[3H]-aldosterone binding sites (type I receptors) in the lateral wall of the cochlea: distribution assessment by quantitative autoradiography

Conditional Ablation of Glucocorticoid and Mineralocorticoid Receptors from Cochlear Supporting Cells Reveals Their Differential Roles for Hearing Sensitivity and Dynamics of Recovery from Noise-Induced Hearing Loss

Endogenous glucocorticoids (GC) are known to modulate basic elements of cochlear physiology. These include both noise-induced injury and circadian rhythms. While GC signaling in the cochlea can directly influence auditory transduction via actions on hair cells and spiral ganglion neurons, evidence also indicates that GC signaling exerts effects via tissue homeostatic processes that can include effects on cochlear immunomodulation. GCs act at both the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). Most cell types in the cochlea express both receptors sensitive to GCs. The GR is associated with acquired sensorineural hearing loss (SNHL) through its effects on both gene expression and immunomodulatory programs. The MR has been associated with age-related hearing loss through dysfunction of ionic homeostatic balance. Cochlear supporting cells maintain local homeostatic requirements, are sensitive to perturbation, and participate in inflammatory signaling. Here, we have used conditional gene manipulation techniques to target Nr3c1 (GR) or Nr3c2 (MR) for tamoxifen-induced gene ablation in Sox9-expressing cochlear supporting cells of adult mice to investigate whether either of the receptors sensitive to GCs plays a role in protecting against (or exacerbating) noise-induced cochlear damage. We have selected mild intensity noise exposure to examine the role of these receptors related to more commonly experienced noise levels. Our results reveal distinct roles of these GC receptors for both basal auditory thresholds prior to noise exposure and during recovery from mild noise exposure. Prior to noise exposure, auditory brainstem responses (ABRs) were measured in mice carrying the floxed allele of interest and the Cre recombinase transgene, but not receiving tamoxifen injections (defined as control (no tamoxifen treatment), versus conditional knockout (cKO) mice, defined as mice having received tamoxifen injections. Results revealed hypersensitive thresholds to mid- to low-frequencies after tamoxifen-induced GR ablation from Sox9-expressing cochlear supporting cells compared to control (no tamoxifen) mice. GR ablation from Sox9-expressing cochlear supporting cells resulted in a permanent threshold shift in mid-basal cochlear frequency regions after mild noise exposure that produced only a temporary threshold shift in both control (no tamoxifen) f/fGR:Sox9iCre+ and heterozygous f/+GR:Sox9iCre+ tamoxifen-treated mice. A similar comparison of basal ABRs measured in control (no tamoxifen) and tamoxifen-treated, floxed MR mice prior to noise exposure indicated no difference in baseline thresholds. After mild noise exposure, MR ablation was initially associated with a complete threshold recovery at 22.6 kHz by 3 days post-noise. Threshold continued to shift to higher sensitivity over time such that by 30 days post-noise exposure the 22.6 kHz ABR threshold was 10 dB more sensitive than baseline. Further, MR ablation produced a temporary reduction in peak 1 neural amplitude one day post-noise. While supporting cell GR ablation trended towards reducing numbers of ribbon synapses, MR ablation reduced ribbon synapse counts but did not exacerbate noise-induced damage including synapse loss at the experimental endpoint. GR ablation from the targeted supporting cells increased the basal resting number of Iba1-positive (innate) immune cells (no noise exposure) and decreased the number of Iba1-positive cells seven days following noise exposure. MR ablation did not alter innate immune cell numbers at seven days post-noise exposure. Taken together, these findings support differential roles of cochlear supporting cell MR and GR expression at basal, resting conditions and especially during recovery from noise exposure.

Expression and dexamethasone-induced nuclear translocation of glucocorticoid and mineralocorticoid receptors in guinea pig cochlear cells

