of normal hearing cats, synapses of ototoxically deafened cats displayed expanded postsynaptic densities, a 35.4% decrease in synaptic vesicle (SV) density, and a reduction in the somatic size of spherical bushy cells (SBCs). In comparison with normal hearing cats, ototoxically deafened cats that received cochlear stimulation had endbulbs that expressed postsynaptic densities (PSDs) that were statistically identical in size, showed a 48.1% reduction in SV density, and whose target SBCs had a 25.5% reduction in soma area. These results demonstrate that electrical stimulation via a cochlear implant in chemically deafened cats preserves PSD size but not other aspects of synapse morphology. This determination further suggests that the effects of ototoxic deafness are not identical to those of hereditary deafness.

Selective vulnerability of adult cochlear nucleus neurons to de-afferentation by mechanical compression

It is well established that the cochlear nucleus (CN) of developing species is susceptible to loss of synaptic connections from the auditory periphery. Less information is known about how de-afferentation affects the adult auditory system. We investigated the effects of de-afferentation to the adult CN by mechanical compression. This experimental model is quantifiable and highly reproducible. Five weeks after mechanical compression to the axons of the auditory neurons, the total number of neurons in the CN was evaluated using un-biased stereological methods. A region-specific degeneration of neurons in the dorsal cochlear nucleus (DCN) and posteroventral cochlear nucleus (PVCN) by 50% was found. Degeneration of neurons in the anteroventral cochlear nucleus (AVCN) was not found. An imbalance between excitatory and inhibitory synaptic transmission after de-afferentation may have played a crucial role in the development of neuronal cell demise in the CN. The occurrence of a region-specific loss of adult CN neurons illustrates the importance of evaluating all regions of the CN to investigate the effects of de-afferentation. Thus, this experimental model may be promising to obtain not only the basic knowledge on auditory nerve/CN degeneration but also the information relevant to the application of cochlear or auditory brainstem implants.

Auditory brainstem activity in children with 9–30 months of bilateral cochlear implant use

Bilateral cochlear implants aim to restore binaural processing along the auditory pathways in children with bilateral deafness. We assessed auditory brainstem activity evoked by single biphasic pulses delivered by an apical or basal electrode from the left, right and both cochlear implants in 13 children. Repeated measures were made over the first 9-30 months of bilateral implant use. In children with short or long periods of unilateral implant use prior to the second implantation, Wave eV of the auditory brainstem response was initially prolonged when evoked by the naïve versus experienced side. These differences tended to resolve in children first implanted <3 years of age but not in children implanted at older ages with long delays between implants. Latency differences were projected to persist for longer periods in children with long delays between implants compared with children with short delays. No differences in right versus left evoked eV latency were found in 2 children receiving bilateral implants simultaneously and their response latencies decreased over time. Binaural interaction responses showed effects of stimulating electrode position (responses were more detectable when evoked by an apical than basal pair of implant electrodes), and duration of delay between implants (measured by latency delays). The trends shown here suggest a negative impact of unilateral implant use on bilateral auditory brainstem plasticity.

Consequences of unilateral hearing loss: time dependent regulation of protein synthesis in auditory brainstem nuclei

Conductive hearing impairment results in marked changes in neuronal activity in the central auditory system, particularly in young animals [Tucci, D.L., Cant, N.B., Durham, D., 1999. Conductive hearing loss results in a decrease in central auditory system activity in the young gerbil. Laryngoscope 109, 1359-1371]. To better understand the effects of conductive hearing loss (CHL) on cellular metabolism, incorporation of (3)H-leucine was used as a measure of protein synthesis in immature postnatal day 21 gerbils subjected to either unilateral CHL by malleus removal or profound sensorineural hearing loss by cochlear ablation. (3)H-leucine uptake was measured after survival times of 6 or 48h. Protein synthesis values were standardized to measurements from the abducens nucleus and compared with measurements from sham animals at similar age/survival times. Protein synthesis in the medial superior olive (MSO) was found to be significantly down-regulated (bilaterally) after CHL in animals surviving 48h. However, 6h after CHL manipulation, protein synthesis is up-regulated in MSO (bilaterally) and in the ipsilateral medial nucleus of the trapezoid body.

Cochlear ablation in mice lacking SHP-1 results in an extended period of cell death of anteroventral cochlear nucleus neurons

Cochlear ablation results in the death of anteroventral cochlear nucleus (AVCN) neurons from birth to approximately postnatal day 14 (P14) in the murine brainstem. It is not known whether microglial activation contributes to AVCN neuronal death following deafferentation. In order to determine whether microglial activation helps to define the period of neuronal susceptibility within AVCN, we performed unilateral cochlear ablation on mice lacking the protein tyrosine phosphatase SHP-1 (me/me). These mice have been shown to have an exaggerated microglial response following ischemic injury. In the present study, the glial and neuronal response to deafferentation within AVCN was examined in wild-type and me/me mice at P5, P14, and P21. Lack of SHP-1 results in robust microglial but not astrocyte activation within the ablated P14 me/me AVCN. These mice also exhibit approximately 28% neuronal death at P14, a time when normal wild-type littermate controls show little cell death. Glial activation and neuronal loss at P5 and P21 were similar between the two phenotypes, suggesting a role of activated microglia in inducing neuronal death beyond P14 but not P21. These results indicate that activated microglia may participate in determining whether neurons in AVCN live or die following deafferentation.

