Transient peripheral vestibular hypofunction measured with vestibular short-latency evoked potentials following noise exposure in rats

Exposure to 120 dB sound pressure level (SPL) band-limited noise results in delayed onset latency and reduced vestibular short-latency evoked potential (VsEP) responses. These changes are still present 4 wk after noise overstimulation. Noise-induced hearing loss (NIHL) has been shown to vary in extent and duration based on the noise intensity. This study investigated whether noise-induced peripheral vestibular hypofunction (NPVH) would also decrease in extent and/or duration with less intense noise exposure. In the present study, rats were exposed to a less intense noise (110 dB SPL) but for the same duration (6 h) and frequency range (500–4,000 Hz) as used in previous studies. The VsEP was assessed 1, 3, 7, 14, 21, and 28 days after noise exposure. In contrast to 120 dB SPL noise exposure, the 110 dB SPL noise exposures produced smaller deficits in VsEP responses that fully recovered in 62% (13/21) of animals within 1 wk. These findings suggest that NPVH, a loss or attenuation of VsEP responses with a requirement for elevated stimulus intensity to elicit measurable responses, is similar to NIHL, that is, lower sound levels produce a smaller or transient deficit. These results show that it will be important to determine the extent and duration of vestibular hypofunction for different noise exposure conditions and their impact on balance.

NEW & NOTEWORTHY This is the first study to show a temporary noise-induced peripheral vestibular hypofunction that recovers following exposure to continuous noise.

Rapamycin Added to Diet in Late Mid-Life Delays Age-Related Hearing Loss in UMHET4 Mice

Our previous study demonstrated rapamycin added to diet at 4 months of age had significantly less age-related outer hair cell loss in the basal half of the cochlea at 22 months of age compared to mice without rapamycin. The present study tested adding rapamycin to diet later in life, at 14 months of age, and added a longitudinal assessment of auditory brain stem response (ABR). The present study used UMHET4 mice, a 4 way cross in which all grandparental strains lack the Cdh23^753A allele that predisposes to early onset, progressive hearing loss. UMHET4 mice typically have normal hearing until 16-17 months, then exhibit threshold shifts at low frequencies/apical cochlea and later in more basal high frequency regions. ABR thresholds at 4, 12, 24, and 48 kHz were assessed at 12, 18, and 24 months of age and compared to baseline ABR thresholds acquired at 5 months of age to determine threshold shifts (TS). There was no TS at 12 months of age at any frequency tested. At 18 months of age mice with rapamycin added to diet at 14 months had a significantly lower mean TS at 4 and 12 kHz compared to mice on control diet with no significant difference at 24 and 48 kHz. At 24 months of age, the mean 4 kHz TS in rapamycin diet group was no longer significantly lower than the control diet group, while the 12 kHz mean remained significantly lower. Mean TS at 24 and 48 kHz in the rapamycin diet group became significantly lower than in the control diet group at 24 months. Hair cell counts at 24 months showed large loss in the apical half of most rapamycin and control diet mice cochleae with no significant difference between groups. There was only mild outer hair cell loss in the basal half of rapamycin and control diet mice cochleae with no significant difference between groups. The results show that a later life addition of rapamycin can decrease age-related hearing loss in the mouse model, however, it also suggests that this decrease is a delay/deceleration rather than a complete prevention.

Exposure to Intense Noise Causes Vestibular Loss

INTRODUCTION

The vestibular system is essential for normal postural control and balance. Because of their proximity to the cochlea, the otolith organs are vulnerable to noise. We previously showed that head jerks that evoke vestibular nerve activity were no longer capable of inducing a response after noise overstimulation. The present study adds a greater range of jerk intensities to determine if the response was abolished or required more intense stimulation (threshold shift).

MATERIALS AND METHODS

Vestibular short-latency evoked potential (VsEP) measurements were taken before noise exposure and compared to repeated measurements taken at specific time points for 28 days after noise exposure. Calretinin was used to identify changes in calyx-only afferents in the sacculus.

RESULTS

Results showed that more intense jerk stimuli could generate a VsEP, although it was severely attenuated relative to prenoise values. When the VsEP was evaluated 4 weeks after noise exposure, partial recovery was observed.

CONCLUSION

These data suggest that noise overstimulation, such as can occur in the military, could introduce an increased risk of imbalance that should be evaluated before returning a subject to situations that require normal agility and motion. Moreover, although there is recovery with time, some dysfunction persists for extended periods.

Effects of Noise Exposure on the Vestibular System: A Systematic Review

Despite our understanding of the impact of noise-induced damage to the auditory system, much less is known about the impact of noise exposure on the vestibular system. In this article, we review the anatomical, physiological, and functional evidence for noise-induced damage to peripheral and central vestibular structures. Morphological studies in several animal models have demonstrated cellular damage throughout the peripheral vestibular system and particularly in the otolith organs; however, there is a paucity of data on the effect of noise exposure on human vestibular end organs. Physiological studies have corroborated morphological studies by demonstrating disruption across vestibular pathways with otolith-mediated pathways impacted more than semicircular canal-mediated pathways. Similar to the temporary threshold shifts observed in the auditory system, physiological studies in animals have suggested a capacity for recovery following noise-induced vestibular damage. Human studies have demonstrated that diminished sacculo-collic responses are related to the severity of noise-induced hearing loss, and dose-dependent vestibular deficits following noise exposure have been corroborated in animal models. Further work is needed to better understand the physiological and functional consequences of noise-induced vestibular impairment in animals and humans.

Intense noise exposure alters peripheral vestibular structures and physiology

The otolith organs play a critical role in detecting linear acceleration and gravity to control posture and balance. Some afferents that innervate these structures can be activated by sound and are at risk for noise overstimulation. A previous report demonstrated that noise exposure can abolish vestibular short-latency evoked potential (VsEP) responses and damage calyceal terminals. However, the stimuli that were used to elicit responses were weaker than those established in previous studies and may have been insufficient to elicit VsEP responses in noise-exposed animals. The goal of this study was to determine the effect of an established noise exposure paradigm on VsEP responses using large head-jerk stimuli to determine if noise induces a stimulus threshold shift and/or if large head-jerks are capable of evoking VsEP responses in noise-exposed rats. An additional goal is to relate these measurements to the number of calyceal terminals and hair cells present in noise-exposed vs. non-noise-exposed tissue. Exposure to intense continuous noise significantly reduced VsEP responses to large stimuli and abolished VsEP responses to small stimuli. This finding confirms that while measurable VsEP responses can be elicited from noise-lesioned rat sacculi, larger head-jerk stimuli are required, suggesting a shift in the minimum stimulus necessary to evoke the VsEP. Additionally, a reduction in labeled calyx-only afferent terminals was observed without a concomitant reduction in the overall number of calyces or hair cells. This finding supports a critical role of calretinin-expressing calyceal-only afferents in the generation of a VsEP response.NEW & NOTEWORTHY This study identifies a change in the minimum stimulus necessary to evoke vestibular short-latency evoked potential (VsEP) responses after noise-induced damage to the vestibular periphery and reduced numbers of calretinin-labeled calyx-only afferent terminals in the striolar region of the sacculus. These data suggest that a single intense noise exposure may impact synaptic function in calyx-only terminals in the striolar region of the sacculus. Reduced calretinin immunolabeling may provide insight into the mechanism underlying noise-induced changes in VsEP responses.

Rapamycin but not acarbose decreases age-related loss of outer hair cells in the mouse Cochlea

Adding rapamycin or acarbose to diet at 9-10 months of age has been shown to significantly increase life span in both male and female UM-HET3 mice. The current study examined cochleae of male and female UM-HET3 mice at 22 months of age to determine if either treatment also influenced age-related loss of cochlear hair cells. A large loss of cochlear outer hair cells was observed at 22 months of age in untreated mice in both apical and basal halves of the cochlear spiral. Addition of acarbose to diet had no significant effect on the amount of outer hair cell loss at 22 months of age or in its pattern, with large loss in both apical and basal halves. The addition of rapamycin to diet, however, significantly reduced outer hair cell loss in the basal half of the cochlea at 22 months of age when compared to untreated mice. There was no significant difference between male and female mice in any of the conditions. Age-related outer hair cell loss in the apical cochlea precedes outer hair cell loss in the base in many mouse strains. The results of the present study suggest that rapamycin but not acarbose treatment can delay age-related loss of outer hair cells at doses at which each drug increases life span.

