GABA-like immunoreactivity in the squirrel monkey organ of corti

The distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity in the squirrel monkey organ of Corti was determined using an antiserum against GABA conjugated to bovine serum albumin. Immunoreactive labeling was seen in the region of the inner spiral bundle, the synaptic region below inner hair cells, in terminals contacting the basal part of outer hair cells, and in tunnel spiral fibers. Examples of each of these immunoreactive components could be observed in all cochlear turns. In the region of inner hair cells, immunoreactive labeling took the form of numerous small puncta randomly distributed below the base of the cells. In the region of outer hair cells, large globular immunoreactive structures reminiscent of terminal endings at the subnuclear level were observed. Since similar structures were seen at the base of outer hair cells in other cochleas processed for AChE, we conclude that GABA-like immunoreactivity was contained in efferent terminals which synapse on outer hair cells. These results strengthen previous evidence for the presence of GABA in the olivocochlear system of the mammalian cochlea.     

Morphology of labeled efferent fibers in the guinea pig cochlea

Efferent axons to the guinea pig cochlea were labeled by extracellular injections of horseradish peroxidase into the intraganglionic spiral bundle within the spiral ganglion. The terminal fibers formed by these axons were classified according to their patterns of termination within the basal turn of the cochlea. A class of terminal fibers designated "autonomic" forms a highly branched plexus in the osseous spiral lamina but does not enter the organ of Corti. The termination of single autonomics includes blood vessels as well as areas of the osseous spiral lamina not adjacent to blood vessels. Two major classes of efferent axons from the olivocochlear bundle enter the cochlea by way of the vestibulocochlear anastomosis and terminate either in areas near inner hair cells (IHC efferents) or onto outer hair cells (OHC efferents). The IHC efferents have thin axons throughout their course within the cochlea and can be divided into two subclasses. The most numerous subclass of IHC efferents (unidirectional) enters the inner spiral bundle and turns to spiral in only one direction for less than 1 mm and then forms a discrete termination including many en passant and terminal swellings that are within both the inner and tunnel spiral bundles. A less common subclass of IHC efferents (bidirectional) bifurcates upon entry into the inner spiral bundle to send branches both apically and basally. These terminal fibers take spiral courses that are greater than 1 mm in extent, often course in the tunnel spiral bundle for a large portion of the spiral, and form terminals throughout their extended spiral course. None of the IHC efferent fibers send branches to cross the tunnel to innervate the outer hair cells. A second major class of olivocochlear fibers, OHC efferent fibers, forms large boutons on the outer hair cells, and although they sometimes spiral beneath the IHCs for some length, they do not give off terminals to this region. The OHC efferent axons are thick and myelinated as they enter the cochlea, and they branch near the spiral ganglion to form several terminal fibers. Some of these terminal fibers are thin as they travel from the intraganglionic spiral bundle across the osseous spiral lamina to the organ of Corti, whereas others are thick and obviously myelinated as far peripheral as the habenula.(ABSTRACT TRUNCATED AT 400 WORDS)     

Immunocytochemical detection of glutamate decarboxylase in the postnatal developing rat organ of corti

Using a fluorescent avidin-biotin technique, we have immunolocalized the GABA synthesizing enzyme, glutamate decarboxylase, in postnatal developing and adult rat organs of Corti. At birth, the glutamate decarboxylase-like immunoreactivity is already present in the basal turn below the inner hair cells, i.e. within lateral olivocochlear efferent fibers of the inner spiral bundle. In the apical turn, the inner spiral bundle displays an immunoreactivity as early as postnatal day 3. Only the outer hair cells of the upper second turn and apex receive fibers immunostained for glutamate decarboxylase that most probably belong to the medial olivocochlear efferent innervation. They first appear at this level at postnatal day 15. Within these two regions of the organ of Corti, the glutamate decarboxylase-like immunoreactivity reaches an adult-like pattern at postnatal days 17-18. These results strengthen the hypothesis that GABA is a putative neurotransmitter that could be used by subpopulations of the two olivocochlear innervations. They also suggest that GABA either plays a neurotrophic function or participates in the regulation of the first cochlear potentials at the level of lateral efferent synapses.     

Early efferent innervation of the developing rat cochlea studied with a carbocyanine dye.

