Remodelling at the calyx of Held-MNTB synapse in mice developing with unilateral conductive hearing loss

Hearing loss-related altered neuronal activity in the inferior colliculus

Hearing loss is well known to cause plastic changes in the central auditory system and pathological changes such as tinnitus and hyperacusis. Impairment of inner ear functions is the main cause of hearing loss. In aged individuals, not only inner ear dysfunction but also senescence of the central nervous system is the cause of malfunction of the auditory system. In most cases of hearing loss, the activity of the auditory nerve is reduced, but that of the successive auditory centers is increased in a compensatory way. It has been reported that activity changes occur in the inferior colliculus (IC), a critical nexus of the auditory pathway. The IC integrates the inputs from the brainstem and drives the higher auditory centers. Since abnormal activity in the IC is likely to affect auditory perception, it is crucial to elucidate the neuronal mechanism to induce the activity changes of IC neurons with hearing loss. This review outlines recent findings on hearing-loss-induced plastic changes in the IC and brainstem auditory neuronal circuits and discusses what neuronal mechanisms underlie hearing-loss-induced changes in the activity of IC neurons. Considering the different causes of hearing loss, we discuss age-related hearing loss separately from other forms of hearing loss (non-age-related hearing loss). In general, the main plastic change of IC neurons caused by both age-related and non-age-related hearing loss is increased central gain. However, plastic changes in the IC caused by age-related hearing loss seem to be more complex than those caused by non-age-related hearing loss.

Group I metabotropic glutamate receptor-triggered temporally patterned action potential-dependent spontaneous synaptic transmission in mouse MNTB neurons

Rhythmic action potentials (AP) are generated via intrinsic ionic mechanisms in pacemaking neurons, producing synaptic responses of regular inter-event intervals (IEIs) in their targets. In auditory processing, evoked temporally patterned activities are induced when neural responses timely lock to a certain phase of the sound stimuli. Spontaneous spike activity, however, is a stochastic process, rendering the prediction of the exact timing of the next event completely based on probability. Furthermore, neuromodulation mediated by metabotropic glutamate receptors (mGluRs) is not commonly associated with patterned neural activities. Here, we report an intriguing phenomenon. In a subpopulation of medial nucleus of the trapezoid body (MNTB) neurons recorded under whole-cell voltage-clamp mode in acute mouse brain slices, temporally patterned AP-dependent glycinergic sIPSCs and glutamatergic sEPSCs were elicited by activation of group I mGluRs with 3,5-DHPG (200 µM). Auto-correlation analyses revealed rhythmogenesis in these synaptic responses. Knockout of mGluR5 largely eliminated the effects of 3,5-DHPG. Cell-attached recordings showed temporally patterned spikes evoked by 3,5-DHPG in potential presynaptic VNTB cells for synaptic inhibition onto MNTB. The amplitudes of sEPSCs enhanced by 3,5-DHPG were larger than quantal size but smaller than spike-driven calyceal inputs, suggesting that non-calyceal inputs to MNTB might be responsible for the temporally patterned sEPSCs. Finally, immunocytochemical studies identified expression and localization of mGluR5 and mGluR1 in the VNTB-MNTB inhibitory pathway. Our results imply a potential central mechanism underlying the generation of patterned spontaneous spike activity in the brainstem sound localization circuit.

Volume electron microscopy reveals age-related circuit remodeling in the auditory brainstem

The medial nucleus of the trapezoid body (MNTB) is an integral component of the auditory brainstem circuitry involved in sound localization. The giant presynaptic nerve terminal with multiple active zones, the calyx of Held (CH), is a hallmark of this nucleus, which mediates fast and synchronized glutamatergic synaptic transmission. To delineate how these synaptic structures adapt to reduced auditory afferents due to aging, we acquired and reconstructed circuitry-level volumes of mouse MNTB at different ages (3 weeks, 6, 18, and 24 months) using serial block-face electron microscopy. We used C57BL/6J, the most widely inbred mouse strain used for transgenic lines, which displays a type of age-related hearing loss. We found that MNTB neurons reduce in density with age. Surprisingly we observed an average of approximately 10% of poly-innervated MNTB neurons along the mouse lifespan, with prevalence in the low frequency region. Moreover, a tonotopy-dependent heterogeneity in CH morphology was observed in young but not in older mice. In conclusion, our data support the notion that age-related hearing impairments can be in part a direct consequence of several structural alterations and circuit remodeling in the brainstem.

