Expression of NMDA, AMPA and GABA(A) receptor subunit mRNAs in the rat auditory brainstem. I. Influence of early auditory deprivation

The effect of maternal diabetes on the expression of gamma-aminobutyric acid and metabotropic glutamate receptors in male newborn rats' inferior colliculi

OBJECTIVES

Few studies have examined the molecular alterations in the auditory pathway of infants of diabetic mothers, notwithstanding the fact that maternal diabetes may have an impact on the development of the neonatal peripheral and central nervous systems. Male newborn rats were studied to determine how maternal diabetes affected the expression of gamma-aminobutyric acid (GABAAα1 and GABAB1) and metabotropic glutamate (mGlu2) receptors in the inferior colliculus (IC) in this research.

METHODS

Female rats were given a single intraperitoneal injection of streptozotocin (STZ) at a 65 mg/kg dose to develop a model of diabetic mothers. The study population was split into sham, diabetes without treatment, and diabetes with insulin groups. Their male neonatal rats were anesthetized on P0, P7, and P14 after mating and delivery. The receptors' distribution pattern was studied using immunohistochemistry (IHC).

RESULTS

Pairwise comparison in the groups revealed that the GABA receptors (Aα1 and B1) were significantly downregulated in the diabetes without treatment group (p<0.001). Furthermore, pairwise comparison in the groups indicated significant mGlu2 upregulation in the diabetes without treatment group (p<0.001). Regarding the concentration of all receptors, there was no discernible distinction between the diabetes with insulin and sham groups.

CONCLUSIONS

This investigation showed that the concentration of GABAAα1 and GABAB1 receptors decreased significantly over time, whereas the concentration of mGlu2 receptors increased significantly over time in male neonatal rats born to streptozotocin-induced diabetic mothers.

Lifelong changes of neurotransmitter receptor expression and debilitation of hippocampal synaptic plasticity following early postnatal blindness

In the weeks immediately after onset of sensory loss, extensive reorganization of both the cortex and hippocampus occurs. Two fundamental characteristics comprise widespread changes in the relative expression of GABA and glutamate receptors and debilitation of hippocampal synaptic plasticity. Here, we explored whether recovery from adaptive changes in the expression of plasticity-related neurotransmitter receptors and hippocampal synaptic plasticity occurs in the time-period of up to 12 months after onset of sensory loss. We compared receptor expression in CBA/J mice that develop hereditary blindness, with CBA/CaOlaHsd mice that have intact vision and no deficits in other sensory modalities throughout adulthood. GluN1-subunit expression was reduced and the GluN2A:GluN2B ratio was persistently altered in cortex and hippocampus. GABA-receptor expression was decreased and metabotropic glutamate receptor expression was altered. Hippocampal synaptic plasticity was persistently compromised in vivo. But although LTP in blind mice was chronically impaired throughout adulthood, a recovery of the early phase of LTP became apparent when the animals reached 12 months of age. These data show that cortical and hippocampal adaptation to early postnatal blindness progresses into advanced adulthood and is a process that compromises hippocampal function. A partial recovery of hippocampal synaptic plasticity emerges in advanced adulthood, however.

Area-dependent change of response in the rat's inferior colliculus to intracochlear electrical stimulation following neonatal cochlear damage

To understand brain changes caused by auditory sensory deprivation, we recorded local-field potentials in the inferior colliculus of young adult rats with neonatal cochlear damage produced by systemic injections of amikacin. The responses were elicited by electrical stimulation of the entire cochlea and recorded at various locations along a dorsolateral-ventromedial axis of the inferior colliculus. We found that hair cells were completely destroyed and spiral ganglion neurons were severely damaged in the basal cochleae of amikacin-treated animals. Hair cells as well as spiral ganglion neurons were damaged also in the middle and apical areas of the cochlea, with the damage being greater in the middle than the apical area. Amplitudes of local-field potentials were reduced in the ventromedial inferior colliculus, but enhanced in the dorsolateral inferior colliculus. Latencies of responses were increased over the entire structure. The enhancement of responses in the dorsolateral inferior colliculus was in contrast with the damage of hair cells and spiral ganglion cells in the apical part of the cochlea. This contrast along with the overall increase of latencies suggests that early cochlear damage can alter neural mechanisms within the inferior colliculus and/or the inputs to this midbrain structure.

Auditory midbrain coding of statistical learning that results from discontinuous sensory stimulation

Detecting regular patterns in the environment, a process known as statistical learning, is essential for survival. Neuronal adaptation is a key mechanism in the detection of patterns that are continuously repeated across short (seconds to minutes) temporal windows. Here, we found in mice that a subcortical structure in the auditory midbrain was sensitive to patterns that were repeated discontinuously, in a temporally sparse manner, across windows of minutes to hours. Using a combination of behavioral, electrophysiological, and molecular approaches, we found changes in neuronal response gain that varied in mechanism with the degree of sound predictability and resulted in changes in frequency coding. Analysis of population activity (structural tuning) revealed an increase in frequency classification accuracy in the context of increased overlap in responses across frequencies. The increase in accuracy and overlap was paralleled at the behavioral level in an increase in generalization in the absence of diminished discrimination. Gain modulation was accompanied by changes in gene and protein expression, indicative of long-term plasticity. Physiological changes were largely independent of corticofugal feedback, and no changes were seen in upstream cochlear nucleus responses, suggesting a key role of the auditory midbrain in sensory gating. Subsequent behavior demonstrated learning of predictable and random patterns and their importance in auditory conditioning. Using longer timescales than previously explored, the combined data show that the auditory midbrain codes statistical learning of temporally sparse patterns, a process that is critical for the detection of relevant stimuli in the constant soundscape that the animal navigates through.

