Gentamicin-induced alterations of succinic dehydrogenase activity in the organ of Corti as revealed by non-decalcified frozen sections of the guinea pig's cochlea

Summary The distribution of succinic dehydrogenase (SDH) activity and the changes of SDH activity after injection of gentamicin (GM) were observed in the organ of Corti using non-decalcified frozen sections of the guinea pig's cochlea. The distribution of SDH activity was found to increase from the apex to the basal turn. At each turn, SDH activity of the inner hair cells, the inner supporting cells and the nerve endings surrounding the supporting cell and on the hair cells presented a greater activity than that found in the outer hair cells, adjacent Deiter's cells and associated nerve endings. It was further observed that GM had a greater effect on SDH activity in the basal turn than the other turns. At each turn, a more sensitive area of response to GM was found on the nerve endings one each hair cell, especially on the outer hair cells of the basal turn.The experimental part of this study was completed at the Laboratory of Autoradiography, Beijing Medical University, Beijing, People's Republic of China     

Immunohistochemical distribution and fine localization of the gamma-aminobutyric acid in the organ of Corti of normal guinea pigs]

This paper presents the distribution and fine localization of the gamma-aminobutyric acid (GABA) in the Corti's organ of normal guinea pigs by using immunohistochemical technique (ABC-GDN method) and immunoelectron microscopy. GABA-IR was found in axons of efferent neurons in all turns of the cochlear spiral, but no positive endings may be found in the apical. The positive immunoreactive products were seen in the efferent components, including inner spiral bundle, tunnel spiral bundle, tunnel-crossing fibers, and large nerve endings on outer hair cell bases. Some of the GABA-IR negative outer hair cells could be seen between the positive endings. One GABA-IR positive nerve fiber could form synapse with six or seven outer hair cell bases. The GABA-IR positive efferent endings, negative efferent endings and negative afferent endings could be seen on the out hair cell bases with transmission electronic microscope. The GABA-IR positive efferent endings and negative afferent endings could form the neuraxon-neurodendron synapse on the inner hair cells bases. These morphological distribution suggests that the GABA may be one of the cochlear efferent neurotransmitter or modulator.     

Acetylcholinesterase in the cochlea on normal guinea pigs by light and electron microscopy

The distribution of AchE activity in the cochleae of 42 normal guinea pigs was studied with thiocholine Cu-ferrocyanide method. Light microscopy revealed the AchE activity in the following areas: 1. the inner spiral bundle, 2. the spiral tunnel bundle, 3. the upper tunnel radial fibers, and 4. the large nerve endings at the base of the outer hair cells. Activity of AchE was most abundant in the basal turn and decreased gradually towards the apex. There was also a decreasing AchE from the first to the third row of outer hair cells. In the fourth turn, AchE activity was demonstrated only in the large nerve endings under the first row of outer hair cells. Under electron microscopy, the reaction was found to be localized to the following structures: 1. the axolemma of the small efferent fibers and their presynaptic enlargements in the inner spiral bundle, 2. the outer surface of the upper tunnel radial fibers and the spiral tunnel bundle, and 3. the large efferent nerve endings in contact with the base of the outer hair cells.     

Neural and infranuclear region changes in outer hair cells in acoustically exposed rabbits

Summary Rabbits were exposed to 100 dB pure-tone sound at 2 kHz for 2 h. Electron microscopy was used to study the relationship between changes in the outer hair cells, afferent nerve endings and efferent nerve endings. There was no relationship found between the degree of changes produced in the afferent nerve endings and that the outer hair cells. However, there was a relationship demonstrable between the degree of changes seen in the efferent nerve endings and the infranuclear region of the outer hair cells. These findings show that acoustic trauma will damage most the infranuclear region of the outer hair cells, while efferent nerve endings are injured next and least affected are afferent nerve endings.     

Localization of efferent neurotransmitters in the inner ear of the homozygous Bronx waltzer mutant mouse

