Frontiers in Cellular Neuroscience

The nasal epithelium is a complex tissue composed of both respiratory and olfactory tissue, and is constantly exposed to environmental insults, including toxins and pathogens. The main olfactory epithelium (MOE) serves as the critical site for olfaction, or sense of smell. Dysfunction at this critical barrier tissue can result in partial or total loss of olfactory function, resulting in significant impact to quality of life. The MOE is heterogeneous, comprised of many cell types including olfactory sensory neurons, support cells, and immune cells. It is not well understood how these diverse cell types in the MOE interact to regulate this tissue during homeostasis, and during times of injury and inflammation. We investigated how environmental olfactory exposures impact cell type specific transcriptional responses in the mouse MOE. We performed single-cell RNA sequencing (scRNA-seq) of the MOE following controlled environmental exposure to both well-known odorants and allergens. We identified major cell types and subtypes within the MOE, and identified transcriptional changes in response to the olfactory exposures. We identified transcriptional changes in OSNs, sustentacular cells, and resident immune cells to each condition. This indicated that environmental olfactory exposures drive changes to multiple cell types in the MOE. To our knowledge, this is the first study to identify effects of environmental olfactory exposures on cell-type specific transcription at homeostasis. These findings highlight the potential importance of multi-cellular interactions and communication in regulation of the olfactory epithelium.

Hypoxic-ischemic injury is a major cause of olfactory dysfunction, yet the cellular and morphological mechanisms underlying this sensory loss remain poorly understood. Here, we investigated the structural, cellular, and functional effects of acute hypoxic exposure on the olfactory system of adult zebrafish (Danio rerio) of both sexes, a model organism with remarkable neuroregenerative capacity. Fish were subjected to 15 minutes of acute severe hypoxia (0.8 mg/L dissolved oxygen) and assessed at 1 and 5 days post-hypoxia (dph). We evaluated olfactory function by means of cadaverine-evoked aversive behavioral assays. Structural and morphological integrity and inflammation of the olfactory epithelium (OE) and olfactory bulb (OB) were characterized using immunohistochemistry, histological stainings, and a 2,3,5-triphenyltetrazolium chloride (TTC) colorimetric assay. Acute hypoxic exposure impaired olfactory-mediated behaviors without affecting locomotion or exploratory behavior. In the peripheral OE, hypoxia caused neurodegeneration, disruption of the nasal mucus layer, and robust leukocytic infiltration. We observed reduced mitochondrial dehydrogenase activity in the olfactory bulb (OB) along with reactive astrogliosis. Olfactory function recovered by 5 days, coinciding with full restoration of OE morphology, and supported by a strong proliferative response. These findings reveal a coordinated degenerative and regenerative response to hypoxia across the olfactory axis, with implications for understanding hypoxia-induced sensory loss and neural repair. SIGNIFICANCEThis work addresses an important gap in knowledge regarding the mechanisms linking hypoxic insult and olfactory dysfunction. By using adult zebrafish, an extraordinarily regenerative vertebrate, it also provides insight into neuronal repair and regenerative processes supporting olfactory recovery. The novelty of our study resides in that, to our knowledge, there are no studies that provide a comprehensive characterization of the effects of hypoxia in the olfactory system across molecular, histological, and functional levels. These findings advance our understanding of hypoxia-induced sensory neurodegeneration and regeneration, and highlight the zebrafish olfactory system as a powerful model for investigating neural repair mechanisms relevant to hypoxic-ischemic brain injury.

Presynaptic homeostatic plasticity (PHP) is a potent form of homeostatic plasticity that has been documented at synapses as diverse as the glutamatergic Drosophila neuromuscular junction (NMJ), cholinergic mammalian NMJ (including human), and glutamatergic synapses in the mammalian brain. Published experimental evidence in favor of PHP in adult hippocampus and cerebellum includes patch-clamp electrophysiology, presynaptic capacitance measurement, calcium imaging, optical reporters of vesicle release and correlated three-dimensional electron microscopy. These studies are grounded in newly optimized experimental protocols that differ substantively from those typically used to study activity-dependent plasticity in neonatal and juvenile slice preparations. Here, we elaborate and extend our assays and methodologies for the study of PHP in the adult mammalian brain. Our assays are designed to optimize synapse, cell and tissue health and minimize the incorporation of unintended adverse experimental conditions that may interfere with the induction and/or expression of PHP. In addition, we provide benchmark criteria for assessment of cell health, necessary for analysis of PHP and, in so doing, advance our understanding of postsynaptic conditions necessary for PHP induction in the adult brain. Our data underscore why PHP may have been previously overlooked, inclusive of a recent manuscript challenging the robust expression of PHP in the mammalian brain (Dou et al., 2026 BioRxiv [preprint]).

