Régis Nouvian

Iron is essential for cellular function, as it is required for oxygen transport and mitochondria oxidative respiration. However, uncoordinated regulation of intracellular iron can produce reactive oxygen species, calling for a tight homeostasis. Here, we examine the role of the iron-binding protein, ferritin, known to store intracellular iron, in the cochlea. Genetic ablation of the ferritin light-chain ( Ftl ) in mouse leads to two different phenotypes. A fraction of the homozygous mice has moderate to profound hearing loss ( Ftl ^{− / −} with High-Threshold, HT), with the other fraction of mice unaffected, i.e., with normal auditory threshold ( Ftl ^{− / −} with Low-Threshold, LT). In the Ftl ^{− / −} HT mice, the outer hair cells, which amplify incoming sound-stimulation, undergo a massive degeneration and the inner hair cells, which converts the mechanical pressure into exocytosis of glutamate, harbor splayed hair bundles. In addition, patch-clamp recordings demonstrated the alteration of calcium current in the IHCs from Ftl ^{− / −} HT mice. Finally, the Ftl ^{− / −} LT mice were found to be more vulnerable to acoustic exposure, suggesting that the difference between the two phenotypes may partly stem from noxious environment. Taken together, our results suggest that ferritin light-chain is a contributing factor to the hair cell function and survival.

There is a substantial body of evidence elucidating the pathophysiological aspects of excitotoxicity in the mammalian cochlea. However, the question of whether the resultant damage is reversible remains unresolved. To replicate an excitotoxic event, we investigated the long-term effects of kainate application in gerbil cochleae. Surprisingly, despite persistent synapse loss, the compound action potential of the auditory nerve fully recovered. This functional retrieval was associated with a phenotypic change in auditory nerve fibers. Thresholds were improved along the tonotopic axis. High-spontaneous rate (SR) fibers largely populated the apical region, while low-SR fibers from the basal region exhibited sound-driven activity indistinguishable from control high-SR fibers. This functional phenotype change may support the full recovery of neural response thresholds and amplitudes after excitotoxicity. Furthermore, hyperresponsiveness of the auditory nerve fibers could be a crucial factor in the development of hyperactivity in the central auditory pathways, a common occurrence following acoustic overstimulation.

Médecine/sciences (M/S), revue internationale dans le domaine de la recherche biologique, médicale et en santé

Summary: Salience attributed to stimuli predicting rewarding or aversive outcomes is critical for adaptive behavior. Dopamine (DA)-neurons play a central role in this process by modulating responses to both rewarding and aversive cues. DA-neurons are tightly and readily modulated by DA D2 autoreceptors (autoD2Rs), but their role in regulating responses to aversive stimuli remains unclear. In this study, we investigated the role of autoD2R in regulating the activity of VTA DA-neurons in response to salient aversive stimuli. Using Drd2 ^Slc6a3 mice, in which Drd2 is selectively deleted in DA-neurons, we observed enhanced excitatory and inhibitory responses of VTA DA-neurons to aversive stimuli, suggesting that autoD2R acts as a critical regulatory brake. Importantly, this modulation occurred independently of either the pacemaker activity of DA-neurons or their coupling to the non-selective sodium leak channel NALCN. Behaviorally, Drd2 ^Slc6a3 male mice showed enhanced discrimination between threat-predicting and non-predicting cues that persisted during extinction learning, highlighting a sex-biased role of autoD2R in threat processing. Our results provide new mechanistic insights through which autoD2R influence behavioral responses to aversive stimuli, with implications for understanding neuropsychiatric disorders characterized by maladaptive threat processing.

Two calcium-binding proteins, CaBP1 and CaBP2, cooperate to keep calcium channels in the hair cells of the inner ear open.

Sound-level coding in the auditory nerve is achieved through the progressive recruitment of auditory nerve fibers (ANFs) that differ in threshold of activation and in the stimulus level at which the spike rate saturates. To investigate the functional state of the ANFs, the electrophysiological tests routinely used in clinics only capture the first action potentials firing in synchrony at the onset of the acoustic stimulation. Assessment of other properties (e.g., spontaneous rate and adaptation time constants) requires single-fiber recordings directly from the nerve, which for ethical reasons is not allowed in humans. By combining neuronal activity measurements at the round window and signal-processing algorithms, we constructed a peristimulus time response (PSTR), with a waveform similar to the peristimulus time histograms (PSTHs) derived from single-fiber recordings in young adult female gerbils. Simultaneous recordings of round-window PSTR and single-fiber PSTH provided models to predict the adaptation kinetics and spontaneous rate of the ANFs tuned at the PSTR probe frequency. The predictive model derived from gerbils was then validated in female mice and finally applied to humans by recording PSTRs from the auditory nerve in normal-hearing patients who underwent cerebellopontine angle surgeries. A rapid adaptation time constant of ∼3 ms and a mean spontaneous rate of ∼22 spikes/s in the 4 kHz frequency range were found. This study offers a promising diagnostic tool to map the human auditory nerve, thus opening new avenues to better understanding auditory neuropathies, tinnitus, and hyperacusis.SIGNIFICANCE STATEMENTNeural adaptation in auditory nerve fibers corresponds to the reduction in the neuronal activity to prolonged or repeated sound stimulation. For obvious ethical reasons, single-fiber recordings from the auditory nerve are not feasible in humans, creating a critical gap in extending data obtained using animal models to humans. Using electrocochleography in rodents, we inferred adaptation kinetics and spontaneous discharge rates of the auditory nerve fibers in humans. Routinely used in basic and clinical laboratories, this tool will provide a better understanding of auditory disorders such as neuropathies, tinnitus, and hyperacusis, and will help to improve hearing-aid fittings.

