Cortical dysmorphology and reduced cortico-collicular projections in an animal model of autism spectrum disorder.

Autism spectrum disorder is a neurodevelopmental disability that includes sensory disturbances. Hearing is frequently affected and ranges from deafness to hypersensitivity. In utero exposure to the antiepileptic valproic acid is associated with increased risk of autism spectrum disorder in humans and timed valproic acid exposure is a biologically relevant and validated animal model of autism spectrum disorder. Valproic acid-exposed rats have fewer neurons in their auditory brainstem and thalamus, fewer calbindin-positive neurons, reduced ascending projections to the midbrain and thalamus, elevated thresholds, and delayed auditory brainstem responses. Additionally, in the auditory cortex, valproic acid exposure results in abnormal responses, decreased phase-locking, elevated thresholds, and abnormal tonotopic maps. We therefore hypothesized that in utero, valproic acid exposure would result in fewer neurons in auditory cortex, neuronal dysmorphology, fewer calbindin-positive neurons, and reduced connectivity. We approached this hypothesis using morphometric analyses, immunohistochemistry, and retrograde tract tracing. We found thinner cortical layers but no changes in the density of neurons, smaller pyramidal and non-pyramidal neurons in several regions, fewer neurons immunoreactive for calbindin-positive, and fewer cortical neurons projecting to the inferior colliculus. These results support the widespread impact of the auditory system in autism spectrum disorder and valproic acid-exposed animals and emphasize the utility of simple, noninvasive auditory screening for autism spectrum disorder.

Impact of cochlear ablation on calbindin and synaptophysin in the gerbil medial nucleus of the trapezoid body before hearing onset.

Spontaneous bursting activity is already generated in the cochlea before hearing onset and represents an important condition of the functional and anatomical organization of auditory brainstem nuclei. In the present study, cochlea ablation induced changes were characterized in auditory brainstem nuclei indirectly innervated by auditory nerve fibers before hearing onset. In Meriones unguiculatus immunohistochemical labeling of calbindin-D28k (CB) and synaptophysin (SYN) were performed. The influence of cochlea-ablation on CB or SYN was analyzed by considering their differential immunoreaction during development. During the normal postnatal development, CB was first detected in somata of the medial nucleus of the trapezoid body (MNTB) at postnatal day (P)4. The immunoreaction increased gradually in parallel to the appearance of CB-immunoreactive terminal fields in distinct superior olivary complex (SOC) nuclei. Cochlear removal at P5 or P9 in animals with 24 and 48 h survival times resulted in an increase in somatic CB-labeling in the lesioned MNTB including terminal fields compared to the non-lesioned MNTB. SYN-immunolabeling was first detected at P0 and began to strongly encircle the MNTB neurons at P4. A further progression was observed with age. Cochlear ablation resulted in a significant reduction of SYN-labeled MNTB areas of P5-cochlea-ablated gerbils after 48 h post-lesion. In P9 cochlea-ablated gerbils, a redistribution of SYN-positive terminals was seen after 24 and 48 h. Taken together, the destruction of cochlea differentially influences CB- and SYN-labeling in the MNTB, which should be considered in association with different critical periods before hearing onset.

Sensory cortical and subcortical auditory neurophysiological changes predict cue-specific extinction behavior enabled by the pharmacological inhibition of an epigenetic regulator during memory formation.

Extinction learning and memory have been broadly investigated at both behavioral and neural levels, but sensory system contributions to extinction processes have been less explored. Using a sound-reward extinction paradigm in male rats, we reveal both cortical and subcortical forms of plasticity associated with the cue-specificity of behavioral extinction memory. In the auditory cortex, frequency tuning narrowed by up to two-thirds of an octave around the remembered extinguished sound cue. Subcortical signals revealed in the auditory brainstem response (ABR) in the same animals developed smaller amplitudes of some (but not all) ABR peaks evoked by the extinguished sound frequency. Interestingly, treatment with an inhibitor of histone deacetylase 3 (HDAC3-i) facilitated both auditory cortical tuning bandwidth changes and changes in subcortical peak amplitude evoked only by the extinguished sound frequency. These neurophysiological changes were correlated to each other, and to the highly precise extinction behavior enabled by HDAC3-i (compared to vehicle controls). Thus, we show for the first time that HDAC3 regulates the specificity of sensory features consolidated in extinction memory. Further, the sensory cortical changes in tuning bandwidth recapitulate known effects of blocking HDAC3 to enhance cue specificity in other behavioral tasks. Therefore, the findings demonstrate how some forms of sensory neuroplasticity may encode specific sensory features of learning experiences in order to enable cue-specific behaviors.

