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Drumming is a non-vocal auditory display producing airborne as well as seismic vibrations by tapping body extremities on a surface. It is mostly described as an alarm signal but is also discussed to signal dominance or mating quality. To clarify the function of drumming in Mongolian gerbils (Meriones unguiculatus), we compared the occurrence of drumming during predator, opposite-sex and same-sex encounters. We tested 48 captive Mongolian gerbils (Meriones unguiculatus) in three experiments. In predator experiments, subjects were exposed alone or with their cagemate to aerial and terrestrial predator dummies. In social encounter experiments, familiar and unfamiliar male-female dyads and same-sex dyads were confronted. For the same-sex encounters, a dominance index was calculated for each subject based on the number of won and lost conflicts. Drumming and drumming-call combinations were counted, and a multi-parametric sound analysis was performed. In all experiments drumming and drumming-call combinations occurred. In predator experiments, more subjects drummed when confronted with the predator stimulus than in the habituation phase. In social encounter experiments, more subjects drummed when facing an unfamiliar than a familiar conspecific. In addition, the accompanying call type and body posture of the sender differed between experiments. Thus, we suggest that whereas drumming signals an increased arousal state of the sender, the accompanying call type and the body posture signal context specific information., (© 2024. The Author(s).)

Based on the auditory periphery and the small head size, Etruscan shrews (Suncus etruscus) approximate ancestral mammalian conditions. The auditory brainstem in this insectivore has not been investigated. Using labelling techniques, we assessed the structures of their superior olivary complex (SOC) and the nuclei of the lateral lemniscus (NLL). There, we identified the position of the major nuclei, their input pattern, transmitter content, expression of calcium binding proteins (CaBPs) and two voltage-gated ion channels. The most prominent SOC structures were the medial nucleus of the trapezoid body (MNTB), the lateral nucleus of the trapezoid body (LNTB), the lateral superior olive (LSO) and the superior paraolivary nucleus (SPN). In the NLL, the ventral (VNLL), a specific ventrolateral VNLL (VNLLvl) cell population, the intermediate (INLL) and dorsal (DNLL) nucleus, as well as the inferior colliculus's central aspect were discerned. INLL and VNLL were clearly separated by the differential distribution of various marker proteins. Most labelled proteins showed expression patterns comparable to rodents. However, SPN neurons were glycinergic and not GABAergic and the overall CaBPs expression was low. Next to the characterisation of the Etruscan shrew's auditory brainstem, our work identifies conserved nuclei and indicates variable structures in a species that approximates ancestral conditions., (© 2024. The Author(s).)

Intracerebral drug delivery is an experimental approach for the treatment of drug-resistant epilepsies that allows for pharmacological intervention in targeted brain regions. Previous studies have shown that targeted pharmacological inhibition of the subthalamic nucleus (STN) via modulators of the GABAergic system produces antiseizure effects. However, with chronic treatment, antiseizure effects are lost as tolerance develops. Here, we report that chronic intrasubthalamic microinfusion of valproate (VPA), an antiseizure medication known for its wide range of mechanisms of action, can produce long-lasting antiseizure effects over three weeks in rats. In the intravenous pentylenetetrazole seizure-threshold test, seizure thresholds were determined before and during chronic VPA application (480 μg/d, 720 μg/d, 960 μg/d) to the bilateral STN. Results indicate a dose-dependent variation in VPA-induced antiseizure effects with mean increases in seizure threshold of up to 33%, and individual increases of up to 150%. The lowest VPA dose showed a complete lack of tolerance development with long-lasting antiseizure effects. Behavioral testing with all doses revealed few, acceptable adverse effects. VPA concentrations were high in STN and low in plasma and liver. In vitro electrophysiology with bath applied VPA revealed a reduction in spontaneous firing rate, increased background membrane potential, decreased input resistance and a significant reduction in peak NMDA, but not AMPA, receptor currents in STN neurons. Our results suggest an advantage of VPA over purely GABAergic modulators in preventing tolerance development with chronic intrasubthalamic drug delivery and provide first mechanistic insights in intracerebral pharmacotherapy targeting the STN., Competing Interests: Declaration of Competing Interest None., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

