Molecular Assembly and Structural Plasticity of Sensory Ribbon Synapses-A Presynaptic Perspective

In the mammalian cochlea, specialized ribbon-type synapses between sensory inner hair cells (IHCs) and postsynaptic spiral ganglion neurons ensure the temporal precision and indefatigability of synaptic sound encoding. These high-through-put synapses are presynaptically characterized by an electron-dense projection-the synaptic ribbon-which provides structural scaffolding and tethers a large pool of synaptic vesicles. While advances have been made in recent years in deciphering the molecular anatomy and function of these specialized active zones, the developmental assembly of this presynaptic interaction hub remains largely elusive. In this review, we discuss the dynamic nature of IHC (pre-) synaptogenesis and highlight molecular key players as well as the transport pathways underlying this process. Since developmental assembly appears to be a highly dynamic process, we further ask if this structural plasticity might be maintained into adulthood, how this may influence the functional properties of a given IHC synapse and how such plasticity could be regulated on the molecular level. To do so, we take a closer look at other ribbon-bearing systems, such as retinal photoreceptors and pinealocytes and aim to infer conserved mechanisms that may mediate these phenomena.

Mapping developmental maturation of inner hair cell ribbon synapses in the apical mouse cochlea

Ribbon synapses of cochlear inner hair cells (IHCs) undergo molecular assembly and extensive functional and structural maturation before hearing onset. Here, we characterized the nanostructure of IHC synapses from late prenatal mouse embryo stages (embryonic days 14-18) into adulthood [postnatal day (P)48] using electron microscopy and tomography as well as optical nanoscopy of apical turn organs of Corti. We find that synaptic ribbon precursors arrive at presynaptic active zones (AZs) after afferent contacts have been established. These ribbon precursors contain the proteins RIBEYE and piccolino, tether synaptic vesicles and their delivery likely involves active, microtubule-based transport pathways. Synaptic contacts undergo a maturational transformation from multiple small to one single, large AZ. This maturation is characterized by the fusion of ribbon precursors with membrane-anchored ribbons that also appear to fuse with each other. Such fusion events are most frequently encountered around P12 and hence, coincide with hearing onset in mice. Thus, these events likely underlie the morphological and functional maturation of the AZ. Moreover, the postsynaptic densities appear to undergo a similar refinement alongside presynaptic maturation. Blockwise addition of ribbon material by fusion as found during AZ maturation might represent a general mechanism for modulating ribbon size.

Stereocilia morphogenesis and maintenance through regulation of actin stability

Stereocilia are actin-based protrusions on auditory and vestibular sensory cells that are required for hearing and balance. They convert physical force from sound, head movement or gravity into an electrical signal, a process that is called mechanoelectrical transduction. This function depends on the ability of sensory cells to grow stereocilia of defined lengths. These protrusions form a bundle with a highly precise geometry that is required to detect nanoscale movements encountered in the inner ear. Congenital or progressive stereocilia degeneration causes hearing loss. Thus, understanding stereocilia hair bundle structure, development, and maintenance is pivotal to understanding the pathogenesis of deafness. Stereocilia cores are made from a tightly packed array of parallel, crosslinked actin filaments, the length and stability of which are regulated in part by myosin motors, actin crosslinkers and capping proteins. This review aims to describe stereocilia actin regulation in the context of an emerging "tip turnover" model where actin assembles and disassembles at stereocilia tips while the remainder of the core is exceptionally stable.

Quantal and nonquantal transmission in calyx-bearing fibers of the turtle posterior crista

Intracellular recordings were made from nerve fibers in the posterior ampullary nerve near the neuroepithelium. Calyx-bearing afferents were identified by their distinctive efferent-mediated responses. Such fibers receive inputs from both type I and type II hair cells. Type II inputs are made by synapses on the outer face of the calyx ending and on the boutons of dimorphic fibers. Quantal activity, consisting of brief mEPSPs, is reduced by lowering the external concentration of Ca2+ and blocked by the AMPA-receptor antagonist CNQX. Poisson statistics govern the timing of mEPSPs, which occur at high rates (250-2,500/s) in the absence of mechanical stimulation. Excitation produced by canal-duct indentation can increase mEPSP rates to nearly 5,000/s. As the rate increases, mEPSPs can change from a monophasic depolarization to a biphasic depolarizing-hyperpolarizing sequence, both of whose components are blocked by CNQX. Blockers of voltage-gated currents affect mEPSP size, which is decreased by TTX and is increased by linopirdine. mEPSP size decreases severalfold after impalement. The size decrease, although it may be triggered by the depolarization occurring during impalement, persists even at hyperpolarized membrane potentials. Nonquantal transmission is indicated by shot-noise calculations and by the presence of voltage modulations after quantal activity is abolished pharmacologically. An ultrastructural study shows that inner-face inputs from type I hair cells outnumber outer-face inputs from type II hair cells by an almost 6:1 ratio.

Mechanisms of efferent-mediated responses in the turtle posterior crista

To study the cellular mechanisms of efferent actions, we recorded from vestibular-nerve afferents close to the turtle posterior crista while efferent fibers were electrically stimulated. Efferent-mediated responses were obtained from calyx-bearing (CD, calyx and dimorphic) afferents and from bouton (B) afferents distinguished by their neuroepithelial locations into BT units near the torus and BM units at intermediate sites. The spike discharge of CD units is strongly excited by efferent stimulation, whereas BT and BM units are inhibited, with BM units also showing a postinhibitory excitation. Synaptic activity was recorded intracellularly after spikes were blocked. Responses of BT/BM units to single efferent shocks consist of a brief depolarization followed by a prolonged hyperpolarization. Both components reflect variations in hair-cell quantal release rates and are eliminated by pharmacological antagonists of alpha9/alpha10 nicotinic receptors. Blocking calcium-dependent SK potassium channels converts the biphasic response into a prolonged depolarization. Results can be explained, as in other hair-cell systems, by the sequential activation of alpha9/alpha10 and SK channels. In BM units, the postinhibitory excitation is based on an increased rate of hair-cell quanta and depends on the preceding inhibition. There is, in addition, an efferent-mediated, direct depolarization of BT/BM and CD fibers. In CD units, it is the exclusive efferent response. Nicotinic antagonists have different effects on hair-cell efferent actions and on the direct depolarization of CD and BT/BM units. Ultrastructural studies, besides confirming the efferent innervation of type II hair cells and calyx endings, show that turtle efferents commonly contact afferent boutons terminating on type II hair cells.

An investigation into the effects of underwater piling noise on salmonids

Underwater piling was undertaken in 2003 in Southampton Water on the South Coast of England. Monitoring was simultaneously undertaken of the waterborne sound from impact and vibropiling and its effects on brown trout in cages at increasing distances from the piling. Brown trout (Salmo trutta) were used as a model for salmon (Salmo salar), which were the species of interest but were not readily available. No obvious signs of trauma that could be attributed to sound exposure were found in any fish examined, from any of the cages. No increase in activity or startle response was seen to vibropiling. Analysis using the dBh, metric indicated that the noise at the nearest cages during impact piling reached levels at which salmon were expected to react strongly. However, the brown trout showed little reaction. An audiogram of the brown trout was measured by the Auditory Brainstem Response method, which indicated that the hearing of the brown trout was less sensitive than that of the salmon. Further analysis indicated that this accounted for the relative lack of reaction, and demonstrated the importance of using the correct species of fish as a model when assessing the effect of noise.

