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Similar to hair cells in the mammalian cochlear system, mechanosensory neurons in the Central American wandering spider Cupiennius salei are strongly innervated by efferent fibers that originate from neurons whose somata are located in the central nervous system (CNS). In both the mammalian and arachnid systems, efferent fibers have been shown to co-express two or more transmitters; however, our understanding regarding co-transmission and how it affects sensory signal transduction and processing in these systems is only fragmentary. The spider model system is exceptionally suitable for this type of investigation due to the large size and easy accessibility of the sensory and efferent neurons in this system. Thus far, GABA and glutamate have been identified as the main fast-acting transmitters in efferent axons that form synaptic contacts onto sensory neurons in slit sense organs. Ultrastructural investigations suggest an abundance of neuropeptides within these peripheral synapses. In an effort to identify these peptides and conduct functional studies, we have employed immunohistochemistry to investigate whether the neuropeptide proctolin is present in neurons of the leg ganglia and in peripheral leg structures. Here, we demonstrate that ~ 73% of all neurons in the CNS of C. salei show proctolin-like immunoreactivity (proc-LIR) including the leg ganglia. We demonstrate that both strongly and weakly labeled neurons can be distinguished. The majority of proc-LIR neurons show weak labeling intensity and ~ 86.2% co-localize with glutamate. In future experiments, we plan to undertake functional studies to investigate the significance of this co-expression, which has yet to be investigated. [ABSTRACT FROM AUTHOR]

Outer hair cells (OHC) act as amplifiers and their function is modified by medial olivocochlear (MOC) efferents. The unique OHC motor protein, prestin, provides the molecular basis for somatic electromotility, which is required for sensitivity and frequency selectivity, the hallmarks of mammalian hearing. Prestin proteins are the major component of the lateral membrane of mature OHCs, which separates apical and basal domains. To investigate the contribution of prestin to this unique arrangement, we compared the distribution of membrane proteins in OHCs of wildtype (WT) and prestin-knockout (KO) mice. In WT, the apical protein PMCA2 was exclusively localized to the hair bundles, while it was also found at the lateral membrane in KOs. Similarly, a basal protein KCNQ4 did not coalesce at the base of OHCs but was widely dispersed in mice lacking prestin. Since the expression levels of PMCA2 and KCNQ4 remained unchanged in KOs, the data indicate that prestin is required for the normal distribution of apical and basal membrane proteins in OHCs. Since OHC synapses predominate in the basal subnuclear region, we also examined the synaptic architecture in prestin-KO mice. Although neurite densities were not affected, MOC efferent terminals in prestin-KO mice were no longer constrained to the basal pole as in WT. This trend was evident as early as at postnatal day 12. Furthermore, terminals were often enlarged and frequently appeared as singlets when compared to the multiple clusters of individual terminals in WT. This abnormality in MOC synaptic morphology in prestin-KO mice is similar to defects in mice lacking MOC pathway proteins such as α9/α10 nicotinic acetylcholine receptors and BK channels, indicating a role for prestin in the proper establishment of MOC synapses. To investigate the contribution of prestin’s electromotility, we also examined OHCs from a mouse model that expresses non-functional prestin (499-prestin). We found no changes in PMCA2 localization and MOC synaptic morphology in OHCs from 499-prestin mice. Taken together, these results indicate that prestin, independent of its motile function, plays an important structural role in membrane compartmentalization, which is required for the formation of normal efferent-OHC synapses in mature OHCs.

Objective:To investigate the influences of contralateral acoustic stimulation on amplitudes and frequencies of spontaneous otoacoustic emissions (SPOAEs).Materials and Methods:Twenty voluntary participants with normal hearing and unilateral SPOAE were included in the study. Emissions were recorded before and during stimulation of contralateral ear with 75 dB SPL white-noise.Results:From 20 cases, thirty SPOAEs with amplitudes ranging between - 6 to 19 dB SPL (mean: 6.3 dB SPL) and with a mean frequency of 1925 Hz were recorded. During contralateral stimulation, the mean amplitude and frequency of SPOAEs were found to be 3.6 dB SPL and 2004 Hz respectively. The suppressive effect of contralateral noise on SPOAEs were present at all frequencies with a higher significance in 1 to 3 kHz frequency range. In all SPOAE frequencies, an upward shift was noted during contralateral acoustic stimulation.Conclusion:Acoustical stimulation of efferent innervation of the cochlea causes alterations in its active micro-mechanics and therefore in amplitudes and frequencies of SPOAEs with a frequency selective manner.