Auditory DUM neurons in a bush-cricket: inhibited inhibitors.

Thoracic ganglia of many hearing insects house the first level of auditory processing. In bush-crickets, the largest population of local auditory neurons in the prothoracic processing centre are dorsal unpaired median (DUM) neurons. It has been suggested that DUM neurons are inhibitory using γ-aminobutyric acid (GABA) as transmitter. Immunohistochemistry reveals a population of about 35-50 GABA-positive somata in the posterior medial cluster of the prothoracic ganglion. Only very few small somata in this cluster remain unstained. At least 10 neurites from 10 neurons can be identified. Intracellularly stained auditory DUM neurons have their soma in the cluster of median GABA positive cells and most of them exhibit GABA-immunoreactivity. Responses of certain DUM neurons show obvious signs of inhibition. Application of picrotoxin (PTX), a chloride-channel blocker in insects, changes the responses of many DUM neurons. They become broader in frequency tuning and broader or narrower in temporal pattern tuning. Furthermore, inhibitory postsynaptic potentials (IPSPs) may be replaced by excitatory postsynaptic potentials. Loss of an IPSP in the rising graded potential after PTX-application leads to a significant reduction of first-spike latency. Therefore, auditory DUM neurons receive effective inhibition and are the best candidates for inhibition in DUM neurons and other auditory interneurons.

In utero exposure to valproic acid disrupts ascending projections to the central nucleus of the inferior colliculus from the auditory brainstem.

Prenatal exposure to the antiepileptic valproic acid (VPA) is associated with an increased risk of autism spectrum disorder (ASD) in humans. Accordingly, in utero exposure to VPA is a validated and biologically relevant animal model of ASD. The majority of individuals with ASD exhibit some degree of auditory dysfunction, ranging from deafness to hypersensitivity. Animals exposed to VPA in utero have abnormal tonotopic maps and responses in the cerebral cortex and hyperactivation, hypoplasia, abnormal neuronal morphology and reduced calcium binding protein expression throughout the auditory brainstem nuclei. Further, our previous work suggests that GABAergic neuronal populations may be more severely impacted by in utero VPA exposure. However, the axonal projection patterns of brainstem nuclei to the inferior colliculus (IC) have not been investigated in VPA-exposed animals. Herein, we use stereotaxic injections of the retrograde tracer Fast Blue into the central nucleus of the IC (CNIC) and examine the proportions of retrogradely labeled neurons in the nuclei of the lateral lemniscus, superior olivary complex and cochlear nuclei. Our results indicate that not only are there fewer neurons in the auditory brainstem after VPA exposure, but also that fewer neurons are retrogradely labeled from the CNIC. Together, our results indicate that in utero VPA exposure may result in altered patterns of input to the auditory midbrain.

Measuring the middle-ear reflex: A quantitative method to assess effects of industrial solvents on central auditory pathways.