Small Arms Fire-like noise: Effects on Hearing Loss, Gap Detection and the Influence of Preventive Treatment

A noise-induced loss of inner hair cell (IHC) - auditory nerve synaptic connections has been suggested as a factor that can trigger the progression of maladaptive plastic changes leading to noise-induced tinnitus. The present study used a military relevant small arms fire (SAF)-like noise (50 biphasic impulses over 2.5 min at 152 dB SPL given unilaterally to the right ear) to induce loss (∼1/3) of IHC synaptic ribbons (associated with synapse loss) in rat cochleae with only minor (less than 10%) loss of outer hair cells. Approximately half of the noise-exposed rats showed poorer Gap Detection post-noise, a behavioral indication suggesting the presence of tinnitus. There was significantly greater loss of IHC ribbons in noise-exposed rats with reduced Gap Detection compared to noise-exposed rats retaining normal Gap Detection. We have previously shown systemic administration of piribedil, memantine, and/or ACEMg significantly reduced loss of IHC ribbons induced by a 3 h 4 kHz octave band 117 dB (SPL) noise. The present study examined if this treatment would also reduce ribbon loss from the SAF-like noise exposure and if this would prevent the reduced Gap Detection. As in the previous study, piribedil, memantine, and ACEMg treatment significantly reduced the noise-induced loss of ribbons, such that it was no longer significantly different from normal. However, it did not prevent development of the reduced Gap Detection indication of tinnitus in all treated noise-exposed rats, reducing the incidence but not reaching significance.

Vestibular short-latency evoked potential abolished by low-frequency noise exposure in rats

The vestibular system plays a critical role in detection of head movements and is essential for normal postural control. Because of their anatomical proximity to the cochlea, the otolith organs are selectively exposed to sound pressure and are at risk for noise overstimulation. Clinical reports suggest a link between noise exposure and balance problems, but the structural and physiological basis for this linkage is not well understood. The goal of this study was to determine the effects of low-frequency noise (LFN) on the otolith organs by correlating changes in vestibular short-latency evoked potentials (VsEPs) with changes in saccular afferent endings following noise exposure. LFN exposure transiently abolished the VsEP and reduced the number of stained calyces within the sacculus. Although some recovery of the VsEP waveform could be observed within 3 days after noise, at 3 wk recovery was only partial in most animals, consistent with a reduced number of afferents with calyceal endings. These data show that a single intense noise exposure is capable of causing a vestibular deficit that appears to mirror the synaptic deficit associated with hidden hearing loss after noise-induced cochlear injury. NEW & NOTEWORTHY This is the first study to explore the effects of low-frequency high-intensity noise on vestibular short-latency evoked potential (VsEP) responses, which shows a linkage between attenuated noise-induced VsEPs and pathological changes to otolith organ afferents. This finding suggests a potential limitation of the VsEP for evaluation of vestibular dysfunction, since the VsEP measurement may assess the activity of a specific class rather than all afferents.

Treatment with Piribedil and Memantine Reduces Noise-Induced Loss of Inner Hair Cell Synaptic Ribbons

Noise overstimulation can induce loss of synaptic ribbons associated with loss of Inner Hair Cell – Auditory Nerve synaptic connections. This study examined if systemic administration of Piribedil, a dopamine agonist that reduces the sound evoked auditory nerve compound action potential and/or Memantine, an NMDA receptor open channel blocker, would reduce noise-induced loss of Inner Hair Cell ribbons. Rats received systemic Memantine and/or Piribedil for 3 days before and 3 days after a 3 hour 4 kHz octave band noise at 117 dB (SPL). At 21 days following the noise there was a 26% and 38% loss of synaptic ribbons in regions 5.5 and 6.5 mm from apex, respectively, elevations in 4-, 8- and 20 kHz tonal ABR thresholds and reduced dynamic output at higher intensities of stimulation. Combined treatment with Piribedil and Memantine produced a significant reduction in the noise-induced loss of ribbons in both regions and changes in ABR sensitivity and dynamic responsiveness. Piribedil alone gave significant reduction in only the 5.5 mm region and Memantine alone did not reach significance in either region. Results identify treatments that could prevent the hearing loss and hearing disorders that result from noise-induced loss of Inner Hair Cell – Auditory Nerve synaptic connections.

Persistence, distribution, and impact of distinctly segmented microparticles on cochlear health following in vivo infusion

Delivery of pharmaceuticals to the cochleae of patients with auditory dysfunction could potentially have many benefits from enhancing auditory nerve survival to protecting remaining sensory cells and their neuronal connections. Treatment would require platforms to enable drug delivery directly to the cochlea and increase the potential efficacy of intervention. Cochlear implant recipients are a specific patient subset that could benefit from local drug delivery as more candidates have residual hearing; and since residual hearing directly contributes to post-implantation hearing outcomes, it requires protection from implant insertion-induced trauma. This study assessed the feasibility of utilizing microparticles for drug delivery into cochlear fluids, testing persistence, distribution, biocompatibility, and drug release characteristics. To allow for delivery of multiple therapeutics, particles were composed of two distinct compartments; one containing polylactide-co-glycolide (PLGA), and one composed of acetal-modified dextran and PLGA. Following in vivo infusion, image analysis revealed microparticle persistence in the cochlea for at least 7 days post-infusion, primarily in the first and second turns. The majority of subjects maintained or had only slight elevation in auditory brainstem response thresholds at 7 days post-infusion compared to pre-infusion baselines. There was only minor to limited loss of cochlear hair cells and negligible immune response based on CD45+ immunolabling. When Piribedil-loaded microparticles were infused, Piribedil was detectable within the cochlear fluids at 7 days post-infusion. These results indicate that segmented microparticles are relatively inert, can persist, release their contents, and be functionally and biologically compatible with cochlear function and therefore are promising vehicles for cochlear drug delivery. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 1510-1522, 2016.

Age-related changes in auditory nerve-inner hair cell connections, hair cell numbers, auditory brain stem response and gap detection in UM-HET4 mice

This study compared the timing of appearance of three components of age-related hearing loss that determine the pattern and severity of presbycusis: the functional and structural pathologies of sensory cells and neurons and changes in gap detection (GD), the latter as an indicator of auditory temporal processing. Using UM-HET4 mice, genetically heterogeneous mice derived from four inbred strains, we studied the integrity of inner and outer hair cells by position along the cochlear spiral, inner hair cell-auditory nerve connections, spiral ganglion neurons (SGN), and determined auditory thresholds, as well as pre-pulse and gap inhibition of the acoustic startle reflex (ASR). Comparisons were made between mice of 5-7, 22-24 and 27-29 months of age. There was individual variability among mice in the onset and extent of age-related auditory pathology. At 22-24 months of age a moderate to large loss of outer hair cells was restricted to the apical third of the cochlea and threshold shifts in the auditory brain stem response were minimal. There was also a large and significant loss of inner hair cell-auditory nerve connections and a significant reduction in GD. The expression of Ntf3 in the cochlea was significantly reduced. At 27-29 months of age there was no further change in the mean number of synaptic connections per inner hair cell or in GD, but a moderate to large loss of outer hair cells was found across all cochlear turns as well as significantly increased ABR threshold shifts at 4, 12, 24 and 48 kHz. A statistical analysis of correlations on an individual animal basis revealed that neither the hair cell loss nor the ABR threshold shifts correlated with loss of GD or with the loss of connections, consistent with independent pathological mechanisms.