Ocsyn and mitochondrial-canalicular complexes in vestibular hair cells

Ocsyn, a syntaxin-interacting protein characterized by Safieddine et al. [Safieddine, S., Ly, C.D., Wang, Y.-X., Kachar, B., Petralia, R.S., Wenthold, R.J., 2002. Ocsyn, a novel syntaxin-interacting protein enriched in the subapical region of inner hair cells. Mol. Cell. Neurosci., 20, 343-353] in the guinea pig organ of Corti was primarily identified in organelles located at the subapical region of inner hair cells. They proposed that in cochlear inner hair cells, ocsyn was involved in protein trafficking associated to recycling endosomes. Ocsyn happens to be highly homologous to syntabulin with an almost identical syntaxin-binding domain. Syntabulin is believed to attach syntaxin-containing vesicles to kinesin for their axonal transport along microtubules. The present study shows the distribution of ocsyn in guinea pig and rat vestibular hair cells using immunocytochemistry and confocal microscopy. Ocsyn was characterized by intense immunolabeled spots distributed exclusively in type I and II vestibular hair cells. The subcuticular region under the cuticular plate exhibited particularly densely packed spots. In the neck region of the sensory cells, where microtubules are abundant, there was no colocalization of ocsyn and alpha-tubulin. Ocsyn labeled spots were also present in the medial and basal hair cell regions, particularly in the supranuclear and infranuclear regions. Mitochondria are particularly numerous in these three regions (subcuticular, supranuclear and infranuclear). Double labeling of ocsyn and cytochrome c showed that ocsyn was present in the same zones that mitochondria. This, together with the great similarity of ocsyn and syntabulin, suggest that, akin to syntabulin, ocsyn is involved in addressing organelles. We propose that ocsyn is involved in the formation of the canalicular-mitochondrial complexes depicted by Spicer et al. [Spicer, S.S., Thomopoulos, G.N., Schulte, B.A., 1999. Novel membranous structures in apical and basal compartments of inner hear cells. J. Comp. Neurol., 409, 424-437].

Regeneration of vestibular otolith afferents after ototoxic damage

Regeneration of receptor cells and subsequent functional recovery after damage in the auditory and vestibular systems of many vertebrates is well known. Spontaneous regeneration of mammalian hair cells does not occur. However, recent approaches provide hope for similar restoration of hearing and balance in humans after loss. Newly regenerated hair cells receive afferent terminal contacts, yet nothing is known about how reinnervation progresses or whether regenerated afferents finally develop normal termination fields. We hypothesized that neural regeneration in the vestibular otolith system would recapitulate the topographic phenotype of afferent innervation so characteristic of normal development. We used an ototoxic agent to produce complete vestibular receptor cell loss and epithelial denervation, and then quantitatively examined afferent regeneration at discrete periods up to 1 year in otolith maculas. Here, we report that bouton, dimorph, and calyx afferents all regenerate slowly at different time epochs, through a progressive temporal sequence. Furthermore, our data suggest that both the hair cells and their innervating afferents transdifferentiate from an early form into more advanced forms during regeneration. Finally, we show that regeneration remarkably recapitulates the topographic organization of afferent macular innervation, comparable with that developed through normative morphogenesis. However, we also show that regenerated terminal morphologies were significantly less complex than normal fibers. Whether these structural fiber changes lead to alterations in afferent responsiveness is unknown. If true, adaptive plasticity in the central neural processing of motion information would be necessitated, because it is known that many vestibular-related behaviors fully recover during regeneration.

Behavioral and Pathological Effects in the Rat Define Two Groups of Neurotoxic Nitriles

Adult male Long-Evans rats (250-350 g) received control vehicles, 3,3'-iminodipropionitrile (IDPN, 400 mg kg(-1) day(-1)), allylnitrile (50 mg kg(-1) day(-1)), cis-crotononitrile (110 mg kg(-1) day(-1)), trans-crotononitrile (250 mg kg(-1) day(-1)), or 2,4-hexadienenitrile (300 mg kg(-1) day(-1)), i.p., for 3 consecutive days. Rats treated with IDPN, allylnitrile, and cis-crotononitrile developed the ECC (excitation with circling and choreiform movements) syndrome, whereas those treated with trans-crotononitrile and hexadienenitrile exhibited a different syndrome, characterized by faltering movements. On quantitative analysis, IDPN, allylnitrile, and cis-crotononitrile induced high scores in a test battery for vestibular dysfunction and hyperactivity in the open field, but they did not significantly decrease stride length. Hexadienenitrile and trans-crotononitrile did not increase the vestibular scores or the locomotor activity, but they caused a marked decrease in stride length; they also decreased holding time on a vertical ladder. In brain and spinal cord tissue from rats exposed to IDPN, allylnitrile, or cis-crotononitrile, Fluoro-Jade B, a selective stain for degenerating neurons, did not reveal any labeling other than that of nerve terminals in the glomeruli of the olfactory bulbs, indicating degeneration of the olfactory mucosa. With the same stain, rats exposed to trans-crotononitrile or hexadienenitrile showed a common pattern of selective neurotoxicity; major targets were the inferior olive and the piriform cortex. Hexadienenitrile did not cause hair cell degeneration in the vestibular and auditory sensory epithelia. Present and previous data indicate that neurotoxic nitriles induce one or the other of two different motor syndromes, through either vestibular hair cell degeneration or neuronal degeneration of the inferior olive.

Progressive degeneration of stereocilia in cochlear hair cells in hearing-impaired kuru2 mice

BACKGROUND

We previously isolated a mouse strain, kuru2, which exhibits abnormal behavior and hearing impairment. To investigate the etiology of this impairment, the ultrastructure of the inner ear was examined.

MATERIALS AND METHODS

The morphologies of the cochlea and the vestibule of control (Jcl:ICR) and mutant mice were analyzed by electron microscopy. In some experiments, the mice were cross-mated and their offspring examined.

RESULTS

The mutant mice displayed progressive degeneration of the stereocilia in the cochlea. The stereocilia started to degenerate on post-natal day 10 and, subsequently, the hair bundles continued to degenerate. On day 18, degeneration of the stereocilia was complete. In contrast, the vestibule was intact.

DISCUSSION

Many mutant mice display hearing impairment These mice demonstrate a characteristic morphology of the inner ear and, since correlations may be made with corresponding human diseases, the current results could contribute to the further understanding of hearing impairment mechanisms.

A Myo7a mutation cosegregates with stereocilia defects and low-frequency hearing impairment

A phenotype-driven approach was adopted in the mouse to identify molecules involved in ear development and function. Mutant mice were obtained using N-ethyl- N-nitrosourea (ENU) mutagenesis and were screened for dominant mutations that affect hearing and/or balance. Heterozygote headbanger ( Hdb/+) mutants display classic behavior indicative of vestibular dysfunction including hyperactivity and head bobbing, and they show a Preyer reflex in response to sound but have raised cochlear thresholds especially at low frequencies. Scanning electron microscopy of the surface of the organ of Corti revealed abnormal stereocilia bundle development from an early age that was more severe in the apex than the base. Utricular stereocilia were long, thin, and wispy. Homozygotes showed a similar but more severe phenotype. The headbanger mutation has been mapped to a 1.5-cM region on mouse Chromosome 7 in the region of the unconventional myosin gene Myo7a, and mutation screening revealed an A>T transversion that is predicted to cause an isoleucine-to-phenylalanine amino acid substitution (I178F) in a conserved region in the motor-encoding domain of the gene. Protein analysis revealed reduced levels of myosin VIIa expression in inner ears of headbanger mice. Headbanger represents a novel inner ear phenotype and provides a potential model for low-frequency-type human hearing loss.

