The efferents interconnecting auditory inner hair cells

Molecular and functional profiling of cell diversity and identity in the lateral superior olive, an auditory brainstem center with ascending and descending projections

The lateral superior olive (LSO), a prominent integration center in the auditory brainstem, contains a remarkably heterogeneous population of neurons. Ascending neurons, predominantly principal neurons (pLSOs), process interaural level differences for sound localization. Descending neurons (lateral olivocochlear neurons, LOCs) provide feedback into the cochlea and are thought to protect against acoustic overload. The molecular determinants of the neuronal diversity in the LSO are largely unknown. Here, we used patch-seq analysis in mice at postnatal days P10-12 to classify developing LSO neurons according to their functional and molecular profiles. Across the entire sample (n = 86 neurons), genes involved in ATP synthesis were particularly highly expressed, confirming the energy expenditure of auditory neurons. Two clusters were identified, pLSOs and LOCs. They were distinguished by 353 differentially expressed genes (DEGs), most of which were novel for the LSO. Electrophysiological analysis confirmed the transcriptomic clustering. We focused on genes affecting neuronal input-output properties and validated some of them by immunohistochemistry, electrophysiology, and pharmacology. These genes encode proteins such as osteopontin, Kv11.3, and Kvβ3 (pLSO-specific), calcitonin-gene-related peptide (LOC-specific), or Kv7.2 and Kv7.3 (no DEGs). We identified 12 "Super DEGs" and 12 genes showing "Cluster similarity." Collectively, we provide fundamental and comprehensive insights into the molecular composition of individual ascending and descending neurons in the juvenile auditory brainstem and how this may relate to their specific functions, including developmental aspects.

Disruption of lateral olivocochlear neurons with a dopaminergic neurotoxin depresses spontaneous auditory nerve activity

Neurons of the lateral olivocochlear (LOC) system project from the auditory brainstem to the cochlea, where they synapse on radial dendrites of auditory nerve fibers. Selective LOC disruption depresses sound-evoked auditory nerve activity in the guinea pig, but enhances it in the mouse. Here, LOC disruption depressed spontaneous auditory nerve activity in the guinea pig. Recordings from single auditory nerve fibers revealed a significantly reduced proportion of fibers with the highest spontaneous firing rates (SRs) and an increased proportion of neurons with lower SRs. Ensemble activity, estimated using round window noise, also decreased after LOC disruption. Decreased spontaneous activity after LOC disruption may be a consequence of reduced tonic release of excitatory transmitters from the LOC terminals in guinea pigs.

Morphological and physiological development of auditory synapses

Acoustic communication requires gathering, transforming, and interpreting diverse sound cues. To achieve this, all the spatial and temporal features of complex sound stimuli must be captured in the firing patterns of the primary sensory neurons and then accurately transmitted along auditory pathways for additional processing. The mammalian auditory system relies on several synapses with unique properties in order to meet this task: the auditory ribbon synapses, the endbulb of Held, and the calyx of Held. Each of these synapses develops morphological and electrophysiological characteristics that enable the remarkably precise signal transmission necessary for conveying the miniscule differences in timing that underly sound localization. In this article, we review the current knowledge of how these synapses develop and mature to acquire the specialized features necessary for the sense of hearing.

Efferent synapses return to inner hair cells in the aging cochlea

Efferent innervation of the cochlea undergoes extensive modification early in development, but it is unclear if efferent synapses are modified by age, hearing loss, or both. Structural alterations in the cochlea affecting information transfer from the auditory periphery to the brain may contribute to age-related hearing deficits. We investigated changes to efferent innervation in the vicinity of inner hair cells (IHCs) in young and old C57BL/6 mice using transmission electron microscopy to reveal increased efferent innervation of IHCs in older animals. Efferent contacts on IHCs contained focal presynaptic accumulations of small vesicles. Synaptic vesicle size and shape were heterogeneous. Postsynaptic cisterns were occasionally observed. Increased IHC efferent innervation was associated with a smaller number of afferent synapses per IHC, increased outer hair cell loss, and elevated auditory brainstem response thresholds. Efferent axons also formed synapses on afferent dendrites but with a reduced prevalence in older animals. Age-related reduction of afferent activity may engage signaling pathways that support the return to an immature state of efferent innervation of the cochlea.

