Regulation of NT-3 and BDNF levels in guinea pig auditory brain stem nuclei after unilateral cochlear ablation

Candidate Key Proteins in Tinnitus-A Bioinformatic Study of Synaptic Transmission in the Cochlear Nucleus

The aim of this study was to identify key proteins of synaptic transmission in the cochlear nucleus (CN) that are involved in normal hearing, acoustic stimulation, and tinnitus. A gene list was compiled from the GeneCards database using the keywords "synaptic transmission" AND "tinnitus" AND "cochlear nucleus" (Tin). For comparison, two gene lists with the keywords "auditory perception" (AP) AND "acoustic stimulation" (AcouStim) were built. The STRING protein-protein interaction (PPI) network and the Cytoscape data analyzer were used to identify the top two high-degree proteins (HDPs) and their high-score interaction proteins (HSIPs), together referred to as key proteins. The top1 key proteins of the Tin-process were BDNF, NTRK1, NTRK3, and NTF3; the top2 key proteins are FOS, JUN, CREB1, EGR1, MAPK1, and MAPK3. Highly significant GO terms in CN in tinnitus were "RNA polymerase II transcription factor complex", "late endosome", cellular response to cadmium ion", "cellular response to reactive oxygen species", and "nerve growth factor signaling pathway", indicating changes in vesicle and cell homeostasis. In contrast to the spiral ganglion, where important changes in tinnitus are characterized by processes at the level of cells, important biological changes in the CN take place at the level of synapses and transcription.

[Central and peripheral aspects of noise-induced hearing loss]

Noise is an important socioeconomic problem in industrialized countries. Development of efficient treatment options for the audiological phenomena resulting from noise-induced hearing loss requires in-depth understanding of the underlying damage mechanisms causing peripheral and central nervous changes. Mechanical damage, ischemia and excitotoxicity are mainly responsible for noise-induced cell death and biophysical changes in the cochlea. Auditory synaptopathy is an additional consequence. Besides these cochlear pathologies, noise exposure leads to extensive changes within the central auditory pathway. Overstimulation causes early cell loss in the ventral cochlear nucleus just after noise exposure, which is in accordance with enhancement of apoptotic mechanisms within the corresponding timeframe. In contrast to the cell loss in lower auditory structures due to overstimulation, the later significant reduction of cell density in higher auditory structures is due to sensory deprivation. Changes in network homeostasis seem to partially compensate structural losses by modulation of spontaneous activity. However, central nervous processing of auditory information is permanently impaired by the neuroplastic changes. Unfortunately, the various noise-induced peripheral and central pathologies are difficult to treat. New therapeutic approaches are required, particularly for treatment of central nervous processing disorders and auditory synaptopathy, which contribute to audiological phenomena such as tinnitus, hyperacusis and poor speech perception in noise.

In ovo Sound Stimulation Mediated Regulation of BDNF in the Auditory Cortex and Hippocampus of Neonatal Chicks

Brain-derived neurotrophic factor (BDNF) is known to mediate activity-dependent changes in the developing auditory system. Its expression in the brainstem auditory nuclei, auditory cortex and hippocampus of neonatal chicks (Gallus gallus domesticus) in response to in ovo high intensity sound exposure at 110 dB (arrhythmic sound: recorded traffic noise, 30-3000 Hz with peak at 2700 Hz, rhythmic sound: sitar music, 100-4000 Hz) was examined to understand the previously reported altered volume and neuronal number in these regions. In the brainstem auditory nuclei, no mature BDNF, but proBDNF at the protein level was detected, and no change in its levels was observed after in ovo sound stimulation (music and noise). Increased ProBDNF protein levels were found in the auditory cortex in response to arrhythmic sound, along with decreased levels of one of the BDNF mRNA transcripts, in response to both rhythmic and arrhythmic sound stimulation. In the hippocampus, increased levels of mature BDNF were found in response to music. Expression microarray analysis was performed to understand changes in gene expression in the hippocampus in response to music and noise, followed by gene ontology analysis showing enrichment of probable signaling pathways. Differentially expressed genes like CAMK1 and STAT1 were found to be involved in downstream signaling on comparing music versus noise-exposed chicks. In conclusion, we report that BDNF is differentially regulated in the auditory cortex at the transcriptional and post-translational level, and in the hippocampus at the post-translational level in response to in ovo sound stimulation.