Oligodendrocyte precursor cells (OPCs) are unique glial cells that communicate bidirectionally with neurons. Neuronal inputs drive various OPC behaviors, including proliferation and differentiation, immunomodulation, blood brain barrier regulation, synapse engulfment and axonal remodeling. OPCs are implicated in numerous stress and pain conditions, where their involvement is likely driven by neuronal activity (ie. neurotransmitter and neuropeptide signaling). One neuropeptide causally involved in chronic pain and stress conditions is calcitonin gene-related peptide (CGRP). Here, we tested the hypothesis that OPCs receive direct inputs from CGRP-containing neurons in the adult brain. Using RNAscope, immunofluorescence and analysis of single-cell datasets, we find that OPCs express receptors for CGRP and we identify close spatial contacts between CGRP and OPCs, with nearly half of CGRP puncta occurring within 1 {micro}m of an OPC. Some of these contacts appear to be synaptic, with CGRP-OPC contacts colocalizing with the presynaptic protein Bassoon and the postsynaptic protein PSD-95. This work suggests the presence of both diffuse and more direct forms of CGRP signaling to OPCs, raising the importance of future experiments to identify both the mode of CGRP release onto OPCs and the functional effects of these different contact types.

The medial olivocochlear (MOC) efferent system modulates outer hair cell (OHC) excitability and protects cochlea from overstimulation. Cholinergic activation of 910 nicotinic acetylcholine receptors (nAChRs) triggers Ca{superscript 2} influx, activating BK and SK2 Ca{superscript 2}-dependent K channels, and K extrusion through KCNQ4 to restore membrane potential. KCNQ4-loss causes chronic depolarization, OHC dysfunction, and hearing loss. Here, we investigated how KCNQ4 deficiency affects cochlear efferent synapse development and organization. Using confocal immunofluorescence, we analyzed efferent innervation in the organ of Corti of Kcnq4-/- (KO) and Kcnq4+/+(WT) mice at 2, 3, 4, and 10 postnatal weeks (W). At 2 W, efferent terminals were similarly distributed between basal and lateral OHC membrane domains in both genotypes. During maturation, WT mice exhibited complete relocation of MOC terminals to the basal domain, whereas KO mice showed delayed maturation, with some terminals laterally displaced up to 10 W. KCNQ4 absence was associated with reduced number and volume of efferent boutons on OHCs. Milder morphometric alterations were observed in efferent boutons within the inner hair cell region. At the molecular level, qPCR revealed downregulation of 10 nAChR subunit, BK, and SK2 transcripts in KO at 4 W, with recovery to 10 W. Despite this recovery, BK protein showed reduced expression, mislocalization, and disorganized synaptic plaques in OHCs. KO also displayed age-dependent upregulation of the calcium-binding proteins calbindin and calretinin, suggesting compensatory responses to altered Ca+{superscript 2} homeostasis. Together, these findings demonstrate that KCNQ4 is essential for OHC repolarization, maturation and maintenance of cochlear efferent synapses. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=179 HEIGHT=200 SRC="FIGDIR/small/700803v1_ufig1.gif" ALT="Figure 1"> View larger version (52K): org.highwire.dtl.DTLVardef@134e2caorg.highwire.dtl.DTLVardef@1155f45org.highwire.dtl.DTLVardef@21b4ccorg.highwire.dtl.DTLVardef@e4ee62_HPS_FORMAT_FIGEXP M_FIG C_FIG