Dopaminergic innervation of the mouse inner ear: evidence for a separate cytochemical group of cochlear efferent fibers

Immunostaining mouse cochleas for tyrosine hydroxylase (TH) and dopamine beta-hydroxylase suggests that there is a rich adrenergic innervation throughout the auditory nerve trunk and a small dopaminergic innervation of the sensory cell areas. Surgical cuts in the brainstem confirm these dopaminergic fibers as part of the olivocochlear efferent bundle. Within the sensory epithelium, TH-positive terminals are seen only in the inner hair cell area, where they intermingle with other olivocochlear terminals expressing cholinergic markers (vesicular acetylcholine transporter; VAT). Double immunostaining suggests little colocalization of TH and VAT; quantification of terminal volumes suggests that TH-positive fibers constitute only 10-20% of the efferent innervation of the inner hair cell area. Immunostaining of mouse brainstem revealed a small population of TH-positive cells in and around the lateral superior olive. Consistent with cochlear projections, double staining for the cholinergic marker acetylcholinesterase suggested that TH-positive somata are not cholinergic and vice versa. All observations are consistent with the view that a small dopaminergic subgroup of lateral olivocochlear neurons 1) projects to the inner hair cell area, 2) is distinct from the larger cholinergic group projecting there, and 3) may correspond to lateral olivocochlear "shell" neurons described by others (Warr et al. [1997] Hear. Res 108:89-111).

Reciprocal synapses between inner hair cell spines and afferent dendrites in the organ of corti of the mouse

We provide, for the first time, ultrastructural evidence for the differentiation of reciprocal synapses between afferent dendrites of spiral ganglion neurons and inner hair cells. Cochlear synaptogenesis of inner hair cells in the mouse occurs in two phases: before and after the onset of hearing at 9-10 postnatal (PN) days. In the first phase, inner hair cells acquire afferent innervation (1-5 PN). Reciprocal synapses form around 9-10 PN on spinous processes emitted by inner hair cells into the dendritic terminals, predominantly in conjunction with ribbon afferent synapses. During the second phase, which lasts up to 14 PN, synaptogenesis is led by the olivocochlear fibers of the lateral bundle, which induce the formation of compound and spinous synapses. The afferent dendrites themselves also develop recurrent presynaptic spines or form mounds of synaptic vesicles apposed directly across inner hair cell ribbon synapses. Thus, in the adult 2-month mouse, afferent dendrites of spiral ganglion neurons are not only postsynaptic but also presynaptic to inner hair cells, providing a synaptic loop for an immediate feedback response. Reciprocal synapses, together with triadic, converging, and serial synapses, are an integral part of the afferent ribbon synapse complex. We define the neuronal circuitry of the inner hair cell and propose that these minicircuits form synaptic trains that provide the neurological basis for local cochlear encoding of the initial acoustic signals.

