bioRxiv Subject Collection: Neuroscience's Journal

Status epilepticus during neurodevelopment increases seizure susceptibility: a model study in zebrafish (Danio rerio)
Epilepsy is among the most common neurological diseases, affecting more than 50 million people worldwide. Unfortunately, one-third of people with epilepsy fail to respond to any current treatment and present a decreased quality of life. Nevertheless, different studies suggest that epilepsies acquired from an initial insult as status epilepticus (SE) followed by epileptogenesis can be prevented. Therefore, it is necessary to establish animal models of epileptogenesis induced by SE. Thus, here we proposed an animal model of epileptogenesis triggered by SE that occurred during the neurodevelopment of zebrafish (Danio rerio). Zebrafish larvae at the 7th day post-fertilization (dpf) were randomly assigned to two experimental groups. One group was exposed to embryo medium. The other group was exposed to pentylenetetrazol (15 mM PTZ) to induce SE. At the 8th dpf, each larva was subjected to the open tank test (OTT) to analyze locomotion and behavioral parameters. After the OTT, each initial group was divided into two groups: animals maintained in embryo medium and animals exposed to PTZ (3 mM), and the susceptibility to PTZ-induced seizure-like behavior was analyzed. Data showed that animals submitted to SE on the 7th dpf showed altered locomotion and behavior 24 hours later. Interestingly, zebrafish larvae submitted to SE on the 7th dpf showed decreased latency to reach seizure-like behavioral stages when exposed to a low concentration of PTZ on the 8th dpf. Concluding, our results show that SE during neurodevelopment increases the susceptibility of zebrafish larvae to present PTZ-induced seizure-like behavior. Our study contributes to the establishment of a model of epileptogenesis in developing zebrafish. Finally, the next step is to improve this model by characterizing molecular, biochemical and physiological markers.

Dueling Endogenous Viral-Like Sequences Control Synaptic Plasticity
The function of a large part of most genomes, generally called 'junk DNA', remains largely unknown. Much of this enigmatic DNA corresponds to transposons, which are considered genomic parasites. Here, we show the protein of the Ty1 retrotransposon Copia is enriched at the Drosophila neuromuscular junction and is transported across synapses. Unexpectedly, disrupting Copia expression results in increases in both synapse development and structural synaptic plasticity. Plasticity is kept in balance as Copia antagonizes the Drosophila Arc (activity-regulated cytoskeleton-associated protein) homolog, which is a transposon-derived gene. Our cryo-EM structure of the Copia capsid shows a shell with large cargo capacity and leads to a hypothesis for mutual antagonism of Arc and Copia capsid assembly. Our findings provide evidence that a fully functional transposon plays a role at synapses, suggesting that transposons and other types of 'junk DNA' are essential to developmental and cellular processes.

Mind the gap: A systematic review and meta-analysisof how social memory is studied
Social recognition is crucial for survival in social species, and necessary for group living, selective reproduction, pair bonding, and dominance hierarchies. Mice and rats are the most commonly used animal models in social memory research, however current paradigms do not account for the complex social dynamics they exhibit in the wild. To assess the range of social memories being studied, we conducted a systematic analysis of neuroscience articles testing the social memory of mice and rats published within the past two decades and analyzed their methods. Our results show that despite these rodent's rich social memory capabilities, the majority of social recognition papers explore short-term memories and short-term familiarity levels with minimal exposure between subject and familiar stimuli - a narrow type of social memory. We have identified several key areas currently understudied or underrepresented: kin relationships, mates, social ranks, sex variabilities, and the effects of aging. Additionally, reporting on social stimulus variables such as housing history, strain, and age, is limited, which may impede reproducibility. Overall, our data highlight large gaps in the diversity of social memories studied and the effects social variables have on social memory mechanisms.

