bioRxiv Subject Collection: Neuroscience's Journal

Neural Sequences Underlying Directed Turning in C. elegans
Complex behaviors like navigation rely on sequenced motor outputs that combine to generate effective movement. The brain-wide organization of the circuits that integrate sensory signals to select and execute appropriate motor sequences is not well understood. Here, we characterize the architecture of neural circuits that control C. elegans olfactory navigation. We identify error-correcting turns during navigation and use whole-brain calcium imaging and cell-specific perturbations to determine their neural underpinnings. These turns occur as motor sequences accompanied by neural sequences, in which defined neurons activate in a stereotyped order during each turn. Distinct neurons in this sequence respond to sensory cues, anticipate upcoming turn directions, and drive movement, linking key features of this sensorimotor behavior across time. The neuromodulator tyramine coordinates these sequential brain dynamics. Our results illustrate how neuromodulation can act on a defined neural architecture to generate sequential patterns of activity that link sensory cues to motor actions.

AmygdalaGo-BOLT3D: A boundary learning transformer for tracing human amygdala
Automated amygdala segmentation is one of the most common tasks in human neuroscience research. However, due to the small volume of the human amygdala, especially in developing brains, the precision and consistency of the segmentation results are often affected by individual differences and inconsistencies in data distribution. To address these challenges, we propose an algorithm for learning boundary contrast of 427 manually traced amygdalae in children and adolescents to generate a transformer, AmygdalaGo-BOLT3D, for automatic segmentation of human amygdala. This method focuses on the boundary to effectively address the issue of false positive recognition and inaccurate edges due to small amygdala volume. Firstly, AmygdalaGo-BOLT3D develops a basic architecture for an adaptive cooperation network with multiple granularities. Secondly, AmygdalaGo-BOLT3D builds the self-attention-based consistency module to address generalizability problems arising from individual differences and inconsistent data distributions. Third, AmygdalaGo-BOLT3D adapts the original sample-mask model for the amygdala scene, which consists of three parts, namely a lightweight volumetric feature encoder, a 3D cue encoder, and a volume mask decoder, to improve the generalized segmentation of the model. Finally, AmygdalaGo-BOLT3D implements a boundary contrastive learning framework that utilizes the interaction mechanism between a prior cue and the embedded magnetic resonance images to achieve effective integration between the two. Experimental results demonstrate that predictions of the overall structure and boundaries of the human amygdala exhibit highly improved precision and help maintain stability in multiple age groups and imaging centers. This verifies the stability and generalization of the algorithm designed for multiple tasks. AmygdalaGo-BOLT3D has been deployed for the community (GITHUB_LINK) to provide an open science foundation for its applications in population neuroscience.

Relative phase of distributed oscillatory dynamics implements a working memory in a simple brain
We report the existence of a working memory system in the nematode C. elegans that is employed for deferred action in a sensory-guided decision-making process. We find that the turn direction of discrete reorientations during navigation is under sensory-guided control and relies on a working memory that can persist over an intervening behavioral sequence. This memory system is implemented by the phasic interaction of two distributed oscillatory dynamical components. The interaction of oscillatory neural ensembles may be a conserved primitive of cognition across the animal kingdom.

YTHDF1 mediates translational control by m6A mRNA methylation in adaptation to environmental challenges
Animals adapt to environmental challenges with long-term changes at the behavioral, circuit, cellular, and synaptic levels which often require new protein synthesis. The discovery of reversible N6-methyladenosine (m6A) modifications of mRNA has revealed an important layer of post-transcriptional regulation which affects almost every phase of mRNA metabolism and therefore translational control. Many in vitro and in vivo studies have demonstrated the significant role of m6A in cell differentiation and survival, but its role in adult neurons is understudied. We used cell-type specific gene deletion of Mettl14, which encodes one of the subunits of the m6A methyltransferase, and Ythdf1, which encodes one of the cytoplasmic m6A reader proteins, in dopamine D1 receptor expressing or D2 receptor expressing neurons. Mettl14 or Ythdf1 deficiency blunted responses to environmental challenges at the behavioral, cellular, and molecular levels. In three different behavioral paradigms, gene deletion of either Mettl14 or Ythdf1 in D1 neurons impaired D1-dependent learning, whereas gene deletion of either Mettl14 or Ythdf1 in D2 neurons impaired D2-dependent learning. At the cellular level, modulation of D1 and D2 neuron firing in response to changes in environments was blunted in all three behavioral paradigms in mutant mice. Ythdf1 deletion resembled impairment caused by Mettl14 deletion in a cell type-specific manner, suggesting YTHDF1 is the main mediator of the functional consequences of m6A mRNA methylation in the striatum. At the molecular level, while striatal neurons in control mice responded to elevated cAMP by increasing de novo protein synthesis, striatal neurons in Ythdf1 knockout mice didnt. Finally, boosting dopamine release by cocaine drastically increased YTHDF1 binding to many mRNA targets in the striatum, especially those that encode structural proteins, suggesting the initiation of long-term neuronal and/or synaptic structural changes. While the m6A-YTHDF1 pathway has similar functional significance at cellular level, its cell type specific deficiency in D1 and D2 neurons often resulted in contrasting behavioral phenotypes, allowing us to cleanly dissociate the opposing yet cooperative roles of D1 and D2 neurons.

