Publications using TDT

Sorting nexins are a large family of evolutionarily conserved proteins that play fundamental roles in endocytosis, endosomal sorting and signaling. As an important member of sorting nexin family, sorting nexin 9 (SNX9) has been shown to participate in coordinating actin polymerization with membrane tubulation and vesicle formation. We previously showed that SNX9 is expressed in mouse auditory hair cells and might regulate actin polymerization in those cells. To further examine the physiological role of SNX9, we generated Snx9 knockout mice using homologous recombination method. Unexpectedly, Snx9 knockout mice have normal viability and fertility, and are morphologically and behaviorally indistinguishable from control mice. Further investigation revealed that the morphology and function of auditory hair cells are not affected by Snx9 inactivation, and Snx9 knockout mice have normal hearing threshold. In conclusion, our data revealed that Snx9-deficient mice do not show defects in development as well as auditory function, suggesting that SNX9 is not essential for mice development and hearing.

It has been reported that the degeneration of cochlear hair cells is the typical cause of presbycusis (or age-related hearing loss). However, the molecular mechanisms that mediate cochlear hair cell apoptosis are not yet fully understood and there is no effective treatment for this disorder. MicroRNAs (miRNAs or miRs) have been increasingly shown to be associated with age-related diseases and are emerging as promising therapeutic targets. In this study, we investigated whether miR-29b is involved in the degeneration of cochlear hair cells. To examine our hypothesis, nuclear staining and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) were used to quantify the hair cell counts. RT-qPCR and western blot analysis were used to examine miR-29b/sirtuin 1 (SIRT1)/proliferator-activated receptor-gamma coactivator 1α (PGC‑1α) signaling in cochlear hair cells. We found that there was a significant degeneration of cochlear hair cells and a higher expression of miR-29b in aged C57BL/6 mice compared with young mice. There was also an age-related decrease in the expression of SIRT1 and PGC‑1α. In the inner ear cell line, HEI-OC1, miR-29b overexpression (by transfection with miR-29b mimic) inhibited SIRT1 and PGC‑1α expression, leading to an increase in mitochondrial dysfunction and apoptosis. Moreover, the inhibition of miR-29b (by transfection with miR-29b inhibitor) increased SIRT1 and PGC‑1α expression, while it decreased apoptosis. Taken together, our findings support a link between age-related cochlear hair cell apoptosis and miR-29b/SIRT1/PGC‑1α signaling, which may present an attractive pharmacological target for the development of novel drugs for the treatment of age-related hearing loss.

The homozygous p.V37I variant in GJB2 is prevalent in East and Southeast Asians and may lead to mild-to-moderate hearing loss with reduced penetrance. To investigate the pathogenic mechanism underlying this variant, we generated a knock-in mouse model of homozygous p.V37I by an embryonic stem cell gene targeting method. Auditory brainstem response test showed that the knock-in mice developed progressive, mild-to-moderate hearing loss over the first 4–9 months. Overall no significant developmental and morphological abnormality was observed in the knock-in mouse cochlea, while confocal immunostaining and electron microscopic scanning revealed minor loss of the outer hair cells. Gene expression microarray analysis identified 105 up-regulated and 43 down-regulated genes in P5 knock-in mouse cochleae ( P < 0.05 adjusted by the Benjamini & Hochberg method), among which four top candidate genes with the highest fold-changes or implication to deafness Fcer1g, Nnmt and Lars2 and Cuedc1 were verified by quantitative real-time PCR. Our study demonstrated that the homozygous p.V37I knock-in mouse modeled the hearing phenotype of the human patients and can serve as a useful animal model for further studies. The differentially expressed genes identified in this study may shed new insights into the understanding of the pathogenic mechanism and the phenotypic modification of homozygous p.V37I.

