A correlative study of evoked otoacoustic emission properties and audiometric thresholds

Correlations were made between the detection thresholds of evoked otoacoustic emissions (EOAEs), subjective click thresholds, and mean audiometric thresholds in 240 normal and hearing-impaired ears. EOAEs have never been observed when subjective click thresholds or mean audiometric thresholds were equal to or greater than 35 dB HL. EOAEs were always found when click thresholds were equal to or lower than 15 dB HL and when mean audiometric thresholds were equal to or lower than 22 dB HL. The incidence of EOAEs decreased and EOAE thresholds increased with increasing click or mean audiometric thresholds.

Evoked otoacoustic emissions: a fundamental and clinical survey

Evoked otoacoustic emissions (EOAEs) hold some promise as a fast, objective and noninvasive procedure to study the cochlea at the outer hair cell level. This paper summarizes the fundamental aspects of EOAEs, in particular their relationship with the active biomechanical properties of the cochlea. The properties of EOAEs are analyzed in normally hearing ears, in ears with sensorineural hearing loss, especially in Ménière's disease, in acoustic neuromas and in central deafness; the properties of EOAEs were also evaluated as to their suitability for screening auditory dysfunction in infants.

Synaptophysin in the developing cochlea

The immunoreactivity to SY38 (anti-synaptophysin antibody) was investigated in rat and guinea-pig cochleas during development. In rat pups SY38 reactivity first appeared in the inner spiral bundle (below inner hair cells) at postnatal day 3. Later on (days 10 and 15) the basal pole of outer hair cells (OHCs) was also reactive. In fetal guinea-pigs, the inner spiral bundle was reactive on day 45 of gestation, while the reactivity occurred below OHCs on day 62 of gestation. A preliminary electron microscopic finding (from a guinea-pig 62 days of gestation) indicated that SY38 immunoreactivity is localized within varicosities of efferent (olivo-cochlear) endings. Synaptophysin is thus present in the cochlea at the level where the two efferent systems terminate. Moreover, the occurrence of SY38 immunoreactivity, first at the ISB then at the OHC levels, is in accordance with the observation that the maturation of lateral efferents precedes that of medial efferents.

Immunoelectron microscopy identifies several types of GABA-containing efferent synapses in the guinea-pig organ of Corti

Using an immunoperoxidase technique, we have localized by light and electron microscopy GABA-immunostained fibers within a component of the efferent innervation of the organ of Corti. At the light microscopic level, GABA-immunostained fibers were observed within the inner spiral bundle (below the inner hair cells) and the tunnel spiral bundle. The immunostaining was clearly more intense in the upper turns than in the basal turns. Mostly in the upper turns, GABA-immunostained fibers were seen crossing the tunnel of Corti to reach the outer hair cells where they formed large immunostained patches at the base of the cells. Unevenly distributed throughout these upper turns, immunostained fibers were seen climbing along the outer hair cells and traveling near the non-sensorineural Hensen's cells. The electron microscopic observations of GABA-immunostained fibers in the upper turns allowed us to identify within the inner spiral bundle vesiculated varicosities synapsing with radial dendrites connected to the inner hair cells. In the outer hair cell area, the GABA-immunostained fibers made several kinds of synaptic contacts. They included a minor population of the large axosomatic synapses with the basal pole of the outer hair cells and many axodendritic synapses with the spiral dendrites connected to these cells. Occasionally, the GABA-immunostained climbing fibers also synapsed with the outer hair cells at a supranuclear level. These result confirm previous light microscopic data dealing with the projection of the GABA-immunostained fibers along the cochlear partition. Moreover, they extend them in characterizing several kinds of GABA-immunostained synapses. These latter findings agree with previous neurochemical electrophysiological data which suggests an efferent neurotransmitter role for GABA. Nevertheless, such an existence of an efferent innervation predominantly projecting to the upper turns of the cochlea adds another criterion distinguishing the "apical" from the "basal" cochlea.