The lateral lemniscus encompasses processing stages for binaural hearing, suppressing spurious frequencies and frequency integration. Within the lemniscal fibres three nuclei can be identified, termed after their location as dorsal, intermediate and ventral nucleus of the lateral lemniscus (DNLL, INLL and VNLL). While the DNLL and VNLL have been functionally and anatomically characterized, less is known about INLL neurons. Here, we quantitatively describe the morphology, the cellular orientation and distribution of synaptic contact sites along dendrites in mature Mongolian gerbils. INLL neurons are largely non-inhibitory and morphologically heterogeneous with an overall perpendicular orientation regarding the lemniscal fibers. Dendritic ranges are heterogeneous and can extend beyond the nucleus border. INLL neurons receive VGluT1/2 containing glutamatergic and a mix of GABA- and glycinergic inputs distributed over the entire dendrite. Input counts suggest that numbers of excitatory exceed the inhibitory contact sites. Axonal projections indicate connectivity to ascending and descending auditory structures. Our data show that INLL neurons form a morphologically heterogeneous continuum and incoming auditory information is processed on thin dendrites of various length and biased to perpendicular orientation. Together with the different axonal projection patterns, this indicates that the INLL is a highly complex structure that might hold many unexplored auditory functions., (© 2023. Springer Nature Limited.)

In mammals, common rules for the encoding of arousal and physical characteristics of the sender are suggested based on a similar vocal production apparatus. In this study, we want to investigate to what extent vocalizations of developing Mongolian gerbil pups fulfill these rules. We recorded vocalizations of 28 Mongolian gerbil pups in four developmental stages using a separation paradigm, suggested to induce different arousal levels. For low arousal, a pup was placed in an arena isolated from its siblings and parents; for high arousal, the pup was additionally stressed through the simulation of a predator. An unsupervised cluster analysis revealed three call types: ultrasonic (USV), audible vocalizations (ADV), and transitions between both (USV-ADV). The USV and USV-ADV rate showed an age-dependent decrease, contrasting an age-dependent increase for ADVs. Vocal correlates for the encoding of arousal were found for USVs and of physical characteristics for USVs and ADVs. However, the pattern of encoding these cues differed between call types and only partly confirmed the common rules suggested for mammals. Our results show that divergent encoding patterns do not only differ between species but also between call types within a species, indicating that coding rules can be shaped by socio-ecological factors or call type specific production mechanisms.

Comparative analysis of evolutionarily conserved neuronal circuits between phylogenetically distant mammals highlights the relevant mechanisms and specific adaptations to information processing. The medial nucleus of the trapezoid body (MNTB) is a conserved mammalian auditory brainstem nucleus relevant for temporal processing. While MNTB neurons have been extensively investigated, a comparative analysis of phylogenetically distant mammals and the spike generation is missing. To understand the suprathreshold precision and firing rate, we examined the membrane, voltage-gated ion channel and synaptic properties in Phyllostomus discolor (bat) and in Meriones unguiculatus (rodent) of either sex. Between the two species, the membrane properties of MNTB neurons were similar at rest with only minor differences, while larger dendrotoxin (DTX)-sensitive potassium currents were found in gerbils. Calyx of Held-mediated EPSCs were smaller and frequency dependence of short-term plasticity (STP) less pronounced in bats. Simulating synaptic train stimulations in dynamic clamp revealed that MNTB neurons fired with decreasing success rate near conductance threshold and at increasing stimulation frequency. Driven by STP-dependent conductance decrease, the latency of evoked action potentials increased during train stimulations. The spike generator showed a temporal adaptation at the beginning of train stimulations that can be explained by sodium current inactivation. Compared with gerbils, the spike generator of bats sustained higher frequency input-output functions and upheld the same temporal precision. Our data mechanistically support that MNTB input-output functions in bats are suited to sustain precise high-frequency rates, while for gerbils, temporal precision appears more relevant and an adaptation to high output-rates can be spared. SIGNIFICANCE STATEMENT Neurons in the mammalian medial nucleus of the trapezoid body (MNTB) convey precise, faithful inhibition vital for binaural hearing and gap detection. The MNTB's structure and function appear evolutionarily well conserved. We compared the cellular physiology of MNTB neurons in bat and gerbil. Because of their adaptations to echolocation or low frequency hearing both species are model systems for hearing research, yet with largely overlapping hearing ranges. We find that bat neurons sustain information transfer with higher ongoing rates and precision based on synaptic and biophysical differences in comparison to gerbils. Thus, even in evolutionarily conserved circuits species-specific adaptations prevail, highlighting the importance for comparative research to differentiate general circuit functions and their specific adaptations., (Copyright © 2023 the authors.)