The organ of Corti, the mammalian sensory epithelium of the inner ear, has two types of mechanoreceptor cells, inner hair cells (IHCs) and outer hair cells (OHCs). In this sensory epithelium, vibrations produced by sound waves are transformed into electrical signals. When depolarized by incoming sounds, IHCs release glutamate and activate auditory nerve fibers innervating them and OHCs, by virtue of their electromotile property, increase the amplification and fine tuning of sound signals. The medial olivocochlear (MOC) system, an efferent feedback system, inhibits OHC activity and thereby reduces the sensitivity and sharp tuning of cochlear afferent fibers. During neonatal development, IHCs fire Ca2C action potentials which evoke glutamate release promoting activity in the immature auditory system in the absence of sensory stimuli. During this period, MOC fibers also innervate IHCs and are thought to modulate their firing rate. Both the MOC-OHC and the MOC-IHC synapses are cholinergic, fast and inhibitory and mediated by the a9a10 nicotinic cholinergic receptor (nAChR) coupled to the activation of calcium-activated potassium channels that hyperpolarize the hair cells. In this review we discuss the biophysical, functional and molecular data which demonstrate that at the synapses between MOC efferent fibers and cochlear hair cells, modulation of transmitter release as well as short term synaptic plasticity mechanisms, operating both at the presynaptic terminal and at the postsynaptic hair-cell, determine the efficacy of these synapses and shape the hair cell response pattern. [ABSTRACT FROM AUTHOR]

Efferent olivocochlear feedback innervation modulates the stream of auditory information from cochlea to brainstem by regulating auditory nerve activity and controlling the contribution of cochlear outer hair cells to basilar membrane motion. In our previous work, we gave a first description of glycine receptors (GlyRs) in the rat cochlea indicating a possible localization at efferent cochlear synapses. Here, we analyze the developmental regulation of GlyR transcripts and protein within the developing murine organ of Corti (postnatal days P0-P21). Using quantitative RT-PCR, GlyRα1 and α2 were identified as the predominant GlyRα subunit transcripts before the onset of hearing (

Abstract: The carotid body (CB) is a chemosensory organ that monitors blood chemicals and initiates compensatory reflex adjustments to maintain homeostasis. The ‘afferent’ sensory discharge induced by changes in blood chemicals, e.g. low PO2 (hypoxia), is relayed by carotid sinus nerve (CSN) fibers and has been well studied. Much less is known, however, about a parallel autonomic (parasympathetic) ‘efferent’ pathway that is the source of CB inhibition. This pathway is the focus of this review which begins with a historical account of the early findings and links them to more recent data on the source of this innervation, and the role of endogenous neurotransmitters in efferent inhibition. We review evidence that these autonomic neurons are embedded in ‘paraganglia’ within the glossopharyngeal (GPN) and CSN nerves, and for the role of nitric oxide (NO) in mediating efferent inhibition. Finally, we discuss recent data linking the action of hypoxia and a key CB neurotransmitter, i.e. ATP, to potential mechanisms for activating this efferent pathway. [Copyright &y& Elsevier]

Conclusion: Current neurotransmission models based on animal studies on the mammalian inner ear not always reflect the situation in human. Rodents and primates show significant differences in characteristics of efferent innervation as well as the distribution of neuroactive substances. Objective: Immunohistochemistry demonstrates the mammalian efferent system as neurochemically complex and diverse: several neuroactive substances may co-exist within the same efferent terminal. Using light and electron microscopic immunohistochemistry, this study presents a comparative overview of the distribution patterns of choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, GABA, CGRP, and enkephalins within the peripheral nerve fiber systems of the human inner ear. Materials and methods: Human temporal bones were obtained post mortem and prepared according to a pre-embedding immunohistochemical technique to detect immunoreactivities to ChAT, GABA, CGRP, leu- and met-enkephalins at the electron microscopic level. Results: Immunoreactivities of all the antigens were present within both the lateral and medial efferent systems of the cochlea, whereas only ChAT, GABA, and CGRP were detected in efferent pathways of the vestibular end organs. [ABSTRACT FROM AUTHOR]

