ERK and SAPK signaling in auditory brainstem neurons after unilateral cochlear ablation

Dynamic Changes in Synaptic Plasticity Genes in Ipsilateral and Contralateral Inferior Colliculus Following Unilateral Noise-induced Hearing Loss

Unilateral noise-induced hearing loss reduces the input to the central auditory pathway disrupting the excitatory and inhibitory inputs to the inferior colliculus (IC), an important binaural processing center. Little is known about the compensatory synaptic changes that occur in the IC as a consequence of unilateral noise-induced hearing loss. To address this issue, Sprague-Dawley rats underwent unilateral noise exposure resulting in severe unilateral hearing loss. IC tissues from the contralateral and ipsilateral IC were evaluated for acute (2-d) and chronic (28-d) changes in the expression of 84 synaptic plasticity genes on a PCR array. Arc and Egr1 genes were further visualized by in situ hybridization to validate the PCR results. None of the genes were upregulated, but many were downregulated post-exposure. At 2-d post-exposure, more than 75% of the genes were significantly downregulated in the contralateral IC, while only two were downregulated in the ipsilateral IC. Many of the downregulated genes were related to long-term depression, long-term potentiation, cell adhesion, immediate early genes, neural receptors and postsynaptic density. At 28-d post-exposure, the gene expression pattern was reversed with more than 85% of genes in the ipsilateral IC now downregulated. Most genes previously downregulated in the contralateral IC 2-d post-exposure had recovered; less than 15% remained downregulated. These time-dependent, asymmetric changes in synaptic plasticity gene expression could shed new light on the perceptual deficits associated with unilateral hearing loss and the dynamic structural and functional changes that occur in the IC days and months following unilateral noise-induced hearing loss.

Changes in the immunohistochemical localization of the glycine receptor in the superior olivary complex of adult circling mice

Circling mice is a mutant model of spontaneous deafness exhibiting degenerated spiral ganglion cells in the cochlea and loss of organ of Corti. The balance between glycinergic inhibition and glutamatergic excitation in the lateral superior olive (LSO) is essential for the detection of interaural level differences. Long term weakening of glycinergic synaptic inhibition in the LSO may lead to the downregulation of synaptic release of glycine in dorsal cochlear nucleus and downregulation of postsynaptic glycine receptor (GlyR) activity in the LSO, which may contribute to hearing loss. The present study utilized an immunohistochemical method to assess changes in GlyR immunoreactivity (IR) and the cell number in the superior olivary complex (SOC) of heterozygote (+/cir) and homozygote (cir/cir) circling mice. A significant decrease in the IR was observed in all nuclei of the SOC of homozygous mice. Loss of GlyR immunoreactive cells and a decrement in cell size was also observed in the homozygotes. A decrease in the GlyR IR in the neurons and neuropils, cell number and size of the cir/cir, may lead to profound changes in inhibitory transmission and the functional properties in the SOC nuclei. Therefore, the functional loss of inhibitory neurotransmitters in the brainstem may result in deafness of adult cir/cir mice.

Central plasticity and dysfunction elicited by aural deprivation in the critical period

The acoustic signal is crucial for animals to obtain information from the surrounding environment. Like other sensory modalities, the central auditory system undergoes adaptive changes (i.e., plasticity) during the developmental stage as well as other stages of life. Owing to its plasticity, auditory centers may be susceptible to various factors, such as medical intervention, variation in ambient acoustic signals and lesion of the peripheral hearing organ. There are critical periods during which auditory centers are vulnerable to abnormal experiences. Particularly in the early postnatal development period, aural inputs are essential for functional maturity of auditory centers. An aural deprivation model, which can be achieved by attenuating or blocking the peripheral acoustic afferent input to the auditory center, is ideal for investigating plastic changes of auditory centers. Generally, auditory plasticity includes structural and functional changes, some of which can be irreversible. Aural deprivation can distort tonotopic maps, disrupt the binaural integration, reorganize the neural network and change the synaptic transmission in the primary auditory cortex or at lower levels of the auditory system. The regulation of specific gene expression and the modified signal pathway may be the deep molecular mechanism of these plastic changes. By studying this model, researchers may explore the pathogenesis of hearing loss and reveal plastic changes of the auditory cortex, facilitating the therapeutic advancement in patients with severe hearing loss. After summarizing developmental features of auditory centers in auditory deprived animals and discussing changes of central auditory remodeling in hearing loss patients, we aim at stressing the significant of an early and well-designed auditory training program for the hearing rehabilitation.

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.

