bioRxiv Subject Collection: Neuroscience's Journal

Potentiation of active locomotor state by spinal-projecting serotonergic neurons
Animals produce diverse motor actions that enable expression of context-appropriate behaviors. Neuromodulators facilitate behavioral flexibility by altering the temporal dynamics and output of neural circuits. Discrete populations of serotonergic (5-HT) neurons target circuits in the brainstem and spinal cord, but their role in the control of motor behavior is unclear. Here we define the pre- and post-synaptic organization of the spinal-projecting serotonergic system and define a role in locomotor control. We show that while forebrain-targeting 5-HT neurons decrease their activity during locomotion, subpopulations of spinal projecting neurons increase their activity in a context-dependent manner. Optogenetic activation of ventrally projecting 5-HT neurons does not trigger initiation of movement, but rather enhances the speed and duration of ongoing locomotion over extended time scales. These findings indicate that the descending serotonergic system potentiates locomotor output and demonstrate a role for serotonergic neurons in modulating the temporal dynamics of motor circuits.

A hierarchical Bayesian model reveals increased precision weighting for afferent cardiac signals, and reduced anxiety, as a function of interoceptive training
Perceptual accuracy for interoceptive signals, such as heartbeats, varies in a trait-like manner across individuals and may influence the capacity for emotion regulation and vulnerability to affective symptoms, notably anxiety. Here, we demonstrate that an interoceptive training protocol improved perceptual accuracy in two tasks of heartbeat perception and reduced both state and trait anxiety in a subclinical sample, extending previous findings in autistic adults. Computational modelling indicated that accuracy improvement in the heartbeat discrimination task was associated with increases in the internal reliability estimate for interoceptive signals - their precision weighting - while a lower-level parameter representing noise in the interoceptive signal itself (which influences speed of learning) moderated this precision weighting improvement. Reductions in both state and trait anxiety in the training group were uniquely explained by computational parameter estimates, and not by conventional accuracy measures. These findings indicate that trait-like differences in interoceptive processing are modifiable and can be targeted to alleviate anxiety symptoms, and that interoceptive interventions may be best guided by a computational phenotyping approach.

Connectivity in the Dorsal Visual Stream is Enhanced in Action Video Game Players
Action video games foster competitive environments that demand rapid spatial navigation and decision-making. Action video gamers often exhibit faster response times and slightly improved accuracy in vision-based sensorimotor tasks. However, the underlying functional and structural changes in the two visual streams of the brain that may be contributing to these cognitive improvements have been unclear. Using functional and diffusion MRI data, this study investigated the differences in connectivity between gamers who play action video games and nongamers in the dorsal and ventral visual streams. We found that action video gamers have enhanced functional and structural connectivity, especially in the dorsal visual stream. Specifically, there is heightened functional connectivity both undirected and directed between the left Superior Occipital Gyrus and the left Superior Parietal Lobule during a moving-dots discrimination decision-making task. This increased connectivity correlates with response time in gamers. The structural connectivity, as quantified by diffusion fractional anisotropy and quantitative anisotropy measures of the axonal fiber pathways between the same regions was also enhanced for gamers compared to nongamers. These findings provide valuable insights into how action video gaming can induce targeted neuroplastic changes, enhancing structural and functional connectivity between specific brain regions in the visual processing pathways. These connectivity changes in the dorsal visual stream underpin the superior performance of action video gamers compared to non-gamers in tasks requiring rapid and accurate vision-based decision-making.

Comparative Analysis of AAV Serotypes for Transduction of Olfactory Sensory Neurons
Olfactory sensory neurons within the nasal epithelium detect volatile odorants and relay odor information to the central nervous system. Unlike other sensory inputs, olfactory sensory neurons interface with the external environment and project their axons directly into the central nervous system. The use of adeno-associated viruses to target these neurons has garnered interest for applications in gene therapy, probing olfactory sensory neuron biology, and modeling disease. To date, there is no consensus on the optimal AAV serotype for efficient and selective transduction of olfactory sensory neurons in vivo. Here we utilized serial confocal imaging and single-nucleus RNA sequencing to evaluate the efficacy of 11 different AAV serotypes in transducing murine olfactory sensory neurons via non-invasive nasal inoculation. Our results reveal that AAV1, while highly effective, exhibited broad tropism, whereas AAV-DJ/8 showed the greatest specificity for olfactory sensory neurons.