Dendritic spines are highly dynamic structures whose morphology and lifespan are modified in response to synaptic efficacy changes. Structural modifications following activity support the long-term encoding of information and could allow for the remodeling of neural circuits. Long-term depression (LTD) is a key mechanism for synaptic weight regulation, yet its structural correlates -- particularly for long-lasting, protein synthesis dependent forms -- remain poorly understood. Furthermore, in humans, this type of plasticity is often disrupted in neurodevelopmental disorders, correlating with cognitive dysfunction and structural abnormalities. Fragile X Syndrome (FXS) is the most common inherited form of intellectual disability and is characterized by excessive metabotropic receptor-mediated synaptic depression, excessive protein synthesis, and spine abnormalities. Here, we investigate the relationship between long lasting synaptic depression and structural plasticity, as well as the role of protein availability in determining how many spines can simultaneously undergo structural changes during LTD in both healthy and FXS mutant neurons. Using high resolution optical methods, we developed and tested a method for inducing metabotropic glutamate receptor (mGluR)-dependent depression at single spines via glutamate uncaging in mouse hippocampal neurons. We found that this form of activity leads to robust spine shrinkage, which requires new protein synthesis. However, when we induced this depression at multiple spines, they competed for structural changes and only one spine shrank. We hypothesized that this was due to limited resources, in the form of newly made proteins, and therefore, we decided to test if spine competition would be altered in the mouse model of FXS, where protein levels are abnormally elevated. Indeed, we found that competition was absent in FXS mutant neurons, and all of the stimulated spines underwent shrinkage following LTD induction. Importantly, we found that single spine structural plasticity in FXS was expressed to the same degree as in WT controls. Taken together, these findings suggest that the hallmark phenotype of excess mGluR LTD in FXS may result from a greater number of inputs undergoing synaptic depression, rather than excessive LTD at individual synapses. By probing plasticity at the level of individual inputs, our findings highlight the importance of evaluating activity across groups of synapses, in order to uncover plasticity interactions that are critical for learning. Understanding how these mechanisms are disrupted in neurodevelopmental disorders such as FXS can inform the development of effective therapeutic strategies.

The replacement of a single codon in the human prion gene, causing the substitution of glycine with valine at position 127 (G127V) of the prion protein (PrP), prevents development of prion disease. We set out to explore if prion disease survival extension manifests in mice if the V127 mutant is delivered through a recombinant adeno-associated virus (rAAV) packaged as a self-complementary DNA. The notorious delivery limitations of rAAVs were overcome using a cross-correction approach that relied on the expression of the mutation in the context of glycosylphosphatidylinositoI-anchorless ({Delta}GPI) PrP. In this proof-of-concept study, we inoculated Rocky Mountain Laboratory (RML) prions into knock-in mice, in which the endogenous murine prion protein gene (Prnp) was replaced with the bank vole prion protein gene (BvPrnp). Prion-inoculated mice that were retro-orbitally transduced with a protective rAAV vector encoding BvPrnpV127{Delta}GPI survived [~]50 days longer than control mice that were unprotected. A deep proteomic analysis revealed that BvPrnpV127{Delta}GPI was protective by slowing perturbations to the proteome observed in late-stage RML prion disease. In addition to capturing details of synaptic decay and depletion of proteins in proximity to PrP, the proteomic dataset revealed the identity of proteins of potential diagnostic value that may be central to the brains attempt to fight prion disease by contributing to astrocytosis or microgliosis, by coping with calcium influx, or by enhancing the endoplasmic reticulum processing of essential proteins. Taken together, our results demonstrate that a gene therapy based on a GPI-anchorless PrP containing the G127V mutation can delay the onset of prion disease in mice, providing a framework for development of a corresponding therapy in humans. AUTHOR SUMMARYA rare change in the human prion protein, involving a single building block, has been linked to strong protection against prion diseases--fatal neurodegenerative disorders. This study tested whether that protective effect could be reproduced using gene therapy in mice. To this end, we exposed the animals to infectious prions and then delivered the protective version of the protein into mice using a viral carrier. Treated mice survived about seven weeks longer than untreated animals, showing that the approach can meaningfully slow disease progression. To understand why, we examined changes in brain proteins during disease and found that treatment helped preserve the normal protein levels of cellular proteins, particularly those involved in communication between nerve cells. The analysis also identified proteins altered in the disease that are linked to the brains defense responses, including inflammation, stress handling, and protein processing, some of which may serve as future disease markers. Importantly, the limited protection observed was not due to poor delivery of the therapy but likely reflects biological limits of the model used. Overall, the findings support the idea that gene therapies based on naturally protective human variants could help slow prion diseases and improve understanding of how the brain responds to them.