Naturally occurring mutant mice provide an excellent model for the study of genetic malformations of the inner ear. Mice homozygous for the Bronx waltzer (bv/bv) mutation are severely hearing impaired or deaf and exhibit a 'waltzing' gait. Functional aspects of cochlear and vestibular efferents in the bv/bv mutant mouse are not well known. The present study was designed to evaluate several candidates of efferent neurotransmitters or neuromodulators including choline acetyltransferase (ChAT), gamma-aminobutyric acid (GABA), and calcitonin gene-related peptide (CGRP) in the inner ear of the bv/bv mutant mouse. Ultrastructural investigations at both light and electron microscopic level were performed. Ultrastructural morphologic evaluations of the cochlea and the vestibular end-organs were also undertaken. It is demonstrated that ChAT, GABA and CGRP immunoreactivities are present in the cochlea and in vestibular end-organs of bv/bv mutant mice. In the organ of Corti, immunoreactivity of ChAT, GABA and CGRP is confined to the inner spiral fibers, tunnel-crossing fibers, and the vesiculated nerve endings synapsing with outer hair cells. Interestingly, immunoreactivity was detectable even where inner hair cells appeared missing. Results also revealed malformations of the outer hair cells with synaptic contacts to efferent nerve endings consistently intact. In the neurosensory epithelia of the vestibular end-organs, the presence of ChAT, GABA, and CGRP immunoreactivity was localized at the vestibular efferents, with the exception of the macula of saccule. In one 8-month-old macula of utricle where the depletion of hair cells appeared highest, ChAT immunostaining was still discernible. Ultrastructural investigation demonstrated that vesiculated efferent nerve endings make synaptic contact with the outer hair cells in the organ of Corti and with type II hair cells in the vestibular end-organs. The present study provides further support that the efferent system in the bv/bv mutant inner ear is morphologically as well as functionally mature. These findings also demonstrate that if and when the onset of efferent degeneration in the bv/bv mutant inner ear occurs, it transpires subsequent to pathological conditions in the hair cells. The present findings give further indication that the efferent systems of the bv/bv mutant inner ear are independent of the afferent systems in many aspects including development, maturation as well as degeneration.     

Aspartate aminotransferase immunoreactivity in cochlea of guinea pig

The distribution of aspartate aminotransferase-like immunoreactivity in the cochlea of the guinea pig was studied at the light microscopy level. Indirect immunofluorescence histochemistry using antisera against cytoplasmic aspartate aminotransferase prepared from pig heart was applied to surface preparations of the organ of Corti and cryostat sections of the cochlea. In the modiolus, immunofluorescence was localized to spiral ganglion cells and myelinated fibers of the auditory nerve and intraganglionic spiral bundles. In the organ of Corti, immunofluorescence was seen in upper tunnel crossing fibers and at the base of outer hair cells, following a distribution similar to that of the efferent innervation of the outer hair cells. Weak immunofluorescence was seen in the inner spiral bundle and tunnel spiral bundle, but was not present in all preparations. Immunofluorescence was not seen in inner hair cells, nor at the base of inner hair cells, and may have been absent from outer hair cells.It is concluded that spiral ganglion cells and myelinated auditory nerve axons contain aspartate aminotransferase-like immunoreactivity; such immunoreactivity has previously been determined in auditory nerve endings in the cochlear nucleus. Olivocochlear neurons that innervate outer hair cells also contain such immunoreactivity while other cochlear efferents contain little or none.     

Ultrastructural findings in a case of Menière's disease

The temporal bones of an individual with documented unilateral Meniere's disease were prepared for light and electron microscopy. A morphometric analysis was performed on hair cells, spiral ganglion cells, dendritic fibers in the osseous spiral lamina, afferent and efferent endings, and afferent synaptic contacts. In the ear with Meniere's disease, we found hair cell damage, including disruption of the cuticular bodies and basalward displacement of some outer hair cells. There was no significant difference in the number of hair cells or spiral ganglion cells on the two sides. There was a significant decrease, however, in the number of afferent nerve endings and afferent synapses at the base of both inner and outer hair cells in the ear with Meniere's disease as compared to the contralateral ear.     

A study of the nerve endings of the outer hair cells in the organ of Corti--using scanning electron microscopy

The nerve endings on the outer hair cells of the guinea pig organ of Corti were studied using a scanning electron microscope. The nerve endings in the basal turn were entirely exposed in the space of Nuel, whereas in the upper turns almost all nerve endings were covered by the Deiters' cell bulge. The number and the size of the nerve endings gradually decreased from the basal turn toward the apical turn. The shape of the nerve endings was spindle-shaped in the basal turn, while it was round in the upper turn. Some nerve endings were found to locate close to the outer hair cell surface. No pathological change was observed in the nerve endings 25 hr after completion of series treatments of streptomycin or kanamycin. However, the outer hair cells consistently showed shrinkage, especially in the basal turn.     

Calpain在噪声损害毛细胞中的角色

目的：了解Calpain在噪声损害耳蜗毛细胞中所扮演的角色。方法观察Calpain免疫细胞化学反应产物在噪声损害中的变化。结果正常对照耳蜗样品中未见Calpain活性显示，但噪声损害耳蜗样品中可见外毛细胞底部及其神经末梢区域出现大量的Calpain。结论噪声引起的耳蜗毛细胞破坏可能与Calpain的激活及其降解功能有关。     

Noise-induced degeneration in the brain and representation of inner and outer hair cells

Noise that destroys receptor cells in the chinchilla cochlea also results in degeneration of axonal endings in the brain from the cochlear nerve fibers and the auditory pathways ascending to the superior olivary complex and inferior colliculus. The patterns of degeneration provide experimental evidence for differential representation of inner and outer hair cells and their functions.     

