Anatomical evidence of synaptic plasticity in the cochlear nuclei of white-deaf cats

The effect of progressive hearing loss on the morphology of endbulbs of Held and bushy cells

Studies of congenital and early-onset deafness have demonstrated that an absence of peripheral sound-evoked activity in the auditory nerve causes pathological changes in central auditory structures. The aim of this study was to establish whether progressive acquired hearing loss could lead to similar brain changes that would degrade the precision of signal transmission. We used complementary physiologic hearing tests and microscopic techniques to study the combined effect of both magnitude and duration of hearing loss on one of the first auditory synapses in the brain, the endbulb of Held (EB), along with its bushy cell (BC) target in the anteroventral cochlear nucleus. We compared two hearing mouse strains (CBA/Ca and heterozygous shaker-2^+/-) against a model of early-onset progressive hearing loss (DBA/2) and a model of congenital deafness (homozygous shaker-2^-/-), examining each strain at 1, 3, and 6 months of age. Furthermore, we employed a frequency model of the mouse cochlear nucleus to constrain our analyses to regions most likely to exhibit graded changes in hearing function with time. No significant differences in the gross morphology of EB or BC structure were observed in 1-month-old animals, indicating uninterrupted development. However, in animals with hearing loss, both EBs and BCs exhibited a graded reduction in size that paralleled the hearing loss, with the most severe pathology seen in deaf 6-month-old shaker-2^-/- mice. Ultrastructural pathologies associated with hearing loss were less dramatic: minor changes were observed in terminal size but mitochondrial fraction and postsynaptic densities remained relatively stable. These results indicate that acquired progressive hearing loss can have consequences on auditory brain structure, with prolonged loss leading to greater pathologies. Our findings suggest a role for early intervention with assistive devices in order to mitigate long-term pathology and loss of function.

Development of brainstem-evoked responses in congenital auditory deprivation

To compare the development of the auditory system in hearing and completely acoustically deprived animals, naive congenitally deaf white cats (CDCs) and hearing controls (HCs) were investigated at different developmental stages from birth till adulthood. The CDCs had no hearing experience before the acute experiment. In both groups of animals, responses to cochlear implant stimulation were acutely assessed. Electrically evoked auditory brainstem responses (E-ABRs) were recorded with monopolar stimulation at different current levels. CDCs demonstrated extensive development of E-ABRs, from first signs of responses at postnatal (p.n.) day 3 through appearance of all waves of brainstem response at day 8 p.n. to mature responses around day 90 p.n.. Wave I of E-ABRs could not be distinguished from the artifact in majority of CDCs, whereas in HCs, it was clearly separated from the stimulus artifact. Waves II, III, and IV demonstrated higher thresholds in CDCs, whereas this difference was not found for wave V. Amplitudes of wave III were significantly higher in HCs, whereas wave V amplitudes were significantly higher in CDCs. No differences in latencies were observed between the animal groups. These data demonstrate significant postnatal subcortical development in absence of hearing, and also divergent effects of deafness on early waves II–IV and wave V of the E-ABR.

Synaptic morphology and the influence of auditory experience

The auditory experience is crucial for the normal development and maturation of brain structure and the maintenance of the auditory pathways. The specific aims of this review are (i) to provide a brief background of the synaptic morphology of the endbulb of Held in hearing and deaf animals; (ii) to argue the importance of this large synaptic ending in linking neural activity along ascending pathways to environmental acoustic events; (iii) to describe how the re-introduction of electrical activity changes this synapse; and (iv) to examine how changes at the endbulb synapse initiate trans-synaptic changes in ascending auditory projections to the superior olivary complex, the inferior complex, and the auditory cortex.

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.