Bats are a natural reservoir for many viruses and are considered to play an important role in the interspecies transmission of viruses. To analyze the susceptibility of bat airway cells to infection by viruses of other mammalian species, we developed an airway organoid culture model derived from airways of Carollia perspicillata. Application of specific antibodies for fluorescent staining indicated that the cell composition of organoids resembled those of bat trachea and lungs as determined by immunohistochemistry. Infection studies indicated that Carollia perspicillata bat airway organoids (AOs) from the trachea or the lung are highly susceptible to infection by two different porcine influenza A viruses. The bat AOs were also used to develop an air-liquid interface (ALI) culture system of filter-grown epithelial cells. Infection of these cells showed the same characteristics, including lower virulence and enhanced replication and release of the H1N1/2006 virus compared to infection with H3N2/2007. These observations agreed with the results obtained by infection of porcine ALI cultures with these two virus strains. Interestingly, lectin staining indicated that bat airway cells only contain a small amount of alpha 2,6-linked sialic acid, the preferred receptor determinant for mammalian influenza A viruses. In contrast, large amounts of alpha 2,3-linked sialic acid, the preferred receptor determinant for avian influenza viruses, are present in bat airway epithelial cells. Therefore, bat airway cells may be susceptible not only to mammalian but also to avian influenza viruses. Our culture models, which can be extended to other parts of the airways and to other species, provide a promising tool to analyze virus infectivity and the transmission of viruses both from bats to other species and from other species to bats. IMPORTANCE We developed an organoid culture system derived from the airways of the bat species Carollia perspicillata . Using this cell system, we showed that the airway epithelium of these bats is highly susceptible to infection by influenza viruses of other mammalian species and thus is not a barrier for interspecies transmission. These organoids provide an almost unlimited supply of airway epithelial cells that can be used to generate well-differentiated epithelial cells and perform infection studies. The establishment of the organoid model required only three animals, and can be extended to other epithelia (nose, intestine) as well as to other species (bat and other animal species). Therefore, organoids promise to be a valuable tool for future zoonosis research on the interspecies transmission of viruses (e.g., bat → intermediate host → human).

Large glutamatergic, somatic synapses mediate temporally precise information transfer. In the ventral nucleus of the lateral lemniscus, an auditory brainstem nucleus, the signal of an excitatory large somatic synapse is sign inverted to generate rapid feedforward inhibition with high temporal acuity at sound onsets, a mechanism involved in the suppression of spurious frequency information. The mechanisms of the synaptically driven input-output functions in the ventral nucleus of the lateral lemniscus are not fully resolved. Here, we show in Mongolian gerbils of both sexes that, for stimulation frequencies up to 200 Hz, the EPSC kinetics together with short-term plasticity allow for faithful transmission with only a small increase in latency. Glutamatergic currents are exclusively mediated by AMPARs and NMDARs. Short-term plasticity is frequency-dependent and composed of an initial facilitation followed by depression. Physiologically relevant output generation is limited by the decrease in synaptic conductance through short-term plasticity (STP). At this endbulb synapse, STP acts as a low pass filter and increases the dynamic range of the conductance dependent input-output relation, while NMDAR signaling slightly increases the sensitivity of the input-output function. Our computational model shows that STP-mediated filtering limits the intensity dependence of the spike output, thus maintaining selectivity to sound transients. Our results highlight the interaction of cellular features that together give rise to the computations in the circuit. SIGNIFICANCE STATEMENT Auditory information processing in the brainstem is a prerequisite for generating our auditory representation of the environment. Thereby, many processing steps rely on temporally precise filtering. Precise feedforward inhibition is a key motif in auditory brainstem processing and produced through sign inversion at several large somatic excitatory synapses. A particular feature of the ventral nucleus of the lateral lemniscus is to produce temporally precise onset inhibition with little temporal variance independent of sound intensity. Our cell-physiology and modeling data explain how the synaptic characteristics of different current components and their short-term plasticity are tuned to establish sound intensity-invariant onset inhibition that is crucial for filtering out spurious frequency information., (Copyright © 2022 the authors.)

A high prevalence of AA-amyloidosis was identified in a breeding colony of northern tree shrews ( Tupaia belangeri ) in a retrospective analysis, with amyloid deposits in different organs being found in 26/36 individuals (72%). Amyloid deposits, confirmed by Congo red staining, were detected in kidneys, intestines, skin, and lymph nodes, characteristic of systemic amyloidosis. Immunohistochemically, the deposited amyloid was intensely positive with anti-AA-antibody (clone mc4), suggesting AA-amyloidosis. The kidneys were predominantly affected (80%), where amyloid deposits ranged from mild to severe and was predominantly located in the renal medulla. In addition, many kidneys contained numerous cysts with atrophy of the renal parenchyma. There was no significant association between concurrent neoplastic or inflammatory processes and amyloidosis. The lack of distinctive predisposing factors suggests a general susceptibility of captive T. belangeri to develop amyloidosis. Clinical and laboratory findings of a female individual with pronounced kidney alterations were indicative of renal failure. The observed tissue tropism with pronounced kidney alterations, corresponding renal dysfunction, and an overall high prevalence suggests amyloidosis as an important disease in captive tree shrews.