The olivocochlear pathway in the developing rat was visualized in fixed material. The fluorescent carbocyanine dye 1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was applied to the cut central axons of the olivocochlear neurones at the floor of the fourth ventricle, and the termination pattern within the cochlea was examined after dye diffusion. From the day of birth (P0) to postnatal day 2 (P2), efferent innervation of the cochlea was exclusively in the region of the inner hair cells. Between P2 and P11, progressive outgrowth of neuronal processes to the outer hair cell region occurred; possible connections with the outer hair cells were occasionally seen at P4 and approached the mature pattern by P6. The efferent innervation of the organ of Corti appeared to mature progressively from the cochlear base to the apex, with outgrowth to the outer hair cells occurring earlier in the basal turn of the cochlea than in the second and third cochlear turns. Numerous blind axonal endings were observed in the spiral lamina especially at early postnatal ages. These findings may be consistent with a sequential pattern of arrival of efferent axons at the organ of Corti and ongoing death of efferent neurones in the brainstem during this period of development.     

A study of cochlear innervation patterns in cats and rats with the Golgi method and Nomarski optics

Cochlear innervation patterns were studied in infant cats and rats with the rapid Golgi method. Examination of thick serial sections and surface preparations with the differential interference contrast microscope (Nomarski optics) allowed direct visualization of individually impregnated spiral ganglion cells, complete with their peripheral processes and endings in the organ of Corti. Individually impregnated efferent fibers could be recognized as heavily varicose axons that project radially to endings beneath inner and outer hair cells after taking a tangential course in the intraganglionic spiral bundle. It was often possible to visualize unimpregnated hair cells in contact with the impregnated endings of both types of fibers. There are at least two types of spiral ganglion cells in the cochlea of the infant cat and rat. One type innervates only inner hair cells by means of radial fibers. These ganglion cells constitute the overwhelming majority of ganglion cells impregnated in our preparations, and each cell typically innervates two inner hair cells. Hence, these ganglion cells establish nearly “point-to-point” connections between the auditory nerve and the organ of Corti. The other type of ganglion cell innervates outer hair cells by means of long spiral fibers; each cell typically innervates many outer hair cells through the numerous angular enlargements and short end branches of its spiral fiber. In addition, a few of these spiral fibers also send branches to inner hair cells by means of short collaterals; it remains to be seen if such fibers also occur in mature cochleas. Efferent fibers have been traced to inner and outer hair cell regions. The simplest pattern is formed by fine beaded axons with only a few branches ending mainly beneath inner hair cells. More complex patterns are formed by larger axons with many branches ending beneath inner or outer hair cells. Many efferent fibers send branches to both inner and outer hair cells. Electrophysiological studies so far have not demonstrated different populations of units that clearly correspond to the spiral and radial fibers. Therefore, the physiological differences between inner and outer hair cell innervation remain undefined.     

Electron microscopic observation of calcitonin gene-related peptide-like immunoreactivity in the organ of Corti of the rat

Calcitonin gene-related peptide (CGRP)-like immunoreactive (CGRP-IR) nerve terminals in the organ of Corti of rats were studied by light and electron microscopy. Surface preparation of the organ of Corti were immunostained using anti-CGRP antiserum for avidin-biotin immunohistochemistry. Dense CGRP-IR fiber bundles were observed by light microscopy in the inner spiral bundles, tunnel spiral bundles and outer spiral bundles. Electron microscopic analysis indicated that CGRP-IR fibers belong to efferent nerves. In the inner spiral bundles, the CGRP-IR fibers showed a direct contact mainly with non-immunoreactive afferent fibers. Some CGRP-IR nerve endings in the inner spiral bundles formed contacts directly with inner hair cells. In the outer spiral bundles, CGRP-IR fibers formed synaptic contacts exclusively with the outer hair cells. It should be noted that the number of synapses of CGRP nerve endings with outer hair cells varied depending upon the sub-row: a falling gradient in number occurred along the inner-outer axis. Our results suggest that CGRP acts as an efferent neuromodulator in the organ of Corti.     

Neuron-specific enolase during the development of the organ of Corti

NSE immunoreactivity has been studied in the organ of Corti of the developing mouse from birth to 21 days. NSE immunohistochemical stain is observed in spiral ganglion cells, in nerve fibers and in nerve endings of inner and outer hair cells, and in both populations of sensory cells. Spiral ganglion cells in lower and central parts of the ganglion stain for NSE at birth, but all nerve cells are stained by day 4. Radial and spiral fibers and the endings on inner hair cells stain at birth, but the nerve endings on outer hair cells develop NSE between days 3 and 6. The inner and outer hair cells are NSE-positive at day 2 but the NSE immunoreactivity in the outer hair cells decreases at the end of the second week until the cells become negative. The NSE stain in the neuronal pathways of the inner and outer hair cell regions increases for about 19 days, showing a predominant accumulation in neuronal endings. The data suggest that the development of NSE expression in the organ of Corti reflects the nascence and maturation of the synaptic contacts. Spiral neurons, their fibers and endings as well as inner and outer hair cells express NSE in the isolated organ of Corti in culture. Variability of stain among the different cell populations indicates a role of local factors in the regulation of NSE expression.     