Disruption of lateral olivocochlear neurons with a dopaminergic neurotoxin depresses spontaneous auditory nerve activity

Neurons of the lateral olivocochlear (LOC) system project from the auditory brainstem to the cochlea, where they synapse on radial dendrites of auditory nerve fibers. Selective LOC disruption depresses sound-evoked auditory nerve activity in the guinea pig, but enhances it in the mouse. Here, LOC disruption depressed spontaneous auditory nerve activity in the guinea pig. Recordings from single auditory nerve fibers revealed a significantly reduced proportion of fibers with the highest spontaneous firing rates (SRs) and an increased proportion of neurons with lower SRs. Ensemble activity, estimated using round window noise, also decreased after LOC disruption. Decreased spontaneous activity after LOC disruption may be a consequence of reduced tonic release of excitatory transmitters from the LOC terminals in guinea pigs.

Netrin-1-mediated axon guidance in mouse embryonic stem cells overexpressing neurogenin-1

Stem cell therapy holds great promise for treating neurodegenerative disease, but major barriers to effective therapeutic strategies remain. A complete understanding of the derived phenotype is required for predicting cell response once introduced into the host tissue. We sought to identify major axonal guidance cues present in neurons derived from the transient overexpression of neurogenin-1 (Neurog1) in mouse embryonic stem cells (ESCs). Neurog1 upregulated the netrin-1 axon guidance receptors DCC (deleted in colorectal cancer) and neogenin (NEO1). Quantitative polymerase chain reaction results showed a 2-fold increase in NEO1 mRNA and a 36-fold increase in DCC mRNA in Neurog1-induced compared with control ESCs. Immunohistochemistry indicated that DCC was primarily expressed on cells positive for the neuronal marker TUJ1. DCC was preferentially localized to the cell soma and growth-cones of induced neurons. In contrast, NEO1 expression showed less specificity, labeling both TUJ1-positive and TUJ1-negative cells as well as uninduced control cells. Axonal outgrowth was directed preferentially toward aggregates of HEK293 cells secreting a recombinant active fragment of netrin-1. These data indicate that DCC and NEO1 are downstream products of Neurog1 and may guide the integration of Neurog1-induced ESCs with target cells secreting netrin-1. Differential expression profiles for netrin receptors could indicate different roles for this guidance cue on neuronal and non-neuronal cells.

Nestin-expressing cells in the developing, mature and noise-exposed cochlear epithelium

The auditory sensory epithelium in non-mammalian vertebrates can replace lost hair cells by transdifferentiation of supporting cells, but this regenerative ability is lost in the mammalian cochlea. Future cell-based treatment of hearing loss may depend on stem cell transplantation or on transdifferentiation of endogenous cells in the cochlea. For both approaches, identification of cells with stem cell features within the mature cochlea may be useful. Here we use a Nestin-β-gal mouse to examine the presence of Nestin positive cells in the mature auditory epithelium, and determine how overstimulation of the ear impacts these cells. Nestin positive cells were found in the apical turn of the cochlea lateral to the outer hair cell area. This pattern of expression persisted into mature age. The area of Nestin positive cells was increased after the noise lesion. This increase in area coincided with an increase in expression of the Nestin mRNA. The data suggest that cells with potential stem cell features remain in the mature mammalian cochlea, restricted to the apical turn, and that an additional set of signals is necessary to trigger their contribution to cell replacement therapy in the ear. As such, this population of cells could serve to generate cochlear stem cells for research and potential therapy, and may be a target for treatments based on induced transdifferentiation of endogenous cochlear cells.

Induction of heat shock proteins by hyperthermia and noise overstimulation in hsf1 -/- mice

Diverse cellular and environmental stresses can activate the heat shock response, an evolutionarily conserved mechanism to protect proteins from denaturation. Stressors activate heat shock transcription factor 1 (HSF1), which binds to heat shock elements in the genes for heat shock proteins, leading to rapid induction of these important molecular chaperones. Both heat and noise stress are known to activate the heat shock response in the cochlea and protect it from subsequent noise trauma. However, the contribution of HSF1 to induction of heat shock proteins following noise trauma has not been investigated at the molecular level. We evaluated the role of HSF1 in the cochlea following noise stress by examining induction of heat shock proteins in Hsf1 ( +/- ) control and Hsf1 ( -/- ) mice. Heat stress rapidly induced expression of Hsp25, Hsp47, Hsp70.1, Hsp70.3, Hsp84, Hsp86, and Hsp110 in the cochleae of wild-type and Hsf1 ( +/- ) mice, but not in Hsf1 ( -/- ) mice, confirming the essential role of HSF1 in mediating the heat shock response. Exposure to broadband noise (2-20 kHz) at 106 dB SPL for 2 h produced partial hearing loss. Maximal induction of heat shock proteins occurred 4 h after the noise. In comparison to heat stress, noise stress resulted in lower induced levels of Hsp25, Hsp70.1, Hsp70.3, Hsp86, and Hsp110 in Hsf1 ( +/- ) mice. Induction of these heat shock proteins was attenuated, but not completely eliminated, in Hsf1 ( -/- ) mice. These same noise exposure conditions induced genes for several immediate early transcription factors and maximum induction occurred earlier than for heat shock proteins. Thus, additional signaling pathways and transcriptional regulators that are activated by noise probably contribute to induction of heat shock proteins in the cochlea.

The intrinsic electrophysiological properties of neurons derived from mouse embryonic stem cells overexpressing neurogenin-1

A mouse embryonic stem (ES) cell line containing an inducible transgene for the proneural gene Neurog1 has been used to generate glutamatergic neurons at a high efficiency. The present study used in vitro electrophysiology to establish the timeline for acquiring a functional neuronal phenotype in Neurog1-induced cells exhibiting a neuronal morphology. TTX-sensitive action potentials could be evoked from over 80% of the cells after only 4.5 days in vitro (DIV). These cells uniformly showed rapidly adapting responses to current injection, firing one to three action potentials at the onset of the stimulus. In the absence of Neurog1, a limited number of ES cells adopted a neuronal morphology, but these cells displayed slow calcium depolarizations rather than sodium-based spikes. Voltage-gated Na(+), K(+), and Ca(2+) currents were present in nearly all induced cells as early as 4.5 DIV. The voltage-dependent properties of these currents changed little from 4 to 12 DIV with half-activation voltage varying by <10 mV for any current type throughout the culture period. This study demonstrates that forced expression of proneural genes can induce ES cells to quickly acquire a functional neuronal phenotype with mature electrophysiological properties. Transient overexpression of Neurog1 may be used in neural repair strategies that require the rapid induction of functional neurons from pluripotent stem cells.

Safety of ciprofloxacin and dexamethasone in the guinea pig middle ear

OBJECTIVE

To investigate the ototoxic potential of ciprofloxacin hydrochloride, 0.3%, plus dexamethasone, 0.1%, after administration to the guinea pig middle ear.

DESIGN

Fifty guinea pigs were randomly assigned to 4 test groups of 10 animals each and 2 control groups of 5 animals each. The 4 test groups were treated twice daily for 4 weeks with 10 muL of (1) ciprofloxacin hydrochloride, 0.3%, plus dexamethasone, 0.1%; (2) ciprofloxacin hydrochloride, 1.0%, plus dexamethasone, 0.3%; (3) ciprofloxacin hydrochloride, 0.3%, or (4) vehicle. The positive and negative control groups were treated with neomycin sulfate, 10%, or isotonic sodium chloride solution, respectively.

SETTING

Academic research laboratory.

INTERVENTIONS

Study animals were implanted with a drug delivery cannula to the middle ear, terminating in the round window niche for direct delivery to the round window membrane.

MAIN OUTCOME MEASURES

Auditory brainstem responses were collected at baseline and following 2 and 4 weeks of dosing. At the termination of the study, inner ear tissues were evaluated microscopically.

RESULTS

No biologically relevant hearing losses were observed after either 2 or 4 weeks of treatment with vehicle, ciprofloxacin alone, or combinations of ciprofloxacin plus dexamethasone. Examination of the organ of Corti revealed normal hair cell counts in all animals that received isotonic sodium chloride solution, vehicle, ciprofloxacin, or combinations of ciprofloxacin and dexamethasone. Conversely, the neomycin sulfate positive control group demonstrated a significant elevation in hearing threshold and profound hair cell loss (P <.001, P = .02, and P <.001 at 2, 8, and 16 kHz, respectively).