Opioid receptors inhibit the adenylate cyclase in guinea pig cochleas

The effects of mu- and delta-preferring agonists on adenylate cyclase activity have been investigated in vitro in homogenates of guinea pig cochleas. Morphine, Leu-enkephalin, D-Ala2, N-methyl-Phe4, Gly-ol5-enkephalin (DAGO) and D-Ser2-Leu-enkephalin-Thr (DSLET) each inhibited the synthesis of cyclic AMP. This effect was reversed by naloxone which had a greater affinity in blocking the effect of the mu-preferring agonists (morphine, DAGO) than in blocking the effect of the delta-preferring agonists (Leu-enkephalin, DSLET). Finally, no additive effects were observed when various combinations of two agonists were used. These results indicate that opioid receptors exist in the guinea pig cochlea and that they are negatively linked to adenylate cyclase. The different affinities shown by naloxone to reverse the inhibition induced by the mu- and delta-preferring agonists suggest that morphine and DAGO act through mu-receptors, whereas Leu-enkephalin and DSLET act through delta-receptors. Since no additive effects have been found when combining two different agonists, it can be hypothesized that the mu- and delta-receptors are coupled to the same pool of adenylate cyclase. It may be proposed from these findings that in vivo enkephalins inhibit the synthesis of cyclic AMP via mu- and delta-receptors. However, whether this effect occurs at a presynaptic level (within opioid-containing olivocochlear varicosities) or at the postsynaptic level (within dendrites of the primary auditory neurons) remains to be determined.

Modulation of the masking phenomenon by the crossed part of the medial olivocochlear bundle

The masking phenomenon is used in clinical investigation of the inner ear to determine the frequency selectivity properties of the auditory system. Sectioning of the crossed part of the medial efferent bundle in a guinea pig model decreases the simultaneous masking phenomenon. The efferent effect seen in the forward masking varied with the masking paradigm used. When the masker onset precedes the maskee onset by more than 40 ms, masking diminishes after sectioning the crossed part of the medial efferent bundle. When this duration was shortest (30 ms), no effect was observed. This phenomenon could be explained by the time necessary to stimulate the medial efferent loop. Our results could explain some differences observed in the compound action potential masking curves with different masking paradigms.

Stereocilia and tectorial membrane development in the rat cochlea. A SEM study

Maturation of the rat cochlea, from postnatal days 2 to 60, was studied using scanning electron microscopy (SEM), with emphasis on stereocilia and tectorial membrane (TM). Two days after birth, the organ of Corti was very immature. An adult appearance of its surface was observed by day 16 in the basal turn, and by the end of the 3rd postnatal week in the apex. Stereocilia started their development first on inner hair cells. By contrast, the apical pole of outer hair cells ended its maturation before that of inner hair cells. Top-links were detected very early in inner hair cell stereociliary development (postnatal day 2). Marginal pillars temporarily attached the TM to the organ of Corti; they disappeared first in the apical region. This transient attachment seems to play a role in the coupling of outer hair cells to the TM, as prints of their longest stereocilia appeared at the undersurface of the TM by the same time. Moreover, these prints were more clear and regular at the base than at the apex of the cochlea. Results are discussed in relation to ultrastructural and functional data on rat cochlea maturation.

Variations of cochlear microphonic potential after sectioning efferent fibers to the cochlea

Cochlear microphonic (CM) potentials were recorded, in guinea pig, with differential electrodes before and after sectioning the medial efferent innervation at the level of the brainstem. Sectioning the crossed part of the medial efferent innervation did not change the CM whatever the frequency or level of stimulation used. Sectioning the medial--crossed and uncrossed--efferent fibers diminished CM amplitude at frequencies above 2 kHz. Thus, the ipsilateral medial efferent tract seems to be involved, through a tonic action, in controlling outer hair cell micromechanics.

Choline acetyltransferase (ChAT) immunoelectron microscopy distinguishes at least three types of efferent synapses in the organ of Corti

