Synaptic Counts Research Articles

ATH‐1105, a small molecule positive modulator of the HGF/MET system, is neuroprotective and attenuates TDP‐43 protein pathology in ALS and frontotemporal dementia‐relevant preclinical models

AbstractBackgroundFrontotemporal dementia (FTD) is a neurodegenerative disorder marked by loss of neurons in the frontal and temporal lobes, leading to cognitive and behavioral changes. Common pathological features include neuroinflammation and intracellular inclusions of TDP‐43, which are found in approximately 50% of people with FTD. Positive modulation of the hepatocyte growth factor (HGF)/MET system has been shown to protect against several drivers of neurodegeneration and may represent a therapeutic strategy for FTD. Here, we assess the effects of ATH‐1105, a small molecule positive modulator of HGF/MET, in preclinical models that capture components of FTD, including neurodegeneration, inflammation, cognitive impairment, and TDP‐43 pathology.MethodTo evaluate the neurotrophic effects of ATH‐1105, neurite length and synaptic count were measured in rat hippocampal neurons after treatment with ATH‐1105. The neuroprotective effects of ATH‐1105 were assessed in rat cortical neurons subjected to neurotoxic insults including 1‐methyl‐4‐phenylpryidinium, glutamate, lipopolysaccharide (LPS), or hydrogen peroxide. Anti‐inflammatory effects of ATH‐1105 were evaluated by measuring secreted proinflammatory cytokines in LPS‐stimulated THP‐1 differentiated macrophages. To assess pro‐cognitive effects after neuroinflammatory insult, adult mice received a single intraperitoneal injection of LPS followed by daily doses of ATH‐1105 for 14 days. Cognitive performance was assessed in the T‐maze spontaneous alternation task. Finally, Prp‐TDP43A315T mice, a model of ALS/FTD, were given daily oral treatment with 20 mg/kg ATH‐1105 or vehicle for two months, starting from one month of age. Plasma was evaluated for levels of proinflammatory cytokines and neurofilament light chain. At study termination, TDP‐43 inclusions in the sciatic nerve were measured by immunohistochemistry.ResultTreatment with ATH‐1105 increased neurite outgrowth and synaptic count in primary hippocampal neurons and protected against all cytotoxic insults in primary cortical neurons. ATH‐1105 mitigated cytokine release in LPS‐stimulated THP‐1 macrophages and attenuated LPS‐induced cognitive impairment in vivo. In TDP‐43A315T mice, treatment with ATH‐1105 reduced plasma biomarkers of inflammation and neurodegeneration. Additionally, after two months of treatment, ATH‐1105 reduced intra‐axonal TDP‐43 inclusions in the sciatic nerve.ConclusionThe data presented provide preliminary evidence that ATH‐1105 is protective against several pathological mechanisms common to FTD, including TDP‐43 protein pathology, and support its continued evaluation as a therapeutic for FTD.

Normative and mechanistic model of an adaptive circuit for efficient encoding and feature extraction

One major question in neuroscience is how to relate connectomes to neural activity, circuit function, and learning. We offer an answer in the peripheral olfactory circuit of the Drosophila larva, composed of olfactory receptor neurons (ORNs) connected through feedback loops with interconnected inhibitory local neurons (LNs). We combine structural and activity data and, using a holistic normative framework based on similarity-matching, we formulate biologically plausible mechanistic models of the circuit. In particular, we consider a linear circuit model, for which we derive an exact theoretical solution, and a nonnegative circuit model, which we examine through simulations. The latter largely predicts the ORN [Formula: see text] LN synaptic weights found in the connectome and demonstrates that they reflect correlations in ORN activity patterns. Furthermore, this model accounts for the relationship between ORN [Formula: see text] LN and LN-LN synaptic counts and the emergence of different LN types. Functionally, we propose that LNs encode soft cluster memberships of ORN activity, and partially whiten and normalize the stimulus representations in ORNs through inhibitory feedback. Such a synaptic organization could, in principle, autonomously arise through Hebbian plasticity and would allow the circuit to adapt to different environments in an unsupervised manner. We thus uncover a general and potent circuit motif that can learn and extract significant input features and render stimulus representations more efficient. Finally, our study provides a unified framework for relating structure, activity, function, and learning in neural circuits and supports the conjecture that similarity-matching shapes the transformation of neural representations.

