Axodendritic and dendrodendritic synapses within outer spiral bundles in a human

GABAergic synapses between auditory efferent neurons and type II spiral ganglion afferent neurons in the mouse cochlea

Cochlear outer hair cells (OHCs) are electromotile and are implicated in mechanisms of amplification of responses to sound that enhance sound sensitivity and frequency tuning. They send information to the brain through glutamatergic synapses onto a small subpopulation of neurons of the ascending auditory nerve, the type II spiral ganglion neurons (SGNs). The OHC synapses onto type II SGNs are sparse and weak, suggesting that type II SGNs respond primarily to loud and possibly damaging levels of sound. OHCs also receive innervation from the brain through the medial olivocochlear (MOC) efferent neurons. MOC neurons are cholinergic yet exert an inhibitory effect on auditory function as they are coupled to alpha9/alpha10 nicotinic acetylcholine receptors (nAChRs) on OHCs, which leads to calcium influx that gates SK potassium channels. The net hyperpolarization exerted by this efferent synapse reduces OHC activity-evoked electromotility and is implicated in cochlear gain control, protection against acoustic trauma, and attention. MOC neurons also label for markers of gamma-aminobutyric acid (GABA) and GABA synthesis. GABAB autoreceptor (GABABR) activation by GABA released from MOC terminals has been demonstrated to reduce ACh release, confirming important negative feedback roles for GABA. However, the full complement of GABAergic activity in the cochlea is not currently understood, including the mechanisms that regulate GABA release from MOC axon terminals, whether GABA diffuses from MOC axon terminals to other postsynaptic cells, and the location and function of GABAA receptors (GABAARs). Previous electron microscopy studies suggest that MOC neurons form contacts onto several other cell types in the cochlea, but whether these contacts form functional synapses, and what neurotransmitters are employed, are unknown. Here we use immunohistochemistry, optical neurotransmitter imaging and patch-clamp electrophysiology from hair cells, afferent dendrites, and efferent axons to demonstrate that in addition to presynaptic GABABR autoreceptor activation, MOC efferent axon terminals release GABA onto type II SGN afferent dendrites with postsynaptic activity mediated by GABAARs. This synapse may have multiple roles including developmental regulation of cochlear innervation, fine tuning of OHC activity, or providing feedback to the brain about MOC and OHC activity.

Dopaminergic and cholinergic innervation in the mouse cochlea after noise-induced or age-related synaptopathy

Cochlear synaptopathy, the loss of or damage to connections between auditory-nerve fibers (ANFs) and inner hair cells (IHCs), is a prominent pathology in noise-induced and age-related hearing loss. Here, we investigated if degeneration of the olivocochlear (OC) efferent innervation is also a major aspect of the synaptopathic ear, by quantifying the volume and spatial organization of its cholinergic and dopaminergic components, using antibodies to vesicular acetylcholine transporter (VAT) and tyrosine hydroxylase (TH), respectively. CBA/CaJ male mice were examined 1 day to 8 months after a synaptopathic noise exposure, and compared to unexposed age-matched controls and unexposed aged mice at 24-28 months. In normal ears, cholinergic lateral (L)OC terminals were denser in the apical half of the cochlea and on the modiolar side of the inner hair cells (IHCs), where ANFs of low-spontaneous rate are typically found, while dopaminergic terminals were more common in the basal third of the cochlea and, re the IHC axes, were offset towards the habenula with respect to cholinergic terminals. The noise had only small and transient effects on the density of LOC innervation, its spatial organization around the IHC axes, or the extent to which TH and VAT signal were colocalized. The synaptopathic noise also had relatively small and transient effects on cholinergic innervation density in the outer hair cell (OHC) area, which normally peaks in the 16 kHz region and falls monotonically towards higher and lower frequencies. In contrast, in the aged ears, there was massive degeneration of OHC efferents, especially in the apical half of the cochlea, where there was also significant loss of OHCs. In the IHC area, there was significant loss of cholinergic terminals in both apical and basal regions and of dopaminergic innervation in the basal half. Furthermore, the cholinergic terminals in the aged ears spread from their normal clustering near the IHC basolateral pole, where the ANF synapses are found, to positions up and down the IHC somata and regions of the neuropil closer to the habenula. This apparent migration was most striking in the apex, where the hair cell pathology was greatest, and may be a harbinger of impending hair cell death.