To counterbalance long-term environmental changes, neuronal circuits adapt the processing of sensory information. In the auditory system, ongoing background noise drives long-lasting adaptive mechanism in binaural coincidence detector neurons in the superior olive. However, the compensatory cellular mechanisms of the binaural neurons in the medial superior olive (MSO) to long-term background changes are unexplored. Here we investigated the cellular properties of MSO neurons during long-lasting adaptations induced by moderate omnidirectional noise exposure. After noise exposure, the input resistance of MSO neurons of mature Mongolian gerbils was reduced, likely due to an upregulation of hyperpolarisation-activated cation and low voltage-activated potassium currents. Functionally, the long-lasting adaptations increased the action potential current threshold and facilitated high frequency output generation. Noise exposure accelerated the occurrence of spontaneous postsynaptic currents. Together, our data suggest that cellular adaptations in coincidence detector neurons of the MSO to continuous noise exposure likely increase the sensitivity to differences in sound pressure levels.

Neurons in the medial superior olive (MSO) detect 10 µs differences in the arrival times of a sound at the two ears. Such acuity requires exquisitely precise integration of binaural synaptic inputs. There is substantial understanding of how neuronal phase locking of afferent MSO structures, and MSO membrane biophysics subserve such high precision. However, we still lack insight into how the entirety of excitatory inputs is integrated along the MSO dendrite under sound stimulation. To understand how the dendrite integrates excitatory inputs as a whole, we combined anatomic quantifications of the afferent innervation in gerbils of both sexes with computational modeling of a single cell. We present anatomic data from confocal and transmission electron microscopy showing that single afferent fibers follow a single dendrite mostly up to the soma and contact it at multiple (median 4) synaptic sites, each containing multiple independent active zones (the overall density of active zones is estimated as 1.375 per μm 2 ). Thus, any presynaptic action potential may elicit temporally highly coordinated synaptic vesicle release at tens of active zones, thereby achieving secure transmission. Computer simulations suggest that such an anatomic arrangement boosts the amplitude and sharpens the time course of excitatory postsynaptic potentials by reducing current sinks and more efficiently recruiting subthreshold potassium channels. Both effects improve binaural coincidence detection compared with single large synapses at the soma. Our anatomic data further allow for estimation of a lower bound of 7 and an upper bound of 70 excitatory fibers per dendrite. SIGNIFICANCE STATEMENT Passive dendritic propagation attenuates the amplitude of postsynaptic potentials and widens their temporal spread. Neurons in the medial superior olive, with their large bilateral dendrites, however, can detect coincidence of binaural auditory inputs with submillisecond precision, a computation that is in stark contrast to passive dendritic processing. Here, we show that dendrites can counteract amplitude attenuation and even decrease the temporal spread of postsynaptic potentials, if active subthreshold potassium conductances are triggered in temporal coordination along the whole dendrite. Our anatomic finding that axons run in parallel to the dendrites and make multiple synaptic contacts support such coordination since incoming action potentials would depolarize the dendrite at multiple sites within a brief time interval., (Copyright © 2021 the authors.)

The auditory system relies on temporal precise information transfer, requiring an interplay of synchronously activated inputs and rapid postsynaptic integration. During late postnatal development synaptic, biophysical, and morphological features change to enable mature auditory neurons to perform their appropriate function. How the number of minimal required input fibers and the relevant EPSC time course integrated for action potential generation changes during late postnatal development is unclear. To answer these questions, we used in vitro electrophysiology in auditory brainstem structures from pre-hearing onset and mature Mongolian gerbils of either sex. Synaptic and biophysical parameters changed distinctively during development in the medial nucleus of the trapezoid body (MNTB), the medial superior olive (MSO), and the ventral and dorsal nucleus of the lateral lemniscus (VNLL and DNLL). Despite a reduction in input resistance in most cell types, all required fewer inputs in the mature stage to drive action potentials. Moreover, the EPSC decay time constant is a good predictor of the EPSC time used for action potential generation in all nuclei but the VNLL. Only in MSO neurons, the full EPSC time course is integrated by the neuron's resistive element, while otherwise, the relevant EPSC time matches only 5-10% of the membrane time constant, indicating membrane charging as a dominant role for output generation. We conclude, that distinct developmental programs lead to a general increase in temporal precision and integration accuracy matched to the information relaying properties of the investigated nuclei., (Copyright © 2020 Kladisios, Fischer and Felmy.)