[Early hearing experience and sensitive developmental periods]

This article reviews the studies on functional deficits in the auditory cortex of congenitally deaf animals. It compares their results with psychophysical and imaging data obtained from prelingually deaf humans. The studies demonstrate that the development of the auditory cortex is affected by the absence of hearing experience. In humans, the restoration of hearing after congenital deafness shows a sensitive period of 4 years, whereas even within this sensitive period cortical plasticity is already decreasing with increasing age. The reasons for the sensitive period are developmental changes of synaptic plasticity, developmentally modified synaptogenesis and synaptic pruning as well as changes in connectivity of the auditory cortex. Absence of top-down interactions from higher order auditory areas is another cardinal reason for the sensitive period. All these mechanisms contribute to the decreasing capacity for cortical plasticity during postnatal development. From the developmental and neurophysiological point of view, an early identification of hearing loss is an important prerequisite for effective therapy.

Cochlear implants: cortical plasticity in congenital deprivation

Congenital auditory deprivation (deafness) leads to a dysfunctional intrinsic cortical microcircuitry. This chapter reviews these deficits with a particular emphasis on layer-specific activity within the primary auditory cortex. Evidence for a delay in activation of supragranular layers and reduction in activity in infragranular layers is discussed. Such deficits indicate the incompetence of the primary auditory cortex to not only properly process thalamic input and generate output within the infragranular layers, but also incorporate top-down modulations from higher order auditory cortex into the processing within primary auditory cortex. Such deficits are the consequence of a misguided postnatal development. Maturation of primary auditory cortex in deaf animals shows evidence of a developmental delay and further alterations in gross synaptic currents, spread of activation, and morphology of local field potentials recorded at the cortical surface. Additionally, degenerative changes can be observed. When hearing is initiated early in life (e.g., by chronic cochlear-implant stimulation), many of these deficits are counterbalanced. However, plasticity of the auditory cortex decreases with increasing age, so that a sensitive period for plastic adaptation can be demonstrated within the second to sixth months of life in the deaf cat. Potential molecular mechanisms of the existence of sensitive period are discussed. Data from animal research may be compared to electroencephalographic data obtained from cochlear-implanted congenitally deaf children. After cochlear implantation in humans, three phases of plastic adaptation can be observed: a fast one, taking place within the first few weeks after implantation, showing no sensitive period; a slower one, taking place within the first months after implantation (a sensitive period up to 4 years of age); and possibly a third, and the longest one, related to increasing activation of higher order cortical areas.

Postnatal Cortical Development in Congenital Auditory Deprivation

The study investigates early postnatal development of local field potentials (LFPs) in the primary auditory cortex of hearing and congenitally deaf cats. In hearing cats, LFPs elicited by electrical intracochlear stimulation demonstrated developmental changes in mid-latency range, including reductions in peak and onset latencies of individual waves and a maturation of their shape and latencies during the first 2 months of life. In long latency range (>80 ms), the P(1)/N(1) response appeared after the fourth week of life and further increased in amplitude and decreased in latency, reaching mature shapes between the fourth and sixth months after birth (p.n.). Cortical activated areas became increasingly smaller during the first 3 months of life, reaching mature values at the fourth month p.n. The layer-specific pattern of synaptic activity matured 4 months p.n. In congenitally deaf cats, the developmental pattern was different. The lowest cortical LFP thresholds were significantly smaller than in hearing controls, demonstrating a "hypersensitivity" to sensory inputs. The development of N(b) waves was delayed and altered and the long latency responses became smaller than in controls at the second and third months. The activated areas remained smaller than in controls until the third month, then they increased rapidly and exceeded the activated areas of age-matched controls. From the fourth month on, the activated areas decreased again and smaller synaptic currents were found in deaf cats than in controls. The presented data demonstrate that functional development of the auditory cortex critically depends on auditory experience.