Fast hierarchical processing of orthographic and semantic parafoveal information during natural reading
Readers extract orthographic and semantic information from parafoveal words before fixating on them. While this has to be achieved within an intersaccadic interval, the neuronal mechanisms supporting this fast parafoveal word processing within the language network remain unknown. We co-registered MEG and eye-tracking data in a natural reading paradigm to uncover the neuronal mechanisms supporting parafoveal processing. Representational similarity analysis (RSA) revealed that parafoveal orthographic neighbours (e.g., "writer" vs. "waiter") showed higher representational similarity than non-neighbours (e.g., "writer" vs. "police"), emerging ~68 ms after fixation onset on the preceding word (e.g., "clever") in the visual word form area. Similarly, parafoveal semantic neighbours (e.g., "writer" vs. "author") exhibited increased representational similarity at ~137 ms in the left inferior frontal gyrus. Importantly, the degree of orthographic and semantic parafoveal processing predicted individual reading speed. Our findings suggest fast hierarchical processing of parafoveal words across distinct brain regions, which enhances reading efficiency.

Natural variations in maternal behaviour shape trait anxiety and hippocampal neurogenesis in offspring
Understanding the early origins of emotional traits and baseline anxiety is crucial for the development of personalized medicine in mood disorders. While previous research primarily focused on extreme conditions like chronic maternal deprivation, in the present study, we investigated how natural variations in maternal care influence anxiety-related behaviour and neurogenesis in C57BL/6J offspring in a longitudinal manner. We observed that mothers engaging in low maternal care (LMC) displayed lower adult neurogenesis in both the olfactory bulb and the dentate gyrus of the hippocampus compared to high maternal care (HMC) mothers. We then observed that LMC-reared pups exhibited increased anxiety-related behaviour at postnatal day (PND) 5, 9, and 22. Furthermore, maternal behaviour induced the development of emotional individuality at early stage. This mood-related phenotype in LMC-reared offspring was associated with decreased neurogenesis after weaning at PND24. In another group of litters, we further examined neurogenesis at an earlier age (PND9) and already found a reduction in the population of adult neural progenitor cells and cell proliferation in the subgranular zone of the dentate gyrus of LMC-reared pups. These results highlight that natural variations in early life experiences such as maternal care, shape long-term brain plasticity and behaviour in offspring. This underscores the relevance of maternal care and adult neurogenesis in shaping personality-like traits related to mood disorders.

Dnmt1-deficiency in PV interneurons alters cortical circuit function and leads to depression-like behavior
Neuropsychiatric disorders, including major depressive disorder (MDD), are highly prevalent in modern society, arising from a complex interplay of genetic and environmental factors. Alterations in the function of cortical inhibitory GABAergic interneurons, along with dysregulations of epigenetic signatures and key regulators such as DNA methyltransferase 1 (DNMT1), have been implicated in these conditions. Through its role in catalyzing DNA methylation, DNMT1 modulates the synaptic activity of parvalbumin-expressing (PV) interneurons, which are essential for cortical inhibition. However, the functional consequences of DNMT1 activity in cortical interneurons at the network level and its impact on behavior remain unknown and must be explored to fully understand the disease implications of dysregulated DNMT1 expression and function. To address this, we utilized a conditional knockout mouse model with Dnmt1 deletion in PV interneurons. Our findings reveal that a Dnmt1 deficiency leads to increased spontaneous firing rates of cortical neurons, reduced cortical gamma oscillations, and altered visually evoked neural responses. Despite intact sensory perception, Dnmt1-deficient mice exhibited reduced physical activity, heightened anxiety-like behavior, and signs of anhedonia and apathy, which represent core features of MDD. These results underscore the critical role of DNMT1 in PV interneuron function and identify it as a potential target for MDD research.