Using anatomical criteria, the olivo-cochlear fibers ending in the organ of Corti (efferent fibers) have recently been separated into two systems: a lateral system innervating principally the inner hair cell (IHC) area and a medial system innervating mainly the outer hair cells (OHCs). Electrophysiological and biochemical experiments suggest that acetylcholine may be a neurotransmitter of these efferent fibers. However, efferent synapses that use acetylcholine as neurotransmitter have not yet been identified at the electron microscopic level. Using a pre-embedding immunoelectron microscopic technique with a monoclonal antibody against choline acetyltransferase (ChAT), we localized ChAT-immunostained fibers below both the IHCs and OHCs. In the inner spiral bundle, one type of ChAT-immunostained fibers was vesiculated and formed axo-dendritic synapses with the afferent auditory dendrites contacting the inner hair cells. A second type of ChAT-immunostained fibers seen in the inner spiral bundle was unvesiculated. Unstained vesiculated varicosities synapsing with the auditory dendrites were also seen in the inner spiral bundle. At the OHC level, ChAT immunostaining was found in nearly all the terminals synapsing with the OHCs. The finding of two types of ChAT-immunostained efferent synapses in the organ of Corti, i.e. axo-dendritic synapses in the inner spiral bundle and axo-somatic synapses with the OHCs, supports the hypothesis that both the lateral and the medial olivo-cochlear systems use acetylcholine as a neurotransmitter. The finding of numerous unstained synapses in the inner spiral bundle, and some below OHCs, together with previous data about putative cochlear neurotransmitters, suggests the possibility of additional non-cholinergic olivo-cochlear systems. It might soon appear useful to reclassify efferents according to the nature of the different neurotransmitters/co-transmitters found in the various efferent synapses of the organ of Corti.

Surface aspects of the developing human organ of Corti

Thirteen cochleas from human fetuses ranging in age from week 9 to week 22 were observed by scanning electron microscopy. The classical 'base-to-apex' and 'internal-to-external' gradients of maturation were confirmed by surface observations of the developing organ of Corti. The tectorial membrane begins to be secreted around week 9, i.e. 2 weeks before the onset of the ciliogenesis. Its structure appears to be first amorphous and then fibrillar. The surface of the organ of Corti looks mature around week 22, but, in fact, its complete maturation, especially at outer hair cell level, probably occurs during the last trimester of pregnancy.

Effects of section of the medial efferent tracts (crossed and uncrossed) on cochlear frequency selectivity

The cochlear innervation of guinea pigs was sectioned midway between the midline and the external side of the cochlear nucleus, in a rostrocaudal direction at the level of the floor of the fourth ventricle, to study the effects of medial efferent pathways on cochlear frequency selectivity estimated with tuning curves of single auditory nerve fibers. Single-unit tuning curves were affected by this type of efferent sectioning. Thus, the ipsi-lateral efferent system seems to be involved, through a tonic action, in the mechanisms responsible for high frequency cochlea selectivity.

Neuron-specific enolase immunoreactivity in the developing mouse cochlea

Onset of neuron-specific enolase reactivity was observed on gestation day 17 in spiral ganglion cells of the basal coil, and 2 days later in cochlear inner (IHC) and outer (OHC) hair cells. IHCs and OHCs were similarly reactive up to postnatal day 7, then the reactivity began to decrease in OHCs.

Development of the auditory hair cell surface in human fetuses. A scanning electron microscopy study

Thirteen human fetal cochleas were investigated by scanning electron microscopy. Our observations concentrated on the hair cell surface. Ciliogenesis appeared to start during the 11th week of gestation on the inner hair cells (IHCs) and one week later on the outer hair cells (OHCs). The earliest stages of stereociliary development were similar on both types of cell and were characterized by the presence of round bundles of cilia arising from the surrounding microvilli. A three-dimensional V-shaped arrangement suddenly appeared, accompanied by the disappearance of short cilia on the internal side. Between the 20th and the 22nd weeks of gestation, both types of hair cell had an adult stereociliary pattern, i.e. a rectilinear arrangement on IHCs and W-shaped on OHCs. However, there were signs of immaturity, such as a disarray of OHCs and the presence of the kinocilium, suggesting that the surface of the auditory hair cells achieves its maturation during the last trimester of pregnancy.

Efferent tracts and cochlear frequency selectivity

The cochlear innervation of guinea pigs was sectioned medially in a rostrocaudal direction at the level of the floor of the fourth ventricle, to study the effects of efferent pathways on cochlear microphonic (CM) suppression, the compound action potential (CAP) masking phenomenon, the input-output CAP function, and cochlear frequency selectivity estimated with tuning curves of single auditory nerve fibers. Sectioning reduced CM suppression without having any effect on absolute CM amplitude; it also reduced CAP masking. The input-output CAP function was not changed at intensities below 75 dB, and the single-unit tuning curves recorded before and after nerve sectioning were unaffected. Thus, the crossed efferent tracts (i.e., mainly the medial system) seems to be involved in the masking function itself, rather than one of the mechanisms responsible for high frequency cochlear selectivity.