Synapsin-like immunoreactivity is present in hair cells and efferent terminals of the toadfish crista ampullaris

The synapsins are presynaptic membrane-associated proteins involved in neurotransmitter release. They are differentially expressed in tissues and cells of the central and peripheral nervous system. In vestibular end organs of mammals, synapsin I-like immunoreactivity has been reported in efferent and afferent terminals and in afferent nerve calyces surrounding type I hair cells. In addition, synapsin I has recently been described in several non-neural cell lines. The present study was conducted to locate synapsin-like immunoreactivity in the neuronal and non-neuronal cells of the fish crista ampullaris, to examine the possibility that the non-neuronal sensory receptor cells express synapsins in vivo. Synapsin-like immunostaining was visualized by immunofluorescence detection in wholemounts of the toadfish crista ampullaris using multiphoton laser scanning microscopy and by electron microscopic visualization of post-embedding immunogold labeling. The results demonstrate that synapsin-like immunoreactivity is present in vestibular hair cells and efferent boutons of the toadfish crista ampullaris. Afferent endings are not labeled. Staining in hair cells is not associated with the synaptic ribbons, suggesting that there is an additional, non-synaptic role for the synapsins in some non-neuronal cells of vertebrates. Moreover, while the cristae of amniote and anamniote species share many functional attributes, differences in their synaptic vesicle-associated protein profiles appear to reflect their disparate hair cell populations.

The structure of tip links and kinocilial links in avian sensory hair bundles

Recent studies have indicated that the tip links and kinocilial links of sensory hair bundles in the inner ear have similar properties and share a common epitope, and that cadherin 23 may also be a component of each link type. Transmission electron microscopy was therefore used to study and compare the fine structure of the tip links and kinocilial links in avian sensory hair bundles. Tannic acid treatment revealed a thin strand, 150-200 nm long and 8-11 nm thick, present in both link types. Fourier analysis of link images showed that the strand of both link types is formed from two filaments coiled in a helix-like arrangement with an axial period of 20-25 nm, with each filament composed of globular structures that are approximately 4 nm in diameter. Differences in the radius and period of the helix-like structure may underlie the observed variation in the length of tip and kinocilial links. The similar helix-like structure of the tip links and kinocilial links is in accord with the presence of a common cell-surface antigen (TLA antigen) and similarities in the physical and chemical properties of the two link types. The spacing of the globular structures comprising each filament of the two link types is similar to the 4.3 nm center-to-center spacing reported for the globular cadherin repeat, and is consistent with the suggestion that cadherin 23 is the tip link.

Ultrastructural observations of efferent terminals in the crista ampullaris of the toadfish, Opsanus tau

The present study was conducted to visualize the ultrastructural features of vestibular efferent boutons in the oyster toadfish, Opsanus tau. The crista ampullaris of the horizontal semicircular canal was processed for and examined by routine transmission electron microscopy. The results demonstrate that such boutons vary in size and shape, and contain a heterogeneous population of lucent vesicles with scattered dense core vesicles. Efferent contacts with hair cells are characterized by local vesicle accumulations in the presynaptic terminal and a subsynaptic cistern in the postsynaptic region of the hair cell. Serial efferent to hair cell to afferent synaptic arrangements are common, particularly in the central portion of the crista. However, direct contacts between efferent terminals and afferent neurites were not observed in our specimens. The existence of serial synaptic contacts, often with a row of vesicles in the efferent boutons lining the efferent-afferent membrane apposition, suggests that the efferent influence on the crista may involve both synaptic and nonsynaptic, secretory mechanisms. Further, it is suggested that differences in more subtle aspects of synaptic architecture and/or transmitter and receptor localization and interaction may render the efferent innervation of the peripheral crista less effective in influencing sensory processing.

Posture, head stability, and orientation recovery during vestibular regeneration in pigeons

Compensatory behavior such as oculomotor, gaze, and postural responses that occur during movement largely depend upon a functioning vestibular system. In the present study, the initial loss and subsequent recovery of postural and head stability in pigeons undergoing vestibular regeneration were examined. Adult pigeons were trained to manipulate a straight run chamber to peck an illuminated key for fluid reward. Six behavioral measures assessing performance, posture, and head stability were quantified. These included run latency, steps (walking), path negotiation (lane changes), gaze saccades, head bobs, and head shakes. Once normative values were obtained for four birds, complete lesion of all receptor cells and denervation of the epithelia in the vestibular endorgans were produced using a single intralabyrinthine application of streptomycin sulfate. Each bird was then tested at specific times during regeneration and the same behavioral measures examined. At 7 days post-streptomycin treatment (PST), all birds exhibited severe postural and head instability, with tremors, head shakes, staggering, and circling predominating. No normal trial runs, walking, gaze saccades, or head bobs were present. Many of these dysfunctions persisted through 3-4 weeks PST. Gradually, tremor and head shakes diminished and were replaced with an increasing number of normal head bobs during steps and gaze saccades. Beginning at 4 weeks PST, but largely inaccurate, was the observed initiation of directed steps, less staggering, and some successful path negotiation. As regeneration progressed, spatial orientation and navigation ability increased and, by 49 days PST, most trials were successful. By 70 days PST, all birds had recovered to pretreatment levels. Thus, it was observed that ataxia must subside, coincident with normalized head and postural stability prior to the recovery of spatial orientation and path navigation recovery. Parallels in recovery were drawn to hair cell regeneration and afferent responsiveness, as inferred from present results and those in other investigations.

Quantitative analysis of stereociliary arrays on vestibular hair cells

We have developed a method for quantifying the number, spacing, and distribution of stereocilia on the apical surface of hair cells using spatial autocorrelation analysis and statistics for directional data. Here, we illustrate the method using idealized hair bundles, and we apply it to scanning micrographs of turtle hair cells from the utricle and posterior canal, and to freeze-fracture preparations of bullfrog saccule. The analysis suggests three common features of stereociliary bundles. First, bundle geometries form a continuum from 'loose' to 'tight' rather than two distinct groups. Second, interciliary spacing along the three hexagon axes is not equal; spacing is usually widest along the hexagon axis closest to the bundle's axis of bilateral symmetry (the presumptive excitatory axis). Third, spacing between stereocilia changes with distance from the kinocilium. All three features will influence predictions of the tip link tensions that accompany bundle deflection.

Circling behavior in the Ecl mouse is caused by lateral semicircular canal defects

The epistatic circler mouse (Ecl mouse) is a preexisting mutant, which displays a circling phenotype and hyperactivity. It has been shown that the circling phenotype in this mutant results from a complex inheritance pattern, but the vestibular pathology has not been analyzed. The present study deals with the morphological and functional basis responsible for the circling behavior in the Ecl mouse. Morphological examination of the inner ears revealed a bilateral malformation of the horizontal (lateral) semicircular canal and duct. No cochlear abnormalities were detected, and auditory brainstem response (ABR) measurements indicated that the auditory system is not affected. Investigation of the vestibuloocular reflex (VOR) in Ecl mice showed that their horizontal VOR on stimulation is virtually absent, which correlates with the morphological findings.