Efferent neurons control hearing sensitivity and protect hearing from noise through the regulation of gap junctions between cochlear supporting cells

It is critical for hearing that the descending cochlear efferent system provides a negative feedback to hair cells to regulate hearing sensitivity and protect hearing from noise. The medial olivocochlear (MOC) efferent nerves project to outer hair cells (OHCs) to regulate OHC electromotility, which is an active cochlear amplifier and can increase hearing sensitivity. Here, we report that the MOC efferent nerves also could innervate supporting cells (SCs) in the vicinity of OHCs to regulate hearing sensitivity. MOC nerve fibers are cholinergic, and acetylcholine (ACh) is a primary neurotransmitter. Immunofluorescent staining showed that MOC nerve endings, presynaptic vesicular acetylcholine transporters (VAChTs), and postsynaptic ACh receptors were visible at SCs and in the SC area. Application of ACh in SCs could evoke a typical inward current and reduce gap junctions (GJs) between them, which consequently enhanced the direct effect of ACh on OHCs to shift but not eliminate OHC electromotility. This indirect, GJ-mediated inhibition had a long-lasting influence. In vivo experiments further demonstrated that deficiency of this GJ-mediated efferent pathway decreased the regulation of active cochlear amplification and compromised the protection against noise. In particular, distortion product otoacoustic emission (DPOAE) showed a delayed reduction after noise exposure. Our findings reveal a new pathway for the MOC efferent system via innervating SCs to control active cochlear amplification and hearing sensitivity. These data also suggest that this SC GJ-mediated efferent pathway may play a critical role in long-term efferent inhibition and is required for protection of hearing from noise trauma.NEW & NOTEWORTHY The cochlear efferent system provides a negative feedback to control hair cell activity and hearing sensitivity and plays a critical role in noise protection. We reveal a new efferent control pathway in which medial olivocochlear efferent fibers have innervations with cochlear supporting cells to control their gap junctions, therefore regulating outer hair cell electromotility and hearing sensitivity. This supporting cell gap junction-mediated efferent control pathway is required for the protection of hearing from noise.

Cochlear Efferent Innervation Is Sparse in Humans and Decreases with Age

The mammalian cochlea is innervated by two cholinergic feedback systems called the medial olivocochlear (MOC) and lateral olivocochlear (LOC) pathways, which send control signals from the brainstem back to the outer hair cells and auditory-nerve fibers, respectively. Despite countless studies of the cochlear projections of these efferent fibers in animal models, comparable data for humans are almost completely lacking. Here, we immunostained the cochlear sensory epithelium from 23 normal-aging humans (14 males and 9 females), 0-86 years of age, with cholinergic markers to quantify the normal density of MOC and LOC projections, and the degree of age-related degeneration. In younger ears, the MOC density peaks in mid-cochlear regions and falls off both apically and basally, whereas the LOC innervation peaks near the apex. In older ears, MOC density decreases dramatically, whereas the LOC density does not. The loss of MOC feedback may contribute to the age-related decrease in word recognition in noise; however, even at its peak, the MOC density is lower than in other mammals, suggesting the MOC pathway is less important for human hearing.SIGNIFICANCE STATEMENT The cochlear epithelium and its sensory innervation are modulated by the olivocochlear (OC) efferent pathway. Although the medial OC (MOC) reflex has been extensively studied in humans, via contralateral sound suppression, the cochlear projections of these cholinergic neurons have not been described in humans. Here, we use immunostaining to quantify the MOC projections to outer hair cells and lateral OC (LOC) projections to the inner hair cell area in humans 0-89 years of age. We show age-related loss of MOC, but not LOC, innervation, which likely contributes to hearing impairments, and a relative paucity of MOC terminals at all ages, which may account for the relative weakness of the human MOC reflex and the difficulty in demonstrating a robust functional role in human experiments.