Therapeutic Potential of Fosgonimeton, a Small-Molecule Positive Modulator of the Neurotrophic HGF/MET Pathway, in Neurodegenerative Conditions (S14.008)

<h3>Objective:</h3> Evaluate the neurotrophic and neuroprotective effects of fosgonimeton, a small-molecule positive modulator of hepatocyte growth factor (HGF)/MET. <h3>Background:</h3> HGF signaling through the MET receptor promotes neural survival and function, making it a compelling target to combat neurodegeneration. We developed a series of small-molecule positive modulators of HGF/MET, including fosgonimeton, to target this neurotrophic system. <h3>Design/Methods:</h3> To demonstrate the neurotrophic effects of fosgonimeton, primary neurons were treated with the active metabolite of fosgonimeton (fosgo-AM) and assessed for synaptic count and neurite outgrowth. To evaluate neuroprotective effects, primary neurons were treated with fosgo-AM before exposure to relevant neurodegenerative insults, namely oxidative stress (hydrogen peroxide [H₂O₂]), neuroinflammation (lipopolysaccharide [LPS]), excitotoxicity (glutamate), and mitochondrial dysfunction (1-methyl-4-phenylpyridinium [MPP+]). The effects of fosgo-AM on protein aggregation in primary neuron cultures were evaluated via levels of phosphorylated tau (p-tau) induced by amyloid β (Aβ) toxicity in cortical neurons and α-synuclein aggregation induced by 6-hydroxydopamine (6-OHDA) in dopaminergic neurons. In vivo, the therapeutic usefulness of fosgonimeton was assessed for LPS-induced cognitive impairment in mice and 6-OHDA–induced motor deficits in rats. <h3>Results:</h3> In vitro, fosgo-AM significantly increased the number of synapses and neurite length in hippocampal and cortical neurons, respectively. Fosgo-AM was neuroprotective of cortical neurons subjected to oxidative, neuroinflammatory, excitotoxic, and mitochondrial challenges. Significant reduction in the number of disease biomarkers was observed with fosgo-AM treatment, including Aβ-induced p-tau accumulation in cortical neurons and 6-OHDA–induced α-synuclein aggregation in dopaminergic neurons. In vivo, fosgonimeton administration significantly attenuated LPS-induced cognitive impairment and 6-OHDA–induced motor deficits in rodent models. <h3>Conclusions:</h3> These results, from in vitro and in vivo systems, consistently support the broad application of fosgonimeton to neurodegenerative conditions. Collectively, our observations highlight the therapeutic potential of HGF/MET modulation in the management of neurodegenerative diseases that include AD and PD, supporting further clinical exploration of this approach. <b>Disclosure:</b> Dr. Reda has received personal compensation for serving as an employee of Athira Pharma, Inc. . Dr. Reda has stock in Athira Pharma, Inc. . Dr. Taylor has received personal compensation for serving as an employee of Athira Pharma, Inc. Dr. Taylor has stock in Athira Pharma, Inc.. Dr. Taylor has received intellectual property interests from a discovery or technology relating to health care. Dr. Johnston has received personal compensation for serving as an employee of Athira Pharma, Inc. Dr. Johnston has stock in Athira Pharma, Inc. An immediate family member of Dr. Johnston has stock in Athira Pharma, Inc. Dr. Johnston has received intellectual property interests from a discovery or technology relating to health care. Dr. Berthiaume has received personal compensation for serving as an employee of Athira Pharma. Dr. Berthiaume has stock in Athira Pharma. An immediate family member of Dr. Berthiaume has stock in Athira Pharma. Dr. Setti has received personal compensation for serving as an employee of Athira Pharma. Dr. Setti has stock in Athira Pharma. Dr. Church has received personal compensation for serving as an employee of Athira Pharma. Dr. Church has stock in Athira Pharma. Dr. Church has received intellectual property interests from a discovery or technology relating to health care.