Abstract: Pituitary adenylyl cyclase-activating polypeptide (PACAP), via its specific receptor pituitary adenylyl cyclase-activating polypeptide receptor 1 (PAC1-R), is known to have roles in neuromodulation and neuroprotection associated with glutamatergic and cholinergic neurotransmission, which, respectively, are believed to form the primary basis for afferent and efferent signaling in the organ of Corti. Previously, we identified transcripts for PACAP preprotein and multiple splice variants of its receptor, PAC1-R, in microdissected cochlear subfractions. In the present work, neural localizations of PACAP and PAC1-R within the organ of Corti and spiral ganglion were examined, defining sites of PACAP action. Immunolocalization of PACAP and PAC1-R in the organ of Corti and spiral ganglion was compared with immunolocalization of choline acetyltransferase (ChAT) and synaptophysin as efferent neuronal markers, and glutamate receptor 2/3 (GluR2/3) and neurofilament 200 as afferent neuronal markers, for each of the three cochlear turns. Brightfield microscopy giving morphological detail for individual immunolocalizations was followed by immunofluorescence detection of co-localizations. PACAP was found to be co-localized with ChAT in nerve fibers of the intraganglionic spiral bundle and beneath the inner and outer hair cells within the organ of Corti. Further, evidence was obtained that PACAP is expressed in type I afferent axons leaving the spiral ganglion en route to the auditory nerve, potentially serving as a neuromodulator in axonal terminals. In contrast to the efferent localization of PACAP within the organ of Corti, PAC1-R immunoreactivity was co-localized with afferent dendritic neuronal marker GluR2/3 in nerve fibers passing beneath and lateral to the inner hair cell and in fibers at supranuclear and basal sites on outer hair cells. Given the known association of PACAP with catecholaminergic neurotransmission in sympathoadrenal function, we also re-examined the issue of whether the organ of Corti receives adrenergic innervation. We now demonstrate the existence of nerve fibers within the organ of Corti which are immunoreactive for the adrenergic marker dopamine β-hydroxylase (DBH). DBH immunoreactivity was particularly prominent in nerve fibers both at the base and near the cuticular plate of outer hair cells of the apical turn, extending to the non-sensory Hensen’s cell region. Evidence was obtained for limited co-localization of DBH with PAC1-R and PACAP. In the process of this investigation, we obtained evidence that efferent and afferent nerve fibers, in addition to adrenergic nerve fibers, are present at supranuclear sites on outer hair cells and distributed within the non-sensory epithelium of the apical cochlear turn for rat, based upon immunoreactivity for the corresponding neuronal markers. Overall, PACAP is hypothesized to act within the organ of Corti as an efferent neuromodulator of afferent signaling via PAC1-R that is present on type I afferent dendrites, in position to afford protection from excitotoxicity. Additionally, PACAP/PAC1-R may modulate secretion of catecholamines from adrenergic terminals within the organ of Corti. [Copyright &y& Elsevier]

In the mature cochlea, inner hair cells (IHCs) transduce acoustic signals into receptor potentials, communicating to the brain by synaptic contacts with afferent fibers. Before the onset of hearing, a transient efferent innervation is found on IHCs, mediated by a nicotinic cholinergic receptor that may contain both α9 and α10 subunits. Calcium influx through that receptor activates calcium-dependent (SK2-containing) potassium channels. This inhibitory synapse is thought to disappear after the onset of hearing [after postnatal day 12 (P12)]. We documented this developmental transition using whole-cell recordings from IHCs in apical turns of the rat organ of Corti. Acetylcholine elicited ionic currents in 88-100% of IHCs between P3 and P14, but in only 1 of 11 IHCs at P16-P22. Potassium depolar- ization of efferent terminals caused IPSCs in 67% of IHCs at P3, in 100% at P7-P9, in 93% at Pl0-Pl2, but in only 40% at P13-P14 and in none of the IHCs tested between P16 and P22. Earlier work had shown by in situ hybridization that α9 mRNA is expressed in adult IHCs but that α10 mRNA disappears after the onset of hearing. In the present study, antibodies to α10 and to the associated calcium-dependent (SK2) potassium channel showed a similar developmental loss. The correlated expression of these gene products with functional inner- ration suggests that Alpha10 and SK2, but not Alpha9, are regulated by synaptic activity. Furthermore, this developmental knock-out of α10, but not α9, supports the hypothesis that functional nicotinic acetylcholine receptors in hair cells are heteromers containing both these subunits. [ABSTRACT FROM AUTHOR]