The micro-architecture of mitochondria at active zones: electron tomography reveals novel anchoring scaffolds and cristae structured for high-rate metabolism

Mitochondria are integral elements of many nerve terminals. They must be appropriately positioned to regulate microdomains of Ca(2+) concentration and metabolic demand, but structures that anchor them in place have not been described. By applying the high resolution of electron tomography (ET) to the study of a central terminal, the calyx of Held, we revealed an elaborate cytoskeletal superstructure that connected a subset of mitochondria to the presynaptic membrane near active zones. This cytoskeletal network extended laterally and was well integrated into the nerve terminal cytoskeleton, which included filamentous linkages among synaptic vesicles. ET revealed novel features of inner membrane for these mitochondria. Crista structure was polarized in that crista junctions, circular openings of the inner membrane under the outer membrane, were aligned with the cytoskeletal superstructure and occurred at higher density in mitochondrial membrane facing the presynaptic membrane. These characteristics represent the first instance where a subcomponent of an organelle is shown to have a specific orientation relative to the polarized structure of a cell. The ratio of cristae to outer membrane surface area is large in these mitochondria relative to other tissues, indicating a high metabolic capacity. These observations suggest general principles for cytoskeletal anchoring of mitochondria in all tissues, reveal potential routes for nonsynaptic communication between presynaptic and postsynaptic partners using this novel cytoskeletal framework, and indicate that crista structure can be specialized for particular functions within cellular microdomains.

Human medial olivocochlear reflex: effects as functions of contralateral, ipsilateral, and bilateral elicitor bandwidths

Animal studies have led to the view that the acoustic medial olivocochlear (MOC) efferent reflex provides sharply tuned frequency-specific feedback that inhibits cochlear amplification. To determine if MOC activation is indeed narrow band, we measured the MOC effects in humans elicited by 60-dB sound pressure level (SPL) contralateral, ipsilateral, and bilateral noise bands as a function of noise bandwidth from 1/2 to 6.7 octaves. MOC effects were quantified by the change in stimulus frequency otoacoustic emissions from 40 dB SPL probe tones near 0.5, 1, and 4 kHz. In a second experiment, the noise bands were centered 2 octaves below probe frequencies near 1 and 4 kHz. In all cases, the MOC effects increased as elicitor bandwidth increased, with the effect saturating at about 4 octaves. Generally, the MOC effects increased as the probe frequency decreased, opposite expectations based on MOC innervation density in the cochlea. Bilateral-elicitor effects were always the largest. The ratio of ipsilateral/contralateral effects depended on elicitor bandwidth; the ratio was large for narrow-band probe-centered elicitors and approximately unity for wide-band elicitors. In another experiment, the MOC effects from increasing elicitor bandwidths were calculated from measurements of the MOC effects from adjacent half-octave noise bands. The predicted bandwidth function agreed well with the measured bandwidth function for contralateral elicitors, but overestimated it for ipsilateral and bilateral elicitors. Overall, the results indicate that (1) the MOC reflexes integrate excitation from almost the entire cochlear length, (2) as elicitor bandwidth is increased, the excitation from newly stimulated cochlear regions more than overcomes the reduced excitation at frequencies in the center of the elicitor band, and (3) contralateral, ipsilateral, and bilateral elicitors show MOC reflex spatial summation over most of the cochlea, but ipsilateral spatial summation is less additive than contralateral.

Reflex control of the human inner ear: a half-octave offset in medial efferent feedback that is consistent with an efferent role in the control of masking

The high sensitivity and frequency selectivity of the mammalian cochlea is due to amplification produced by outer hair cells (OHCs) and controlled by medial olivocochlear (MOC) efferents. Data from animals led to the view that MOC fibers provide frequency-specific inhibitory feedback; however, these studies did not measure intact MOC reflexes. To test whether MOC inhibition is primarily at the frequency that elicits the MOC activity, acoustically elicited MOC effects were quantified in humans by the change in otoacoustic emissions produced by 60-dB SPL tone and half-octave-band noise elicitors at different frequencies relative to a 40-dB SPL, 1-kHz probe tone. On average, all elicitors produced MOC effects that were skewed (elicitor frequencies -1 octave below the probe produced larger effects than those -1 octave above). The largest MOC effects were from elicitors below the probe frequency for contra- and bilateral elicitors but were from elicitors centered at the probe frequency for ipsilateral elicitors. Typically, ipsilateral elicitors produced larger effects than contralateral elicitors and bilateral elicitors produced effects near the ipsi+contra sum. Elicitors at levels down to 30-dB SPL produced similar patterns. Tuning curves (TCs) interpolated from these data were V-shaped with Q10s approximately 2. These are sharper than MOC-fiber TCs found near 1 kHz in cats and guinea pigs. Because cochlear amplification is skewed (more below the best frequency of a cochlear region), these data are consistent with an anti-masking role of MOC efferents that reduces masking by reducing the cochlear amplification seen at 1 kHz.