Some things are learned simply because they are there and not because they are relevant at that moment in time. This is particularly true of surrounding sounds, which we process automatically and continuously, detecting their repetitive patterns or singularities. Learning about rewards and punishment is typically attributed to cortical structures in the brain and known to occur over long time windows. Learning of surrounding regularities, on the other hand, is attributed to subcortical structures and has been shown to occur in seconds. The brain can, however, also detect the regularity in sounds that are discontinuously repeated across intervals of minutes and hours. For example, we learn to identify people by the sound of their steps through an unconscious process involving repeated but isolated exposures to the coappearance of sound and person. Here, we show that a subcortical structure, the auditory midbrain, can code such temporally spread regularities. Neurons in the auditory midbrain changed their response pattern in mice that heard a fixed tone whenever they went into one room in the environment they lived in. Learning of temporally spread sound patterns can, therefore, occur in subcortical structures.

Tinnitus: Maladaptive auditory-somatosensory plasticity

Tinnitus, the phantom perception of sound, is physiologically characterized by an increase in spontaneous neural activity in the central auditory system. However, as tinnitus is often associated with hearing impairment, it is unclear how a decrease of afferent drive can result in central hyperactivity. In this review, we first assess methods for tinnitus induction and objective measures of the tinnitus percept in animal models. From animal studies, we discuss evidence that tinnitus originates in the cochlear nucleus (CN), and hypothesize mechanisms whereby hyperactivity may develop in the CN after peripheral auditory nerve damage. We elaborate how this process is likely mediated by plasticity of auditory-somatosensory integration in the CN: the circuitry in normal circumstances maintains a balance of auditory and somatosensory activities, and loss of auditory inputs alters the balance of auditory somatosensory integration in a stimulus timing dependent manner, which propels the circuit towards hyperactivity. Understanding the mechanisms underlying tinnitus generation is essential for its prevention and treatment. This article is part of a Special Issue entitled <Tinnitus>.

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.

Stimulus-timing-dependent modifications of rate-level functions in animals with and without tinnitus

Tinnitus has been associated with enhanced central gain manifested by increased spontaneous activity and sound-evoked firing rates of principal neurons at various stations of the auditory pathway. Yet, the mechanisms leading to these modifications are not well understood. In a recent in vivo study, we demonstrated that stimulus-timing-dependent bimodal plasticity mediates modifications of spontaneous and tone-evoked responses of fusiform cells in the dorsal cochlear nucleus (DCN) of the guinea pig. Fusiform cells from sham animals showed primarily Hebbian learning rules while noise-exposed animals showed primarily anti-Hebbian rules, with broadened profiles for the animals with behaviorally verified tinnitus (Koehler SD, Shore SE. J Neurosci 33: 19647-19656, 2013a). In the present study we show that well-timed bimodal stimulation induces alterations in the rate-level functions (RLFs) of fusiform cells. The RLF gains and maximum amplitudes show Hebbian modifications in sham and no-tinnitus animals but anti-Hebbian modifications in noise-exposed animals with evidence for tinnitus. These findings suggest that stimulus-timing bimodal plasticity produced by the DCN circuitry is a contributing mechanism to enhanced central gain associated with tinnitus.

Acoustical stimulus changes the expression of stromal cell-derived factor-1 in the spiral ganglion neurons of the rat cochlea

Neural stem cell (NSC) transplantation into the cochlea has been tested as a treatment for spiral ganglion neuron (SGN) degenerative disease and injury in various animal models. A recent study has shown evidence of functional recovery after transplantation of the stem cells into a degenerated-SGN model. Chemokine stromal cell-derived factor-1 (SDF-1, or known as CXC chemokine ligand-12, CXCL-12) signaling through CXCR4 has previously been identified as a key step in the homing of the stem cells within the injury areas; meanwhile, studies have revealed that the SDF-1/CXCR4 axis is also involved in axon guidance and pathfinding. A study found that transplanted neural precursor cells can migrate to the root of the auditory nerve when animals are subjected to an augmented acoustic environment (AAE). In accordance with these studies, we hypothesize that AAE will up-regulate the expression of SDF-1 in acoustic nerves. We tested our hypothesis by examining the expression of SDF-1 in different acoustic environments, and the results were confirmed by the auditory brainstem response (ABR), immunohistochemical and RT-PCR analyses. The results showed that SDF-1 was expressed at a relatively low level in the SGNs under normal animal unit acoustic conditions (40-50 dB). Moreover, it was significantly up-regulated in the SGNs under the 75 dB (augmented physiological process without hearing loss) and 90 dB AAE (pathological process with light hearing loss) conditions; however, under the 115 dB AAE (pathological process with severe hearing loss) condition, the expression of SDF-1 was not up-regulated. The results confirmed that appropriately augmented acoustical stimuli lead to the up-regulation of SDF-1, which may assist in the migration of the transplanted cells and the subsequent establishment of essential synaptic contacts between the exogenous cells and the host auditory pathway.

