Topographic organization of the cochlear spiral ganglion demonstrated by restricted lesions of the anteroventral cochlear nucleus

Analog Transmission of Action Potential Fine Structure in Spiral Ganglion Axons

Action potential waveforms generated at the axon initial segment (AIS) are specialized between and within neuronal classes. But is the fine structure of each electrical event retained when transmitted along myelinated axons or is it rapidly and uniformly transmitted to be modified again at the axon terminal? To address this issue action potential axonal transmission was evaluated in a class of primary sensory afferents that possess numerous types of voltage-gated ion channels underlying a complex repertoire of endogenous firing patterns. In addition to their signature intrinsic electrophysiological heterogeneity, spiral ganglion neurons are uniquely designed. The bipolar, myelinated somata of type I neurons are located within the conduction pathway, requiring that action potentials generated at the first heminode must be conducted through their electrically excitable membrane. We utilized this unusual axonal-like morphology to serve as a window into action potential transmission to compare locally-evoked action potential profiles to those generated peripherally at their glutamatergic synaptic connections with hair cell receptors. These comparisons showed that the distinctively-shaped somatic action potentials were highly correlated with the nodally-generated, invading ones for each neuron. This result indicates that the fine structure of the action potential waveform is maintained axonally, thus supporting the concept that analog signaling is incorporated into each digitally-transmitted action potential in the specialized primary auditory afferents.

Calretinin and calbindin distribution patterns specify subpopulations of type I and type II spiral ganglion neurons in postnatal murine cochlea

As the first neural element in the auditory pathway, neurons in the spiral ganglion shape the initial coding of sound stimuli for subsequent processing. Within the ganglion, type I and type II neurons form divergent and convergent innervation patterns, respectively, with their hair cell sensory receptors, indicating that very different information is gathered and conveyed. Layered onto these basic innervation patterns are structural and electrophysiological features that provide additional levels of processing multifaceted sound stimuli. To understand the nature of this additional complexity of signal coding, we characterized the distribution of calretinin and calbindin, two regulators of intracellular calcium that serve as markers for neuronal subpopulations. We showed in acute preparations and in vitro that calretinin and calbindin staining levels were heterogeneous. Immunocytochemical analysis of colocalization further showed that high levels of staining for the two molecules rarely overlapped. Although varied amounts of calbindin and calretinin were found within each tonotopic location and neuronal type, some distinct subdistributions were noted. For example, calretinin levels were highest in neurons innervating the midcochlea region, whereas calbindin levels were similar across the entire ganglion. Furthermore, we noted that apical type II neurons, identified by antiperipherin labeling, had significantly lower levels of calretinin and higher levels of calbindin. We also established that the endogenous firing feature of onset tau of the subthreshold response showed a pattern related to quantified calretinin and calbindin staining levels. Taken together, our results suggest an additional dimension of complexity within the spiral ganglion beyond that currently categorized.

Combining cell-based therapies and neural prostheses to promote neural survival

Cochlear implants provide partial restoration of hearing for profoundly deaf patients by electrically stimulating spiral ganglion neurons (SGNs); however, these neurons gradually degenerate following the onset of deafness. Although the exogenous application of neurotrophins (NTs) can prevent SGN loss, current techniques to administer NTs for long periods of time have limited clinical applicability. We have used encapsulated choroid plexus cells (NTCells; Living Cell Technologies, Auckland, New Zealand) to provide NTs in a clinically viable manner that can be combined with a cochlear implant. Neonatal cats were deafened and unilaterally implanted with NTCells and a cochlear implant. Animals received chronic electrical stimulation (ES) alone, NTs alone, or combined NTs and ES (ES + NT) for a period of as much as 8 months. The opposite ear served as a deafened unimplanted control. Chronic ES alone did not result in increased survival of SGNs or their peripheral processes. NT treatment alone resulted in greater SGN survival restricted to the upper basal cochlear region and an increased density of SGN peripheral processes. Importantly, chronic ES in combination with NTs provided significant SGN survival throughout a wider extent of the cochlea, in addition to an increased peripheral process density. Re-sprouting peripheral processes were observed in the scala media and scala tympani, raising the possibility of direct contact between peripheral processes and a cochlear implant electrode array. We conclude that cell-based therapy is clinically viable and effective in promoting SGN survival for extended durations of cochlear implant use. These findings have important implications for the safe delivery of therapeutic drugs to the cochlea.