Breed differences in cochlear integrity in adult, commercially raised chickens

Two types of chickens are commercially available. Broiler birds are bred to develop quickly for meat production, while egg layers are bred to attain a smaller adult size. Because we have observed breed differences in the response of central auditory neurons to cochlear ablation in adult birds [Edmonds et al. (1999) Hear. Res. 127, 62-76], we examined cochleae from the two breeds for differences in integrity. We evaluated cochlear hair cell structure using scanning electron microscopy and cochlear hair cell function using distortion product otoacoustic emissions (DPOAEs) and the auditory brainstem response. We observed striking breed differences in cochlear integrity in adult but not hatchling birds. In adult broiler birds, all cochleae showed damage, encompassing at least the basal 29% of the cochlea. In 15 of 18 broiler ears, damage was observed throughout the basal 60% of the cochlea. In contrast, cochleae from egg layer adults were largely normal. Two thirds of egg layer ears showed no anatomical abnormalities, while in the remainder cochlear damage was seen within the basal 48% of the cochlea. DPOAEs recorded from egg layer birds showed loss of high frequency emissions in every ear for which the cochlea displayed anatomical damage. Average sound pressure levels in both commercial facilities were 90 dB, suggesting these two breeds may exhibit differential susceptibility to noise damage.

The effects of congenital deafness on auditory nerve synapses and globular bushy cells in cats

It is well known that auditory deprivation affects the structure and function of the central nervous system. Congenital deafness represents one form of deprivation, and in the adult white cat, it has been shown to have a clear effect upon the synaptic interface between endbulbs of Held and spherical bushy cells. It is not known, however, whether all primary synapses are affected and/or whether they are affected in the same way and to the same extent. Thus, we studied a second neuronal circuit in the deaf white cat involving modified (small) endbulbs and globular bushy cells. Compared to normal hearing cats, modified endbulbs of congenitally deaf cats were 52.2% smaller but unchanged in structural complexity. There was also a striking loss of extracellular space between ending and cell body. The somata of postsynaptic globular bushy cells were 13.4% smaller and had enlarged postsynaptic densities. These data reveal that axosomatic synapses demonstrate abnormal structure as a consequence of deafness and that the extent of the abnormalities can vary with respect to the circuits involved. The implication of these observations is that synaptic anomalies would introduce differential delays within separate circuits, thereby desynchronizing neural activity from sound stimuli. This loss of synchronization could in turn disrupt temporal processing and compromise a host of related functions, including language comprehension.

Congenital auditory deprivation reduces synaptic activity within the auditory cortex in a layer-specific manner

The present study investigates the functional deficits of naive auditory cortices in adult congenitally deaf cats. For this purpose, their auditory system was stimulated electrically using cochlear implants. Synaptic currents in cortical layers were revealed using current source density analyses. They were compared with synaptic currents found in electrically stimulated hearing cats. The naive auditory cortex showed significant deficits in synaptic activity in infragranular cortical layers. Furthermore, there was also a deficit of synaptic activities at longer latencies (>30 ms). The 'cortical column' was not activated in the well-defined sequence found in normal hearing cats. These results demonstrate functional deficits as a consequence of congenital auditory deprivation. Similar deficits are likely in congenitally deaf children.

Recruitment of the auditory cortex in congenitally deaf cats by long-term cochlear electrostimulation

In congenitally deaf cats, the central auditory system is deprived of acoustic input because of degeneration of the organ of Corti before the onset of hearing. Primary auditory afferents survive and can be stimulated electrically. By means of an intracochlear implant and an accompanying sound processor, congenitally deaf kittens were exposed to sounds and conditioned to respond to tones. After months of exposure to meaningful stimuli, the cortical activity in chronically implanted cats produced field potentials of higher amplitudes, expanded in area, developed long latency responses indicative of intracortical information processing, and showed more synaptic efficacy than in naïve, unstimulated deaf cats. The activity established by auditory experience resembles activity in hearing animals.