Effects of early and late treatment with L-baclofen on the development and maintenance of tinnitus caused by acoustic trauma in rats

Subjective tinnitus is a chronic neurological disorder in which phantom sounds are perceived. Recent evidence supports the hypothesis that tinnitus is related to neuronal hyperactivity in auditory brain regions, and consequently drugs that increase GABAergic neurotransmission in the CNS, such as the GABA(B) receptor agonist L-baclofen, may be effective as a treatment. The aim of this study was to investigate the effects of early (5 mg/kg s.c., 30 min and then every 24 h for 5 days following noise exposure) and late treatment (3 mg/kg/day s.c. for 4.5 weeks starting at 17.5 weeks following noise exposure) with l-baclofen on the psychophysical attributes of tinnitus in a conditioned lick suppression model following acoustic trauma in rats. Acoustic trauma (a 16-kHz, 115-dB pure tone presented unilaterally for 1h) resulted in a significant decrease in the suppression ratio (SR) compared to sham controls in response to 20-kHz tones at 2, 10 and 17.5 weeks post-exposure (P ≤ 0.009, P ≤ 0.02 and P ≤ 0.03, respectively). However, l-baclofen failed to prevent the development of tinnitus when administered during the first 5 days following the acoustic trauma and also failed to reverse it when treatment was carried out every day for 4.5 weeks. We also found that treatment with L-baclofen did not alter the expression of the GABA(B)-R2 subunit in the cochlear nucleus of noise-exposed animals.

Biological impact of preschool music classes on processing speech in noise

Musicians have increased resilience to the effects of noise on speech perception and its neural underpinnings. We do not know, however, how early in life these enhancements arise. We compared auditory brainstem responses to speech in noise in 32 preschool children, half of whom were engaged in music training. Thirteen children returned for testing one year later, permitting the first longitudinal assessment of subcortical auditory function with music training. Results indicate emerging neural enhancements in musically trained preschoolers for processing speech in noise. Longitudinal outcomes reveal that children enrolled in music classes experience further increased neural resilience to background noise following one year of continued training compared to nonmusician peers. Together, these data reveal enhanced development of neural mechanisms undergirding speech-in-noise perception in preschoolers undergoing music training and may indicate a biological impact of music training on auditory function during early childhood.

The level and distribution of the GABA(B)R1 and GABA(B)R2 receptor subunits in the rat's inferior colliculus

The type B γ-aminobutyric acid receptor (GABA(B) receptor) is an important neurotransmitter receptor in the midbrain auditory structure, the inferior colliculus (IC). A functional GABA(B) receptor is a heterodimer consisting of two subunits, GABA(B)R1 and GABA(B)R2. Western blotting and immunohistochemical experiments were conducted to examine the expression of the two subunits over the IC including its central nucleus, dorsal cortex, and external cortex (ICc, ICd, and ICx). Results revealed that the two subunits existed in both cell bodies and the neuropil throughout the IC. The two subunits had similar regional distributions over the IC. The combined level of cell body and neuropil labeling was higher in the ICd than the other two subdivisions. Labeling in the ICc and ICx was stronger in the dorsal than the ventral regions. In spite of regional differences, no defined boundaries were formed between different areas. For both subunits, the regional distribution of immunoreactivity in the neuropil was parallel to that of combined immunoreactivity in the neuropil and cell bodies. The density of labeled cell bodies tended to be higher but sizes of cell bodies tended to be smaller in the ICd than in the other subdivisions. No systematic regional changes were found in the level of cell body immunoreactivity, except that GABA(B)R2-immunoreactive cell bodies in the ICd had slightly higher optic density (OD) than in other regions. Elongated cell bodies existed throughout the IC. Many labeled cell bodies along the outline of the IC were oriented in parallel to the outline. No strong tendency of orientation was found in labeled cell bodies in ICc. Regional distributions of the subunits in ICc correlated well with inputs to this subdivision. Our finding regarding the contrast in the level of neuropil immunoreactivity among different subdivisions is consistent with the fact that the GABA(B) receptor has different pre- and postsynaptic functions in different IC regions.

A computational model of inferior colliculus responses to amplitude modulated sounds in young and aged rats