Muscarinic receptor subtypes are differentially distributed in the rat cochlea

Five different genes encode the muscarinic acetylcholine receptors. The muscarinic receptor subtypes M1, M3, and M5 are typically coupled to activation of the Galpha(q/11)-phosphatidyl inositol pathway, whereas the M2 and M4 subtypes are typically linked to Galpha(i) and adenylyl cyclase inhibition. In order to localize muscarinic receptors in the rat cochlea, we applied polyclonal antibodies for subtypes M1, M2, M3, and M5, and monoclonal antibody for subtype M4 to paraffin sections. In the organ of Corti, outer hair cells exhibited strong immunoreactivity for M3 and weak immunoreactivity for M1. Deiters' cells were strongly immunoreactive to antibodies for the M1 and M2 subtypes, with weak staining observed for M3, and weaker yet for M5. Inner hair cells showed moderate immunoreactivity for the M1 subtype, weaker staining for the M5 subtype, and slight staining for the M3 subtype. Among the spiral ganglion neurons, weak to moderate immunoreactivity was detected for M3 and M5 subtypes and weak staining was observed for the M1 subtype. The efferent fibers of the intraganglionic spiral bundle were positive for M2 and M5. In the lateral wall, weak to moderate staining was detected for M5 in the stria vascularis corresponding in position to the basolateral extensions of marginal cells. Staining for M3 was observed associated with capillaries. Fibrocytes of the spiral ligament exhibited limited but selective subtype immunoreactivity. No immunoreactivity was detected in the cochlea for the M4 subtype. From the present findings we suggest that M3 is the primary muscarinic receptor subtype in outer hair cells mediating a postsynaptic response to the medial olivocochlear cholinergic efferent input. The muscarinic receptor subtypes M1, M3, and M5 appear to subserve the action of cholinergic lateral olivocochlear efferent stimulation on postsynaptic responses in type I afferents. Whether M1, M3, and M5 protein in inner hair cells indicates constitutive or vestigial expression remaining from development is unknown. M2 and M5 muscarinic receptors expressed presynaptically may modulate the efferent signal. Finally, expression by Deiters' cells of several muscarinic subtypes raises the possibility that cholinergic efferents couple to these non-sensory cells through muscarinic receptors.

Differentiation of spinous synapses in the mouse organ of corti

The inner hair cells, the primary auditory receptors, are perceived only as a means for transfer of sound signals via the auditory nerve to the central nervous system. During initial synaptogenesis, they receive relatively few and mainly somatic synapses. However, around the onset of hearing (10-14 postnatal days in the mouse), a complex network of local spinous synapses differentiates, involving inner hair cells, their afferent dendrites, and lateral olivocochlear terminals. Inner hair cell spines participate in triadic synapses between olivocochlear terminals and afferent dendrites. Triadic synapses have not yet been confirmed in the adult. Synaptic spines of afferent dendrites form axodendritic synapses with olivocochlear terminals and somatodendritic synapses with inner hair cells. The latter are of two types: ribbon-dendritic spines and stout dendritic spines surrounded only by a crown of synaptic vesicles. Formation of spinous afferent synapses results from sprouting of dendritic filopodia that intussuscept inner hair cell cytoplasm. This process continues in the adult, indicating ongoing synaptogenesis. Spinous processes of olivocochlear synaptic terminals contact adjacent afferent dendrites, thus integrating their connectivity. They develop about 14 postnatal days, but their presence in the adult has yet to be confirmed. Differentiation of spinous synapses in the organ of Corti results in a total increase of synaptic contacts and in a complexity of synaptic arrangements and connectivity. We propose that spinous synapses provide the morphological substrate for local processing of initial auditory signals within the cochlea.

Fast Ca2+ signals at mouse inner hair cell synapse: a role for Ca2+-induced Ca2+ release