The expression of two calcium-binding proteins of the parvalbumin (PV) family, the α isoform (αPV) and the β isoform known as oncomodulin (OM), was investigated in the rat cochlea during postnatal development and related to cholinergic efferent innervation. Using RT-PCR analysis, we found that OM expression begins between postnatal day 2 (P2) and P4, and peaks as early as P10, while αPV mRNA begins expression before birth and remains highly expressed into the adult period. Both in situ hybridization and immunoreactivity confirm that OM is uniquely expressed by the outer hair cells (OHCs) in the rat cochlea and occurs after efferent innervation along the cochlear spiral between P2 and P4. In contrast to OM expression, αPV immunoreactivity is expressed in both inner hair cells (IHCs) and OHCs at birth. Following olivocochlear efferent innervation, OHCs demonstrate weak OM immunoreactivity beginning at P5 and diminished αPV immunoreactivity after P10. In organ cultures isolated prior to the efferent innervation of OHCs, OM immunoreactivity failed to develop in OHCs, but αPV immunoreactivity remained present in both IHCs and OHCs. In contrast, organ cultures isolated after efferent innervation of OHCs show OHCs with low levels of OM immunoreactivity and high levels of αPV immunoreactivity. This study suggests that OM and αPV are differentially regulated in OHCs during cochlear development. Our findings further raise the possibility that the expression of PV proteins in OHCs may be influenced by efferent innervation. © 2003 Wiley Periodicals, Inc. J Neurobiol 58: 479-492, 2004 [ABSTRACT FROM AUTHOR]

There is evidence that the inhibitory neurotransmitter γ-aminobutyric acid (GABA) is released from some efferent olivocochlear nerve endings terminating at outer hair cells (OHCs). Using monoclonal antibodies against postsynaptic GABA receptor from bovine cerebral cortex we confirm the presence of GABA and benzodiazepine bindings sites of α- and β-subunits of GABA receptors at the basal pole of isolated OHCs. Whole-cell recording with viable OHCs revealed that the application of 10-10 M GABA to the cell surface was followed by a concentration-dependent hyperpolarization of the outer cell membrane. Hyperpolarization was increased in the presence of 2.5 × 10 M chlorazepate, a benzodiazepine derivative. Electrophysiological effects caused by GABA alone or in combination with chlorazepate were specifically inhibited by 10 M of the GABA-receptor antagonist picrotoxin. Moreover, 10-10 M GABA caused reversible slow elongation of the cylindrical hair cell body in OHCs examined. These neurotransmitter-induced motile responses were specifically blocked by 10 M picrotoxin. The results suggest that a subpopulation of OHCs express α- and β-subunits of GABA receptors which both form a GABA/benzodiazepine-receptor complex at the basal pole of isolated OHCs. These receptors are thought to allow GABA which is released from efferent auditory nerve terminals to bind to the cell surface of OHCs, resulting in GABAreceptor activation. This probably gates a GABA-receptor-associated chloride channel in the postsynaptic OHC membrane, allowing hyperpolarization and elongation of the cell. [ABSTRACT FROM AUTHOR]

γ-Aminobutyric acid (GABA)-ergic efferent nerve fibers were studied during the postnatal development of the rat cochlea, using light microscopic immunocytochemical techniques. Antibodies against GABA and its synthesizing enzyme, glutamate decarboxylase (GAD), were used. Immunoreactivity to GAD is already present at birth (postnatal day 1) and could be found below the inner hair cells of the basal turn. Immunoreactivity progressively extends toward the apical turn until day 3. GAD-like immunoreactivity appears under the outer hair cells on postnatal day 15 and is only found in the upper part of the second turn and in the apical turn. The distribution of GABA-like immunoreactivity closely corresponds to that observed with the anti-glutamate decarboxylase antibody. However, the GABA-like immunoreactivity appears about 1-2 days after GAD-like immunoreactivity. At the beginning of the 3rd postnatal week, an adult pattern of GABA- and GAD-like immunoreactivity is established. These results suggest that GABA, which appears under the inner hair cells largely before the onset of hearing, may play a neurotrophic function during cochlear maturation and participate in the regulation of the first cochlear potentials as soon as they appear. [ABSTRACT FROM AUTHOR]

Copyright of Pflügers Archiv: European Journal of Physiology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Pflügers Archiv: European Journal of Physiology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Pflügers Archiv: European Journal of Physiology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Recent studies reveal great diversity in the structure, function, and efferent innervation of afferent synaptic connections between the cochlear inner hair cells (IHCs) and spiral ganglion neurons (SGNs), which likely enables audition to process a wide range of sound pressures. By performing an extensive electron microscopic (EM) reconstruction of the neural circuitry in the mature mouse organ of Corti, we demonstrate that afferent SGN dendrites differ in abundance and composition of efferent innervation in a manner dependent on their afferent synaptic connectivity with IHCs. SGNs that sample glutamate release from several presynaptic ribbons receive more efferent innervation from lateral olivocochlear projections than those driven by a single ribbon. Next to the prevailing unbranched SGN dendrites, we found branched SGN dendrites that can contact several ribbons of 1-2 IHCs. Unexpectedly, medial olivocochlear neurons provide efferent innervation of SGN dendrites, preferring those forming single-ribbon, pillar-side synapses. We propose a fine-tuning of afferent and efferent SGN innervation., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Author(s). Published by Elsevier Inc. All rights reserved.)