CONCLUSION

The results of this preclinical study support the safety of ciprofloxacin hydrochloride, 0.3%, plus dexamethasone, 0.1%, for clinical use in the open middle ear cavity.

Stem cell transplantation for auditory nerve replacement

The successful function of cochlear prostheses depends on activation of auditory nerve. The survival of auditory nerve neurons, however, can vary widely in candidates for cochlear implants and influence implant efficacy. Stem cells offer the potential for improving the function of cochlear prostheses and increasing the candidate pool by replacing lost auditory nerve. The first phase of studies for stem cell replacement of auditory nerve has examined the in vitro survival and differentiation as well as in vivo differentiation and survival of exogenous embryonic and tissue stem cells placed into scala tympani and/or modiolus. These studies are reviewed and new results on in vivo placement of B-5 mouse embryonic stem cells into scala tympani of the guinea pig cochleae with differentiation into a glutamatergic neuronal phenotype are presented. Research on the integration and connections of stem cell derived neurons in the cochlea is described. Finally, an alternative approach is considered, based on the use of endogenous progenitors rather than exogenous stem cells, with a review of promising findings that have identified stem cell-like progenitors in cochlear and vestibular tissues to provide the potential for auditory nerve replacement.

Immunolocalization of vesicular glutamate transporters 1 and 2 in the rat inferior colliculus

The inferior colliculus is a major relay nucleus in the ascending auditory pathways that receives multiple glutamatergic inputs. Vesicular glutamate transporters 1 and 2 (VGLUT1, VGLUT2) most often have complementary non-overlapping distributions and can be used to differentiate glutamatergic inputs. The present study therefore examined co-immunolabeling of VGLUT1 and VGLUT2 in three divisions of the rat inferior colliculus. Additional co-immunolabeling of microtubule-associated protein 2 and neuronal class III beta-tubulin provided visualization of neuronal soma and processes and allowed identification of axo-somatic versus axo-dendritic contacts. Results showed numerous VGLUT1 and 2 immunolabeled terminals in the central nucleus, lateral cortex and dorsal cortex. In all three divisions there was little to no co-containment of the two vesicular glutamate transporters indicating a complementary distribution. VGLUT1 made predominantly axo-dendritic connections in the neuropil, while VGLUT2 had many axo-somatic contacts in addition to axo-dendritic contacts. VGLUT2 immunolabeled terminals were numerous on the soma and proximal dendrites of many medium-to-large and large neurons in the central nucleus and medium to large neurons in the dorsal cortex. There were more VGLUT2 terminals than VGLUT1 in all divisions and more VGLUT2 terminals in dorsal and lateral cortices than in the central nucleus. This study shows that VGLUT1 and VGLUT2 differentiate complementary patterns of glutamatergic inputs into the central nucleus, lateral and dorsal cortex of the inferior colliculus with VGLUT1 endings predominantly on the dendrites and VGLUT2 on both dendrites and somas.

Delayed neurotrophin treatment following deafness rescues spiral ganglion cells from death and promotes regrowth of auditory nerve peripheral processes: effects of brain-derived neurotrophic factor and fibroblast growth factor

The extent to which neurotrophic factors are able to not only rescue the auditory nerve from deafferentation-induced degeneration but also promote process regrowth is of basic and clinical interest, as regrowth may enhance the therapeutic efficacy of cochlear prostheses. The use of neurotrophic factors is also relevant to interventions to promote regrowth and repair at other sites of nerve trauma. Therefore, auditory nerve survival and peripheral process regrowth were assessed in the guinea pig cochlea following chronic infusion of BDNF + FGF(1) into scala tympani, with treatment initiated 4 days, 3 weeks, or 6 weeks after deafferentation from deafening. Survival of auditory nerve somata (spiral ganglion neurons) was assessed from midmodiolar sections. Peripheral process regrowth was assessed using pan-Trk immunostaining to selectively label afferent fibers. Significantly enhanced survival was seen in each of the treatment groups compared to controls receiving artificial perilymph. A large increase in peripheral processes was found with BDNF + FGF(1) treatment after a 3-week delay compared to the artificial perilymph controls and a smaller enhancement after a 6-week delay. Neurotrophic factor treatment therefore has the potential to improve the benefits of cochlear implants by maintaining a larger excitable population of neurons and inducing neural regrowth.

Math5 expression and function in the central auditory system

The basic helix-loop-helix (bHLH) transcription factor Math5 (Atoh7) is required for retinal ganglion cell (RGC) and optic nerve development. Using Math5-lacZ knockout mice, we have identified an additional expression domain for Math5 outside the eye, in functionally connected structures of the central auditory system. In the adult hindbrain, the cytoplasmic Math5-lacZ reporter is expressed within the ventral cochlear nucleus (VCN), in a subpopulation of neurons that project to medial nucleus of the trapezoid body (MNTB), lateral superior olive (LSO), and lateral lemniscus (LL). These cells were identified as globular and small spherical bushy cells based on their morphology, abundance, distribution within the cochlear nucleus (CN), co-expression of Kv1.1, Kv3.1b and Kcnq4 potassium channels, and projection patterns within the auditory brainstem. Math5-lacZ is also expressed by cochlear root neurons in the auditory nerve. During embryonic development, Math5-lacZ was detected in precursor cells emerging from the caudal rhombic lip from embryonic day (E)12 onwards, consistent with the time course of CN neurogenesis. These cells co-express MafB and are post-mitotic. Math5 expression in the CN was verified by mRNA in situ hybridization, and the identity of positive neurons was confirmed morphologically using a Math5-Cre BAC transgene with an alkaline phosphatase reporter. The hindbrains of Math5 mutants appear grossly normal, with the exception of the CN. Although overall CN dimensions are unchanged, the lacZ-positive cells are significantly smaller in Math5 -/- mice compared to Math5 +/- mice, suggesting these neurons may function abnormally. The auditory brainstem response (ABR) of Math5 mutants was evaluated in a BALB/cJ congenic background. ABR thresholds of Math5 -/- mice were similar to those of wild-type and heterozygous mice, but the interpeak latencies for Peaks II-IV were significantly altered. These temporal changes are consistent with a higher-level auditory processing disorder involving the CN, potentially affecting the integration of binaural sensory information.

Deafness associated changes in two-pore domain potassium channels in the rat inferior colliculus

Two-pore potassium channels can influence neuronal excitability by regulating background leakage of potassium ions and resting membrane potential. The present study used quantitative real time PCR and in situ hybridization to determine if the decreased activity from deafness would induce changes in two-pore potassium channel subunit expression in the rat inferior colliculus (IC). Ten subunits were assessed with quantitative real-time PCR at 3 days, 3 weeks and 3 months following bilateral cochlear ablation. TASK-1, TASK-5 and THIK-2 showed significant decreases in expression at all three times assessed. TASK-5, relatively specific to auditory neurons, had the greatest decrease. TWIK-1 was significantly decreased at 3 weeks and 3 months following deafness and TREK-2 was only significantly decreased at 3 days. TASK-3, TWIK-2, THIK-1, TRAAK and TREK-1 did not show any significant changes in gene expression. In situ hybridization was used to examine TASK-1, TASK-5, TWIK-1 and THIK-2 in the central nucleus, dorsal cortex and lateral (external) cortex of the IC in normal hearing animals and at 3 weeks following deafening. All four subunits showed expression in neurons throughout IC subdivisions in normal hearing rats, with TASK-5 having the greatest overall number of labeled neurons. There was no co-localization of subunit expression with glial fibrillary acidic protein immunostaining, indicating no expression in glia. Three weeks following deafening there was a significant decrease in the number of neurons expressing TASK-1 and THIK-2 in the IC, while TASK-5 had significant decreases in the central nucleus and dorsal cortex and TWIK-1 in the lateral and dorsal cortices.