A single allele of any of the [~]1100 functional mouse odorant receptor (OR) genes is expressed by mature olfactory sensory neurons (mOSNs) through a poorly understood mechanism of singular gene choice. We were interested in developing an expression system to study OR selection in a mouse embryonic stem cell (mESC) carrying an Olfr151-IRES-CRE knock-in and a CRE-recombination-dependent ROSA26-MTMG reporter. Recombination of the ROSA reporter could not be observed after exposing this mESC to thousands of chemical compounds, including 175 compounds known to interfere with epigenetic regulatory processes nor by transfection with high probability Olfr151 promoter minigenes coexpressing CRE. The only two known regulatory elements that control OR promoters are not olfactory specific. Still, our inability to elicit even leaky CRE recombination in mESCs suggests that a specific transcriptional machinery within the OSN lineage is needed to drive OR promoter activation. Author SummaryThe Olfr151 promoter is inactive in mESCs.

Unraveling how adult neurons reshape their architecture is key to understanding post-developmental plasticity. Drosophila clock neurons, which remodel their terminals on a daily basis, offer a unique model to examine the mechanisms underlying structural plasticity. In this study, we examine the impact of the experimental design on the remodeling process. We established a simple fixation protocol that preserves tissue integrity and prevents its deformation while enabling the fixation of a larger number of individuals within the appropriate time window. We show that intrinsic (i.e., targeting fluorescent reporters to the membrane) or extrinsic (i.e., temperature) variables may influence this dynamic process. Examining ex vivo preparations, we found that the s-LNv terminals display numerous thin filopodia extending from their synaptic boutons. However, these fine membrane protrusions are lost upon fixation, as they could only be accurately visualized ex vivo. Finally, we present MorphoScope, a Python-based interface that eliminates observer bias in complexity measurements. Altogether, we present a powerful and robust model to investigate the principles of adult neuronal plasticity, with implications extending beyond circadian biology.

Despite a long history of studying presynaptic inhibition of the Ia afferent synapse that produces the monosynaptic EPSP on motoneurons, recent evidence has upset the conventional idea that GABAA receptors mediate this inhibition and instead suggests that there are mainly GABAB receptors at this synapse. However, without targeted access to the GABAergic neurons that activate these receptors, quantifying their functional contribution to presynaptic inhibition has proven difficult. We demonstrate here that focal optogenetic activation of terminals of a subpopulation of GAD2+ GABAergic neurons that exclusively project ventrally to Ia afferent synapses produce long-lasting presynaptic inhibition that is entirely mediated by GABAB receptors and simultaneously produces a characteristic brief GABAA receptor-mediated IPSP on the motoneurons. These ventral GAD2 neurons are recurrently activated by Ia afferents, contributing to post-activation depression with repeated afferent reflex testing, with a similar long time-course to post-activation depression of the H-reflex induced in humans from either repetitive activation of the same Ia afferents or from antagonist nerve conditioning. In contrast, focal activation of dorsally projecting GAD2 neurons does not directly cause presynaptic inhibition or postsynaptic IPSPs but does produce primary afferent depolarization. Following chronic spinal cord injury (SCI), the expression of GABAB receptors on the Ia terminal is halved, and in mice and humans, is associated with a similar decrease of GABAB receptor-mediated post-activation depression of Ia-EPSPs transmission, which is reversed by the GABAB receptor agonist baclofen. In summary, GABAB receptors mediate presynaptic inhibition, but are down regulated with SCI, contributing to reflex hyperexcitability associated with spasticity. Key Points SummaryO_LIPresynaptic inhibition of Ia afferents is mediated by the recurrent activation of terminal GABAB receptors by a subpopulation of ventrally projecting GAD2+ interneurons. C_LIO_LIIn contrast, dorsally projecting GAD2+ interneurons activate GABAA receptors on Ia afferent nodes to facilitate action potential conduction through branchpoints. C_LIO_LIRepetitive activation of Ia afferents at rates of every 10 s or faster produces post-activation depression via neurotransmitter depletion and from activation of terminal GABAB receptors. C_LIO_LIThese ventrally projecting GAD2+ interneurons can also be activated by other afferents that then produce PAD-evoked spikes to produce post-activation depression from conditioning nerve stimulation. C_LIO_LIThe reduction of GABAB receptors on the Ia terminal in spinal cord injury results in reduced presynaptic inhibition and post-activation depression, contributing to reflex hyperexcitability. C_LI O_FIG O_LINKSMALLFIG WIDTH=189 HEIGHT=200 SRC="FIGDIR/small/700955v2_ufig1.gif" ALT="Figure 1"> View larger version (39K): org.highwire.dtl.DTLVardef@17abd51org.highwire.dtl.DTLVardef@12316baorg.highwire.dtl.DTLVardef@a92168org.highwire.dtl.DTLVardef@1d06ca0_HPS_FORMAT_FIGEXP M_FIG Abstract legend: Schematic of GABAergic circuit producing presynaptic inhibition and primary afferent depolarization (PAD) in proprioceptive Ia afferents. We propose two populations of GAD2+ GABAergic interneurons, one with dorsal projections (purple) that activate GABAA receptors on the nodes of Ia afferents to produce PAD and subsequent facilitation of Ia afferent conduction, and another ventrally projecting population (pink) that activates GABAB receptors on the Ia afferent terminal to produce presynaptic inhibition via inhibition of VCa2+ channels and reduction of neurotransmitter release and replenishment. Both are activated by first order interneurons (grey). Repetitive activation of Ia afferents (green extensor) recurrently activates twhe ventrally projecting GAD2+ neurons to activate terminal GABAB receptors and long-lasting post-activation depression of Ia EPSPs and reflexes as measured from ventral root recordings. Strong conditioning stimulation of other afferents (blue flexor) activates dorsal GAD2+ neurons that can produce PAD-evoked spikes in extensor afferents that orthodromically activate motoneurons to set up post-activation depression of subsequent extensor reflexes. Here, PAD is also evoked in other afferents (flexor) by dorsally projecting GAD2+ neurons (light pink branch) but without activation of the ventrally projecting GAD2+ neurons or presynaptic inhibition. C_FIG