Auditory system development: primary auditory neurons and their targets

The neurons of the cochlear ganglion transmit acoustic information between the inner ear and the brain. These placodally derived neurons must produce a topographically precise pattern of connections in both the inner ear and the brain. In this review, we consider the current state of knowledge concerning the development of these neurons, their peripheral and central connections, and their influences on peripheral and central target cells. Relatively little is known about the cellular and molecular regulation of migration or the establishment of precise topographic connection to the hair cells or cochlear nucleus (CN) neurons. Studies of mice with neurotrophin deletions are beginning to yield increasing understanding of variations in ganglion cell survival and resulting innervation patterns, however. Finally, existing evidence suggests that while ganglion cells have little influence on the differentiation of their hair cell targets, quite the opposite is true in the brain. Ganglion cell innervation and synaptic activity are essential for normal development of neurons in the cochlear nucleus.

Conductive hearing loss results in changes in cytochrome oxidase activity in gerbil central auditory system

Conductive hearing loss (CHL) restricts auditory input to an intact peripheral auditory system. Effects of deprivation on the central auditory system (CAS) have been debated, although a number of studies support the hypothesis that CHL can cause modification of CAS structure and function. The present study was designed to test the hypothesis that unilateral CHL results in a decrease in cytochrome oxidase (CO) activity in CAS nuclei that receive major afferent input from the affected ear. Gerbils at postnatal day 12 (P21) or 6-8 weeks underwent left unilateral CHL (malleus removal), cochlear ablation, or a sham surgical procedure. After a survival time of 48 hours or 3 weeks, animals were sacrificed and tissue was processed for cytochrome oxidase histochemistry. Optical density (OD) measurements were made from individual neurons in the anteroventral cochlear nucleus (AVCN) and from medial and lateral dendritic fields in the medial superior olivary nucleus (MSO), the lateral superior olivary nucleus, and the inferior colliculus. The width of the CO-stained neuropil in MSO was also measured as an estimate of dendritic length. OD measures were corrected to neutral areas of the brain. Cochlear ablation caused significant decreases in CO activity in left lower brainstem nuclei, particularly in adult animals. Following CHL, a significant decrease in CO activity was observed in the ipsilateral AVCN and a significant increase was observed in the contralateral AVCN. Cochlear ablation resulted in decreased width of MSO neuropil containing dendrites that receive primary input from the ablated ear. CHL resulted in a significant increase in the width of MSO neuropil on both sides of the brain in the P21 animals that survived 3 weeks but not in P21 animals that survived only 48 hours or in the adult animals. Unilateral CHL is associated with changes in CO activity in the AVCN and may affect MSO dendritic length in younger animals.

Effects of conductive hearing loss on auditory nerve activity in gerbil

The goal of this research was to define the effects of conductive hearing loss (CHL) produced by malleus removal (MR) on auditory nerve activity in the absence of acoustic stimulation. Subjects were gerbils in two age groups: 21 days (P21) and 42 days (P42). Ensemble background activity (EBA) of the auditory nerve was measured by spectral analysis of a signal recorded from the round window. The EBA measure was studied following MR in an acute setting in P42 animals, and in chronic experiments either 5 days (P21 and P42 animals) or 3 weeks (P21 animals) after MR. Findings were: (1) an EBA can be reliably measured in the gerbil, and this measure is highly stable over time; (2) the EBA is eliminated by application of tetrodotoxin, confirming the auditory nerve as the source of this measure; (3) MR results in a statistically significant decrease in the magnitude of the EBA in adult animals; and (4) the EBA measure is age-dependent. The EBA was significantly lower in P21 than in P42 animals, and response to MR varied as a function of age. We conclude that CHL decreases 'spontaneous' activity of the auditory nerve in young adult gerbils.

The Colorado newborn hearing screening project, 1992-1999: on the threshold of effective population-based universal newborn hearing screening

OBJECTIVE

Although previous studies have documented the feasibility and benefits of universal newborn hearing screening in selected hospitals, none have reviewed the effectiveness of regionally mandated participation of large numbers of hospitals with variable levels of motivation to succeed. The purpose of this study was to measure hospital participation and overall screening success in a statewide program for universal newborn hearing screening and to track improvements in program establishment and outpatient follow-up over time.

METHODS

Four Colorado hospitals began voluntarily performing hearing screening before hospital discharge on all newborns in 1992. By 1996, 26 Colorado hospitals were participating in universal newborn hearing screening. The publication of screening results from these early years served as a catalyst for legislation requiring increased hospital participation in establishing universal screening programs. Data systems were subsequently developed to improve statistical tracking and follow-up. Eight years' worth of cumulative study data as well as the results from calendar year 1999 (the year of greatest hospital participation) were reviewed for collective measures of successful screening and follow-up. Three hospitals did not initiate newborn hearing screening programs until after the study period ended in 1999. Of the 57 hospitals that were screening newborns in 1999, the chosen method of screening at 52 hospitals was automated auditory brainstem response testing; 3 hospitals used otoacoustic emission testing, and the remaining 2 hospitals used 2-stage screening. Hearing loss was defined as a threshold of 35 decibels or greater in 1 or both ears at the time of confirmatory testing.