Glucocorticoids (GC) are powerful anti-inflammatory agents frequently used to protect the auditory organ against damage associated with a variety of conditions, including noise exposure and ototoxic drugs as well as bacterial and viral infections. In addition to glucocorticoid receptors (GC-R), natural and synthetic GC are known to bind mineralocorticoid receptors (MC-R) with great affinity. We used light and laser scanning confocal microscopy to investigate the expression of GC-R and MC-R in different cell populations of the guinea pig cochlea, and their translocation to different cell compartments after treatment with the synthetic GC dexamethasone. We found expression of both types of receptors in the cytoplasm and nucleus of sensory inner and outer hair cells as well as pillar, Hensen and Deiters cells in the organ of Corti, inner and outer sulcus cells, spiral ganglion neurons and several types of spiral ligament and spiral limbus cells; stria vascularis cells expressed mostly MC-R whereas fibrocytes type IV were positive for GC-R only. GC-R and MC-R were also localized at or near the plasma membrane of pillar cells and outer hair cells, whereas GC-R were found at or near the plasma membrane of Hensen cells only. We investigated the relative levels of receptor expression in the cytoplasm and the nucleus of Hensen cells treated with dexamethasone, and found they varied in a way suggestive of dose-induced translocation. These results suggest that the oto-protective effects of GC could be associated with the concerted activation of genomic and non-genomic, GC-R and MC-R mediated signaling pathways in different regions of the cochlea.

The role of glucocorticoids for spiral ganglion neuron survival

Glucocorticoids, which are steroidal stress hormones, have a broad array of biological functions. Synthetic glucocorticoids are frequently used therapeutically for many pathologic conditions, including diseases of the inner ear; however, their exact functions in the cochlea are not completely understood. Recent work has clearly demonstrated the presence of glucocorticoid signaling pathways in the cochlea and elucidated their protective roles against noise-induced hearing loss. Furthermore, indirect evidence suggests the involvement of glucocorticoids in age-related loss of spiral ganglion neurons and extensive studies in the central nervous system demonstrate profound effects of glucocorticoids on neuronal functions. With the advancement of recent pharmacologic and genetic tools, the role of these pathways in the survival of spiral ganglion neurons after noise exposure and during aging should be revealed.

Stria vascularis and vestibular dark cells: characterisation of main structures responsible for inner-ear homeostasis, and their pathophysiological relations

The regulation of inner-ear fluid homeostasis, with its parameters volume, concentration, osmolarity and pressure, is the basis for adequate response to stimulation. Many structures are involved in the complex process of inner-ear homeostasis. The stria vascularis and vestibular dark cells are the two main structures responsible for endolymph secretion, and possess many similarities. The characteristics of these structures are the basis for regulation of inner-ear homeostasis, while impaired function is related to various diseases. Their distinct morphology and function are described, and related to current knowledge of associated inner-ear diseases. Further research on the distinct function and regulation of these structures is necessary in order to develop future clinical interventions.

Expression and function of the human mineralocorticoid receptor: lessons from transgenic mouse models

The human mineralocorticoid receptor (hMR), a ligand-dependent transcription factor (NR3C2) which belongs to the nuclear receptor superfamily, mediates most of the known effects of aldosterone. Beside its involvement in the regulation of sodium balance and the control of blood pressure, aldosterone-hMR tandem also exerts important regulatory functions on the cardiovascular and central nervous systems. To study the molecular mechanisms involved in the tissue-specific expression of hMR and explore its functional implication in pathophysiology, transgenic mouse models have been generated using both targeted oncogenesis and MR overexpression. We have previously demonstrated that the transcription of hMR is directed by two alternative promoters, P1 and P2, which correspond to the 5'-flanking regions of the untranslated exons 1alpha and 1beta of the hMR gene, respectively. Utilization of P1 and P2 to drive expression of the SV40 large T antigen (TAg) in transgenic mice led us (i) to determine distinct tissue-specific patterns of promoter usage; (ii) to identify novel sites of MR expression including brown adipose tissue, thus providing a new functional link between aldosterone and energy homeostasis; (iii) to generate original immortalized cell lines derived from numerous aldosterone-sensitive tissues most notably distal nephron, brown fat, skin, liver, lung, brain, heart, blood vessels and inner ear. These differentiated cell lines represent suitable models to further explore cell-specific mineralocorticoid responses and cross-talk with other signaling pathways. Generation of transgenic mice in which hMR expression was directed by P1 promoter demonstrated the importance of MR in the cardiac and renal function. Morphological and functional alterations of the renal tubule were observed with basal decreased sodium/potassium ratio exacerbated under sodium depletion. Hypokinetic dilated cardiomyopathies were associated with tachycardia, arrhythmia but normal arterial blood pressure emphasizing the direct role of MR on cardiomyocyte function. Taken together, transgenic animal models constitute valuable experimental systems to gain new insights into the widespread and pleiotropic in vivo functions of MR.