Traditional oriental medicine for sensorineural hearing loss: Can ethnopharmacology contribute to potential drug discovery?

ETHNOPHARMACOLOGICAL RELEVANCE

In Traditional Oriental Medicine (TOM), the development of hearing pathologies is related to an inadequate nourishment of the ears by the kidney and other organs involved in regulation of bodily fluids and nutrients. Several herbal species have historically been prescribed for promoting the production of bodily fluids or as antiaging agents to treat deficiencies in hearing.

AIM OF REVIEW

The prevalence of hearing loss has been increasing in the last decade and is projected to grow considerably in the coming years. Recently, several herbal-derived products prescribed in TOM have demonstrated a therapeutic potential for acquired sensorineural hearing loss and tinnitus. Therefore, the aims of this review are to provide a comprehensive overview of the current known efficacy of the herbs used in TOM for preventing different forms of acquired sensorineural hearing loss and tinnitus, and associate the traditional principle with the demonstrated pharmacological mechanisms to establish a solid foundation for directing future research.

METHODS

The present review collected the literature related to herbs used in TOM or related compounds on hearing from Chinese, Korean, and Japanese herbal classics; library catalogs; and scientific databases (PubMed, Scopus, Google Scholar; and Science Direct).

RESULTS

This review shows that approximately 25 herbal species and 40 active compounds prescribed in TOM for hearing loss and tinnitus have shown in vitro or in vivo beneficial effects for acquired sensorineural hearing loss produced by noise, aging, ototoxic drugs or diabetes. The inner ear is highly vulnerable to ischemia and oxidative damage, where several TOM agents have revealed a direct effect on the auditory system by normalizing the blood supply to the cochlea and increasing the antioxidant defense in sensory hair cells. These strategies have shown a positive impact on maintaining the inner ear potential, sustaining the production of endolymph, reducing the accumulation of toxic and inflammatory substances, preventing sensory cell death and preserving sensory transmission. There are still several herbal species with demonstrated therapeutic efficacy whose mechanisms have not been deeply studied and others that have been traditionally used in hearing loss but have not been tested experimentally. In clinical studies, Ginkgo biloba, Panax ginseng, and Astragalus propinquus have demonstrated to improve hearing thresholds in patients with sensorineural hearing loss and alleviated the symptoms of tinnitus. However, some of these clinical studies have been limited by small sample sizes, lack of an adequate control group or contradictory results.

CONCLUSIONS

Current therapeutic strategies have proven that the goal of the traditional oriental medicine principle of increasing bodily fluids is a relevant approach for reducing the development of hearing loss by improving microcirculation in the blood-labyrinth barrier and increasing cochlear blood flow. The potential benefits of TOM agents expand to a multi-target approach on different auditory structures of the inner ear related to increased cochlear blood flow, antioxidant, anti-inflammatory, anti-apoptotic and neuroprotective activities. However, more research is required, given the evidence is very limited in terms of the mechanism of action at the preclinical in vivo level and the scarce number of clinical studies published.

Protective Effects of Acupuncture Against Gentamicin-Induced Ototoxicity in Rats: Possible Role of Neurotrophin-3

BACKGROUND The aim of this study was to investigate the protective effects of acupuncture against gentamicin-induced ototoxicity and explore the possible protective role of neurotrophin-3 (NT-3). MATERIAL AND METHODS Twenty-four rats were divided randomly into 4 groups: control group, gentamicin group, neitinggong group, and tinggong group. Rats in the gentamicin, neitinggong, and tinggong groups received intraperitoneal injection of gentamicin (100 mg/kg) for 14 consecutive days. Rats in the neitinggong and tinggong groups further received acupuncture at neitinggong or tinggong acupoints once every 2 days for 20 days. Rats in the control group received intraperitoneal injection of saline. Auditory brainstem response (ABR) was tested in all rats on the day before treatment (day 0), and again on day 14 and day 20 to determine the average threshold value of ABR for each treatment group. The expression of NT-3 in the cochlear nucleus and the inferior colliculus nucleus were detected by immunohistochemical staining. RESULTS The average threshold value of ABR was significantly higher in the gentamicin group as compared with that of the control group on day 14 (P<0.05). On day 20, the average threshold values of ABR in the neitinggong and tinggong groups were significantly lower than that of the gentamicin group (P<0.05). No statistically significant differences in NT-3 expression in the cochlear nucleus were observed among the groups (P>0.05). However, the expression of NT-3 in the inferior colliculus nucleus in both the neitinggong and tinggong groups was significantly higher than that of the gentamicin group (P<0.01). CONCLUSIONS A decrease in NT-3 expression in the inferior colliculus nucleus may contribute to gentamicin-induced ototoxicity in rats. Acupuncture at neitinggong or tinggong acupoints effectively improved hearing, which was attributed partially to the rescue of NT-3 expression in the inferior colliculus nucleus. Therefore, preserving NT-3 expression in the auditory system may be a viable strategy to counteract gentamicin-induced ototoxicity.