Computer-aided three-dimensional reconstruction of the inner hair cells and their nerve endings in the guinea pig cochlea

The inner hair cells and their nerve endings in the basal and third turns of the guinea pig cochlea were reconstructed three-dimensionally from serial thin sections by means of computer graphics. The inner hair cells showed narrowing below the cuticular plate and gradual increase in size toward the base where the surface was indented by nerve attachments. The sizes of the afferent nerve endings were in the range of 0.9 to 3.2 microns in cross section. Large endings were fewer and were located mainly on the pillar cell side, whereas small endings were numerous and were located on the modiolar side. The number of afferent nerve endings on a single cell in the basal turn averaged 26 and in the third turn, 17. All the afferent fibers were unbranched, most of them innervated a single cell, and 81% of them formed synapses with distinct synaptic bodies in the inner hair cells. A few afferent endings formed membrane specializations with two adjacent inner hair cells but the synaptic body was seen in only one cell. The number of efferent endings, from 0.08 to 2.0 micron in diameter, was about nine in the basal turn and was seven in the third turn. No significant difference was seen between the basal and third turns in cell shape, proportion of afferent to efferent endings, nor in morphology of synapses.     

Neuroanatomical basis of cochlear coding mechanisms.

The afferent cochlear neurons to outer and inner hair cells differ not only in their relative numbers (95% to inner hair cells and only 5% to outer hair cells) and distribution pattern (great convergence for outer hair cells and divergence for inner hair cells) but also in their degeneration behaviour and metabolism. Some special afferent fibres seem to exist. The afferent neurons present three types of ganglion cells in the spiral ganglion with no morphological evidence for an extensive direct interaction between afferent neurons of the outer and the inner hair cell system at any level in the cochlea and even their efferent nerve supply is essentially separated.     

CGRP- and cholinergic-containing fibers project to guinea pig outer hair cells

MU33, an antibody to calcitonin gene-related peptide (CGRP), was used to investigate the magnitude of CGRP-containing nerve fiber endings, compared to acetylcholinesterase (AChE)-containing nerve fiber endings on outer hair cells (OHCs). Results showed that CGRP-containing nerve fiber immunoreactivity mimicked the AChE fiber OHC staining pattern across the cochlea suggesting that CGRP can function as both a medial efferent (contacting primarily OHCs) and as a lateral efferent (contacting primarily inner hair cell afferents) neurotransmitter.     

Innervation densities of inner and outer hair cells of the human organ of Corti. Evidence for auditory neural degeneration in a case of Usher's syndrome

The innervation densities, i.e. the number of afferent endings per inner and outer hair cell, number of synaptic specializations per inner hair cell and the number of efferent endings per outer hair cell, were evaluated by serial section electron microscopy in the normal human inner ear and a case of Usher's syndrome (retinitis pigmentosa and sensorineural hearing loss). There was a significant decrease in the innervation density in Usher's syndrome as compared to normative controls, which could not be explained by the decrease in density of spiral ganglion cells. This was interpreted as suggesting a decrease in the number of neural processes and endings within the dendritic arborization of remaining spiral ganglion cells in this disorder.     

小鸡基底乳头听毛细胞突触的超微结构观察

目的；进一步研究鸡的内耳毛细胞及其神经支配的形态学特征。方法：应用透射分别观察５只小鸡基底乳头的４个部位（距基底乳头远端起始部０．６ｍｍ、１．６ｍｍ，２．６ｍｍ，３．６ｍｍ）毛细胞突触的形态学特征。结果：高毛细胞（ＴＨＣ）、矮毛细胞（ＳＨＣ）与传入性和传出性神经末梢形成突触。ＴＨＣ下方与大的传入性神经末梢和小的传出性神经末梢形成突触；ＳＨＣＧ民大的传出性神经末梢和小的传入性神经末梢构成突触。结论：     

Localization of soluble guanylate cyclase activity in the guinea pig inner ear

The aim of this study was to characterize the nitric oxide (NO) receptor soluble guanylate cyclase (sGC), to determine the cells targeted by NO and to elucidate the function of the NO/cGMP pathway in the inner ear. sGC activity in the inner ear was localized by immunohistochemical detection of NO-stimulated cGMP. Soluble guanylate cyclase activity in the cochlea was detected in the nerve endings underneath the outer and inner hair cells, supporting cells, stria vascularis and vessels. In the vestibular organs, sGC activity was detected in the cytoplasm of sensory cells, nerve fibres, dark cells and transitional cells and vessels. These findings suggest that the NO/cGMP pathway may be involved in regulatory processes in neurotransmission, blood flow and inner ear fluid homeostasis.     