RESULTS

During the full 8-year study period, 1992 to 1999, 148 240 newborns were screened. A total of 291 infants who were born during the study period received a diagnosis of congenital hearing loss. In this cohort of 291 children, the cumulative frequency of bilateral hearing loss was 71% (range: 48%-94% by calendar year), the frequency of sensorineural hearing loss was 82% (range: 67%-88%), and the frequency of 1 or more risk factors was 47% (range: 37%-61%). During calendar year 1999, a total of 63 590 births were recorded at 60 birthing hospitals in Colorado. The families of 263 (0.4%) of these newborns refused newborn hearing screening. Of the remaining 63 327 newborns, 87% (55 324 infants) were screened for hearing acuity before hospital discharge, a far greater percentage than the 19% of all newborns screened during the first 5 years of voluntary hospital participation, and approaching the American Academy of Pediatrics's recommendation of 95% of newborns completing hospital-based testing in a successful screening program. As a result of this statewide hearing screening program, congenital hearing loss was diagnosed in 86 Colorado newborns during 1999, representing an occurrence rate of approximately 1 affected child in every 650 newborns. In this group of 86 infants, 59 had bilateral sensorineural hearing loss, 17 had unilateral sensorineural hearing loss, 4 had bilateral conductive hearing loss, and 6 had unilateral conductive hearing loss. Mild hearing loss was present in 6 infants, moderate hearing loss was present in 42 infants, severe hearing loss was present in 33 infants, and profound hearing loss was present in the remaining 5 infants. Only 32 of the 86 affected newborns in 1999 had 1 or more risk factors for hearing loss subsequently identified. After failing an initial hospital-based screening at 1 of the 57 participating hospitals in 1999, 2.3% of infants screened (1283 newborns) were referred for follow-up testing, easily exceeding the standard of <4% recommended by the American Academy of Pediatrics. Similarly, the false-positive rate of 2.2% during 1999 exceeded the recommended standard of <3%. Of the infants who failed their initial screening, 76% (978 infants) had documented follow-up testing to confirm or exclude congenital hearing loss, a percentage significantly improved from a follow-up rate of 48% during the first 5 years of screening, although not yet achieving the standard of 95% recommended by the American Academy of Pediatrics. Nine participating hospitals, however, were able to document appropriate follow-up for 95% or more of the infants who failed their initial screening tests. The median age of diagnosis of congenital hearing loss during 1999 was 2.1 months; 71% of affected infants were identified by 3 months of age (the recommended standard for age of diagnosis), and 92% of affected newborns were identified by 5 months of age. Measures of screening success were compared for large, mid-sized, and small hospitals. Increasing hospital size, as measured by the number of births per year, was associated with an increasing percentage of newborns who were successfully screened. It was notable that smaller hospital size was associated with increased referral rates for follow-up testing, whereas larger hospital size was associated with the highest recapture rate for follow-up testing.

CONCLUSIONS

Universal screening for congenital hearing loss is demonstrated to be feasible in a large regional effort of legislatively mandated participation. The success of such an endeavor is dependent on educational efforts for community professionals, commitment on the part of program planners, and data systems that more accurately track and recall infants who fail initial hospital-based screening.

Effects of conductive hearing loss on gerbil central auditory system activity in silence

Animal models of conductive hearing loss (CHL) show altered structure and function in the central auditory system (CAS), particularly following unilateral deprivation. Assessment of neuronal activity as measured by 2-deoxyglucose (2-DG) uptake following CHL has been reported by two groups of investigators, with different findings. Woolf and colleagues [Brain Res. 274 (1983) 119] found that 2-DG uptake increased in the cochlear nucleus ipsilateral to the CHL, while Tucci et al. [Laryngoscope 109 (1999) 1359] found a decrease in 2-DG uptake in the ipsilateral cochlear nucleus. One significant difference between the protocols in the two studies was that, in the first study, animals were maintained in silence following 2-DG injection, whereas in the Tucci et al. study, animals were exposed to sound. The current study was designed to replicate the protocol used by Woolf et al. Young adult gerbils underwent unilateral malleus removal with bilateral canal ligation (n=6) or a sham procedure (n=7) 48 h prior to 2-DG administration and sacrifice. Optical density measurements were made from CAS nuclei. 2-DG uptake decreased in the ipsilateral cochlear nucleus and contralateral inferior colliculus, and in nuclei of the superior olivary complex bilaterally, supporting the finding that CHL is associated with a decrease in CAS neuronal activity.

Patterns of cell death in mouse anteroventral cochlear nucleus neurons after unilateral cochlea removal

Developmental changes that influence the results of removal of afferent input on the survival of neurons of the anteroventral cochlear nucleus (AVCN) of mice were examined with the hope of providing a suitable model for understanding the cellular and molecular basis for these developmental changes in susceptibility. We performed unilateral cochlear ablation on wild-type mice at a variety of ages around the time of hearing onset to determine developmental changes in the sensitivity of AVCN neurons to afferent deprivation. In postnatal day 5 (P5) mice, cochlea removal resulted in 61% neuronal loss in the AVCN. By age P14, fewer than 1% of AVCN neurons were lost after this manipulation. This reveals a rather abrupt change in the sensitivity to disruption of afferent input, a critical period. We next investigated the temporal events associated with neuron loss after cochlea removal in susceptible animals. We demonstrate that significant cell loss occurs within 48 hours of cochlea removal in P7 animals. Furthermore, evidence of apoptosis was observed within 12 hours of cochlea removal, suggesting that the molecular events leading to cell loss after afferent deprivation begin to occur within hours of cochlea removal. Finally, we began to examine the role of the bcl-2 gene family in regulating afferent deprivation-induced cell death in the mouse AVCN. AVCN neurons in mature bcl-2 knockout mice demonstrate susceptibility to removal of afferent input comparable to neonatal sensitivity of wild-type controls. These data suggest that bcl-2 is one effector of cell survival as these cells switch from afferent-dependent to -independent survival mechanisms.

Conductive hearing loss results in a decrease in central auditory system activity in the young gerbil

OBJECTIVES/HYPOTHESIS

The impact of childhood conductive HL (CHL) on development of auditory function has long been debated. The present study was conducted to define and compare the consequences of CHL and cochlear ablation (CA) in young and adult animals, using 2-deoxyglucose (2-DG) uptake as a measure of metabolic activity. It was hypothesized that, for both ages, CHL would result in a decrease in activity in the major ascending central auditory system pathway of the manipulated ear, but that this decrease would be significantly less than that observed with CA.

STUDY DESIGN

Sham-controlled study of metabolic effects of CHL during sound stimulation.