Signaling pathway for apoptosis of vestibular hair cells of mice due to aminoglycosides

Previous studies on regeneration of mammalian vestibular hair cells have indicated the potential for self-repair of damaged hair cells. The rescue of damaged hair cells from cell death may therefore increase regenerated hair cells in affected vestibular epithelia. The role of apoptosis in the degradation of vestibular hair cells following aminoglycoside treatment has been elucidated. To seek a method of protecting vestibular hair cells from aminoglycoside toxicity, we examined the apoptosis signaling pathway of vestibular hair cells due to aminoglycoside toxicity. Induction of apoptosis in hair cells of mouse ampullar cristae damaged by local application of neomycin was evaluated by the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) method and transmission electron microscopy (TEM). Immunohistochemistry for apoptosis-related proteins was employed to determine the signaling pathway of apoptosis of hair cells. The occurrence of apoptosis in hair cells was demonstrated by TUNEL staining and TEM. In apoptotic hair cells, activation of caspase-3 and -9, and redistribution of cytochrome c was identified, while there was no expression of activated caspase-8 or apoptosis-inducing factor. In conclusion, these findings indicate that the mitochondria-mediated pathway of apoptosis may play a role in inducing the apoptosis of vestibular hair cells due to aminoglycoside toxicity. Stabilization of the mitochondrial membrane may therefore rescue vestibular hair cells from apoptosis, leading to an increase in self-repaired hair cells in affected vestibular epithelia.

Functional and Anatomic Alterations in the Gentamicin-Damaged Vestibular System in the Guinea Pig

HYPOTHESIS

The purpose of this study was to investigate the expected functional and morphologic effect of gentamicin on the vestibular system simultaneously by measurement of vestibular evoked potentials and electron microscopic evaluation.

BACKGROUND

Vestibular short-latency evoked potentials to linear acceleration have been shown to be a useful parameter of vestibular function. In gentamicin-treated animals, the morphologic damage has been well documented, although this has seldom been quantified.

METHODS

Fifteen guinea pigs were divided into three equal groups. Two groups received different dosages of intramuscular gentamicin for 3 weeks; the third group was the control group. Vestibular short-latency evoked potentials to linear acceleration pulses were measured. After the last gentamicin dose, the utricles were prepared for scanning and transmission electron microscopy. On scanning electron microscopy photographs, the surface area damage ratio of the utricles, a simple method of quantifying gross morphologic damage, was calculated.

RESULTS

The vestibular short-latency evoked potential of gentamicin-treated guinea pigs showed a slow-developing, damaging, dose-response effect on the function of the vestibular system (p = 0.01). Scanning electron microscopy and transmission electron microscopy showed severe morphologic damage in the sensory hair cells of the utricle. The surface area damage ratio showed a dose-response relationship (p = 0.01).

CONCLUSION

Functional and anatomic alterations in the gentamicin-damaged vestibular system in the guinea pig are related.

Differing effects on the inner ear of three gentamicin compounds: GM-C1, -C2 and -C1a

OBJECTIVE

Gentamicin (GM) has been used to ablate the vestibular system function as a form of treatment for Ménière's disease. Generally, clinicians have not recognized that commercial GM comprises three major compounds--GM-C1, -C2 and -C1a--and the effects of the individual compounds on the inner ear have not been elucidated. In this study we investigated differences in cochleo- and vestibulotoxicity between the three compounds

MATERIAL AND METHODS

Hartley guinea pigs were treated for 2 weeks with daily subcutaneous injections (100 mg/kg) of the GM complex or the GM-C1, -C2 or -C1a compounds. After a further 2 weeks, we assessed the inner ear by measuring the auditory brainstem response (ABR) and performing a sinusoidal rotation test (SRT) and morphological analyses.

RESULTS

The ABR thresholds in the C2 group were found to be more impaired than those in the other groups In the SRT, the nystagmus counts in the C2 group were lower than those in the controls and in the C1 group. The counts in the C1a group were lower than those in the C1 group. Morphological analyses revealed that the proportion of remaining outer hair cells was less in the C2 group than in the other groups.

CONCLUSION

Our results suggest that characterization of differences in ototoxicity may be useful for treatment of Ménière's disease with GM.

Ultrastructural analysis of [3H]thymidine-labeled cells in the rat utricular macula

Ototoxic drugs stimulate cell proliferation in adult rat vestibular sensory epithelia, as does the infusion of transforming growth factor alpha (TGFalpha) plus insulin. We sought to determine whether new hair cells can be regenerated by means of a mitotic pathway. Previously, studies have shown that the nuclei of some newly generated cells are located in the lumenal half of the sensory epithelium, suggesting that some may be newly generated sensory hair cells. The aim of this study was to examine the ultrastructural characteristics of newly proliferated cells after TGFalpha stimulation and/or aminoglycoside damage in the utricular sensory epithelium of the adult rat. The cell proliferation marker tritiated-thymidine was infused, with or without TGFalpha plus insulin, into the inner ears of normal or aminoglycoside-damaged rats for 3 or 7 days by means of osmotic pumps. Autoradiographic techniques and light microscopy were used to identify cells synthesizing DNA. Sections with labeled cells were re-embedded, processed for transmission electron microscopy, and the ultrastructural characteristics of the labeled cells were examined. The following five classes of tritiated-thymidine labeled cells were identified in the sensory epithelium: (1) labeled cells with synaptic specializations that appeared to be newly generated hair cells, (2) labeled supporting cells, (3) labeled leukocytes, (4) labeled cells that we have classified as "active cells" in that they are relatively nondescript but contain massive numbers of polyribosomes, and (5) labeled degenerating hair cells. These findings suggest that new hair cells can be generated in situ by means of a mitotic mechanism in the vestibular sensory epithelium of adult mammals.

Caspase inhibitors promote vestibular hair cell survival and function after aminoglycoside treatment in vivo

The sensory hair cells of the inner ear undergo apoptosis after acoustic trauma or aminoglycoside antibiotic treatment, causing permanent auditory and vestibular deficits in humans. Previous studies have demonstrated a role for caspase activation in hair cell death and ototoxic injury that can be reduced by concurrent treatment with caspase inhibitors in vitro. In this study, we examined the protective effects of caspase inhibition on hair cell death in vivo after systemic injections of aminoglycosides. In one series of experiments, chickens were implanted with osmotic pumps that administrated the pan-caspase inhibitor z-Val-Ala-Asp(Ome)-fluoromethylketone (zVAD) into inner ear fluids. One day after the surgery, the animals received a 5 d course of treatment with streptomycin, a vestibulotoxic aminoglycoside. Direct infusion of zVAD into the vestibule significantly increased hair cell survival after streptomycin treatment. A second series of experiments determined whether rescued hair cells could function as sensory receptors. Animals treated with streptomycin displayed vestibular system impairment as measured by a greatly reduced vestibulo-ocular response (VOR). In contrast, animals that received concurrent systemic administration of zVAD with streptomycin had both significantly greater hair cell survival and significantly increased VOR responses, as compared with animals treated with streptomycin alone. These findings suggest that inhibiting the activation of caspases promotes the survival of hair cells and protects against vestibular function deficits after aminoglycoside treatment.

