Adolescent Parvalbumin Expression in the Left Orbitofrontal Cortex Shapes Sociability in Female Mice

Discovering functional interactions among schizophrenia-risk genes by combining behavioral genetics with cell biology

Despite much progress in identifying risk genes for polygenic brain disorders, their core pathogenic mechanisms remain poorly understood. In particular, functions of many proteins encoded by schizophrenia risk genes appear diverse and unrelated, complicating the efforts to establish the causal relationship between genes and behavior. Using various mouse lines, recent studies indicate that alterations of parvalbumin-positive (PV+) GABAergic interneurons can lead to schizophrenia-like behavior. PV+ interneurons display fast spiking and contribute to excitation-inhibition balance and network oscillations via feedback and feedforward inhibition. Here, we first summarize different lines of genetically modified mice that display motor, cognitive, emotional, and social impairments used to model schizophrenia and related mental disorders. We highlight ten genes, encoding either a nuclear, cytosolic, or membrane protein. Next, we discuss their functional relationship in regulating fast spiking and other aspects of PV+ interneurons and in the context of other domains of schizophrenia. Future investigations combining behavioral genetics and cell biology should elucidate functional relationships among risk genes to identify the core pathogenic mechanisms underlying polygenic brain disorders.

Whole-brain Mapping of Inputs and Outputs of Specific Orbitofrontal Cortical Neurons in Mice

The orbitofrontal cortex (ORB), a region crucial for stimulus-reward association, decision-making, and flexible behaviors, extensively connects with other brain areas. However, brain-wide inputs to projection-defined ORB neurons and the distribution of inhibitory neurons postsynaptic to neurons in specific ORB subregions remain poorly characterized. Here we mapped the inputs of five types of projection-specific ORB neurons and ORB outputs to two types of inhibitory neurons. We found that different projection-defined ORB neurons received inputs from similar cortical and thalamic regions, albeit with quantitative variations, particularly in somatomotor areas and medial groups of the dorsal thalamus. By counting parvalbumin (PV) or somatostatin (SST) interneurons innervated by neurons in specific ORB subregions, we found a higher fraction of PV neurons in sensory cortices and a higher fraction of SST neurons in subcortical regions targeted by medial ORB neurons. These results provide insights into understanding and investigating the function of specific ORB neurons.

Parvalbumin interneuron deficits in schizophrenia

Parvalbumin-expressing (PV+) interneurons represent one of the most abundant subclasses of cortical interneurons. Owing to their specific electrophysiological and synaptic properties, PV+ interneurons are essential for gating and pacing the activity of excitatory neurons. In particular, PV+ interneurons are critically involved in generating and maintaining cortical rhythms in the gamma frequency, which are essential for complex cognitive functions. Deficits in PV+ interneurons have been frequently reported in postmortem studies of schizophrenia patients, and alterations in gamma oscillations are a prominent electrophysiological feature of the disease. Here, I summarise the main features of PV+ interneurons and review clinical and preclinical studies linking the developmental dysfunction of cortical PV+ interneurons with the pathophysiology of schizophrenia.

Short- and Long-Term High-Fat Diet Exposure Differentially Alters Phasic and Tonic GABAergic Signaling onto Lateral Orbitofrontal Pyramidal Neurons

The chronic consumption of caloric dense high-fat foods is a major contributor to increased body weight, obesity, and other chronic health conditions. The orbitofrontal cortex (OFC) is critical in guiding decisions about food intake and is altered with diet-induced obesity. Obese rodents have altered morphologic and synaptic electrophysiological properties in the lateral orbitofrontal cortex (lOFC). Yet the time course by which exposure to a high-fat diet (HFD) induces these changes is poorly understood. Here, male mice are exposed to either short-term (7 d) or long-term (90 d) HFD. Long-term HFD exposure increases body weight, and glucose signaling compared with short-term HFD or a standard control diet (SCD). Both short and long-term HFD exposure increased the excitability of lOFC pyramidal neurons. However, phasic and tonic GABAergic signaling was differentially altered depending on HFD exposure length, such that tonic GABAergic signaling was decreased with early exposure to the HFD and phasic signaling was changed with long-term diet exposure. Furthermore, alterations in the short-term diet exposure were transient, as removal of the diet restored electrophysiological characteristics similar to mice fed SCD, whereas long-term HFD electrophysiological changes were persistent and remained after HFD removal. Finally, we demonstrate that changes in reward devaluation occur early with diet exposure. Together, these results suggest that the duration of HFD exposure differentially alters lOFC function and provides mechanistic insights into the susceptibility of the OFC to impairments in outcome devaluation.SIGNIFICANCE STATEMENT This study provides mechanistic insight on the impact of short-term and long-term high-fat diet (HFD) exposure on GABAergic function in the lateral orbitofrontal cortex (lOFC), a region known to guide decision-making. We find short-term HFD exposure induces transient changes in firing and tonic GABA action on lOFC pyramidal neurons, whereas long-term HFD induces obesity and has lasting changes on firing, tonic GABA and inhibitory synaptic transmission onto lOFC neurons. Given that GABAergic signaling in the lOFC can influence decision-making around food, these results have important implications in present society as palatable energy dense foods are abundantly available.