Effects of antioxidants on auditory nerve function and survival in deafened guinea pigs

Based on in vitro studies, it is hypothesized that neurotrophic factor deprivation following deafferentation elicits an oxidative state change in the deafferented neuron and the formation of free radicals that then signal cell death pathways. This pathway to cell death was tested in vivo by assessing the efficacy of antioxidants (AOs) to prevent degeneration of deafferented CNVIII spiral ganglion cells (SGCs) in deafened guinea pigs. Following destruction of sensory cells, guinea pigs were treated immediately with Trolox (a water soluble vitamin E analogue)+ascorbic acid (vitamin C) administered either locally, directly in the inner ear, or systemically. Electrical auditory brainstem response (EABR) thresholds were recorded to assess nerve function and showed a large increase following deafness. In treated animals EABR thresholds decreased and surviving SGCs were increased significantly compared to untreated animals. These results indicate that a change in oxidative state following deafferentation plays a role in nerve cell death and antioxidant therapy may rescue SGCs from deafferentation-induced degeneration.

Deafness associated changes in expression of two-pore domain potassium channels in the rat cochlear nucleus

Two-pore domain potassium channels (K(2PD)+) play an important role in setting resting membrane potential by regulating background leakage of potassium ions, which in turn controls neuronal excitability. To determine whether these channels contribute to activity-dependent plasticity following deafness, we used quantitative real-time PCR to examine the expression of 10 K(2PD)+ subunits in the rat cochlear nucleus at 3 days, 3 weeks and 3 months after bilateral cochlear ablation. There was a large sustained decrease in the expression of TASK-5, a subunit that is predominantly expressed in auditory brain stem neurons, and in the TASK-1 subunit which is highly expressed in several types of cochlear nucleus neurons. TWIK-1 and THIK-2 also showed significant decreases in expression that were maintained across all time points. TWIK-2, TREK-1 and TREK-2 showed no significant change in expression at 3 days but showed large decreases at 3 weeks and 3 months following deafness. TRAAK and TASK-3 subunits showed significant decreases at 3 days and 3 weeks following deafness, but these differences were no longer significant at 3 months. Dramatic changes in expression of K(2PD)+ subunits suggest these channels may play a role in deafness-associated changes in the excitability of cochlear nucleus neurons.

Changes in glycine immunoreactivity in the rat superior olivary complex following deafness

The balance between inhibitory and excitatory amino acid neurotransmitters contributes to the control of normal functioning of the auditory brainstem. Changes in the level of neuronal activity within the auditory brainstem pathways influence the balance between inhibition and excitation. Activity-dependent plasticity in the auditory pathways can be studied by creating a large decrease in activity through peripheral deafening. Deafness-related decreases in GABA have previously been shown in the inferior colliculus. However, glycine is a more prevalent inhibitory transmitter in the mature superior olivary complex (SOC). The present study therefore examined if there were deafness-related changes in glycine in the SOC using postembedding immunocytochemistry. Animals were bilaterally deafened by an intrascalar injection of neomycin. Five nuclei in the SOC, the lateral superior olive (LSO), superior paraolivary nucleus (SPoN), and the medial, lateral, and ventral nuclei of the trapezoid body (MNTB, LNTB, and VNTB) were examined 14 days following the deafening and compared to normal hearing age-matched controls. The LSO and SPoN were divided into high and low frequency regions. The number of glycine immunoreactive puncta on the somata of principal cells showed significant decreases in all regions assessed, with changes ranging from 50% in the VNTB to 23% in the LSO.

Tyrosine hydroxylase in rat auditory midbrain: distribution and changes following deafness

Tyrosine hydroxylase (TH), a key enzyme in the catecholaminergic pathway, allows for the differentiation of dopaminergic neurons. We previously showed decreases in TH gene expression in the rat inferior colliculus (IC) 3 and 21 days following deafness. In the present study, we characterized the normal distribution of TH as well as changes following deafness (bilateral cochlear ablation) in the IC and nuclei of the lateral lemniscus. Immunostaining was compared in three groups of rats: normal hearing (n=8), 21 day deaf (n=5) and 90 days following deafening (n=5). Many TH immunoreactive fibers and puncta were identified in the IC and nuclei of the lateral lemniscus of normal hearing animals and labeling was most dense in the external cortex of the IC. We also identified immunolabeling for fibers and puncta for another catecholaminergic enzyme, dopamine beta hydroxylase (DBH), but not phenylethanolamine-N-methyltranferase (PNMT). Neurons immunopositive for TH but not DBH or PNMT were observed in the dorsal cortex and dorsal horn of the central nucleus of the IC and ventral and intermediate lemniscus. In the central nucleus of the IC and dorsal lateral lemniscus many lightly labeled TH neurons were also DBH positive. Although the number of immunopositive cells in the IC and lemniscus declined 3 weeks and 3 months after deafening, the decline was not significant at three weeks in the VNLL nor after three months in the dorsal cortex. Immunolabeling for TH decreased significantly in IC and lemniscus 3 weeks and 3 months following deafening. These results suggest a role for dopaminergic neurons and fibers in deafness-related plasticity.

Deafness-related decreases in glycine-immunoreactive labeling in the rat cochlear nucleus

There is increasing evidence of activity-related plasticity in auditory pathways. The present study examined the effects of decreased activity on immunolocalization of the inhibitory neurotransmitter glycine in the cochlear nucleus of the rat after bilateral cochlear ablation. Specifically, glycine-immunoreactive puncta adjacent to somatic profiles were compared in normal hearing animals and animals deafened for 14 days. The number of glycine-immunoreactive puncta surrounding somatic profiles of spherical and globular bushy cells, glycine-immunoreactive type I stellate multipolar cells, radiate neurons (type II stellate multipolar cells), and fusiform cells decreased significantly. In addition, the number of glycine immunopositive tuberculoventral (vertical or corn) cells in the deep layer of the dorsal cochlear nucleus also decreased significantly. These results suggest that decreased inhibition reported in cochlear nucleus after deafness may be due to decreases in glycine.

Heat shock factor 1-deficient mice exhibit decreased recovery of hearing following noise overstimulation

Heat shock proteins (Hsps) can enhance cell survival in response to stress. Heat shock factor 1 (Hsf1) is the major transcription factor that regulates stress-inducible Hsp expression. We previously demonstrated the presence of Hsf1 in the rodent cochlea and also demonstrated that a heat shock known to precondition the cochlea against noise trauma results in Hsf1 activation in the rodent cochlea. In the present study, we used an Hsf1-deficient (Hsf1-/- mouse model to determine whether eliminating the Hsf1-dependent stress pathway would influence hearing loss and/or recovery from a moderate-intensity noise. Hsf1-/- mice and their normal littermates (Hsf1+/+) were exposed to a 98-dB, broadband (2-20 kHz) noise for 2 hr, and auditory brainstem response thresholds were measured at three frequencies (4, 12, and 20 kHz) 3 hr, 3 days, and 2 weeks after noise. Hsf1-/- mice had greater hearing loss than Hsf1+/+ mice, with significant differences in recovery observed at all frequencies tested by 2 weeks after noise. Increased outer hair cell loss was also observed in Hsf1-/- mice following noise. These studies provide evidence for the importance of Hsf1 in cochlear protection, recovery, and/or repair following noise overstimulation.