Inner hair cells of the mammalian cochlea translate acoustic stimuli into 'phase-locked' nerve impulses with frequencies of up to at least 1 kHz. Little is known about the intracellular Ca2+ signal that links transduction to the release of neurotransmitter at the afferent synapse. Here, we use confocal microscopy to provide evidence that Ca2+-induced Ca2+ release (CICR) may contribute to the mechanism. Line scan images (2 ms repetition rate) of neonatal mouse inner hair cells filled with the fluorescent indicator FLUO-3, revealed a transient increase in intracellular Ca2+ concentration ([Ca2+]i) during brief (5-50 ms) depolarizing commands under voltage clamp. The amplitude of the [Ca2+]i transient depended upon the Ca2+ concentration in the bathing medium in the range 0-1.3 mM. [Ca2+]i transients were confined to a region near the plasma membrane at the base of the cell in the vicinity of the afferent synapses. The change in [Ca2+]i appeared uniform throughout the entire basal sub-membrane space and we were unable to observe hotspots of activity. Both the amplitude and the rate of rise of the [Ca2+]i transient was reduced by external ryanodine (20 microM), an agent that blocks Ca2+ release from the endoplasmic reticulum. Intracellular Cs+, commonly used to record at presynaptic sites, produced a similar effect. We conclude that both ryanodine and intracellular Cs+ block CICR in inner hair cells. We discuss the contribution of CICR to the measured [Ca2+]i transient, the implications for synaptic transmission at the afferent synapse and the significance of its sensitivity to intracellular Cs+.

Postnatal development of efferent synapses in the rat cochlea

The development of olivocochlear efferent axons and their contacts in the postnatal cochlea was studied after DiI applications to the olivocochlear bundle in the ipsilateral brainstem of rats from 0 to 10 days of age (P0-10). Light microscopic analyses showed that labeled axons reached the vicinity of inner hair cells by P0 and outer hair cells by P2. Electron microscopic analyses demonstrated that labeled immature efferent axons are present among supporting cells of the greater epithelial ridge as well as inner hair cells at P0. The first efferent contacts that contacted inner hair cells contained a few irregularly sized vesicles and, occasionally, mitochondria. Postsynaptic specializations within inner hair cells apposed to labeled efferent axons included subsynaptic cisterns, irregularly sized vesicles, and synaptic bodies. Similar features were present in unlabeled profiles, presumed to be afferents, indicating that immature efferent axons could not be reliably distinguished from afferents without positive labeling. Efferent axons synapsed with outer hair cells by P4 and had synapse-like contacts at the bases of Deiters' cells at P4 and P6. Contacts between afferents and efferents were observed frequently in the inner spiral bundle from P6. As they matured, efferent axon terminals contacting hair cells contained increasing numbers of synaptic vesicles and were typically apposed by well-defined postsynaptic cisterns, thus acquiring distinctive profiles.

Cholinergic synaptic inhibition of inner hair cells in the neonatal mammalian cochlea

Efferent feedback onto sensory organs provides a means to modulate input to the central nervous system. In the developing mammalian cochlea, inner hair cells are transiently innervated by efferent fibers, even before sensory function begins. Here, we show that neonatal inner hair cells are inhibited by cholinergic synaptic input before the onset of hearing. The synaptic currents, as well as the inner hair cell's response to acetylcholine, are mediated by a nicotinic (alpha9-containing) receptor and result in the activation of small-conductance calcium-dependent potassium channels.

The glutamate receptor subunit delta1 is highly expressed in hair cells of the auditory and vestibular systems

In the inner ear, fast excitatory synaptic transmission is mediated by ionotropic glutamate receptors, including AMPA, kainate, and NMDA receptors. The recently identified delta1 and delta2 glutamate receptors share low homology with the other three types, and no clear response or ligand binding has been obtained from cells transfected with delta alone or in combination with other ionotropic receptors. Studies of mice lacking expression of delta2 show that this subunit plays a crucial role in plasticity of cerebellar glutamatergic synapses. In addition, these mice show a deficit in vestibular compensation. These findings and the nature of glutamatergic synapses between vestibulocochlear hair cells and primary afferent dendrites suggest that delta receptors may be functionally important in the inner ear and prompted us to investigate the expression of delta receptors in the cochlea and peripheral vestibular system. Reverse transcription and DNA amplification by PCR combined with immunocytochemistry and in situ hybridization were used. Our results show that the expression of delta1 in the organ of Corti is intense and restricted to the inner hair cells, whereas delta1 is expressed in all spiral ganglion neurons as well as in their satellite glial cells. In the vestibular end organ, delta1 was highly expressed in both hair cell types and also was expressed in the vestibular ganglion neurons. The prominent expression of delta1 in inner hair cells and in type I and type II vestibular hair cells suggests a functional role in hair cell neurotransmission.