Effects of brain-derived neurotrophic factor (BDNF) on the cochlear nucleus in cats deafened as neonates

Many previous studies have shown significant neurotrophic effects of intracochlear delivery of BDNF in preventing degeneration of cochlear spiral ganglion (SG) neurons after deafness in rodents and our laboratory has shown similar results in developing cats deafened prior to hearing onset. This study examined the morphology of the cochlear nucleus (CN) in a group of neonatally deafened cats from a previous study in which infusion of BDNF elicited a significant improvement in survival of the SG neurons. Five cats were deafened by systemic injections of neomycin sulfate (60 mg/kg, SQ, SID) starting one day after birth, and continuing for 16-18 days until auditory brainstem response (ABR) testing demonstrated profound bilateral hearing loss. The animals were implanted unilaterally at about 1 month of age using custom-designed electrodes with a drug-delivery cannula connected to an osmotic pump. BDNF (94 μg/ml; 0.25 μl/hr) was delivered for 10 weeks. The animals were euthanized and studied at 14-23 weeks of age. Consistent with the neurotrophic effects of BDNF on SG survival, the total CN volume in these animals was significantly larger on the BDNF-treated side than on the contralateral side. However, total CN volume, both ipsi- and contralateral to the implants in these deafened juvenile animals, was markedly smaller than the CN in normal adult animals, reflecting the severe effects of deafness on the central auditory system during development. Data from the individual major CN subdivisions (DCN, Dorsal Cochlear Nucleus; PVCN, Posteroventral Cochlear Nucleus; AVCN, Anteroventral Cochlear Nucleus) also were analyzed. A significant difference was observed between the BDNF-treated and control sides only in the AVCN. Measurements of the cross-sectional areas of spherical cells showed that cells were significantly larger in the AVCN ipsilateral to the implant than on the contralateral side. Further, the numerical density of spherical cells was significantly lower in the AVCN ipsilateral to the implant than on the contralateral side, consistent with the larger AVCN volume observed with BDNF treatment. Together, findings indicate significant neurotrophic effects of intracochlear BDNF infusion on the developing CN.

Neural Hyperactivity of the Central Auditory System in Response to Peripheral Damage

It is increasingly appreciated that cochlear pathology is accompanied by adaptive responses in the central auditory system. The cause of cochlear pathology varies widely, and it seems that few commonalities can be drawn. In fact, despite intricate internal neuroplasticity and diverse external symptoms, several classical injury models provide a feasible path to locate responses to different peripheral cochlear lesions. In these cases, hair cell damage may lead to considerable hyperactivity in the central auditory pathways, mediated by a reduction in inhibition, which may underlie some clinical symptoms associated with hearing loss, such as tinnitus. Homeostatic plasticity, the most discussed and acknowledged mechanism in recent years, is most likely responsible for excited central activity following cochlear damage.