G M1 -gangliosidosis is caused by a reduced activity of β-galactosidase ( Glb1 ), resulting in intralysosomal accumulations of G M1 . The aim of this study was to reveal the pathogenic mechanisms of G M1 -gangliosidosis in a new Glb1 knockout mouse model. Glb1 -/- mice were analyzed clinically, histologically, immunohistochemically, electrophysiologically and biochemically. Morphological lesions in the central nervous system were already observed in two-month-old mice, whereas functional deficits, including ataxia and tremor, did not start before 3.5-months of age. This was most likely due to a reduced membrane resistance as a compensatory mechanism. Swollen neurons exhibited intralysosomal storage of lipids extending into axons and amyloid precursor protein positive spheroids. Additionally, axons showed a higher kinesin and lower dynein immunoreactivity compared to wildtype controls. Glb1 -/- mice also demonstrated loss of phosphorylated neurofilament positive axons and a mild increase in non-phosphorylated neurofilament positive axons. Moreover, marked astrogliosis and microgliosis were found, but no demyelination. In addition to the main storage material G M1 , G A1 , sphingomyelin, phosphatidylcholine and phosphatidylserine were elevated in the brain. In summary, the current Glb1 -/- mice exhibit a so far undescribed axonopathy and a reduced membrane resistance to compensate the functional effects of structural changes. They can be used for detailed examinations of axon-glial interactions and therapy trials of lysosomal storage diseases.

The development of sensory circuits is partially guided by sensory experience. In the medial superior olive (MSO), these refinements generate precise coincidence detection to localize sounds in the azimuthal plane. Glycinergic inhibitory inputs to the MSO, which tune the sensitivity to interaural time differences, undergo substantial structural and functional refinements after hearing onset. Whether excitation and calcium signaling in the MSO are similarly affected by the onset of acoustic experience is unresolved. To assess the time window and mechanism of excitatory and calcium-dependent refinements during late postnatal development, we quantified EPSCs and calcium entry in MSO neurons of Mongolian gerbils of either sex raised in a normal and in an activity altered, omnidirectional white noise environment. Global dendritic calcium transients elicited by action potentials disappeared rapidly after hearing onset. Local synaptic calcium transients decreased, leaving a GluR2 lacking AMPAR-mediated influx as the only activity-dependent source in adulthood. Exposure to omnidirectional white noise accelerated the decrease in calcium entry, leaving membrane properties unaffected. Thus, sound-driven activity accelerates the excitatory refinement and shortens the period of activity-dependent calcium signaling around hearing onset. Together with earlier reports, our findings highlight that excitation, inhibition, and biophysical properties are differentially sensitive to distinct features of sensory experience. SIGNIFICANCE STATEMENT Neurons in the medial superior olive, an ultra-fast coincidence detector for sound source localization, acquire their specialized function through refinements during late postnatal development. The refinement of inhibitory inputs that convey sensitivity to relevant interaural time differences is instructed by the experience of sound localization cues. Which cues instruct the refinement of excitatory inputs, calcium signaling, and biophysical properties is unknown. Here we demonstrate a time window for activity- and calcium-dependent refinements limited to shortly after hearing onset. Exposure to omnidirectional white noise, which suppresses sound localization cues but increases overall activity, accelerates the refinement of calcium signaling and excitatory inputs without affecting biophysical membrane properties. Thus, the refinement of excitation, inhibition, and intrinsic properties is instructed by distinct cues., (Copyright © 2020 the authors.)

Coincidence detector neurons of the medial superior olive (MSO) are sensitive to interaural time differences in the range of a few tens of microseconds. The biophysical basis for this remarkable acuity is a short integration time constant of the membrane, which is achieved by large low voltage-activated potassium and hyperpolarization-activated inward cation conductances. Additional temporal precision is thought to be achieved through a sub-cellular distribution of low voltage-activated potassium channel expression biased to the soma. To evaluate the contribution of potassium channels, we investigated the presence and sub-cellular distribution profile of seven potassium channel sub-units in adult MSO neurons of gerbils. We find that low- and high voltage-activated potassium channels are present with distinct sub-cellular distributions. Overall, low voltage-activated potassium channels appear to be biased to the soma while high voltage-activated potassium channels are more evenly distributed and show a clear expression at distal dendrites. Additionally, low voltage-activated potassium channel sub-units co-localize with glycinergic inputs while HCN1 channels co-localize more with high voltage-activated potassium channels. Functionally, high voltage-activated potassium currents are already active at low voltages near the resting potential. We describe a possible role of high voltage-activated potassium channels in modulating EPSPs in a computational model and contributing to setting the integration time window of coincidental inputs. Our data shows that MSO neurons express a large set of different potassium channels with distinct functional relevance.