Single unit recordings in the auditory nerve of congenitally deaf white cats: morphological correlates in the cochlea and cochlear nucleus

It is well known that experimentally induced cochlear damage produces structural, physiological, and biochemical alterations in neurons of the cochlear nucleus. In contrast, much less is known with respect to the naturally occurring cochlear pathology presented by congenital deafness. The present study attempts to relate organ of Corti structure and auditory nerve activity to the morphology of primary synaptic endings in the cochlear nucleus of congenitally deaf white cats. Our observations reveal that the amount of sound-evoked spike activity in auditory nerve fibers influences terminal morphology and synaptic structure in the anteroventral cochlear nucleus. Some white cats had no hearing. They exhibited severely reduced spontaneous activity and no sound-evoked activity in auditory nerve fibers. They had no recognizable organ of Corti, presented >90% loss of spiral ganglion cells, and displayed marked structural abnormalities of endbulbs of Held and their synapses. Other white cats had partial hearing and possessed auditory nerve fibers with a wide range of spontaneous activity but elevated sound-evoked thresholds (60-70 dB SPL). They also exhibited obvious abnormalities in the tectorial membrane, supporting cells, and Reissner's membrane throughout the cochlear duct and had complete inner and outer hair cell loss in the base. The spatial distribution of spiral ganglion cell loss correlated with the pattern of hair cell loss. Primary neurons of hearing-impaired cats displayed structural abnormalities of their endbulbs and synapses in the cochlear nucleus which were intermediate in form compared to normal and totally deaf cats. Changes in endbulb structure appear to correspond to relative levels of deafness. These data suggest that endbulb structure is significantly influenced by sound-evoked auditory nerve activity.

A model for prelingual deafness, the congenitally deaf white cat--population statistics and degenerative changes

Cochlear implantation in congenitally deaf children leads to electrical stimulation of an entirely naive central auditory system. In this case, processes of central auditory maturation are induced by the electric stimuli. For the study of these processes the deaf white cat (DWC) appears to be an appropriate model. However, a knowledge of the basic data of these animals is necessary before such a model may be used. This paper presents these data and is one of a series of publications concerning congenital deafness in children and cochlear implantation. In our strain 72% of the animals are totally deaf as judged by the absence of any brain stem evoked potentials at click intensities up to 120 dB SPL peak equivalent. Primarily, there is a degeneration of the entire organ of Corti during the first postnatal weeks. An absence of acoustically evoked brain stem responses in the early postnatal weeks shows that DWCs probably never have any hearing experience. Months after the degeneration of the organ of Corti, the spiral ganglion starts to degenerate from the midportion of the cochlea. However, even in adult cats (2 years), a sufficient number of functionally intact auditory afferents remain, which are suitable for electrical cochlear stimulation.

Response of the primary auditory cortex to electrical stimulation of the auditory nerve in the congenitally deaf white cat

Neural activity plays an important role in the development and maintenance of sensory pathways. However, while there is considerable experience using cochlear implants in both congenitally deaf adults and children, little is known of the effects of a hearing loss on the development of the auditory cortex. In the present study, cortical evoked potentials, field potentials, and multi- and single-unit activity evoked by electrical stimulation of the auditory nerve were used to study the functional organisation of the auditory cortex in the adult congenitally deaf white cat. The absence of click-evoked auditory brainstem responses during the first weeks of life demonstrated that these animals had no auditory experience. Under barbiturate anaesthesia, cortical potentials could be recorded from the contralateral auditory cortex in response to bipolar electrical stimulation of the cochlea in spite of total auditory deprivation. Threshold, morphology and latency of the evoked potentials varied with the location of the recording electrode, with response latency varying from 10 to 20 ms. There was evidence of threshold shifts with site of the cochlear stimulation in accordance with the known cochleotopic organisation of AI. Thresholds also varied with the configuration of the stimulating electrodes in accordance with changes previously observed in normal hearing animals. Single-unit recordings exhibited properties similar to the evoked potentials. Increasing stimulus intensity resulted in an increase in spike rate and a decrease in latency to a minimum of approximately 8 ms, consistent with latencies recorded in AI of previously normal animals (Raggio and Schreiner, 1994). Single-unit thresholds also varied with the configuration of the stimulating electrodes. Strongly driven responses were followed by a suppression of spontaneous activity. Even at saturation intensities the degree of synchronisation was less than observed when recording from auditory brainstem nuclei. Taken together, in these auditory deprived animals basic response properties of the auditory cortex of the congenitally deaf white cat appear similar to those reported in normal hearing animals in response to electrical stimulation of the auditory nerve. In addition, it seems that the auditory cortex retains at least some rudimentary level of cochleotopic organisation.