BDNF signaling in the rat cerebello-vestibular pathway during vestibular compensation: BDNF signaling in vestibular compensation

Vestibular compensation, which is the behavioral recovery from lesions to the peripheral vestibular system, is attributed to plasticity of the central vestibular system. It has been reported that brain-derived neurotrophic factor (BDNF) is expressed and released in an activity-dependent manner. Upon binding to the tyrosine receptor kinase B (TrkB), BDNF can acutely modulate synaptic transmission and plasticity in the central nervous system. To assess the possible contribution of BDNF to this recovery process, we studied the expression of BDNF, TrkB.FL, TrkB.T1 and KCC2 (K(+) -Cl(-) cotransporter isoform 2) in the bilateral medial vestibular nucleus (MVN) and the flocculus of rats at 4 h, 8 h, 1, 3 and 7 days following unilateral labyrinthectomy (UL) using immunohistochemistry, quantitative real-time PCR and western blotting. Our results have shown that, compared with the sham controls and the contra-lesional side, (a) the expression of BDNF and TrkB.FL increased at 4 h in the ipsi-lesional flocculus after UL; (b) the expression of TrkB.T1 decreased at 4 h and KCC2 decreased at 8 h and 1 day in the ipsi-lesional flocculus after UL; and (c) BDNF and TrkB.FL expression was enhanced and KCC2 expression was reduced in the ipsi-lesional MVN at 8 h after UL. Our data supported the hypothesis that BDNF upregulation may reduce the inhibitory effects of the flocculus and commissural inhibition system by regulating inhibitory GABAergic synaptic transmission in floccular Purkinje cells and Purkinje cell terminals in the MVN. Additionally, KCC2 may be a switch in this process.

Cochlear nucleus whole mount explants promote the differentiation of neuronal stem cells from the cochlear nucleus in co-culture experiments

The cochlear nucleus is the first brainstem nucleus to receive sensory input from the cochlea. Depriving this nucleus of auditory input leads to cellular and molecular disorganization which may potentially be counteracted by the activation or application of stem cells. Neuronal stem cells (NSCs) have recently been identified in the neonatal cochlear nucleus and a persistent neurogenic niche was demonstrated in this brainstem nucleus until adulthood. The present work investigates whether the neurogenic environment of the cochlear nucleus can promote the survival of engrafted NSCs and whether cochlear nucleus-derived NSCs can differentiate into neurons and glia in brain tissue. Therefore, cochlear nucleus whole-mount explants were co-cultured with NSCs extracted from either the cochlear nucleus or the hippocampus and compared to a second environment using whole-mount explants from the hippocampus. Factors that are known to induce neuronal differentiation were also investigated in these NSC-explant experiments. NSCs derived from the cochlear nucleus engrafted in the brain tissue and differentiated into all cells of the neuronal lineage. Hippocampal NSCs also immigrated in cochlear nucleus explants and differentiated into neurons, astrocytes and oligodendrocytes. Laminin expression was up-regulated in the cochlear nucleus whole-mounts and regulated the in vitro differentiation of NSCs from the cochlear nucleus. These experiments confirm a neurogenic environment in the cochlear nucleus and the capacity of cochlear nucleus-derived NSCs to differentiate into neurons and glia. Consequently, the presented results provide a first step for the possible application of stem cells to repair the disorganization of the cochlear nucleus, which occurs after hearing loss.

Effects of the neurotrophic factors BDNF, NT-3, and FGF2 on dissociated neurons of the cochlear nucleus

The cochlear nucleus is the first relay station for acoustic information in the auditory pathway and its cellular integrity is affected by hearing loss. Neurotrophic factors, which are known to regulate fundamental processes in the brain, are expressed in the cochlear nucleus and are regulated by the changes in the stimulation. The aim of this study was to evaluate the effect of the neurotrophins Brain derived neurotrophic factor (BDNF) and Neurotrophin 3 (NT-3) and the neurotrophic factor Fibroblast growth factor 2 (FGF2) on primary cultured cells of the mouse cochlear nucleus. No effect on overall cell growth was detected after 8 days in culture by the factors applied. NT-3 had a strong impact on enhancement of neuronal survival, whereas BDNF stimulated neuronal survival and axonal outgrowth. Axonal branching was negatively affected by the administration of BDNF. FGF2 did not show any effect. The results presented represent fundamental research on auditory neurons, but might be one step toward defining novel therapeutic strategies in the future to prevent cochlear nucleus degeneration induced by hearing loss.