Synaptic arrangements between inner hair cells and tunnel fibers in the mouse cochlea

Hair cells, the sensory cells of the organ of Corti, receive afferent innervation from the spiral ganglion neurons and efferent innervation from the superior olivary complex. The inner and outer hair cells are innervated by distinctive fiber systems. Our electron microscopical studies demonstrate, however, that inner hair cells, in addition to their own innervation, are also synaptically engaged with the fibers destined specifically to innervate outer hair cells, within both the afferent and efferent innervation. Serial sections of the afferent tunnel fibers (destined to innervate outer hair cells) in the apical turn demonstrate that, while crossing toward the tunnel of Corti, they receive en passant synapses from inner hair cells. Each inner hair cell (in a series of five in the apical turn) was innervated by two tunnel fibers, one on each side. We show here for the first time that, in the adult, the afferent tunnel fibers receive a ribbon synapse from inner hair cells and form reciprocal contacts on their spines. Vesiculated efferent fibers from the inner pillar bundle (which carries the innervation to outer hair cells) form triadic synapses with inner hair cells and their synaptic afferent dendrites; the vesiculated terminals of the lateral olivocochlear fibers from the inner spiral bundle synapse extensively on the afferent tunnel fibers, forming triadic synapses with both afferent tunnel fibers and their synaptic inner hair cells. This intense synaptic activity involving inner hair cells and both afferent and efferent tunnel fibers, at their crossroad, implies functional connections between both inner and outer hair cells in the process of hearing.     

Immunocytochemical localization of choline acetyltransferase-like immunoreactivity in the guinea pig cochlea

The immunocytochemical localization of the enzyme choline acetyltransferase (ChAT) was examined in the guinea pig organ of Corti to determine if both lateral and medial systems of efferents would show immunoreactive labeling for this specific enzyme marker of cholinergic neurons. Cochleae were also examined after lesion of efferents to determine if ChAT-like immunoreactivity is confined to efferents. ChAT-like immunoreactivity was seen in the inner spiral bundle, tunnel spiral bundle and by the bases of inner hair cells corresponding to the lateral system of efferents. ChAT-like immunoreactivity was also seen in crossing fibers and puncta at the bases and by the nuclei of outer hair cells corresponding to the medial system of efferents. With the use of video enhanced contrast microscopy more than 9 ChAT-like immunoreactive puncta at the bases of outer hair cells could be resolved. In cochleae examined 6 weeks after ipsilateral lesion of efferents, no ChAT-like immunoreactivity was observed. These results add strong evidence that acetylcholine is a transmitter of both the medial and lateral systems of efferents.     

Differential expression of PKC beta II in the rat organ of Corti

To investigate a possible involvement of protein kinase C (PKC) in cochlear efferent neurotransmission, we studied the expression of the calcium-dependent PKC beta II isoform in the rat organ of Corti at different postnatal ages using immunofluorescence and immunoelectron microscopy. We found evidence of PKC beta II as early as postnatal day (PND) 5 in efferent axons running in the inner spiral bundle and in Hensen cells. At PND 8, we also found PKC beta II in efferents targeting outer hair cells (OHCs), and a slight detection at the synaptic pole in the first row of the basal and middle cochlear turns. At PND 12, PKC beta II expression declined in the efferent fibres contacting OHCs, whereas expression was concentrated at the postsynaptic membrane, from the basal and middle turns. The adult-like pattern of PKC beta II distribution was observed at PND 20. Throughout the cochlea, we found PKC beta II expression in the Hensen cells, non-sensory cells involved in potassium re-cycling, and lateral efferent terminals of the inner spiral bundle. In addition, we observed expression in OHCs at the postsynaptic membrane facing the endings of the medial efferent system, with the exception of some OHCs located in the most apical region of the cochlea. These data therefore suggest an involvement of PKC beta II in both cochlear efferent neurotransmission and ion homeostasis. Among other functions, PKC beta II could play a role in the efferent control of OHC activity.     