Previous research demonstrated that while selectively attending to relevant aspects of the external world, the brain extracts pertinent information by aligning its neuronal oscillations to key time points of stimuli or their sampling by sensory organs. This alignment mechanism is termed oscillatory entrainment. We investigated the global, long-timescale dynamics of this mechanism in the primary auditory cortex of nonhuman primates, and hypothesized that lapses of entrainment would correspond to lapses of attention. By examining electrophysiological and behavioral measures, we observed that besides the lack of entrainment by external stimuli, attentional lapses were also characterized by high-amplitude alpha oscillations, with alpha frequency structuring of neuronal ensemble and single-unit operations. Entrainment and alpha-oscillation-dominated periods were strongly anticorrelated and fluctuated rhythmically at an ultra-slow rate. Our results indicate that these two distinct brain states represent externally versus internally oriented computational resources engaged by large-scale task-positive and task-negative functional networks.

In the auditory system, early neural stations such as brain stem are characterized by strict tonotopy, which is used to deconstruct sounds to their basic frequencies. But higher along the auditory hierarchy, as early as primary auditory cortex (A1), tonotopy starts breaking down at local circuits. Here, we studied the response properties of both excitatory and inhibitory neurons in the auditory cortex of anesthetized mice. We used in vivo two photon-targeted cell-attached recordings from identified parvalbumin-positive neurons (PVNs) and their excitatory pyramidal neighbors (PyrNs). We show that PyrNs are locally heterogeneous as characterized by diverse best frequencies, pairwise signal correlations, and response timing. In marked contrast, neighboring PVNs exhibited homogenous response properties in pairwise signal correlations and temporal responses. The distinct physiological microarchitecture of different cell types is maintained qualitatively in response to natural sounds. Excitatory heterogeneity and inhibitory homogeneity within the same circuit suggest different roles for each population in coding natural stimuli.

Trimethyltin (TMT) is an occupational and environmental health hazard behaving as a potent neurotoxin known to affect the central nervous system as well as the peripheral auditory system. However, the mechanisms underlying TMT-induced ototoxicity are poorly understood. To elucidate the effects of TMT on the cochlea, a single injection of 4 or 8 mg/kg TMT was administered intraperitoneally to adult rats. The compound action potential (CAP) threshold was used to assess the functional status of the cochlea and histological techniques were used to assess the condition of the hair cells and auditory nerve fibers. TMT at 4 mg/kg produced a temporary CAP threshold elevation of 25-60 dB that recovered by 28 d post-treatment. Although there was no hair cell loss with the 4 mg/kg dose, there was a noticeable loss of auditory nerve fibers particularly beneath the inner hair cells. TMT at 8 mg/kg produced a large permanent CAP threshold shift that was greatest at the high frequencies. The CAP threshold shift was associated with the loss of outer hair cells and inner hair cells in the basal, high-frequency region of the cochlea, considerable loss of auditory nerve fibers and a significant loss of spiral ganglion neurons in the basal turn. Spiral ganglion neurons showed evidence of soma shrinkage and nuclear condensation and fragmentation, morphological features of apoptotic cell death. TMT-induced damage was greatest in the high-frequency, basal region of the cochlea and the nerve fibers beneath the inner hair cells were the most vulnerable structures.

Otitis media (OM) is an inflammatory or infectious disease of the middle ear. Acute otitis media (AOM) and otitis media with effusion (OME) are the two major types of OM. However, the tympanic membrane (TM) motion differences induced by AOM and OME have not been quantified in animal models in the literature. In this study, the guinea pig AOM and OME models were created by transbullar injection of Streptococcus pneumoniae type 3 and lipopolysaccharide, respectively. To explore the effects of OM on the entire TM vibration, the measurements of full-field TM motions were performed in the AOM, OME and untreated control ears by using scanning laser Doppler vibrometry (SLDV). The results showed that both AOM and OME generally reduced the displacement peak and produced the traveling-wave-like motions at relatively low frequencies. Compared with the normal ear, OME resulted in a significant change of the TM displacement mainly in the inferior portion of the TM, and AOM significantly affected the surface motion across four quadrants. The SLDV measurements provide more insight into sound-induced TM vibration in diseased ears.