Tactile Adaptation to Orientation Produces a Robust Tilt Aftereffect and Exhibits Crossmodal Transfer When Tested in Vision
Orientation processing is one of the most fundamental functions in both visual and somatosensory perception. Converging findings suggest that orientation processing in both modalities is closely linked: somatosensory neurons share a similar orientation organisation as visual neurons, and the visual cortex has been found to be heavily involved in tactile orientation perception. The tilt aftereffect (TAE) is a demonstration of orientation adaptation and is used widely in behavioural experiments to investigate orientation mechanisms in vision. By testing the classic TAE paradigm in both tactile and crossmodal orientation tasks between vision and touch we were able to show that tactile perception of orientation shows a very robust TAE, similar to its visual counterpart. We further show that orientation adaptation in touch transfers to produce a TAE when tested in vision, but not vice versa. We also observed a similar asymmetricity in the intramodal serial dependence effect within the test sequence. These findings provide concrete evidence that vision and touch engage a similar orientation processing mechanism, but the asymmetry in the crossmodal transfer of TAE and serial dependence provides more insights into the underlying mechanism of this link.

Mapping the bioimaging marker of Alzheimer's disease based on pupillary light response-driven brain-wide fMRI in awake mice
Pupil dynamics has emerged as a critical non-invasive indicator of brain state changes. In particular, pupillary-light-responses (PLR) in Alzheimer's disease (AD) patients may be used as biomarkers of brain degeneration. To characterize AD-specific PLR and its underlying neuromodulatory sources, we combined high-resolution awake mouse fMRI with real-time pupillometry to map brain-wide event-related correlation patterns based on illumination-driven pupil constriction (Pc) and post-illumination pupil dilation recovery (amplitude, Pd, and time, T). The Pc-driven differential analysis revealed altered visual signal processing coupled with reduced thalamocortical activation in AD mice compared with the wild-type normal mice. In contrast, the post-illumination pupil dilation recovery-based fMRI highlighted multiple brain areas related to AD brain degeneration, including the cingulate cortex, hippocampus, septal area of the basal forebrain, medial raphe nucleus, and pontine reticular nuclei (PRN). Also, brain-wide functional connectivity analysis highlighted the most significant changes in PRN of AD mice, which serves as the major subcortical relay nuclei underlying oculomotor function. This work combined non-invasive pupil-fMRI measurements in preclinical models to identify pupillary biomarkers based on neuromodulatory dysfunction coupled with AD brain degeneration.

Beyond conventional wisdom: unveiling quantitative insights in fluorescence lifetime imaging via realistic simulation of biological systems
Fluorescence lifetime imaging microscopy (FLIM) and photometry (FLiP) are illuminating the dynamics of biological signals. Because fluorescence lifetime is an intensive property of a fluorophore that is insensitive to sensor expression levels, it excels over fluorescence intensity measurements by allowing comparison across animals, over chronic time periods, and quantitation of the absolute levels of biological signals. However, the insensitivity of lifetime to sensor expression level does not always hold true in biological experiments where autofluorescence, ambient light, dark currents and afterpulses of the detectors are present. To quantitatively evaluate the potential and limitations of fluorescence lifetime measurements, we introduce FLiSimBA, a flexible platform enabling realistic Fluorescence Lifetime Simulation for Biological Applications. FLiSimBA accurately recapitulates experimental data and provides quantitative analyses. Using FLiSimBA, we determine the photons required for minimum detectable differences in lifetime and quantify the impact of hardware innovation. Furthermore, we challenge the conventional view that fluorescence lifetime is insensitive to sensor expression levels and define the conditions in which sensor express levels do not result in statistically significant difference in biological experiments. Thus, we introduce an adaptable simulation tool that allows systematic exploration of parameters to define experimental advantages and limitations in biological applications. Moreover, we provide a statistical framework and quantitative insights into the impact of key experimental parameters on signal-to-noise ratio and fluorescence lifetime responses. Our tool and results will enable the growing community of FLIM users and developers to optimize FLIM experiments, expose limitations, and identify opportunities for future innovation of fluorescence lifetime technologies.