Afferent innervation of the utricular macula in pigeons

Biotinylated dextran amine (BDA) was used to retrogradely label afferents innervating the utricular macula in adult pigeons. The pigeon utriclar macula consists of a large rectangular-shaped neuroepithelium with a dorsally curved anterior edge and an extended medioposterior tail. The macula could be demarcated into several regions based on cytoarchitectural differences. The striola occupied 30% of the macula and contained a large density of type I hair cells with fewer type II hair cells. Medial and lateral extrastriola zones were located outside the striola and contained only type II hair cells. A six- to eight-cell-wide band of type II hair cells existed near the center of the striola. The reversal line marked by the morphological polarization of hair cells coursed throughout the epithelium, near the peripheral margin, and through the center of the type II band. Calyx afferents innervated type I hair cells with calyceal terminals that contained between 2 and 15 receptor cells. Calyx afferents were located only in the striola region, exclusive of the type II band, had small total fiber innervation areas and low innervation densities. Dimorph afferents innervated both type I and type II hair cells with calyceal and bouton terminals and were primarily located in the striola region. Dimorph afferents had smaller calyceal terminals with few type I hair cells, extended fiber branches with bouton terminals and larger innervation areas. Bouton afferents innervated only type II hair cells in the extrastriola and type II band regions. Bouton afferents innervating the type II band had smaller terminal fields with fewer bouton terminals and smaller innervation areas than fibers located in the extrastriolar zones. Bouton afferents had the most bouton terminals on the longest fibers, the largest innervation areas with the highest innervation densities of all afferents. Among all afferents, smaller terminal innervation fields were observed in the striola and large fields were located in the extrastriola. The cellular organization and innervation patterns of the utricular maculae in birds appear to represent an organ in adaptive evolution, different from that observed for amphibians or mammals.

Afferent innervation patterns of the saccule in pigeons

The innervation patterns of vestibular saccular afferents were quantitatively investigated in pigeons using biotinylated dextran amine as a neural tracer and three-dimensional computer reconstruction. Type I hair cells were found throughout a large portion of the macula, with the highest density observed in the striola. Type II hair cells were located throughout the macula, with the highest density in the extrastriola. Three classes of afferent innervation patterns were observed, including calyx, dimorph, and bouton units, with 137 afferents being anatomically reconstructed and used for quantitative comparisons. Calyx afferents were located primarily in the striola, innervated a number of type I hair cells, and had small innervation areas. Most calyx afferent terminal fields were oriented parallel to the anterior-posterior axis and the morphological polarization reversal line. Dimorph afferents were located throughout the macula, contained fewer type I hair cells in a calyceal terminal than calyx afferents and had medium sized innervation areas. Bouton afferents were restricted to the extrastriola, with multi-branching fibers and large innervation areas. Most of the dimorph and bouton afferents had innervation fields that were oriented dorso-ventrally but were parallel to the neighboring reversal line. The organizational morphology of the saccule was found to be distinctly different from that of the avian utricle or lagena otolith organs and appears to represent a receptor organ undergoing evolutionary adaptation toward sensing linear motion in terrestrial and aerial species.

Apoptosis during chick inner ear development: some observations by TEM and TUNEL techniques

In order to clarify the occurrence, distribution and possible role of apoptosis during inner ear development, the ultrastructural aspects (by TEM) (at 9-19 incubation day and 1 day after hatching) and the distribution of the apoptotic phenomenon (by the TdT-mediated dUTP nick end-labeling technique), were studied in the crista ampullaris of chick embryo at 5-19 days of incubation to hatching and of postnatal 1-day old chick. We found, in the sensorial epithelium, dark supporting cells in chick embryos and mainly dark hair cells in postnatal chicks, both with ultrastructural features consistent with those of apoptosis. The presence of apoptotic phenomena was confirmed by the TUNEL technique. According to our findings, it is hypothesized that apoptosis in the inner ear may be involved: 1) at first, in macroscopic remodelling of the membranous labyrinth in early developmental stages, 2) later, in the correct differentiation of the hair and of the supporting cells, leading to characteristic cellular pattern formation and 3) finally, in physiological cell turnover of the postnatal chicken sensorial epithelium of the crista.

Depolarization redistributes synaptic membrane and creates a gradient of vesicles on the synaptic body at a ribbon synapse

We used electron tomography of frog saccular hair cells to reconstruct presynaptic ultrastructure at synapses specialized for sustained transmitter release. Synaptic vesicles at inhibited synapses were abundant in the cytoplasm and covered the synaptic body at high density. Continuous maximal stimulation depleted 73% of the vesicles within 800 nm of the synapse, with a concomitant increase in surface area of intracellular cisterns and plasmalemmal infoldings. Docked vesicles were depleted 60%-80% regardless of their distance from the active zone. Vesicles on the synaptic body were depleted primarily in the hemisphere facing the plasmalemma, creating a gradient of vesicles on its surface. We conclude that formation of new synaptic vesicles from cisterns is rate limiting in the vesicle cycle.

Subcellular immunolocalization of NMDA receptor subunit NR1, 2A, 2B in the rat vestibular periphery

The immunohistochemical localization of the NMDA glutamate receptor subunits NR1, NR2A, and NR2B was investigated in the rat vestibular periphery at the light and electron microscopy level using specific antipeptide antibodies. The afferent calyceal terminals and nerve fibers innervating type I vestibular hair cells were strongly NR1, NR2A, and NR2B immunoreactive. Under electron microscopy, the basolateral type I hair cell membrane was NR1 immunoreactive. The type II hair cell and its afferent boutons were NR1, NR2A, and NR2B non-immunoreactive. Nearly all of Scarpa's ganglion neurons were NR1 immunoreactive, but there was a subset of NR2A non-immunoreactive neurons. Additionally, the larger sized Scarpa's ganglia neurons were NR2B immunoreactive, while the smaller neurons were non-immunoreactive. These findings are strong evidence for functional NMDA receptor mediation or modulation of afferent excitatory neurotransmission from type I but not type II vestibular hair cells to the primary afferent nerve. The receptor subtype(s) may be a combination of NR1/NR2A, NR1/NR2B, and/or NR1/NR2A/NR2B.

Survival of bundleless hair cells and subsequent bundle replacement in the bullfrog's saccule

Our senses of hearing and balance depend upon hair cells, the sensory receptors of the inner ear. Millions of people suffer from hearing and balance deficits caused by damage to hair cells as a result of exposure to noise, aminoglycoside antibiotics, and antitumor drugs. In some species such damage can be reversed through the production of new cells. This proliferative response is limited in mammals but it has been hypothesized that damaged hair cells might survive and undergo intracellular repair. We examined the fate of bullfrog saccular hair cells after exposure to a low dose of the aminoglycoside antibiotic gentamicin to determine whether hair cells could survive such treatment and subsequently be repaired. In organ cultures of the bullfrog saccule a combination of time-lapse video microscopy, two-photon microscopy, electron microscopy, and immunocytochemistry showed that hair cells can lose their hair bundle and survive as bundleless cells for at least 1 week. Time-lapse and electron microscopy revealed stages in the separation of the bundle from the cell body. Scanning electron microscopy (SEM) of cultures fixed 2, 4, and 7 days after antibiotic treatment showed that numerous new hair bundles were produced between 4 and 7 days of culture. Further examination revealed hair cells with small repaired hair bundles alongside damaged remnants of larger surviving bundles. The results indicate that sensory hair cells can undergo intracellular self-repair in the absence of mitosis, offering new possibilities for functional hair cell recovery and an explanation for non-proliferative recovery.