The inferior colliculus (IC) receives ascending excitatory and inhibitory inputs from multiple sources, but how these auditory inputs converge to generate IC spike patterns is poorly understood. Simulating patterns of in vivo spike train data from cellular and synaptic models creates a powerful framework to identify factors that contribute to changes in IC responses, such as those resulting in age-related loss of temporal processing. A conductance-based single neuron IC model was constructed, and its responses were compared to those observed during in vivo IC recordings in rats. IC spike patterns were evoked using amplitude-modulated tone or noise carriers at 20–40 dB above threshold and were classified as low-pass, band-pass, band-reject, all-pass, or complex based on their rate modulation transfer function tuning shape. Their temporal modulation transfer functions were also measured. These spike patterns provided experimental measures of rate, vector strength, and firing pattern for comparison with model outputs. Patterns of excitatory and inhibitory synaptic convergence to IC neurons were based on anatomical studies and generalized input tuning for modulation frequency. Responses of modeled ascending inputs were derived from experimental data from previous studies. Adapting and sustained IC intrinsic models were created, with adaptation created via calcium-activated potassium currents. Short-term synaptic plasticity was incorporated into the model in the form of synaptic depression, which was shown to have a substantial effect on the magnitude and time course of the IC response. The most commonly observed IC response sub-types were recreated and enabled dissociation of inherited response properties from those that were generated in IC. Furthermore, the model was used to make predictions about the consequences of reduction in inhibition for age-related loss of temporal processing due to a reduction in GABA seen anatomically with age.

Tracking the expression of excitatory and inhibitory neurotransmission-related proteins and neuroplasticity markers after noise induced hearing loss

Excessive exposure to loud noise can damage the cochlea and create a hearing loss. These pathologies coincide with a range of CNS changes including reorganisation of frequency representation, alterations in the pattern of spontaneous activity and changed expression of excitatory and inhibitory neurotransmitters. Moreover, damage to the cochlea is often accompanied by acoustic disorders such as hyperacusis and tinnitus, suggesting that one or more of these neuronal changes may be involved in these disorders, although the mechanisms remain unknown. We tested the hypothesis that excessive noise exposure increases expression of markers of excitation and plasticity, and decreases expression of inhibitory markers over a 32-day recovery period. Adult rats (n = 25) were monaurally exposed to a loud noise (16 kHz, 1/10(th) octave band pass (115 dB SPL)) for 1-hour, or left as non-exposed controls (n = 5). Animals were euthanased at either 0, 4, 8, 16 or 32 days following acoustic trauma. We used Western Blots to quantify protein levels of GABA(A) receptor subunit α1 (GABA(A)α1), Glutamic-Acid Decarboxylase-67 (GAD-67), N-Methyl-D-Aspartate receptor subunit 2A (NR2A), Calbindin (Calb1) and Growth Associated Protein 43 (GAP-43) in the Auditory Cortex (AC), Inferior Colliculus (IC) and Dorsal Cochlear Nucleus (DCN). Compared to sham-exposed controls, noise-exposed animals had significantly (p<0.05): lower levels of GABA(A)α1 in the contralateral AC at day-16 and day-32, lower levels of GAD-67 in the ipsilateral DCN at day-4, lower levels of Calb1 in the ipsilateral DCN at day-0, lower levels of GABA(A)α1 in the ipsilateral AC at day-4 and day-32. GAP-43 was reduced in the ipsilateral AC for the duration of the experiment. These complex fluctuations in protein expression suggests that for at least a month following acoustic trauma the auditory system is adapting to a new pattern of sensory input.

Short-term plasticity and auditory processing in the ventral cochlear nucleus of normal and hearing-impaired animals

The dynamics of synaptic transmission between neurons plays a major role in neural information processing. In the cochlear nucleus, auditory nerve synapses have a relatively high release probability and show pronounced synaptic depression that, in conjunction with the variability of interspike intervals, shapes the information transmitted to the postsynaptic cells. Cellular mechanisms have been best analyzed at the endbulb synapses, revealing that the recent history of presynaptic activity plays a complex, non-linear, role in regulating release. Emerging evidence suggests that the dynamics of synaptic function differs according to the target neuron within the cochlear nucleus. One consequence of hearing loss is changes in evoked release at surviving auditory nerve synapses, and in some situations spontaneous release is greatly enhanced. In contrast, even with cochlear ablation, postsynaptic excitability is less affected. The existing evidence suggests that different modes of hearing loss can result in different dynamic patterns of synaptic transmission between the auditory nerve and postsynaptic neurons. These changes in dynamics in turn will affect the efficacy with which different kinds of information about the acoustic environment can be processed by the parallel pathways in the cochlear nucleus.

Early auditory experience-induced composition/ratio changes of N-methyl-D-aspartate receptor subunit expression and effects of D-2-amino-5-phosphonovaleric acid chronic blockade in rat auditory cortex

Auditory function can be affected by many factors, including environment and experience. In this study, we investigated whether early auditory experience mediates the regulation of the composition/ratio changes of the N-methyl-D-aspartic acid (NMDA) receptor subunits during development of the rat auditory cortex. We found that early sound exposure can increase expression of the NMDA receptor subunits and increase the composition/ratios of NMDA receptor subunits during the postnatal critical period. D-2-amino-5-phosphonovaleric acid (D-APV) could block and reverse the auditory experience-mediated changes, and there were marked reductions in expression levels and the composition/ratios of NMDA receptor subunits. These results indicate that the experience-dependent plasticity of the auditory cortex in the critical period during postnatal development has a marked influence on NMDA receptor expression in the rat and that changes in NMDA receptor subunit composition/ratios might mediate the early auditory experience-dependent plasticity crucial to auditory function.