Thalamo-Cortical Interaction for Incremental Binding in Mental Contour-Tracing
Visual object-based attention marks a key process of mammalian perception. By which mechanisms this process is implemented and how it can be interacted with by means of attentional control is not completely understood yet. Incremental binding is a mechanism required in more demanding scenarios of object-based attention and is likewise experimentally investigated quite well. Attention spreads across a representation of the visual object and labels bound elements by constant up-modulation of neural activity. The speed of incremental binding was found to be dependent on the spatial arrangement of distracting elements in the scene and to be scale invariant giving rise to the growth-cone hypothesis. In this work, we propose a neural dynamical model of incremental binding that provides a mechanistic account for these findings. Through simulations, we investigate the model properties and demonstrate how an attentional spreading mechanism tags neurons that participate in the object binding process. They utilize Gestalt properties and eventually show growth-cone characteristics labeling perceptual items by delayed activity enhancement of neuronal firing rates. We discuss the algorithmic process underlying incremental binding and relate it to the model's computation. This theoretical investigation encompasses complexity considerations and finds the model to be not only of explanatory value in terms of neurohpysiological evidence, but also to be an efficient implementation of incremental binding striving to establish a normative account. By relating the connectivity motifs of the model to neuroanatomical evidence, we suggest thalamo-cortical interactions to be a likely candidate for the flexible and efficient realization suggested by the model. There, pyramidal cells are proposed to serve as the processors of incremental grouping information. Local bottom-up evidence about stimulus features is integrated via basal dendritic sites. It is combined with an apical signal consisting of contextual grouping information which is gated by attentional task-relevance selection mediated via higher-order thalamic representations.

General Anesthesia Activates a Central Anxiolytic Center in the BNST
Low doses of general anesthetics like ketamine and dexmedetomidine have anxiolytic properties independent of their sedative effects. How these different drugs exert these anxiolytic effects is not well understood. We discovered a population of GABAergic neurons in the oval division of the bed nucleus of the stria terminalis that is activated by multiple anesthetics and the anxiolytic drug diazepam (ovBNSTGA). A majority of ovBNSTGA neurons express neurotensin receptor 1 (Ntsr1) and innervate brain regions known to regulate anxiety and stress responses. Optogenetic activation ovBNSTGA or ovBNSTNtsr1 neurons significantly attenuated anxiety-like behaviors in both naive animals and mice with inflammatory pain, while inhibition of these cells increased anxiety. Notably, activation of these neurons decreased heart rate and increased heart rate variability, suggesting that they reduce anxiety through modulation of the autonomic nervous system. Our study identifies ovBNSTGA/ovBNSTNtsr1 neurons as one of the brain's endogenous anxiolytic centers and a potential therapeutic target for treating anxiety-related disorders.

Thalamic nuclei in primary trigeminal neuralgia: gray matter volume patterns before and after surgery
Trigeminal neuralgia is a prevalent chronic pain disorder characterized by recurring episodes of intense facial pain, which significantly impairs patients' quality of life. MRI-based biomarkers have consistently demonstrated their ability to predict pain intensity and treatment outcomes. However, most studies have primarily focused on the trigeminal system, disregarding the extensive neural reorganization that occurs throughout the brain in response to chronic pain. In this study, we aimed to examine the thalamus, a key brain structure involved in information processing, and provide a detailed perspective on thalamic remodeling in response to chronic pain at the level of individual thalamic nuclei. We analyzed a sample of 61 patients with primary trigeminal neuralgia undergoing surgical treatment, along with 28 healthy participants. Our results revealed significant gray matter volume changes in thalamic nuclei among patients with trigeminal neuralgia. Notably, the intralaminar nuclei (centromedian/parafascicular) and nuclei associated with visual and auditory signal processing (lateral and medial geniculate bodies) exhibited significant alterations, contrasting with the ventral group nuclei involved in nociceptive processing. Additionally, we found no substantial volume increase in any of the studied nuclei following successful surgical intervention. The volumes of thalamic nuclei were negatively correlated with pain intensity and disease duration, while positively correlated with working memory scores. The findings obtained in this study, albeit preliminary, have promising clinical implications as they unveil previously unknown facets of chronic pain development and provide valuable guidance for clinical decision-making.

