Protecting the auditory system with glucocorticoids

In vitro and in vivo evaluation of a hydrogel reservoir as a continuous drug delivery system for inner ear treatment

Fibrous tissue growth and loss of residual hearing after cochlear implantation can be reduced by application of the glucocorticoid dexamethasone-21-phosphate-disodium-salt (DEX). To date, sustained delivery of this agent to the cochlea using a number of pharmaceutical technologies has not been entirely successful. In this study we examine a novel way of continuous local drug application into the inner ear using a refillable hydrogel functionalized silicone reservoir. A PEG-based hydrogel made of reactive NCO-sP(EO-stat-PO) prepolymers was evaluated as a drug conveying and delivery system in vitro and in vivo. Encapsulating the free form hydrogel into a silicone tube with a small opening for the drug diffusion resulted in delayed drug release but unaffected diffusion of DEX through the gel compared to the free form hydrogel. Additionally, controlled DEX release over several weeks could be demonstrated using the hydrogel filled reservoir. Using a guinea-pig cochlear trauma model the reservoir delivery of DEX significantly protected residual hearing and reduced fibrosis. As well as being used as a device in its own right or in combination with cochlear implants, the hydrogel-filled reservoir represents a new drug delivery system that feasibly could be replenished with therapeutic agents to provide sustained treatment of the inner ear.

Expression and nuclear translocation of glucocorticoid receptors in type 2 taste receptor cells

Stress increases the secretion of glucocorticoids (GCs), potent steroid hormones that exert their effects on numerous target tissues by acting through glucocorticoid receptors (GRs). GC signaling significantly affects ingestive behavior and taste preferences in humans and rodent models, but far less is known about the hormonal modulation of the peripheral sensory system that detects and assesses nutrient content of foods. A previous study linked restraint stress in rats to diminished expression of mRNA for one subunit of the sweet taste receptor (Tas1r3) in taste tissue and reduced gustatory nerve excitation by sweet compounds. Using RT-PCR, we detected mRNAs for GRα in circumvallate taste papillae and in oral epithelium devoid of taste buds ("non-taste" tissue). Further, circumvallate tissue was significantly enriched in GR mRNA compared to non-taste tissue based on quantitative PCR. Histologically, GR protein was expressed in all taste bud populations examined (circumvallate, foliate and fungiform papillae). Using transgenic mice expressing green fluorescent protein, almost all (97%) Tas1r3-positive taste cells (sweet-/umami-sensitive) expressed GR compared to a significantly smaller percentage (89%) of TrpM5-positive taste cells (sweet-, umami- and bitter-sensitive). When mice (n=4) were restrain stressed, GR protein mobilized to the nucleus in Tas1r3-GFP taste cells (1.7-fold over controls). Our results suggest that GR can be activated in taste receptor cells and may play a role in specific taste qualities (e.g., sweet, umami, and bitter) to shape how the taste system responds to stress.

Sound preconditioning therapy inhibits ototoxic hearing loss in mice

Therapeutic drugs with ototoxic side effects cause significant hearing loss for thousands of patients annually. Two major classes of ototoxic drugs are cisplatin and the aminoglycoside antibiotics, both of which are toxic to mechanosensory hair cells, the receptor cells of the inner ear. A critical need exists for therapies that protect the inner ear without inhibiting the therapeutic efficacy of these drugs. The induction of heat shock proteins (HSPs) inhibits both aminoglycoside- and cisplatin-induced hair cell death and hearing loss. We hypothesized that exposure to sound that is titrated to stress the inner ear without causing permanent damage would induce HSPs in the cochlea and inhibit ototoxic drug–induced hearing loss. We developed a sound exposure protocol that induces HSPs without causing permanent hearing loss. We used this protocol in conjunction with a newly developed mouse model of cisplatin ototoxicity and found that preconditioning mouse inner ears with sound has a robust protective effect against cisplatin-induced hearing loss and hair cell death. Sound therapy also provided protection against aminoglycoside-induced hearing loss. These data indicate that sound preconditioning protects against both classes of ototoxic drugs, and they suggest that sound therapy holds promise for preventing hearing loss in patients receiving these drugs.