Expression of the nicotinic acetylcholine receptor subunit, alpha9, in the guinea pig cochlea

Acetylcholine is a major neurotransmitter of the cochlear efferent system. Based on its high level of expression in hair cells, the recently cloned nicotinic receptor subunit, alpha9 [Elgoyhen et al., Cell 79 (1994) 705-715], is likely to be the postsynaptic receptor for acetylcholine in hair cells either as a homomeric complex or with other subunits yet to be identified. To further study this receptor, we cloned and sequenced alpha9 cDNA from the guinea pig organ of Corti library [Wilcox and Fex, Hear. Res. 62 (1992) 124-126]. The sequence of the guinea pig alpha9 cDNA is similar to that of the rat, with identities of 85% and 89% at the nucleotide and amino acid levels, respectively. Most differences are in the cytoplasmic loop domain between the transmembrane segments 3 and 4. We also observed minor differences in the putative ligand binding regions. Pharmacological differences between acetylcholine receptors on outer hair cells of rat and guinea pig have been reported, and the minor structural changes we observe could account for these differences. Reverse transcription-polymerase chain reaction analysis showed a high expression of alpha9 in the organ of Corti while expression was low or not detected in the spiral ganglion. In situ hybridization histochemistry showed expression of alpha9 mRNA in both inner and outer hair cells, with much higher expression in outer hair cells than in inner hair cells. In the inner hair cell, silver grains were more abundant over the basal part of the cell than over the apical part. Immunocytochemistry showed a pattern of distribution of the alpha9 protein similar to that seen for mRNA with in situ hybridization. Immunolabeling was most intense at the bases of both inner and outer hair cells. To determine the effect of hair cell loss on alpha9 expression, hair cells were destroyed by either systemic or local application of kanamycin. This treatment led to a down regulation of alpha9 in hair cells; this down regulation appeared to precede hair cell degeneration. In the spiral ganglion, a transient up regulation of alpha9, as determined by RT-PCR, was seen 4-6 weeks after kanamycin treatment.

The development of vestibulocochlear efferents and cochlear afferents in mice

We have reinvestigated the embryonic development of the vestibulocochlear system in mice using anterograde and retrograde tracing techniques. Our studies reveal that rhombomeres 4 and 5 include five motor neuron populations. One of these, the abducens nucleus, will not be dealt with here. Rhombomere 4 gives rise to three of the remaining populations: the facial branchial motor neurons; the vestibular efferents; and the cochlear efferents. The migration of the facial branchial motor neurons away from the otic efferents is completed by 13.5 days post coitum (dpc). Subsequently the otic efferents separate into the vestibular and cochlear efferents, and complete their migration by 14.5 dpc. In addition to their common origin, all three populations have perikarya that migrate via translocation through secondary processes, form a continuous column upon completion of their migrations, and form axonal tracts that run in the internal facial genu. Some otic efferent axons travel with the facial branchial motor nerve from the internal facial genu and exit the brain with that nerve. These data suggest that facial branchial motor neurons and otic efferents are derived from a common precursor population and use similar cues for pathway recognition within the brain. In contrast, rhombomere 5 gives rise to the fourth population to be considered here, the superior salivatory nucleus, a visceral motor neuron group. Other differences between this group and those derived from rhombomere 4 include perikaryal migration as a result of translocation first through primary processes and only then through secondary processes, a final location lateral to the branchial motor/otic efferent column, and axonal tracts that are completely segregated from those of the facial branchial and otic efferents throughout their course inside the brain. Analysis of the peripheral distribution of the cochlear efferents and afferents show that efferents reach the spiral ganglion at 12.5 dpc when postmitotic ganglion cells are migrating away from the cochlear anlage. The efferents begin to form the intraganglionic spiral bundle by 14.5 dpc and the inner spiral bundle by 16.5 dpc in the basal turn. They have extensive collaterals among supporting cells of the greater epithelial ridge from 16.5 dpc onwards. Afferents and efferents in the basal turn of the cochlea extend through all three rows of outer hair cells by 18.5 dpc. Selective labeling of afferent fibers at 20.5 dpc (postnatal day 1) shows that although some afferents are still in early developmental stages, some type II spiral ganglion cells already extend for long distances along the outer hair cells, and some type I spiral ganglion cells end on a single inner hair cell. These data support previous evidence that in mice the early outgrowth of afferent and efferent fibers is essentially achieved by birth.