Ultrastructural evaluation of calcitonin gene-related peptide immunoreactivity in the human cochlea and vestibular endorgans

Calcitonin gene-related peptide (CGRP) is a neuropeptide widely distributed in the peripheral and central nervous system. Demonstrated in the efferent systems of the mammalian cochlea and vestibule, immunoreactive patterns of CGRP may vary by species. There is, however, no information in the literature investigating CGRP localization in the human cochlea. In the present study, the ultrastructural localization of CGRP immunoreactivity was evaluated in the human inner ear with immunoelectron microscopy. It was found that, in human cochlea, CGRP immunoreactivity was located in unmyelinated nerve fibres of the spiral lamina, inner spiral fibres beneath inner hair cells, tunnel spiral fibres, tunnel crossing fibres and outer radial fibres. In endorgans of human vestibule, CGRP immunoreactivity was located in vesiculated nerve fibres and bouton-type nerve terminals which were seen to contact afferent nerve chalices surrounding type I sensory cells and afferent nerve fibres, or to form an en passant contact with afferent dendrites. CGRP immunoreactivity appeared to be confined to efferent systems in all cases. This study presents evidence that CGRP could serve a role in neurotransmission or neuroregulation in both cochlear and vestibular efferent systems of human.

Functional morphology of the crista ampullaris: with special interests in sensory hairs and cupula: a review

The functional significance of the ciliary interconnections and cupula has been reviewed. The ciliary interconnecting systems are divided into 2 types, i.e. side links and tip links. The side links acts to maintain the regular distance between the cilia thereby keeping the geometrical arrangement of the entire sensory hair bundle intact as well as to prevent close contact between neighbouring cilia. The tip links, stretching upwards from the tips of the shorter stereocilia to their taller neighbouring shafts, are actually involved in mechanoelectrical transduction. The cupula is composed of the cupula and subcupular meshwork. The subcupular meshwork consists of long branching filaments cross-bridged to one another. The cupula would function as a rigid plate and equally distribute the shear force of the cupula to all the ciliary bundles. The subcupular meshwork may play a role in the transmission of the shear strain force of the cupula to the ciliary bundle and may also exert an additional damping effect in order to prevent unwanted vibrations.

Pathology of the rat vestibular sensory epithelia during subchronic 3,3'-iminodipropionitrile exposure: hair cells may not be the primary target of toxicity

3,3'-Iminodipropionitrile (IDPN) is a neurotoxic compound that causes proximal neurofilamentous axonopathy and loss of the vestibular sensory hair cells. During subchronic exposure, the hair cells are eliminated by extrusion of the virtually intact cell from the sensory epithelia towards the luminal cavity. We describe the alterations of the vestibular epithelia before and during hair cell extrusion. Adult male Long-Evans rats were exposed to 0.2% IDPN in the drinking water for 1, 3, 5, 8 or 14 weeks, or to 0.1% IDPN for 14 weeks. Protrusion and subsequent extrusion of hair cells were observed in the cristae and utricular maculae of rats exposed to 0.2% IDPN for at least 5 weeks. At earlier time points and at lower doses, we observed the following pathological signs: blebbing of hair cells, swelling, retraction and fragmentation of the afferent nerve terminals, detachment of hair cells from the surrounding structures and loss of the pre- and post-synaptic membrane thickenings between hair cells and their afferent terminals. Widespread enlargement of the intercellular spaces also preceded and accompanied the extrusion process. The present data challenge the hypothesis that IDPN specifically affects hair cells.

The inner ear macular sensory epithelia of the Daubenton's bat

The inner ear macular sensory epithelia of the Daubenton's bat were examined quantitatively to estimate the area and total number of hair cells. Ultrastructural examination of the sensory epithelium reveals two main types of hair cells: the chalice-innervated hair cell and the bouton-innervated hair cell. The existence of an intermediate type, with a nerve ending covering the lateral side of the hair cell, indicates that the chalice-innervated hair cells are derived from bouton-innervated hair cells. Thus, at least a part of the bouton-innervated hair cells forms a transitional stage. A number of immature as well as apoptotic hair cells were observed. It is suggested that a continuous production of new hair cells takes place in mature individuals, probably based on transdifferentiation of supporting cells.

Vestibular receptor cells and signal detection: bioaccelerometers and the hexagonal sampling of two-dimensional signals

The inner ear contains sensory organs which signal changes in head movement. The vestibular sacs, in particular, are sensitive to linear accelerations. Electron microscopic images have revealed the structure of tiny sensory hair bundles, whose mechanical deformation results in the initiation of neuronal activity and the transmission of electrical signals to the brain. The structure of the hair bundles is shown in this paper to be that of the most efficient two-dimensional phased-array signal processors.

Gravity receptors: an ultrastructural basis for peripheral sensory processing

Our ultrastructural study of serial sections has shown that type II hair cells of the anterior part of the utricular macula are integrated into the afferent neural circuitry of type I cells, which are arranged in clusters. Additionally, there exists a complex system of intramacularly originating efferent-type nerve fibers and terminals. The findings taken together suggest that, on morphological grounds, complex processing of sensory information occurs in gravity receptors. Asymmetry of such a complex system may contribute to motion and Space-motion sickness.

Growth factor treatment enhances vestibular hair cell renewal and results in improved vestibular function

The vestibules of adult guinea pigs were lesioned with gentamicin and then treated with perilymphatic infusion of either of two growth factor mixtures (i.e., GF I or GF II). GF I contained transforming growth factor alpha (TGFalpha), insulin-like growth factor type one (IGF-1), and retinoic acid (RA), whereas GF II contained those three factors and brain-derived neurotrophic factor. Treatment with GF I significantly enhanced vestibular hair cell renewal in ototoxin-damaged utricles and the maturation of stereociliary bundle morphology. The addition of brain-derived neurotrophic factor to the GF II infusion mixture resulted in the return of type 1 vestibular hair cells in ototoxin-damaged cristae, and improved vestibular function. These results suggest that growth factor therapy may be an effective treatment for balance disorders that are the result of hair cell dysfunction and/or loss.

Lectin from Griffonia simplicifolia identifies an immature-appearing subpopulation of sensory hair cells in the avian utricle

The sensory hair cells of the inner ear are coated with a variety of glycoproteins and glycolipids which can be identified by the binding of specific lectins. The present study examined the binding patterns of three lectins-Wheat Germ Agglutinin, Peanut Agglutinin, and lectin from Griffonia simplicifolia (Isoform B(4))-in the avian utricle. Each of the lectins exhibited a distinct pattern of hair cell labeling. Wheat Germ Agglutinin (WGA) appeared to label the ciliary bundles of all sensory hair cells. In contrast, the binding of Peanut Agglutinin (PNA) was mainly confined to the ciliary bundles of extrastriolar hair cells. Finally, lectin from Griffonia simplicifolia (GS-IB(4)) labeled a subpopulation of hair cells in all regions of the chick utricle. Those bundles were much smaller than the majority of ciliary bundles labeled by either WGA or PNA, and the density of GS-IB(4)-labeled bundles in the normal mature utricle was relatively low. Increased densities of GS-IB(4)-labeled hair cells were observed in the embryonic utricle and during the process of hair cell regeneration. The observations suggest that GS-IB(4) labels a glycoprotein that is expressed preferentially on the ciliary bundles of immature hair cells.