METHODS

Gerbils (aged 21 days or adult), underwent malleus removal, CA, or a sham procedure. Young animals survived either 48 hours or 3 weeks; adults survived 3 weeks. Each age/survival CHL group contained eight animals; otherwise, each group (CA and sham) contained five animals, for a total number of 54. At the appropriate survival time, animals were given an intracardiac injection of 14C-2-DG, and sacrificed under anesthesia after 45 minutes of exposure to normal laboratory sounds. Tissue sections were prepared for exposure to x-ray film for optical density measurements, and alternate sections stained for identification of nuclei. Measurements from auditory nuclei of experimental animals were corrected against an unaffected control area (abducens nucleus) and compared with measurements taken from animals in the sham group. Auditory evoked potential thresholds to both air- and bone-conducted stimuli were obtained in a second group of neonatal and adult animals.

RESULTS

Both CHL and CA resulted in a marked decrease in 2-DG uptake in the major ascending projection of the manipulated ear, in both the neonatal and adult animals. In young animals, effects of CHL and CA were similar. Effects of CHL in adult animals were less marked and significantly different from either effects of CHL in young animals or effects of CA in adult animals. HL following malleus removal only was purely conductive and ranged from 38 to 55 dB across frequency.

CONCLUSIONS

Results suggest that, particularly in young animals, a unilateral CHL may have profound effects on metabolic activity in the central auditory system.

The case for early identification of hearing loss in children. Auditory system development, experimental auditory deprivation, and development of speech perception and hearing

Human infants spend the first year of life learning about their environment through experience. Although it is not visible to observers, infants with hearing are learning to process speech and understand language and are quite linguistically sophisticated by 1 year of age. At this same time, the neurons in the auditory brain stem are maturing, and billions of major neural connections are being formed. During this time, the auditory brain stem and thalamus are just beginning to connect to the auditory cortex. When sensory input to the auditory nervous system is interrupted, especially during early development, the morphology and functional properties of neurons in the central auditory system can break down. In some instances, these deleterious effects of lack of sound input can be ameliorated by reintroduction of stimulation, but critical periods may exist for intervention. Hearing loss in newborn infants can go undetected until as late as 2 years of age without specialized testing. When hearing loss is detected in the newborn period, infants can benefit from amplification (hearing aids) and intervention to facilitate speech and language development. All evidence regarding neural development supports such early intervention for maximum development of communication ability and hearing in infants.

The efficacy of utilizing the P300 as a measure of auditory deprivation in monaurally aided profoundly hearing-impaired children

Auditory event-related responses have been successfully used to differentiate the effects of auditory deprivation in laboratory animals and could provide an alternative and less subjective method of testing auditory deprivation. The purpose of this study was to investigate the efficacy of using the P300 response to measure auditory deprivation in monaurally aided children with hearing impairment. Seventeen children (8 female, 9 male) aged 9 to 18 years (M = 13.1 years) participated in the study. Ten subjects with bilateral congenital severe to profound sensorineural hearing losses (M = 87 dB) were recruited from a residential school for the deaf. All had worn monaural amplification for at least 8 years. Seven children with normal hearing sensitivity were matched for age effects. A two-tone auditory oddball paradigm was utilized to elicit the P300 responses. Statistical analysis revealed that the P300 amplitude was significantly greater in the aided ear compared to the unaided ear of the hearing-impaired group (p < 0.05). Two unaided ears of this group had no measurable responses. No other significant latency or amplitude differences were found within or between groups. Results of this investigation serve to reconfirm that binaural amplification should routinely be recommended for hearing-impaired children.

Observations on the Use of SCAN to Identify Children at Risk for Central Auditory Processing Disorder

The SCAN: A Screening Test for Auditory Processing Disorders was designed for administration in a quiet school test setting, although it is also administered by audiologists in the audiometric booth in a study seeking to screen for the presence of central auditory processing disorder (CAPD) in children with a history of otitis media (OM), 14 children with a history of OM and an equal number without this risk factor for CAPD (non-OM) were tested in a school setting using the SCAN and the Peabody Picture Vocabulary Test-Revised (PPVT-R). Forty-three percent of the OM group failed the SCAN, as did 29% of the non-OM group; hence, the groups were not differentiated using the SCAN. To prove the effects of environment on test results, a second experiment was conducted in which six additional children were administered the SCAN in both a school setting and an audiometric test booth. Individual data revealed that subjects performed more poorly on the SCAN administered in the school setting than in audiometric test booth. The marked difference in SCAN scores between the two environments raises methodological concerns regarding the use of this instrument for indentification of children at risk for central auditory processing disorders.

Auditory brainstem responses of CBA/J mice with neonatal conductive hearing losses and treatment with GM1 ganglioside

Exogenous administration of GM1 ganglioside to CBA/J mice with a neonatal conductive hearing loss ameliorates the atrophy of spiral ganglion neurons, ventral cochlear nucleus neurons, and ventral cochlear nucleus volume. The present investigation demonstrates the extent of a conductive loss caused by atresia and tests the hypothesis that GM1 ganglioside treatment will ameliorate the conductive hearing loss. Auditory brainstem responses were recorded from four groups of seven mice each: two groups received daily subcutaneous injections of saline (one group had normal hearing; the other had a conductive hearing loss); the other two groups received daily subcutaneous injections of GM1 ganglioside (one group had normal hearing; the other had a conductive hearing loss). In mice with a conductive loss, decreases in hearing sensitivity were greatest at high frequencies. The decreases were determined by comparing mean ABR thresholds of the conductive loss mice with those of normal hearing mice. The conductive hearing loss induced in the mice in this study was similar to that seen in humans with congenital aural atresias. GM1 ganglioside treatment had no significant effect on ABR wave I thresholds or latencies in either group.

