Off-channel effect of high-frequency overstimulation on duration tuning of low-frequency inferior colliculus neurons in guinea pigs

CONCLUSION

High-frequency overstimulation can cause the loss of duration selectivity in low-frequency inferior colliculus neurons in guinea pigs.

OBJECTIVE

To investigate the effect of high-frequency overstimulation on duration tuning in low-frequency inferior colliculus neurons in guinea pigs.

MATERIALS AND METHODS

Duration tuning pattern was recorded by measuring the spikes of single neurons in response to the best frequency (BF) of different durations. The effect of high-frequency overstimulation was verified by comparing the responses before and after the tone exposure.

RESULTS

In total, 40 duration-tuned neurons were successfully recorded before and after the tone exposure. After the high-frequency tone trauma, a total of 29 neurons (72.5%) became non-duration-tuned.

Neurodegeneration in thiamine deficient rats-A longitudinal MRI study

Selective neurodegeneration accompanied by mitochondrial dysfunction characterizes neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Thiamine deficiency (TD) in rats is a model for the study of cellular and molecular mechanisms that lead to selective neuronal loss caused by chronic oxidative deficits. Neurodegeneration in TD-rats develops over a period of 12 to 14 days and can be partially reversed by thiamine administration. The aim of this study was to characterize the in-vivo progression of neurodegeneration and the neuronal rescue processes in TD using T(2) magnetic resonance mapping and diffusion tensor imaging (DTI). Each rat was scanned prior to TD induction (day 0), before the appearance of neurological symptoms (day 10), during the symptomatic stage (days 12 and 14) and during the recuperation period (days 31 and 87). Time-dependent lesions were revealed mainly in the thalamus and the inferior colliculi. Early decrease in the fractional anisotropy (FA) was found on day 10 in the inferior colliculi and to a lesser degree in the thalamus, while the earliest detectable changes in the T(2) parameter occurred only on day 12. FA values in the thalamus remained significantly low after thiamine restoration, suggesting irreversible disarrangement and replacement of neuronal structures. While T(2) values in the frontal cortex demonstrated no lesions, FA values significantly increased on days 14 and 31. An enlargement of the lateral ventricles was observed and persevered during the recovery period. This longitudinal MRI study demonstrated that in TD MRI can detect neurodegeneration and neuronal recovery. DTI is more sensitive than T(2) mapping in the early detection of TD lesions.

Auditory midbrain implant: a review

The auditory midbrain implant (AMI) is a new hearing prosthesis designed for stimulation of the inferior colliculus in deaf patients who cannot sufficiently benefit from cochlear implants. The authors have begun clinical trials in which five patients have been implanted with a single shank AMI array (20 electrodes). The goal of this review is to summarize the development and research that has led to the translation of the AMI from a concept into the first patients. This study presents the rationale and design concept for the AMI as well a summary of the animal safety and feasibility studies that were required for clinical approval. The authors also present the initial surgical, psychophysical, and speech results from the first three implanted patients. Overall, the results have been encouraging in terms of the safety and functionality of the implant. All patients obtain improvements in hearing capabilities on a daily basis. However, performance varies dramatically across patients depending on the implant location within the midbrain with the best performer still not able to achieve open set speech perception without lip-reading cues. Stimulation of the auditory midbrain provides a wide range of level, spectral, and temporal cues, all of which are important for speech understanding, but they do not appear to sufficiently fuse together to enable open set speech perception with the currently used stimulation strategies. Finally, several issues and hypotheses for why current patients obtain limited speech perception along with several feasible solutions for improving AMI implementation are presented.