Amphetamine increases timing variability by degrading prefrontal ramping activity
Background: Amphetamine is a commonly abused psychostimulant that increases synaptic catecholamine levels and impairs executive functions. However, it is unknown how acute amphetamine affects brain areas involved in executive control, such as the prefrontal cortex. We studied this problem in mice using interval timing, which requires participants to estimate an interval of several seconds with a motor response. Rodent prefrontal cortex ensembles are required for interval timing. We tested the hypothesis that amphetamine disrupts interval timing by degrading prefrontal cortex temporal encoding. Methods: We first quantified the effects of amphetamine on interval timing performance by conducting a meta analysis of 11 prior rodent studies. We also implanted multielectrode recording arrays in the dorsomedial prefrontal cortex of 7 mice and then examined the effects of 1.5 mg/kg D amphetamine injected intraperitoneally on interval timing behavior and prefrontal neuronal ensemble activity. Results: A meta analysis of previous literature revealed that amphetamine produces a large effect size on interval timing variability across studies but only a medium effect size on central tendencies of interval timing. We found a similar effect on interval timing variability in our task, which was accompanied by greater trial to trial variability in prefrontal ramping, attenuated interactions between pairs of ramping neurons, and dampened low frequency oscillations. Conclusions: These findings suggest that amphetamine alters prefrontal temporal processing by increasing the variability of prefrontal ramping. Our work provides insight into how amphetamine affects timing related brain activity, which may be useful in developing new neurophysiological markers for amphetamine use and novel treatments targeting the prefrontal cortex.

Cannabis smoke and oral Δ9THC enhance working memory in aged but not young adult subjects
With increased legalization of recreational and medical cannabis, use of this drug is growing rapidly among older adults. As cannabis use can impair cognition in young adults, it is critically important to understand how consumption interacts with the cognitive profile of aged individuals, who are already at increased risk of decline. The current study was designed to determine how cannabis influences multiple forms of cognition in young adult and aged rats of both sexes when delivered via two translationally-relevant routes of administration. Acute exposure to cannabis smoke enhanced prefrontal cortex-dependent working memory accuracy in aged males, but impaired accuracy in aged females, while having no effects in young adults of either sex. In contrast, the same cannabis smoke exposure regimen had minimal effects on a hippocampus-dependent trial-unique non-matching to location mnemonic task, irrespective of age or sex. In a second set of experiments, chronic oral consumption of {Delta}9-tetrahydrocannabinol ({Delta}9THC) enhanced working memory in aged rats of both sexes, while having no effects in young adults. In contrast, the same oral {Delta}9THC regimen did not affect spatial learning and memory in either age group. Minimal age differences were observed in {Delta}9THC pharmacokinetics with either route of administration. Together, these results show that cannabis and {Delta}9THC can attenuate working memory impairments that emerge in aging. While these enhancing effects do not extend to hippocampus-dependent cognition, cannabis does not appear to exacerbate age-associated impairments in this cognitive domain.

Enhancer AAV toolbox for accessing and perturbing striatal cell types and circuits
We present an enhancer AAV toolbox for accessing and perturbing striatal cell types and circuits. Best-in-class vectors were curated for accessing major striatal neuron populations including medium spiny neurons (MSNs), direct and indirect pathway MSNs, as well as Sst-Chodl, Pvalb-Pthlh, and cholinergic interneurons. Specificity was evaluated by multiple modes of molecular validation, three different routes of virus delivery, and with diverse transgene cargos. Importantly, we provide detailed information necessary to achieve reliable cell type specific labeling under different experimental contexts. We demonstrate direct pathway circuit-selective optogenetic perturbation of behavior and multiplex labeling of striatal interneuron types for targeted analysis of cellular features. Lastly, we show conserved in vivo activity for exemplary MSN enhancers in rat and macaque. This collection of striatal enhancer AAVs offers greater versatility compared to available transgenic lines and can readily be applied for cell type and circuit studies in diverse mammalian species beyond the mouse model.

Molecular and Neural Circuit Mechanisms Underlying Sexual Experience-dependent Long-Term Memory in Drosophila.
The neural and molecular underpinnings of sexual experience-dependent long-term memory (SELTM) in male Drosophila melanogaster remain poorly understood, despite its significance for reproductive success. Here, we dissect the role of a specific class of neurons, termed 'Yuelao' (YL) neurons, in the formation of SELTM and the associated molecular pathways. We employed a combination of genetic manipulations, immunohistochemistry, and behavioral assays in Drosophila to investigate the function of YL neurons and their molecular interactors in SELTM. Utilizing the RNA sequencing data and transgenic tools, we delineated the neural circuits involved in taste and pheromone processing relevant to SELTM. Our findings reveal that the YL neurons, expressing the Orb2 scaffolding protein, are indispensable for the formation of SELTM following sexual experience. These neurons are regulated by the neuromodulator short neuropeptide F (sNPF) and its receptor (sNPF-R), which modulate glutamate release via NMDAR2. We demonstrate that sexual experience triggers synaptic plasticity in YL neurons, characterized by an increase in dendritic and presynaptic terminal areas, and a decrease in intracellular calcium levels. Furthermore, we show that YL neurons are specialized for the generation of appetitive sexual experience-dependent memory and project to brain regions implicated in memory formation. Our study uncovers the YL neurons as a key neural substrate for SELTM in Drosophila, shedding light on the molecular and circuit mechanisms that mediate the formation of long-term memories following sexual experience. These findings provide novel insights into the neural basis of taste-related memory and have broader implications for understanding the interplay between experience, memory formation, and behavior.