Volatile organic solvents are frequently present in industrial atmospheres. Their lipophilic properties mean they quickly reach the brain following inhalation. Acute exposure to some solvents perturbs the middle ear reflex, which could jeopardize cochlear protection against loud noises. As the physiological mechanisms involved in this protective reflex are highly complex, in vivo rodent models are required to allow rapid and reliable identification of any adverse effects of solvents on the middle ear reflex (MER). In this study, MER amplitude was measured in anesthetized Brown-Norway rats by monitoring the decrease in distortion product otoacoustic emissions (DPOAEs) caused by a contralateral stimulation. Our screening test consisted in measuring the impact of inhalation of solvent vapors at 3000 ppm for 15 min on the MER amplitude. We had previously studied a selection of aromatic solvents with this model; here, we extended the analysis to volatile compounds from other chemical families. The results obtained shed light on the mechanisms involved in the interactions between solvents and their neuronal targets. Thus, benzene and chlorobenzene had the greatest effect on MER (≥ + 1.8 dB), followed by a group composed of toluene, styrene, p-xylene, m-xylene, tetrachloroethylene and cyclohexane, which had a moderate effect on the MER (between + 0.3 and + 0.7 dB). Finally, trichloroethylene, n-hexane, methyl-ethyl-ketone, acetone, o-xylene, and ethylbenzene had no effect on the MER. Thus, the effect of solvents on the MER is not simply linked to their lipophilicity, rather it depends on specific interactions with neuronal targets. These interactions appear to be governed by the compound's chemical structure, e.g. the presence of an aromatic ring and its steric hindrance. In addition, perturbation of the MER by a solvent is independent of its toxic effects on cochlear cells. As the MER plays a protective role against exposure to high-intensity noises, these findings could have a significant impact in terms of prevention for subjects exposed to both noise and solvents.

Effect of perinatal biotin deficiency on auditory pathway of the Wistar-Albino rats.

Aim: Severe biotin deficiency associated with biotinidase enzyme deficiency in newborns is seen as severe neurological problems and hearing loss. However, the effect on the infant of deficiencies in the maternal diet during pregnancy are not clear. Material and methods: The study included 16 female Wistar albino rats and 4 male Wistar albino rats, that were mated and then the females were separated into 4 groups. At 40 days after the birth, 3 pups were selected from each group, and these 12 pups were evaluated with DPOAE and ABR electrophysiologically and the cochlea was examined ultrastructurally with electron microscopy. Results: In the DPOAE evaluation, At 8000 and 11,000 Hz, the signal-noise ratios in the B-N and B-B groups were statistically significantly higher (p < .05). In ABR, lengthening of the latency periods was determined in all the waves at both 8 and 16 kHz in the B-B group. When the IPL periods were examined, lengthening in IPL 1-5 was statistically significant in the B-B group only at 8 kHz. Conclusions: Biotin can be said to have an effect on hearing pathways. However, specifically where on the hearing pathways that biotin is involved has not been clarified.

Effect of levodopa on cochlear mechanics and efferent auditory system of Parkinson's disease individuals. / Efeito da levodopa na mecânica coclear e no sistema auditivo eferente de indivíduos com doença de Parkinson.

PURPOSE: To evaluate the effect of levodopa on cochlear dynamics and on the medial olivocochlear efferent pathway of idiopathic Parkinson's disease (PD) individuals. METHODS: Individuals with and without PD, followed at a University Hospital, were submitted to Distortion Product Otoacoustic Emissions (DPOAE) and DPOAE Inhibitory Effect (OAEIE) in the presence of contralateral noise. Correlation measures between DPOAE and OAEIE results with Hoehn&Yahr (H&Y) stage, daily dose of levodopa and PD diagnosis period were established. Furthermore, electroacoustic measures were compared between individuals without and those with PD, stratified by dose of levodopa daily administered. RESULTS: Weak negative correlation between DPOAE amplitude and daily dose of levodopa was found, as well positive correlations between EIEOA with daily dose of levodopa and time of PD diagnosis, respectively. Higher DPOAE amplitude was found in individuals with PD using daily doses of levodopa ≤ 600 milligrams when compared to individuals without PD and those with PD using higher doses. EIEOA was lower in individuals using doses ≤ 600 milligrams, when compared to the other groups. CONCLUSION: Daily doses of levodopa up to 600 mg / day increase the cochlear mechanical-transducer responses in 2 and 3 kHz frequencies, while the action of olivocochlear efferent systems is reduced in this region.