Functional Electrical Stimulation and Brain-Machine Interfaces for Simultaneous Control of Wrist and Finger Flexion
Brain-machine interface (BMI) controlled functional electrical stimulation (FES) is a promising treatment to restore hand movements to people with cervical spinal cord injury. Recent intracortical BMIs have shown unprecedented successes in decoding user intentions, however the hand movements restored by FES have largely been limited to predetermined grasps. Restoring dexterous hand movements will require continuous control of many biomechanically linked degrees-of-freedom in the hand, such as wrist and finger flexion, that would form the basis of those movements. Here we investigate the ability to restore simultaneous wrist and finger flexion, which would enable grasping with a controlled hand posture and assist in manipulating objects once grasped. We demonstrate that intramuscular FES can enable monkeys with temporarily paralyzed hands to move their fingers and wrist across a functional range of motion, spanning an average 88.6 degrees at the metacarpophalangeal joint flexion and 71.3 degrees of wrist flexion, and intramuscular FES can control both joints simultaneously in a real-time task. Additionally, we demonstrate a monkey using an intracortical BMI to control the wrist and finger flexion in a virtual hand, both before and after the hand is temporarily paralyzed, even achieving success rates and acquisition times equivalent to able-bodied control with BMI control after temporary paralysis in two sessions. Together, this outlines a method using an artificial brain-to-body interface that could restore continuous wrist and finger movements after spinal cord injury.

Efficient modular system identification provides a high-resolution assay of temporal processing and reveals the multilevel effects of attention along the human auditory pathway
Human studies of auditory temporal processing and the effects therein of aging, hearing loss, musicianship, and other auditory processing disorders have conventionally employed brainstem evoked potentials (e.g., FFRs/EFRs targeting specific modulation frequencies). Studies of temporal processing in forebrain structures are fewer and are often restricted to the 40 Hz steady-state response. One factor contributing to the limited investigation is the lack of a fast and reliable method to characterize temporal processing non-invasively in humans over a wide range of modulation frequencies. Here, we use a system-identification approach where white noise, modulated using an extended maximum-length sequence (em-seq), is employed to target stimulus energy toward a modulation-frequency range of interest and efficiently obtain a robust auditory modulation-temporal response function or mod-TRF. The mod-TRF can capture activity from sources in the early processing pathway (5-7 ms latency), middle-latency region (MLR), and late latency region (LLR). The mod-TRF is a high-resolution, modular assay of the temporal modulation transfer function (tMTF) in that the distinct neural components contributing to the tMTF can be separated on the basis of their latency, modulation frequency band, and scalp topography. This decomposition provides the insight that the seemingly random individual variation in the shape of the tMTF can be understood as arising from individual differences in the weighting and latency of similar underlying neural sources in the composite scalp response. We measured the mod-TRF under different states of attention and found a reduction in latency or enhancement of amplitude of the response from specific sources. Surprisingly, we found that attention effects can extend to the earliest parts of the processing pathway (5ms) in highly demanding tasks. Taken together, the mod-TRF is a promising tool for dissecting auditory temporal processing and obtain further insight into a variety of phenomenon such as aging, hearing loss, and neural pathology.

