GABA visualized by immunocytochemistry in the guinea pig cochlea in axons and endings of efferent neurons

Antiserum raised against GABA coupled with glutaraldehyde to bovine serum albumin was applied to the guinea pig cochlea. Immunoreactivity was visualized as horseradish peroxidase reaction product in surface preparations of the organ of Corti using immunocytochemical techniques. Bright-field, differential interference contrast and video-enhanced contrast light microscopy were used. GABA-like immunoreactivity was found in axons and endings of efferent neurons in all turns of the cochlear spiral, but predominantly in the third turn and first half of the fourth turn. In these apical turns, immunoreactivity was seen in the efferent components: inner spiral bundle, tunnel spiral bundle, tunnel-crossing fibers, large nerve endings synapsing on outer hair cell bases, nerve endings high up on outer hair cells, nerve endings or varicosities close to outer hair cells, and outer spiral fibers. Some immunoreactive large nerve endings at outer hair cells were found in the apical half of the fourth turn. This study shows that axons and endings of efferent neurons in the organ of Corti of guinea pig contain GABA-like immunoreactivity with a distribution similar to that of GAD-like immunoreactivity as shown in a previous study. In both studies, many efferent nerve axons and endings were unstained, even in regions of maximal density of immunoreactivity in the apical turns. The evidence indicates that a subpopulation of efferent neurons projecting to the organ of Corti is GABAergic and very likely different from the lateral and the medial olivocochlear efferent systems.     

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.     

The pattern of sensorineural degeneration in the cochlea of the deaf shaker-1 mouse: ultrastructural observations

Experiments were done to extend existing knowledge on the nature and course of postnatal sensory and neural changes in the inner ear of shaker-1 mutant mice. Mice, 3-, 6-, 10-, 12-, 18-, and 30 days old, homozygous for the sh-1 gene, were studied using transmission electron microscopy. The data indicate retarded development coupled with the early onset of progressive degeneration in Corti's organ its nerve supply, and the cells of the spiral ganglion. Especially noteworthy are the following: in 3-day-old mice both outer hair cells and spiral ganglion cells are already abnormal. The latter are especially loosely ensheathed by glial cells and are in direct contact with nerve fibers. Outer hair cells contain vacuoles and lysosomes. By 6 days of age inner hair cells come to be similarly affected. By 18 days most of the afferent nerve supply of the organ of Corti has degenerated. The behavior of efferents within the organ is complex. Efferents arrive late (day 12) at the outer hair cells, they are few in number, form only immature synapses with the cell, and they subsequently degenerate. In contrast, the efferent nerve supply of the inner hair cell appears normal, if not over-abundant. The simultaneous occurrence of organ of Corti and spiral ganglion cell anomalies is discussed in terms of the role of sensorineural interactions in the expression of genetic defects affecting the inner ear. The selective degeneration of efferents to outer hair cells is viewed as being consistent with the hypothesis that there are two independent efferent systems which innervate the two types of cochlear sensory hair cells.     

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.     

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.     

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.     

Neuron-specific enolase-like immunoreactivity in inner hair cells but not outer hair cells in the guinea pig organ of Corti

Neuron-specific enolase (NSE) has been localized only in neurons and cells with characteristics of neurons. The immunocytochemical localization of NSE was examined in guinea pig cochleae to determine if hair cells, which have some neuronal characteristics, would show NSE-like immunoreactive labeling. NSE-like immunoreactivity was seen in inner hair cells but not in outer hair cells. This is the first report of NSE-like immunoreactivity in a receptor cell. NSE-like immunoreactivity was also seen in efferent fibers and terminals and in both type I and type II spiral ganglion cells. The finding of NSE-like immunoreactivity in inner but not outer cells adds to the number of differences found between them and may be related to differences in function and action.     

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.     

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)     

Mild carbon monoxide exposure diminishes selectively the integrity of the cochlea of the developing rat