Stimulation by cochlear implant in unilaterally deaf rats reverses the decrease of inhibitory transmission in the inferior colliculus

In the last decade, numerous studies have investigated synaptic transmission changes in various auditory nuclei after unilateral cochlear injury. However, few data are available concerning the potential effect of electrical stimulation of the deafferented auditory nerve on the inhibitory neurotransmission in these nuclei. We report here for the first time the effect of chronic electrical stimulation of the deafferented auditory nerve on alpha1 subunit of the glycinergic receptor (GlyRalpha1) and glutamic acid decarboxylase (GAD)67 expression in the central nucleus of inferior colliculus (CIC). Adult rats were unilaterally cochleectomized by intracochlear neomycin sulphate injection. Fifteen days later, the ipsilateral auditory nerve was chronically stimulated either 4, 8 or 22 h daily, for 5 days using intracochlear bipolar electrodes. GlyRalpha1 and GAD67 mRNA and protein were quantified in the CIC using in situ hybridization and immunohistofluorescence methods. Our data showed that as after surgical ablation, GlyRalpha1 and GAD67 expression were strongly decreased in the contralateral CIC after unilateral chemical cochleectomy. Most importantly, these postlesional down-modulations were significantly reversed by chronic electrical stimulation of the deafferented auditory nerve. This recovery, however, did not persist for more than 5 days after the cessation of the deafferented auditory nerve electrical stimulation. Thus, downregulations of GlyRalpha1 and GAD67 may be involved both in the increased excitability observed in the CIC after unilateral deafness and consequently in the tinnitus frequently observed in unilateral adult deaf patients. Electrical stimulation of the deafferented auditory nerve in patients may be a potential new approach for treating tinnitus with unilateral hearing loss.

Early auditory deprivation alters expression of NMDA receptor subunit NR1 mRNA in the rat auditory cortex

The expression of NMDA receptor NR1 subunit mRNA was studied in rat auditory cortex (AC) on different postnatal days using digoxigenin-labeled oligonucleotide probes. The results showed that NR1 expression increased from birth to postnatal day 35 (P35) and remained constant until P56. The most significant increases occurred between P7 and P14. Changes in NR1 mRNA expression in rats subjected to monaural hearing deprivation on P7, P21, P35, and P49 were examined on P56. Between P7 and P21, when the rat auditory system was still in a critical period of development, NR1 mRNA expression was lower in the contralateral AC, which received auditory signals from the plugged ear, than in the ipsilateral AC. However, no significant difference was observed between the rats deprived of hearing on P35 and those deprived of hearing on P42, the end of the critical period of auditory development. These results showed that monaural hearing deprivation during early postnatal development was associated with decreased NR1 mRNA expression in the contralateral AC and suggested the involvement of NR1 in auditory function during development. They also indicated that, during postnatal development, environmental factors changed the functional plasticity of neurons in the AC through NR1 receptor expression. Taken together, these findings provide a possible underlying mechanism for the development of postnatal auditory function.

The effects of different auditory activity on the expression of phosphorylated c-Jun in the auditory system

CONCLUSION

The data revealed that calcium influx via the NMDA receptor up-regulated the expression of phosphorylated c-Jun in the primary auditory cortices following sensory stimulation and after different neural injury stimulations which guide activity-dependent changes in gene expression and neural plasticity.

OBJECTIVES

Activator protein-1 (AP-1) transcription factor, which is mainly composed of c-Fos and c-Jun proteins, is believed to be a key participant in molecular processes that guide activity-dependent changes in gene expression. Our previous study had shown that the expression of NMDAR2A gene on synaptosomes membrane of auditory cortical neurons varied by electrical intracochlear stimulation (EIS) and neural injury induced by acoustic trauma. In this study, we investigated the role of the NMDA receptor (NMDAR) in regulating the expression of phosphorylated c-Jun in the primary auditory cortex (AI). The modulation factors observed for gene expression included EIS and noise traumas.

MATERIALS AND METHODS

EIS was applied in rats with early postnatal auditory deprivation. The impact of the noise traumas on the ultrastructures of spiral ganglion neurons (SGNs) and their innervations to inner hair cells (IHCs) were verified by transmission electron microscopy (EM). These changes include a decrease in subcellular organelles, the swelling of mitochondria and endoplasmic reticulum, the morphological changes in cell nuclei, and damage in the afferent synapse.

RESULTS

Immunohistochemistry observations showed that the expression of phosphorylated c-jun and active caspase-3 in hair cells and SGNs varied with amount of noise. Immunocytochemistry and Western blotting showed that the auditory cortex began to express phosphorylated c-jun 24 h after 2 h of EIS. However, this expression was not changed by EIS if NMDAR antagonist was applied. The level of phosphorylated c-Jun was remarkably increased in AI after noise overstimulation at 115 dB SPL for 3 h. Again, such an increase was not seen if NMDAR antagonist 3-(2 carboxypiperazin-4yl) propyl-1-phosphonic acid (CPP, 10 mg/kg, i.p.) was applied 30 min before the noise exposure.