The auditory midbrain of people with tinnitus: abnormal sound-evoked activity revisited

Sound-evoked fMRI activation of the inferior colliculi (IC) was compared between tinnitus and non-tinnitus subjects matched in threshold (normal), age, depression, and anxiety. Subjects were stimulated with broadband sound in an "on/off" fMRI paradigm with and without on-going sound from the scanner coolant pump. (1) With pump sounds off, the tinnitus group showed greater stimulus-evoked activation of the IC than the non-tinnitus group, suggesting abnormal gain within the auditory pathway of tinnitus subjects. (2) Having pump sounds on reduced activation in the tinnitus, but not the non-tinnitus group. This result suggests response saturation in tinnitus subjects, possibly occurring because abnormal gain increased response amplitude to an upper limit. (3) In contrast to Melcher et al. (2000), the ratio of activation between right and left IC did not differ significantly between tinnitus and non-tinnitus subjects or in a manner dependent on tinnitus laterality. However, new data from subjects imaged previously by Melcher et al. suggest a possible tinnitus subgroup with abnormally asymmetric function of the IC. The present and previous data together suggest elevated responses to sound in the IC are common among those with tinnitus and normal thresholds, while abnormally asymmetric activation is not, even among those with lateralized tinnitus.

Protein expression of small conductance calcium-activated potassium channels is altered in inferior colliculus neurons of the genetically epilepsy-prone rat

The genetically epilepsy-prone rat (GEPR) exhibits inherited predisposition to sound stimuli-induced generalized tonic-clonic seizures (audiogenic reflex seizures) and is a valid model to study the physiopathology of epilepsy. In this model, the inferior colliculus (IC) exhibits enhanced neuronal firing that is critical in the initiation of reflex audiogenic seizures. The mechanisms underlying IC neuronal hyperexcitability that leads to seizure susceptibility are not as yet fully understood. The present report shows that the levels of protein expression of SK1 and SK3 subtypes of the small conductance Ca2+-activated K+ channels were significantly decreased, while SK2 channel proteins were increased in IC neurons of seizure-naive GEPR-3s (SN-GEPR-3), as compared to control Sprague-Dawley rats. No significant change was found in the expression of BK channel proteins in IC neurons of SN-GEPR-3s. Single episode of reflex audiogenic seizures in the GEPR-3s did not significantly alter the protein expression of SK1-3 and BK channels in IC neurons compared to SN-GEPR-3s. Thus, downregulation of SK1 and SK3 channels and upregulation of SK2 channels provide direct evidence that these Ca2+-activated K+ channels play important roles in IC neuronal hyperexcitability that leads to inherited seizure susceptibility in the GEPR.

[Effect of electrical stimulation of the primary auditory cortex on the spontaneous activities of the external nucleus of the inferior colliculus in a rat model of tinnitus induced by salicylate acid.]

This study aims to investigate corticofugal modulation on spontaneous activities of the external nucleus of the inferior colliculus (ICx) in a salicylate acid (SA) induced tinnitus rat model by the stimulation of the primary auditory cortex (AI). Extracellular recording techniques and stereotaxic method were used. The spontaneous activities of a single unit were recorded from the left ICx after electrical stimulation was given to the left AI of the rats duplicated as acute SA models. The average rate of spontaneous discharge and the interspike interval histogram of spontaneous activities were used as indices. The single unit spontaneous discharges of the same unit of ICx before and after AI stimulation were observed. There was an inhibitory effect of AI stimulation on the activities of the high discharge unit [(8.75+/-2.70) Hz vs (5.06+/-2.01) Hz] and a facilitatory effect on the low discharge unit [(1.41+/-0.45) Hz vs (2.46+/-0.79) Hz]. In the normal group, there was a restraining effect on the average rate of spontaneous discharge of the ICx after AI stimulation. The average rate of spontaneous discharge changed from (3.66+/-0.84) Hz to (2.47+/-0.43) Hz in the first hour after AI stimulation and then recovered within 2-4 h. And the discharge rate of the spike interval within 0-20 ms decreased (17% vs 7.3%, 11.2%) in the first 2 h and recovered 3-4 h after AI stimulation. The discharge rate of the spike interval within 0-6 ms (short interval) recovered 2 h after AI stimulation. In the acute SA model group, the average rate of spontaneous discharge recorded from the ICx decreased from (7.48+/-0.85) Hz to (3.38+/-0.39) Hz in the first hour after AI stimulation and the suppression effect remained 4 h (P<0.05). There was no difference in the average rate of spontaneous discharge between the acute SA model group and the normal group at 2-4 h after AI stimulation. The suppression effect after AI stimulation on the 0-20 ms interval spikes in the ICx lasted 4 h, while that on the shorter interval (0-6 ms) spikes recovered in the 3rd hour after AI stimulation in acute SA model group. It is concluded that the high average spontaneous discharge rate of ICx in acute SA model decreases significantly by AI stimulation, and the suppression effects on the shorter interval spikes recovers in the 3rd hour after AI stimulation. It might be inferred that stimulation of AI, through exciting the auditory descending projections, could remit the increased spontaneous discharge of ICx induced by SA that may relate with tinnitus in a period of time.