Reproducible Sex Differences in Personalized Functional Network Topography in Youth
A key step towards understanding psychiatric disorders that disproportionately impact female mental health is delineating the emergence of sex-specific patterns of brain organization at the critical transition from childhood to adolescence. Prior work suggests that individual differences in the spatial organization of functional brain networks across the cortex are associated with psychopathology and differ systematically by sex. We aimed to evaluate the impact of sex on the spatial organization of person-specific functional brain networks. We leveraged person-specific atlases of functional brain networks defined using non-negative matrix factorization in a sample of n = 6437 youths from the Adolescent Brain Cognitive Development Study. Across independent discovery and replication samples, we used generalized additive models to uncover associations between sex and the spatial layout ('topography') of personalized functional networks (PFNs). Next, we trained support vector machines to classify participants' sex from multivariate patterns of PFN topography. Finally, we leveraged transcriptomic data from the Allen Human Brain Atlas to evaluate spatial correlations between sex differences in PFN topography and gene expression. Sex differences in PFN topography were greatest in association networks including the fronto-parietal, ventral attention, and default mode networks. Machine learning models trained on participants' PFNs were able to classify participant sex with high accuracy. Brain regions with the greatest sex differences in PFN topography were enriched in expression of X-linked genes as well as genes expressed in astrocytes and excitatory neurons. Sex differences in PFN topography are robust, replicate across large-scale samples of youth, and are associated with expression patterns of X-linked genes. These results suggest a potential contributor to the female-biased risk in depressive and anxiety disorders that emerge at the transition from childhood to adolescence.

A Kinematic Deviation Index (KDI) for Evaluation of Forelimb Function in Rodents
Rodent models are widely used to study neurological conditions and assess forelimb movement to measure function performance, deficit, recovery and treatment effectiveness. Traditional assessment methods based on endpoints such as whether the task is accomplished, while easy to implement, provide limited information on movement patterns important to assess different functional strategies. On the other side, detailed kinematic analysis provides granular information on the movement patterns but is difficult to compare across laboratories, and may not translate to clinical metrics of upper limb function. To address these limitations, we developed and validated a kinematic deviation index (KDI) for rodents that mimics current trends in clinical research. The KDI is a unitless summary score that quantifies the difference between an animal movement during a task and its optimal performance derived from spatiotemporal marker sequences without pre-specifying movements. We demonstrate the utility of KDI in assessing reaching and grasping in mice and validate its discrimination between trial endpoints in healthy animals. Furthermore, we show KDI sensitivity to interventions, including acute and chronic spinal cord injury and optogenetic disruption of sensorimotor circuits. The KDI provides a comprehensive measure of motor function that bridges the gap between detailed kinematic analysis and simple success/failure metrics, offering a valuable tool for assessing recovery and compensation in rodent models of neurological disorders.

Spatial learning in feature-impoverished environments in Drosophila
The ability to return to memorized goal locations is essential for animal survival. While it is well documented that animals use visual landmarks to locate goals, how they navigate spatial learning tasks in environments lacking such landmarks remains poorly understood. Here, using a high-throughput spatial learning task we developed to investigate this question in Drosophila, we found that Drosophila can simultaneously use self-generated olfactory cues and self-motion cues to learn a spatial goal under visually challenging conditions. Specifically, flies mark a rewarded goal location with self-deposited scents, to which they assign a positive value to, and use these scents and their self-motion cues to guide them back to the goal. This learning process is mediated by the mushroom body(MB) - an olfactory learning center responsible for associating odors with reinforcement - and by PFN neurons, which encode egocentric translational velocity, a self-motion cue. Intriguingly, when the environment is enriched with prominent external olfactory landmarks, flies shift their strategy, prioritizing these landmarks over self-generated cues. Our findings demonstrate that Drosophila can dynamically adapt to environmental complexities when solving spatial learning tasks by creating and integrating internal and external cues, revealing an unexpected level of sophistication in their cognitive capacities.