OBJETIVO: Analisar o efeito da levodopa na dinâmica coclear, bem como na via eferente olivococlear medial de indivíduos com doença de Parkinson idiopática (DP). MÉTODO: Indivíduos com e sem DP, acompanhados em um hospital universitário, realizaram a pesquisa das emissões otoacústicas por produto de distorção (EOAPD) e do efeito inibitório das EOAPD (EIEOA) na presença de ruído contralateral. Foram estabelecidas as medidas de correlação entre os resultados das EOAPD e do EIEOA com estágio Hoehn&Yahr (H&Y), dose diária de levodopa e tempo de diagnóstico da DP. Além disso, as medidas eletroacústicas foram comparadas entre os indivíduos sem DP e com DP, estratificados de acordo com a dose de levodopa administrada diariamente. RESULTADOS: Foi identificada correlação fraca e negativa entre a amplitude das EOAPD com a dose diária de levodopa e correlações positivas, de força moderada e fraca, entre o EIEOA com a dose diária de levodopa e o tempo de diagnóstico da DP, respectivamente. A amplitude das EOAPD foi maior nos indivíduos com DP em uso de levodopa ≤ 600 miligramas quando comparada à de indivíduos sem DP e com DP, em uso de dose superior. Já o EIEOA foi menor nos indivíduos em uso de doses ≤ 600 miligramas, quando comparado aos demais grupos. CONCLUSÃO: Doses diárias de levodopa iguais ou inferiores a 600 mg/dia aumentam as respostas mecanotransdutoras cocleares nas frequências de 2 e 3 kHz, enquanto que a ação dos sistemas eferentes olivococleares é reduzida nesta região.

Auditory Midbrain Hypoplasia and Dysmorphology after Prenatal Valproic Acid Exposure.

Prenatal exposure to the antiepileptic valproic acid (VPA) is associated with an increased risk of autism spectrum disorder (ASD) in humans and is used as an animal model of ASD. The majority of individuals with ASD exhibit adverse reactions to sensory stimuli and auditory dysfunction. Previous studies of animals exposed to VPA reveal abnormal neuronal responses to sound and mapping of sound frequency in the cerebral cortex and hyperactivation, hypoplasia and abnormal neuronal morphology in the cochlear nuclei (CN) and superior olivary complex (SOC). Herein, we examine the neuronal populations in the lateral lemniscus and inferior colliculus in animals exposed in utero to VPA. We used a combination of morphometric techniques, histochemistry and immunofluorescence to examine the nuclei of the lateral lemniscus (NLL) and the central nucleus of the inferior colliculus (CNIC). We found that the VPA exposure resulted in larger neurons in the CNIC and the dorsal nucleus of the lateral lemniscus (DNLL). However, we found that there were significantly fewer neurons throughout all nuclei examined in the auditory brainstem of VPA-exposed animals. Additionally, we found significantly fewer calbindin-immunopositive neurons in the DNLL. VPA exposure had no impact on the proportions of perineuronal nets in the NLL or CNIC. Finally, consistent with our observations in the CN and SOC, VPA exposure resulted in fewer dopaminergic terminals in the CNIC. Together, these results indicate that in utero VPA exposure significantly impacts structure and function of nearly the entire central auditory pathway.

Age-Related Changes in Ang II Receptor Localization and Expression in the Developing Auditory Pathway.

We studied Ang II receptor localization in different nuclei of the auditory system, by means of binding autoradiography, during brain development. The inferior colliculus (IC), a large midbrain structure which serves as an obligatory synaptic station in both the ascending and descending auditory pathways, exhibited high Ang II AT2 binding at all ages (P0, P8, P15, P30), being maximal at P15. These observations were confirmed by in situ hybridization and immunofluorescence at P15, demonstrating that AT2 receptor mRNA localized at the same area recognized by AT2 antibodies and anti ß III-tubulin suggesting the neuronal nature of the reactive cells. Ang II AT1 receptors were absent at early developmental ages (P0) in all nuclei of the auditory system and a low level was observed in the IC at the age P8. AT2 receptors were present at ventral cochlear nucleus and superior olivary complex, being higher at P15 and P8, respectively. We also explored the effect of prenatal administration of Ang II or PD123319 (AT2 antagonist) on binding of Ang II receptors at P0, P8, P15. Both treatments increased significantly the level of AT2 receptors at P0 and P8 in the IC. Although total binding in the whole IC from P15 animals showed no difference between treatments, the central nucleus of the IC exhibited higher binding. Our results supports a correlation between the timing of the higher expression of Ang II AT2 receptors in different nuclei, the onset of audition and the establishment of neuronal circuits of the auditory pathway.