Interactions of hearing loss and diabetes mellitus in the middle age CBA/CaJ mouse model of presbycusis

Recently, we characterized the more severe nature of hearing loss in aged Type 2 diabetic human subjects [Frisina, S.T., Mapes, F., Kim, S., Frisina, D.R., Frisina, R.D., 2006. Characterization of hearing loss in aged type II diabetics. Hear. Res. 211, 103-113]. The current study prospectively assessed hearing abilities in middle age CBA/CaJ mice with Type 1 diabetes mellitus (T1DM) (STZ injection) or Type 2 diabetes mellitus (T2DM) (high fat diet), for a period of 6 months. Blood glucose, body weight and auditory tests (Auditory Brainstem Response-ABR, Distortion Product Otoacoustic Emissions-DPOAE) were evaluated at baseline and every 2 months. Tone and broad-band noise-burst responses in the inferior colliculus were obtained at 6 months. Body weights of controls did not change over 6 months (approximately 32 g), but there was a significant (approximately 5 g) decline in the T1DM, while T2DM exhibited approximately 10 g weight gain. Blood glucose levels significantly increased: 3-fold for T1DM, 1.3-fold for T2DM; with no significant changes in controls. ABR threshold elevations were found for both types of diabetes, but were most pronounced in the T2DM, starting as early as 2 months after induction of diabetes. A decline of mean DPOAE amplitudes was observed in both diabetic groups at high frequencies, and for the T2DM at low frequencies. In contrast to ABR thresholds, tone and noise thresholds in the inferior colliculus were lower for both diabetic groups. Induction of diabetes in middle-aged CBA/CaJ mice promotes amplification of age-related peripheral hearing loss which makes it a suitable model for studying the interaction of age-related hearing loss and diabetes. On the other hand, initial results of effects from very high blood glucose level (T1DM) on the auditory midbrain showed disruption of central inhibition, increased response synchrony or enhanced excitation in the inferior colliculus.

Cross-modal interactions of auditory and somatic inputs in the brainstem and midbrain and their imbalance in tinnitus and deafness

PURPOSE

This review outlines the anatomical and functional bases of somatosensory influences on auditory processing in the normal brainstem and midbrain. It then explores how interactions between the auditory and somatosensory system are modified through deafness, and their impact on tinnitus is discussed.

METHOD

Literature review, tract tracing, immunohistochemistry, and in vivo electrophysiological recordings were used.

RESULTS

Somatosensory input originates in the dorsal root ganglia and trigeminal ganglia, and is transmitted directly and indirectly through 2nd-order nuclei to the ventral cochlear nucleus, dorsal cochlear nucleus (DCN), and inferior colliculus. The glutamatergic somatosensory afferents can be segregated from auditory nerve inputs by the type of vesicular glutamate transporters present in their terminals. Electrical stimulation of the somatosensory input results in a complex combination of excitation and inhibition, and alters the rate and timing of responses to acoustic stimulation. Deafness increases the spontaneous rates of those neurons that receive excitatory somatosensory input and results in a greater sensitivity of DCN neurons to trigeminal stimulation.

CONCLUSIONS

Auditory-somatosensory bimodal integration is already present in 1st-order auditory nuclei. The balance of excitation and inhibition elicited by somatosensory input is altered following deafness. The increase in somatosensory influence on auditory neurons when their auditory input is diminished could be due to cross-modal reinnervation or increased synaptic strength, and may contribute to mechanisms underlying somatic tinnitus.

Tinnitus: a philosophical problem

Anatomical, physiological and metabolic properties of tinnitus have been identified and a comprehensive theory is immerging. The key elements and their interaction are presented in a general fashion highlighting areas of concern such as needed details of individual biosusceptibility and the need for continued tinnitus modeling for predictions as an aid in the development of effective treatment modalities. Nonetheless, there remains something of the uniqueness of tinnitus as a personal experience. The use of the final common pathway (FCP) as a unifying principle in diagnosis and treatment is presented.