Rapid reconstruction of neural circuits using tissue expansion and light sheet microscopy.

Brain function is mediated by the physiological coordination of a vast, intricately connected network of molecular and cellular components. The physiological properties of neural network components can be quantified with high throughput. The ability to assess many animals per study has been critical in relating physiological properties to behavior. By contrast, the synaptic structure of neural circuits is presently quantifiable only with low throughput. This low throughput hampers efforts to understand how variations in network structure relate to variations in behavior. For neuroanatomical reconstruction, there is a methodological gulf between electron microscopic (EM) methods, which yield dense connectomes at considerable expense and low throughput, and light microscopic (LM) methods, which provide molecular and cell-type specificity at high throughput but without synaptic resolution. To bridge this gulf, we developed a high-throughput analysis pipeline and imaging protocol using tissue expansion and light sheet microscopy (ExLLSM) to rapidly reconstruct selected circuits across many animals with single-synapse resolution and molecular contrast. Using Drosophila to validate this approach, we demonstrate that it yields synaptic counts similar to those obtained by EM, enables synaptic connectivity to be compared across sex and experience, and can be used to correlate structural connectivity, functional connectivity, and behavior. This approach fills a critical methodological gap in studying variability in the structure and function of neural circuits across individuals within and between species.

Synaptic counts approximate synaptic contact area in Drosophila.

The pattern of synaptic connections among neurons defines the circuit structure, which constrains the computations that a circuit can perform. The strength of synaptic connections is costly to measure yet important for accurate circuit modeling. Synaptic surface area has been shown to correlate with synaptic strength, yet in the emerging field of connectomics, most studies rely instead on the counts of synaptic contacts between two neurons. Here we quantified the relationship between synaptic count and synaptic area as measured from volume electron microscopy of the larval Drosophila central nervous system. We found that the total synaptic surface area, summed across all synaptic contacts from one presynaptic neuron to a postsynaptic one, can be accurately predicted solely from the number of synaptic contacts, for a variety of neurotransmitters. Our findings support the use of synaptic counts for approximating synaptic strength when modeling neural circuits.

Endolymphatic Hydrops is a Marker of Synaptopathy Following Traumatic Noise Exposure.

After acoustic trauma, there can be loss of synaptic connections between inner hair cells and auditory neurons in the cochlea, which may lead to hearing abnormalities including speech-in-noise difficulties, tinnitus, and hyperacusis. We have previously studied mice with blast-induced cochlear synaptopathy and found that they also developed a build-up of endolymph, termed endolymphatic hydrops. In this study, we used optical coherence tomography to measure endolymph volume in live CBA/CaJ mice exposed to various noise intensities. We quantified the number of synaptic ribbons and postsynaptic densities under the inner hair cells 1 week after noise exposure to determine if they correlated with acute changes in endolymph volume measured in the hours after the noise exposure. After 2 h of noise at an intensity of 95 dB SPL or below, both endolymph volume and synaptic counts remained normal. After exposure to 2 h of 100 dB SPL noise, mice developed endolymphatic hydrops and had reduced synaptic counts in the basal and middle regions of the cochlea. Furthermore, round-window application of hypertonic saline reduced the degree of endolymphatic hydrops that developed after 100 dB SPL noise exposure and partially prevented the reduction in synaptic counts in the cochlear base. Taken together, these results indicate that endolymphatic hydrops correlates with noise-induced cochlear synaptopathy, suggesting that these two pathologic findings have a common mechanistic basis.