Rat pups were chronically exposed to carbon monoxide (CO) concentrations (12 or 25 ppm) in air starting at day 8, through 22 days of age, to examine the changes in the peripheral auditory system. Gastrostomy-reared rat pups, with or without CO exposure, were used and compared with mother-reared pups. The organ of Corti and the neurons of the spiral ganglion were analyzed for their morphology by using immunochemical and histological techniques. The inner and outer hair cells in the organ of Corti of animals exposed to 12 and 25 ppm CO were not different from the controls. However, at 25 ppm CO exposure, the nerve terminals innervating the inner hair cells were swollen. The somata of neurons in the spiral ganglion showed mild changes in the cytoplasm, and signs of mild vacuolization were observed in myelin covering their central processes. Synaptophysin, a marker for synaptic vesicles, and choline acetyltransferase, a marker for cholinergic terminals, showed no difference in immunoreactivity in CO exposed animals at 12 and at 25 ppm when compared with their age-matched controls. Also, Na(+)K(+) ATPase immunoreactivity patterns were normal compared with controls. Three enzymes were significantly reduced at the 25 ppm CO exposure: Cytochrome oxidase, NADH-TR, and calcium ATPase were decreased in both the organ of Corti and the neurons of the spiral ganglion, and decreased immunostaining for the neurofilament and myelin basic proteins was found. We conclude that components of the cochlea are selectively affected by mild chronic CO exposure during development.     

NADPH-diaphorase histochemistry reveals an autonomic-like innervation in the postnatal hamster cochlea.

Previous studies used nicotinamide adenine diphosphate (NADPH)-diaphorase histochemistry as an indicator of nitric oxide synthase (NOS) expression in the adult mammalian cochlea. In this study, we investigated the early postnatal expression of diaphorase activity in the hamster cochlea. Two types of extrinsic fibers were intensely labeled as early as postnatal day 3 (P3) in the portion of the cochlear nerve that innervates the base of the modiolus. By P10, these fibers had reached the spiral ganglion and were projecting toward the organ of Corti. The perivascular type of fiber did not project into the organ of Corti; however, the nonperivascular type could be traced among the supporting cells below the outer hair cells. Spiral ganglion cell somata were also labeled as early as P3. The onset of diaphorase expression in the spiral ganglion cells corresponds to a critical period of synaptogenesis for these sensorineural cells. If NADPH-diaphorase activity is an indicator of NOS, then our results suggest that NO may play a role during postnatal cochlear development.     

Cholecalcin (28-kDa CaBP) in the rat cochlea. Development in normal and hypothyroid animals. An immunocytochemical study

The distribution of cholecalcin (28-kDa calcium-binding protein) in the cochlea of developing rats was followed by immunocytochemistry. In normal animals, the protein first appeared in utero in the cells of K?lliker's organ, a structure involved in the secretion of the tectorial membrane. The inner hair cells, followed by the outer hair cells, then became immunoreactive from the base of the cochlea to the apex. Their cuticular plate, the anchoring structure for stereocilia, was particularly labeled. The cells of K?lliker's organ lost their immunoreactivity after the first postnatal week, the time when they lose their secretory activity. During the same period, when the tunnel of Corti and the space of Nuel open, labeling appeared in the supporting cells. The neurons of the spiral ganglion were stained from the second postnatal week and the fibers of the cochlear nerve after the end of the first month. No difference was induced by thyroid deficiency until the second postnatal week. Thereafter, K?lliker's organ did not transform and its cholecalcin immunoreactivity persisted, together with the secretory activity. As a result, the tectorial membrane was greatly distorted and the mechanical properties of the organ of Corti were dramatically impaired. The supporting cells were labeled although the tunnel of Corti and the space of Nuel did not open. Staining of the spiral ganglion neurons was delayed. All the nervous structures of the cochlea were, however, stained at the end of the first postnatal month, as in normal animals, despite the irreversible complete deafness. Cholecalcin is thus present during development of the cochlea in both non-neuronal and neuronal structures, and is probably involved in very different processes in various types of cells.     

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.     

Characterization of nitric oxide synthase isoforms expressed in different structures of the guinea pig cochlea

Nitric oxide synthase (NOS) activity and NADPH diaphorase staining has previously been reported in mammalian cochlea. Here we demonstrate immunoreactivity for neuronal-type NOS I and endothelial-type NOS III in the cochlea of the guinea pig. NOS I immunoreactivity was seen in inner and outer hair cells, and spiral ganglion cells. Staining for NOS I was also shown in basal and intermediate cells of the stria vascularis, spiral ligament cells, and the media of vessels near the modiolus. An antibody to NOS III stained primarily vascular endothelial cells. Some NOS III immunoreactivity was also detected in spiral ganglion cells. An antibody to the inducible-type NOS II did not stain any structure of the guinea pig cochlea, suggesting that this isoform is not expressed under physiological conditions. Nitric oxide produced by NOS I and/or NOS III may act as a neuromodulator in the organ of Corti and could also play a role as a regulator of cochlear blood flow.     