Loss of miR-183/96 Alters Synaptic Strength via Presynaptic and Postsynaptic Mechanisms at a Central Synapse

A point mutation in miR-96 causes non-syndromic progressive peripheral hearing loss and alters structure and physiology of the central auditory system. To gain further insight into the functions of microRNAs (miRNAs) within the central auditory system, we investigated constitutive Mir-183/96dko mice of both sexes. In this mouse model, the genomically clustered miR-183 and miR-96 are constitutively deleted. It shows significantly and specifically reduced volumes of auditory hindbrain nuclei, because of decreases in cell number and soma size. Electrophysiological analysis of the calyx of Held synapse in the medial nucleus of the trapezoid body (MNTB) demonstrated strongly altered synaptic transmission in young-adult mice. We observed an increase in quantal content and readily releasable vesicle pool size in the presynapse while the overall morphology of the calyx was unchanged. Detailed analysis of the active zones (AZs) revealed differences in its molecular composition and synaptic vesicle (SV) distribution. Postsynaptically, altered clustering and increased synaptic abundancy of the AMPA receptor subunit GluA1 was observed resulting in an increase in quantal amplitude. Together, these presynaptic and postsynaptic alterations led to a 2-fold increase of the evoked excitatory postsynaptic currents in MNTB neurons. None of these changes were observed in deaf Cldn14ko mice, confirming an on-site role of miR-183 and miR-96 in the auditory hindbrain. Our data suggest that the Mir-183/96 cluster plays a key role for proper synaptic transmission at the calyx of Held and for the development of the auditory hindbrain.SIGNIFICANCE STATEMENT The calyx of Held is the outstanding model system to study basic synaptic physiology. Yet, genetic factors driving its morphologic and functional maturation are largely unknown. Here, we identify the Mir-183/96 cluster as an important factor to regulate its synaptic strength. Presynaptically, Mir-183/96dko calyces show an increase in release-ready synaptic vesicles (SVs), quantal content and abundance of the proteins Bassoon and Piccolo. Postsynaptically, the quantal size as well as number and size of GluA1 puncta were increased. The two microRNAs (miRNAs) are thus attractive candidates for regulation of synaptic maturation and long-term adaptations to sound levels. Moreover, the different phenotypic outcomes of different types of mutations in the Mir-183 cluster corroborate the requirement of mutation-tailored therapies in patients with hearing loss.

Developmental plasticity of NMDA receptors at the calyx of Held synapse

Excitatory synaptic transmission is largely mediated by glutamate receptors in central synapses, such as the calyx of Held synapse in the auditory brainstem. This synapse is best known for undergoing extensive morphological and functional changes throughout early development and thereby has served as a prominent model system to study presynaptic mechanisms of neurotransmitter release. However, the pivotal roles of N-methyl-d-aspartate receptors (NMDARs) in gating acute forms of activity-dependent, persistent plasticity in vitro and chronic developmental remodeling in vivo are hardly noted. This article will provide a retrospective review of key experimental evidence to conceptualize the impact of a transient abundance of NMDARs during the early postnatal stage on the functionality of fast-spiking central synapses while raising a series of outstanding questions that are of general significance for understanding the developing brain in health and diseases. This article is part of the special Issue on "Glutamate Receptors - NMDA receptors".