Synaptic proteins are tonotopically graded in postnatal and adult type I and type II spiral ganglion neurons

Inherent in the design of the mammalian auditory system is the precision necessary to transduce complex sounds and transmit the resulting electrical signals to higher neural centers. Unique specializations in the organ of Corti are required to make this conversion, such that mechanical and electrical properties of hair cell receptors are tailored to their specific role in signal coding. Electrophysiological and immunocytochemical characterizations have shown that this principle also applies to neurons of the spiral ganglion, as evidenced by distinctly different firing features and synaptic protein distributions of neurons that innervate high- and low-frequency regions of the cochlea. However, understanding the fine structure of how these properties are distributed along the cochlear partition and within the type I and type II classes of spiral ganglion neurons is necessary to appreciate their functional significance fully. To address this issue, we assessed the localization of the postsynaptic AMPA receptor subunits GluR2 and GluR3 and the presynaptic protein synaptophysin by using immunocytochemical labeling in both postnatal and adult tissue. We report that these presynaptic and postsynaptic proteins are distributed oppositely in relation to the tonotopic map and that they are equally distributed in each neuronal class, thus having an overall gradation from one end of the cochlea to the other. For synaptophysin, an additional layer of heterogeneity was superimposed orthogonal to the tonotopic axis. The highest anti-synaptophysin antibody levels were observed within neurons located close to the scala tympani compared with those located close to the scala vestibuli. Furthermore, we noted that the protein distribution patterns observed in postnatal preparations were largely retained in adult tissue sections, indicating that these features characterize spiral ganglion neurons in the fully developed ear.

Complex primary afferents: What the distribution of electrophysiologically-relevant phenotypes within the spiral ganglion tells us about peripheral neural coding

Spiral ganglion neurons are the first neural element of the auditory system. They receive precise synaptic signals which represent features of sound stimuli encoded by hair cell receptors and they deliver a digital representation of this information to the central nervous system. It is well known that spiral ganglion neurons are selectively responsive to specific sound frequencies, and that numerous structural and physiological specializations in the inner ear increase the quality of this tuning, beyond what could be accomplished by the passive properties of the basilar membrane. Further, consistent with what we know about other sensory systems, it is becoming clear that the parallel divergent innervation pattern of type I spiral ganglion neurons has the potential to encode additional features of sound stimuli. To date, we understand the most about the sub-modalities of frequency and intensity coding in the peripheral auditory system. Work reviewed herein will address the issue of how intrinsic electrophysiological features of the neurons themselves have the potential to contribute to the precision of coding and transmitting information about these two parameters to higher auditory centers for further processing.

Atoh1-lineal neurons are required for hearing and for the survival of neurons in the spiral ganglion and brainstem accessory auditory nuclei

Atoh1 is a basic helix-loop-helix transcription factor necessary for the specification of inner ear hair cells and central auditory system neurons derived from the rhombic lip. We used the Cre-loxP system and two Cre-driver lines (Egr2(Cre) and Hoxb1(Cre)) to delete Atoh1 from different regions of the cochlear nucleus (CN) and accessory auditory nuclei (AAN). Adult Atoh1-conditional knock-out mice (Atoh1(CKO)) are behaviorally deaf, have diminished auditory brainstem evoked responses, and have disrupted CN and AAN morphology and connectivity. In addition, Egr2; Atoh1(CKO) mice lose spiral ganglion neurons in the cochlea and AAN neurons during the first 3 d of life, revealing a novel critical period in the development of these neurons. These new mouse models of predominantly central deafness illuminate the importance of the CN for support of a subset of peripheral and central auditory neurons.