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

It is well known that experimentally induced cochlear damage produces structural, physiological, and biochemical alterations in neurons of the cochlear nucleus. In contrast, much less is known with respect to the naturally occurring cochlear pathology presented by congenital deafness. The present study attempts to relate organ of Corti structure and auditory nerve activity to the morphology of primary synaptic endings in the cochlear nucleus of congenitally deaf white cats. Our observations reveal that the amount of sound-evoked spike activity in auditory nerve fibers influences terminal morphology and synaptic structure in the anteroventral cochlear nucleus. Some white cats had no hearing. They exhibited severely reduced spontaneous activity and no sound-evoked activity in auditory nerve fibers. They had no recognizable organ of Corti, presented >90% loss of spiral ganglion cells, and displayed marked structural abnormalities of endbulbs of Held and their synapses. Other white cats had partial hearing and possessed auditory nerve fibers with a wide range of spontaneous activity but elevated sound-evoked thresholds (60–70 dB SPL). They also exhibited obvious abnormalities in the tectorial membrane, supporting cells, and Reissner's membrane throughout the cochlear duct and had complete inner and outer hair cell loss in the base. The spatial distribution of spiral ganglion cell loss correlated with the pattern of hair cell loss. Primary neurons of hearing-impaired cats displayed structural abnormalities of their endbulbs and synapses in the cochlear nucleus which were intermediate in form compared to normal and totally deaf cats. Changes in endbulb structure appear to correspond to relative levels of deafness. These data suggest that endbulb structure is significantly influenced by sound-evoked auditory nerve activity. J. Comp. Neurol. 397:532–548, 1998. © 1998 Wiley-Liss, Inc.     

Cochlear innervation in the developing rat: an immunocytochemical study of neurofilament and spectrin proteins

We have studied the innervation of the developing cochlea by immunocytochemical staining of the cytoskeletal proteins, neurofilament (NF), and spectrin (brain spectrin and erythrocyte spectrin). NF immunoreactivity was seen in spiral ganglion cell bodies and their processes and in fibers of the intraganglionic spiral bundle (IGSB) on gestational day 16. NF immunoreactivity with monoclonal antibodies to NF160 and NF68 was present beneath both inner hair cells (the IHC) and outer hair cells (OHCs) on gestational day 20. NF200 immunostaining was located only in the IGSB and in fibers reaching the IHC. The first NF200 immunoreactivity beneath the OHCs was seen in the basal turn at birth. NF labelling began to decrease on postnatal day 9 and its intensity became more like that of the adult. Brain spectrin immunostaining was first seen in the IGSB of the basal turn on gestational day 18. It reached the fibers between the spiral ganglion and the IHC on gestational day 20. Brain spectrin immunoreactivity was first seen beneath the OHCs in the basal turn at birth. It reached all the OHCs of the cochlea by postnatal day 4, and began to decrease 9 days after birth. Erythrocyte spectrin immunostaining was first observed during the second postnatal week, when it labelled spiral ganglion cells. The distribution of NF200 and brain spectrin immunoreactivity suggested that efferent innervation of OHCs is present at birth in the rat, and confirms previous studies showing the early efferent innervation of the OHCs of the mouse and the rat at birth, and the time lag between the appearance of the two spectrin isoforms during development.     

Tenascin-C in the cochlea of the developing mouse

Tenascin-C is a glycoprotein of the extracellular matrix that acts in vitro as both a permissive and a nonpermissive substrate for neurite growth. We analyzed, by immunocytochemistry, the distribution of tenascin-C along neural growth pathways in the developing mouse cochlea. In the spiral lamina, tenascin-C coexists in a region where nerve bundles arborize. In the organ of Corti, tenascin-C lines the neural pathways along pillar and Deiters' cells before and during the time of nerve fiber ingrowth. By embryonic day 16, tenascin-C is abundant on the pillar side of the inner hair cell but does not accumulate on the modiolar side until about birth, a time after the arrival of afferent fibers. The synaptic zones beneath outer hair cells are strongly labeled during the time when early events in afferent synaptogenesis are progressing but not during the time of efferent synaptogenesis. At the age when most neural growth ceases, tenascin-C immunoreactivity disappears. Faint tenascin-C immunolabeling of normal hair cells, strong tenascin immunolabeling in pathological hair cells of Bronx waltzer (bv/bv) mice, and staining for beta-galactosidase, whose gene replaces tenascin in a "knockout" mouse, indicate that hair cells supply at least part of the tenascin-C. The changing composition of the extracellular matrix in the synaptic region during afferent and efferent synaptogenesis is consistent with a role for tenascin in synaptogenesis. The presence of tenascin-C along the growth routes of nerve fibers, particularly toward the outer hair cells, raises the possibility that growth cone interactions with tenascin-C helps to guide nerve fibers in the cochlea.     

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.     

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)