Synergistic Transcriptional Changes in AMPA and GABAA Receptor Genes Support Compensatory Plasticity Following Unilateral Hearing Loss.

Debilitating perceptual disorders including tinnitus, hyperacusis, phantom limb pain and visual release hallucinations may reflect aberrant patterns of neural activity in central sensory pathways following a loss of peripheral sensory input. Here, we explore short- and long-term changes in gene expression that may contribute to hyperexcitability following a sudden, profound loss of auditory input from one ear. We used fluorescence in situ hybridization to quantify mRNA levels for genes encoding AMPA and GABAA receptor subunits (Gria2 and Gabra1, respectively) in single neurons from the inferior colliculus (IC) and auditory cortex (ACtx). Thirty days after unilateral hearing loss, Gria2 levels were significantly increased while Gabra1 levels were significantly decreased. Transcriptional rebalancing was more pronounced in ACtx than IC and bore no obvious relationship to the degree of hearing loss. By contrast to the opposing, synergistic shifts in Gria2 and Gabra1 observed 30 days after hearing loss, we found that transcription levels for both genes were equivalently reduced after 5 days of hearing loss, producing no net change in the excitatory/inhibitory transcriptional balance. Opposing transcriptional shifts in AMPA and GABA receptor genes that emerge several weeks after a peripheral insult could promote both sensitization and disinhibition to support a homeostatic recovery of neural activity following auditory deprivation. Imprecise transcriptional changes could also drive the system toward perceptual hypersensitivity, degraded temporal processing and the irrepressible perception of non-existent environmental stimuli, a trio of perceptual impairments that often accompany chronic sensory deprivation.

Dorsal Cochlear Nucleus Fusiform-cell Plasticity is Altered in Salicylate-induced Tinnitus.

Following noise overexposure and tinnitus-induction, fusiform cells of the dorsal cochlear nucleus (DCN) show increased spontaneous firing rates (SFR), increased spontaneous synchrony and altered stimulus-timing-dependent plasticity (StDP), which correlate with behavioral measures of tinnitus. Sodium salicylate, the active ingredient in aspirin, which is commonly used to induce tinnitus, increases SFR and activates NMDA receptors in the ascending auditory pathway. NMDA receptor activation is required for StDP in many brain regions, including the DCN. Blocking NMDA receptors can alter StDP timing rules and decrease synchrony in DCN fusiform cells. Thus, systemic activation of NMDA receptors with sodium salicylate should elicit pathological changes to StDP, thereby increasing SFR and synchrony and induce tinnitus. Herein, we examined the action of salicylate in tinnitus generation in guinea pigs in vivo by measuring tinnitus using two behavioral measures and recording single-unit responses from DCN fusiform cells pre- and post-salicylate administration in the same animals. First, we show that animals administered salicylate show evidence of tinnitus using both behavioral paradigms, cross-validating the tests. Second, fusiform cells in animals with tinnitus showed increased SFR, synchrony and altered StDP timing rules, like animals with noise-induced tinnitus. These findings suggest that alterations to fusiform-cell plasticity are an essential component of tinnitus, regardless of induction technique.

Muscarinic receptors regulate auditory and prefrontal cortical communication during auditory processing.