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'.

The very distal part of the basilar papilla in the chicken: a morphological approach

The very distal part of the chicken basilar papilla was investigated by light and electron (scanning and transmission) microscopy. The rostral tip of the basilar papilla has a lenticular area with atypical sensory hair cells which are more similar to vestibular than to auditory cells. The structure of the lenticular area appears to be suitable for vestibular function or, more likely, for auditory perception at very low frequencies. Several hypotheses can be proposed to explain this very peculiar portion of the avian cochlea. It is difficult to consider it a continuously growing area since it remains stable in adulthood. A better explanation would be that there is an incomplete ontogenetic or phylogenetic process.

Kainic acid selectively alters auditory dendrites connected with cochlear inner hair cells

Cochleas of adult guinea pigs and rats, and 6-day-old rat pups, were injected, through the round window, with 2 microliters of artificial classical Konishi perilymph containing 1 nmol kainic acid (KA). 5 min later, they were fixed, removed, and processed for electron microscopy. In all KA-treated cochleas, the injection resulted in a severe swelling of auditory dendrites below the inner hair cells (IHCs). Below the outer hair cells (OHCs), the swelling appeared only in the 6-day-old rats, not in adult animals. These results are significant in three different ways: (1) They confirm the strong difference between afferents innervating the IHCs and the OHCs in adult cochleas. (2) They shed some light on the synaptic plasticity found at the OHC level during synaptogenesis. (3) They support the hypothesis that glutamate, or a related substance, is the IHC neurotransmitter.

Met-enkephalin-Arg6-Gly7-Leu8 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.

Early stages of innervation and sensory cell differentiation in the human fetal organ of Corti

Early development of the human organ of Corti was investigated at the light and electron microscopic level. In 9-week fetal material the cochlea was completely coiled and nerve fibers penetrated into an undifferentiated Corti's primordium. By week 10 a single cell in radial sections, presumed to be the inner hair cell (IHC), could be found with many nerve fibers surrounding and contacting its base. It is possible to identify outer hair cells by the end of the 11th week while IHC development continues. During the 12th week stereocilia appear on IHCs and synaptic specializations could be found at the junctions between both types of hair cell and afferent dendrites. The first appearance of the efferent fibers beneath IHCs were also observed during week twelve.

Morphology, synaptology and electrophysiology of the developing cochlea

The development of synapses clearly differentiates the two kinds of cochlear receptors inner (IHC) and outer (OHC) hair cells. An adult-like pattern of innervation can be seen very early below the IHCs: both afferent and efferent (mainly axo-dendritic) synapses are formed before the onset of function. At the OHC level, drastic changes occur: numerous afferent dendrites retract as the large efferent endings arrive and synapse with the hair cell. This synaptic OHC remodelling coincides with maturation of compound action potential (AP) parameters, such as the "S" -shape of the input-output curves and the sharpening of AP tuning curves.

Met-enkephalin characterization in the cochlea: high-performance liquid chromatography and immunoelectron microscopy

The present paper extends and refines previous observations of enkephalin-like immunoreactivity in the guinea-pig cochlea. Firstly, Met-enkephalin was identified and a quantitative evaluation was made by combining high-performance liquid chromatography (HPLC) and a specific radioimmunoassay. Both the antibody specificity and the HPLC purification allowed us to demonstrate the co-existence, in the cochlea, of at least 3 opioid peptides: Met-enkephalin, Leu-enkephalin and Met-enkephalin-Arg6-Phe7. Secondly, a pre-embedding immunoperoxidase technique was used on whole or dissected cochleas. Immunoreactivity was localized in efferent fibers (coming from the brainstem) in the inner spiral bundle, tunnel spiral bundle and intraganglionic spiral bundle. In the inner spiral bundle vesiculated Met-enkephalin-immunoreactive fibers could be seen synapsing with afferent auditory dendrites. It is hypothesized that these Met-enkephalin immunoreactive fibers (belonging to the lateral efferent system) could be responsible for an inhibitory effect upon the gross cochlear action potential.