Forskolin induced increase in spontaneous activity of auditory brainstem neurons is comparable to acoustic stimulus evoked responses

Contemporary proposals for the pathophysiology of tinnitus due to cochlear damage underscore increased spontaneous activity of auditory brainstem neurons. One of the several consequences of the cochlear injury is the activation of the ERK pathway, suppression of phosphodiestase E activity, and putatively setting a long-term increase in intracellular levels of cyclic AMP at central auditory neurons. Local application of forskolin also increases intracellular cyclic AMP and spontaneous neural activity. We measured the effects of locally applied forskolin on spontaneous firing rate of isolated neurons in the peri-olivary region of the superior olive complex in anesthetized adult Long Evan rats. Forskolin induced increase in spontaneous neural activity was comparable to supra-threshold tone evoke neural responses. These results are viewed in context of hyperexcitability as a correlate of tinnitus.

Damage-induced activation of ERK1/2 in cochlear supporting cells is a hair cell death-promoting signal that depends on extracellular ATP and calcium

Acoustic overstimulation and ototoxic drugs can cause permanent hearing loss as a result of the damage and death of cochlear hair cells. Relatively little is known about the signaling pathways triggered by such trauma, although a significant role has been described for the c-Jun N-terminal kinase [one of the mitogen-activated protein kinases (MAPKs)] pathway. We investigated the role of another MAPK family, the extracellularly regulated kinases 1 and 2 (ERK1/2) during hair cell damage in neonatal cochlear explants. Within minutes of subjecting explants to mechanical damage, ERK1/2 were transiently activated in Deiters' and phalangeal cells but not in hair cells. The activation of ERK1/2 spread along the length of the cochlea, reaching its peak 5-10 min after damage onset. Release of extracellular ATP and the presence of functional connexin proteins were critical for the activation and spread of ERK1/2. Damage elicited an intercellular Ca(2+) wave in the hair cell region in the first seconds after damage. In the absence of Ca(2+) influx, the intercellular Ca(2+) wave and the magnitude and spread of ERK1/2 activation were reduced. Treatment with the aminoglycoside neomycin produced a similar pattern of ERK1/2 activation in supporting cells surrounding pyknotic hair cells. When ERK1/2 activation was prevented, there was a reduction in the number of pyknotic hair cells. Thus, activation of ERK1/2 in cochlear supporting cells in vitro is a common damage signaling mechanism that acts to promote hair cell death, indicating a direct role for supporting cells in regulating hair cell death.

Consequences of unilateral hearing loss: time dependent regulation of protein synthesis in auditory brainstem nuclei

Conductive hearing impairment results in marked changes in neuronal activity in the central auditory system, particularly in young animals [Tucci, D.L., Cant, N.B., Durham, D., 1999. Conductive hearing loss results in a decrease in central auditory system activity in the young gerbil. Laryngoscope 109, 1359-1371]. To better understand the effects of conductive hearing loss (CHL) on cellular metabolism, incorporation of (3)H-leucine was used as a measure of protein synthesis in immature postnatal day 21 gerbils subjected to either unilateral CHL by malleus removal or profound sensorineural hearing loss by cochlear ablation. (3)H-leucine uptake was measured after survival times of 6 or 48h. Protein synthesis values were standardized to measurements from the abducens nucleus and compared with measurements from sham animals at similar age/survival times. Protein synthesis in the medial superior olive (MSO) was found to be significantly down-regulated (bilaterally) after CHL in animals surviving 48h. However, 6h after CHL manipulation, protein synthesis is up-regulated in MSO (bilaterally) and in the ipsilateral medial nucleus of the trapezoid body.

Glucocorticoid receptors modulate auditory sensitivity to acoustic trauma

Glucocorticoids are widely used to treat different hearing disorders yet the exact mechanisms of glucocorticoid action on the inner ear are not known. The inner ear of both humans and experimental animals demonstrate an abundance of glucocorticoid receptors (GRs) in both neuronal and non-neuronal tissues. In this review, we discuss how activation of the hypothalamic-pituitary-adrenal axis can directly modulate hearing sensitivity. Recent findings indicate that several factors define the responsiveness of the peripheral auditory system to glucocorticoids including the concentration of agonist, availability of the GR, and the activation of GR and NF-kappaB. These findings will further our understanding of individual glucocorticoid responsiveness to steroid treatment, and will help improve the development of pharmaceuticals to selectively target GR in the inner ear for individuals with increased sensitivity to acoustic trauma.

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.

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.

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.