Spatial selectivity to intracochlear electrical stimulation in the inferior colliculus is degraded after long-term deafness in cats

In an animal model of electrical hearing in prelingually deaf adults, this study examined the effects of deafness duration on response thresholds and spatial selectivity (i.e., cochleotopic organization, spatial tuning and dynamic range) in the central auditory system to intracochlear electrical stimulation. Electrically evoked auditory brain stem response (EABR) thresholds and neural response thresholds in the external (ICX) and central (ICC) nuclei of the inferior colliculus were estimated in cats after varying durations of neonatally induced deafness: in animals deafened <1.5 yr (short-deafened unstimulated, SDU cats) with a mean spiral ganglion cell (SGC) density of approximately 45% of normal and in animals deafened >2.5 yr (long-deafened, LD cats) with severe cochlear pathology (mean SGC density <7% of normal). LD animals were subdivided into unstimulated cats and those that received chronic intracochlear electrical stimulation via a feline cochlear implant. Acutely deafened, implanted adult cats served as controls. Independent of their stimulation history, LD animals had significantly higher EABR and ICC thresholds than SDU and control animals. Moreover, the spread of electrical excitation was significantly broader and the dynamic range significantly reduced in LD animals. Despite the prolonged durations of deafness the fundamental cochleotopic organization was maintained in both the ICX and the ICC of LD animals. There was no difference between SDU and control cats in any of the response properties tested. These findings suggest that long-term auditory deprivation results in a significant and possibly irreversible degradation of response thresholds and spatial selectivity to intracochlear electrical stimulation in the auditory midbrain.

[Level of glutamate on the auditory pathway of inferior colliculus after exposure to noise observed by microdialysis in vivo]

OBJECTIVE

To explore the quality and quantity of glutamate(Glu) on the auditory pathway of inferior colliculus when exposed to conditioning noise or traumatic sound to find the relationship between the change of neurotransmitters and the protection of hearing, and the mechanism in the phenomena.

METHODS

Twenty Sprague-Dawley rats were divided into four groups: control group, conditioning group, intensive noise group and combining group. The hearing function was measured by ABR. Glu was detected by microdialysis and HPLC.

RESULTS

The ABR levels after exposure were (47.0+/-2.7) dB SPL (intensive noise group ),(31.0+/-2.2) dB SPL (conditioning group ) and (36.0+/-2.2) dB SPL (combining group ).The levels of Glu were (310.5+/-78.5)x 10(-7)mol/L (intense noise group ),(162.9+/-64.1)x10(-7)mol/L (conditioning group ); (113.6+/-38.1)x10(-7)mol/L (combining group),(56.3+/-23.9)x10(-7)mol/L (control group).

CONCLUSION

The level of Glu on the auditory pathway of inferior colliculus was higher when exposed to noise . Glu was exhausted when given conditioning sound, and the level of Glu was lower when given following intense noise. High level of Glu can damage hearing, so exhaustion of Glu in sound condition may contribute to the protection.

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

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

Central neural activity in rats with tinnitus evaluated with manganese-enhanced magnetic resonance imaging (MEMRI)