Several distinct receptor binding enkephalins in olivocochlear fibers and terminals in the organ of corti

Biochemical studies centering on the use of reverse-phase high-performance liquid chromatography (HPLC) and radioimmunoassays (RIA) demonstrate the presence in the guinea pig organ of Corti of at least 3 enkephalin-related peptides, two of which are identified as Met- and Leu-enkephalin, respectively. Enkephalins were identified and quantitated by HPLC-RIA in the isolated second turn of the organ of Corti, but were not found in stria vascularis or auditory nerve dissected from the cochlea. Three enkephalin-immunoreactive HPLC fractions inhibited the binding of labeled naloxone to rat brain membranes. All enkephalins identified by the combined HPLC-RIA procedure had an apparent molecular weight similar to that of Met- and Leu-enkephalin peptide standards. Immunocytochemistry, performed with the best-characterized Met-enkephalin antiserum used in the RIAs, localized the enkephalin-like immunoreactivity to lateral efferent fibers and terminals under inner hair cells of the organ of Corti. Other antisera raised against Met-enkephalin, not used for RIA, visualized enkephalin-like immunoreactivity in medial efferent fibers under outer hair cells as well. This enkephalin-like immunoreactivity may reflect the presence in the medial efferent system of other structurally similar peptides in addition to those detected biochemisally. Efferent fiber lesion, by evulsion of the vestibular nerve close to the vestibulocochlear anastomosis in which the olivocochlear fibers run, eliminated enkephalin-like immunoreactivity and the enkephalin-related peptides identified by HPLC-RIA.     

Immunohistochemical localization of adrenergic receptors in the rat organ of corti and spiral ganglion

Alpha(1)-, beta(1)-, and beta(2)-adrenergic receptors (ARs), which mediate responses to adrenergic input, have been immunohistochemically identified within the organ of Corti and spiral ganglion with polyclonal antibodies of established specificity. Alpha(1)-AR was immunolocalized to sites overlapping supranuclear regions of inner hair cells as well as to nerve fibers approaching the base of inner hair cells, most evident in the basal cochlear turn. A similar preponderance across cochlear turns for alpha(1)-AR in afferent cell bodies in the spiral ganglion pointed to type I afferent dendrites as a possible neural source of alpha(1)-AR beneath the inner hair cell. Foci of immunoreactivity for alpha(1)-AR, putatively neural, were found overlapping supranuclear and basal sites of outer hair cells for all turns. Beta(1)- and beta(2)-ARs were immunolocalized to sites overlapping apical and basal poles of the inner and outer hair cells, putatively neural in part, with immunoreactive nerve fibers observed passing through the habenula perforata. Beta(1)- and beta(2)-ARs were also detected in the cell bodies of Deiters' and Hensen's cells. Within the spiral ganglion, beta(1)- and beta(2)-ARs were immunolocalized to afferent cell bodies, with highest expression in the basal cochlear turn, constituting one possible neural source of receptors within the organ of Corti, specifically on type I afferent dendrites. Beta(1)- and beta(2)-ARs in Hensen's and Deiters' cells would couple to Galphas, known to be present specifically in the supporting cells. Overall, adrenergic modulation of neural/supporting cell function within the organ of Corti represents a newly considered mechanism for modifying afferent signaling.     

Triple Immunofluorescence Evidence for the Coexistence of Acetylcholine,Enkephalins and Calcitonin Gene-related Peptide Within Efferent (Olivocochlear) Neurons of Rats and Guinea-pigs

The efferent (olivocochlear) nerve supply to the cochlea is subdivided into a lateral and a medial innervation according to several criteria, e.g. locus of origin in the superior olivary complex and type of synaptic connections established in the organ of Corti. We have used a triple immunofluorescence colocalization approach to determine whether putative cholinergic neurons from the lateral innervation contain both metenkephalin and calcitonin gene-related peptide (CGRP), and whether those from the medial innervation also contain CGRP. About 80% of the choline acetyltransferase (ChAT)-like immunostained lateral efferent neurons within the lateral superior olive were CGRP- and metenkephalin-like immunostained. In the organ of Corti, colocalization of the three antigens within the inner spiral bundle was also found. This bundle contains the lateral efferent synapses, with the dendrites of the primary auditory neurons innervating the sensory inner hair cells. Most of the medial efferent neurons in the ventral nucleus of the trapezoid body were only immunoreactive for ChAT. However, in the rostral part of the nucleus, a minority of ChAT-like immunostained neurons were also CGRP-like immunostained. None of the ChAT-like immunostained medial efferent neurons presented metenkephalin-like immunostaining. In agreement with these brainstem data, partial colocalization of the ChAT- and CGRP-like immunostaining and a lack of metenkephalin immunoreactivity was noted below the sensory outer hair cells, which are the synaptic targets of medial efferent terminals in the organ of Corti. This distinction in the coexistence pattern of the two efferent innervations probably reflects distinct modes of action for acetylcholine in the cochlea. In one case, the effects of acetylcholine on the primary auditory neurons innervating the inner hair cells may require balanced modulation by metenkephalin and CGRP. In the other case, modulation of the effects of acetylcholine on the outer hair cells by neuropeptides would be less critical.     