Localization of Soluble Guanylate Cyclase Activity in the Guinea Pig Inner Ear

The aim of this study was to characterize the nitric oxide (NO) receptor soluble guanylate cyclase (sGC), to determine the cells targeted by NO and to elucidate the function of the NO/cGMP pathway in the inner ear. sGC activity in the inner ear was localized by immunohistochemical detection of NO-stimulated cGMP. Soluble guanylate cyclase activity in the cochlea was detected in the nerve endings underneath the outer and inner hair cells, supporting cells, stria vascularis and vessels. In the vestibular organs, sGC activity was detected in the cytoplasm of sensory cells, nerve fibres, dark cells and transitional cells and vessels. These findings suggest that the NO/cGMP pathway may be involved in regulatory processes in neurotransmission, blood flow and inner ear fluid homeostasis.     

Glutaminase-like immunoreactivity in the organ of Corti of guinea pig

The distribution of glutaminase (GLNase)-like immunoreactivity (IR) in the normal and surgically de-efferented organ of Corti of guinea pig was studied. Primary antisera were against phosphate-dependent GLNase from rat kidney. Indirect immunocytochemical techniques were used; IR was visualized in cryostat sections through immunofluorescence, and through immunofluorescence or with horseradish peroxidase reaction product in surface preparations. Standard microscopy and video-enhanced light microscopy with asymmetric illumination contrast were used. GLNase-like IR was found at inner hair cells (IHCs) in the normal and in the de-efferented organ of Corti, in the tunnel spiral bundle, in tunnel-crossing fibers, in endings high up on outer hair cells (OHCs), in outer spiral bundles, in puncta close to OHCs, and in large, efferent endings at OHC bases. There was no GLNase-like IR at OHCs in the de-efferented organ of Corti. It is concluded that GLNase-like IR is present in auditory nerve dendrites at IHCs and in olivocochlear efferents of the medial system, and that future studies are needed to determine whether also the lateral system of olivocochlear efferents contains GLNase-like IR. A diagram is included depicting the relation between OHCs and efferent nerve endings along the cochlear spiral, showing that in the apicalmost 3/4 turn of the spiral OHCs have no efferent endings.     

Incorporation of glycerol in the organ of Corti: predominance in hair cells and effects of kanamycin

A biochemical study revealed that [3H]glycerol was incorporated into various kinds of phospholipids in the guinea pig cochlea. The sites of incorporation of [3H]glycerol injected into the scala tympani were localized autoradiographically. The most active incorporation occurred in the hair cells, especially in the outer ones, followed by the inner ones and the nerve endings attached to hair cells. Glycerol incorporation into hair cells appeared to increase as the apex was approached. Chronic intoxication with kanamycin selectively suppressed glycerol incorporation in the hair cells. The site of glycerol incorporation shown by the present autoradiographic study is likely to reflect the site of phospholipid synthesis. Autoradiographic studies could constitute a new approach to understanding the possible involvement of phospholipid metabolism in the function and pathology of the cochlea.     

Comparative anatomy of the cochlea and auditory nerve in mammals

The numbers and structure of hair cells; afferent, efferent, and reciprocal synapses as seen at the base of hair cells; innervation patterns of first order cochlear neurons; and number and morphology of spiral ganglion cells will be discussed and compared in the guinea pig, rat, cat, monkey and man. Despite many similarities both in the organ of Corti and the spiral ganglion in these species, there are a number of differences which may have important physiologic implications. In the organ of Corti, the major differences among species are the length and width of the basilar membrane, the number of inner and outer hair cells, and the length of hairs on both inner and outer hair cells. Significant differences in the innervation pattern of the inner hair cell among these species include the number of afferent nerve terminals per inner hair cell, the degree of branching of afferent fibers, and the number of synapses per afferent nerve terminal. Among outer hair cells, the number of afferent nerve terminals per outer hair cell, presence or absence of a pre-synaptic body, presence or absence of reciprocal synapses, the number of efferent terminals per outer hair cell, and the presence of dendodendritic synapses in outer spiral bundles may be differences important physiologically. In the spiral ganglion, there are significant differences in the number of spiral ganglion cells, the number of cochlear nerve fibers, the percentage of spiral ganglion cells which are myelinated, and the presence of synapses on spiral ganglion cells.