Deafness induced cell size changes in rostral AVCN of the guinea pig

The right cochleae of 250-350 g guinea pigs were lesioned by topical administration of neomycin in the middle ear cavity. Eight weeks after the lesion, the cochleae and cochlear nuclei were analyzed. Cochlear hair cell loss was assessed, and cell areas of spherical bushy cells in the rostral anteroventral cochlear nucleus (AVCN) were compared between the lesioned and normal hearing sides for each animal. In five animals with both inner and outer hair cell loss in the lesioned cochlea, the average area of neuronal somata in the rostral AVCN in the lesioned side was 22% smaller than the average area of these cells in the normal hearing side. In two animals with outer hair cell loss but inner hair cells remaining, there was no difference in cell size between the lesioned and non-lesioned AVCN. These results provide evidence that there is significant shrinkage in AVCN cell size in the mature mammal after hearing loss associated with inner hair cell loss.

Exogenous GM1 ganglioside effects on conductive and sensorineural hearing losses

Both CBA/J mice with neonatal cochlea removals and CBA/J mice with neonatal atresias of the external auditory meatus have significantly smaller ventral cochlear nucleus is greater in the mice with cochlea removals, but the soma area reduction is greater in the mice with external auditory meatus atresias. GM1 gangliosides were subcutaneously injected daily into a group of CBA/J mice with conductive hearing loss caused by neonatal removals of their left external auditory meatus, and into a group of CBA/J mice unilaterally deafened by left cochlea removals. In the mice with conductive hearing loss, the ganglioside treatment significantly ameliorated the atrophy of spiral ganglion neurons, ventral cochlear nucleus neurons, and ventral cochlear nucleus volume. In unilaterally deafened mice, the ganglioside treatment had no measurable effect on the atrophy of ventral cochlear nucleus neurons or of ventral cochlear volume. It is suggested that GM1 ganglioside treatment potentiates growth factors which sustain spiral ganglion integrity and that this sustained activity of the spiral ganglion in turn maintains the integrity of the cochlear nuclei.

Changes in the cat cochlear nucleus following neonatal deafening and chronic intracochlear electrical stimulation

The effects of chronic intracochlear electrical stimulation on the cochlear nucleus (CN) were studied in eight cats that were neonatally deafened by daily intramuscular injections of neomycin. Profound hearing loss was confirmed in each animal by auditory brainstem response (ABR) and frequency following response (500 Hz) testing. Five of the kittens were implanted unilaterally with a scala tympani electrode array at ages 8-16 weeks. These kittens were stimulated daily for four hours at 2 dB above the evoked ABR threshold, over a period of three months, and subsequently euthanized for histological analysis at 26-32 weeks of age. The three remaining deaf kittens were maintained without stimulation over prolonged periods in order to study the long-term consequences of neonatal deafening, and were euthanized at 66-133 weeks of age. This study compares the CN of these deafened experimental animals and the CN of normal adult cats. Three experimental parameters were examined: CN volume, cross-sectional area of spherical cells in the rostral anteroventral cochlear nucleus (AVCN), and spherical cell density in this same region. The CN in animals that received electrical stimulation showed significant bilateral degenerative changes in all three measured parameters. Total nuclear volume was reduced by 35-36%, spherical cell size was reduced by 20-26%, and spherical cell density decreased by 36-42%, as compared to the normal cat CN. Comparisons were also made in the stimulated animals between CN ipsilateral to the stimulated cochlea and the contralateral, unstimulated CN.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroanatomical alterations in the rat nucleus of the solitary tract following early maternal NaCl deprivation and subsequent NaCl repletion

Restricting the NaCl content in the rat maternal and preweaning diet results in a significant and specific reduction (60%) of chorda tympani nerve responsiveness to sodium stimuli in the offspring. Repletion of dietary sodium at any time during postnatal development results in a complete and persistent recovery of chorda tympani nerve function. To learn whether the maturation of postsynaptic cells are also affected by the early dietary manipulation, dendritic morphology, neuronal and glial densities and numbers were studied within the area of the nucleus of the solitary tract (NTS). Examination of dendritic morphologies in Golgi-Cox stained neurons revealed that cells with multipolar and fusiform somata in the rostral NTS exhibited longer dendrites following dietary NaCl deprivation during development (deprived rats) than in controls. These changes were generally maintained in rats initially deprived of NaCl and then fed a NaCl-replete diet postweaning ("recovered" rats). In contrast, ovoid neurons were not affected by NaCl deprivation but had increases in the lengths of their dendrites following "recovery." Along with dendritic alterations, the packing density of neurons in the rostral NTS was greater in NaCl-deprived rats than in controls, but was similar to controls following "recovery." Glial packing density also increased following deprivation and remained high in "recovered" rats. These results indicate that activity-dependent events as well as events not related to afferent activity (e.g., hormonal changes) may influence the morphological development of NTS neurons. In addition, significant interactions among primary afferent fibers, central neurons, and glia may direct development within the central gustatory system.

A quantitative method for measuring regional in vivo fatty-acid incorporation into and turnover within brain phospholipids: review and critical analysis