Reciprocal synapses between outer hair cells and their afferent terminals: evidence for a local neural network in the mammalian cochlea

Cochlear outer hair cells (OHCs) serve both as sensory receptors and biological motors. Their sensory function is poorly understood because their afferent innervation, the type-II spiral ganglion cell, has small unmyelinated axons and constitutes only 5% of the cochlear nerve. Reciprocal synapses between OHCs and their type-II terminals, consisting of paired afferent and efferent specialization, have been described in the primate cochlea. Here, we use serial and semi-serial-section transmission electron microscopy to quantify the nature and number of synaptic interactions in the OHC area of adult cats. Reciprocal synapses were found in all OHC rows and all cochlear frequency regions. They were more common among third-row OHCs and in the apical half of the cochlea, where 86% of synapses were reciprocal. The relative frequency of reciprocal synapses was unchanged following surgical transection of the olivocochlear bundle in one cat, confirming that reciprocal synapses were not formed by efferent fibers. In the normal ear, axo-dendritic synapses between olivocochlear terminals and type-II terminals and/or dendrites were as common as synapses between olivocochlear terminals and OHCs, especially in the first row, where, on average, almost 30 such synapses were seen in the region under a single OHC. The results suggest that a complex local neuronal circuitry in the OHC area, formed by the dendrites of type-II neurons and modulated by the olivocochlear system, may be a fundamental property of the mammalian cochlea, rather than a curiosity of the primate ear. This network may mediate local feedback control of, and bidirectional communication among, OHCs throughout the cochlear spiral.

A novel effect of cochlear efferents: in vivo response enhancement does not require alpha9 cholinergic receptors

Outer hair cells in the mammalian cochlea receive a cholinergic efferent innervation that constitutes the effector arm of a sound-evoked negative feedback loop. The well-studied suppressive effects of acetylcholine (ACh) release from efferent terminals are mediated by alpha9/alpha10 ACh receptors and are potently blocked by strychnine. Here, we report a novel, efferent-mediated enhancement of cochlear sound-evoked neural responses and otoacoustic emissions in mice. In controls, a slow enhancement of response amplitude to supranormal levels appears after recovery from the classic suppressive effects seen during a 70-s epoch of efferent shocks. The magnitude of post-shock enhancement can be as great as 10 dB and tends to be greater for high-frequency acoustic stimuli. Systemic strychnine at 10 mg/kg eliminates efferent-induced suppression, revealing a purely enhancing effect of efferent shocks, which peaks within 5 s after efferent-stimulation onset, maintains a constant level through the stimulation epoch, and slowly decays back to baseline with a time constant of approximately 100 s. In mice with targeted deletion of the alpha9 ACh receptor subunit, efferent-evoked effects resemble those in wild types with strychnine blockade, further showing that this novel efferent effect is fundamentally different from all cholinergic effects previously reported.

The existence of opioid receptors in the cochlea of guinea pigs

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

Simultaneous measurement of noise-activated middle-ear muscle reflex and stimulus frequency otoacoustic emissions

Otoacoustic emissions serve as a noninvasive probe of the medial olivocochlear (MOC) reflex. Stimulus frequency otoacoustic emissions (SFOAEs) elicited by a low-level probe tone may be the optimal type of emission for studying MOC effects because at low levels, the probe itself does not elicit the MOC reflex [Guinan et al. (2003) J. Assoc. Res. Otolaryngol. 4:521]. Based on anatomical considerations, the MOC reflex activated by ipsilateral acoustic stimulation (mediated by the crossed olivocochlear bundle) is predicted to be stronger than the reflex to contralateral stimulation. Broadband noise is an effective activator of the MOC reflex; however, it is also an effective activator of the middle-ear muscle (MEM) reflex, which can make results difficult to interpret. The MEM reflex may be activated at lower levels than measured clinically, and most previous human studies have not explicitly included measurements to rule out MEM reflex contamination. The current study addressed these issues using a higher-frequency SFOAE probe tone to test for cochlear changes mediated by the MOC reflex, while simultaneously monitoring the MEM reflex using a low-frequency probe tone. Broadband notched noise was presented ipsilaterally at various levels to elicit probe-tone shifts. Measurements are reported for 15 normal-hearing subjects. With the higher-frequency probe near 1.5 kHz, only 20% of subjects showed shifts consistent with an MOC reflex in the absence of an MEM-induced shift. With the higher-frequency probe near 3.5 kHz, up to 40% of subjects showed shifts in the absence of an MEM-induced shift. However, these responses had longer time courses than expected for MOC-induced shifts, and may have been dominated by other cochlear processes, rather than MOC reflex. These results suggest caution in the interpretation of effects observed using ipsilaterally presented acoustic activators intended to excite the MOC reflex.