AO_SCPLOWBSTRACTC_SCPLOWSpiral ganglion neurons (SGNs) constitute the sole afferent connection between cochlear hair cells and central auditory nuclei. SGNs die during postnatal developmental pruning, and also following hair cell death, which can be triggered by ototoxic agents such as aminoglycoside antibiotics, including kanamycin. After hair cell loss, animal models show extensive SGN degeneration occurring gradually over a period of weeks to months. Here, we compared spatial and temporal patterns of SGN loss and immune cell involvement in these two cases of cell death in rats. Developmental SGN pruning occurred from postnatal day 5 (P5) to P8 in the basal half of the cochlea, and from P5 to P12 in the apical half. This was accompanied by a transient increase in spiral ganglion macrophages temporally and spatially correlated with SGN death, consistent with a role clearing degenerating neurons. After deafening neonatal rats with kanamycin injections, SGN death became evident at approximately 5.5 weeks of age and persisted throughout the ganglion, with greatest loss in the middle regions; less in the base and apex. Macrophage numbers also increased but neither temporally nor spatially correlated with SGN death. Rather, increased macrophage number and activation began approximately three weeks before SGN death and was highest in the apex. Additionally, T-cells and NK cells appeared in the ganglion concurrently with SGN degeneration. These observations suggest fundamentally different roles for macrophages post-deafening than during developmental pruning and, with prior observations that anti-inflammatory drugs reduce SGN death, support a causal role for immune responses in SGN death post-deafening.

Neuronal populations connected by gap junctions can be revealed via dye coupling of small molecules like neurobiotin and lucifer yellow. However, the extent of dye diffusion between neurons varies with connexin subtype, loading method, and neuromodulation. Due to the increasing availability of GCaMP transgenic animals, we explore the possibility of revealing gap junctional coupling using Ca2+ imaging in the Xenopus laevis tadpole motor system. Reliable axo-axonal electrical coupling was previously found in excitatory descending interneurons (dINs) using paired recordings but not with neurobiotin dye coupling. Here, we made whole-cell patch-clamp recordings with Ca2+-supplemented intracellular solution to load Ca2+ into GCaMP6s-expressing neurons, followed by Ca2+ imaging to detect potential Ca2+ diffusion across coupled neurons. Successful membrane breakthroughs led to transient fluorescence increases in the patched neuron. However, increasing the Ca2+ concentration promoted membrane resealing and rapid loss of whole-cell recordings. Regardless of recording duration, loading-triggered fluorescence only lasted up to three minutes, suggesting rapid Ca2+ clearance. Pharmacologically blocking sarcoplasmic /endoplasmic reticulum Ca2+-ATPases and plasma membrane Na+/Ca2+ exchangers did not prolong fluorescence, although sustained fluorescence was achieved with positive current injections. Counter to our expectations, fluorescence increases in Ca2+-loaded dINs did not spread to neighboring dINs. Robust intracellular Ca2+ regulation mechanisms, membrane resealing, and long dIN axons likely hindered intercellular Ca2+ diffusion. Therefore, this approach is not appropriate for revealing electrical coupling within this system.