Auditory nerve input is not an absolute requirement for the expression, distribution and calcium permeability of AMPA receptors in the adult rat ventral cochlear nucleus

In order to understand whether glutamatergic excitatory presynaptic input is an absolute requirement for the adult regulation of postsynaptic glutamate receptors we analyzed if a period of 11 days of excitatory deprivation affects the expression, distribution and Ca(2+) permeability of AMPA receptor subunits in the ventral cochlear nucleus of the rat. Bilateral cochlear ablations were performed in 30-day-old rats. After 11 days of survival, immunohistochemistry for GluR1, GluR2/3 and GluR4 AMPA receptor subunits showed no changes in the normal pattern of distribution, with GluR2/3 and GluR4 immunoreactivity predominating, and little GluR1. No changes in the amount of these AMPA receptor subunits were found between normal and cochleotomized rats in Western blots. AMPA receptors lacking the GluR2 subunit are Ca(2+) permeable. Kainate-induced Co(2+) uptake histochemistry, which labels AMPA Ca(2+) permeable receptors, demonstrated no changes in somatic labeling intensity for Co(2+), 11 days after cochleotomy. Therefore, our data indicate that excitatory input is not an absolute requirement to maintain AMPA receptor subunit expression, distribution and functional properties such as Ca(2+) permeability in VCN neurons. Nevertheless, subtle changes in AMPA receptors through regulatory post-transductional mechanisms cannot be ruled out.

The effect of early auditory deprivation on the age-dependent expression pattern of NR2B mRNA in rat auditory cortex

NMDA receptors have been well shown to be involved in neuronal plasticity. In order to understand the role of NR2B subtype NMDA receptors in auditory function development, the present study investigated the effect of early auditory deprivation on the expression of NR2B mRNA in rat auditory cortex (AC) during postnatal development. For normal rats, the NR2B mRNA expression was highest at birth (postnatal day 1 [P1]) and declined rapidly to low level during adulthood. However, during the critical period of rat auditory development (two to three weeks after birth), there was a transient NR2B expression peak on postnatal day 21 (P21). For the auditory-deprived rats, the general declining trend of NR2B mRNA expression from birth to adult was similar to that observed in the normal group, whereas the expression level from P15 to P27 was significantly lower than normal and the transient peak on P21 disappeared. In both groups, the distribution pattern of NR2B mRNA-positive neurons was also examined in various layers and dorsal, medial and ventral subdistricts of AC. There is no significant effect on the spatial expression of the NR2B mRNA in the AC between normal and deprived group. Our results indicated that the early auditory deprivation decreased the expression levels of NR2B mRNA in AC during the critical period of rat auditory development, suggesting that NR2B plays an important role in the developmental plasticity of auditory function in rats.

The inferior colliculus of the rat: quantitative immunocytochemical study of GABA and glycine

Both GABA and glycine (Gly) containing neurons send inhibitory projections to the inferior colliculus (IC), whereas inhibitory neurons within the IC are primarily GABAergic. To date, however, a quantitative description of the topographic distribution of GABAergic neurons in the rat's IC and their GABAergic or glycinergic inputs is lacking. Accordingly, here we present detailed maps of GABAergic and glycinergic neurons and terminals in the rat's IC. Semithin serial sections of the IC were obtained and stained for GABA and Gly. Images of the tissue were digitized and used for a quantitative densitometric analysis of GABA immunostaining. The optical density, perimeter, and number of GABA- and Gly immunoreactive boutons apposed to the somata were measured. Data analysis included comparisons across IC subdivisions and across frequency regions within the central nucleus of the IC. The results show that: 1) 25% of the IC neurons are GABAergic; 2) there are more GABAergic neurons in the central nucleus of the IC than previously estimated; 3) GABAergic neurons are larger than non-GABAergic; 4) GABAergic neurons receive less GABA and glycine puncta than non-GABAergic; 5) differences across frequency regions are minor, except that the non-GABAergic neurons from high frequency regions are larger than their counterparts in low frequency regions; 6) differences within the laminae are greater along the dorsomedial-ventrolateral axis than along the rostrocaudal axis; 7) GABA and non-GABAergic neurons receive different numbers of puncta in different IC subdivisions; and 8) GABAergic puncta are both apposed to the somata and in the neuropil, glycinergic puncta are mostly confined to the neuropil.

N-methyl-D-aspartate receptor and apoptosis in Alzheimer's disease and multiinfarct dementia

This study investigates the role of excitotoxicity in Alzheimer's disease and in multiinfarct dementia by examining, via immunohistochemical methods, the number of cells that are positive for N-methyl-D-aspartate (NMDA) receptor and the degree of colocalization between NMDA receptor and apoptosis markers such as TUNEL or activated caspase-3 in the frontal cortex of individuals with these two conditions, comparing the results with those from subjects who died of normal aging. We showed an increased number of NMDA receptor-positive cells and an increased number of TUNEL-labeled cells in the frontal cortex of subjects with Alzheimer's disease, especially in the deeper layers of the cortex. However, only about 10% of cells showed colocalization of NMDA receptor with the apoptosis markers studied, suggesting that NMDA-mediated excitotoxicity does not play a major role in neuronal apoptosis in Alzheimer's disease or in multiinfarct dementia.