Auditory nerve synapses persist in ventral cochlear nucleus long after loss of acoustic input in mice with early-onset progressive hearing loss

Perceptual performance in persons with hearing loss, especially those using devices to restore hearing, is not fully predicted by traditional audiometric measurements designed to evaluate the status of peripheral function. The integrity of auditory brainstem synapses may vary with different forms of hearing loss, and differential effects on the auditory nerve-brain interface may have particularly profound consequences for the transfer of sound from ear to brain. Loss of auditory nerve synapses in ventral cochlear nucleus (VCN) has been reported after acoustic trauma, ablation of the organ of Corti, and administration of ototoxic compounds. The effects of gradually acquired forms deafness on these synapses are less well understood. We investigated VCN gross morphology and auditory nerve synapse integrity in DBA/2J mice with early-onset progressive sensorineural hearing loss. Hearing status was confirmed using auditory brainstem response audiometry and acoustic startle responses. We found no change in VCN volume, number of macroneurons, or number of VGLUT1-positive auditory nerve terminals between young adult and older, deaf DBA/2J. Cell-type specific analysis revealed no difference in the number of VGLUT1 puncta contacting bushy and multipolar cell body profiles, but the terminals were smaller in deaf DBA/2J mice. Transmission electron microscopy confirmed the presence of numerous healthy, vesicle-filled auditory nerve synapses in older, deaf DBA/2J mice. The present results suggest that synapses can be preserved over a relatively long time-course in gradually acquired deafness. Elucidating the mechanisms supporting survival of central auditory nerve synapses in models of acquired deafness may reveal new opportunities for therapeutic intervention.

Glia-related mechanisms in the anteroventral cochlear nucleus of the adult rat in response to unilateral conductive hearing loss

Conductive hearing loss causes a progressive decline in cochlear activity that may result in functional and structural modifications in auditory neurons. However, whether these activity-dependent changes are accompanied by a glial response involving microglia, astrocytes, or both has not yet been fully elucidated. Accordingly, the present study was designed to determine the involvement of glial related mechanisms in the anteroventral cochlear nucleus (AVCN) of adult rats at 1, 4, 7, and 15 d after removing middle ear ossicles. Quantitative immunohistochemistry analyses at light microscopy with specific markers of microglia or astroglia along with immunocytochemistry at the electron microscopy level were used. Also, in order to test whether trophic support by neurotrophins is modulated in glial cells by auditory activity, the expression and distribution of neurotrophin-3 (NT-3) and its colocalization with microglial or astroglial markers was investigated. Diminished cochlear activity after middle ear ossicle removal leads to a significant ipsilateral increase in the mean gray levels and stained area of microglial cells but not astrocytes in the AVCN at 1 and 4 d post-lesion as compared to the contralateral side and control animals. These results suggest that microglial cells but not astrocytes may act as dynamic modulators of synaptic transmission in the cochlear nucleus immediately following unilateral hearing loss. On the other hand, NT-3 immunostaining was localized mainly in neuronal cell bodies and axons and was upregulated at 1, 4 and 7 d post-lesion. Very few glial cells expressed this neurotrophin in both control and experimental rats, suggesting that NT-3 is primarily activated in neurons and not as much in glia after limiting auditory activity in the AVCN by conductive hearing loss.

Insult-induced adaptive plasticity of the auditory system

The brain displays a remarkable capacity for both widespread and region-specific modifications in response to environmental challenges, with adaptive processes bringing about the reweighing of connections in neural networks putatively required for optimizing performance and behavior. As an avenue for investigation, studies centered around changes in the mammalian auditory system, extending from the brainstem to the cortex, have revealed a plethora of mechanisms that operate in the context of sensory disruption after insult, be it lesion-, noise trauma, drug-, or age-related. Of particular interest in recent work are those aspects of auditory processing which, after sensory disruption, change at multiple—if not all—levels of the auditory hierarchy. These include changes in excitatory, inhibitory and neuromodulatory networks, consistent with theories of homeostatic plasticity; functional alterations in gene expression and in protein levels; as well as broader network processing effects with cognitive and behavioral implications. Nevertheless, there abounds substantial debate regarding which of these processes may only be sequelae of the original insult, and which may, in fact, be maladaptively compelling further degradation of the organism's competence to cope with its disrupted sensory context. In this review, we aim to examine how the mammalian auditory system responds in the wake of particular insults, and to disambiguate how the changes that develop might underlie a correlated class of phantom disorders, including tinnitus and hyperacusis, which putatively are brought about through maladaptive neuroplastic disruptions to auditory networks governing the spatial and temporal processing of acoustic sensory information.