Spontaneous discharge patterns in cochlear spiral ganglion cells before the onset of hearing in cats

Spontaneous neural activity has been recorded in the auditory nerve of cats as early as 2 days postnatal (P2), yet individual auditory neurons do not respond to ambient sound levels <90-100 dB SPL until about P10. Significant refinement of the central projections from the spiral ganglion to the cochlear nucleus occurs during this neonatal period. This refinement may be dependent on peripheral spontaneous discharge activity. We recorded from single spiral ganglion cells in kittens aged P3-P9. The spiral ganglion was accessed through the round window through the spiral lamina. A total of 112 ganglion cells were isolated for study in nine animals. Spike rates in neonates were very low, ranging from 0.06 to 56 spikes/s, with a mean of 3.09 +/- 8.24 spikes/s. Ganglion cells in neonatal kittens exhibited remarkable repetitive spontaneous bursting discharge patterns. The unusual patterns were evident in the large mean interval CV (CV(i) = 2.9 +/- 1.6) and burst index of 5.2 +/- 3.5 across ganglion cells. Spontaneous bursting patterns in these neonatal mammals were similar to those reported for cochlear ganglion cells of the embryonic chicken, suggesting this may be a general phenomenon that is common across animal classes. Rhythmic spontaneous discharge of retinal ganglion cells has been shown to be important in the development of central retinotopic projections and normal binocular vision. Bursting rhythms in cochlear ganglion cells may play a similar role in the auditory system during prehearing periods.

Postnatal refinement of auditory nerve projections to the cochlear nucleus in cats

Studies of visual system development have suggested that competition driven by activity is essential for refinement of initial topographically diffuse neuronal projections into their precise adult patterns. This has led to the assertion that this process may shape development of topographic connections throughout the nervous system. Because the cat auditory system is very immature at birth, with auditory nerve neurons initially exhibiting very low or no spontaneous activity, we hypothesized that the auditory nerve fibers might initially form topographically broad projections within the cochlear nuclei (CN), which later would become topographically precise at the time when adult-like frequency selectivity develops. In this study, we made restricted injections of Neurobiotin, which labeled small sectors (300-500 microm) of the cochlear spiral ganglion, to study the projections of auditory nerve fibers representing a narrow band of frequencies. Results showed that projections from the basal cochlea to the CN are tonotopically organized in neonates, many days before the onset of functional hearing and even prior to the development of spontaneous activity in the auditory nerve. However, results also demonstrated that significant refinement of the topographic specificity of the primary afferent axons of the auditory nerve occurs in late gestation or early postnatal development. Projections to all three subdivisions of the CN exhibit clear tonotopic organization at or before birth, but the topographic restriction of fibers into frequency band laminae is significantly less precise in perinatal kittens than in adult cats. Two injections spaced > or = 2 mm apart in the cochlea resulted in labeled bands of projecting axons in the anteroventral CN that were 53% broader than would be expected if they were proportional to those in adults, and the two projections were incompletely segregated in the youngest animals studied. Posteroventral CN (PVCN) projections (normalized for CN size) were 36% broader in neonates than in adults, and projections from double injections in the youngest subjects were nearly fused in the PVCN. Projections to the dorsal division of the CN were 32% broader in neonates than in adults when normalized, but the dorsal CN projections were always discrete, even at the earliest ages studied.