Development of a robust central auditory synapse in congenital deafness

Within the medial nucleus of the trapezoid body (MNTB) in the auditory brain stem, there is a large central synapse known as the calyx of Held, which mediates high-fidelity glutamatergic transmission. We investigated the effects of congenital deafness on the development of pre- and postsynaptic parameters of synaptic strength at the calyx of Held. Whole cell recordings of evoked excitatory postsynaptic currents (EPSCs) and immunohistochemistry of GluR1-4 subunits were performed using brain stem slices from congenitally deaf or hearing mice at postnatal days P5 and P12. In both hearing and deaf mice there was a similar developmental decrease in the NMDA component of the evoked EPSC. There was a concurrent increase in release probability and number of release sites, contributing to a fivefold increase in evoked AMPA-mediated EPSC amplitude. The increase in release probability is opposite to that found in previous studies at the calyx of Held in the rat. There was also a seven- to eightfold increase in the size of the readily releasable pool of vesicles and a decrease in tetanic depression. The postsynaptic glutamate receptor subunits were similarly developmentally regulated and unaffected by deafness. GluR1 and 4 dominated at both ages. There was a decrease in expression of GluR1-3 from P5 to P12 and a shift from GluR2 to GluR3, indicating that AMPA receptor complexes at P12 are predominantly calcium-permeable. These results demonstrate that early development at this robust synapse proceeds normally with congenital deafness, suggesting that auditory nerve activity does not affect the development of synaptic strength at the calyx of Held.

Targeting hearing genes in mice

The rate of identification of genes for hearing has clearly outpaced the rate of determination of the functions of these genes' products. The use of transgenic and knock-out mouse models is a powerful approach to the elucidation of gene function in the ear. A large number of gene-targeted mice with auditory defects have recently been created and characterized, and nine independent mouse lines in which Cre recombinase activity begins to be expressed during early embryonic development of the ear or is specifically expressed in hair cells during postnatal development will be useful for ear-specific gene manipulation when combined with mouse lines that have loxP sites flanking the genes of interest. Existing gene-trapped embryonic stem (ES) cells and existing targeting constructs are readily available; new targeting constructs can easily be created by modifying bacterial artificial chromosomes and using them to directly transfect and screen ES cells; and N-ethyl-N-nitrosourea mutagenesis of ES cells can create point mutations in specific genes. To minimize variation in hearing phenotypes and avoid undesired hearing defects, mutant mice in the common gene-targeting background strains (129 and C57BL/6) should be transferred into congenic CBA/CaJ, a strain with "gold standard" normal hearing. Valuable mutant strains can be maintained, distributed, and cryopreserved in one of four NIH-sponsored Mutant Mouse Regional Resource Centers. Targeting hearing genes in mice will provide unprecedented opportunities for collaboration and new directions in the hearing research community.

Cerebral expression of the α2-subunit of soluble guanylyl cyclase is linked to cerebral maturation and sensory pathway refinement during postnatal development

Soluble guanylyl cylase (sGC) has been identified for being a receptor for the gaseous transmitters nitric oxide and carbon monoxide. Currently four subunits alpha1, alpha2, beta1, and beta2 have been characterized. Heterodimers of alpha and beta-subunits as well as homodimers of the beta2-subunit are known to constitute functional sGC which use GTP to form cGMP a potent signal molecule in a multitude of second messenger cascades. Since NO-cGMP signaling plays a pivotal role in neuronal development we analyzed the maturational expression pattern of the newly characterized alpha2-subunit of sGC within the brain of Wistar rats by means of RNase protection assay and immunohistochemistry. alpha2-subunit mRNA as well as immunoreactive alpha2-protein increased during postnatal cerebral development. Topographical analysis revealed a selective high expression of the alpha2-subunit in the choroid plexus and within developing sensory systems involving the olfactory and somatosensory system of the forebrain as well as parts of the auditory and visual system within the hindbrain. In cultured cortical neurons the alpha2-subunit was localized to the cell membrane, especially along neuronal processes. During the first 11 days of postnatal development several cerebral regions showed a distinct expression of the alpha2-subunit which was not paralleled by the alpha1/beta1-subunits especially within the developing thalamo-cortical circuitries of the somatosensory system. However, at later developmental stages all three subunits became more homogenously distributed among most cerebral regions, indicating that functional alpha1/beta1 and alpha2/beta1 heterodimers of sGC could be formed. Our findings indicate that the alpha2-subunit is an essential developmentally regulated constituent of cerebral sensory systems during maturation. In addition the alpha2-subunit may serve other functions than forming a functional heterodimer of sGC during the early phases of sensory pathway refinement.

Influence of auditory deprivation upon the tonopic organization in the inferior colliculus: a Fos immunocytochemical study in the rat

The frequency organization in the inferior colliculus of neonatally-deafened rats was investigated using electrical stimulation of the cochlea and immunoreactivity for Fos as a marker of neuronal activity. An electrode implanted either at the base or at the apex of the right cochlea delivered a unique 45-min stimulation at two different level intensities and at two time points, i.e. either at 4 weeks or at 4 months. In 4-week-old rats stimulated at 5x threshold, a site-for-site organization was observed since basal or apical stimulation induced a strong labelling in the ventro-medial or in the dorsolateral part of the left inferior colliculus, respectively. In 4-month-old rats, stimulation of the base induced an extremely weak Fos labelling without any specific location in the left inferior colliculus while stimulation of the apex induced a diffuse labelling with two discrete bands being distinguishable in the left inferior colliculus. In 4-week-old rats stimulated at 15x threshold, basal stimulation elicited a diffuse Fos-like immunoreactivity in the left inferior colliculus while apical stimulation yielded a response restricted to the dorsal part of the left inferior colliculus. In 4-month-old rats, no response was detected in the left inferior colliculus after stimulation of the basal part of the cochlea. Stimulation of the apex could still induce a labelling in the dorsolateral left inferior colliculus. Thus, the inferior colliculus exhibits an adult-like tonotopic organization early on independently of any acoustic stimulation. Prolonged absence of auditory input dramatically alters this organization in the inferior colliculus, especially for high frequencies. From a clinical standpoint, these results could argue for early implantation in deaf children.