Chronic Conductive Hearing Loss Is Associated With Speech Intelligibility Deficits in Patients With Normal Bone Conduction Thresholds

OBJECTIVES

The main objective of this study is to determine whether chronic sound deprivation leads to poorer speech discrimination in humans.

DESIGN

We reviewed the audiologic profile of 240 patients presenting normal and symmetrical bone conduction thresholds bilaterally, associated with either an acute or chronic unilateral conductive hearing loss of different etiologies.

RESULTS

Patients with chronic conductive impairment and a moderate, to moderately severe, hearing loss had lower speech recognition scores on the side of the pathology when compared with the healthy side. The degree of impairment was significantly correlated with the speech recognition performance, particularly in patients with a congenital malformation. Speech recognition scores were not significantly altered when the conductive impairment was acute or mild.

CONCLUSIONS

This retrospective study shows that chronic conductive hearing loss was associated with speech intelligibility deficits in patients with normal bone conduction thresholds. These results are as predicted by a recent animal study showing that prolonged, adult-onset conductive hearing loss causes cochlear synaptopathy.

Auditory Brainstem Deficits from Early Treatment with a CSF1R Inhibitor Largely Recover with Microglial Repopulation

Signaling between neurons and glia is necessary for the formation of functional neural circuits. A role for microglia in the maturation of connections in the medial nucleus of the trapezoid body (MNTB) was previously demonstrated by postnatal microglial elimination using a colony stimulating factor 1 receptor (CSF1R). Defective pruning of calyces of Held and significant reduction of the mature astrocyte marker glial fibrillary acidic protein (GFAP) were observed after hearing onset. Here, we investigated the time course required for microglia to populate the mouse MNTB after cessation of CSF1R inhibitor treatment. We then examined whether defects seen after microglial depletion were rectified by microglial repopulation. We found that microglia returned to control levels at four weeks of age (18 d postcessation of treatment). Calyceal innervation of MNTB neurons was comparable to control levels at four weeks and GFAP expression recovered by seven weeks. We further investigated the effects of microglia elimination and repopulation on auditory function using auditory brainstem recordings (ABRs). Temporary microglial depletion significantly elevated auditory thresholds in response to 4. 8, and 12 kHz at four weeks. Treatment significantly affected latencies, interpeak latencies, and amplitudes of all the ABR peaks in response to many of the frequencies tested. These effects largely recovered by seven weeks. These findings highlight the functions of microglia in the formation of auditory neural circuits early in development. Further, the results suggest that microglia retain their developmental functions beyond the period of circuit refinement.

Impact of Auditory Experience on the Structural Plasticity of the AIS in the Mouse Brainstem Throughout the Lifespan

Sound input critically influences the development and maintenance of neuronal circuits in the mammalian brain throughout life. We investigate the structural and functional plasticity of auditory neurons in response to various auditory experiences during development, adulthood, and aging. Using electrophysiology, computer simulation, and immunohistochemistry, we study the structural plasticity of the axon initial segment (AIS) in the medial nucleus of the trapezoid body (MNTB) from the auditory brainstem of the mice (either sex), in different ages and auditory environments. The structure and spatial location of the AIS of MNTB neurons depend on their functional topographic location along the tonotopic axis, aligning high- to low-frequency sound-responding neurons (HF or LF neurons). HF neurons dramatically undergo structural remodeling of the AIS throughout life. The AIS progressively shortens during development, is stabilized in adulthood, and becomes longer in aging. Sound inputs are critically associated with setting and maintaining AIS plasticity and tonotopy at various ages. Sound stimulation increases the excitability of auditory neurons. Computer simulation shows that modification of the AIS length, location, and diameter can affect firing properties of MNTB neurons in the developing brainstem. The adaptive capability of axonal structure in response to various auditory experiences at different ages suggests that sound input is important for the development and maintenance of the structural and functional properties of the auditory brain throughout life.