Upregulation of insulin-like growth factor and interleukin 1β occurs in neurons but not in glial cells in the cochlear nucleus following cochlear ablation

One of the main mechanisms used by neurons and glial cells to promote repair following brain injury is to upregulate activity-dependent molecules such as insulin-like growth factor 1 (IGF-1) and interleukin-1β (IL-1β). In the auditory system, IGF-1 is crucial for restoring synaptic transmission following hearing loss; however, whether IL-1β is also involved in this process is unknown. In this study, we evaluated the expression of IGF-1 and IL-1β within neurons and glial cells of the ventral cochlear nucleus in adult rats at 1, 7, 15, and 30 days following bilateral cochlear ablation. After the lesion, significant increases in both the overall mean gray levels of IGF-1 immunostaining and the mean gray levels within cells of the cochlear nucleus were observed at 1, 7, and 15 days compared with control animals. The expression and distribution of IL-1β in the ventral cochlear nucleus of ablated animals was temporally and spatially correlated with IGF-1. We also observed a lack of colocalization between IGF-1 and IL-1β with either astrocytes or microglia at any of the time points following ablation. These results suggest that the upregulation of IGF-1 and IL-1β levels within neurons-but not within glial cells-may reflect a plastic mechanism involved in repairing synaptic homeostasis of the overall cellular environment of the cochlear nucleus following bilateral cochlear ablation.

Differential regulation of the expression of neurotrophin receptors in rat extraocular motoneurons after lesion

Neurotrophins acting through high-affinity tyrosine kinase receptors (trkA, trkB, and trkC) play a crucial role in regulating survival and maintenance of specific neuronal functions after injury. Adult motoneurons supplying extraocular muscles survive after disconnection from the target, but suffer dramatic changes in morphological and physiological properties, due in part to the loss of their trophic support from the muscle. To investigate the dependence of the adult rat extraocular motoneurons on neurotrophins, we examined trkA, trkB, and trkC mRNA expression after axotomy by in situ hybridization. trkA mRNA expression was detectable at low levels in unlesioned motoneurons, and its expression was downregulated 1 and 3 days after injury. Expression of trkB and trkC mRNAs was stronger, and after axotomy a simultaneous, but inverse regulation of both receptors was observed. Thus, whereas a considerable increase in trkB expression was seen about 2 weeks after axotomy, the expression of trkC mRNA had decreased at the same post-lesion period. Injured extraocular motoneurons also experienced an initial induction in expression of calcitonin gene-related peptide and a transient downregulation of cholinergic characteristics, indicating a switch in the phenotype from a transmitter-specific to a regenerative state. These results suggest that specific neurotrophins may contribute differentially to the survival and regenerative responses of extraocular motoneurons after lesion.

Impact of sound exposure and aging on brain-derived neurotrophic factor and tyrosine kinase B receptors levels in dorsal cochlear nucleus 80 days following sound exposure

Recent studies suggested that acute sound exposure resulting in a temporary threshold shift in young adult animals within a series of maladaptive plasticity changes in central auditory structures. Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, is involved in post-trauma peripheral hair cell and spiral ganglion cell survival and has been shown to modulate synaptic strength in cochlear nucleus following sound exposure. The present study evaluated levels of BDNF and its receptor (tyrosine kinase B, [TrkB]) in the dorsal cochlear nucleus (DCN) following a unilateral moderate sound exposure in young (7-8 months) and aged (28-29 months) Fischer Brown Norway (FBN) rats. Eighty days post-exposure, auditory brainstem response (ABR) thresholds for young exposed rats approached control values while aged exposed rats showed residual permanent threshold shifts (PTS) relative to aged controls. BDNF protein levels were significantly up-regulated by 9% in young exposed fusiform cells ipsilateral to the exposure. BDNF levels in aged sound-exposed fusiform cells increased 31% ipsilateral to the exposure. Protein levels of the BDNF receptor, TrkB, were also significantly increased in aged but not in young sound-exposed DCN fusiform cells. The present findings suggest a relationship between the up-regulation of BDNF/TrkB and the increase in spontaneous and driven activity previously observed for aged and sound-exposed fusiform cells. This might be due to a selective maladaptive compensatory down-regulation of glycinergic inhibition in DCN fusiform cells.