Excitability of auditory brainstem neurons, in vivo, is increased by cyclic-AMP

Physiological control of auditory neural responses is critical for accurate representation of acoustic information, such as sound source localization and speech perception. Central auditory neural responses are almost certainly regulated by a range of mechanisms, including second messenger systems, such as the cAMP pathway. An increase in spontaneous neural discharge is known to accompany cochlear insults. Here we report that an increase in spontaneous as well as tone-evoked discharge can also be induced by pressure application of forskolin, a pharmacological agent that elevates intracellular cAMP level by activating adenyl cyclase. The forskolin induced increase in superior olivary complex (SOC) brainstem neurons is specific, dose-dependent, and reversible, whereas application of artificial cerebrospinal fluid (aCSF, the vehicle) does not alter activity. Forskolin-application also has a relatively greater effect on spontaneous activity compared to tone evoked responses. Blockade of the hyperpolarization-activated current, Ih, by ZD7288, consistently reversed the effects of forskolin. Based on these findings, we propose that the second messenger, cAMP, can significantly modulate neural excitability and spontaneous discharge in SOC neurons, principally by shifting the activation of Ih channels.

Tinnitus as a plastic phenomenon and its possible neural underpinnings in the dorsal cochlear nucleus

Tinnitus displays many features suggestive of plastic changes in the nervous system. These can be categorized based on the types of manipulations that induce them. We have categorized the various forms of plasticity that characterize tinnitus and searched for their neural underpinnings in the dorsal cochlear nucleus (DCN). This structure has been implicated as a possible site for the generation of tinnitus-producing signals owing to its tendency to become hyperactive following exposure to tinnitus inducing agents such as intense sound and cisplatin. In this paper, we review the many forms of plasticity that have been uncovered in anatomical, physiological and neurochemical studies of the DCN. Some of these plastic changes have been observed as consequences of peripheral injury or as fluctuations in the behavior and chemical activities of DCN neurons, while others can be induced by stimulation of auditory or even non-auditory structures. We show that many parallels can be drawn between the various forms of plasticity displayed by tinnitus and the various forms of neural plasticity which have been defined in the DCN. These parallels lend further support to the hypothesis that the DCN is an important site for the generation and modulation of tinnitus-producing signals.

Reconnecting neuronal networks in the auditory brainstem following unilateral deafening

When we disturbed the auditory input of the adult rat by cochleotomy or noise trauma on one side, several substantial anatomical, cellular, and molecular changes took place in the auditory brainstem. We found that: (1) cochleotomy or severe noise trauma both lead to a considerable increase of immunoreactivity of the growth-associated protein GAP-43 in the ventral cochlear nucleus (VCN) of the affected side; (2) the expression of GAP-43 in VCN is restricted to presynaptic endings and short fiber segments; (3) axon collaterals of the cholinergic medial olivocochlear (MOC) neurons are the path along which GAP-43 reaches VCN; (4) partial cochlear lesions induce the emergence of GAP-43 positive presynaptic endings only in regions tonotopically corresponding to the extent of the lesion; (5) judging from the presence of immature fibers and growth cones in VCN on the deafened side, at least part of the GAP-43 positive presynaptic endings appear to be newly formed neuronal contacts following axonal sprouting while others may be modified pre-existing contacts; and (6) GAP-43 positive synapses are formed only on specific postsynaptic profiles, i.e., glutamatergic, glycinergic and calretinin containing cell bodies, but not GABAergic cell bodies. We conclude that unilateral deafening, be it partial or total, induces complex patterns of reconnecting neurons in the adult auditory brainstem, and we evaluate the possibility that the deafness-induced chain of events is optimized to remedy the loss of a bilaterally balanced activity in the auditory brainstem.

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

Injury to areas of the central nervous system can alter neurotrophin levels, which may influence postlesion neuronal survival and plasticity. To determine if sensorineural hearing loss induces such changes, we used an enzyme-linked immunosorbent assay (ELISA) to measure neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) levels in adult guinea pig brain stem auditory nuclei 3-60 days after a unilateral cochlear ablation (UCA). After UCA, which destroyed the cochlea and cochlear nerve on one side, NT-3 levels were usually depressed at 3 days by 22-44% but became elevated transiently at 7 days by 28-124%. BDNF levels were elevated transiently by 50% on the ablated side in the anteroventral (AVCN) and posteroventral (PVCN) cochlear nucleus at 3 days and may have signaled support for the survival of deafferented neurons. Coincident elevation at 3 and 7 days of BDNF or NT-3 and phosphorylated extracellular signal-regulated protein kinase 2 (ERK2-P) suggested a relationship to stimulated signal transduction activity. Elevated neurotrophin levels may have contributed to synaptogenesis in the AVCN and the superior olive and to changes in the synaptic biochemistry in the auditory nuclei after UCA. In contrast, deficiencies or failure to elevate neurotrophin levels within several days of the UCA correlated with upregulation of phosphorylated stress-activated protein kinase (SAPK-P), suggesting a relationship with stress-activated signal transduction and with the sparse degeneration of fibers observed in some of the auditory nuclei after UCA.