The role of oxidative stress in the susceptibility of noise-impaired cochleae to synaptic loss induced by intracochlear electrical stimulation

Intracochlear electrical stimulation (ES) generated by cochlear implants (CIs) is used to activate auditory nerves to restore hearing perception in deaf subjects and those with residual hearing who use electroacoustic stimulation (EAS) technology. Approximately 1/3 of EAS recipients experience loss of residual hearing a few months after ES activation, but the underlying mechanism is unknown. Clinical evidence indicates that the loss is related to the previous history of noise-induced hearing loss (NIHL). In this report, we investigated the impact of intracochlear ES on oxidative stress levels and synaptic counts in inner hair cells (IHCs) of the apical, middle and basal regions of guinea pigs with normal hearing (NH) and NIHL. Our results demonstrated that intracochlear ES with an intensity of 6 dB above the thresholds of electrically evoked compound action potentials (ECAPs) could induce the elevation of oxidative stress levels, resulting in a loss of IHC synapses near the electrodes in the basal and middle regions of the NH cochleae. Furthermore, the apical region of cochleae with NIHL were more susceptible to synaptic loss induced by relatively low-intensity ES than that of NH cochleae, resulting from the additional elevation of oxidative stress levels and the reduced antioxidant capability throughout the whole cochlea.

Acute and chronic synaptic pathology in multiple sclerosis gray matter

Objectives: To investigate the extent of synaptic loss, and the contribution of gray matter (GM) inflammation and demyelination to synaptic loss, in multiple sclerosis (MS) brain tissue. Methods: This study was performed on two different post-mortem series of MS and control brains, including deep GM and cortical GM. MS brain samples had been specifically selected for the presence of active demyelinating GM lesions. Over 1,000,000 individual synapses were identified and counted using confocal microscopy, and further characterized as glutamatergic/GABAergic. Synaptic counts were also correlated with neuronal/axonal loss. Results: Important synaptic loss was observed in active demyelinating GM lesions (−58.9%), while in chronic inactive GM lesions, synaptic density was only mildly reduced compared to adjacent non-lesional gray matter (NLGM) (−12.6%). Synaptic loss equally affected glutamatergic and GABAergic synapses. Diffuse synaptic loss was observed in MS NLGM compared to control GM (−21.2% overall). Conclusion: This study provides evidence, in MS brain tissue, of acute synaptic damage/loss during active GM inflammatory demyelination and of synaptic reorganization in chronically demyelinated GM, affecting equally glutamatergic and GABAergic synapses. Furthermore, this study provides a strong indication of widespread synaptic loss in MS NLGM also independently from focal GM demyelination.

Trk agonist drugs rescue noise-induced hidden hearing loss.

TrkB agonist drugs are shown here to have a significant effect on the regeneration of afferent cochlear synapses after noise-induced synaptopathy. The effects were consistent with regeneration of cochlear synapses that we observed in vitro after synaptic loss due to kainic acid–induced glutamate toxicity and were elicited by administration of TrkB agonists, amitriptyline, and 7,8-dihydroxyflavone, directly into the cochlea via the posterior semicircular canal 48 hours after exposure to noise. Synaptic counts at the inner hair cell and wave 1 amplitudes in the auditory brainstem response (ABR) were partially restored 2 weeks after drug treatment. Effects of amitriptyline on wave 1 amplitude and afferent auditory synapse numbers in noise-exposed ears after systemic (as opposed to local) delivery were profound and long-lasting; synapses in the treated animals remained intact 1 year after the treatment. However, the effect of systemically delivered amitriptyline on synaptic rescue was dependent on dose and the time window of administration: it was only effective when given before noise exposure at the highest injected dose. The long-lasting effect and the efficacy of postexposure treatment indicate a potential broad application for the treatment of synaptopathy, which often goes undetected until well after the original damaging exposures.