Differential impact of temporary and permanent noise-induced hearing loss on neuronal cell density in the mouse central auditory pathway

Although acoustic overstimulation has a major pathophysiological influence on the inner ear, central components of the auditory pathway can also be affected by noise-induced hearing loss (NIHL). The present study investigates the influence of a noise-induced temporary threshold shift (TTS) and/or permanent threshold shift (PTS) on neuronal cell densities in key structures of the central auditory pathway. Mice were noise-exposed (3 h, 5-20 kHz) at 115 dB sound pressure level (SPL) under anesthesia, and were investigated immediately (TTS group, n = 5) after the exposure, or 1 week later (PTS group, n = 6). Unexposed animals were used as controls (n = 7). Frequency-specific auditory brainstem responses (ABR) were recorded to examine auditory thresholds. Cell density was determined within the dorsal (DCN) and ventral (VCN) cochlear nucleus; the central nucleus of the inferior colliculus (ICC); the dorsal, ventral, and medial subdivisions of the medial geniculate body (MGBd, MGBv, and MGBm); and layer I to VI of the primary auditory cortex (AI I-VI). ABR thresholds were significantly elevated in the TTS group (52-69 dB SPL) and in the PTS group (33-42 dB SPL) compared to controls. There was a significant decrease in cell density only in the VCN of the TTS group (-10%), most likely induced by the acute overstimulation of neurons. Cell density was significantly reduced in all investigated auditory structures at 1 week post-exposure (PTS group), except in layer II of the AI (VCN: -30% and DCN: -30% (high-frequency); -39% (low-frequency); ICC: -31%; MGBd: -31%; MGBm: -28%; MGBv: -31%; AI: -10 to 14%). Thus there were dramatic changes within the neuronal cytoarchitecture of the central auditory pathway following a single noise exposure. The present findings should help clinicians to better understand the complex psychoacoustic phenomena of NIHL.

Maturation of auditory brainstem projections and calyces in the congenitally deaf (dn/dn) mouse

The deaf dn/dn mouse is a valuable model of human congenital deafness. In this study we used the lipophylic dye DiA to trace auditory nerve and cochlear nucleus projections in the dn/dn mouse. In both normal and deaf mice, the ipsilateral projections from the anteroventral cochlear nucleus (AVCN) to the lateral superior olive (LSO), and the contralateral projections from the AVCN to the medial nucleus of the trapezoid body (MNTB) were intact. With age, there was a noted increase in the fenestration of the endbulb and calyx of Held, and this morphological maturation was also observed in the deaf mice, although there was a significant difference in total endbulb volume at P20 between normal and deaf mice. However, total calyceal volume was not significantly different between normal and deaf mice. There was electrophysiological evidence of in vivo spontaneous ventral cochlear nucleus activity in normal and deaf animals, indicating that this activity may be responsible for the appropriate connectivity in the deaf mice. Our results indicate that congenital deafness caused by the dn/dn mutation does not result in aberrant projections between the AVCN and the ipsilateral MNTB and contralateral LSO but can cause abnormalities in endbulb size.

Cochlear ablation in adult ferrets results in changes in insulin-like growth factor-1 and synaptophysin immunostaining in the cochlear nucleus

Afferent activity modulates synaptic plasticity as well as the levels of activity-dependent molecules such as growth factors. Disruption of this activity due to deafferentation has been shown to result in an altered trophic support and consequently in changes in neuronal excitability and synaptic transmission. In the present study, to test whether lack of cochlear integrity results in changes in insulin-growth factor-1 (IGF-1) and synaptophysin immunostaining in the cochlear nucleus, the first relay structure in the auditory pathway, unilateral cochlear ablations were performed in adult ferrets. Changes in IGF-1 and synaptophysin immunostaining were assessed in the anteroventral (AVCN), posteroventral (PVCN) and dorsal cochlear nucleus (DCN) at 1, 20 and 90 days after deafferentation. An increase in IGF-1 immunostaining within AVCN, PVCN and DCN was observed ipsilaterally at all survival times after cochlear ablation when compared with the contralateral side and unoperated animals. This increase was accompanied by a significant ipsilateral increase in the mean gray level of synaptophysin immunostaining as well as a decrease in the area of synaptophysin immunostaining at 1 and 20 days after the ablation in AVCN, PVCN and DCN compared with the contralateral side and control animals. These changes in synaptophysin immunostaining were no longer present 90 days after cochlear ablation. The present results provide evidence of a persistent upregulation in IGF-1 and a transitory upregulation in synaptophysin levels in the cochlear nucleus that may reflect neuroprotective mechanisms following the loss of trophic support from spiral ganglion neurons.