Microglia Regulate Pruning of Specialized Synapses in the Auditory Brainstem

The assembly of uniquely organized sound localization circuits in the brainstem requires precise developmental mechanisms. Glial cells have been shown to shape synaptic connections in the retinogeniculate system during development, but their contributions to specialized auditory synapses have not been identified. Here we investigated the role of microglia in auditory brainstem circuit assembly, focusing on the formation and pruning of the calyx of Held in the medial nucleus of the trapezoid body (MNTB). Microglia were pharmacologically depleted in mice early in development using subcutaneous injections of an inhibitor of colony stimulating factor 1 receptor, which is essential for microglia survival. Brainstems were examined prior to and just after hearing onset, at postnatal days (P) 8 and P13, respectively. We found that at P13 there were significantly more polyinnervated MNTB neurons when microglia were depleted, consistent with a defect in pruning. Expression of glial fibrillary acidic protein (GFAP), a mature astrocyte marker that normally appears in the MNTB late in development, was significantly decreased in microglia-depleted mice at P13, suggesting a delay in astrocyte maturation. Our results demonstrate that monoinnervation of MNTB neurons by the calyx of Held is significantly disrupted or delayed in the absence of microglia. This finding may reflect a direct role for microglia in synaptic pruning. A secondary role for microglia may be in the maturation of astrocytes in MNTB. These findings highlight the significant function of glia in pruning during calyx of Held development.

Cochlear ablation in neonatal rats disrupts inhibitory transmission in the medial nucleus of the trapezoid body

Inhibitory circuits in the auditory brainstem undergo multiple postnatal changes that are both activity-dependent and activity-independent. We tested to see if the shift from GABA- to glycinergic transmission, which occurs in the rat medial nucleus of the trapezoid body (MNTB) around the onset of hearing, depends on sound-evoked neuronal activity. We prevented the activity by bilateral cochlear ablations in early postnatal rats and studied ionotropic GABA and glycine receptors in MNTB neurons after hearing onset. The removal of the cochlea decreased responses of GABA_A and glycine receptors to exogenous agonists as well as the amplitudes of inhibitory postsynaptic currents. The reduction was accompanied by a decrease in the number of glycine receptor- or vesicular GABA transporter-immunopositive puncta. Furthermore, the ablations markedly affected the switch in presynaptic GABA_A to glycine receptors. The increase in the expression of postsynaptic glycine receptors and the shift in inhibitory transmitters were not prevented. The results suggest that inhibitory transmission in the MNTB is subject to multiple developmental signals and support the idea that auditory experience plays a role in the maturation of the brainstem glycinergic circuits.

Subtle differences in synaptic transmission in medial nucleus of trapezoid body neurons between wild-type and Fmr1 knockout mice

In animal models for fragile X syndrome where the gene for fragile X mental retardation protein is knocked out (Fmr1 KO), neurotransmission in multiple brain regions shifts excitation/inhibition balance, resulting in hyperexcitability in neural circuits. Here, using whole-cell recordings from brainstem slices, we investigated synaptic transmission at the medial nucleus of trapezoid body (MNTB, a critical nucleus in the brainstem sound localization circuit), in Fmr1 KO and wild-type (WT) mice 2-3 weeks of age in both sexes. Surprisingly, neither synaptic excitation nor inhibition in KO neurons was significantly changed. The synaptic strength, kinetics, and short-term plasticity of synaptic excitation remained largely unaltered. Subtle differences were observed in response patterns, with KO neurons displaying less all-or-none eEPSCs. Similarly, synaptic inhibition mediated by glycine and GABA remains largely unchanged, except for a slower kinetics of mixed sIPSCs. In pharmacologically isolated glycinergic and GABAergic inhibition, no significant differences in synaptic strength and kinetics were detected between the two genotypes. These results demonstrate that at the cellular level synaptic transmission at MNTB is largely unaffected in Fmr1 KO mice by 2-3 weeks after birth, suggesting the existence of compensatory mechanisms that maintain the inhibitory output of MNTB to its targets in the auditory brainstem.