Much of our understanding about how acetylcholine modulates prefrontal cortical (PFC) networks comes from behavioral experiments that examine cortical dynamics during highly attentive states. However, much less is known about how PFC is recruited during passive sensory processing and how acetylcholine may regulate connectivity between cortical areas outside of task performance. To investigate the involvement of PFC and cholinergic neuromodulation in passive auditory processing, we performed simultaneous recordings in the auditory cortex (AC) and PFC in awake head fixed mice presented with a white noise auditory stimulus in the presence or absence of local cholinergic antagonists in AC. We found that a subset of PFC neurons were strongly driven by auditory stimuli even when the stimulus had no associative meaning, suggesting PFC monitors stimuli under passive conditions. We also found that cholinergic signaling in AC shapes the strength of auditory driven responses in PFC, by modulating the intra-cortical sensory response through muscarinic interactions in AC. Taken together, these findings provide novel evidence that cholinergic mechanisms have a continuous role in cortical gating through muscarinic receptors during passive processing and expand traditional views of prefrontal cortical function and the contributions of cholinergic modulation in cortical communication.

Efeito da levodopa na mecânica coclear e no sistema auditivo eferente de indivíduos com doença de Parkinson / Effect of levodopa on cochlear mechanics and efferent auditory system of Parkinson's disease individuals

RESUMO Objetivo Analisar o efeito da levodopa na dinâmica coclear, bem como na via eferente olivococlear medial de indivíduos com doença de Parkinson idiopática (DP). Método Indivíduos com e sem DP, acompanhados em um hospital universitário, realizaram a pesquisa das emissões otoacústicas por produto de distorção (EOAPD) e do efeito inibitório das EOAPD (EIEOA) na presença de ruído contralateral. Foram estabelecidas as medidas de correlação entre os resultados das EOAPD e do EIEOA com estágio Hoehn&Yahr (H&Y), dose diária de levodopa e tempo de diagnóstico da DP. Além disso, as medidas eletroacústicas foram comparadas entre os indivíduos sem DP e com DP, estratificados de acordo com a dose de levodopa administrada diariamente. Resultados Foi identificada correlação fraca e negativa entre a amplitude das EOAPD com a dose diária de levodopa e correlações positivas, de força moderada e fraca, entre o EIEOA com a dose diária de levodopa e o tempo de diagnóstico da DP, respectivamente. A amplitude das EOAPD foi maior nos indivíduos com DP em uso de levodopa ≤ 600 miligramas quando comparada à de indivíduos sem DP e com DP, em uso de dose superior. Já o EIEOA foi menor nos indivíduos em uso de doses ≤ 600 miligramas, quando comparado aos demais grupos. Conclusão Doses diárias de levodopa iguais ou inferiores a 600 mg/dia aumentam as respostas mecanotransdutoras cocleares nas frequências de 2 e 3 kHz, enquanto que a ação dos sistemas eferentes olivococleares é reduzida nesta região.

ABSTRACT Purpose To evaluate the effect of levodopa on cochlear dynamics and on the medial olivocochlear efferent pathway of idiopathic Parkinson's disease (PD) individuals. Methods Individuals with and without PD, followed at a University Hospital, were submitted to Distortion Product Otoacoustic Emissions (DPOAE) and DPOAE Inhibitory Effect (OAEIE) in the presence of contralateral noise. Correlation measures between DPOAE and OAEIE results with Hoehn&Yahr (H&Y) stage, daily dose of levodopa and PD diagnosis period were established. Furthermore, electroacoustic measures were compared between individuals without and those with PD, stratified by dose of levodopa daily administered. Results Weak negative correlation between DPOAE amplitude and daily dose of levodopa was found, as well positive correlations between EIEOA with daily dose of levodopa and time of PD diagnosis, respectively. Higher DPOAE amplitude was found in individuals with PD using daily doses of levodopa ≤ 600 milligrams when compared to individuals without PD and those with PD using higher doses. EIEOA was lower in individuals using doses ≤ 600 milligrams, when compared to the other groups. Conclusion Daily doses of levodopa up to 600 mg / day increase the cochlear mechanical-transducer responses in 2 and 3 kHz frequencies, while the action of olivocochlear efferent systems is reduced in this region.