Intensive remodeling of Purkinje cell spines after climbing fibers deafferentation does not involve MAPK and Akt activation

Subtotal lesion of the inferior olive (IO) achieved by treating experimental animals with 3-acetylpyridine (3AP) induces partial Purkinje cells (PCs) deafferentation that leads to PC hyperactivity and new spine formation. Coincidentally, the olivary terminals belonging to the few survived olivary neurons undergo an extensive collateral sprouting resulting in reinnervation of the neighboring denervated PCs. We obtained chemical deafferentation of PCs in adult rats (body weight, 120-170 g; age, 35-40 days) by a single intraperitoneal injection of 3AP (65 mg/kg body weight), and as early as 3 days after 3AP treatment, important morphological changes could be observed on PCs. Mitogen-activated protein kinase (MAPK) cascades and more specifically extracellular signal-regulated kinases 1/2 (ERK1/2) play a critical role in the signaling events underlying synaptic plasticity. For instance, long-term depression (LTD) in the adult hippocampus and long-term potentiation (LTP) in cerebellum both involve ERK activation. Since PCs deprived of their climbing fibers (CFs) afferents initiate an intensive remodeling of the spines and rapid recall of the remaining CFs, it prompted us to see whether the observed phenomena correlated with MAPK and Akt activation. Immunohistochemistry and Western blotting were done at various time points after 3AP application (from 24 h to 6 days), as the exact dynamics of CF loss is not precisely known. As judged by Western blotting, there was no increase of activated ERK in the cerebellum. However, immunohistochemistry revealed increased ERK phosphorylation in the "pinceaux" of basket cells in 3AP animals. Similarly, stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), p38 MAPK, and Akt activation were also studied by means of Western blotting and immunohistochemistry. Upon 3AP treatment no changes in phosphorylation status could be seen in the different kinases subjected to analysis. Our results suggest that activation of MAPK and Akt cascades is not essential in this model of neuronal plasticity.

Phosphorylated cAMP response element-binding protein levels in guinea pig brainstem auditory nuclei after unilateral cochlear ablation

After left unilateral cochlear ablation (UCA) in young adult guinea pigs, the appearance of plasticities in auditory pathways suggested altered gene expression and modified phenotypic behaviors of auditory neurons. Because phosphorylated cyclic-AMP response element-binding protein (CREB-P) is a transcription factor that binds to certain genes to facilitate their expression, CREB-P levels were measured after UCA and correlated with postablation plasticities. After UCA, Western blotting was employed to quantify CREB-P levels and illustrate CREB levels in the anteroventral (AVCN), posteroventral (PVCN), and dorsal (DCN) cochlear nucleus; the lateral (LSO) and medial superior olive (MSO); the medial nucleus of the trapezoid body (MNTB); and the central nucleus of the inferior colliculus (ICc) for up to 145 days. We also quantified the levels of several protein synthesis regulators and synaptic markers in the AVCN at 60 days. Sucrose-based extraction buffer improved CREB-P recovery. CREB-P levels became depressed at 3 and 7 postablation days, except in the PVCN, where they were elevated at 7 days, and in the ICc, where they were elevated at both times. At 60 days, CREB-P levels in all the nuclei were elevated. In the AVCN, levels of the protein synthesis regulators and synaptic markers were also elevated at 60 days. By 145 days, CREB-P levels again declined, except in the AVCN, where elevations persisted and increased on the ablated side, and in the ICc, where CREB-P elevations remained. The changes in CREB-P levels coincided with several plasticities in glutamatergic and glycinergic transmitter release and receptor activities, and alterations in neurotrophic support, that developed after UCA. These findings suggest that UCA altered CREB-P levels, which in turn might have contributed to plasticities that appear after UCA.