Immunofluorescence with Met-enkephalin and Leu-enkephalin antibodies in the guinea pig cochlea

We have applied an indirect immunofluorescence technique to cryostat sections of guinea pig cochleas using Met-enkephalin and Leu-enkephalin antisera. With both antisera, high immunofluorescence was seen in the inner spiral bundle, the tunnel spiral bundle and the intraganglionic spiral bundle. Immunoreactivity persisted to a dilution of 1/50 000. In three out of the twelve guinea pigs, immunoreactivity was observed under the outer hair cells, but only with the Met-enkephalin antiserum. Immunoreactivity of medium intensity was seen when the Met-enkephalin antiserum was pre-adsorbed by Leu-enkephalin. No immunoreactivity was seen when the Leu-enkephalin antiserum was pre-adsorbed by Met-enkephalin. After treatment of cochlear sections with acidic permanganate, immunoreactivity of medium intensity was still observed with the Leu-enkephalin antiserum. These observations indicate that Met-enkephalin and Leu-enkephalin co-exist in the cochlea in regions where efferent terminals are located, particularly in the inner spiral bundle below the inner hair cells.

Age-related structural investigation of the Bronx waltzer mutant mouse cochlea: scanning and transmission electron microscopy

Cochleas of mice homozygous for the Bronx waltzer gene (symbol bv) were investigated using scanning (SEM) and transmission (TEM) electron microscopy. An age-related study was done from birth to postnatal day 100. With SEM, the arrangement of hair cells confirmed the unique feature of the bv/bv cochlea: the inner hair cells (IHCs) were either absent or abnormally haired but the outer hair cells (OHCs) appeared normal. No significant difference was observed with age. Using TEM, the remaining (20-25%) IHCs could be divided into two groups: normal-looking IHCs but with an abnormal synaptic pole, and abnormal, abortive-like IHCs. Very little if any sign of degeneration was observed whatever the age. OHCs displayed an almost normal cytology and pattern of innervation. The neurons of the spiral ganglion were very rare, even at birth. These findings suggest that the bv mutation should rather be classified in another group, than 'degenerative'. The persistence of normal structures at OHC level is discussed in light of the cochlear physiology of the bv/bv: it again raises the question of the real role of OHCs in the peripheral auditory mechanisms.

Incidence and clinical significance of anti-ENA antibodies in systemic lupus erythematosus. Estimation by counterimmunoelectrophoresis

Eighty-two patients with systemic lupus erythematosus (SLE) were investigated for the presence and significance of serum antibodies to Extractable Nuclear Antigen (ENA) and its major components, RNP, Sm and SS-B (or Ha). The counterimmunoelectrophoresis assay allowed independent detection and measurement of antibodies to the different components. Forty patients had anti-ENA antibodies, 25 (30%) were of anti-RNP type alone or anti-RNP associated with anti-Sm, and 12 (15%) were of anti-RNP type alone. Anti-ENA antibodies distinguished a subset of patients with less common incidence of renal disease, positive Coombs test, anticoagulant serum factors and high titres of anti-DNAds antibodies, with higher incidence of Raynaud's phenomenon, swollen hands, hypergammaglobulinemia and high titres of antinuclear antibodies with speckled pattern on immunofluorescence. SLE patients with anti-RNP antibodies had in addition a high frequency of normal complement values. All but one SLE patient with only anti-RNP antibodies fulfilled at least four or more criteria for the diagnosis of SLE. We conclude that anti-ENA antibodies in SLE patients are associated with a low prevalence of nephritis and a clinical and laboratory profile similar to that of the MCTD syndrome. These findings demonstrate the difficulty of inferring rigid differences between MCTD and SLE. The MCTD syndrome probably represents only one segment of the whole clinical spectrum of SLE.