Association between electrophysiological phenotypes and Kv2.1 potassium channel expression explained by geometrical analysis
Excitable cells exhibit different electrophysiological profiles while responding to current stimulation in current-clamp experiments. In theory, the differences could be explained by changes in the expression of proteins mediating transmembrane ion transport. Experimental verification by performing systematic, controlled variations in the expression of proteins of the same type (e.g. voltage-dependent, noninactivating Kv2.1 channels) is difficult to achieve in the absence of other changes. However, biophysical models enable this possibility and allow us to assess and characterise the electrophysiological phenotypes associated to different levels of expression of non-inactivating voltage-dependent K-channels of type Kv2.1. To do so, we use a 2-dimensional biophysical model of neuronal membrane potential and study the phase plane geometry and bifurcation structures associated with different levels of Kv2.1 expression with the input current as bifurcation parameter. We find that increasing the expression of Kv2.1 channels reduces the size of the region of the phase plane from which action potentials can be initiated. The changes in expression can also be related to different transitions between rest and repetitive firing in current clamp experiments. For instance, increasing the number of Kv2.1 channels shifts the rheobase current to higher levels, but also expands the dynamic range in which excitatory external current produces repetitive spiking. Our analysis shows that changes in the responses to increasing input currents can be associated to different sequences of fixed point bifurcations. In general, the fixed points are attracting, then repulsive, and later become attracting again as the input current increases, but the bifurcation sequences also include changes in fixed point type, and change qualitatively with the expression of Kv2.1 channels. In the non-repetitive spiking regime with low current stimulation, low expression of Kv2.1 channels yields bifurcation sequences that include transitions between 3 and 1 fixed points, and repetitive firing starts with delays that decrease with increasing current (aggregation). For higher expression of Kv2.1 channels there is only one fixed point that changes in type and attractivity, convergence to rest tends to be oscillatory (resonance), and repetitive spiking starts without noticeable delays. Our models explain how the same neuron is theoretically be capable of including both aggregating and resonant modes of integration for synaptic input, as shown in current clamp experiments.

Effects of oscillation phase on discrimination performance in a visual tilt illusion
Neural oscillations reflect fluctuations in the relative excitation/inhibition of neural systems and are theorised to play a critical role in several canonical neural computations and cognitive processes. These theories have been supported by findings that detection of visual stimuli fluctuates with the phase of oscillations at the time of stimulus onset. However, null results have emerged in studies seeking to demonstrate these effects in visual discrimination tasks, raising questions about the generalisability of these phenomena to wider neural processes. Recently, we suggested that methodological limitations may mask effects of oscillation phase in higher-level sensory processing. Thus, to test the generality of phasic influences requires a task that requires stimulus discrimination but depends on early sensory processing. Here, we examined the influence of oscillation phase in the visual tilt illusion, in which an oriented centre grating is perceived titled away from the orientation of a surround grating. This illusion is produced by lateral inhibitory interactions in early visual processing. We presented centre gratings at participants' titrated subjective vertical angle and had participants report whether the grating appeared tilted leftward or rightward of vertical on each trial while measuring their brain activity with EEG. We observed a robust fluctuation in orientation perception across different phases of posterior alpha and theta oscillations, consistent with fluctuating illusion magnitude across the oscillatory cycle. These results confirm that oscillation phase affects complex processing involved in stimulus discrimination, consistent with their purported role in canonical computations that underpin cognition.

Olfactory ensheathing cells are hybrid glial cells that promote neural repair
Olfactory ensheathing cells (OECs) are unique glial cells found in both the central and peripheral nervous systems where they support the continuous axonal outgrowth of immature olfactory sensory neurons to their targets. Here we show that following severe spinal cord injury, olfactory bulb-derived OECs transplanted near the injury site modify the normally inhibitory glial scar and facilitate axon regeneration past the scar border and into the lesion center. To understand the mechanisms underlying the reparative properties of such transplanted OECs, we used single-cell RNA-sequencing to study their gene expression programs. Our analyses revealed five diverse subtypes of OECs, each expressing novel marker genes and pathways indicative of progenitor, axonal regeneration and repair, secreted molecules, or microglia-like functions. As expected, we found substantial overlap of OEC genes with those of Schwann cells, but also with astrocytes, oligodendrocytes and microglia. We confirmed established markers on cultured OECs, and then localized select top genes of OEC subtypes in rat olfactory bulb tissue. In addition, we present evidence that OECs secrete both Reelin and Connective tissue growth factor, extracellular matrix molecules which are important for neural repair and axonal outgrowth. Our results support that adult OECs are a unique hybrid glia, some with progenitor characteristics, and that their gene expression patterns indicate diverse functions related to wound healing, injury repair and axonal regeneration.