In natural environments our auditory system is exposed to multiple and diverse signals of fluctuating amplitudes. Therefore, to detect, localize, and single out individual sounds the auditory system has to process and filter spectral and temporal information from both ears. It is known that the overall sound pressure level affects sensory signal transduction and therefore the temporal response pattern of auditory neurons. We hypothesize that the mammalian binaural system utilizes a dynamic mechanism to adjust the temporal filters in neuronal circuits to different overall sound pressure levels. Previous studies proposed an inhibitory mechanism generated by the reciprocally coupled dorsal nuclei of the lateral lemniscus (DNLL) as a temporal neuronal-network filter that suppresses rapid binaural fluctuations. Here we investigated the consequence of different sound levels on this filter during binaural processing. Our in vivo and in vitro electrophysiology in Mongolian gerbils shows that the integration of ascending excitation and contralateral inhibition defines the temporal properties of this inhibitory filter. The time course of this filter depends on the synaptic drive, which is modulated by the overall sound pressure level and N-methyl-D-aspartate receptor (NMDAR) signaling. In psychophysical experiments we tested the temporal perception of humans and show that detection and localization of two subsequent tones changes with the sound pressure level consistent with our physiological results. Together our data support the hypothesis that mammals dynamically adjust their time window for sound detection and localization within the binaural system in a sound level dependent manner.

The peripheral deafness gene Mir96 is expressed in both the cochlea and central auditory circuits. To investigate whether it plays a role in the auditory system beyond the cochlea, we characterized homozygous Dmdo/Dmdo mice with a point mutation in miR-96. Anatomical analysis demonstrated a significant decrease in volume of auditory nuclei in Dmdo/Dmdo mice. This decrease resulted from decreased cell size. Non-auditory structures in the brainstem of Dmdo/Dmdo mice or auditory nuclei of the congenital deaf Cldn14-/- mice revealed no such differences. Electrophysiological analysis in the medial nucleus of the trapezoid body (MNTB) showed that principal neurons fired preferentially multiple action potentials upon depolarization, in contrast to the single firing pattern prevalent in controls and Cldn14-/- mice. Immunohistochemistry identified significantly reduced expression of two predicted targets of the mutated miR-96, Kv1.6 and BK channel proteins, possibly contributing to the electrophysiological phenotype. Microscopic analysis of the Dmdo/Dmdo calyx of Held revealed a largely absent compartmentalized morphology, as judged by SV2-labeling. Furthermore, MNTB neurons from Dmdo/Dmdo mice displayed larger synaptic short-term depression, slower AMPA-receptor decay kinetics and a larger NMDA-receptor component, reflecting a less matured stage. Again, these synaptic differences were not present between controls and Cldn14-/- mice. Thus, deafness genes differentially affect the auditory brainstem. Furthermore, our study identifies miR-96 as an essential gene regulatory network element of the auditory system which is required for functional maturation in the peripheral and central auditory system alike.

Auditory signals carry relevant information on a large range of time scales from below milliseconds to several seconds. Different stages in the auditory brainstem are specialized to extract information in specific frequency domains. One biophysical mechanism to facilitate frequency specific processing are membrane potential resonances. Here, we provide data from three different brainstem nuclei that all exhibit high-frequency subthreshold membrane resonances that are all most likely based on low-threshold potassium currents. Fitting a linear model, we argue that, as long as neurons possess active subthreshold channels, the main determinant for their resonance behavior is the steady state membrane time constant. Tuning this leak conductance can shift membrane resonance frequencies over more than a magnitude and therefore provide a flexible mechanism to tune frequency-specific auditory processing.

Animals employ mechano-sensory systems to detect and explore their environment. Mechano-sensation encompasses stimuli such as constant pressure, surface movement or vibrations at various intensities that need to be segregated in the central nervous system. Besides different receptor structures, sensory filtering via intrinsic response properties could provide a convenient way to solve this problem. In leech, three major mechano-sensory cell types can be distinguished, according to their stimulus sensitivity, as nociceptive, pressure and touch cells. Using intracellular recordings, we show that the different mechano-sensory neuron classes in Hirudo medicinalis differentially respond supra-threshold to distinct frequencies of sinusoidal current injections between 0.2 and 20 Hz. Nociceptive cells responded with a low-pass filter characteristic, pressure cells as high-pass filters and touch cells as an intermediate band-pass filter. Each class of mechano-sensory neurons is thus intrinsically tuned to a specific frequency range of voltage oscillation that could help segregate mechano-sensory information centrally., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2017. Published by The Company of Biologists Ltd.)