Underpinning heterogeneity in synaptic transmission by presynaptic ensembles of distinct morphological modules

Synaptic heterogeneity is widely observed but its underpinnings remain elusive. We addressed this issue using mature calyx of Held synapses whose numbers of bouton-like swellings on stalks of the nerve terminals inversely correlate with release probability (Pr). We examined presynaptic Ca2+ currents and transients, topology of fluorescently tagged knock-in Ca2+ channels, and Ca2+ channel-synaptic vesicle (SV) coupling distance using Ca2+ chelator and inhibitor of septin cytomatrix in morphologically diverse synapses. We found that larger clusters of Ca2+ channels with tighter coupling distance to SVs elevate Pr in stalks, while smaller clusters with looser coupling distance lower Pr in swellings. Septin is a molecular determinant of the differences in coupling distance. Supported by numerical simulations, we propose that varying the ensemble of two morphological modules containing distinct Ca2+ channel-SV topographies diversifies Pr in the terminal, thereby establishing a morpho-functional continuum that expands the coding capacity within a single synapse population.

Effects of Non-traumatic Noise and Conductive Hearing Loss on Auditory System Function

The effects of traumatic noise-exposure and deafening on auditory system function have received a great deal of attention. However, lower levels of noise as well as temporary conductive hearing loss also have consequences on auditory physiology and hearing. Here we review how abnormal acoustic experience at early ages affects the ascending and descending auditory pathways, as well as hearing behavior.

Changes in Properties of Auditory Nerve Synapses following Conductive Hearing Loss

Auditory activity plays an important role in the development of the auditory system. Decreased activity can result from conductive hearing loss (CHL) associated with otitis media, which may lead to long-term perceptual deficits. The effects of CHL have been mainly studied at later stages of the auditory pathway, but early stages remain less examined. However, changes in early stages could be important because they would affect how information about sounds is conveyed to higher-order areas for further processing and localization. We examined the effects of CHL at auditory nerve synapses onto bushy cells in the mouse anteroventral cochlear nucleus following occlusion of the ear canal. These synapses, called endbulbs of Held, normally show strong depression in voltage-clamp recordings in brain slices. After 1 week of CHL, endbulbs showed even greater depression, reflecting higher release probability. We observed no differences in quantal size between control and occluded mice. We confirmed these observations using mean-variance analysis and the integration method, which also revealed that the number of release sites decreased after occlusion. Consistent with this, synaptic puncta immunopositive for VGLUT1 decreased in area after occlusion. The level of depression and number of release sites both showed recovery after returning to normal conditions. Finally, bushy cells fired fewer action potentials in response to evoked synaptic activity after occlusion, likely because of increased depression and decreased input resistance. These effects appear to reflect a homeostatic, adaptive response of auditory nerve synapses to reduced activity. These effects may have important implications for perceptual changes following CHL.

SIGNIFICANCE STATEMENT

Normal hearing is important to everyday life, but abnormal auditory experience during development can lead to processing disorders. For example, otitis media reduces sound to the ear, which can cause long-lasting deficits in language skills and verbal production, but the location of the problem is unknown. Here, we show that occluding the ear causes synapses at the very first stage of the auditory pathway to modify their properties, by decreasing in size and increasing the likelihood of releasing neurotransmitter. This causes synapses to deplete faster, which reduces fidelity at central targets of the auditory nerve, which could affect perception. Temporary hearing loss could cause similar changes at later stages of the auditory pathway, which could contribute to disorders in behavior.