Subsets of extraocular motoneurons produce kinematically distinct saccades during hunting and exploration.
Animals construct diverse behavioural repertoires by moving a limited number of body parts with varied kinematics and patterns of coordination. There is evidence that distinct movements can be generated by changes in activity dynamics within a common pool of motoneurons, or by selectively engaging specific subsets of motoneurons in a task-dependent manner. However, in most cases we have an incomplete understanding of the patterns of motoneuron activity that generate distinct actions and how upstream premotor circuits select and assemble such motor programmes. In this study, we used two closely related but kinematically distinct types of saccadic eye movement in larval zebrafish as a model to examine circuit control of movement diversity. In contrast to the prevailing view of a final common pathway, we found that in oculomotor nucleus, distinct subsets of motoneurons were engaged for each saccade type. This type-specific recruitment was topographically organised and aligned with ultrastructural differ-ences in motoneuron morphology and afferent synaptic innervation. Medially located motoneu-rons were active for both saccade types and circuit tracing revealed a type-agnostic premotor pathway that appears to control their recruitment. By contrast, a laterally located subset of motoneurons was specifically active for hunting-associated saccades and received premotor in-put from pretectal hunting command neurons. Our data support a model in which generalist and action-specific premotor pathways engage distinct subsets of motoneurons to elicit varied movements of the same body part that subserve distinct behavioural functions.

Local Synthesis of Reticulon-1C Lessens the Outgrowth of Injured Axons by Controlling Spastin Activity
The regenerative potential of developing cortical axons following injury depends on intrinsic mechanisms, such as axon-autonomous protein synthesis, that are still not fully understood. An emerging factor in this regenerative process is the bi-directional interplay between microtubule dynamics and structural proteins of the axonal endoplasmic reticulum. Therefore, we hypothesize that locally synthesized structural proteins of the endoplasmic reticulum may regulate microtubule dynamics and the outgrowth of injured cortical axons. This hypothesis is supported by RNA data-mining, which identified Reticulon-1 as the sole ER-shaping protein consistently present in axonal transcriptomes and found it to be downregulated following cortical axon injury. Using compartmentalized microfluidic chambers, we demonstrate that local knockdown of Reticulon-1 mRNA enhances outgrowth while reducing the distal tubulin levels of injured cortical axons. Additionally, live cell imaging shows injury-induced reductions in microtubule growth rate and length, which are fully restored by axonal Reticulon-1 knockdown. Interestingly, axonal inhibition of the microtubule-severing protein Spastin fully prevents the effects of local Reticulon-1 knockdown on outgrowth and tubulin levels, while not affecting microtubule dynamics. Furthermore, we provide evidence supporting that the Reticulon-1C isoform is locally synthesized in injured axons and associates with Spastin to inhibit its severing activity. Our findings reveal a novel injury-dependent mechanism in which a locally synthesized ER-shaping protein lessens microtubule dynamics and the outgrowth of cortical axons.

Serpina1e mediates the exercise-induced enhancement of hippocampal memory
The exercise induced enhancement of learning and memory is thought to be regulated by interactions between body and brain via secretory proteins in the blood plasma. Given the prominent role that skeletal muscle plays during exercise, the beneficial effects of exercise on cognitive functions appear to be mediated by muscle derived secretory factors including myokines. However, the specific myokines that exert beneficial effects on cognitive functions remain to be elucidated. Here, we reveal that a novel myokine, Serpina1e, acts a molecular mediator that directly supports long term memory formation in the hippocampus. Using an in vivo myokine labeling mouse model, proteomic analysis revealed that the Serpina1 family of proteins are the myokines whose levels increased the most in plasma after chronic aerobic exercise for 4 weeks. Systemic delivery of recombinant Serpina1e into sedentary mice was sufficient for reproducing the beneficial effect of exercise on hippocampus associated cognitive functions. Moreover, plasma Serpina1e can cross the blood cerebral spinal fluid (CSF) barrier and blood brain barrier to reach the brain, thereby influencing hippocampal function. Indeed, an increase in the plasma level of Serpina1e promoted hippocampal neurogenesis, increased the levels of brain-derived neurotrophic factor (BDNF) and induced neurite growth. Our findings reveal that Serpina1e is a myokine that migrates to the brain and mediates exercise induced memory enhancement by triggering neurotrophic growth signaling in the hippocampus. This discovery elucidates the molecular mechanisms underlying the beneficial effects of exercise on cognitive function and may have implications for the development of novel therapeutic interventions for alleviating cognitive disorders.