Protein kinase A and calcium/calmodulin-dependent protein kinase II regulate glycine and GABA release in auditory brain stem nuclei

We reported previously that unilateral cochlear ablation (UCA) in young adult guinea pigs induced protein kinase C (PKC)-dependent plastic changes in the electrically evoked release of exogenous [14C]glycine ([14C]Gly) or [14C]-gamma-aminobutyric acid ([14C]GABA) in several brain stem auditory nuclei. The present study assessed whether such changes depended on protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII). In the major subdivisions of the cochlear nucleus (CN) and the main nuclei of the superior olivary complex (SOC) dissected from intact animals, dibutyryl-cyclic adenosine monophosphate (DBcAMP) (0.2 mM), a PKA activator, elevated release by 1.6-2.3-fold. The PKA inhibitor, H-89 (2 microM), did not alter the release but blocked the stimulatory effects of DBcAMP. These findings suggested that PKA could positively regulate glycinergic and GABAergic release. After UCA, PKA regulation declined and failed in the ventral CN but persisted in the SOC nuclei. After 145 postablation days, H-89 reversed elevations of [14C]GABA release in the medial nucleus of the trapezoid body (MNTB). A CaMKII inhibitor, KN-93, reversed depressions of [14C]Gly release in the DCN. Thus, the postablation plasticities in these nuclei probably depended on PKA or CaMKII. Both H-89 and KN-93 depressed [14C]Gly release in the lateral superior olive (LSO) and ipsilateral medial superior olive (MSO), suggesting that either kinase was used by endogenous mechanisms in these nuclei to upregulate glycinergic release. In contrast, KN-93 elevated [14C]GABA release in the contralateral MNTB, suggesting a downregulatory action of CaMKII, an action opposite to that of PKA.

Quantitative study of degeneration and new growth of axons and synaptic endings in the chinchilla cochlear nucleus after acoustic overstimulation

To determine if acoustic overstimulation altered synaptic connections in the cochlear nucleus, anesthetized adult chinchillas, with one ear protected by a silicone plug, were exposed for 3 hr to a 108-dB octave-band noise, centered at 4 kHz, and allowed to survive for periods up to 32 weeks. This exposure led to cochlear damage in the unprotected ear, mainly in the basal regions of the organ of Corti. The anterior part of the ipsilateral posteroventral cochlear nucleus consistently contained a band of degenerating axons and terminals, in which electron microscopic analysis revealed substantial losses of axons and synaptic terminals with excitatory and inhibitory cytology. The losses were significant after 1 week's survival and progressed for 16-24 weeks after exposure. By 24-32 weeks, a new growth of these structures produced a resurgence in the number of axons and terminals. The net number of excitatory endings fully recovered, but the quantity with inhibitory cytology was only partially recouped. Neuronal somata lost both excitatory and inhibitory endings at first and later recovered a full complement of excitatory but not inhibitory terminals. Dendrites suffered a net loss of both excitatory and inhibitory endings. Excitatory and inhibitory terminals with unidentified postsynaptic targets in the neuropil declined, then increased in number, with excitatory terminals exhibiting a greater recovery. These findings are consistent with a loss and regrowth of synaptic endings and with a reorganization of synaptic connections that favors excitation.

Protein kinase A and calcium/calmodulin-dependent protein kinase II regulate D-[3H]aspartate release in auditory brain stem nuclei

We noted previously that after unilateral cochlear ablation (UCA) in young adult guinea pigs, plastic changes in glutamatergic transmitter release in several brain stem auditory nuclei depended on protein kinase C. In this study, we assessed whether such changes depended on protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII). The electrically-evoked release of D-[3H]aspartate (D-[3H]Asp) was quantified in vitro as an index of glutamatergic transmitter release in the major subdivisions of the cochlear nucleus (CN) and the main nuclei of the superior olivary complex (SOC). In tissues from intact animals, dibutyryl-cyclic adenosine monophosphate (DBcAMP), a PKA activator, elevated D-[3H]Asp release by 1.9-3.7-fold. The PKA inhibitor, H-89 (2 microM), did not alter the evoked release but blocked the stimulatory effects of DBcAMP. These findings suggested that PKA could positively regulate glutamatergic transmitter release. Seven days after the ablation of one cochlea and its cochlear nerve, the stimulatory effect of DBcAMP remained evident. After 145 postablation days, H-89 blocked the plastic elevations of D-[3H]Asp release in the ipsilateral CN and lateral (LSO) and medial (MSO) superior olive. A CaMKII inhibitor, KN-93, produced similar blocks, suggesting that the postablation plasticities in these nuclei depended on PKA or CaMKII. Both H-89 and KN-93 elevated release in the medial nucleus of the trapezoid body (MNTB) and the contralateral MSO, suggesting that either kinase could be used by endogenous mechanisms in these nuclei to downregulate glutamatergic release.