DNA damage induced neuroinflammation in neurodegenerative disease

AbstractBackgroundThe neurological symptom is highly related to the activation of innate immune system in the brain in neurodegenerative disease. The robust activation of microglia, which includes an overproduction of cytokines, creates a pro‐inflammatory environment that greatly contributes to the neuronal cell damage.MethodDNA damage was induced by either direct triggers of double strands break or blocking DNA damage repair. Conditioned media from microglia culture was collected and treated to the neuronal culture. Different neuronal parameters, including the morphological changes, synaptic counts and death markers, were measured afterwards.ResultIn microglia, DNA damage induces elevated level of cytoplasmic DNA, which primes the anti‐virus innate immune response via the cytosolic DNA sensor, STING. Knockdown of STING or inhibition of STING activity rescued the inflammatory response, specifically, the overproduction of cytokines. Cytosolic DNA species also works through the AIM2‐containing inflammasome to process pro‐IL‐1β to its active form, IL‐1β. DNA damage in microglia leads to the secretion of pro‐inflammatory cytokines to the conditioned with sever neuronal cell damage.ConclusionThe present work pointed out the fundamentality of accumulated DNA damage in triggering the activation of pro‐inflammatory pathway in the immune cells. Upon DNA damage, brain microglia, the major immune cells of the central nervous system become activated and assume a neurotoxic pro‐inflammatory phenotype. This also calls attention to the importance of the microglia‐neuron interaction in brain diseases. Meanwhile, the evidence for the presence of cytoplasmic DNA in other cell types under different disease conditions are also relevant to our understanding of a wider range of neurodegenerative diseases.

Synaptic migration and reorganization after noise exposure suggests regeneration in a mature mammalian cochlea

Overexposure to intense noise can destroy the synapses between auditory nerve fibers and their hair cell targets without destroying the hair cells themselves. In adult mice, this synaptopathy is immediate and largely irreversible, whereas, in guinea pigs, counts of immunostained synaptic puncta can recover with increasing post-exposure survival. Here, we asked whether this recovery simply reflects changes in synaptic immunostaining, or whether there is actual retraction and extension of neurites and/or synaptogenesis. Analysis of the numbers, sizes and spatial distribution of pre- and post-synaptic markers on cochlear inner hair cells, in guinea pigs surviving from 1 day to 6 months after a synaptopathic exposure, shows dramatic synaptic re-organization during the recovery period in which synapse counts recover from 16 to 91% of normal in the most affected regions. Synaptic puncta move all over the hair cell membrane during recovery, translocating far from their normal positions at the basolateral pole, and auditory-nerve terminals extend towards the hair cell’s apical end to re-establish contact with them. These observations provide stronger evidence for spontaneous neural regeneration in a mature mammalian cochlea than can be inferred from synaptic counts alone.

Role of cochlear synaptopathy in cytomegalovirus infected mice and in children

ObjectivesDetermine whether a murine model of cytomegalovirus (CMV) and CMV- infected children show evidence of synaptopathy. Study designMurine model of CMV infection and case series. Subjects and methodsC57 BL/6 mice were inoculated with murine-CMV (mCMV). Auditory function was assessed using Auditory Brainstem Response (ABR) and distortion product otoacoustic emission (DPOAE) testing. Temporal bones from mCMV-infected mice were used for both ribbon synapse and hair cell quantification. Four groups of children (non-CMV normal hearing, non-CMV hearing impaired, CMV normal hearing and CMV hearing impaired) underwent ABRs between 2014 and 2018. The outcomes included raw amplitude, wave I:V amplitude ratio, absolute latency, and interpeak latency. ResultsMice at 8 weeks post mCMV infection had higher ABR and DPOAE (P < 0.05) thresholds and increased outer hair cell loss compared to uninfected mice and mCMV-infected mice at 4 and 6 weeks post infection, indicating progressive hearing loss. A reduction in the wave I amplitude and synaptic counts were noted earlier at 4 weeks in CMV-infected mice (P < 0.05). The human data indicated that the wave I:V amplitude ratio was lower on average in CMV-infected groups when compared to the uninfected cohorts. The wave I:V amplitude ratio for the click and 4k stimuli were not significantly different between the congenital CMV-infected and uninfected children with normal or with hearing loss. ConclusionThis study suggests mCMV infection results in a synaptopathy before hair cell damage. Additional studies need to be performed to determine whether this effect is also observed in CMV-infected children. Level of evidenceAnimal studies and basic science- NA; human studies: level 4.