Multiple sclerosis (MS) is an autoimmune and neurological disorder characterized by myelin disruption and neuronal degeneration. Currently approved therapies focus on symptom relief but do not promote central nervous system (CNS) repair. In contrast, astrocytes proliferate and repopulate MS-related lesions. Moreover, in active lesions, they hinder regenerative processes such as neural progenitor migration. Here, we propose astrocytes as a potential target for myelin repair in the human diseased brain. To achieve this aim, we investigated whether glial fibrillary acidic protein (GFAP)+ astrocytes can be transdifferentiated into oligodendrocyte lineage cells through forced overexpression of transcription factors both in vitro and ex vivo organotypic cultures of human adult cortex. Our results show that overexpression of OLIG2 and SOX10 in human induced pluripotent stem cell-derived astrocytes gives rise to oligodendrocyte progenitor cells 12 days post-induction, as shown by morphological changes and O4 marker expression. Importantly, transdifferentiation of GFAP-expressing endogenous astrocytes in human adult cortical tissue give rise to mature oligodendrocytes, as shown by expression of CC1, after only 12 days of overexpression of OLIG2 and SOX10. To our knowledge, this is the first study to assess direct astrocyte-to-oligodendrocyte reprogramming in a human platform preserving the native three-dimensional architecture of the brain. Further work will be required to determine whether the reprogrammed cells can myelinate axons and to evaluate the potential of this approach for structural and functional repair in the demyelinated human CNS.

Amacrine cells (ACs) are retinal interneurons that regulate synaptic transmission from bipolar cells to retinal ganglion cells (RGCs) and play essential roles in object motion detection, contrast sensitivity, and light adaptation. A subtype of GABAergic ACs identified using a tyrosine hydroxylase (TH) promoter-driven green fluorescent protein (GFP) mouse line has been termed TH2 amacrine cells (TH2-ACs). Although TH2-ACs contribute to the feature selectivity of object-motion signals in the adult retina, their functional properties during early postnatal development remain unclear. Using genetic mouse models, electrophysiology, immunohistochemistry, and calcium imaging, we show that TH2-ACs exhibit spontaneous rhythmic depolarizations during development. In the first postnatal week, these depolarizations were abolished by acetylcholine receptor antagonists, indicating that TH2-ACs are excited by starburst amacrine cells (SACs) via spontaneous cholinergic retinal waves. During the second postnatal week, rhythmic depolarizations persisted but were blocked by glutamate receptor antagonists, demonstrating that TH2-ACs are subsequently driven by bipolar cells through glutamatergic waves. Calcium imaging further revealed that this activity propagates across the TH2-AC network in a wave-like manner, potentially resulting in spatially and temporally patterned GABA release. Pharmacological blockades of GABAA receptors significantly enhanced glutamatergic wave activity in SACs and RGCs, indicating that GABAergic signaling from TH2-ACs participates in exerting inhibitory control over retinal waves. Together, these findings identify TH2-ACs as active participants in the development of retinal wave circuits and suggest that this participation via GABA signaling could contribute to activity-dependent refinement of retinal circuits underlying object motion processing. Key pointsO_LITH2 amacrine cells are excited by starburst amacrine cells through cholinergic retinal wave activity during the first postnatal week. C_LIO_LIDuring the second postnatal week, TH2 amacrine cells are driven by bipolar cells via glutamatergic retinal wave activity. C_LIO_LIThe dense dendritic arborization of TH2 amacrine cells enables their participation in the propagation of both cholinergic and glutamatergic waves. C_LIO_LIWave-like GABA release from TH2 amacrine cells contributes to the modulation of retinal wave activity through activation of GABAA receptors. C_LI