To capture the context of sensory information, neural networks must process input signals across multiple timescales. In the auditory system, a prominent change in temporal processing takes place at an inhibitory GABAergic synapse in the dorsal nucleus of the lateral lemniscus (DNLL). At this synapse, inhibition outlasts the stimulus by tens of milliseconds, such that it suppresses responses to lagging sounds, and is therefore implicated in echo suppression. Here, we untangle the cellular basis of this inhibition. We demonstrate with in vivo whole-cell patch-clamp recordings in Mongolian gerbils that the duration of inhibition increases with sound intensity. Activity-dependent spillover and asynchronous release translate the high presynaptic firing rates found in vivo into a prolonged synaptic output in acute slice recordings. A key mechanism controlling the inhibitory time course is the passive integration of the hyperpolarizing inhibitory conductance. This prolongation depends on the synaptic conductance amplitude. Computational modeling shows that this prolongation is a general mechanism and relies on a non-linear effect caused by synaptic conductance saturation when approaching the GABA reversal potential. The resulting hyperpolarization generates an efficient activity-dependent suppression of action potentials without affecting the threshold or gain of the input-output function. Taken together, the GABAergic inhibition in the DNLL is adjusted to the physiologically relevant duration by passive integration of inhibition with activity-dependent synaptic kinetics. This change in processing timescale combined with the reciprocal connectivity between the DNLLs implements a mechanism to suppress the distracting localization cues of echoes and helps to localize the initial sound source reliably., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Passive and active membrane properties determine the voltage responses of neurons. Within the auditory brain stem, refinements in these intrinsic properties during late postnatal development usually generate short integration times and precise action-potential generation. This developmentally acquired temporal precision is crucial for auditory signal processing. How the interactions of these intrinsic properties develop in concert to enable auditory neurons to transfer information with high temporal precision has not yet been elucidated in detail. Here, we show how the developmental interaction of intrinsic membrane parameters generates high firing precision. We performed in vitro recordings from neurons of postnatal days 9-28 in the ventral nucleus of the lateral lemniscus of Mongolian gerbils, an auditory brain stem structure that converts excitatory to inhibitory information with high temporal precision. During this developmental period, the input resistance and capacitance decrease, and action potentials acquire faster kinetics and enhanced precision. Depending on the stimulation time course, the input resistance and capacitance contribute differentially to action-potential thresholds. The decrease in input resistance, however, is sufficient to explain the enhanced action-potential precision. Alterations in passive membrane properties also interact with a developmental change in potassium currents to generate the emergence of the mature firing pattern, characteristic of coincidence-detector neurons. Cholinergic receptor-mediated depolarizations further modulate this intrinsic excitability profile by eliciting changes in the threshold and firing pattern, irrespective of the developmental stage. Thus our findings reveal how intrinsic membrane properties interact developmentally to promote temporally precise information processing., (Copyright © 2015 the American Physiological Society.)

Neurons in the medial superior olive (MSO) detect microsecond differences in the arrival time of sounds between the ears (interaural time differences or ITDs), a crucial binaural cue for sound localization. Synaptic inhibition has been implicated in tuning ITD sensitivity, but the cellular mechanisms underlying its influence on coincidence detection are debated. Here we determine the impact of inhibition on coincidence detection in adult Mongolian gerbil MSO brain slices by testing precise temporal integration of measured synaptic responses using conductance-clamp. We find that inhibition dynamically shifts the peak timing of excitation, depending on its relative arrival time, which in turn modulates the timing of best coincidence detection. Inhibitory control of coincidence detection timing is consistent with the diversity of ITD functions observed in vivo and is robust under physiologically relevant conditions. Our results provide strong evidence that temporal interactions between excitation and inhibition on microsecond timescales are critical for binaural processing.

Neurons of the medial superior olive (MSO) code for the azimuthal location of low-frequency sound sources via a binaural coincidence detection system operating on microsecond time scales. These neurons are morphologically simple and stereotyped, and anatomical studies have indicated a functional segregation of excitatory and inhibitory inputs between cellular compartments. It is thought that this morphological arrangement holds important implications for the computational task of these cells. To date, however, there has been no functional investigation into synaptic input sites or functional receptor distributions on mature neurons of the MSO. Here, functional neurotransmitter receptor maps for amino-3-hydroxyl-5-methyl-4-isoxazole propionate (AMPA), N-methyl-D-aspartate (NMDA), glycine (Gly), and ionotropic γ-aminobutyric acid (GABA(A)) receptors (Rs) were compared and complemented by their corresponding synaptic input map. We find in MSO neurons from postnatal day 20-35 gerbils that AMPARs and their excitatory inputs target the soma and dendrites. Functional GlyRs and their inhibitory inputs are predominantly refined to the somata, although a pool of functional GlyRs is present extrasynaptically on MSO dendrites. GABA(A)R responses are present throughout the cell but lack direct synaptic contact indicating an involvement in volume transmission. NMDARs are present both synaptically and extrasynaptically with an overall distribution similar to GlyRs. Interestingly, even at physiological temperatures these functional NMDARs can be potentiated by synaptically released Gly. The functional receptor and synaptic input maps produced here led to the identification of a cross talk between transmitter systems and raises the possibility that extrasynaptic receptors could be modulating leak conductances as a homeostatic mechanism.