Key points The vesicular glutamate transporter type 3 (VGLUT3) loads glutamate into synaptic vesicles of auditory sensory cells, the inner hair cells (IHCs). The VGLUT3-p.A211V variant is associated with the human deafness DFNA25 Mutant mice carrying the VGLUT3-p.A211V variant show a progressive hearing loss. IHCs from mutant mice harbor distorted stereocilary bundles, which detect incoming sound-stimulation followed by oversized synaptic ribbons, which release glutamate onto the afferent nerve fibers. These results suggest that DFNA25 stems from the failure of auditory sensory cells to faithfully transduce acoustic cues into neural message. Abstract DFNA25 is an autosomal-dominant and progressive form of human deafness caused by mutations in the SLC17A8 gene, which encodes the vesicular glutamate transporter type 3 (VGLUT3). To resolve the mechanisms underlying DFNA25, we studied phenotypes of mice harboring the p.A221V mutation in human (corresponding to p.A224V in mouse). Using auditory brainstem response and distortion products otoacoustic emissions, we showed progressive hearing loss with intact cochlear amplification in the VGLUT3^A224V/A224V mouse. The summating potential was reduced indicating the alteration of inner hair cells (IHCs) receptor potential. Scanning electron microscopy examinations demonstrated the collapse of stereocilia bundle in IHCs, leaving those from outer hair cells unaffected. In addition, IHC ribbon synapses underwent structural and functional modifications at later stages. Using super-resolution microscopy, we observed oversized synaptic ribbons and patch-clamp membrane capacitance measurements showed an increase in the rate of the sustained releasable pool of exocytosis. These results suggest that DFNA25 stems from a failure in the mechano-transduction followed by a change in synaptic transfer. The VGLUT3^A224V/A224V mouse model opens the way to a deeper understanding and to a potential treatment of DFNA25. This article is protected by copyright. All rights reserved

Information in sound stimuli is conveyed from sensory hair cells to the cochlear nuclei by the firing of auditory nerve fibers (ANFs). For obvious ethical reasons, single unit recordings from the cochlear nerve have never been performed in human, thus functional hallmarks of ANFs are unknown. By filtering and rectifying the electrical signal recorded at the round window of gerbil cochleae, we reconstructed a peri-stimulus time response (PSTR), with a waveform similar to the peri-stimulus time histograms (PSTHs) recorded from single ANFs. Pair-by-pair analysis of simultaneous PSTR and PSTH recordings in gerbil provided a model to predict the rapid adaptation and spontaneous discharge rates (SR) in a population of ANFs according to their location in the cochlea. We then probed the model in the mouse, in which the SR-based distribution of ANFs differs from the gerbil. We show that the PSTR-based predictions of the rapid adaptation time constant and mean SR across frequency again matched those obtained by recordings from single ANFs. Using PSTR recorded from the human cochlear nerve in 8 normal-hearing patients who underwent cerebellopontine angle surgeries for a functional cranial-nerve disorders (trigeminal neuralgia or hemifacial spasm), we predicted a rapid adaptation of about 3 milliseconds and a mean SR of 23 spikes/s in the 4 kHz frequency range in human ANFs. Together, our results support the use of PSTR as a promising diagnostic tool to map the auditory nerve in humans, thus opening new avenues to better understanding neuropathies, tinnitus, and hyperacusis.

Summary: The ability to efficiently switch from one defensive strategy to another maximizes an animal’s chance of survival. Here, we demonstrate that the selection of active defensive behaviors requires the coordinated activation of dopamine D2 receptor (D2R) signaling within the central extended amygdala (EA) comprising the nucleus accumbens, the oval bed nucleus stria terminals and the central amygdala. We find that discriminative learning between predictive and non-predictive threat auditory stimuli is unaltered in mice carrying a temporally-controlled deletion of D2R within output neurons of the EA. In contrast, intact EA D2R signaling is required for active avoidance learning and innate flight responses triggered by a visual threat stimulus (looming). Consequently, conditional D2R knockout mice biased defensive responses toward passive defensive strategies. Altogether, these findings identify EA D2R signaling as an important mechanism by which DA regulates the switch from passive to active defensive behaviors, regardless whether of learned or innate threat.

DFNA25 is an autosomal-dominant and progressive form of human deafness caused by mutations in the SLC17A8 gene, which encodes the vesicular glutamate transporter type 3 (VGLUT3). To resolve the mechanisms underlying DFNA25, we studied the phenotype of the mouse harboring the p.A221V mutation in human (corresponding to p.A224V in mouse). Using auditory brainstem response and distortion products of otoacoustic emissions, we showed that VGLUT3 ^A224V/A224V mouse replicates the DFNA25 progressive hearing loss with intact cochlear amplification. Scanning electron microscopy examinations demonstrated fused stereocilia bundle of the inner hair cells (IHCs) as the primary cause for DFNA25. In addition, the IHC ribbon synapses undergo structural and functional modifications at later stages. Using super-resolution microscopy, we observed oversized synaptic ribbons associated with an increase in the rate of the sustained releasable pool of exocytosis. These results indicate that the primary defect in DFNA25 stems from a failure in the mechano-transduction followed by a change in synaptic transfer. The VGLUT3 ^A224V/A224V mouse model opens the way to a deeper understanding and to a potential treatment of DFNA25.