Activity Dependent and Independent Determinants of Synaptic Size Diversity.

The extraordinary diversity of excitatory synapse sizes is commonly attributed to activity-dependent processes that drive synaptic growth and diminution. Recent studies also point to activity-independent size fluctuations, possibly driven by innate synaptic molecule dynamics, as important generators of size diversity. To examine the contributions of activity-dependent and independent processes to excitatory synapse size diversity, we studied glutamatergic synapse size dynamics and diversification in cultured rat cortical neurons (both sexes), silenced from plating. We found that in networks with no history of activity whatsoever, synaptic size diversity was no less extensive than that observed in spontaneously active networks. Synapses in silenced networks were larger, size distributions were broader, yet these were rightward-skewed and similar in shape when scaled by mean synaptic size. Silencing reduced the magnitude of size fluctuations and weakened constraints on size distributions, yet these were sufficient to explain synaptic size diversity in silenced networks. Model-based exploration followed by experimental testing indicated that silencing-associated changes in innate molecular dynamics and fluctuation characteristics might negatively impact synaptic persistence, resulting in reduced synaptic numbers. This, in turn, would increase synaptic molecule availability, promote synaptic enlargement, and ultimately alter fluctuation characteristics. These findings suggest that activity-independent size fluctuations are sufficient to fully diversify glutamatergic synaptic sizes, with activity-dependent processes primarily setting the scale rather than the shape of size distributions. Moreover, they point to reciprocal relationships between synaptic size fluctuations, size distributions, and synaptic numbers mediated by the innate dynamics of synaptic molecules as they move in, out, and between synapses.SIGNIFICANCE STATEMENT Sizes of glutamatergic synapses vary tremendously, even when formed on the same neuron. This diversity is commonly thought to reflect the outcome of activity-dependent forms of synaptic plasticity, yet activity-independent processes might also play some part. Here we show that in neurons with no history of activity whatsoever, synaptic sizes are no less diverse. We show that this diversity is the product of activity-independent size fluctuations, which are sufficient to generate a full repertoire of synaptic sizes at correct proportions. By combining modeling and experimentation we expose reciprocal relationships between size fluctuations, synaptic sizes and synaptic counts, and show how these phenomena might be connected through the dynamics of synaptic molecules as they move in, out, and between synapses.

TSP1 and TSP2 Have Unique and Overlapping Roles in Protecting against Noise-Induced Auditory Synaptopathy

Thrombospondins (TSPs) are cell adhesion molecules that play an important role in the maintenance of hearing and afferent synaptic connections. Based on their reported function in restoring synaptic connections after stroke, we tested a potential role for TSP1 and TSP2 genes in repairing cochlear synapses following noise injury. We observed a tonotopic gradient in the expression of TSP1 and TSP2 mRNA in control mouse cochleae and an upregulation of these genes following noise exposure. Examining the functional sequelae of these changes revealed that afferent synaptic counts and auditory brainstem responses (ABRs) in noise-exposed TSP1 and TSP2 knockout (−/−) mice exhibited a worst recovery when compared to controls. Consistent with their tonotopic expression, TSP1−/− mice showed greater susceptibility to noise-induced hearing loss (NIHL) at 8 kHz and 16 kHz frequencies, whereas NIHL in TSP2−/− mice occurred only at mid and high frequencies. Further analysis of the ABR waveforms indicated peripheral neuronal damage in TSP2−/− but not in TSP1−/− mice. Noise trauma affecting mid to high frequencies triggered severe seizures in the TSP2−/− mice. We found that decreased susceptibility to audiogenic seizures in TSP1−/− mice was correlated with increased TSP2 protein levels in their inner ears, suggesting that TSP2 might functionally compensate for the loss of TSP1 in these mice. Our data indicate that TSP1 and TSP2 are both involved in susceptibility to NIHL, with TSP2 playing a more prominent role.