Local field potentials (LFPs) measured in the extracellular matrix of the brain are postulated to arise from the integration of synaptic ionic currents and spread by volume conduction. However, there is a lack of consensus on whether these spatiotemporal voltage gradients are just an epiphenomenon of spiking or if the LFPs play a functional role in information processing. To examine a potential functional role of LFPs in information processing, we developed a microfluidic device that allows neurons from the hippocampal formation to self-wire through microfluidic channels, effectively isolating the activity of single axons between subregions of the network. We recorded spontaneous theta-band activity (4-10 Hz) in these axons whose power spectra were independent of simultaneous spiking activity. A sparse set of axons from the CA3 into the CA1 had the highest theta amplitudes. Source neurons for the axonal theta were identified through cross correlation. Functionally, sparse axonal theta phase and amplitude correlated with target subregional spiking and more strongly with burst length. These results suggest that theta voltage oscillations in axons may contribute to activation of slow voltage-gated calcium channels to drive stronger synaptic release of transmitter to coordinate hippocampal activity between subregions. We propose that theta oscillations are controlled by specific ion channels distinct from those that generate spikes, a multiplex coding mechanism for inter-regional communication with implications for routing, executive control, disease states and artificial neural networks.

Oligodendrocyte progenitor cells (OPCs) have the capacity to self-renew, differentiate, and remyelinate the CNS. Aging is associated with a reduction in the functional capacity of OPCs even in the absence of an autoimmune insult. To determine how aging affects the response of oligodendroglia to a strong inflammatory insult comparable to an immune-mediated demyelinating event in multiple sclerosis (MS), we performed adoptive transfer of young myelin-reactive Th17 T cells into young and aged OPC lineage tracing mice. After adoptive transfer, OPCs were enriched within spinal cord lesions of both young and aged mice. However differentiated oligodendrocytes (OLs) were significantly reduced after adoptive transfer. Both young and aged OPCs differentiated into mature OLs during adoptive transfer. Transmission electron microscopy revealed thinly myelinated axons without degenerative features that likely represent remyelinated axons in lesions of both age groups. Young and aged OPCs rise to the challenge after a strong auto-immune attack, suggesting that compensatory strategies permit both young and aged oligodendroglia to survive despite an inflammatory environment. Identifying pathways that promote resilience of oligodendroglia in the face of an inflammatory challenge will facilitate the development of remyelinating therapies for people with MS.

Previously, electrophysiological differences between subpopulations of midbrain dopamine (DA) neurons were identified based on projection targets, including distinct responses to hyperpolarization and in the regularity of pacemaking. Here we explored single-compartment models of three subpopulations of DA neurons, projecting to medial shell of the nucleus accumbens (VTA-mNAcc), dorsomedial striatum (SNc-DMS) or dorsolateral striatum (SNc-DLS). We reduced the dimensionality to a phase plane consisting of membrane potential and one slow variable, either total slow potassium conductance or Kv4 channel inactivation. Nullclines are curves on which the rate of change of each variable is zero, given the value of the other variable. The voltage nullclines had three branches: upper spiking, unstable middle, and lower quiescent branch. Recruitment of Kv4 channels by the more prominent after-hyperpolarizing potential (AHP) in the DA-DMS and DA-DLS models channels stabilized pacemaking by creating a restorative moving fixed point along the quiescent branch. The slow inactivation of KV4 channels dominated and regularized the dynamics during the interspike interval; a dominant slow process may be a general mechanisn for stable regular pacemaking in a frequency range between 1-10 Hz. In contrast, the smaller AHP in VTA-mNAcc models prevented recruitment of this Kv4-mediated moving fixed point, which increased the sensitivity to synaptic inputs. On rebound from hyperpolarization the ability to produce robust ramps reverses between the DA neurons: now VTA-mNAcc projecting DA models fully recruited Kv4 channels and produced stable ramp-like pauses, whereas SNc-DLS projecting cells recruited significant regenerative inward CaV3 channels that overwhelmed Kv4 channels and produced rebound bursts. Author SummaryMidbraim dopamine (DA) neurons in the mammalian midbrain are linked to motivation, control of voluntary movement initiation, and reward-based learning. Their dysfunction is implicated in major disorders like Parkinsons disease, schizophrenia or substance use disorders. Firing patters like bursts or pauses in most DA subpopulations are thought to signal better or worse than expected outcomes. Here we use dynamic systems analysis to capture how functional diversity of DA neurons of their intrinsic properties results in differences of synaptic input integration leading to the generation of burst and pause patterns of electrical activity.