Vitamins A, C, and E and selenium in the treatment of idiopathic sudden sensorineural hearing loss

This study evaluated the effectiveness of vitamins A, C, and E, with selenium, in the treatment of idiopathic sudden sensorineural hearing loss (ISSNHL). This was a prospective, controlled study performed at a tertiary teaching and research hospital. Over a 32-month period, patients were treated with either our standard ISSNHL treatment regimen plus vitamins A, C, and E and selenium (ACE+ group) or with only our standard ISSNHL treatment regimen (ACE- group). The demographics, additional symptoms, mean initial and final hearing levels, mean hearing gain, and recovery data were compared between the two groups. The ACE+ group, consisting of 70 (55.5 %) patients, received vitamin A (natural beta-carotene, 26,000 IU), vitamin C (ascorbic acid, 200 mg), vitamin E (d-alpha-tocopherol, 200 IU), and selenium (50 μg) twice daily for 30 days in addition to our ISSNHL treatment regimen: methylprednisolone at an initial dose of 1 mg/kg body weight per day, tapered over 14 days; Rheomacrodex(®) [(10 g of dextran and 0.9 g of NaCl)/100 ml] 500 ml daily for 5 days; Vastarel(®) 20-mg tablet (20 mg of trimetazidine dihydrochloride) three times daily for 30 days; and ten 60-min hyperbaric oxygen (HBO) sessions (2.5 absolute atmospheres of 100 % O2), once daily, starting the day of hospitalization. The ACE- group comprised 56 (44.4 %) patients, who received only our ISSNHL treatment regimen. The mean hearing gains were 36.2 ± 20.3 dB in the ACE+ group and 27.1 ± 20.6 dB in the ACE- group. The mean hearing gain rates were significantly higher in the ACE+ group than in the ACE- group (p = 0.014). Treatment with vitamins A, C, and E and selenium was effective in ISSNHL patients undergoing treatment with methylprednisolone, dextran, trimetazidine dihydrochloride, and HBO, and might be more effective when the initial hearing level is below 46 dB.

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.

Repeated restraint stress impairs auditory attention and GABAergic synaptic efficacy in the rat auditory cortex

Chronic stress induces dendritic atrophy in the rat primary auditory cortex (A1), a key brain area for auditory attention. The aim of this study was to determine whether repeated restraint stress affects auditory attention and synaptic transmission in A1. Male Sprague-Dawley rats were trained in a two-alternative choice task (2-ACT), a behavioral paradigm to study auditory attention in rats. Trained animals that reached a performance over 80% of correct trials in the 2-ACT were randomly assigned to control and restraint stress experimental groups. To analyze the effects of restraint stress on the auditory attention, trained rats of both groups were subjected to 50 2-ACT trials one day before and one day after of the stress period. A difference score was determined by subtracting the number of correct trials after from those before the stress protocol. Another set of rats was used to study the synaptic transmission in A1. Restraint stress decreased the number of correct trials by 28% compared to the performance of control animals (p < 0.001). Furthermore, stress reduced the frequency of spontaneous inhibitory postsynaptic currents (sIPSC) and miniature IPSC in A1, whereas glutamatergic efficacy was not affected. Our results demonstrate that restraint stress decreased auditory attention and GABAergic synaptic efficacy in A1.

Corticosterone treatment impairs auditory fear learning and the dendritic morphology of the rat inferior colliculus

Stress leads to secretion of the adrenal steroid hormone corticosterone (CORT). The aim of this study was to determine the effects of chronic CORT administration on auditory and visual fear conditioning. Male Sprague-Dawley rats received CORT (400 mg/ml) in their drinking water for 10 consecutive days; this treatment induces stress levels of serum CORT. CORT impaired fear conditioning (F((1,28)) = 11.52, p < 0.01) and extinction (F((1,28)) = 4.86, p < 0.05) of auditory fear learning, but did not affect visual fear conditioning. In addition, we analyzed the CORT effects on the neuronal morphology of the inferior colliculus (flat neurons, auditory mesencephalon, a key brain area for auditory processing) and superior colliculus (wide-field neurons, related to visual processing) by Golgi stain. CORT decreased dendritic arborization of inferior colliculus neurons by approximately 50%, but did not affect superior colliculus neurons. Thus, CORT had more deleterious effects on the auditory fear processing than the visual system in the brain.

Stress and tinnitus-from bedside to bench and back

The aim of this review is to focus the attention of clinicians and basic researchers on the association between psycho-social stress and tinnitus. Although tinnitus is an auditory symptom, its onset and progression often associates with emotional strain. Recent epidemiological studies have provided evidence for a direct relationship between the emotional status of subjects and tinnitus. In addition, studies of function, morphology, and gene and protein expression in the auditory system of animals exposed to stress support the notion that the emotional status can influence the auditory system. The data provided by clinical and basic research with use of animal stress models offers valuable clues for an improvement in diagnosis and more effective treatment of tinnitus.