Brain-like border ownership signals support prediction of natural videos
To make sense of visual scenes, the brain must segment foreground from background. This is thought to be facilitated by neurons in the primate visual system that encode border ownership (BOS), i.e. whether a local border is part of an object on one or the other side of the border. It is unclear how these signals emerge in neural networks without a teaching signal of what is foreground and background. In this study, we investigated whether BOS signals exist in PredNet, a self-supervised artificial neural network trained to predict the next image frame of natural video sequences. We found that a significant number of units in PredNet are selective for BOS. Moreover these units share several other properties with the BOS neurons in the brain, including robustness to scene variations that constitute common object transformations in natural videos, and hysteresis of BOS signals. Finally, we performed ablation experiments and found that BOS units contribute more to prediction than non-BOS units for videos with moving objects. Our findings indicate that BOS units are especially useful to predict future input in natural videos, even when networks are not required to segment foreground from background. This suggests that BOS neurons in the brain might be the result of evolutionary or developmental pressure to predict future input in natural, complex dynamic visual environments.

Perceived multisensory common cause relations shape the ventriloquism effect but only marginally the trial-wise aftereffect
Combining multisensory cues is fundamental for perception and action, and reflected by two frequently-studied phenomena: multisensory integration and sensory recalibration. In the context of audio-visual spatial signals, these are exemplified by the ventriloquism effect and its aftereffect. The ventriloquism effect occurs when the perceived location of a sound is biased by a concurrent visual stimulus, while the aftereffect manifests as a recalibration of sound localization after exposure to spatially discrepant stimuli. The relationship between these processes--whether recalibration is a direct consequence of integration or operates independently--remains debated. This study investigates the role of causal inference in these processes by examining whether trial-wise judgments of audio-visual stimuli as originating from a common cause influence both the ventriloquism effect and the immediate aftereffect. Using a spatial paradigm, participants made explicit judgments about the common cause of stimulus pairs, and their influence on both perceptual biases was assessed. Our results indicate that while multisensory integration is contingent on common cause judgments, the immediate recalibration effect is not. This suggests that recalibration can occur independently of the perceived commonality of the multisensory stimuli, challenging the notion that recalibration is solely a byproduct of integration.

Time Makes Space: Emergence of Place Fields in Networks Encoding Temporally Continuous Sensory Experiences
The vertebrate hippocampus is believed to use recurrent connectivity in area CA3 to support episodic memory recall from partial cues. This brain area also contains place cells, whose location-selective firing fields implement maps supporting spatial memory. Here we show that place cells emerge in networks trained to remember temporally continuous sensory episodes. We model CA3 as a recurrent autoencoder that recalls and reconstructs sensory experiences from noisy and partially occluded observations by agents traversing simulated arenas. The agents move in realistic trajectories modeled from rodents and environments are modeled as continuously varying, high-dimensional, sensory experience maps (spatially smoothed Gaussian random fields). Training our autoencoder to accurately pattern-complete and reconstruct sensory experiences with a constraint on total activity causes spatially localized firing fields, i.e., place cells, to emerge in the encoding layer. The emergent place fields reproduce key aspects of hippocampal phenomenology: a) remapping (maintenance of and reversion to distinct learned maps in different environments), implemented via repositioning of experience manifolds in the networks hidden layer, b) orthogonality of spatial representations in different arenas, c) robust place field emergence in differently shaped rooms, with single units showing multiple place fields in large or complex spaces, and d) slow representational drift of place fields. We argue that these results arise because continuous traversal of space makes sensory experience temporally continuous. We make testable predictions: a) rapidly changing sensory context will disrupt place fields, b) place fields will form even if recurrent connections are blocked, but reversion to previously learned representations upon remapping will be abolished, c) the dimension of temporally smooth experience sets the dimensionality of place fields, including during virtual navigation of abstract spaces.

A posture subspace in primary motor cortex
To generate movements, the brain must combine information about movement goal and body posture. Motor cortex (M1) is a key node for the convergence of these information streams. How are posture and goal information organized within M1s activity to permit the flexible generation of movement commands? To answer this question, we recorded M1 activity while monkeys performed a variety of tasks with the forearm in a range of postures. We found that posture- and goal-related components of neural population activity were separable and resided in nearly orthogonal subspaces. The posture subspace was stable across tasks. Within each task, neural trajectories for each goal had similar shapes across postures. Our results reveal a simpler organization of posture information in M1 than previously recognized. The compartmentalization of posture and goal information might allow the two to be flexibly combined in the service of our broad repertoire of actions.