The transduction process in the cochlea requires patent hair cells. Population responses that reflect this patency are the cochlear microphonic (CM) and summating potential (SP). They can be measured using electrocochleography (ECochG). The CM reflects the sound waveform in the form of outer hair cell (OHC) depolarization and hyperpolarization, and the SP reflects the average voltage difference of the OHC membrane potential for depolarization and hyperpolarization. The CM can be measured using ECochG or via the so-called otoacoustic emissions, using a sensitive microphone in the ear canal. Neural population responses are called the compound action potentials (CAPs), which by frequency selective masking can be decomposed into narrow-band action potentials (NAPs) reflecting CAPs evoked by activity from small cochlear regions. Presence of CM and absence of CAPs are the diagnostic hallmarks of auditory neuropathy. Increased and prolonged SPs are often found in Ménière's disease but are too often in the normal range to be diagnostic. When including NAP waveforms, Ménière's disease can be differentiated from vestibular schwannomas, which often feature overlapping symptoms such as dizziness, hearing loss, and tinnitus. The patency of the efferent system, particularly the olivocochlear bundle, can be tested using the suppressive effect of contralateral stimulation on the otoacoustic emission amplitude., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Outer hair cells (OHC) act as amplifiers and their function is modified by medial olivocochlear (MOC) efferents. The unique OHC motor protein, prestin, provides the molecular basis for somatic electromotility, which is required for sensitivity and frequency selectivity, the hallmarks of mammalian hearing. Prestin proteins are the major component of the lateral membrane of mature OHCs, which separates apical and basal domains. To investigate the contribution of prestin to this unique arrangement, we compared the distribution of membrane proteins in OHCs of wildtype (WT) and prestin-knockout (KO) mice. In WT, the apical protein PMCA2 was exclusively localized to the hair bundles, while it was also found at the lateral membrane in KOs. Similarly, a basal protein KCNQ4 did not coalesce at the base of OHCs but was widely dispersed in mice lacking prestin. Since the expression levels of PMCA2 and KCNQ4 remained unchanged in KOs, the data indicate that prestin is required for the normal distribution of apical and basal membrane proteins in OHCs. Since OHC synapses predominate in the basal subnuclear region, we also examined the synaptic architecture in prestin-KO mice. Although neurite densities were not affected, MOC efferent terminals in prestin-KO mice were no longer constrained to the basal pole as in WT. This trend was evident as early as at postnatal day 12. Furthermore, terminals were often enlarged and frequently appeared as singlets when compared to the multiple clusters of individual terminals in WT. This abnormality in MOC synaptic morphology in prestin-KO mice is similar to defects in mice lacking MOC pathway proteins such as α9/α10 nicotinic acetylcholine receptors and BK channels, indicating a role for prestin in the proper establishment of MOC synapses. To investigate the contribution of prestin's electromotility, we also examined OHCs from a mouse model that expresses non-functional prestin (499-prestin). We found no changes in PMCA2 localization and MOC synaptic morphology in OHCs from 499-prestin mice. Taken together, these results indicate that prestin, independent of its motile function, plays an important structural role in membrane compartmentalization, which is required for the formation of normal efferent-OHC synapses in mature OHCs.

Damaging effects on the cochlea of high-intensity acoustic overexposures have been extensively documented, but only few works have focused on the danger of moderate noise levels. Using scanning and transmission electron microscopy, we explored the noise-induced neuroepithelial changes that occur in the cochlea of rats subjected to moderate intensities, 70 and 85 dB SPL, for an extended period of time (6 hr/day over 3 months). Although the full quota of outer and inner sensory hair cells remained present, we detected discrete abnormalities, likely resulting from metabolic impairment, in both types of hair cell within the basal region of the cochlea. In contrast, important noise-dependent losses of spiral ganglion neurons had occurred. In addition, we found cytoplasmic accumulations of lipofuscin-like aggregates in most of the surviving cochlear neurons. These results strongly suggest that noise levels comparable to those of certain working environments, with sufficient exposure duration, pose a severe risk to the cochlea. Moreover, our data support the notion that long-duration exposure to moderate noise is a causative factor of presbycusis., (© 2015 Wiley Periodicals, Inc.)