The pathophysiology of tinnitus, the perception of sound in the absence of acoustic stimulation, is largely unknown, although several lines of research implicate long-term neuroplastic loss of inhibition. The evidence to date suggests that the neuroplastic alterations are likely to be found in multiple brain structures. The present study used manganese-enhanced magnetic resonance imaging (MEMRI) to assess the pattern of neural activity in the central auditory pathway of rats with psychophysical evidence of chronic acoustic-exposure-induced tinnitus. Manganese, an activity-dependent paramagnetic contrast agent, accumulates in active neurons through voltage-gated calcium channels, primarily at synapses, and serves as both a structural and functional indicator. Comparison images were obtained from normal subjects exposed to external tinnitus-like sound, and from tinnitus subjects treated with vigabatrin, a GABA agonist shown to eliminate the psychophysical evidence of tinnitus in rats. MEMRI indicated: (1) In rats with evidence of tinnitus, activity was generally elevated in the auditory brainstem, with significant elevation in the cerebellar paraflocculus, the posterior ventral cochlear nucleus, and the inferior colliculus; in general forebrain structures showed decreased activity, although MEMRI may be a less sensitive indicator of forebrain activity than brainstem activity; (2) in normal rats exposed to a tinnitus-like sound, a similar pattern of elevated brainstem activity and decreased forebrain activity was evident, with the notable exception of the paraflocculus, where artificial tinnitus had no effect and (3) vigabatrin, decreased brainstem activity to control levels, in rats with prior evidence of tinnitus, and decreased forebrain activity to below control levels. It was concluded that chronic tinnitus in rats is associated with focal activity elevation in the auditory brainstem and increased activity in the paraflocculus that may be unique to tinnitus.

Reorganization of receptive fields following hearing loss in inferior colliculus neurons

We explored frequency and intensity encoding in the inferior colliculus (IC) of the C57 mouse model of sensorineural hearing loss. Consistent with plasticity reported in the IC of other models of hearing loss, frequency response areas (FRAs) in hearing-impaired (HI) mice were broader with fewer high-frequency units than normal-hearing (NH) mice. The broad FRAs recorded from HI mice had lower cutoffs on the low frequency edge of the FRA. Characteristic frequency (CF) and sharpness of tuning (Q10) calculated from the FRA were used to divide the sample into four categories: low-CF sharp-FRA, low-CF broad-FRA, high-CF sharp-FRA, and high-CF broad-FRA units. Rate-intensity functions (RIFs) for CF tones and noise were used to determine the minimum and maximum response counts as well as the sound pressure levels resulting in 10%, 50%, and 90% of the maximum spike count. Tone RIFs of broad FRA units were shifted to the right of tone RIFs of sharp FRA units in both NH and HI mouse IC, regardless of the unit CF. The main effects of hearing loss were seen in the noise RIFs. The low-CF broad-FRA units in HI mice had elevated responses to noise, and the high-CF sharp-FRA units in HI mice had lower maximum rates, as compared with the units recorded from NH mice. These results suggest that, as the IC responds to peripheral hearing loss with changes in the representation of frequency, an altered balance between inhibitory and excitatory inputs to the neurons recorded from the HI mice alters aspects of the units' intensity encoding. This altered balance likely occurs, at least in part, outside of the IC.

Excitatory, inhibitory and facilitatory frequency response areas in the inferior colliculus of hearing impaired mice

Individuals with age-related hearing loss often have difficulty understanding complex sounds such as basic speech. The C57BL/6 mouse suffers from progressive sensorineural hearing loss and thus is an effective tool for dissecting the neural mechanisms underlying changes in complex sound processing observed in humans. Neural mechanisms important for processing complex sounds include multiple tuning and combination sensitivity, and these responses are common in the inferior colliculus (IC) of normal hearing mice. We examined neural responses in the IC of C57Bl/6 mice to single and combinations of tones to examine the extent of spectral integration in the IC after age-related high frequency hearing loss. Ten percent of the neurons were tuned to multiple frequency bands and an additional 10% displayed non-linear facilitation to the combination of two different tones (combination sensitivity). No combination-sensitive inhibition was observed. By comparing these findings to spectral integration properties in the IC of normal hearing CBA/CaJ mice, we suggest that high frequency hearing loss affects some of the neural mechanisms in the IC that underlie the processing of complex sounds. The loss of spectral integration properties in the IC during aging likely impairs the central auditory system's ability to process complex sounds such as speech.

Influence of sympathetic fibers on noise-induced hearing loss in the chinchilla

The influence of the sympathetic efferent fibers on cochlear susceptibility to noise-induced hearing loss is still an open question. In the current study, we explore the effects of unilateral and bilateral Superior Cervical Ganglion (SCG) ablation in the chinchilla on hearing loss from noise exposure, as measured with inferior colliculus (IC) evoked potentials, distortion product otoacoustic emissions (DPOAE), and outer hair cell (OHC) loss. The SCG was isolated at the level of the bifurcation of the carotid artery and removed unilaterally in 15 chinchillas. Another eight chinchillas underwent bilateral ablation. Twelve animals were employed as sham controls. Noise exposure was a 4kHz octave band noise for 1h at 110dB SPL. Results showed improved recovery of DPOAE amplitudes after noise exposure in ears that underwent SCGectomy, as well as lower evoked potential threshold shifts relative to sham controls. Effects of SCGectomy on OHC loss were small. Results of the study suggest that sympathetic fibers do exert some influence on susceptibility to noise, but the influence may not be a major one.