Continuous hair cell turnover in the inner ear vestibular organs of a mammal, the Daubenton's bat (Myotis daubentonii)

In both humans and mice the number of hair cells in the inner ear sensory epithelia declines with age, indicating cell death (Park et al. 1987; Rosenhall 1973). However, recent reports demonstrate the ability of the vestibular sensory epithelia to regenerate after injury (Forge et al. 1993, 1998; Kuntz and Oesterle 1998; Li and Forge 1997; Rubel et al. 1995; Tanyeri et al. 1995). Still, a continuous hair cell turnover in the vestibular epithelia has not previously been demonstrated in mature mammals. Bats are the only flying mammals, and they are known to live to a higher age than animals of equal size. The maximum age of many species is 20 years, with average lifespans of 4-6 years (Schober and Grimmberger 1989). Further, the young are fully developed and able to fly at the age of 2 months, and thus the vestibular organs are thought to be differentiated at that age. Consequently, long-lived mammals such as bats might compensate for the loss of hair cells by producing new hair cells in their postembryonic life. Here we show that the utricular macula of adult Daubenton's bats (more than 6 months old) contains innervated immature hair cells as well as apoptotic hair cells, which strongly indicates a continuous turnover of hair cells, as previously demonstrated in birds.

A finite element method for mechanical response of hair cell ciliary bundles

This paper describes the development of a methodology for performing a mechanical analysis of hair cell ciliary bundles. The cilia were modeled as shear deformable beams, and interconnections were modeled as two-force members. These models were incorporated into software, which performs a finite element analysis of a user-defined bundle. The algorithm incorporates aspects of the bundle such as geometric realignment and buckling of compressed side links. A sample bundle is introduced and results of modeling it are presented.

Neuronal organization of the utricular macula concerned with innervation of single vestibular neurons in the cat

We investigated whether cross-striolar inhibition, which may increase sensitivity to linear acceleration, contributed to utricular (UT) afferent innervation of single vestibular neurons (VNs). Excitatory and inhibitory postsynaptic potentials (EPSPs, IPSPs, respectively) were recorded from VNs after focal stimulation of the UT macula (M). From a total of 83 VNs, 25 (30%) neurons received inputs from both sides of the UTM, and the response patterns were opposite, i.e. cross-striolar inhibition was observed. In roughly 2/3 of these neurons, stimulation of the medial side of the UTM evoked EPSPs, while stimulation of the lateral side evoked IPSPs. In the remaining 1/3 neurons, the response patterns were opposite. Thirty-two (39%) of the 83 neurons received the identical pattern of inputs from both sides of the UTM: EPSPs in 26 neurons and IPSPs in six neurons. Twenty-six (31%) of the 83 neurons received inputs from either the medial or the lateral side of the UTM. These findings suggest that cross-striolar inhibition existed in the UT system, although it was not a dominant circuit that increased the sensitivity as in the saccular system [15].

Simulation studies of vestibular macular afferent-discharge patterns using a new, quasi-3-D finite volume method

A quasi-three-dimensional finite-volume numerical simulator was developed to study passive voltage spread in vestibular macular afferents. The method, borrowed from computational fluid dynamics, discretizes events transpiring in small volumes over time. The afferent simulated had three calyces with processes. The number of processes and synapses, and direction and timing of synapse activation, were varied. Simultaneous synapse activation resulted in shortest latency, while directional activation (proximal to distal and distal to proximal) yielded most regular discharges. Color-coded visualizations showed that the simulator discretized events and demonstrated that discharge produced a distal spread of voltage from the spike initiator into the ending. The simulations indicate that directional input, morphology, and timing of synapse activation can affect discharge properties, as must also distal spread of voltage from the spike initiator. The finite volume method has generality and can be applied to more complex neurons to explore discrete synaptic effects in four dimensions.

Distribution and time course of hair cell regeneration in the pigeon utricle

Vestibular and cochlear regeneration following ototoxic insult from aminoglycoside antibiotics has been well documented, particularly in birds. In the present study, intraotic application of a 2 mg streptomycin paste was used to achieve complete vestibular hair cell destruction in pigeons (Columba livia) while preserving regenerative ability. Scanning electron microscopy was used to quantify hair cell density longitudinally during regeneration in three different utricular macula locations, including the striola, central and peripheral regions. The utricular epithelium was void of stereocilia (indicating hair cell loss) at 4 days after intraotic treatment with streptomycin. At 2 weeks the stereocilia began to appear randomly and mostly in an immature form. However, when present most kinocilia were polarized toward the developing striola. Initially, regeneration occurred more rapidly in the central and peripheral regions of the utricle as compared to the striola. As regeneration proceeded from 2 to 12 weeks, hair cell density in the striola region equaled the density noted in the central and peripheral regions. At 24 weeks, hair cell density of the central and peripheral regions was equal to normal values, however the striola region had a slightly greater hair cell density than that observed for normal animals.

Glutamate-like immunoreactivity during hair cell recovery after gentamicin exposure in the chinchilla vestibular sensory periphery

OBJECTIVE

Determine the expression of glutamate by immunohistochemistry in normal and recovering vestibular hair cells in the chinchilla crista ampullaris after gentamicin ototoxicity.

STUDY DESIGN

In five groups of three animals each, ototoxicity was produced by placing gentamicin (50 microg)-impregnated Gelfoam pellets within the perilymphatic space of the superior semicircular canal. Animals were sacrificed at 1, 2, 4, 8, and 16 weeks after treatment. A group of normal (n=3) animals was also processed.

METHODS

For the detection of glutamate the inner ears of these animals were dissected, and the horizontal cristae ampullaris embedded in plastic. Two-micron-thick tissue sections were obtained and incubated with monoclonal antibodies against glutamate. The immunoreaction was detected using the avidinbiotinylated-complex technique and diaminobenzidine was the chromogen.

RESULTS

Normal sensory epithelia demonstrated type I and type II hair cells with moderate glutamate-like immunoreactivity. Supporting cells demonstrated no glutamate-like immunoreactivity. Afferent nerve fibers and calyxes surrounding type I hair cells demonstrated strong glutamate-like immunoreactivity. At 1 and 2 weeks after treatment the few type II hair cells surviving ototoxic treatment (15%-18%) contained moderate glutamate-like immunoreactivity, supporting cells showed no immunoreactivity, and nerve terminals and fibers displayed strong immunoreactivity. At 4 and 8 weeks after treatment, recovered hair cells (80%) had greater glutamate-like immunoreactivity when compared with normal hair cells, supporting cells displayed no glutamate-like immunoreactivity, and afferent fibers contained strong glutamate-like immunoreactivity. At 16 weeks, glutamate-like immunoreactivity in hair cells returned to normal level.

CONCLUSION

Glutamate may be used as an indicator of hair cell differentiation and as an index of the molecular recovery of hair cells after ototoxicity.