Ultrastructure of the normal human organ of corti. new anatomical findings in surgical specimens

CONCLUSION

A thorough scanning electron microscopy (SEM) investigation of immediately fixed human adult cochleae obtained during surgery for petro-clival meningiomas conveyed new information about morphology.

OBJECTIVE

To investigate the ultrastructure of human adult cochleae using SEM.

MATERIAL AND METHODS

Two human cochleae were decalcified, fixed with glutaraldehyde and osmium and prepared for SEM.

RESULTS

The excellently preserved morphology showed the pathways of nerve fibres through the organ of Corti. Undulating lateral cell membranes of Hensen and Claudius cells created an enlarged surface that may be important for homoeostasis. The distal attachment of the tectorial membrane to the reticular lamina was present in the shape of a marginal net, which was extended through marginal pillars. Stereocilia imprints extended as far as the distal end of the marginal pillars. The presence of an irregularly distributed fourth row of outer hair cells attached to the marginal pillars raises questions about differences in the excitation of the last row of outer hair cells as a result of differences in the composition of the tectorial membrane.

High resolution scanning electron microscopy of the human organ of Corti

Scanning electron microscopy on immediately fixed human cochleae obtained during surgery for life-threatening petro-clival meningioma showed excellently preserved morphology. We compared the morphological findings with those from transmission electron microscopic sections of well preserved human and animal tissue. The characteristics of neural innervation, the pathways of the nerves through the organ of Corti and the intimate relation of nerves to supporting cells along their route could be studied in detail. The lateral membranes of Hensen and Claudius cells were folded creating a surface enlargement. Marginal pillars extended the distal end of the tectorial membrane and correspond to the marginal net or "randfasernetz" described earlier. Stereocilia imprints at the undersurface of the tectorial membrane go as far as to the distal end of the marginal pillars. The presence of an irregularly distributed fourth row of outer hair cell, attached to the marginal pillars, raises questions about differences in the excitation of the last row of outer hair cells. The complex nature of many supporting cells, stria vascularis and Reissner's membrane, intracellular complexities as well as surface features are described. Supernumerary inner hair cells were observed and the different arrangement of outer spiral fibres in contrast to findings in animals and variations of nerve fibres within the organ of Corti between apex and base are discussed.

Medial olivocochlear efferent reflex in humans: otoacoustic emission (OAE) measurement issues and the advantages of stimulus frequency OAEs

Otoacoustic emissions (OAEs) are useful for studying medial olivocochlear (MOC) efferents, but several unresolved methodological issues cloud the interpretation of the data they produce. Most efferent assays use a "probe stimulus" to produce an OAE and an "elicitor stimulus" to evoke efferent activity and thereby change the OAE. However, little attention has been given to whether the probe stimulus itself elicits efferent activity. In addition, most studies use only contralateral ( re the probe) elicitors and do not include measurements to rule out middle-ear muscle (MEM) contractions. Here we describe methods to deal with these problems and present a new efferent assay based on stimulus frequency OAEs (SFOAEs) that incorporates these methods. By using a postelicitor window, we make measurements in individual subjects of efferent effects from contralateral, ipsilateral, and bilateral elicitors. Using our SFOAE assay, we demonstrate that commonly used probe sounds (clicks, tone pips, and tone pairs) elicit efferent activity, by themselves. Thus, results of efferent assays using these probe stimuli can be confounded by unwanted efferent activation. In contrast, the single 40 dB SPL tone used as the probe sound for SFOAE-based measurements evoked little or no efferent activity. Since they evoke efferent activation, clicks, tone pips, and tone pairs can be used in an adaptation efferent assay, but such paradigms are limited in measurement scope compared to paradigms that separate probe and elicitor stimuli. Finally, we describe tests to distinguish middle-ear muscle (MEM) effects from MOC effects for a number of OAE assays and show results from SFOAE-based tests. The SFOAE assay used in this study provides a sensitive, flexible, frequency-specific assay of medial efferent activation that uses a low-level probe sound that elicits little or no efferent activity, and thus provides results that can be interpreted without the confound of unintended efferent activation.