Glutamate-related gene expression changes with age in the mouse auditory midbrain

Glutamate is the main excitatory neurotransmitter in both the peripheral and central auditory systems. Changes of glutamate and glutamate-related genes with age may be an important factor in the pathogenesis of age-related hearing loss-presbycusis. In this study, changes in glutamate-related mRNA gene expression in the CBA mouse inferior colliculus with age and hearing loss were examined and correlations were sought between these changes and functional hearing measures, such as the auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAEs). Gene expression of 68 glutamate-related genes was investigated using both genechip microarray and real-time PCR (qPCR) molecular techniques for four different age/hearing loss CBA mouse subject groups. Two genes showed consistent differences between groups for both the genechip and qPCR. Pyrroline-5-carboxylate synthetase enzyme (Pycs) showed down-regulation with age and a high-affinity glutamate transporter (Slc1a3) showed up-regulation with age and hearing loss. Since Pycs plays a role in converting glutamate to proline, its deficiency in old age may lead to both glutamate increases and proline deficiencies in the auditory midbrain, playing a role in the subsequent inducement of glutamate toxicity and loss of proline neuroprotective effects. The up-regulation of Slc1a3 gene expression may reflect a cellular compensatory mechanism to protect against age-related glutamate or calcium excitoxicity.

Interactions between opioid-peptides-containing pathways and GABAA-receptors-mediated systems modulate panic-like-induced behaviors elicited by electric and chemical stimulation of the inferior colliculus

Aiming to clarify the effect of interactive interconnections between the endogenous opioid peptides-neural links and GABAergic pathways on panic-like responses, in the present work, the effect of the peripheral and central administration of morphine or the non-specific opioid receptors antagonist naloxone was evaluated on the fear-induced responses (defensive attention, defensive immobility and escape behavior) elicited by electric and chemical stimulation of the inferior colliculus. Central microinjections of opioid drugs in the inferior colliculus were also performed followed by local administration of the GABA(A)-receptor antagonist bicuculline. The defensive behavior elicited by the blockade of GABAergic receptors in the inferior colliculus had been quantitatively analyzed, recording the number of crossing, jump, rotation and rearing, in each minute, during 30 min, in the open-field test. The opioid receptors stimulation with morphine decreased the defensive attention, the defensive immobility and escape behavior thresholds, and the non-specific opioid receptors blockade caused opposite effects, enhancing the defensive behavior thresholds. These effects were corroborated by either the stimulation or the inhibition of opioid receptors followed by the GABA(A) receptor blockade with bicuculline, microinjected into the inferior colliculus. There was a significant increase in the diverse fear-induced responses caused by bicuculline with the pretreatment of the inferior colliculus with morphine, and the opposite effect was recorded after the pretreatment of the inferior colliculus nuclei with naloxone followed by bicuculline local administration. These findings suggest an interaction between endogenous opioid-peptides-containing connections and GABA(A)-receptor-mediated system with direct influence on the organization of the panic-like or fear-induced responses elaborated in the inferior colliculus during critical emotional states.

Ethanol withdrawal is accompanied by downregulation of calcium channel alpha 1B subunit in rat inferior colliculus neurons

Ethanol withdrawal enhances the current density of calcium (Ca(2+)) channels in inferior colliculus (IC) neurons. The present report shows that ethanol withdrawal markedly enhanced the susceptibility to seizures as it decreased significantly the protein levels of alpha(1B) subunit associated with N-type Ca(2+) channel in IC neurons of animals not tested for seizures. Thus, remodeling of N-type Ca(2+) channels may play an important role in neuronal hyperexcitability that leads to ethanol withdrawal seizures.