Ultrastructural analysis of primary endings in deaf white cats: morphologic alterations in endbulbs of Held

Changes in structure and function of the auditory system can be produced by experimentally manipulating the sensory environment, and especially dramatic effects result from deprivation procedures. An alternative deprivation strategy utilizes naturally occurring lesions. The congenitally deaf white cat represents an animal model of sensory deprivation because it mimics a form of human deafness called the Scheibe deformity and permits studies of how central neurons react to early-onset cochlear degeneration. We studied the synaptic characteristics of the endbulb of Held, a prominent auditory nerve terminal in the cochlear nucleus. Endbulbs arise from the ascending branch of the auditory nerve fiber and contact the cell body of spherical bushy cells. After 6 months, endbulbs of deaf white cats exhibit alterations in structure that are clearly distinguishable from those of normal hearing cats, including a diminution in terminal branching, a reduction in synaptic vesicle density, structural abnormalities in mitochondria, thickening of the pre- and postsynaptic densities, and enlargement of synapse size. The hypertrophied membrane densities are suggestive of a compensatory response to diminished transmitter release. These data reveal that early-onset, long-term deafness produces unambiguous alterations in synaptic structure and may be relevant to rehabilitation strategies that promote aural/oral communication.

Ultrastructural changes in primary endings of deaf white cats

Changes in brain structure occur as a consequence of altered experience. During maturation of the auditory nervous system, sensory deprivation is known to cause cell loss, abnormal axonal projections, and synaptic alterations. These animal data may be relevant to clinical observations that cochlear implants provide superior benefit for individuals who become deaf postlingually compared with those who become deaf prelingually. That is, implantation appears most efficacious if it occurs after functional connections are established but before deprivation-induced changes in the central auditory system. After this period, synaptic reorganization may underlie the diminished effectiveness of cochlear implants.

Morphological changes in the cochlear nucleus of congenitally deaf white cats

Investigations in animal models and humans have indicated that congenital deafness produces degenerative changes in the central auditory pathway. The cochlear nucleus is the first central structure that receives cochlear input, and may be considered the origin of ascending auditory pathways. In this context, we studied congenitally deaf white cats, who express early onset cochlear receptor loss, in order to assess the nature of structural changes in cells of the cochlear nucleus. It is conceivable that pathologic alterations in higher auditory structures are transneuronally distributed through this nucleus. The cochlear nuclei of nonwhite cats with normal hearing were compared to those of deaf white cats exhibiting hearing loss in excess of 70 dB SPL. The cochlear nuclei of the deaf white cats were smaller in volume by roughly 50%, with the ventral and dorsal divisions being equally affected. Cell body silhouette area was determined for spherical bushy cells of the anteroventral cochlear nucleus (AVCN), pyramidal cells of the dorsal cochlear nucleus (DCN), sensory neurons from the principal trigeminal nucleus, and motoneurons of the facial nucleus. We found no statistical difference in neuronal cell body size between nonauditory neurons of these two groups of cats, whereas auditory neurons of deaf white cats were 30.8-39.4% smaller than those of normal cats. These data imply that neuronal changes in congenitally deaf cats are specific to the auditory pathway. Although cochlear nucleus volume loss was uniform for both divisions, there was a differential effect on cell density: AVCN cell density increased by 40%, whereas DCN cell density was relatively unaffected (10% increase). Astrocyte density was also greater in the AVCN (52%) compared to that in the DCN (5%). These observations reveal a differential impact on cells in the cochlear nucleus to congenital deafness, suggesting selective processing impairment at this level. If similar patterns of degeneration occur in humans, such pathologies may underlie reduced processing of input from cochlear implants in congenitally deaf adults.