An experimental method and its associated mathematical model are described to quantitate in vivo incorporation rates into and turnovers of fatty acids (FAs) within stable brain metabolic compartments, particularly phospholipids. A radiolabeled FA is injected i.v. in a rat, and arterial plasma unacylated FA radioactivities and unlabeled concentrations are sampled until the animal is killed after 15 min, when the brain is analyzed biochemically or with quantitative autoradiography. Unbound unacylated label in blood easily crosses the blood-brain barrier; rapidly equilibrates in the unacylated FA, acyl-CoA and phosphatidate-diacylglycerol brain pools; then is incorporated into phospholipids and other stable metabolic compartments. Uptake and incorporation of labeled FAs are independent of cerebral blood flow at constant brain blood volume. Different labeled FAs enter specific sn positions of different brain phospholipids, suggesting that a combination of probes can be used to investigate metabolism of these phospholipids. Thus, [9,10-3-H]palmitate preferentially labels the sn1 position of phosphatidylcholine; [1-14C]arachidonate the sn2 positions of phosphatidylinositol and phosphatidylcholine; and [1-14C]docosahexaenoate the sn2 positions of phosphatidylethanolamine and phosphatidylcholine. The FA model provides an operational equation for rates of incorporation of FAs into brain phospholipids, taking into account intracerebral recycling and de novo synthesis of the FA, as well as entry into brain of FA from acylated blood sources. The equation is essentially independent of specific details of the proposed model, and can be used to calculate turnovers and half-lives of FAs within different phospholipid classes. For the model to be most applicable, experiments should satisfy conditions for pulse-labeling of the phospholipids, with brain sampling times short enough to minimize exchange of label between stable metabolic compartments. A 15-20 min sampling time satisfies these criteria. The FA method has been used to elucidate the dynamics of brain phospholipids metabolism in relation to brain development, brain tumor, chronically reduced auditory input, transient ischemic insult, axotomy with and without nerve regeneration, and cholinergic stimulation in animals with or without a chronic unilateral lesion of the nucleus basalis magnocellularis.

Prolonged conductive hearing loss in rat pups causes shorter brainstem transmission time

A carefully controlled study was conducted in rats to determine whether a reversible conductive hearing loss during the neonatal period could induce changes in central conduction and thereby perhaps contribute to an understanding of learning problems seen in children following conductive hearing loss in infancy. Ear plugs were inserted from post-natal day 9 in rat pups and auditory nerve brainstem evoked responses were recorded in the presence of ear plugs and following their removal on post-natal day 23. The I-IV interpeak latency (brainstem transmission time) was significantly shorter in the experimental rats several days after plug removal (day 28) compared to untreated control rats. This difference was not related to residual conductive loss, nor to different body weights and was present even in response to equal sensation level stimuli, and was not present in adult rats with similarly induced conductive hearing losses. It seems therefore that conductive hearing loss in the young animal (critical period?) can induce changes in central auditory conduction and may be related to the findings of smaller brainstem auditory neurons in sound deprived animals. These results have implications for neonatal hearing loss in humans.

Effects of neonatal deafening and chronic intracochlear electrical stimulation on the cochlear nucleus in cats

Four newborn kittens were deafened by daily intramuscular injections of neomycin sulfate, beginning the day after birth and continuing for 14-16 days. At 10-16 weeks of age the deaf kittens were implanted unilaterally with a four wire intracochlear electrode array. The animals were stimulated daily (starting at 13-18 weeks of age), for a period of one hour, at 6 dB above the electrically evoked auditory brainstem response threshold. After 3 months of chronic intracochlear electrical stimulation, animals were studied in acute electrophysiological experiments and euthanized for histological studies. This study compares the stimulated and control cochlear nuclei (CN) of these deafened animals to the CN of four normal adult cats. Statistical comparisons of spherical cell densities in the anteroventral cochlear nucleus (AVCN), cross-sectional spherical cell areas, and volumes of the cochlear nucleus subdivisions were included in the analysis. The results indicate that, by all of these measures, the cochlear nuclei in neonatally deafened animals were significantly different from the cochlear nuclei of control animals. As a result of deafening, the density of spherical cells was decreased by 30%, the cross-sectional areas of spherical cells were reduced by 20%, and the volume of the cochlear nucleus was reduced by 25%. These changes were observed in both cochlear nuclei (ipsilateral to both stimulated and unstimulated ears) of the deafened animals. With the measures employed, no significant difference was demonstrated in comparisons between the deafened/unstimulated and the deafened/stimulated cochlear nuclei. That is, no reversal of the profound effects of deafening was observed in the cochlear nuclei as a consequence of chronic intracochlear electrical stimulation which was begun 11 to 16 weeks after deafening.

Neonatal conductive hearing loss does not compromise brainstem auditory function and structure in rhesus monkeys

The effect of conductive hearing loss on the maturation of the auditory pathway was evaluated using the auditory brainstem response (ABR) in rhesus monkeys. Ten newborn rhesus monkeys were assigned to control (N = 4), unilateral hearing loss (N = 3), or bilateral hearing loss (N = 3) groups. Hearing loss was created by surgically excising a 3 mm section of the external auditory canal and suturing the canal. Auditory brainstem responses to click stimuli were recorded prior to and after the surgical procedure and bi-monthly or monthly for a 14 month follow-up period. Results showed that after surgery all ears developed an estimated 30-50 dB conductive hearing loss which was retained throughout the follow-up period. Contrary to expectations, the latencies of the ABR component waves decreased with age in all ears. When adjusted for hearing level, there were no differences between ears in maturation of the component waves of the ABR. These data suggest that, in primates, a conductive hearing loss does not affect the maturation of those aspects of the auditory pathway reflected in the ABR. Furthermore, the conductive losses were not accompanied by any discernible change in the neuronal sizes of brainstem auditory neurons or the volume of the cochlear nuclei.

Effects of profound sensorineural loss on gerbilline auditory encephalopathy

To investigate the role of acoustic stimulation in the development of spongiform degeneration in the cochlear nuclei of Mongolian gerbils, the right cochlea in 8 juvenile gerbils was chemically treated by placing sodium chloride (NaCl) crystals on the cochlear round window membrane. Sixty days after NaCl treatment there was extensive damage to the strial, sensorineural and supporting cells of the treated inner ear. The cochlear damage was accompanied by a dramatic decrease in the number and the extent of the spongioid lesions in the ipsilateral cochlear nuclei compared to the contralateral (control) cochlear nuclei. These results lend further support to the hypothesis that the progress of this disorder is related to auditory function.