Severe noise-induced damage to the inner ear leads to auditory nerve fiber degeneration thereby reducing the neural input to the cochlear nucleus (CN). Paradoxically, this leads to a significant increase in spontaneous activity in the CN which has been linked to tinnitus, hyperacusis and ear pain. The biological mechanisms that lead to an increased spontaneous activity are largely unknown, but could arise from changes in glutamatergic or GABAergic neurotransmission or neuroinflammation. To test this hypothesis, we unilaterally exposed rats for 2h to a 126dB SPL narrow band noise centered at 12kHz. Hearing loss measured by auditory brainstem responses exceeded 55dB from 6 to 32kHz. The mRNA from the exposed CN was harvested at 14 or 28days post-exposure and qRT-PCR analysis was performed on 168 genes involved in neural inflammation, neuropathic pain and glutamatergic or GABAergic neurotransmission. Expression levels of mRNA of Slc17a6 and Gabrg3, involved in excitation and inhibition respectively, were significantly increased at 28days post-exposure, suggesting a possible role in the CN spontaneous hyperactivity associated with tinnitus and hyperacusis. In the pain and inflammatory array, noise exposure upregulated mRNA expression levels of four pain/inflammatory genes, Tlr2, Oprd1, Kcnq3 and Ntrk1 and decreased mRNA expression levels of two more genes, Ccl12 and Il1β. Pain/inflammatory gene expression changes via Ntrk1 signaling may induce sterile inflammation, neuropathic pain, microglial activation and migration of nerve fibers from the trigeminal, cuneate and vestibular nuclei into the CN. These changes could contribute to somatic tinnitus, hyperacusis and otalgia.

The development of the cerebellar system depends in part on the emergence of functional connectivity in its input and output pathways. Characterization of spontaneous activity within these pathways provides insight into their functional status in early development. In the present study we recorded extracellular activity from the interpositus nucleus (IP) and its primary downstream target, the red nucleus (RN), in unanesthetized rats at postnatal days 8 (P8) and P12, a period of dramatic change in cerebellar circuitry. The two structures exhibited state-dependent activity patterns and age-related changes in rhythmicity and overall firing rate. Importantly, sensory feedback (i.e., reafference) from myoclonic twitches (spontaneous, self-generated movements that are produced exclusively during active sleep) drove neural activity in the IP and RN at both ages. Additionally, anatomic tracing confirmed the presence of cerebellorubral connections as early as P8. Finally, inactivation of the IP and adjacent nuclei using the GABA A receptor agonist muscimol caused a substantial decrease in neural activity in the contralateral RN at both ages, as well as the disappearance of rhythmicity; twitch-related activity in the RN, however, was preserved after IP inactivation, indicating that twitch-related reafference activates the two structures in parallel. Overall, the present findings point to the contributions of sleep-related spontaneous activity to the development of cerebellar networks.

The high prevalence of noise-induced and age-related hearing loss in the general population has warranted the use of animal models to study the etiology of these pathologies. Quick and accurate auditory threshold determination is a prerequisite for experimental manipulations targeting hearing loss in animal models. The standard auditory brainstem response (ABR) measurement is fairly quick and translational across species, but is limited by the need for anesthesia and a lack of perceptual assessment. The goal of this study was to develop a new method of hearing assessment utilizing prepulse inhibition (PPI) of the acoustic startle reflex, a commonly used tool that measures detection thresholds in awake animals, and can be performed on multiple animals simultaneously. We found that in control mice PPI audiometric functions are similar to both ABR and traditional operant conditioning audiograms. The hearing thresholds assessed with PPI audiometry in sound exposed mice were also similar to those detected by ABR thresholds one day after exposure. However, three months after exposure PPI threshold shifts were still evident at and near the frequency of exposure whereas ABR thresholds recovered to the pre-exposed level. In contrast, PPI audiometry and ABR wave one amplitudes detected similar losses. PPI audiometry provides a high throughput automated behavioral screening tool of hearing in awake animals. Overall, PPI audiometry and ABR assessments of the auditory system are robust techniques with distinct advantages and limitations, which when combined, can provide ample information about the functionality of the auditory system.