The role of TRPV4 in acute sleep deprivation-induced fear memory impairment
Acute sleep deprivation (ASD) negatively impacts fear memory, but the underlying mechanisms are not fully understood. Transient receptor potential vanilloid 4 (TRPV4), a cation channel which is closely correlated with the concentration of Ca2+, and neuronal Ca2+ overloading is a crucial inducement of learning and memory impairment. This study utilized an acute sleep-deprived mouse model combined with fear conditioning to investigate these mechanisms. mRNA sequencing revealed increased expression of TRPV4 in mice with ASD-induced fear memory impairment. Notably, knockdown of TRPV4 reversed ASD-induced fear memory impairment. ASD leads to the increased concentration of Ca2+. Additionally, we observed a reduction in spine density and a significant decrease in postsynaptic density protein 95 (PSD95), which is associated with synaptic plasticity, in sleep-deprived fear memory impairment mice. This indicates that ASD may cause overloaded Ca2+, disrupting synaptic plasticity and impairing fear memory. Moreover, TRPV4 knockdown significantly decreased Ca2+ concentration, mitigated the loss of dendritic spines and reduction of PSD95, contributing to the restoration of fear memory. These findings indicate a potential protective role of TRPV4 knockdown in counteracting ASD-induced fear memory deficits. Collectively, our results highlight that TRPV4 may be a potential therapeutic target in mediating fear memory impairment due to ASD and underscore the importance of sleep management for conditions like PTSD.

Attentional priority and limbic activity favor gains over losses
Prospect theory has suggested that decisions reflect a bias toward avoidance of loss compared to equivalent gains. In our study we examine whether a similar bias is found in decisions regarding whether a loss associated stimulus or a gain associated stimulus is given priority in perception. We find that for most people, gains are given priority over loss in the decision of which of the two stimuli occurs first. We also ask whether gains are reflected in greater activity in limbic systems related to emotion. In an fMRI study, we find that most people show greater emotional response to gains, not losses. We consider how these findings might be related to risk aversion in studies of decision making.

Mnemonic brain state engagement is diminished in healthy aging
Healthy older adults typically show impaired episodic memory - memory for when and where an event oc-curred - but intact semantic memory - knowledge for general information and facts. As older adults also have difficulty inhibiting the retrieval of prior knowledge from memory, their selective decline in episodic memory may be due to a tendency to over engage the retrieval state, a brain state in which attention is focused internally in an attempt to access prior knowledge. The retrieval state trades off with the encoding state, a brain state which supports the formation of new memories. Therefore, episodic memory declines in older adults may be the result of differential engagement in mnemonic brain states. Our hypothesis is that older adults are biased toward a retrieval state. We recorded scalp electroencephalography while young, middle-aged and older adults performed a memory task in which they were explicitly directed to either encode the currently presented object stimulus or retrieve a previously presented, categorically-related object stimulus. We used multivariate pattern analysis of spectral activity to decode engagement in the retrieval vs. encoding state. We find that all three age groups can follow top-down instructions to selectively engage in encoding or retrieval and that we can decode mnemonic states for all age groups. However, we find that mnemonic brain state engagement is diminished for older adults relative to middle-aged adults. Our interpretation is that a combination of executive control deficits and a modest bias to retrieve modulates older adults mnemonic state engagement. Together, these findings suggest that dif-ferential mnemonic state engagement may underlie age-related memory changes.