Dynamin 1- and 3-Mediated Endocytosis Is Essential for the Development of a Large Central Synapse In Vivo

Dynamin is a large GTPase crucial for endocytosis and sustained neurotransmission, but its role in synapse development in the mammalian brain has received little attention. We addressed this question using the calyx of Held (CH), a large nerve terminal in the auditory brainstem in mice. Tissue-specific ablation of different dynamin isoforms bypasses the early lethality of conventional knock-outs and allows us to examine CH development in a native brain circuit. Individual gene deletion of dynamin 1, a primary dynamin isoform in neurons, as well as dynamin 2 and 3, did not affect CH development. However, combined tissue-specific knock-out of both dynamin 1 and 3 (cDKO) severely impaired CH formation and growth during the first postnatal week, and the phenotypes were exacerbated by further additive conditional knock-out of dynamin 2. The developmental defect of CH in cDKO first became evident on postnatal day 3 (P3), a time point when CH forms and grows abruptly. This is followed by a progressive loss of postsynaptic neurons and increased glial infiltration late in development. However, early CH synaptogenesis before protocalyx formation was not altered in cDKO. Functional maturation of synaptic transmission in the medial nucleus of the trapezoid body in cDKO was impeded during development and accompanied by an increase in the membrane excitability of medial nucleus of the trapezoid body neurons. This study provides compelling genetic evidence that CH formation requires dynamin 1- and 3-mediated endocytosis in vivo, indicating a critical role of dynamin in synaptic development, maturation, and subsequent maintenance in the mammalian brain.

SIGNIFICANCE STATEMENT

Synaptic development has been increasingly implicated in numerous brain disorders. Dynamin plays a crucial role in clathrin-mediated endocytosis and synaptic transmission at nerve terminals, but its potential role in synaptic development in the native brain circuitry is unclear. Using the calyx of Held, a giant nerve terminal in the mouse brainstem, we evaluated the role of dynamin in this process by using tissue-specific knock-out (KO) of three different dynamin isoforms (dynamin 1, 2, and 3) individually and in combination. Our data demonstrated that dynamin is required for the formation, functional maturation, and subsequent survival of the calyx of Held. This study highlights the important role of dynamin-mediated endocytosis in the development of central synapses in the mammalian brain.

Activity-dependent formation and location of voltage-gated sodium channel clusters at a CNS nerve terminal during postnatal development

In auditory pathways, the precision of action potential (AP) propagation depends on axon myelination and high densities of voltage-gated Na (Na_v) channels clustered at nodes of Ranvier. Changes in Na_v channel expression at the heminode, the final node before the nerve terminal, can alter AP invasion into the presynaptic terminal. We studied the activity-dependent formation of Na_v channel clusters before and after hearing onset at postnatal day 12 in the rat and mouse auditory brain stem. In rats, the Na_v channel cluster at the heminode formed progressively during the second postnatal week, around hearing onset, whereas the Na_v channel cluster at the nodes was present before hearing onset. Initiation of heminodal Na_v channel clustering was correlated with the expression of scaffolding protein ankyrinG and paranodal protein Caspr. However, in whirler mice with congenital deafness, heminodal Na_v channels did not form clusters and maintained broad expression, but Na_v channel clustering was normal at the nodes. In addition, a clear difference in the distance from the heminodal Na_v channel to the calyx across the mediolateral axis of the medial nucleus of the trapezoid body (MNTB) developed after hearing onset. In the medial MNTB, where neurons respond best to high-frequency sounds, the heminodal Na_v channel cluster was located closer to the terminal than in the lateral MNTB, where neurons respond best to low-frequency sounds. Thus sound-mediated neuronal activities are potentially associated with the refinement of the heminode adjacent to the presynaptic terminal in the auditory brain stem.

NEW & NOTEWORTHY

Clustering of voltage-gated sodium (Na_v) channels and their distribution along the axon, specifically at the unmyelinated axon segment next to the nerve terminal, are essential for tuning propagated action potentials. Na_v channel clusters near the nerve terminal and their location as a function of neuronal position along the mediolateral axis are controlled by auditory inputs after hearing onset. Thus sound-mediated neuronal activity influences the tonotopic organization of Na_v channels at the nerve terminal in the auditory brain stem.