Immunoelectron microscopy of AMPA receptor subunits reveals three types of putative glutamatergic synapse in the rat vestibular end organs

To characterize the synapses between hair cells and afferent nerve endings in the rat vestibular end organs, the ultrastructural localization of AMPA receptor subunits (GluR1-4) was examined by postembedding immunogold cytochemistry. Immunoreactivities for GluR2/3 and GluR4 were associated with the synapses between type I hair cells and the surrounding chaliceal nerve endings and with the bouton type nerve endings contacting type II hair cells. There was no detectable immunoreactivity for GluR1. A third type of immunoreactive synapse was found between the outer face of chalices and type II hair cells. While the linear densities of gold particles (particles per micrometer postsynaptic specialization) of bouton type endings and chaliceal nerve endings were the same, the former type of ending showed larger postsynaptic specializations and, hence, a higher number of receptor molecules. These data indicate that there are three types of putative glutamatergic synapse in the vestibular end organ.

Hair cell recovery in the chinchilla crista ampullaris after gentamicin treatment: a quantitative approach

The mechanisms for hair cell recovery were investigated after intraortic application of 50 microg gentamicin into the perilymphatic space of the superior semicircular canal of the chinchilla. Histologic evaluation of one normal group and four posttreatment groups (7, 14, 28, and 56 days) was made with light and transmission electron microscopic techniques. The numeric changes of hair cells and supporting cells was quantified with the dissector technique. At 7 and 14 days after treatment, no type I hair cells were present, and 85% and 88% of type II hair cells were lost. Supporting cells decreased to 76% at 7 days, but they recovered to 91% at 14 days. Recovery of the epithelia was evident 28 days after treatment; 83% were type II hair cells, and 3% were type I hair cells. The supporting cell number remained close to normal (86%). Between 14 and 28 days after treatment, there was an increase of 1758 of type II hair cells, representing approximately 125 new hair cells per day. At the same time interval the number of supporting cells remained near normal. These results suggest that new hair cells might be the result of supporting cell mitotic division and differentiation.

Localization of Ca-ATPase in frog crista ampullaris

The distribution of Ca-ATPase in frog crista ampullaris was mapped ultracytochemically by using a one-step lead citrate reaction. Electron-dense precipitates, as an expression of Ca-ATPase activity, were observed on the surface of stereocilia and on the apical membrane surrounding the cuticular plate of hair cells. Sensory cells of the isthmus region showed more reactivity than those of the peripheral regions of the crista. No reaction products were detectable on the basolateral membranes and in cytoplasmatic organelles. Supporting cells of the crista showed a quite variable Ca-ATPase reaction on microvilli and on basolateral membranes. The presence of an evident reactivity on the stereocilia is consistent with the existence of an apical calcium microdomain involved in the mechano-transduction process and supports the current view that calcium ions enter the stereocilia during natural stimulation. On the other hand, the lack of an observable reactivity on the basolateral membrane of hair cells suggests that in semicircular canals other mechanisms of active transport of calcium ions across the plasma membrane, such as Na-Ca exchange, may be involved in homeostasis of the ion.

Immunocytochemical localization of calmodulin in the vestibular end-organs of the gerbil

This study demonstrates the presence of calmodulin in the vestibular end-organs of the gerbil by use of immunocytochemistry. Using fluorescence microscopy, calmodulin was localized to the cytoplasm, cuticular plate, and stereocilia of both type I and type II hair cells in the sensory epithelia of the utricle and cristae ampullaris; no label was found in the supporting cells, the dark cells, or the nerve fibers. There was no immunoreactive distinction between the labeling of type I and type II hair cells in the striolar or extrastriolar regions. Thus, immunocytochemical labeling for calmodulin provides a good marker for hair cells in gerbil vestibular epithelium. The presence of calmodulin in the stereocilia was confirmed by immunoelectron microscopy using secondary antibodies coupled to colloidal gold.

Electrical properties of frog saccular hair cells: distortion by enzymatic dissociation

Although it is widely accepted that the electrical resonance seen in many types of auditory and vestibular hair cells contributes to frequency selectivity in these sensory systems, unexplained discrepancies in the frequency (f) and sharpness (Q) of tuning have raised serious questions. For example, enzymatically dissociated hair cells from bullfrog (Rana catesbeiana) sacculus resonate at frequencies well above the range of auditory and seismic stimuli to which the sacculus is most responsive. Such disparities, in addition to others, have led to the proposal that electrical resonance alone cannot account for frequency tuning. Using grassfrog (Rana pipiens) saccular hair cells, we show that the reported discrepancies in f and Q in this organ can be explained by the deleterious effects of enzyme (papain) exposure during cell dissociation. In patch-clamp studies of hair cells in a semi-intact epithelial preparation, we observed a variety of voltage behaviors with frequencies of 35-75 Hz. This range is well below the range of resonant frequencies observed in enzymatically dissociated hair cells and more in tune with the frequency range of natural stimuli to which the sacculus is maximally responsive. The sharpness of tuning also agreed with previous studies using natural stimuli. In contrast to results from enzymatically dissociated hair cells, both a calcium-activated K+ (KCa) current and a voltage-dependent K+ (KV) current contributed to the oscillatory responses of hair cells in the semi-intact preparation. The properties of the KCa and the Ca2+ current were altered by enzymatic dissociation. KV and a small-conductance calcium-activated K+ current were apparently eliminated.

Sensory cells determine afferent terminal morphology in cross-innervated electroreceptor organs: implications for hair cells

Type I and type II hair cells of the vestibular system are innervated by afferents that form calyceal and bouton terminals, respectively. These cannot be experimentally cross-innervated in the inner ear to determine how they influence each other. However, analogous organs are accessible for transplantation and cross-innervation in the brown ghost electric fish. These fish possess three types of electroreceptor organs. Of these, the sensory receptors of the type I tuberous organ are S-100- and parvalbumin-positive with a calbindin-positive afferent that forms a large calyx around the organ. Neither the sensory receptors nor the afferents of the ampullary organs label with these antibodies, and the afferent branches form a single large bouton beneath each receptor cell. In controls, when cut ampullary afferents reinnervate transplanted ampullary organs, they have characteristic calbindin-negative terminals with large boutons. When type I tuberous afferents reinnervate ampullary organs, receptor cells remain S-100- and parvalbumin-negative, and the tuberous afferents still express calbindin. The nerve terminals, however, make large ampullary-like boutons on the receptor cells. These results suggest that (1) afferent terminal morphology is dictated by the receptor organ; (2) expression of calbindin by the afferent is not suppressed by innervation of the incorrect end organ; (3) ampullary organs generate ampullary receptor cells although innervated by tuberous afferents; and (4) ampullary receptor cells can be trophically supported by tuberous afferents.

[Immunocytochemical study of cholinergic innervation in the neurosensory epithelia of human vestibule]

OBJECTIVE

To investigate the cholinergic innervation of the neurosensory epithelia of human vestibule.

METHODS

A modified preembedding immunostaining technique for immunoelectronmicroscopy was applied to this study. A polyclonal antibody to choline acetyltransferase (ChAT) was used as the marker of cholinergic fibers.

RESULTS

ChAT-immunoreactive products were restricted to the nerve fibers and terminals which were rich in synaptic vesicles. The ChAT-immunoreactive fibers synaps with afferent chalice as well as with type II sensory hair cells.

CONCLUSION

This study demonstrates that cholinergic fibers innervate the neurosensory epithelia of human vestible. The cholinergic fibers of human vestibular sensory epithelia belong to the vestibular efferent system.