Rapid detection and diagnosis of a traumatic brain injury (TBI) can significantly improve the prognosis for recovery. Helmet-mounted sensors that detect impact severity based on measurements of acceleration or pressure show promise for aiding triage and transport decisions in active, field environments such as professional sports or military combat. The detected signals, however, report on the mechanics of an impact rather than directly indicating the presence and severity of an injury. We explored the use of cortical somatosensory evoked electroencephalographic potentials (SSEPs) to detect and track, in real-time, neural electrophysiological abnormalities within the first hour following head injury in an animal model. To study the immediate electrophysiological effects of injury in vivo, we developed an experimental paradigm involving focused ultrasound that permits continuous, real-time measurements and minimizes mechanical artifact. Injury was associated with a dramatic reduction of amplitude over the damaged hemisphere directly after the injury. The amplitude systematically improved over time but remained significantly decreased at one hour, compared with baseline. In contrast, at one hour there was a concomitant enhancement of the cortical SSEP amplitude evoked from the uninjured hemisphere. Analysis of the inter-trial electroencephalogram (EEG) also revealed significant changes in low-frequency components and an increase in EEG entropy up to 30 minutes after injury, likely reflecting altered EEG reactivity to somatosensory stimuli. Injury-induced alterations in SSEPs were also observed using noninvasive epidermal electrodes, demonstrating viability of practical implementation. These results suggest cortical SSEPs recorded at just a few locations by head-mounted sensors and associated multiparametric analyses could potentially be used to rapidly detect and monitor brain injury in settings that normally present significant levels of mechanical and electrical noise.

Sensory processing in the lower auditory pathway is generally considered to be rigid and thus less subject to modulation than central processing. However, in addition to the powerful bottom-up excitation by auditory nerve fibers, the ventral cochlear nucleus also receives efferent cholinergic innervation from both auditory and nonauditory top–down sources. We thus tested the influence of cholinergic modulation on highly precise time-coding neurons in the cochlear nucleus of the Mongolian gerbil. By combining electrophysiological recordings with pharmacological application in vitro and in vivo , we found 55–72% of spherical bushy cells (SBCs) to be depolarized by carbachol on two time scales, ranging from hundreds of milliseconds to minutes. These effects were mediated by nicotinic and muscarinic acetylcholine receptors, respectively. Pharmacological block of muscarinic receptors hyperpolarized the resting membrane potential, suggesting a novel mechanism of setting the resting membrane potential for SBC. The cholinergic depolarization led to an increase of spike probability in SBCs without compromising the temporal precision of the SBC output in vitro . In vivo , iontophoretic application of carbachol resulted in an increase in spontaneous SBC activity. The inclusion of cholinergic modulation in an SBC model predicted an expansion of the dynamic range of sound responses and increased temporal acuity. Our results thus suggest of a top–down modulatory system mediated by acetylcholine which influences temporally precise information processing in the lower auditory pathway.

Objective: This study examined the effect of microphone placement on the interaural level differences (ILDs) available to bilateral cochlear implant (BiCI) users, and the subsequent effects on horizontal-plane sound localization. Design: Virtual acoustic stimuli for sound localization testing were created individually for eight BiCI users by making acoustic transfer function measurements for microphones placed in the ear (ITE), behind the ear (BTE), and on the shoulders (SHD). The ILDs across source locations were calculated for each placement to analyze their effect on sound localization performance. Sound localization was tested using a repeated-measures, within-participant design for the three microphone placements. Results: The ITE microphone placement provided significantly larger ILDs compared to BTE and SHD placements, which correlated with overall localization errors. However, differences in localization errors across the microphone conditions were small. Conclusions: The BTE microphones worn by many BiCI users in everyday life do not capture the full range of acoustic ILDs available, and also reduce the change in cue magnitudes for sound sources across the horizontal plane. Acute testing with an ITE placement reduced sound localization errors along the horizontal plane compared to the other placements in some patients. Larger improvements may be observed if patients had more experience with the new ILD cues provided by an ITE placement.