Postnatal maturation of auditory-nerve heterogeneity, as seen in spatial gradients of synapse morphology in the inner hair cell area

Auditory nerve fibers in the adult ear are divided into functional subgroups according to spontaneous rate (SR) and threshold sensitivity. The high-threshold, low-SR fibers are morphologically and spatially distinct from the low-threshold high-SR fibers at their synaptic contacts with inner hair cells. This distinction between SR groups in the adult ear is visible in confocal microscopy as complementary size gradients of presynaptic ribbons and post-synaptic glutamate receptor patches across the modiolar-pillar and habenular-cuticular axes in the inner hair cell area. The aim of the present study was to track the post-natal development of this morphological gradient, in mouse, to determine the earliest age at which this important aspect of cochlear organization is fully mature. Here we show, using morphometric analysis of the organ of Corti immunostained for pre- and post-synaptic markers of efferent and afferent innervation, that this SR-based morphological gradient is not fully established until postnatal day 28, well after other features, such as synaptic counts and efferent innervation density in both the inner and outer hair cell areas, appear fully mature.

Perinatal thiamine deficiency causes cochlear innervation abnormalities in mice

Neonatal thiamine deficiency can cause auditory neuropathy in humans. To probe the underlying cochlear pathology, mice were maintained on a thiamine-free or low-thiamine diet during fetal development or early postnatal life. At postnatal ages from 18 days to 22 wks, cochlear function was tested and cochlear histopathology analyzed by plastic sections and cochlear epithelial whole-mounts immunostained for neuronal and synaptic markers. Although none of the thiamine-deprivation protocols resulted in any loss of hair cells or any obvious abnormalities in the non-sensory structures of the cochlear duct, all the experimental groups showed significant anomalies in the afferent or efferent innervation. Afferent synaptic counts in the inner and outer hair cell areas were reduced, as was the efferent innervation density in both the outer and inner hair cell areas. As expected for primary neural degeneration, the thresholds for distortion product otoacoustic emissions were not affected, and as expected for subtotal hair cell de-afferentation, the suprathreshold amplitudes of auditory brainstem responses were more affected than the response thresholds. We conclude that the auditory neuropathy from thiamine deprivation could be produced by loss of inner hair cell synapses.

Cochlear neuropathy in human presbycusis: Confocal analysis of hidden hearing loss in post-mortem tissue

Recent animal work has suggested that cochlear synapses are more vulnerable than hair cells in both noise-induced and age-related hearing loss. This synaptopathy is invisible in conventional histopathological analysis, because cochlear nerve cell bodies in the spiral ganglion survive for years, and synaptic analysis requires special immunostaining or serial-section electron microscopy. Here, we show that the same quadruple-immunostaining protocols that allow synaptic counts, hair cell counts, neuronal counts and differentiation of afferent and efferent fibers in mouse can be applied to human temporal bones, when harvested within 9 h post-mortem and prepared as dissected whole mounts of the sensory epithelium and osseous spiral lamina. Quantitative analysis of five “normal” ears, aged 54–89 yrs, without any history of otologic disease, suggests that cochlear synaptopathy and the degeneration of cochlear nerve peripheral axons, despite a near-normal hair cell population, may be an important component of human presbycusis. Although primary cochlear nerve degeneration is not expected to affect audiometric thresholds, it may be key to problems with hearing in noise that are characteristic of declining hearing abilities in the aging ear.