Somatostatin interneurons control the timing of developmental desynchronization in cortical networks
Synchronous neuronal activity is a hallmark of the early developing brain. In the mouse cerebral cortex, activity decorrelates during the second week of postnatal development, progressively acquiring the characteristic pattern of sparse coding underlying the integration of multidimensional sensory information. The maturation of inhibition seems critical for this process, but the specific types of interneurons involved in this crucial transition of network activity in the developing cortex remain unknown. Using in vivo volumetric and longitudinal two-photon calcium imaging during the period that precedes the change from highly synchronous to decorrelated activity, we identify somatostatin-expressing (SST+) interneurons as critical modulators of this switch. Modulation of the activity of SST+ cells accelerates or delays the decorrelation of cortical network activity, a process that involves regulating the degree of maturation of parvalbumin-expressing (PV+) interneurons. SST+ cells critically link sensory inputs with local circuits controlling the neural dynamics in the developing cortex while modulating the integration of other interneurons into nascent cortical circuits.

STAT3 regulates the generation of astroglia inhuman brain organoids with high mTORC1 activity
During brain development, neural progenitor cells first produce neurons, then astrocytes and other glial cell types, which provide important trophic support and shape neuronal development and function. Intrinsic genetic programs interact with extracellular signals to control progenitor fate, resulting in temporally segregated periods of neurogenesis and gliogenesis. Animal models have implicated STAT3 as an important driver of astrogenesis; however, the signaling pathways that control glial differentiation during human brain development are less well understood. Prior work demonstrated that constitutive activation of mTORC1 signaling in human brain organoid models resulted in the precocious generation of glial-lineage cells. In this study, we tested whether mTORC1 acts via STAT3 to control astrogenesis in brain organoids. We show that knockdown of STAT3 reduces astrogenesis in wild-type organoids and in organoids with constitutively high mTORC1 signaling caused by deletion of the negative regulator TSC2. However, mTORC1 is not required for cytokine-induced activation of STAT3 and expression of the astrocytic protein GFAP. Together, these results show that mTORC1 acts through STAT3 to control astroglia production in human brain organoid models, but that mTOR signaling is dispensable for STAT3-driven astrogenesis.

Reward Expectation Reduces Representational Drift in the Hippocampus
Spatial memory in the hippocampus involves dynamic neural patterns that change over days, termed representational drift. While drift may aid memory updating, excessive drift could impede retrieval. Memory retrieval is influenced by reward expectation during encoding, so we hypothesized that diminished reward expectation would exacerbate representational drift. We found that high reward expectation limited drift, with CA1 representations on one day gradually re-emerging over successive trials the following day. Conversely, the absence of reward expectation resulted in increased drift, as the gradual re-emergence of the previous day's representation did not occur. At the single cell level, lowering reward expectation caused an immediate increase in the proportion of place-fields with low trial-to-trial reliability. These place fields were less likely to be reinstated the following day, underlying increased drift in this condition. In conclusion, heightened reward expectation improves memory encoding and retrieval by maintaining reliable place fields that are gradually reinstated across days, thereby minimizing representational drift.

The multiscale topological organization of the functional brain network in adolescent PTSD
The experience of an extremely aversive event can produce enduring deleterious behavioral and neural consequences, among which posttraumatic stress disorder (PTSD) is a representative example. In this work, we aim to study the whole-cortex functional organization of adolescents with PTSD without the a priori selection of specific regions of interest or functional networks. To do so, we built on the network neuroscience framework and specifically on multisubject community analysis to study the functional connectivity of the brain. We show, across different topological scales (the number of communities composing the cortex), an increased coupling between regions belonging to unimodal (sensory) regions and a reduced coupling between transmodal (association) regions in the adolescent PTSD group. These results open up an intriguing possibility concerning an altered large-scale cortical organization in adolescent PTSD.