Innervation of the chicken basilar papilla during its development

Afferent and efferent innervation of the chicken basilar papilla was studied during development. Observations from the 10-day embryonic stage (E 10) and the 8-day post-hatch (P 8), were focussed on some characteristic features of the synaptogenesis. The presynaptic specializations (synaptic bodies: SB) appear early (E 10) in hair cells, opposite afferent dendrites. They are found at all developmental stages in tall hair cells (THC), whereas they become rare in short hair cells (SHC). The maturation of afferent endings, especially those connected to THC, is characterized around the E 15 stage, by an over abundant sprouting and branching. The development of the efferent system indicates two stages. Starting at E 14, some small efferent synapses are seen at the base of the THC and SHC. From E 18, the large synapses extend to the base of SHC. These results are discussed in the light of homologies with the mammalian inner and outer cochlear hair cells.

Early degeneration of sensory and ganglion cells in the inner ear of mice with uncomplicated genetic deafness (dn): preliminary observations

Deafness (dn/dn) mouse cochlea was investigated by electron microscopy in order to detect the first postnatal signs of histopathology. At birth, the inner hair cells (IHCs) showed abnormal vacuolization, afferent dendrites at IHC level appeared swollen and devoid of cytoplasmic content, and most of the neurons of the spiral ganglion contained an abnormal smooth endoplasmic. At day 7, these abnormalities had greatly increased, especially in the spiral ganglion neurons where large patches of filamentous material were found. These observations can account for the permanent electrophysiological deafness of dn/dn mice. Moreover, these data, showing early cytological abnormalities in both the IHCs and the spiral ganglion neurons, indicate that it is difficult to simply classify the deafness mutation as being 'degenerative'; some 'morphogenetic' processes are likely also involved.

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.

Radioautographic study of the symphathetic fibers in the cochlea

The noradrenergic fibers of the cochlea have been investigated by radioautography after an in vitro labeling by 3H-Noradrenalin (3H-NA). Unmyelinated, perivascular and non-perivascular fibers took up and retained 3H-NA. Non-perivascular fibers formed small bundles together with unmyelinated efferents in the intraganglionic spiral bundle and the osseous spiral lamina. No labeled fibers were detectable beyond the habenula inside the organ of Corti. The labeled fibers, which are probably noradrenergic, were characterized by numerous varicosities and discontinuous Schwann sheathes. These morphological characteristics could associate them with a general role in cochlear function. Sometimes, labeled fibers were also found very close to Ranvier's node of certain myelinated fibers, which could suggest a specific effect on these fibers.

The Gunn rat: an experimental model for central deafness

The Gunn rat which develops neonatal hyperbilirubinemia has been used as an experimental model to evaluate the effect of bilirubin on the auditory system. Electrocochleographic and morphological studies (including light microscopy, surface preparations and transmission electron microscopy) did not reveal any cochlear abnormality in homozygous Gunn rats. Brainstem auditory evoked potentials showed morphological and amplitude changes suggesting a functional damage in the brainstem auditory pathways. These results suggest that hearing loss, when observed in kernicterus, is primarily due to neuronal damage at the level of brainstem auditory nuclei.

Cochlear synaptogenesis in the hypothyroid rat

The effects of congenital hypothyroidism on cochlear synaptogenesis were studied using transmission electron microscopy in 30 propylthiouracil-treated rat pups 1-35 days of age. No difference with controls was observed at the level of the inner hair cells which are known to mature essentially during the prenatal period. On the contrary, hypothyroidism resulted in numerous abnormalities in synaptogenesis at the level of the outer hair cells: abnormal persistence of numerous afferent dendrites and presynaptic specializations, incomplete development of efferent terminals and absence of formation of postsynaptic cisterns. It can be concluded that hypothyroidism results in severe retardation in the postnatal development of the innervation of outer hair cells.

Sectioning the efferent bundle decreases cochlear frequency selectivity

Cochlear frequency selectivity was tested in rats and cats before and after sectioning the vestibular nerve which carries the efferent bundle to the cochlea. Compound action potential tuning curve were obtained by a simultaneous masking procedure. Transection of the efferent bundle resulted, in all group of animals, in an enlargement of the tip segment of the tuning curve. The Q10 dB value decreased by about 30% without any significant threshold change. These results taken together with previous developmental findings, suggest that efferents to outer hair cells could play a role in sharpening the frequency selectivity of the cochlea.