Efferent innervation of the inner hair cell region: origins and terminations of two lateral olivocochlear systems

The projections of lateral olivocochlear neurons (LOC), which terminate beneath inner hair cells (IHCs), were investigated by injecting biotinylated dextran amine into the lateral superior olivary nucleus (LSO) and the surrounding region in the rat. This region has been definitively shown to contain two types of olivocochlear neurons: small cells within the LSO (intrinsic neurons) and large cells (shell neurons) surrounding it (Vetter, D.E., Mugnaini, E., 1992. Distribution and dendritic features of three groups of rat olivocochlear neurons. Anat. Embryol. 185, 1-16). Labeled efferent axons were studied by light microscopy in whole mounts and radial sections of the organ of Corti (OC). It was found that injections confined to the LSO, which presumably affected mainly intrinsic neurons, labeled a cluster of axons in the osseous spiral lamina that entered the inner spiral bundle (ISB) and terminated in one or more dense patches that, in total basal-apical extent, spanned no more than 10-20% (1-2 mm) of the total length of the OC (10 mm). In contrast, injections affecting shell neurons produced labeled axons that entered the OC over a span of more than 50% of its length and which, as a group, coursed in the ISB for at least 80%, and sometimes more than 95% of total cochlear length. Study of individual axons in the OC revealed that intrinsic axons did not bifurcate upon entering the OC and traveled less than 1 mm before terminating in a discrete, dense arbor. In contrast, shell axons typically bifurcated into basal and apical branches that, in toto, traveled between 1 and 2 mm beneath the IHCs, forming numerous en passant swellings and a few terminal branches en route. The fact that localized injections of intrinsic neurons produced focal peaks of labeling in the cochlea, whereas similar injections of shell neurons produced a diffuse, non-focal projection that could extend for nearly the entire length of the cochlea, suggests that significant differences exist between these two populations in their capacity to influence localized, frequency-specific regions of the OC, and thus in their probable functional roles. The present findings in the rat not only confirm a previous study in the guinea pig which found a similar dual efferent innervation beneath the IHCs (Brown, M.C., 1987. Morphology of labeled efferent fibers in the guinea pig cochlea. J. Comp. Neurol. 260, 605-618), but extend those observations by linking two axonal types beneath the IHCs to their respective cell bodies of origin in the lateral zone of the superior olivary complex.

Expression of adenylyl cyclase type I in cochlear inner hair cells

Expression of calcium/calmodulin-activated adenylyl cyclase type I (ACI) mRNA has been determined in the cochlea and in an organ-of-Corti subdissected tissue fraction by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Amplification products of predicted size were obtained from the mouse cochlea and rat organ of Corti with nucleotide sequences corresponding to respective ACI brain transcripts. In addition, ACI template was detected in a rat inner hair cell cDNA library by PCR. Immunoreactivity to ACI has been localized within the organ of Corti to the inner hair cell, with diaminobenzidine staining found in both the cell body and in the stereocilia. Evidence, thus, has been obtained that both ACI transcript and protein are expressed in the inner hair cell, the primary mechanosensory receptor cell of the cochlea. We hypothesize that ACI is activated by calcium influx through a calcium/calmodulin interaction and that this adenytyl cyclase isoform may have a role in modulation of receptoneural afferent transmission and/or mechanosensory transduction in the cochlea.