Study of afferent nerve terminals and fibers in the gerbil cochlea: distribution by size

The purpose of the present study was to determine if the synaptic terminals and nerve fibers in the gerbil cochlea fall into morphologically and spatially classified groups. In cats and guinea pigs, these groups, based on size, location on inner hair cell (IHC) and stratification within the osseous spiral lamina, have been found to correlate with spontaneous rate, threshold sensitivity and projection pattern to the cochlear nucleus. Thus, there may be anatomical data to suggest mechanisms for intensity coding of different frequencies of sound. Afferent nerve terminals contacting IHCs in the gerbil cochlea were analyzed with regard to size and location. Data were obtained from serial thin sections (700 for each IHC) cut perpendicular to the long axis of eight IHCs (two apical and two basal IHCs from two cochleas), observed and photographed using a transmission electron microscope. Results indicate that the percentage of modiolar versus pillar-side terminals around each IHC varies from cell to cell. In some cases, the smallest fibers were located on the modiolar side, but a consistent distribution of the smallest fibers on this side of the cell was not characteristic. While a size-based segregation of terminals does not appear around the perimeter of the IHC, modest size-based segregation of nerve fibers is found in the osseous spiral lamina. Perimeter measurements were made from myelinated fibers cut in cross-section, obtained from semi-thin sections in the distal (near the IHCs) and proximal (near the spiral ganglion) regions of the osseous spiral lamina. Best-fit line analysis indicates there is a modest nerve fiber size/vertical organization along the scala tympani/scala vestibuli (SV) axis of the nerve bundles within the osseous spiral lamina such that more of the smaller perimeter fibers are located on the SV side and more of the larger perimeter fibers are located on the ST side. Our data for terminals at the IHC are different from those seen in the cat; our data for nerve fibers in the osseous spiral lamina support those seen in the cat and guinea pig.

Spatial organization of the reciprocal connections between the cat dorsal and anteroventral cochlear nuclei

We are studying the interconnections between the anteroventral cochlear nucleus (AVCN) and the dorsal cochlear nucleus (DCN). Biotinylated dextran was injected into the DCN, where the best frequency of responses was also recorded. Ventrotubercular neurons in AVCN were labeled, along with cochlear nerve fibers and the axons of cells in DCN. In AVCN, a central band of labeled cochlear nerve axons and large endbulbs was labeled. Bordering this band was a 'fringe' of smaller tuberculoventral axonal endings forming pericellular nests. Most AVCN neurons projecting to DCN were stellate, elongate, or giant cells, located in the posterior division of AVCN, regardless of the DCN injection site. About 75% of the labeled AVCN cells lay within the bands of labeled cochlear nerve fibers. Another 15% were in the outer fringes on either side of these bands, while 10% were outside the bands and the fringes. These findings suggest that most AVCN neurons projecting to the DCN conform to the tonotopic map. A significant portion of the ventrotubercular neurons occupy side-bands in AVCN. Reciprocally, the tuberculoventral tract forms a robust fringe of axonal endings flanking the central bands. The neuronal and axonal bands and side-bands may underlie excitatory and inhibitory signal transformations.

THE ROLE OF TIMING IN THE BRAIN STEM AUDITORY NUCLEI OF VERTEBRATES

Vertebrate animals gain biologically important information from environmental sounds. Localization of sound sources enables animals to detect and respond appropriately to danger, and it allows predators to detect and localize prey. In many species, rapidly fluctuating sounds are also the basis of communication between conspecifics. This information is not provided directly by the output of the ear but requires processing of the temporal pattern of firing in the tonotopic array of auditory nerve fibers. The auditory nerve feeds information through several parallel ascending pathways. Anatomical and electrophysiological specializations for conveying precise timing, including calyceal synaptic terminals and matching axonal conduction times, are evident in several of the major ascending auditory pathways through the ventral cochlear nucleus and its nonmammalian homologues. One pathway that is shared by all higher vertebrates makes an ongoing comparison of interaural phase for the localization of sound in the azimuth. Another pathway is specifically associated with higher frequency hearing in mammals and is thought to make use of interaural intensity differences for localizing high-frequency sounds. Balancing excitation from one ear with inhibition from the other in rapidly fluctuating signals requires that the timing of these synaptic inputs be matched and constant for widely varying sound stimuli in this pathway. The monaural nuclei of the lateral lemniscus, whose roles are not understood (although they are ubiquitous in higher vertebrates), receive input from multiple pathways that encode timing with precision, some through calyceal endings.