Longitudinal deformation based morphometry pipeline to study neuroanatomical differences in structural MRI based on SyN unbiased templates
Morphometric measures in humans derived from magnetic resonance imaging (MRI) have provided important insights into brain differences and changes associated with development and disease in vivo. Deformation-based morphometry (DBM) is a registration-based technique that has been shown to be useful in detecting local volume differences and longitudinal brain changes while not requiring a priori segmentation or tissue classification. Typically, DBM measures are derived from registration to common template brain space (one-level DBM). Here, we present a two-level DBM technique: first, the Jacobian determinants are calculated for each individual input MRI at the subject level to capture longitudinal individual brain changes; then, in a second step, an unbiased common group space is created, and the Jacobians co-registered to enable the comparison of individual morphological changes across subjects or groups. This two-level DBM is particularly suitable for capturing longitudinal intra-individual changes in vivo, as calculating the Jacobians within-subject space leads to superior accuracy. Using artificially induced volume differences, we demonstrate that this two-level DBM pipeline is 4.5x more sensitive in detecting longitudinal within-subject volume changes compared to a typical one-level DBM approach. It also captures the magnitude of the induced volume change much more accurately. Using 150 subjects from the OASIS-2 dataset, we demonstrate that the two-level DBM is superior in capturing cortical volume changes associated with cognitive decline across patients with dementia and cognitively healthy individuals. This pipeline provides researchers with a powerful tool to study longitudinal brain changes with superior accuracy and sensitivity. It is publicly available and has already been used successfully, proving its utility.

The BDNF/TrkB pathway in Somatostatin-expressing neurons suppresses cocaine-seeking behaviour
Cocaine addiction is a highly debilitating condition consisting of compulsive self-administration and seek for the substance of abuse, and its most challenging feature is the high rate of relapse. Addiction and relapse share similarities with neural plasticity which acts through the Brain-Derived Neurotrophic Factor and its receptor TrkB. Somatostatin (SST) expressing interneurons are involved in neuronal plasticity and are important in modulating cocaine-seeking behaviour in mice. We tested the role of TrkB in Somatostatin (SST)-expressing neurons in the extinction of cocaine-seeking behaviour, using mice in which TrkB has been knocked out specifically in SST neurons. We have observed that in these mice, once a cocaine-conditioned place preference is acquired, its extinction through seven days of extinction training is impaired, showing how this process relies on neural plasticity in SST neurons. When we promoted plasticity during extinction training using a light-activable TrkB in SST neurons in the prefrontal cortex of cocaine-conditioned mice, relapse of cocaine-seeking was prevented. Our data identify the critical role of TrkB-mediated plasticity within SST neurons in the extinction of and relapse to cocaine addiction.

Modelling inflammation-induced peripheral sensitization in a dish - more complex than expected?
Peripheral sensitization of nociceptors is believed to be a key driver of chronic pain states. Here, we sought to study the effects of a modified version of inflammatory soup on the excitability of human stem-cell derived sensory neurons. For this, we used a pre-existing and a novel stem cell line, modified to stably express the calcium sensor GCamP6f.

Upon treatment with inflammatory soup, we observed no changes in neuronal transcription or functional responses upon calcium imaging, and only a very minor increase in resting membrane potential via whole cell patch clamping. Similarly small changes were observed when treating mouse primary sensory neurons with inflammatory soup. A semi-systematic re-examination of past literature further indicated that observed effects of inflammatory mediators on dissociated sensory neuron cultures are generally very small.

We conclude that modelling inflammation-induced peripheral sensitization in vitro is non-trivial and will require careful selection of mediators and/or more complex, longitudinal multi-cellular setups. Especially in the latter, our novel GCamP6f induced-pluripotent stem cell line may be of value.

EEG-based analysis of intrinsic brain network function in chronic pain: Insights from a comprehensive multi-data set study
Chronic pain is associated with alterations in brain function. A better understanding of these alterations might help to develop new approaches for the diagnosis, prediction, and treatment of chronic pain. Here, we analyzed associations between chronic pain and alterations of intrinsic brain network function using resting-state electroencephalography. We included data from 537 people with chronic pain obtained from various research groups worldwide. We found strong evidence for associations between pain intensity and intrinsic brain network connectivity, but the replicability of these associations in independent data was mostly low. However, a mega-analysis revealed associations of chronic pain with salience-somatomotor network connectivity at theta frequencies and with more complex patterns of intrinsic brain network connectivity. These findings provide novel insights into brain network function in chronic pain. Moreover, they highlight the need for collaborative multi-center studies, which can be guided by the present approach to promote replicability and consistency of findings.