ERG Channels Regulate Excitability in Stellate and Bushy Cells of Mice Ventral Cochlear Nucleus.

ERG (ether-a-go-go-related gene) channels are the members of the voltage-dependent potassium channel family, which have three subtypes, as ERG1 (Kv 11.1), ERG2 (Kv 11.2), and ERG3 (Kv11.3). There is no information on ERG channels in the cochlear nucleus (CN) neurons, which is the first relay station of the auditory pathway. As occur in some of congenital long QT Syndromes (LQTS), mutation of the KCNQ11 genes for ERG channel has been reported to be accompanied by hearing loss. For that reason, we aimed to study biophysical properties and physiological importance, and contribution of ERG K+ currents to the formation of action potentials in the stellate and bushy neurons of the ventral cochlear nucleus (VCN). A total of 70 mice at 14-17 days old were used for this study. Electrophysiological characterization of ERG channels was performed using patch-clamp technique in the CN slices. In current clamp, ERG channel blockers, terfenadine (10 µM) and E-4031 (10 µM), were applied in both cell types. The activation, inactivation, and deactivation kinetics of the ERG channels were determined by voltage clamp. In conclusion, the findings obtained in the present study suggest that stellate and bushy neurons express ERG channels and ERG channels appear to contribute to setting action potential (AP) frequency, threshold for AP induction, and, possibly, resting membrane potentials in this cells.

Activation in the auditory pathway of the gerbil studied with 18F-FDG PET: effects of anesthesia.

Here, we present results from an 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) study in the Mongolian gerbil, a preferred animal model in auditory research. One major issue in preclinical nuclear imaging, as well as in most of the neurophysiological methods investigating auditory processing, is the need of anesthesia. We compared the usability of two types of anesthesia which are frequently employed in electrophysiology, ketamine/xylazine (KX), and fentanyl/midazolam/medetomidine (FMM), for valid measurements of auditory activation with 18F-FDG PET. Gerbils were placed in a sound-shielding box and injected with 18F-FDG. Two acoustic free-field conditions were used: (1) baseline (no stimulation, 25 dB background noise) and (2) 90 dB frequency-modulated tones (FM). After 40 min of 18F-FDG uptake, a 30 min acquisition was performed using a small animal PET/CT system. Blood glucose levels were measured after the uptake phase before scanning. Standardized uptake value ratios for relevant regions were determined after implementing image and volume of interest templates. Scans demonstrated a significantly higher uptake in the inferior colliculus with FM stimulation compared to baseline in awake subjects (+ 12%; p = 0.02) and with FMM anesthesia (+ 13%; p = 0.0012), but not with KX anesthesia. In non-auditory brain regions, no significant difference was detected. Blood glucose levels were significantly higher under KX compared to FMM anesthesia (17.29 ± 0.42 mmol/l vs. 14.30 ± 1.91 mmol/l; p = 0.024). These results suggest that valid 18F-FDG PET measurements of auditory activation comparable to electrophysiology can be obtained from gerbils during opioid-based anesthesia due to its limited effects on interfering blood glucose levels.

Chronic lead exposure induces cochlear oxidative stress and potentiates noise-induced hearing loss.

Acquired hearing loss is caused by complex interactions of multiple environmental risk factors, such as elevated levels of lead and noise, which are prevalent in urban communities. This study delineates the mechanism underlying lead-induced auditory dysfunction and its potential interaction with noise exposure. Young-adult C57BL/6 mice were exposed to: 1) control conditions; 2) 2 mM lead acetate in drinking water for 28 days; 3) 90 dB broadband noise 2 h/day for two weeks; and 4) both lead and noise. Blood lead levels were measured by inductively coupled plasma mass spectrometry analysis (ICP-MS) lead-induced cochlear oxidative stress signaling was assessed using targeted gene arrays, and the hearing thresholds were assessed by recording auditory brainstem responses. Chronic lead exposure downregulated cochlear Sod1, Gpx1, and Gstk1, which encode critical antioxidant enzymes, and upregulated ApoE, Hspa1a, Ercc2, Prnp, Ccl5, and Sqstm1, which are indicative of cellular apoptosis. Isolated exposure to lead or noise induced 8-12 dB and 11-25 dB shifts in hearing thresholds, respectively. Combined exposure induced 18-30 dB shifts, which was significantly higher than that observed with isolated exposures. This study suggests that chronic exposure to lead induces cochlear oxidative stress and potentiates noise-induced hearing impairment, possibly through parallel pathways.