Reduced sensory stimulation alters the molecular make-up of glutamatergic hair cell synapses in the developing cochlea

Neural activity during early development is known to alter innervation pathways in the central and peripheral nervous systems. We sought to examine how reduced sound-induced sensory activity in the cochlea affected the consolidation of glutamatergic synapses between inner hair cells (IHC) and the primary auditory neurons as these synapses play a primary role in transmitting sound information to the brain. A unilateral conductive hearing loss was induced prior to the onset of sound-mediated stimulation of the sensory hair cells, by rupturing the tympanic membrane and dislocating the auditory ossicles in the left ear of P11 mice. Auditory brainstem responses at P15 and P21 showed a 40-50-dB increase in thresholds for frequencies 8-32kHz in the dislocated ear relative to the control ear. Immunohistochemistry and confocal microscopy were subsequently used to examine the effect of this attenuation of sound stimulation on the expression of RIBEYE, which comprises the presynaptic ribbons, Shank-1, a postsynaptic scaffolding protein, and the GluA2/3 and 4 subunits of postsynaptic AMPA receptors. Our results show that dislocation did not alter the number of pre- or postsynaptic protein puncta. However, dislocation did increase the size of RIBEYE, GluA4, GluA2/3 and Shank-1 puncta, with postsynaptic changes preceding presynaptic changes. Our data suggest that a reduction in sound stimulation during auditory development induces plasticity in the molecular make-up of IHC glutamatergic synapses, but does not affect the number of these synapses. Up-regulation of synaptic proteins with sound attenuation may facilitate a compensatory increase in synaptic transmission due to the reduced sensory stimulation of the IHC.

Molecular Machines Regulating the Release Probability of Synaptic Vesicles at the Active Zone

The fusion of synaptic vesicles (SVs) with the plasma membrane of the active zone (AZ) upon arrival of an action potential (AP) at the presynaptic compartment is a tightly regulated probabilistic process crucial for information transfer. The probability of a SV to release its transmitter content in response to an AP, termed release probability (Pr), is highly diverse both at the level of entire synapses and individual SVs at a given synapse. Differences in Pr exist between different types of synapses, between synapses of the same type, synapses originating from the same axon and even between different SV subpopulations within the same presynaptic terminal. The Pr of SVs at the AZ is set by a complex interplay of different presynaptic properties including the availability of release-ready SVs, the location of the SVs relative to the voltage-gated calcium channels (VGCCs) at the AZ, the magnitude of calcium influx upon arrival of the AP, the buffering of calcium ions as well as the identity and sensitivity of the calcium sensor. These properties are not only interconnected, but can also be regulated dynamically to match the requirements of activity patterns mediated by the synapse. Here, we review recent advances in identifying molecules and molecular machines taking part in the determination of vesicular Pr at the AZ.

Noradrenergic refinement of glutamatergic neuronal circuits in the lateral superior olivary nucleus before hearing onset

Neuronal circuit plasticity during development is fundamental for precise network formation. Pioneering studies of the developmental visual cortex indicated that noradrenaline (NA) is crucial for ocular dominance plasticity during the critical period in the visual cortex. Recent research demonstrated tonotopic map formation by NA during the critical period in the auditory system, indicating that NA also contributes to synaptic plasticity in this system. The lateral superior olive (LSO) in the auditory system receives glutamatergic input from the ventral cochlear nucleus (VCN) and undergoes circuit remodeling during postnatal development. LSO is innervated by noradrenergic afferents and is therefore a suitable model to study the function of NA in refinement of neuronal circuits. Chemical lesions of the noradrenergic system and chronic inhibition of α2-adrenoceptors in vivo during postnatal development in mice disrupted functional elimination and strengthening of VCN-LSO afferents. This was potentially mediated by activation of presynaptic α2-adrenoceptors and inhibition of glutamate release because NA presynaptically suppressed excitatory postsynaptic current (EPSC) through α2-adrenoceptors during the first two postnatal weeks in an in vitro study. Furthermore, NA and α2-adrenoceptor agonist induced long-term suppression of EPSCs and decreased glutamate release. These results suggest that NA has a critical role in synaptic refinement of the VCN-LSO glutamatergic pathway through failure of synaptic transmission. Because of the ubiquitous distribution of NA afferents and the extensive expression of α2-adrenoceptors throughout the immature brain, this phenomenon might be widespread in the developing central nervous system.