Compound synapses within the GABAergic innervation of the auditory inner hair cells in the adolescent mouse

Ultrastructural investigation of the gamma-aminobutyric acid (GABA) component of the inner spiral bundle in adolescent mice revealed a pathway of glutamic acid decarboxylase (GAD)-positive and -negative fibers and vesiculated endings that contact inner hair cells and their afferents through a complex of axosomatic and axodendritic synapses. Ultrastructural details were investigated by using conventional electron microscopy. Several synaptic arrangements were observed: Main axosomatic synapses form between vesiculated endings and individual or adjoining inner hair cells (interreceptor synapses). Spinous synapses form on long, spinelike processes that protrude from inner hair cells to reach distant efferent endings. The efferent endings associate with inner hair cells and their synaptic afferents through compound synapses-serial, "converging," and triadic-otherwise characteristic of sensory relay nuclei. Serial synapses form by the sequential presynaptic alignment of the efferent-->receptor-->afferent components. Converging synapses result from the simultaneous apposition of a receptor ribbon synapse and a presynaptic efferent terminal on a recipient afferent dendrite. Triadic synapses comprise a vesiculated efferent ending in contact with an inner hair cell and with its synaptic afferent. Additionally, efferent endings may form simple axodendritic and axoaxonal synapses with GAD-negative vesiculated endings. The combination of different synaptic arrangements leads to short chains of compound synapses. It is assumed that these synaptic patterns seen in the adolescent mouse represent adult synaptology. The patterns of synaptic connectivity suggest an integrative role for the GABA/GAD lateral efferent system, and imply its involvement in the pre- and postsynaptic modulation of auditory signals.

Cell-specific expression of the alpha 9 n-ACh receptor subunit in auditory hair cells revealed by single-cell RT-PCR

Single-cell reverse transcription polymerase chain reaction was carried out in three different cell types from the organ of Corti of the four-day old rat. For this purpose, pieces of the organ of Corti were mounted under a differential-interference contrast video microscope. Two different mounting configurations were used to allow imaging of cells from two almost orthogonal angles. This method afforded unequivocal recognition of various cell types in the vital tissue, and extraction of nucleus and cytoplasm of specified individual cells with a patch pipette. Messenger RNA encoding the alpha 9 acetylcholine (ACh) receptor subunit was detected and sequenced from individual outer hair cells and inner hair cells, but was not found in Deiters' cells. The identical Deiters' cells were positive for a P2x receptor subunit. This indicates cell-specific expression of the alpha 9 subunit in inner hair cells and outer hair cells and supports the hypothesis that this subunit contributes to calcium (Ca2+) permeable ionotropic ACh receptors (ACh-R). ACh-dependent Ca2+ concentration increase has been observed in both outer hair cells and inner hair cells.

Synaptophysin and GAP-43 proteins in efferent fibers of the inner ear during postnatal development

A rearrangement of afferent and efferent fibers occurs in the postnatal development of the inner ear. Growth and synaptogenesis was explored during this critical period by immunohistochemically monitoring the expression of GAP-43 and synaptophysin. Both proteins were colocalized in efferent fibers beyond postnatal day 3 (pn3). Two distinct synaptophysin- and GAP-43-positive fibers innervated different parts of inner hair cells in the first and second postnatal weeks, respectively. GAP-43-positive efferents projecting to outer hair cells upregulated synaptophysin with base to apex gradient between postnatal day 5 and postnatal day 14. In efferents projecting to outer hair cells GAP-43 was downregulated about 6 days beyond synaptogenesis. In efferents projecting to inner hair cells, however, GAP-43 remained upregulated even beyond pn18, indicating continuous synapse replacement of this fiber type. Both proteins thus improved as excellent markers for growth and synaptogenesis of distinct postnatal efferent fibers.