Deafness-related plasticity in the inferior colliculus: gene expression profiling following removal of peripheral activity

The inferior colliculus (IC) is a major center of integration in the ascending as well as descending auditory pathways, where both excitatory and inhibitory amino acid neurotransmitters play a key role. When normal input to the auditory system is decreased, the balance between excitation and inhibition in the IC is disturbed. We examined global changes in gene expression in the rat IC 3 and 21 days following bilateral deafening, using Affymetrix GeneChip arrays and focused our analysis on changes in expression of neurotransmission-related genes. Over 1400 probe sets in the Affymetrix Rat Genome U34A Array were identified as genes that were differentially expressed. These genes encoded proteins previously reported to change as a consequence of deafness, such as calbindin, as well as proteins not previously reported to be modulated by deafness, such as clathrin. A subset of 19 differentially expressed genes was further examined using quantitative RT-PCR at 3, 21 and 90 days following deafness. These included several GABA, glycine, glutamate receptor and neuropeptide-related genes. Expression of genes for GABA-A receptor subunits beta2, beta3, and gamma2, plus ionotropic glutamate receptor subunits AMPA 2, AMPA 3, and kainate 2, increased at all three times. Expression of glycine receptor alpha1 initially declined and then later increased, while alpha2 increased sharply at 21 days. Glycine receptor alpha3 increased between 3 and 21 days, but decreased at 90 days. Of the neuropeptide-related genes tested with qRT-PCR, tyrosine hydroxylase decreased approximately 50% at all times tested. Serotonin receptor 2C increased at 3, 21, and 90 days. The 5B serotonin receptor decreased at 3 and 21 days and returned to normal by 90 days. Of the genes tested with qRT-PCR, only glycine receptor alpha2 and serotonin receptor 5B returned to normal levels of expression at 90 days. Changes in GABA receptor beta3, GABA receptor gamma2, glutamate receptor 2/3, enkephalin, and tyrosine hydroxylase were further confirmed using immunocytochemistry.

Noise overstimulation induces immediate early genes in the rat cochlea

In mammals, exposure to intense noise produces a permanent hearing loss called permanent threshold shift (PTS), whereas a moderate noise produces only a temporary threshold shift (TTS). Little is known about the molecular responses to such high intensity noise exposures. In this study we used gene arrays to examine the early response to acoustic overstimulation in the rat cochlea. We compared cochlear RNA from noise-exposed rats with RNA from unexposed controls. The intense PTS noise induced several immediate early genes encoding both transcription factors (c-FOS, EGR1, NUR77/TR3) and cytokines (PC3/BTG2, LIF and IP10). In contrast, the TTS noise down-regulated the gene for growth hormone. The response of these genes to different noise intensities was examined by quantitative RT-PCR 2.5 h after the 90-min noise exposure. For most genes, the extent of induction correlates with the intensity of the noise exposure. Three proteins (EGR1, NUR77/TR3, and IP10) were detected in many regions of the unexposed cochlea. After exposure to 120 dB noise, these proteins were present at higher levels or showed extended expression in additional regions of the cochlea. LIF was undetectable in the cochlea of unexposed rats, but could be seen in the organ of Corti and spiral ganglion neurons following noise. NUR77/TR3 was a nuclear protein before noise, but following noise translocated to the cytoplasm. These studies provide new insights into the molecular response to noise overstimulation in the mammalian cochlea.

Delayed neurotrophic treatment preserves nerve survival and electrophysiological responsiveness in neomycin-deafened guinea pigs

Benefits of cochlear prostheses for the deaf are dependent on survival and excitability of the auditory nerve. Degeneration of deafferented auditory nerve fibers is prevented and excitability maintained by immediate replacement therapy with exogenous neurotrophic factors, in vivo. It is important to know whether such interventions are effective after a delay following deafness, typical for the human situation. This study evaluated the efficacy of brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor axokine-1 analogue (CNTF Ax1) application, 2 or 6 weeks postdeafening, in preventing further degeneration and a decrease in excitability. Guinea pigs were deafened and implanted with intracochlear stimulating electrodes, a scala tympani cannula-osmotic pump system, and auditory brainstem response (ABR) recording electrodes. Subjects received BDNF + CNTF Ax1 or artificial perilymph (AP) treatment for 27 days, beginning at 2 or 6 weeks following deafening. Electrical (E) ABR thresholds increased following deafening. After 1 week, in the 2-weeks-delayed neurotrophic factor treatment group, EABR thresholds decreased relative to AP controls, which were statistically significant at 2 weeks. In the 6-week delay group, a tendency to enhanced EABR sensitivity began at 2 weeks of treatment and increased thereafter, with a significant difference between neurotrophic factor- and AP-treated groups across the treatment period. A clear, statistically significant, enhanced survival of spiral ganglion cells was seen in both neurotrophic factor treatment groups relative to AP controls. These findings demonstrate that BDNF + CNTF Ax1 can act to delay or possibly even reverse degenerative and, likely apoptotic, processes well after they have been activated. These survival factors can rescue cells from death and enhance electrical excitability, even during the period of degeneration and cell loss when the spiral ganglion cell population is reduced by >50% (6 weeks). It is noteworthy that this same degree of ganglion cell loss, secondary to receptor damage, is typically observed after a period equivalent to some years of deafness in humans.

Charakterisierung der Genexpression im Innenohr und der Hörbahn hörender und ertaubter Ratten mittels Gene-Arrays

BACKGROUND

The phenotype of deafness and its mechanisms are morphologically and electrophysiologically well characterised. However, the molecular mechanisms and the consequences of deafness are poorly understood.

METHODS

In this study we investigated changes in gene expression profiles in subfractions of the cochlea and the colliculus inferior, a non-cochlear tissue, of normal and deafened (10 % Neomycin) rats using the gene-array-technology. RNA was prepared from modiolus (Mo) und sensorineural epithel/lateral wall (SnE/Lw) und Colliculus inferior (IC), reverse transcribed with gene specific primers, labeled with (32)P-dATP and hybridised with its complementary sequences of 1200 rat ESTs.

RESULTS

Similar gene expression profiles were detected in Mo- and SnE/Lw in normal as well in deafened rats differing significantly from those found in IC. In deafened animals differences in mRNA levels were determined in IC for 8 genes, in Mo für 17 genes and in SnE/Lw for 25 genes in comparison to those of normal rats. By using gene-arrays many genes described in the literature previously could be detected. Otherwise most of the genes found in the cochlea are unknown.

CONCLUSIONS

The gene-array-technology is a valuable tool in otological research for gene expression analysis and, therefore, for comprehensive understanding of molecular processes in the inner ear. Furthermore gene screening for candidate genes could be a big step ahead in developing therapies of diseases of the inner ear.

Neonatal deafening causes changes in Fos protein induced by cochlear electrical stimulation

The influence of neonatal deafness on cochlear electrically evoked Fos expression in the auditory brainstem was examined. Newborn rats were deafened by systemic injection of kanamycin, 1 mg/g daily for 12 days. At 4, 5, 6 or 8 weeks of age, these animals received cochlear electrical stimulation with a basal monopolar electrode for 90 minutes. Age-matched untreated control animals received similar stimulation. Experimental and control animals were assessed for spiral ganglion cell densities and Fos immunoreactive staining in the central nucleus of the inferior colliculus. Spiral ganglion cell assessments showed significant decreases in spiral ganglion cell densities in deafened rats compared to age-matched controls, at 5 weeks of age in lower turns and 6 and 8 weeks in all turns. Cochlear electrical stimulation induced Fos immunoreactive staining in the nucleus of auditory brain stem neurons in treatment and control groups. A significantly greater number of Fos immunoreactive neurons was found in the contralateral central nucleus of inferior colliculus in 5, 6 and 8 week old deafened animals compared to age-matched controls. The increases were larger with a longer duration of deafness. These results suggest that there are changes in auditory processing as a consequence of neonatal deafness.

Induction of heat shock protein 32 (Hsp32) in the rat cochlea following hyperthermia

The genes for heat shock proteins (Hsps) can be upregulated in response to cellular trauma, resulting in enhanced cell survival and protection. Hsp32, also known as heme oxygenase 1, catalyzes the degradation of heme to produce carbon monoxide and bilirubin, which play a variety of cytoprotective functions at physiological concentrations, and iron, which is rapidly sequestered by the iron-binding protein ferritin. In the present study we examined the expression and localization of Hsp32 in the rat cochlea after heat shock using semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), Western blot, and immunocytochemistry. Low levels of constitutive Hsp32 expression were observed in the normal rat cochlea by RT-PCR and Western blot. Hsp32 mRNA (messenger RNA) was present at higher levels in a subfraction containing sensorineural epithelium and lateral wall than in a subfraction containing modiolus. Western blot revealed that Hsp32 protein levels increase in the rat cochlea following heat shock. Immunocytochemistry showed scattered staining of outer hair cells in the organ of Corti of normal untreated rats. Following heat shock Hsp32 is upregulated in outer hair cells and the cells of the stria vascularis. These results suggest a potential role for Hsp32 as a component of the oxidative stress response pathway in the rat cochlea.