Aging after noise exposure: acceleration of cochlear synaptopathy in "recovered" ears.

Cochlear synaptic loss, rather than hair cell death, is the earliest sign of damage in both noise- and age-related hearing impairment (Kujawa and Liberman, 2009; Sergeyenko et al., 2013). Here, we compare cochlear aging after two types of noise exposure: one producing permanent synaptic damage without hair cell loss and another producing neither synaptopathy nor hair cell loss. Adult mice were exposed (8-16 kHz, 100 or 91 dB SPL for 2 h) and then evaluated from 1 h to ∼ 20 months after exposure. Cochlear function was assessed via distortion product otoacoustic emissions and auditory brainstem responses (ABRs). Cochlear whole mounts and plastic sections were studied to quantify hair cells, cochlear neurons, and the synapses connecting them. The synaptopathic noise (100 dB) caused 35-50 dB threshold shifts at 24 h. By 2 weeks, thresholds had recovered, but synaptic counts and ABR amplitudes at high frequencies were reduced by up to ∼ 45%. As exposed animals aged, synaptopathy was exacerbated compared with controls and spread to lower frequencies. Proportional ganglion cell losses followed. Threshold shifts first appeared >1 year after exposure and, by ∼ 20 months, were up to 18 dB greater in the synaptopathic noise group. Outer hair cell losses were exacerbated in the same time frame (∼ 10% at 32 kHz). In contrast, the 91 dB exposure, producing transient threshold shift without acute synaptopathy, showed no acceleration of synaptic loss or cochlear dysfunction as animals aged, at least to ∼ 1 year after exposure. Therefore, interactions between noise and aging may require an acute synaptopathy, but a single synaptopathic exposure can accelerate cochlear aging.

Connectome of the fly visual circuitry.

Recent powerful tools for reconstructing connectomes using electron microscopy (EM) have made outstanding contributions to the field of neuroscience. As a prime example, the detection of visual motion is a classic problem of neural computation, yet our understanding of the exact mechanism has been frustrated by our incomplete knowledge of the relevant neurons and synapses. Recent connectomic studies have successfully identified the concrete neuronal circuit in the fly's visual system that computes the motion signals. This identification was greatly aided by the comprehensiveness of the EM reconstruction. Compared with light microscopy, which gives estimated connections from arbor overlap, EM gives unequivocal connections with precise synaptic counts. This paper reviews the recent study of connectomics in a brain of the fruit fly Drosophila and highlights how connectomes can provide a foundation for understanding the mechanism of neuronal functions by identifying the underlying neural circuits.

Dexmedetomidine protects spatial learning and memory ability in rats.

The authors tested the hypothesis that combined use of dexmedetomidine on fetal rats during isoflurane exposure in maternal anesthesia can attenuate the abnormal spatial learning and memory abilities in adults via the renin-angiotensin-aldosterone system. Fifty timed-pregnancy rats were randomly assigned to five groups (Dex+Iso, Sal+Iso, Sal+Oxy, Dex+Oxy, and a control group ) on embryonic day 14 to receive five different dispositions, i.e. combined injection of dexmedetomidine (Dex) or saline (Sal) and inhalation of isoflurane (Iso), oxygen (Oxy), or normal air for 4 h (n = 10). The latency time(s) from day 1 to day 4 all showed a decreasing tendency in all four groups. The synaptic count of the Sal+Iso group was significantly lower than the Control group (p < 0.05), suggesting that severe neurodegeneration occurred under the influence of fetal isoflurane exposure. In contrast, the synapse count of the Dex+Iso group was near to that of Control group. The rats are protected in neurodevelopmental, normal development. Combine use of dexmedetomidine during exposure to isoflurane in utero during middle-pregnancy can attenuate the impairment of spatial learning and memory abilities for the rats in adulthood.