A method for examining turnover and synthesis of palmitate-containing brain lipids in vivo

1. A theoretical three compartment model is presented which gives the rate of incorporation of plasma palmitate into brain, Jpalm, in terms of turnover and synthesis of palmitate-containing lipids, de novo synthesis of palmitate from acetate, and recycling of palmitate within lipids. 2. Jpalm equals 4 h brain radioactivity following intravenous injection of [U-14C]-palmitate (determined with quantitative autoradiography), divided by integrated plasma specific activity of palmitate. Jpalm follows the time course of brain lipid synthesis during development of the rat, but is age-invariant in the adult. 3. At 1-7 days after 5 min of bilateral carotid occlusion in the awake gerbil, intravascular [14C]-palmitate incorporation is reduced in the CA1 pyramidal layer of the hippocampus, consistent with delayed neuronal death, but is elevated in the CA3 and CA4 pyramidal layers and dentate gyrus, suggesting synthesis of new membrane during recovery from the ischaemic insult. 4. Several weeks after unilateral destruction of the cochlea in 11 day old rats, incorporation of [14C]-palmitate from plasma into appropriate central auditory regions is reduced, corresponding to reduced cell size and altered morphology. 5. [14C]-palmitate incorporation into the left hypoglossal nucleus is increased during and following axonal regeneration (up to 23% compared with control side) following transection of the left hypoglossal nerve in Fischer-344 rats, whereas incorporation is decreased 6-7% when regeneration is prevented. Time courses of incorporation in both cases correspond to histological changes. 6. The results show that the palmitate method can be used to examine regional turnover and synthesis of brain lipids following injury, sensory deprivation, development, regeneration and ageing.

The effects of auditory deprivation on morphological maturation of the ventral cochlear nucleus

The volumes of the auditory brainstem nuclei and age-related auditory brainstem response (ABR) thresholds were analyzed in homozygote (je/je) and heterozygote (je/+) jerker mutant mice. Altogether 97 mice were used in the study. Je/je mice never develop any hearing. The dorsal (DCN) and ventral (VCN) cochlear nuclei were found to have stopped their growth at 56 days after birth. In je/+ mutants, ABR thresholds remained normal or near-normal for 3-6 months, whereas VCN and DCN volumes remained unchanged at least after 56 days after birth. There is no significant difference in DCN volume in je/je and je/+ mice. However, the VCN volume and the cross-sectional area of globular cells were both significantly larger in je/+ than in je/je mice (P less than 0.01). These findings show that auditory deprivation during the maturation of hearing in je/je mutants causes an incomplete maturation of only the ventral cochlear nucleus.

Regional cerebral palmitate incorporation after unilateral auditory deprivation in immature and adult Fischer-344 rats

Regional cerebral incorporation of intravenously injected [U-14C]palmitate was measured from 1 day to 13 weeks after left cochlear destruction in 11-day- and 3 month-old, awake Fischer-344 rats. In 11-day-old animals, statistically significant left-right differences in incorporation were absent 1 day after cochlear destruction and were found only in parts of the cochlear nucleus and inferior colliculus after 1 week. After 6 to 13 weeks, consistent with functional neuroanatomy of central auditory regions, incorporation was reduced by 6 to 9% in the left cochlear nucleus and left lateral superior olivary nucleus, compared with corresponding right-side regions. The right medial superior olivary nucleus, medial nucleus of the trapezoid body, lateral lemniscus nucleus, inferior colliculus, medial geniculate body, and auditory cortex had 5 to 9% less incorporation than did corresponding left-side regions. Fewer significant differences after chronic auditory deprivation occurred in 3-month-old rats than in 11-day-old rats following cochlear destruction. Reduced incorporation corresponded to reported changes in cell morphology, which also were greater in immature than mature rats following auditory deprivation. The results suggest that the palmitate method can be used to identify long-term regional changes in the turnover of brain lipids after sensory deprivation.

Reversible conductive hearing loss in mice

Fibrin tissue adhesive was injected into the right ears of four 7-week-old CBA/J mice. Auditory brain stem responses (ABRs) were used to monitor changes in auditory sensitivity over the next 26 days, after which the middle ears and cochleas were examined histologically. Mean maximum ABR threshold shifts were as great as 36 dB, and were larger for higher frequencies than for lower. Maximum shifts occurred between 1 and 8 days; by 26 days, thresholds had returned to intensity levels observed before injection.

Stimulation-dependent development of neuronal cytoplasm in mouse cochlear nucleus

The extent of neuronal development in the mouse ventral cochlear nucleus was examined in normal and developmentally auditory deprived mice. Mice were unilaterally deprived on postnatal day three by external auditory meatus removal and sacrificed with controls on day 45, which is after the developmental period. Light microscopic morphometry of neuronal nucleus and cytoplasm areas demonstrated that the normal spherical and globular cells were larger in their low frequency regions than in their high frequency regions. This size difference occurred mainly in the cytoplasm. Developmental deprivation reduced neuronal cytoplasm areas similarly in the high and low frequency regions of both neuronal types, but had no effect on the size of the neuronal nucleus. It was interpreted that cytoplasmic metabolic events are dependent on developmental levels of stimulation and high and low frequency regions normally are differentially stimulated. Furthermore, these stimulation-dependent cytoplasmic events are impaired by developmental hypostimulation, which retards neuronal metabolism and growth. No changes were observed in the cochlear nucleus contralateral to the deprived side, suggesting that compensatory changes, such as hypertrophy, did not occur.