Variability of the hereditary deafness in the white cat. I. Physiology

Electrophysiological investigations, at different levels of the auditory pathway, were performed on 54 white cats. Hearing tests generally consisted of recording electrocochleograms or making audiograms from collicular or cortical responses. Some investigations were performed with chronically implanted electrodes to detect the first appearance of a hearing defect. Results showed no evident relationship between the age of the white cat and the appearance, severity or completeness of hearing loss. A large variety of hearing remnants was encountered in partly deaf animals which is tentatively related to a similar variety of histological damage of the cochlea described in a companion paper (Rebillard, M., Pujol, R. and Rebillard, G. (1981): Hearing Res. 5, 189-200).

Variability of the hereditary deafness in the white cat. II. Histology

Cochlear degeneration in white cats was investigated by light- and electron-microscopic examinations. A great variety of histological damage was encountered both in completely and partially deaf animals. These variable features are discussed mainly with regard to the rate of degeneration and the site of the first damage. Atypical findings, such as primary degeneration of spiral ganglion neurons, are presented. The possibility for some damaged cochleas to work without hair cells is described. These results clearly demonstrate that hereditary degeneration in the cochlea of white cats is not a unique and regular process.

Age-related changes in the C57BL/6J mouse cochlea. I. Physiological findings

The development and degeneration of cochlear output was studied in C57BL/6J mice from the time of hearing onset (12 days of age) through adulthood (50 days of age) using the eighth nerve compound action potential (N1). Mice were stimulated in free-field, using short rise-time tone bursts (2-35 kHz). Sensitivity to tones and threshold tuning curve sharpness (Q10) increased markedly between 12 and 20 days of age. Response latencies changed, in a complex way, to attain minimum values by 20 days of age. The form of intensity function was essentially identical in 12- and 16-day-old mice. In 30- and 40-day-old mice signs of both hearing development and degeneration were seen. For example, while N1 threshold intensities to low-frequency tones continued to decline, those to high frequencies began to increase. The results indicate that some of the previously observed age-related changes in central auditory function in C57BL/6J mice can be accounted for in terms of cochlear evolution. The observed changed in cochlear function are discussed in relation to outer-, middle-, and inner-ear modifications. A basis for the limits of the critical period for audiogenic seizure "priming' is hypothesized.

Age-related changes in the C57BL/6J mouse cochlea. II. Ultrastructural findings

The C57BL/6J mouse organ of Corti was studied using transmission electron microscopy. The basal coil of the cochlea was examined in mice 1-50 days of age. At birth the cochlea was very immature but both types of hair cells were innervated. Inner hair cells (IHC) were connected to afferent and efferent processes, and efferent endings synapsed on IHC afferents. Outer hair cells (OHC) were innervated exclusively by afferents, which made well-defined synapses with the cell. Maturation at IHC took place rapidly and was essentially over by about 12 days of age. Changes were largely restricted to an increase in presynaptic specializations opposite afferents and a decrease in postsynaptic specializations (cisterns) opposite efferents. Ontogeny at OHC took place over a longer, 2.5 week, period. Afferent synapses lost their presynaptic specializations (synaptic bodies), and then efferent fibers arrived below OHC. The efferents subsequently made temporary axo-dendritic synapses with the afferents before replacing most of them at OHC. The first synapses between efferent endings and OHC were seen at 9 days of age, but it was not until about 20 days of age that mature synapses were seen. Some evidence of hair cell degeneration was seen in 30- and 50-day-old mice. The results are discussed in terms of sensory hair cell differentiation, the disappearance of OHC synaptic bodies, and age-related changes in auditory system function.

High resolution radioautographic study of the inner ear following in vivo tritiated deoxyglucose administration

Light and high resolution radioautography were performed on cat and guinea pig inner ear after an in vivo administration of tritiated deoxyglucose and conventional histological treatments. In the guinea pig cochlea a diffuse radioautographic reaction product appeared on all the neurosensory and surrounding structures, with a more intense labelling of stria vascularis and Reissner's membrane. In the cat vestibular organ a diffuse reaction was also noted, with an intense clear-cut labelling of some dark cells at the base of the lateral crista and on some cells limiting the endolymphatic space, opposite the utricular macula. At the EM level, the silver grain distribution preferentially appeared to be localized on the cytoplasmic glycogen granules. In addition, some sensory cells were densely marked, the silver grains neither occurring on the nucleus nor on the surrounding nerve calyx, and rarely upon the mitochondria. These data indicate that using classical histological treatments, it is possible to retain a part of the radioactive molecules around the injection site. The possible physiological meaning of the selective labeling of some cellular and subcellular compartments is discussed.