Mice often produce ultrasonic vocalizations (USVs) that sweep upwards in frequency from around 60 to around 80 kHz and downwards in frequency from 80 to 60 kHz. Whether or not these USVs are used for communication purposes is still unknown. Here, mice were trained and tested using operant conditioning procedures and positive reinforcement to discriminate between synthetic upsweeps and downsweeps. The stimuli varied in bandwidth, duration, and direction of sweep. The mice performed significantly worse when discriminating between background and test stimuli when the stimuli all occupied the same bandwidths. Further, the mice's discrimination performance became much worse for stimuli that had durations similar to those natural vocalizations of the mice. Sweeps composed of different frequency ranges and longer durations had improved discrimination. These results collected using artificial stimuli created to mimic natural USVs indicate that the bandwidth of the vocalizations may be much more important for communication than the frequency contours of the vocalizations.

To efficiently detect a wide range of light-intensity changes, visual neurons must adapt to ambient luminance. However, how neurons in the primary visual cortex (V1) code the distribution of luminance remains unknown. We designed stimuli that represent rapid changes in luminance under different luminance distributions and investigated V1 neuron responses to these novel stimuli. We demonstrate that V1 neurons represent luminance changes by dynamically adjusting their responses when the luminance distribution changes. Many cells (35%) detected luminance changes by responding to dark stimuli when the distribution was dominated by bright stimuli, bright stimuli when dominated by dark stimuli, and both dark and bright stimuli when dominated by intermediate luminance stimuli; 13% of cells signaled the mean luminance that was varied with different distributions; the remaining 52% of cells gradually shifted the responses that were most sensitive to luminance changes when the luminance distribution varied. The remarkable response changes of the former two cell groups suggest their crucial roles in detecting luminance changes. These response characteristics demonstrate that V1 neurons are not only sensitive to luminance change, but also luminance distribution change. They encode luminance changes according to the luminance distribution. Mean cells represent the prevailing luminance and reversal cells represent the salient stimuli in the environment.

The Speech Intelligibility Index (SII) assumes additivity of the importance of acoustically independent bands of speech. To further evaluate this assumption, open-set speech recognition was measured for words and sentences, in quiet and in noise, when the speech stimuli were presented to the listener in selected frequency bands. The filter passbands were constructed from various combinations of 20 bands having equivalent (0.05) importance in the SII framework. This permitted the construction of a variety of equal-SII band patterns that were then evaluated by nine different groups of young adults with normal hearing. For monosyllabic words, a similar dependence on band pattern was observed for SII values of 0.4, 0.5, and 0.6 in both quiet and noise conditions. Specifically, band patterns concentrated toward the lower and upper frequency range tended to yield significantly lower scores than those more evenly sampling a broader frequency range. For all stimuli and test conditions, equal SII values did not yield equal performance. Because the spectral distortions of speech evaluated here may not commonly occur in everyday listening conditions, this finding does not necessarily represent a serious deficit for the application of the SII. These findings, however, challenge the band-independence assumption of the theory underlying the SII.

The neurotransmitter serotonin (5-HT) is considered a powerful modulator of sensory system organization and function in a wide range of animals. The olfactory system is innervated by midbrain 5-HT neurons into both its primary and secondary odor-processing stages. Facilitated by this circuitry, 5-HT and its receptors modulate olfactory system function, including odor information input to the olfactory bulb. It is unknown, however, whether the olfactory system requires 5-HT for even its most basic behavioral functions. To address this question, we established a conditional genetic approach to specifically target adult brain tryptophan hydroxylase 2 ( Tph2 ), encoding the rate-limiting enzyme in brain 5-HT synthesis, and nearly eliminate 5-HT from the mouse forebrain. Using this novel model, we investigated the behavior of 5-HT–depleted mice during performance in an olfactory go/no-go task. Surprisingly, the near elimination of 5-HT from the forebrain, including the olfactory bulbs, had no detectable effect on the ability of mice to perform the odor-based task. Tph2 -targeted mice not only were able to learn the task, but also had levels of odor acuity similar to those of control mice when performing coarse odor discrimination. Both groups of mice spent similar amounts of time sampling odors during decision-making. Furthermore, odor reversal learning was identical between 5-HT–depleted and control mice. These results suggest that 5-HT neurotransmission is not necessary for the most essential aspects of olfaction, including odor learning, discrimination, and certain forms of cognitive flexibility.