Denervation study of synapses of organ of corti of old world monkeys

Fine structural characteristics of synapses in the spiral organ of Corti were examined, with reference to differences between inner and outer haircell systems, and to location of neurons of origin of efferent axons. Surgical interruption of crossed olivocochlear bundle, of vestibular nerve, of facial nerve, and excision of superior cervical ganglia were used to determine the pathways of efferent axons. Interruption of the vestibular nerve near the brainstem results in degeneration of all efferent terminals on outer hair cells. Mid-line lesions at, and caudal to, the facial colliculus result in degeneration of about half of these efferent terminals. Efferent synaptic bulbs to the inner hair-cell system are small, of the order of one micron, and form type 2 junctions with afferent dendrites. They tend to have more large dense-core vesicles (about 80 nm) than the large efferent terminals of the outer hair-cell system, and appear to be the terminals of axons in the habenula perforata, which exhibit varicosities laden with large dense core vesicles. The varicosities are unaffected by excision of the superior cervical ganglia. So far as our material can reveal, it appears that the varicosities in the habenula perforata do not survive vestibular root interruption, nor do the efferent processes in the internal spiral bundle or at the base of inner hair cells. Most interestingly, the afferent processes of the inner hair-cell system, as identified for example by their relation to pre-synaptic bodies in the inner hair cells, are subject to a trans-synaptic reaction after severance of the vestibular root. They undergo a dramatic cytological transformation, characterized by increase of volume, engorgement with microtubules, microfilaments, microvesicles of various sizes, and clusters of lysosomes. Thus, both the efferent and afferent terminals of the inner hair-cell system show marked cytological differences from the corresponding terminals of the outer hair cell system.     

Morphology of labeled afferent fibers in the guinea pig cochlea

Cochlear afferent and efferent fibers in the guinea pig were labeled by focal extracellular injections of horseradish peroxidase into the spiral ganglion of the basal turn. The morphology and pattern of termination of these fibers were studied by light microscopy. Fibers labeled by injections into the peripheral side of the ganglion could be grouped on the basis of their courses and terminations in the cochlea into two classes of afferent fibers, two classes of efferent (olivocochlear) fibers, and other presumably autonomic fibers. This paper describes the characteristics of labeled afferent fibers and their parent ganglion cells. Peripheral afferent fibers were grouped into two major classes: thick (mean diameter 1.7 micron) radial fibers projecting in a primarily radial fashion from the spiral ganglion and terminating on single inner hair cells and thin (mean diameter 0.5 micron) outer spiral fibers that spiral basalward in the organ of Corti to terminate on outer hair cells, usually in one row. For outer spiral fibers, the number of outer hair cells contacted and the length of the terminal region depend on the row of outer hair cells contacted, with third-row fibers forming, on the average, the most extensive region of termination. Within the spiral ganglion, two types of ganglion cells could be distinguished: type-I ganglion cells of large size (mean soma area = 216 microns 2) with a ratio of central process diameter to peripheral process diameter greater than one and type-II ganglion cells of smaller size (mean soma area = 100 microns 2) and a central to peripheral process ratio near one. In three cochleae in which injections were made central to the ganglion, 11 type-I ganglion cells have been traced to radial fibers contacting inner hair cells and eight type-II ganglion cells have been traced to outer spiral fibers contacting outer hair cells. Thus the afferent innervation of the guinea pig cochlea is similar to the pattern described in other mammals, in which there is separate innervation of the inner and outer hair cells by the two types of ganglion cells. The central axons of both types of ganglion cells were traced individually through serial sections of a block of tissue containing the cochlea, the auditory nerve, and the cochlear nucleus. They followed similar courses in the auditory nerve, and the axons followed into the cochlear nucleus bifurcated in similar regions of the interstitial portion.(ABSTRACT TRUNCATED AT 400 WORDS)     