Cochlear receptor development in the rat with emphasis on synaptogenesis

Maturation of the albino rat cochlea was studied using light and electron microscopy. Critical stages of receptor morphology were examined. At birth, cochlear structures are very immature, but even at this early stage synapses are recognizable. Under inner hair cells (IHCs) both afferent and efferent synapses are present. Under outer hair cells (OHCs) only afferent endings are seen. During the first postnatal week, synaptic development proceeds slowly. Between 6 to 12 days of age, substantial changes occur in the pattern of hair cell innervation. There are fewer efferent synapses at the IHC level and the first efferent junctions form on OHCs. In addition, a pattern of temporary innervation is seen under the OHC, with axo-dendritic synapses between efferent endings and afferent fibres. Between 12 and 16 days of age the main changes in hair cell innervation are at OHC level where afferent junctions regress and large efferent synapses form. By 16 days of age sensory-neural relationships seem adult-like. The results are discussed in relation to rat cochlear electrophysiological development and the period of supra-normal sensitivity to acoustic trauma.

Ontogenetic approach to inner and outer hair cell function

Anatomo-functional relationships made during ontogenesis seem to offer possibilities of a better understanding of cochlear function. The asymmetrical kinetics of inner and outer hair cells (IHCs and OHCs) structural development can assist in determining their functional role. Light- and electron-microscopic observations of cochlear development in kittens, guinea pigs, hamsters and rats reveal that the maturation of IHC and their afferent and efferent innervation occurs first. The OHCs develop more slowly and the pattern of their innervation changes dramatically, the last stage involving the enlargement of efferent synapses. The timing of this maturation correlates well with the development of cochlear function. The findings suggest that IHCs are required for the onset of auditory function while mature OHCs are necessary for the more discriminative properties of the cochlea.

[Cellular detection of the in vivo incorporation of tritiated 2-deoxyglucose. Radioautographic study in the inner ear]

Inner ear injection of (3H) deoxyglucose was followed by in situ glutaraldehyde fixation and, after removal, by traditional preparation of the tissue for electron microscopy. Radioautography of semi-thin sections revealed preferential tracer accumulation in metabolically active structures such as the cochlear stria vascularis and the dark cells surrounding the basis of the vestibular cristae. Furthermore, in the vestibule, the perikaryon of some sensory cells was intensely labelled. These results show that an in vivo injection of deoxyglucose, followed by conventional histological processing can be used to mark active cells at both cellular and subcellular levels.

Supra-normal sensitivity to ototoxic antibiotic of the developing rat cochlea

A period of increased sensitivity to the ototoxic antibiotic amikacin was found in the cochlea of the developing rat. Starting either at birth, 10 or 30 days of age, rat were injected i.p. daily with 200 mg/kg of amikacine for 10 days. Only in the group injected with drug from 10 to 20 days of age was there evidence of a substantial (40-100 dB) permanent threshold shift (PTS). To characterize the period of supra-normal sensitivity more precisely, independent groups of pups were injected with 50 mg/kg of amikacin from 10 to 20, 15 to 25, 20 to 30, and 30 to 40 days of age. Only those pups injeced from 10 to 20 or 15 to 25 days of age showed PTS (30-60 dB). These data, taken together with those on rat cochlear maturation indicate that the sensitive period to ototoxicity starts at the age of onset of cochlear function (around day 10) and ends few days after the cochlea shows adult-like characteristics (3rd postnatal week).

Sensitive period to acoustic trauma in the rat pup cochlea. Histological findings

In a previous paper we demonstrated physiologically a sensitive period for acoustic trauma in the rat pup cochlea, with a maximum at 22 days of age. Seven or 60 days after noise exposure these 22-day exposed rats were used for light and electron microscopy. At 7 days, surface preparation revealed restricted damage to the basal cochlear coil, while with electron microscopy signs of cytoplasmic degeneration appeared in the great majority of basal coil structures. Two months after noise exposure these structures have completely degenerated. These results were discussed in terms of mechanical versus metabolic explanations of the critical period for acoustic trauma.