The emotional ear in stress

Stress of some kind is encountered everyday and release of stress hormones is essential for adaptation to change. Stress can be physical (pain, noise exposure, etc.), psychological (apprehension to impending events, acoustic conditioning, etc.) or due to homeostatic disturbance (hunger, blood pressure, inner ear pressure, etc.). Persistent elevated levels of stress hormones can lead to disease states. The aim of the present review is to bring together data describing morphological or functional evidence for hormones of stress within the inner ear. The present review describes possible multiple interactions between the sympathetic and the complex feed-back neuroendocrine systems which interact with the immune system and so could contribute to various inner ear dysfunctions such as tinnitus, vertigo, hearing losses. Since there is a rapidly expanding list of genes specifically expressed within the inner ear this clearly allows for possible genomic and non-genomic local action of steroid hormones. Since stress can be encountered at any time throughout the life-time, the effects might be manifested starting from in-utero. These are avenues of research which remain relatively unexplored which merit further consideration. Progress in this domain could lead towards integration of stress concept into the overall clinical management of various inner ear pathologies.

Sodium- and potassium-activated ATPase in the mammalian vestibular system

Autoradiographic and cytochemical procedures were employed to determine the cellular distribution of the Na,K-ATPase enzyme in the mammalian vestibular system. A light-microscope survey of vestibular tissues incubated with [(3)H]ouabain shows high densities of ouabain binding sites within the dark cell epithelium (DC) of the ampullae of the semi-circular canals, and to a lesser extent, the DC of the utricular macula. A moderate number of binding sites was found in nerve fibers penetrating the connective tissue beneath the sensory epithelium (SE) of the ampullae and the maculae. A small number of binding sites is distributed in the deep portion of the SE, both in the ampullae and in the maculae. These latter binding sites seem to be associated with nerve terminals and receptor cells. At the ultrastructural level, the vestibular dark cells exhibit extensive basolateral membrane infolding, a morphological hallmark of cells engaged in trans-epithelial ion transport. The cytochemical reaction product is K(+)-dependent, ouabain inhibitable, and is restricted to the basolateral membrane extensions, with little or no product on the luminal membrane. The extent of membrane infolding in dark cells of the utricle is less pronounced than that of the ampullar dark cells and the intensity of the cytochemical reaction appears to correlate with the extent of membrane infolding. The results support the widely held hypothesis that the vestibular dark cells play a role in endolymph production. They also suggest that the vestibular sensory epithelia may be a site of ion exchange.

Absence of mRNA encoding estrogen receptor in the rat cochlea

Based on changes in hearing thresholds and tinnitus that are co-related with the menstrual cycle, it has been suggested that the cochlea may respond directly to estrogen. For this to occur, the cochlea should express estrogen receptors. In situ mRNA hybridization was performed on normal female rat cochleas, using radiolabeled RNA probes complementary to mRNA encoding estrogen receptor, to determine whether estrogen receptors are present in the cochlea. Strong hybridization of the riboprobes to sections of uterus and hypothalamus indicated that the technique detected estrogen receptor mRNA. No hybridization to any cochlear tissues was observed. The results indicate that estrogen receptors are not expressed on cochlear cells, at least in rats. This in turn suggests that variation in cochlear responses during the estrus cycle are not the result of the direct effect of estrogen on the cochlea. Such variation may, however, be caused by systemic changes in fluid regulation induced by estrogen receptors at a distant site, or by other hormone receptors.

Histochemical demonstration of 11 beta-hydroxysteroid dehydrogenase in ampullae of semicircular canals

The enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) plays a major role in the protection of the mineralocorticoid (type I) receptor. The cellular mechanism of aldosterone selectivity relies on the coexpression of mineralocorticoid receptors and 11 beta HSD in the same cells. An 11 beta HSD activity has been localized in the ampullae of the semicircular canal by histochemical technique which links steroid metabolism with the deposition of formazan deposition. Oxidation of 11 beta-hydroxyandrostenedione was observed in the dark cell region. Oxidation of 11 beta-hydroxyandrostenedione was nicotine-adenine dinucleotide (NAD) dependent. These results demonstrate the presence of a dehydrogenase activity separate from the nicotineamide-adenine dinucleotide phosphate (NADP) dependent 11 beta HSD.