Upregulation of glial cell line-derived neurotrophic factor and artemin mRNA in the auditory nerve of deafened rats

Nerve growth factors play key roles in spiral ganglion cells survival and excitability. Our aim was to determine gene expression patterns of glial cell line-derived neurotrophic factor family (GDNF) members and their receptors in the auditory nerve and inferior colliculus of deafened rats. The gene expression of GDNF, persephin, artemin and neurturin, and their receptors GFRalpha1, GFRalpha2, GFRalpha3 and Ret, was determined by semiquantitative reverse transcriptase-polymerase chain reaction using GAPDH expression as an internal standard. Following deafness, no significant changes in expression of GDNF family genes were found in inferior colliculus. In contrast, artemin, GDNF, GFRalpha1-3 and Ret RNA expression were strongly upregulated in the auditory nerve following deafness, indicating their importance in protecting the auditory nerve against cell damage.

Age-related structural and functional changes in the cochlear nucleus

Presbycusis - age-related hearing loss - is a key communication disorder and chronic medical condition of our aged population. The cochlear nucleus is the major site of projections from the auditory portion of the inner ear. Relative to other levels of the peripheral and central auditory systems, relatively few studies have been conducted examining age-related changes in the cochlear nucleus. The neurophysiological investigations suggest declines in glycine-mediated inhibition, reflected in increased firing rates in cochlear nucleus neurons from old animals relative to young adults. Biochemical investigations of glycine inhibition in the cochlear nucleus are consistent with the functional aging declines of this inhibitory neurotransmitter system that affect complex sound processing. Anatomical reductions in neurons of the cochlear nucleus and their output pathways can occur due to aging changes in the brain, as well as due to age-dependent plasticity of the cochlear nucleus in response to the age-related loss of inputs from the cochlea, particularly from the basal, high-frequency regions. Novel preventative and curative biomedical interventions in the future aimed at alleviating the hearing loss that comes with age, will likely emanate from increasing our knowledge and understanding of its neural and molecular bases. To the extent that this sensory deficit resides in the central auditory system, including the cochlear nucleus, future neural therapies will be able to improve hearing in the elderly.

Spontaneous activity in the inferior colliculus of CBA/J mice after manipulations that induce tinnitus

Several physiological studies have linked experimentally induced tinnitus to increases in the spontaneous activity of auditory neurons. These results have led to the proposal of hyperactivity models of tinnitus in which elevated neural activity in the absence of auditory stimulation is perceived as phantom sound. Such models are appealing in their simplicity but remain controversial because a generalized elevation of spontaneous rates may not be observed after treatments that induce tinnitus in humans and experimental animals. Our study addressed these issues by characterizing the effects of common methods of tinnitus induction on spontaneous activity in the central nucleus of the inferior colliculus (ICC). The ICC is an interesting structure in tinnitus research because its diverse inputs include putative generator sites in the dorsal cochlear nucleus, as well as brainstem sources that appear to remain normal after tinnitus induction. Groups of CBA/J mice were subjected to one of three induction methods: bilateral or unilateral sound exposure, and acute salicylate intoxication. Relative to normal baselines, bilaterally exposed mice showed increases in the spontaneous rates of neurons with tuning near the exposure frequency. When the sample was separated into physiologically defined response classes, exposure effects were strongest among neurons with broad excitatory bandwidths. By contrast, salicylate decreased the spontaneous rates of low-frequency neurons with transient sound-evoked activity. Our results suggest that the disordered processes of hearing that give rise to tinnitus do not involve a pervasive elevation of spontaneous activity or a single mode of induction.

Cross-modal innervation of primary visual cortex by auditory fibers in congenitally anophthalmic mice

Auditory-visual cross-modal innervation was examined in control (sighted, ZRDCT-N) and congenitally anophthalmic (eyeless, ZRDCT-AN) mice using electrophysiological recording and pathway tracing with carbocyanine dyes. Electrophysiological data demonstrate that the primary visual cortex of congenitally eyeless, blind, mice receives auditory stimuli. Neuroanatomical data demonstrate a direct connection between the inferior colliculus (IC) and visual cortex. Our experiments provide new information about how the brain adapts to the loss of sight.