Pathways for protection from noise induced hearing loss

There is increasing evidence that at least one function of both the medial and the lateral olivocochlear efferent systems is to provide adjustment of the set point of activity in their postsynaptic target, the outer hair cells and afferent processes, respectively. New results, summarized in this review, suggest that both efferent systems can provide protection from noise through this mechanism. There are also intracellular pathways that can provide protection from noise-induced cellular damage in the cochlea. This review also summarizes new results on the pathways that regulate and react to levels of reactive oxygen species in the cochlea as well as the role of stress pathways for the heat shock proteins and for neurotrophic factors in protection, recovery and repair.

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

Differential Gene Expression Following Noise Trauma in Birds and Mammals

Acoustic overstimulation has very different outcomes in birds and mammals. When noise exposure kills hair cells in birds, these cells can regenerate and hearing will recover. In mammals, however, the hair cell loss, and resulting hearing loss, is permanent. Changes in gene expression form the basis for important biological processes, including repair, regeneration, and plasticity. We are therefore using a battery of molecular approaches to identify and compare changes in gene expression following noise trauma in birds and mammals. Both differential display and subtractive hybridisation were used to identify genes whose expression increased in the chick basilar papilla immediately following exposure to an octave band noise (118 dB, centre frequency 1.5 kHz) for 4-6 hr. Among those upregulated genes were two involved in actin signalling: the CDC42 gene encoding a Rho GTPase, and WDR1, which encodes a protein involved in actin dynamics. A third gene, UBE3B, encodes an E3 ubiquitin ligase involved in protein turnover. A fourth gene encodes a cystein-rich secreted protein that may interact with calcium channels. To examine the mammalian response, gene microarrays on nylon membranes (Clontech Atlas Gene Arrays) were used to examine global changes in gene expression 30 minutes after TTS (110 dB broadband noise 50% duty cycle) or PTS (125 dB, 100% duty cycle) noise overstimulation (each for 90 minutes) in the rat cochlea. Several genes, including classic immediate early response genes such as c-fos, EGR1/NGFI-A, and NGFI-B, were upregulated at this early time point following the PTS exposure, but were not upregulated following the TTS exposure.

Absence of hair cell protection by exogenous FGF-1 and FGF-2 delivered to guinea pig cochlea in vivo

Recent findings that glial cell line-derived neurotrophic factor (GDNF), neurotrophin-3 (NT-3), and transforming growth factor a can protect the auditory hair cells from acoustic trauma or aminoglycoside ototoxicity in vivo raise the question of whether other neurotrophic factors can also protect the hair cells in vivo. Fibroblast growth factor-2 (FGF-2) can protect hair cells from neomycin ototoxicity in vitro, and in vivo study has shown upregulation of FGF receptor-3 in the cochlea following noise exposure, suggesting that some FGF family members might play a role in protection or repair of the cochlea from damage. We therefore examined if FGF-1 and FGF-2 chronically delivered to the cochlea prior to noise overstimulation can attenuate noise-induced hair cell damage in vivo under conditions in which GDNF and NT-3 were effective. Pigmented female guinea pigs underwent left scala tympani implantation of a microcannula attached to an osmotic pump filled with artificial perilymph only or containing FGFs (10 or 1 mg/ml FGF-1 or 10 mg/ml FGF-2). They were exposed to noise (4 kHz octave band, 115 dB SPL, 5 hr) 4 days after surgery. Threshold shifts 10 days postexposure were essentially equivalent at all frequencies tested across different treatment groups. No significant difference in threshold shifts was observed between the treated and untreated ears in any of the groups. The extent of hair cell damage was also comparable among the different treatment groups. These findings indicate that exogenous FGF-1 or FGF-2 does not influence noise-induced hair cell damage under the experimental conditions used in this study, suggesting that these FGFs are not good candidates as auditory hair cell protectors in vivo.

Mechanism of electrical stimulation-induced neuroprotection: effects of verapamil on protection of primary auditory afferents

In order to assess the role of L-type voltage-gated calcium channels in electrical stimulation-mediated neuroprotection in vivo, we assessed survival of primary auditory afferents (spiral ganglion cells) in systemically deafened guinea pigs following chronic electrical stimulation with or without intracochlear infusion of verapamil, an L-type voltage-gated calcium channel antagonist. Continuous intracochlear drug delivery (0.5 microl/h) was provided using a delivery system developed previously in our laboratory using Alzet mini-osmotic pumps. In the absence of chronic stimulation, spiral ganglion cell survival was relatively symmetric in animals treated unilaterally with either artificial perilymph or verapamil (50 microg/ml). In the presence of unilateral chronic electrical stimulation, spiral ganglion cell survival was significantly greater in stimulated, perilymph-infused ears, relative to the contralateral ear. In contrast, spiral ganglion cell survival was bilaterally symmetric in chronically stimulated, verapamil-infused animals. The difference in symmetry of spiral ganglion cell survival between the two groups was statistically significant. In vitro, passive depolarization has been demonstrated to enhance survival of cultured neurons via activation of L-type calcium channels. The results of this study indicate that, as suggested by in vitro depolarization models, in vivo electrical stimulation-mediated neuroprotection requires the activation of L-type voltage-gated calcium channels. Chronic electrical stimulation of the deaf ear is an ideal preparation for further studies in which to extrapolate findings from in vitro depolarization models.

In vivo studies of polypyrrole/peptide coated neural probes

Neural probes are micromachined multichannel electrode arrays that facilitate the functional stimulation and recording of neurons in the peripheral and central nervous system. For long-term implantations, surface modification is necessary for maintaining the stable connection between electrodes and neurons. The conductive polymer polypyrrole (PPy) and synthetic peptide DCDPGYIGSR were co-deposited on the electrode surface by electrochemical polymerization. The stability of PPy/DCDPGYIGSR coatings was tested in soaking experiments. It was found that the peptide was entrapped in the PPy film and did not diffuse away within 7 weeks of soaking in DI water. Coated probes were implanted in guinea pig brain for periods of 1, 2 and 3 weeks. Recording tests were performed and the impedance was monitored. The explanted probes and tissue were examined by immunocytochemical studies. Significantly more neurofilament positive staining was found on the coated electrode which indicated that the coatings had established strong connections with the neuronal structure in vivo. Good recordings were obtained from the coated sites that had neurons attached. First week tissue sections had no significant gliosis. In week 2, a layer of non-neuronal tissue consisting of mostly meningeal fibroblasts and ECM protein including at least fibronectin was formed around the probe tracks of both coated and uncoated probes. Astrocytes started to form a loosely organized layer by the end of the third week.

Glial cell line-derived neurotrophic factor and chronic electrical stimulation prevent VIII cranial nerve degeneration following denervation

As with other cranial nerves and many CNS neurons, primary auditory neurons degenerate as a consequence of loss of input from their target cells, the inner hair cells (IHCs). Electrical stimulation (ES) of spiral ganglion cells (SGCs) has been shown to enhance their survival. Glial cell line-derived neurotrophic factor (GDNF) has also been shown to increase survival of SGCs following IHC loss. In this study, the combined effects of the GDNF transgene delivered by adenoviral vectors (Ad-GDNF) and ES were tested on SGCs after first eliminating the IHCs. Animal groups received Ad-GDNF or ES or both. Ad-GDNF was inoculated into the cochlea of guinea pigs after deafening, to overexpress human GDNF. ES-treated animals were implanted with a cochlear implant electrode and chronically stimulated. A third group of animals received both Ad-GDNF and ES (GDNF/ES). Electrically evoked auditory brainstem responses were recorded from ES-treated animals at the start and end of the stimulation period. Animals were sacrificed 43 days after deafening and their ears prepared for evaluation of IHC survival and SGC counts. Treated ears exhibited significantly greater SGC survival than nontreated ears. The GDNF/ES combination provided significantly better preservation of SGC density than either treatment alone. Insofar as ES parameters were optimized for maximal protection (saturated effect), the further augmentation of the protection by GDNF suggests that the mechanisms of GDNF- and ES-mediated SGC protection are, at least in part, independent. We suggest that GDNF/ES combined treatment in cochlear implant recipients will improve auditory perception. These findings may have implications for the prevention and treatment of other neurodegenerative processes. .