Microglia are Required for Developmental Specification of AgRP Innervation in the Hypothalamus of Offspring Exposed to Maternal High Fat Diet During Lactation
Nutritional fluctuations that occur early in life dictate metabolic adaptations that will affect susceptibility to weight gain and obesity later in life. The postnatal period in mice represents a time of dynamic changes in hypothalamic development and maternal consumption of a high fat diet during the lactation period (MHFD) changes the composition of milk and leads to enhanced susceptibility to obesity in offspring. Agouti-related peptide (AgRP) neurons in the arcuate nucleus of the hypothalamus (ARH) react to changes in multiple metabolic signals and distribute neuroendocrine information to other brain regions, such as the paraventricular hypothalamic nucleus (PVH), which is known to integrate a variety of signals that regulate body weight. Development of neural projections from AgRP neurons to the PVH occurs during the lactation period and these projections are reduced in MHFD offspring, but underlying developmental mechanisms remain largely unknown. Microglia are the resident immune cells of the central nervous system and are involved in refinement of neural connections and modulation of synaptic transmission. Because high fat diet exposure causes activation of microglia in adults, a similar activation may occur in offspring exposed to MHFD and play a role in sculpting hypothalamic feeding circuitry. Genetically targeted axonal labeling and immunohistochemistry were used to visualize AgRP axons and microglia in postnatal mice derived from MHFD dams and morphological changes quantified. The results demonstrate regionally localized changes to microglial morphology in the PVH of MHFD offspring that suggest enhanced surveillance activity and are temporally restricted to the period when AgRP neurons innervate the PVH. In addition, axon labeling experiments confirm a significant decrease in AgRP innervation of the PVH in MHFD offspring and provide direct evidence of synaptic pruning of AgRP inputs to the PVH. Microglial depletion with the Colony-stimulating factor 1 receptor inhibitor PLX5622 determined that the decrease in AgRP innervation observed in MHFD offspring is dependent on microglia, and that microglia are required for weight gain that emerges as early as weaning in offspring of MHFD dams. However, these changes do not appear to be dependent on the degree of microglial mediated synaptic pruning. Together, these findings suggest that microglia are activated by exposure to MHFD and interact directly with AgRP axons during development to permanently alter their density, with implications for developmental programming of metabolic phenotype.

Significance StatementMaternal high fat diet exposure results in enhanced risk for negative health outcomes in humans and multiple animal models. Here we demonstrate that microglia are required for changes in body weight and perturbations to hypothalamic circuits caused by maternal high fat diet exposure that is limited to the lactational period. We identified spatially and temporally limited morphological changes to microglia that reflect an enhancement of surveillance activity and align with a critical period of hypothalamic circuit formation. We also identify direct cellular interactions between microglia and developing axons, as well as evidence for synaptic engulfment, although this mechanism does not appear to be responsible for changes to neural patterning caused by maternal high fat diet exposure. Together these findings identify an essential role for microglia in specifying patterns of hypothalamic innervation during development in response to maternal high fat diet exposure, which may contribute to developmental programming of metabolic phenotype.

Superiority of Rhythmic Auditory Signals over Electrical Stimulation to Entrain Behavior
Neural tracking (entrainment) of auditory rhythms enhances perception. We previously demonstrated that transcranial alternating current stimulation (tACS) can enhance or suppress entrainment to rhythmic auditory stimuli, depending on the timing between the electrical and auditory signals, although tACS effects are primarily modulatory. This study further investigated entrainment to tACS and auditory rhythms when the electrical and auditory signals were presented together (Experiment 1, N = 34) or independently (Experiment 2, N = 24; Experiment 3, N = 12). We hypothesized that tACS effects would be more pronounced when the auditory rhythm was made less perceptually salient to reduce the competition with the electrical rhythm. Participants detected silent gaps in modulated or unmodulated noise stimuli. In Experiment 1, auditory stimuli predominated in entraining behavior. While behavioral entrainment to sound rhythms was affected by the modulation depth of the auditory stimulus, entrainment to tACS was not. In Experiment 2, with no rhythmic information from the sound, 16 of 24 participants showed significant behavioral entrainment to tACS, although the most effective tACS frequency varied across participants. An oscillator model with a free parameter for the individual resonance frequency produced profiles similar to those we observed behaviorally. In Experiment 3, both neural and behavioral entrainment to rhythmic sounds were affected by the auditory stimulus frequency, but again the most effective entraining frequency varied across participants. Our findings suggest that tACS effects depend on the individuals preferred frequency when there is no competition with sensory stimuli, emphasizing the importance of targeting individual frequencies in tACS experiments. When both sensory and electrical stimuli are rhythmic and compete, sensory stimuli prevail, indicating the superiority of sensory stimulation in modulating behavior.