A complex mTOR response in habituation paradigms for a social signal in adult songbirds.

Nonassociative learning is considered simple because it depends on presentation of a single stimulus, but it likely reflects complex molecular signaling. To advance understanding of the molecular mechanisms of one form of nonassociative learning, habituation, for ethologically relevant signals we examined song recognition learning in adult zebra finches. These colonial songbirds learn the unique song of individuals, which helps establish and maintain mate and other social bonds, and informs appropriate behavioral interactions with specific birds. We leveraged prior work demonstrating behavioral habituation for individual songs, and extended the molecular framework correlated with this behavior by investigating the mechanistic Target of Rapamycin (mTOR) signaling cascade. We hypothesized that mTOR may contribute to habituation because it integrates a variety of upstream signals and enhances associative learning, and it crosstalks with another cascade previously associated with habituation, ERK/ZENK. To begin probing for a possible role for mTOR in song recognition learning, we used a combination of song playback paradigms and bidirectional dysregulation of mTORC1 activation. We found that mTOR demonstrates the molecular signatures of a habituation mechanism, and that its manipulation reveals the complexity of processes that may be invoked during nonassociative learning. These results thus expand the molecular targets for habituation studies and raise new questions about neural processing of complex natural signals.

Increased spontaneous firing rates in auditory midbrain following noise exposure are specifically abolished by a Kv3 channel modulator.

Noise exposure has been shown to produce long-lasting increases in spontaneous activity in central auditory structures in animal models, and similar pathologies are thought to contribute to clinical phenomena such as hyperacusis or tinnitus in humans. Here we demonstrate that multi-unit spontaneous neuronal activity in the inferior colliculus (IC) of mice is significantly elevated four weeks following noise exposure at recording sites with frequency tuning within or near the noise exposure band, and this selective central auditory pathology can be normalised through administration of a novel compound that modulates activity of Kv3 voltage-gated ion channels. The compound had no statistically significant effect on IC spontaneous activity without noise exposure, nor on thresholds or frequency tuning of tone-evoked responses either with or without noise exposure. Administration of the compound produced some reduction in the magnitude of evoked responses to a broadband noise, but unlike effects on spontaneous rates, these effects on evoked responses were not specific to recording sites with frequency tuning within the noise exposure band. Thus, the results suggest that modulators of Kv3 channels can selectively counteract increases in spontaneous activity in the auditory midbrain associated with noise exposure.

Age-related defects in short-term plasticity are reversed by acetyl-L-carnitine at the mouse calyx of Held.

Hearing acuity and sound localization are affected by aging and may contribute to cognitive dementias. Although loss of sensorineural conduction is well documented to occur with age, little is known regarding short-term synaptic plasticity in central auditory nuclei. Age-related changes in synaptic transmission properties were evaluated at the mouse calyx of Held, a sign-inverting relay synapse in the circuit for sound localization, in juvenile adults (1 month old) and late middle-aged (18-21 months old) mice. Synaptic timing and short-term plasticity were severely disrupted in older mice. Surprisingly, acetyl-l-carnitine (ALCAR), an anti-inflammatory agent that facilitates mitochondrial function, fully reversed synaptic transmission delays and defects in short-term plasticity in aged mice to reflect transmission similar to that seen in juvenile adults. These findings support ALCAR supplementation as an adjuvant to improve short-term plasticity and potentially central nervous system performance in animals compromised by age and/or neurodegenerative disease.