Expression and localization of heat shock factor (Hsf) 1 in the rodent cochlea

Activation of heat shock factors (Hsfs) is one of the potential mechanisms for regulating the transcription of the heat shock proteins (Hsps) and certain other stress-responsive genes. Reverse transcription polymerase chain reaction (RT-PCR), Western blot and immunocytochemistry were used to examine the expression and localization of Hsf1, the stress-responsive member of the Hsf family, in the rat and mouse cochlea. Cerebellum was used as a positive control. Semi-quantitative RT-PCR of cochlear RNA revealed that Hsf1 was more highly expressed in a subfraction containing sensorineural epithelium and lateral wall than in a subfraction containing modiolus, with the alpha splice form predominant over the beta in both subfractions. Immunocytochemistry showed selective staining in the rodent cochlea. Hsf1 immunostaining was found in the nuclei of inner and outer hair cells in the organ of Corti, spiral ganglion cells in the modiolus, and cells in the marginal and intermediate layers of the stria vascularis. This is largely consistent with where Hsp70 induction is reported. Hsf1 activation following heat shock was examined by Western blot. Hyperthermia resulted in stress-induced Hsf1 hyperphosphorylation in cochlea as well as cerebellum. This hyperphosphorylation as well as the correlation of its localization with Hsp70 induction supports a role for Hsf1 in the cochlear stress response.

Stress pathways in the rat cochlea and potential for protection from acquired deafness

Noise overstimulation will induce or influence intracellular molecular pathways in the cochlea. One of these is the 'classical' stress response pathway involving heat shock proteins. Hsp70 is induced in the cochlea by a wide variety of stresses including noise, hyperthermia and ototoxic drugs. When a stress that induces Hsp70 is applied to the cochlea, there is protection from a subsequent noise that would normally cause a permanent hearing loss. An upstream regulator of heat shock protein transcription, heat shock factor 1, is expressed in the cochlea and activated by stress. Mice lacking this heat shock factor have reduced recovery from noise-induced hearing loss. The same noise exposure that induces Hsp70 also increases the level of glial cell line-derived neurotrophic factor in the cochlea. Moreover, when this neurotrophic factor is applied into the perilymph of scala tympani prior to a noise exposure there is a significant reduction in hair cell loss and hearing loss. With the potential for activation of multiple pathways in the response to noise, gene microarrays can be useful to examine global gene expression. Initial studies examined differential gene expression immediately following a mild noise exposure (from which there is complete recovery) versus an intense noise (giving profound permanent deafness). Differential expression of several immediate early genes was found following the intense but not the mild noise exposure.

Protection and regrowth of the auditory nerve after deafness: neurotrophins, antioxidants and depolarization are effective in vivo

Deafness, with loss of sensory (hair) cells, results in progressive pathophysiological changes ending in the degeneration of most auditory nerve neurons. It is now possible to consider these events in the broader context of anti-apoptotic survival factors in the peripheral and central nervous system. One consequence of deafferentation of a neuron is the loss of neurotrophins that can lead to a change in oxidative state (formation of free radicals), changes in intracellular Ca(2+), and an up-regulation of apoptotic genes. Interventions that can modify availability of neurotrophins, [Ca(2+)](I), and/or free radical formation or their destructive effects, may preserve the auditory nerve. Some interventions (neurotrophins) may also lead to a regrowth of neurites. Studies in this area are of basic value and also of immediate clinical interest for the application of the cochlear prosthesis to the severe and profoundly deaf, since the benefits of this prosthesis are directly dependent on auditory nerve survival and the proximity of stimulating electrode to neuron. We, and others, have found that auditory nerve degeneration can be prevented by chronic electrical stimulation. We have demonstrated in vivo that this effect can be blocked by tetrodotoxin, thus indicating that propagated action potentials are a necessary condition, and by verapamil (Ca(2+) channel blocker), supporting in vitro studies by others, indicating that L-type Ca(2+) channels are necessary for stimulation-induced rescue of the deafferented auditory nerve. The intensities of electrical stimulation required for rescue are at levels sufficient to express the intermediate-early gene c-fos which can initiate transcription of anti-apoptotic genes and pathways, and up-regulate expression of neurotrophins that may act in an autocrine manner to protect the nerve from death. We, and others, have found that chronic local delivery (osmotic pump and microcannulation of the inner ear fluid spaces) of individual neurotrophins and cocktails of factors can also enhance survival of the deafferented nerve, and some can also initiate a regrowth of degenerated peripheral processes of the nerve into the region of the destroyed sensory epitheliae. Recently, we have shown that this rescue can occur with delayed intervention, after degeneration of some neurons has begun, more closely mimicking the human clinical situation. Finally, we have shown that interventions with antioxidants may also be effective in preventing pathophysiological changes of the auditory nerve following deafness. These studies in the auditory periphery support the 'neurotrophic factor hypothesis' as proposed as a general mechanism underlying neurodegenerative and age-related pathology of the central nervous system. Additional animal studies can yield a rational scientific basis to justify human trials, with a goal to maintain auditory cell survival and initiate and direct fiber growth to the next generation of prosthesis. Intimate contact between electrode and a dense population of auditory neurons should greatly enhance the benefits of these devices for the profoundly deaf.

Gene expression profiles of the rat cochlea, cochlear nucleus, and inferior colliculus

High-throughput DNA microarray technology allows for the assessment of large numbers of genes and can reveal gene expression in a specific region, differential gene expression between regions, as well as changes in gene expression under changing experimental conditions or with a particular disease. The present study used a gene array to profile normal gene expression in the rat whole cochlea, two subregions of the cochlea (modiolar and sensorineural epithelium), and the cochlear nucleus and inferior colliculus of the auditory brainstem. The hippocampus was also assessed as a well-characterized reference tissue. Approximately 40% of the 588 genes on the array showed expression over background. When the criterion for a signal threshold was set conservatively at twice background, the number of genes above the signal threshold ranged from approximately 20% in the cochlea to 30% in the inferior colliculus. While much of the gene expression pattern was expected based on the literature, gene profiles also revealed expression of genes that had not been reported previously. Many genes were expressed in all regions while others were differentially expressed (defined as greater than a twofold difference in expression between regions). A greater number of differentially expressed genes were found when comparing peripheral (cochlear) and central nervous system regions than when comparing the central auditory regions and the hippocampus. Several families of insulin-like growth factor binding proteins, matrix metalloproteinases, and tissue inhibitor of metalloproteinases were among the genes expressed at much higher levels in the cochlea compared with the central nervous system regions.

Constitutive expression of Hsp27 in the rat cochlea

Heat shock protein-27 (Hsp27) is known to function as both a stress-inducible molecular chaperone and regulator of actin polymerization. For many cells in the cochlea, actin is part of the cytoskeleton and plays an important role in the maintenance of cochlear function. To understand the molecular processes by which the cochlear actin cytoskeleton is maintained and regulated during normal auditory function, we examined the expression and localization of Hsp27 in the normal rat cochlea. Reverse transcription-polymerase chain reaction and Western blot showed constitutive expression of Hsp27 in the normal rat cochlea. Immunofluorescence microscopy showed Hsp27-like staining is localized to the cuticular plate and lateral wall of outer hair cells. Hsp27-like immunostaining is also found in tension fibroblasts, in the root cells of the spiral limbus and in Reissner's membrane. The presence of Hsp27 in the actin-rich tension fibroblasts and outer hair cells suggests a potential role in the regulation and maintenance of the actin cytoskeleton in these cells. The presence of high levels of constitutive Hsp27 may also provide a mechanism for pre-protecting these cells against environmental stressors.