Glutamatergic neuronal synapses in the mouse neocortex mature during the first two months after birth. A key event during synaptic maturation is a change in short-term synaptic plasticity (STP), i.e. a switch from strong synaptic depression to a weaker depression or even facilitation. Glutamatergic pyramidal neurons located in the cortical layers II/III, layer V, and layer VI project axons through the corpus callosum where they release glutamate along their shafts and form glutamatergic synapses with oligodendrocyte precursor cells (OPCs). Here, we used single-cell electrophysiological recordings in brain slices to investigate synaptic plasticity at neuron-OPC synapses along axonal shafts in the white matter, and applied computation approaches to pinpoint the mechanisms of this plasticity. We found that during postnatal development of mice, there is a switch from short-term synaptic depression to short-term synaptic facilitation at glutamatergic neuron-OPC synapses in the corpus callosum. Synaptic delay of phasic neuron-OPC excitatory postsynaptic current shortens, and the amount of asynchronous release at neuron-OPC synapses decrease as animals mature, indicating that glutamate release becomes more synchronized. Our computational modelling suggests that both pre- and postsynaptic changes may contribute to the functional development and changes of plasticity at neuron-OPC synapses in the white matter. Taking together, our findings indicate that synaptic release machineries located at different sites along the same axon (i.e. axonal shaft in the white matter vs synaptic boutons in the grey matter) mature in a very similar fashion, STP occurs at both synaptic sites, and STP dynamics represent an important event during brain maturation.

The rat vestibular system plays a critical role in anti-gravity responses such as the tail-lift reflex and the air-righting reflex. In a previous study in male rats, we obtained evidence that these two reflexes depend on the function of non-identical populations of vestibular sensory hair cells (HC). Here, we caused graded lesions in the vestibular system of female rats by exposing the animals to several different doses of an ototoxic chemical, 3,3-iminodipropionitrile (IDPN). After exposure, we assessed the anti-gravity responses of the rats and then assessed the loss of type I HC (HCI) and type II HC (HCII) in the central and peripheral regions of the crista, utricle and saccule. As expected, we recorded a dose-dependent loss of vestibular function and loss of HCs. The relationship between hair cell loss and functional loss was examined using non-linear models fitted by orthogonal distance regression. The results indicated that both the tail-lift reflex and the air-righting reflexes mostly depend on HCI function. However, a different dependency was found on the epithelium triggering the reflex: while the tail-lift response is sensitive to loss of crista and/or utricle HCIs, the air-righting response rather depends on utricular and/or saccular integrity.

Hidden hearing loss (HHL) is an auditory neuropathy characterized by altered auditory nerve responses despite normal hearing thresholds. Recent experimental and computational studies suggest that permanent disruptions to heminode positions in spiral ganglion neuron (SGN) fibers can contribute to these deficits. However, the interaction between heminode disruption and noisy backgrounds ubiquitous in daily listening remains unexplored. This study investigates how background noise affects auditory processing with these peripheral disorders and how deficits propagate to downstream sound localization circuits in the superior olivary complex. We developed computational models of SGN fibers with mild and severe degrees of heminode disruption, subjected to sinusoidal tone stimuli in the presence of background noise with varying spectral characteristics. We analyzed the phase-locking of SGN fiber responses to the stimulus tone and modeled the subsequent effects on interaural time difference (ITD) sensitivity in the medial superior olive (MSO) using a binaural localization network. We found that near-tone-frequency noise disrupted SGN phase locking through cycle-to-cycle variability in spike phases, with effects consistent across tone frequencies. Mild heminode disruption produced frequency-dependent degradation in SGN phase locking, with effects observed only at higher frequencies tested (600-1000 Hz), without reducing overall firing rates. Critically, the effects of noise and heminode disruption were additive, with combined exposure leading to reduced ITD sensitivity and large temporal fluctuations in MSO responses. Severe heminode disruption, which additionally reduced firing rates at the SGN fibers and subsequent stages, produced profound localization deficits across all frequencies tested. Thus, our model results suggest that noisy environments exacerbate auditory deficits from peripheral disorders implicated in HHL and could potentially impair speech intelligibility through degradation in localization ability. This model may be useful for understanding the downstream impacts of SGN neuropathies.