Objectives: The binaural interaction component (BIC) is the residual auditory brainstem response (ABR) obtained after subtracting the sum of monaurally evoked from binaurally evoked ABRs. The DN1 peak—the first negative peak of the BIC—has been postulated to have diagnostic value as a biomarker for binaural hearing abilities. Indeed, not only do DN1 amplitudes depend systematically upon binaural cues to location (interaural time and level differences), but they are also predictive of central hearing deficits in humans. A prominent issue in using BIC measures as a diagnostic biomarker is that DN1 amplitudes not only exhibit considerable variability across subjects, but also within subjects across different measurement sessions. Design: In this study, the authors investigate the DN1 amplitude measurement reliability by conducting repeated measurements on different days in eight adult guinea pigs. Results: Despite consistent ABR thresholds, ABR and DN1 amplitudes varied between and within subjects across recording sessions. However, the study analysis reveals that DN1 amplitudes varied proportionally with parent monaural ABR amplitudes, suggesting that common experimental factors likely account for the variability in both waveforms. Despite this variability, the authors show that the shape of the dependence between DN1 amplitude and interaural time difference is preserved. The authors then provide a BIC normalization strategy using monaural ABR amplitude that reduces the variability of DN1 peak measurements. Finally, the authors evaluate this normalization strategy in the context of detecting changes of the DN1 amplitude-to-interaural time difference relationship. Conclusions: The study results indicate that the BIC measurement variability can be reduced by a factor of two by performing a simple and objective normalization operation. The authors discuss the potential for this normalized BIC measure as a biomarker for binaural hearing.

Contralateral suppression of otoacoustic emissions (OAEs) is frequently used to assess the medial olivocochlear (MOC) efferent system, and may have clinical utility. However, OAEs are weak or absent in hearing-impaired ears, so little is known about MOC function in the presence of hearing loss. A potential alternative measure is contralateral suppression of the auditory steady-state response (ASSR) because ASSRs are measurable in many hearing-impaired ears. This study compared contralateral suppression of both transient-evoked otoacoustic emissions (TEOAEs) and ASSRs in a group of ten primarily older adults with either normal hearing or mild sensorineural hearing loss. Responses were elicited using 75-dB peak sound pressure level clicks. The MOC was activated using contralateral broadband noise at 60 dB sound pressure level. Measurements were made concurrently to ensure a consistent attentional state between the two measures. The magnitude of contralateral suppression of ASSRs was significantly larger than contralateral suppression of TEOAEs. Both measures usually exhibited high test–retest reliability within a session. However, there was no significant correlation between the magnitude of contralateral suppression of TEOAEs and of ASSRs. Further work is needed to understand the role of the MOC in contralateral suppression of ASSRs.

Sound recognition relies not only on spectral cues, but also on temporal cues, as demonstrated by the profound impact of time reversals on perception of common sounds. To address the coding principles underlying such auditory asymmetries, we recorded a large sample of auditory cortex neurons using two-photon calcium imaging in awake mice, while playing sounds ramping up or down in intensity. We observed clear asymmetries in cortical population responses, including stronger cortical activity for up-ramping sounds, which matches perceptual saliency assessments in mice and previous measures in humans. Analysis of cortical activity patterns revealed that auditory cortex implements a map of spatially clustered neuronal ensembles, detecting specific combinations of spectral and intensity modulation features. Comparing different models, we show that cortical responses result from multi-layered nonlinearities, which, contrary to standard receptive field models of auditory cortex function, build divergent representations of sounds with similar spectral content, but different temporal structure.