In vivo synaptic transmission and morphology in mouse models of Tuberous sclerosis, Fragile X syndrome, Neurofibromatosis type 1, and Costello syndrome

Defects in the rat sarcoma viral oncogene homolog (Ras)/extracellular-signal-regulated kinase and the phosphatidylinositol 3-kinase-mammalian target of rapamycin (mTOR) signaling pathways are responsible for several neurodevelopmental disorders. These disorders are an important cause for intellectual disability; additional manifestations include autism spectrum disorder, seizures, and brain malformations. Changes in synaptic function are thought to underlie the neurological conditions associated with these syndromes. We therefore studied morphology and in vivo synaptic transmission of the calyx of Held synapse, a relay synapse in the medial nucleus of the trapezoid body (MNTB) of the auditory brainstem, in mouse models of tuberous sclerosis complex (TSC), Fragile X syndrome (FXS), Neurofibromatosis type 1 (NF1), and Costello syndrome. Calyces from both Tsc1^+/- and from Fmr1 knock-out (KO) mice showed increased volume and surface area compared to wild-type (WT) controls. In addition, in Fmr1 KO animals a larger fraction of calyces showed complex morphology. In MNTB principal neurons of Nf1^+/- mice the average delay between EPSPs and APs was slightly smaller compared to WT controls, which could indicate an increased excitability. Otherwise, no obvious changes in synaptic transmission, or short-term plasticity were observed during juxtacellular recordings in any of the four lines. Our results in these four mutants thus indicate that abnormalities of mTOR or Ras signaling do not necessarily result in changes in in vivo synaptic transmission.

Synaptic plasticity in the auditory system: a review

Synaptic transmission via chemical synapses is dynamic, i.e., the strength of postsynaptic responses may change considerably in response to repeated synaptic activation. Synaptic strength is increased during facilitation, augmentation and potentiation, whereas a decrease in synaptic strength is characteristic for depression and attenuation. This review attempts to discuss the literature on short-term and long-term synaptic plasticity in the auditory brainstem of mammals and birds. One hallmark of the auditory system, particularly the inner ear and lower brainstem stations, is information transfer through neurons that fire action potentials at very high frequency, thereby activating synapses >500 times per second. Some auditory synapses display morphological specializations of the presynaptic terminals, e.g., calyceal extensions, whereas other auditory synapses do not. The review focuses on short-term depression and short-term facilitation, i.e., plastic changes with durations in the millisecond range. Other types of short-term synaptic plasticity, e.g., posttetanic potentiation and depolarization-induced suppression of excitation, will be discussed much more briefly. The same holds true for subtypes of long-term plasticity, like prolonged depolarizations and spike-time-dependent plasticity. We also address forms of plasticity in the auditory brainstem that do not comprise synaptic plasticity in a strict sense, namely short-term suppression, paired tone facilitation, short-term adaptation, synaptic adaptation and neural adaptation. Finally, we perform a meta-analysis of 61 studies in which short-term depression (STD) in the auditory system is opposed to short-term depression at non-auditory synapses in order to compare high-frequency neurons with those that fire action potentials at a lower rate. This meta-analysis reveals considerably less STD in most auditory synapses than in non-auditory ones, enabling reliable, failure-free synaptic transmission even at frequencies >100 Hz. Surprisingly, the calyx of Held, arguably the best-investigated synapse in the central nervous system, depresses most robustly. It will be exciting to reveal the molecular mechanisms that set high-fidelity synapses apart from other synapses that function much less reliably.