Conductive hearing loss affects the growth of the cochlear nuclei over an extended period of time

During normal growth in CBA/J mice, the volume of dorsal and ventral cochlear nuclei change very little between 1 and 3 days of age; then more than double between 6 and 12 days of age. After 12 days, the rate of growth declines, but growth continues through at least 90 days. The globular cells of the ventral cochlear nucleus also double their soma areas between 6 and 12 days, but then grow no more. The number of ventral cochlear nucleus neurons containing Nissl substance doubles between 6 and 12 days of age and then remains stable. This increase in neuronal numbers is probably caused by differentiation of neuroblasts into neurons, not by mitoses. Conductive losses from 4 to 45 days, and from 24 to 45 days, both result in reduced volume of the ventral cochlear nucleus, but have no effect on the volume of the dorsal cochlear nucleus. Globular cell area is affected by a conductive loss from 4 to 45 days of age, but not by a conductive loss from 24 to 45 days. Therefore, conductive losses affect neuropil growth beyond the time when soma size is no longer affected by these losses.

Sound amplification negates central effects of a neonatal conductive hearing loss

Neonatal CBA/J mice with external auditory meati removed were raised in a sound-amplified environment. If amplification continued until sacrifice at 24 or 45 days of age, both cochlear nuclear volumes and cross-sectional areas of VCN globular cells were of normal size; without amplification both measurements were significantly reduced.

Morphological and electrophysiological study of the inner ear and the central auditory pathways following whole body fetal irradiation

Pregnant CBA/CBA mice were whole body irradiated with 2 Gy on the 13th or 16th day of gestation, respectively. The exposed fetuses were raised to an age of 21 postnatal days. Auditory brainstem recordings of threshold levels showed a considerable elevation independent of if irradiation had been performed on either the 13th gestational day or the 16th gestational day. In exposed animals a latency difference occurs in the peaks that increases from peak 1 to peak 5, measuring in peak 5 up to 1.16 ms. Also the peak-to-peak length of waves 1-5 increases in irradiated animals. Scanning electron microscopy of the cochleae showed varying degrees of stereociliary derangement of both outer and inner hair cells, particularly in cochleae where irradiation had been performed on the 13th gestational day, but not loss of hair cells. Light microscopic analysis of auditory brainstem nuclei revealed normal conditions except that in inner ears exposed on the 16th gestational day the flocculus was fused to the lateral surface of the anterior ventral cochlear nucleus. It is concluded that the elevated threshold levels in irradiated animals are most likely due to pathological changes in the peripheral receptor organ whereas the increased latencies and the increased peak-to-peak length likewise reflect functional changes in the brainstem auditory nuclei.

Incomplete maturation of brainstem auditory nuclei in genetically induced early postnatal cochlear degeneration

In the Shaker-2 mouse mutant, a cochleo-saccular type of genetically induced inner ear degeneration occurs. Morphological signs of degeneration are evident in the 3rd postnatal week and a severe and almost total degeneration has occurred by the age of 6-9 weeks. There are no qualitative differences between the brain of the normal CBA/J mouse and that of the Shaker-2 mouse. The growth of the auditory brainstem nuclei (dorsal cochlear nucleus and ventral cochlear nucleus) in the mutant Sh-2 mouse has stopped by 14 days of age--but not in the normal CBA/J mouse. The brainstem as a whole continues to grow between 14 and 140 days of age in both strains. The early cochlear degeneration coincides with the critical period when a normal input from the peripheral receptor organ is necessary for the maturation of the central auditory pathway.

Permanent defects in rat peripheral auditory function following perinatal hypothyroidism: determination of a critical period

Rats were treated with propylthiouracil (PTU) for 10-day periods beginning at different ages. Daily injections of L-thyroxine (T4) were administered concurrently with PTU to a group of rats which served as one control group. Peripheral auditory function was evaluated by the brainstem response audiometry (BSRA) technique performed at 12, 16, 25 and 120 days of age. PTU treatment significantly increased wave I latency (cochlear nerve compound action potential) in adult rats when administered from 3 days before delivery through 6 days of age, but was without permanent effect (wave I latencies and thresholds) when administered for 10 days starting at 10 days after birth. T4 replacement during the first 10 postnatal days prevented permanent abnormalities. These data suggest that the period of greatest vulnerability to thyroid hormone depletion in the peripheral auditory system extends from at least 3 days before delivery through between 5 and 10 days of age.

Afferent influences on brain stem auditory nuclei of the chicken: effects of conductive and sensorineural hearing loss on n. magnocellularis

Nucleus magnocellularis is the avian homologue of the spherical cell region of the mammalian anteroventral cochlear nucleus. Its primary excitatory synaptic input is from large end bulbs of Held from the eighth nerve ganglion cells. We have examined the effects of three peripheral manipulations--middle ear ossicle (columella) removal (monaural and binaural), columella removal and oval window puncture (monaural), and monaural earplug--on cross-sectional cell area ("cell size") of second-order auditory neurons in n. magnocellularis of the chicken. Manipulations were performed between embryonic day 19 and posthatch day 4. Survival time was varied from 2 to 60 days. Air conduction and bone conduction thresholds were determined to assess for conductive and sensorineural hearing loss associated with each of these manipulations. Hair cell counts were made from basilar papillae of each experimental group. We found that a columella removal alone, which produced a 50-55-dB purely conductive hearing loss, was not associated with changes in cell size of n. magnocellularis neurons. Similarly, chronic monaural earplugging did not affect the cross-sectional area of these second-order auditory neurons. Conversely, a combined columella removal and oval window puncture, which produced a mixed hearing loss with a 15-40-dB sensorineural component was associated with an 18-20% reduction in n. magnocellularis cell area. Hair cell counts for experimental ears were not significantly different from control ears. These results, in conjunction with measurements of multiunit activity recorded in n. magnocellularis, suggest that manipulations which markedly attenuate extrinsic auditory stimulation, but do not result in chronic change in the average activity levels, also do not influence the size of n. magnocellularis cell bodies. On the other hand, a manipulation which influences overall activity levels, but does not result in degeneration of receptor cells, resulted in marked changes in n. magnocellularis cell size.