Comprehensive Assessment of Ischemic Stroke in Nonhuman Primates: Neuroimaging, Behavioral, and Serum Proteomic Analysis
Ischemic strokes, prevalence and impactful, underscore the necessity of advanced research models closely resembling human physiology. O integrating n ur study in nonhuman primates (NHPs) offers a comprehensive exploration of ischemic stroke, integrating neuroimaging data, behavioral outcomes, and serum proteomics to elucidate the complex interplay of factors involved in stroke pathophysiology. We observed a consistent pattern in infarct volume, peaking at 1-month post-middle cerebral artery occlusion (MCAO) and stabilizing thereafter. This trend was closely correlated with notable changes in motor function and working memory performance. Using diffusion tensor imaging (DTI), we detected significant alterations in fractional anisotropy (FA) and mean diffusivity (MD) values, indicative of microstructural changes in the brain. These findings were strongly correlated with the observed neurological and cognitive deficits, highlighting the sensitivity of DTI metrics in stroke assessment. Behaviorally, the monkeys exhibited a reliance on their unaffected limb for compensatory movements, a response commonly observed in stroke impairment. This adaptation, alongside the consistent findings in DTI metrics, suggests a substantial impact of stroke on motor function and spatial perception. Proteomic analysis through MS/MS functional enrichment revealed two distinct groups of proteins with significant changes post-MCAO. Notably, MMP9, THBS1, MB, PFN1, and YWHAZ emerged as potential biomarkers and therapeutic targets in ischemic stroke. Our findings underscore the complex nature of stroke and the potential of an integrated approach, combining neuroimaging, behavioral studies, and proteomics, for advancing our understanding and treatment of this condition.

Neurotrophin NT-4/5 Promotes Structural Changes in Neurons of the Developing Visual Cortex
Current hypotheses on the mechanisms underlying the development and plasticity of the ocular dominance system through competitive interactions between pathways serving the two eyes strongly suggest the involvement of neurotrophins and their high affinity receptors. In the cat, infusion of the tyrosine kinase B ligand (trkB), neurotrophin-4/5 (NT-4/5), abolishes ocular dominance plasticity that follows monocular deprivation (Gillespie et al., 2000), while tyrosine kinase A and C ligands (trkA and trkC) do not have this effect. One interpretation of this finding is that NT-4/5 causes overgrowth and sprouting of thalamocortical and/or corticocortical terminals, leading to promiscuous neuronal connections which override the experience-dependent fine tuning of connections based on correlated activity. The present study tested whether neurons in cortical regions infused with NT-4/5 showed anatomical changes compatible with this hypothesis. Cats at the peak of the critical period received chronic infusion NT-4/5 into visual cortical areas 17/18 via an osmotic minipump. Visual cortical neurons were labeled in fixed slices using the DiOlistics methods (Gan et al., 2000) and analyzed in confocal microscopy. Infusion of NT-4/5 induced a significant increase of spine-like processes on primary dendrites and a distinctive sprouting of protuberances from neuronal somata in all layers. The increase of neuronal membrane was paralleled by an increase in density of the presynaptic marker synaptophysin in infused areas, suggesting an increase in the numbers of synapses. A contingent of these newly formed synapses may feed into inhibitory circuits, as suggested by an increase of GAD-65 immunostaining in NT-4/5 affected areas. These anatomical changes are consistent with the physiological changes in such animals, suggesting that excess trkB neurotrophin can stimulate the formation of promiscuous connections during the critical period.

Daily dynamics of resting-state EEG theta and gamma fluctuations are associated with cognitive performance in healthy aging
Healthy age-related cognitive changes are highly heterogeneous across individuals. This variability is increasingly explained through the lens of spontaneous fluctuations of brain activity, now considered as powerful index of age-related changes. However, brain activity is a biological process modulated by circadian rhythms, and how these fluctuations evolve throughout the day is under investigated. Assessing the daily dynamics of brain fluctuations involves the use of techniques measuring the temporal dynamics of synchronized communications between brain regions, such as electroencephalography. We found that theta and gamma daily fluctuations in the salience-control executive inter-network (SN-CEN) are associated with distinct mechanisms underlying cognitive heterogeneity in aging. Higher levels of SN-CEN theta daily fluctuations appear to be deleterious for memory performance and were associated with higher tau/neuroinflammation rates. In contrast, higher levels of gamma daily fluctuations are positively associated with executive performance, and were associated with lower rate of B-amyloid deposition. Thus, accounting for daily EEG fluctuations of brain activity contributes to better understand subtle brain changes underlying individuals cognitive performance in healthy aging.