A Novel Head-Fixed Assay for Social Touch in Mice Uncovers Aversive Responses in Two Autism Models

Social touch, an important aspect of social interaction and communication, is essential to kinship across animal species. How animals experience and respond to social touch has not been thoroughly investigated, in part because of the lack of appropriate assays. Previous studies that examined social touch in freely moving rodents lacked the necessary temporal and spatial control over individual touch interactions. We designed a novel head-fixed assay for social touch in mice, in which the experimenter has complete control to elicit highly stereotyped bouts of social touch between two animals. The user determines the number, duration, context, and type of social touch interactions, while monitoring an array of complex behavioral responses with high resolution cameras. We focused on social touch to the face because of its high translational relevance to humans. We validated this assay in two different models of autism spectrum disorder (ASD), the Fmr1 knock-out (KO) model of Fragile X syndrome (FXS) and maternal immune activation (MIA) mice. We observed higher rates of avoidance running, hyperarousal, and aversive facial expressions (AFEs) to social touch than to object touch, in both ASD models compared with controls. Fmr1 KO mice showed more AFEs to mice of the same sex but whether they were stranger or familiar mice mattered less. Because this new social touch assay for head-fixed mice can be used to record neural activity during repeated bouts of social touch it could be used to uncover underlying circuit differences.SIGNIFICANCE STATEMENT Social touch is important for communication in animals and humans. However, it has not been extensively studied and current assays to measure animals' responses to social touch have limitations. We present a novel head-fixed assay to quantify how mice respond to social facial touch with another mouse. We validated this assay in autism mouse models since autistic individuals exhibit differences in social interaction and touch sensitivity. We find that mouse models of autism exhibit more avoidance, hyperarousal, and aversive facial expressions (AFEs) to social touch compared with controls. Thus, this novel assay can be used to investigate behavioral responses to social touch and the underlying brain mechanisms in rodent models of neurodevelopmental conditions, and to evaluate therapeutic responses in preclinical studies.

Stress induces behavioral abnormalities by increasing expression of phagocytic receptor MERTK in astrocytes to promote synapse phagocytosis

Childhood neglect and/or abuse can induce mental health conditions with unknown mechanisms. Here, we identified stress hormones as strong inducers of astrocyte-mediated synapse phagocytosis. Using in vitro, in vivo, and human brain organoid experiments, we showed that stress hormones increased the expression of the Mertk phagocytic receptor in astrocytes through glucocorticoid receptor (GR). In post-natal mice, exposure to early social deprivation (ESD) specifically activated the GR-MERTK pathway in astrocytes, but not in microglia. The excitatory post-synaptic density in cortical regions was reduced in ESD mice, and there was an increase in the astrocytic engulfment of these synapses. The loss of excitatory synapses, abnormal neuronal network activities, and behavioral abnormalities in ESD mice were largely prevented by ablating GR or MERTK in astrocytes. Our work reveals the critical roles of astrocytic GR-MERTK activation in evoking stress-induced abnormal behaviors in mice, suggesting GR-MERTK signaling as a therapeutic target for stress-induced mental health conditions.

FriendlyClearMap: an optimized toolkit for mouse brain mapping and analysis

BACKGROUND

Tissue clearing is currently revolutionizing neuroanatomy by enabling organ-level imaging with cellular resolution. However, currently available tools for data analysis require a significant time investment for training and adaptation to each laboratory's use case, which limits productivity. Here, we present FriendlyClearMap, an integrated toolset that makes ClearMap1 and ClearMap2's CellMap pipeline easier to use, extends its functions, and provides Docker Images from which it can be run with minimal time investment. We also provide detailed tutorials for each step of the pipeline.

FINDINGS

For more precise alignment, we add a landmark-based atlas registration to ClearMap's functions as well as include young mouse reference atlases for developmental studies. We provide an alternative cell segmentation method besides ClearMap's threshold-based approach: Ilastik's Pixel Classification, importing segmentations from commercial image analysis packages and even manual annotations. Finally, we integrate BrainRender, a recently released visualization tool for advanced 3-dimensional visualization of the annotated cells.

CONCLUSIONS

As a proof of principle, we use FriendlyClearMap to quantify the distribution of the 3 main GABAergic interneuron subclasses (parvalbumin+ [PV+], somatostatin+, and vasoactive intestinal peptide+) in the mouse forebrain and midbrain. For PV+ neurons, we provide an additional dataset with adolescent vs. adult PV+ neuron density, showcasing the use for developmental studies. When combined with the analysis pipeline outlined above, our toolkit improves on the state-of-the-art packages by extending their function and making them easier to deploy at scale.