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.

Topography of spiral ganglion projections to cochlear nucleus during postnatal development in cats

A fundamenntal organizational principle of the central auditory system is that virtually all areas are tonotopically organized. However, we know very little about the timing or mechanisms that are responsible for the development of this organization. When cats are born, their auditory nervous systems are extremely immature, and their hearing thresholds are very high. Until postnatal days 7-10 (P7-10), cats have behavioral and physiological thresholds which are near or above the pain threshold for adults and also have poor frequency selectivity. Physiological thresholds for auditory nerve fibers and cochlear nucleus neurons are typically above 100-120 dB SPL (sound pressure level re 20 microPa). Three weeks later (at approximately P31), the sensitivity and frequency discrimination (tuning) of these neurons approximate adult values. This study examines the development of the tonotopic projections from the spiral ganglion to the cochlear nucleus during the period in cat development in which the auditory system undergoes the transition from being essentially nonfunctional to having adult-like function. With the animals heavily anesthetized, the cochleas were surgically exposed in kittens ranging in age from P6 to P45. Focal injections of Neurobiotin (NB) were made into Rosenthal's canal, labeling a small cluster of cells in the spiral ganglion of each cochlea. The projections of these labeled cells were visualized as frequency-specific bands of labeled axons and terminals in all major subdivisions of the cochlear nucleus. The thickness of these bands (i.e., the dimension of the bands orthogonal to the isofrequency representation and across the frequency gradient) were measured and compared to similar projections in adults. As in adult cats, the thickness of the bands varied only slightly with the location of the injection site (frequency representation) over a range of 1-7 mm from the cochlear base (45-13 kHz). Moreover, band thickness did not vary significantly with age. These data indicate that the tonotopic organization of spiral ganglion projections to the cochlear nucleus is as precise in kittens as young as P6 as it is in adults.

Spatial organization of inner hair cell synapses and cochlear spiral ganglion neurons

The morphological organization of the central projections of the cat cochlear spiral ganglion into the cochlear nucleus was previously investigated by creating restricted lesions in the anteroventral cochlear nucleus (AVCN) to ablate selectively either the lateral or the medial aspect of isofrequency projection laminae. Such lesions resulted in highly selective retrograde degeneration of spiral ganglion cells. Ablation of the lateral part of the AVCN caused degeneration of cells within the scala tympani part of the ganglion, whereas medial ablations within the AVCN induced degeneration of the scala vestibuli aspect of the ganglion. The peripheral axons also degenerated and this fiber loss exhibited selective topographies that paralleled the cell loss within the spiral ganglion, although this phenomenon was more prominent in the proximal part of the osseous spiral lamina near the ganglion and less obvious more distally near the habenula perforata. In this investigation, inner hair cells (IHCs) from these selective lesion cases were evaluated by electron microscopy of serial sections through the basal synaptic regions. Results demonstrated differential degeneration of afferent synapses, with greater (but not completely selective) loss of pillar synapses after lateral AVCN lesions and greater loss of modiolar synapses after medial lesions. Because auditory nerve fibers of different spontaneous discharge rates (SRs) have different spatial distributions on the IHC (Liberman, Science 216:1239, 1982), our results suggest that this SR-based organization is maintained in a topographic organization across the vertical (scala tympani-to-scala vestibuli) dimension of the spiral ganglion cell cluster and carried into the ventral cochlear nuclei (VCN). Thus, in addition to the spiral frequency organization represented by the dorsal-to-ventral frequency map in the VCN, there is also an orderly organization of inputs from high- and low-SR fibers across the lateral-to-medial dimension of the VCN such that the lateral isofrequency laminae receive a proportionately greater input from high-SR fibers, whereas medial isofrequency laminae receive preferential input from low- and medium-SR fibers.