Neurons in the medial superior olive (MSO) process microsecond interaural time differences, the major cue for localizing low-frequency sounds, by comparing the relative arrival time of binaural, glutamatergic excitatory inputs. This coincidence detection mechanism is additionally shaped by highly specialized glycinergic inhibition. Traditionally, it is assumed that the binaural inputs are conveyed by many independent fibers, but such an anatomical arrangement may decrease temporal precision. Short-term depression on the other hand might enhance temporal fidelity during ongoing activity. For the first time we show that binaural coincidence detection in MSO neurons may require surprisingly few but strong inputs, challenging long-held assumptions about mammalian coincidence detection. This study exclusively uses adult gerbils for in vitro electrophysiology, single-cell electroporation and immunohistochemistry to characterize the size and short-term plasticity of inputs to the MSO. We find that the excitatory and inhibitory inputs to the MSO are well balanced both in strength and short-term dynamics, redefining this fastest of all mammalian coincidence detector circuits.

During regulated fusion of secretory granules with the plasma membrane, a fusion pore first opens and then dilates. The dilating pore allows cargo proteins from the dense core to be released into the extracellular space. Using real-time evanescent field fluorescence microscopy of live PC12 cells, it was determined how rapidly proteins of different sizes escape from single granules after fusion. Tissue plasminogen activator (tPA)-Venus is released 40-fold slower than the three times smaller neuropeptide Y [NPY-monomeric GFP (mGFP)]. An NPY bearing two mGFPs in tandem [NPY-(mGFP)(2)] as an intermediate-sized fusion probe is released most slowly. Although, the time-course of release varies substantially for a given probe. Coexpression of beta-actin, actin-related protein 3 or mAbp1 slowed the release of the two larger cargo molecules but did not affect release of NPY-mGFP or of the granule-membrane-bound probe Vamp-pHluorin. Additionally, high concentrations of cytochalasin D slowed release of the tPA-Venus. Together these results suggest that fusion pore dilation is not the only determinate of release time-course and that actin rearrangements similar to those mediating actin-mediated motility influences the time-course of release without directly interfering with the granule membrane to cell membrane connection.

Ca2+ influx through voltage-gated Ca2+ channels and the resulting elevation of intracellular Ca2+ concentration, [Ca2+]i, triggers transmitter release in nerve terminals. However, it is controversial whether in addition to the opening of Ca2+ channels, membrane potential directly affects transmitter release. Here, we assayed the influence of membrane potential on transmitter release at the calyx of Held nerve terminals. Transmitter release was evoked by presynaptic Ca2+ uncaging, or by presynaptic Ca2+ uncaging paired with presynaptic voltage-clamp depolarizations to +80 mV, under pharmacological block of voltage-gated Ca2+ channels. Such a change in membrane potential did not alter the Ca2+ dependence of transmitter release rates or synaptic delays. We also found, by varying the amount of Ca2+ influx during Ca2+ tail-currents, that the time course of phasic transmitter release is not invariant to changes in release probability. Rather, the time difference between peak Ca2+ current and peak transmitter release became progressively shorter with increasing Ca2+ current amplitude. When this time difference was plotted as a function of the estimated local [Ca2+]i at the sites of vesicle fusion, a slope of approximately 100 micros per 10 microM [Ca2+]i was found, in reasonable agreement with a model of cooperative Ca2+ binding and vesicle fusion. Thus, the amplitude and time course of the [Ca2+]i signal at the sites of vesicle fusion controls the timing and the amount of transmitter release, both under conditions of brief periods of Ca2+ influx, as well as during step-like elevations of [Ca2+]i produced by Ca2+ uncaging.

In nerve terminals, residual Ca(2+) remaining from previous activity can cause facilitation of transmitter release by a mechanism that is still under debate. Here we show that the intracellular Ca(2+) sensitivity of transmitter release at the calyx of Held is largely unchanged during facilitation, which leaves an increased microdomain Ca(2+) signal as a possible mechanism for facilitation. We measured the Ca(2+) dependencies of facilitation, as well as of transmitter release, to estimate the required increment in microdomain Ca(2+). These measurements show that linear summation of residual and microdomain Ca(2+) accounts for only 30% of the observed facilitation. However, a small degree of supralinearity in the summation of intracellular Ca(2+) signals, which might be caused by saturation of cytosolic Ca(2+) buffer(s), is sufficient to explain facilitation at this CNS synapse.