Early postnatal sound exposure induces lasting neuronal changes in the inferior colliculus of senescence accelerated mice (SAMP8): a morphometric study on GABAergic neurons and NMDA expression

Senescence-acceleration-prone mice (SAMP8) provide a model to study the influence of early postnatal sound exposure upon the aging auditory midbrain. SAMP8 were exposed to a 9-kHz monotone of either 53- or 65-dB sound pressure level during the first 30 postnatal days, the neurons in the auditory midbrain responding selectively to 9 kHz were localized by c-fos immunohistochemistry and the following parameters were compared to control SAMP8 not exposed to sound: mortality after sound exposure, dendritic spine density, and quantitative neurochemical alterations in this 9-kHz isofrequency lamina. For morphometric analysis, animals were examined at 1, 4, and 8 months of age. Serial sections of the inferior colliculus were Golgi impregnated or stained immunohistochemically for the expression of epsilon1 subunit of NMDA receptor or GABA. Mortality after exposure to 53 dB was the same as in controls, but was markedly increased from 7 months of age onward after postnatal exposure to 65 dB. No gross morphological alterations were observed in the auditory midbrain after sound exposure. However, sound exposure to 53 or 65 dB significantly reduced dendritic spine density by 11% at 4 months or by 11-17% both at 1 and 4 months of age, respectively. The effect of sound exposure upon neurons expressing the NMDAepsilon1 subunit was dose-dependent. Increasing with age until 4 months in control mice and remaining essentially stable thereafter, the percentage of NMDAepsilon1-immunoreactive neurons was significantly elevated by 40-66% in 1- and 8-month-old SAMP8 exposed to 53 dB, whereas no significant effect of 65 dB was apparent. The proportion of GABAergic cells declined with age in controls. It was significantly decreased at 1 month after 53 and 65 dB sound exposure. In contrast, it was elevated at later stages, being significantly increased at 4 months after exposure to 53 dB and at 8 months after exposure to 65 dB. The total cell number in the 9-kHz isofrequency lamina of SAMP8 decreased with age, but was not affected by exposure to either 53 or 65 dB. The present results indicate that early postnatal exposure to a monotone of mild intensity has long-term effects upon the aging auditory brain stem. Some of the changes induced by sound exposure, e.g., decline in spine density, are interpreted as accelerations of the normal aging process, whereas other effects, e.g., increased NMDAepsilon1 expression after 53 dB and elevated GABA expression after both 53 and 65 dB, are not merely explicable by accelerated aging.

Evolution and development of time coding systems

Precise temporal coding is a hallmark of both the electrosensory and auditory systems. Selective pressures to improve accuracy or encode more rapid changes have produced a suite of convergent physiological and morphological features that contribute to temporal coding. Comparative studies of temporal coding can also point to shared computational strategies, and suggest how selection might act to improve coding.

Deafness-induced changes in the auditory pathway: implications for cochlear implants

A profound sensorineural hearing loss induces significant pathological and atrophic changes within the cochlea and central auditory pathway. We describe these deafness-induced morphological and functional changes following controlled lesions of the cochlea in experimental animals. Such changes are generally consistent with the limited number of reports describing deafness-induced changes observed in human material. The implications of these pathophysiological changes within the auditory pathway on cochlear implant function are discussed. Finally, the plastic response of the deafened auditory system to electrical stimulation of the auditory nerve is reviewed in light of the clinical implications for cochlear implant recipients.

Cochlear electrical stimulation: influence of age of implantation on Fos immunocytochemical reactions in inferior colliculi and dorsal cochlear nuclei of the rat

The influence of age at the time of implantation of a stimulating electrode unilaterally in the inner ear on central auditory pathways was investigated in rats deafened shortly after birth. Immunoreactivity for Fos served as a functional marker of neuronal activity. Electrodes were implanted in the left cochlea of rats aged 3 weeks or 4 months. Stimulation lasted 45 minutes, then rats were sacrificed and tissues processed for immunocytochemistry. The younger animals showed significantly more neurons with Fos immunoreactivity bilaterally in the dorsal cochlear nuclei (DCN) and inferior colliculi (IC) than the older rats or control animals with normal hearing receiving the same stimulation. Activity was more prominent in the left DCN and right IC. The results show that electrical stimulation of the inner ear is more effective in younger animals in eliciting gene expression associated with development of a functional network in the auditory pathways. This suggests that deaf children should be provided with cochlear implants as early as possible.