The existence of opioid receptors in the cochlea of guinea pigs

Several independent investigations have demonstrated the presence of opioid peptides in the inner ear organ of Corti and in particular in the efferent nerve fibers innervating the cochlear hair cells. However, the precise innervation pattern of opioid fibers remains to be investigated. In the present study the expression of opioid receptors and their peptides is demonstrated in young adult guinea pig cochlea. Opioid receptors are mainly expressed in hair cells of the organ of Corti and in inner and outer spiral bundles with different characteristics for each type of receptor. Co-localization studies were employed to compare the distribution of mu-, delta- and kappa-opioid receptors and their respective peptides, beta-endorphin, leu-enkephalin and dynorphin. Additionally, immunostaining of synaptophysin was used in this study to identify the presynaptic site. Immunoreactivity for enkephalin and dynorphin was found in the organ of Corti. Leu-enkephalin was co-localized with synaptophysin prominently in the inner spiral bundle (ISB). Dynorphin was co-localized with synaptophysin in both inner and outer spiral bundles. Delta-opioid receptor was most prominently co-localized with its peptide in the ISB bundle. Kappa-opioid receptor was seemingly present with dynorphin in both inner and outer spiral bundles. The co-staining of both peptides and receptors with synaptophysin in the same areas suggests that some of the opioid receptors may act as auto-receptors. The results provide further evidence that opioids may function as neurotransmitters or neuromodulators in the cochlea establishing the basis for further electrophysiological and pharmacological investigations to understand better the roles of the opioid system in auditory function.     

Met-enkephalin-Arg-Gly-Leu in the organ of Corti: high performance liquid chromatography and immunoelectron microscopy

The opioid octapeptide Met-enkephalin-Arg⁶-Gly⁷-Leu⁸ (MERGL) was identified and quantified in the guinea pig cochlea using high performance liquid chromatography and a specific radioimmunoassay. The presence of MERGL immunostaining in efferent endings (coming from the brainstem) within the inner spiral bundle and the tunnel spiral bundle was demonstrated using a pre-embedding immunoelectronmicroscopic technique. Axo-dendritic synapses were observed between the MERGL immunostained varicosities and auditory dendrites. It is hypothesized that MERGL could act, together with Met-enkephalin and other pro-enkephalin A-related peptides, as an efferent neuromodulator or neurotransmitter in the organ of Corti.     

Electron microscopic localization of N-terminal proenkephalin (synenkephalin) immunostaining in the guinea pig organ of Corti

Using a pre-embedding immunoelectron microscopic technique, anti-synenkephalin immunostaining has been demonstrated within efferent varicosities (originating from the brainstem) of the inner spiral bundle and the tunnel spiral bundle of the organ of Corti. Axodendritic synapses were observed between the anti-synenkephalin immunostained varicosities and auditory dendrites connected to inner hair cells. No anti-synenkephalin immunostaining was found in any efferents at the outer hair cell level. We suggest that this immunolocalization of synenkephalin in the organ of Corti allows a better differentiation of the cochlear efferent systems on a neurochemical basis. The whole lateral system, or at least a significant part of it, could be referred to as the 'enkephalin-containing efferent system'.     

Immunohistochemical localization of neurocalcin in the rat inner ear

Localization in the rat inner ear of neurocalcin, a three EF-hand calcium-binding protein, was examined immunohistochemically. Neurocalcin-like immunoreactivity was restricted to neurons in neuroepithelial receptor organs, while hair cells and supporting cells showed no such immunoreactivity. In the organ of Corti, both afferent and efferent nerve terminals, which formed synapses on both inner and outer hair cells, showed distinct immunoreactions. Spiral ganglion neurons and cochlear nerves were immunopositive. In the cristae ampullaris, macula utriculi and macula sacculi, afferent nerve terminals forming nerve calices or terminal boutons were strongly immunopositive. Efferent nerve terminals making synapses either on nerve calices or on hair cells showed an intense immunoreactivity. Vestibular ganglion neurons were strongly immunopositive. In electron microscopy, immunoreaction products were diffuse in the cytoplasm of ganglion neurons and nerve terminals. Neurocalcin-like immunoreactivity occurred in association with microtubules, outer mitochondrial membranes, synaptic vesicles and synaptic membranes. It is thus likely that neurocalcin is involved in neural functions in each type of afferent and efferent transmission in the inner ear.     

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.