The Claustrum Supports Resilience to Distraction

Claustrum projections to the anterior cingulate modulate nociceptive and pain-associated behavior

The anterior cingulate cortex (ACC) is critical for the perception and unpleasantness of pain.1,2,3,4,5,6 It receives nociceptive information from regions such as the thalamus and amygdala and projects to several cortical and subcortical regions of the pain neuromatrix.7,8 ACC hyperexcitability is one of many functional changes associated with chronic pain, and experimental activation of ACC pyramidal cells produces hypersensitivity to innocuous stimuli (i.e., allodynia).9,10,11,12,13,14 A less-well-studied projection to the ACC arises from a small forebrain region, the claustrum.15,16,17,18,19,20 Stimulation of excitatory claustrum projection neurons preferentially activates GABAergic interneurons, generating feed-forward inhibition onto excitatory cortical networks.21,22,23,24 Previous work has shown that claustrocingulate projections display altered activity in prolonged pain25,26,27; however, it remains unclear whether and how the claustrum participates in nociceptive processing and high-order pain behaviors. Inhibition of ACC activity reverses mechanical allodynia in animal models of persistent and neuropathic pain,1,9,28 suggesting claustrum inputs may function to attenuate pain processing. In this study, we sought to define claustrum function in acute and chronic pain. We found enhanced claustrum activity after a painful stimulus that was attenuated in chronic inflammatory pain. Selective inhibition of claustrocingulate projection neurons enhanced acute nociception but blocked pain learning. Inversely, chemogenetic activation of claustrocingulate neurons had no effect on basal nociception but rescued inflammation-induced mechanical allodynia. Together, these results suggest that claustrocingulate neurons are a critical component of the pain neuromatrix, and dysregulation of this connection may contribute to chronic pain.

Pathological claustrum activity drives aberrant cognitive network processing in human chronic pain

Aberrant cognitive network activity and cognitive deficits are established features of chronic pain. However, the nature of cognitive network alterations associated with chronic pain and their underlying mechanisms require elucidation. Here, we report that the claustrum, a subcortical nucleus implicated in cognitive network modulation, is activated by acute painful stimulation and pain-predictive cues in healthy participants. Moreover, we discover pathological activity of the claustrum and a region near the posterior inferior frontal sulcus of the right dorsolateral prefrontal cortex (piDLPFC) in migraine patients during acute pain and cognitive task performance. Dynamic causal modeling suggests a directional influence of the claustrum on activity in this piDLPFC region, and diffusion weighted imaging verifies their structural connectivity. These findings advance understanding of claustrum function during acute pain and provide evidence of a possible circuit mechanism driving cognitive impairments in chronic pain.

Neurotoxic lesions of the anterior claustrum influence cued fear memory in rats

Background

The claustrum (CLA), a subcortical area between the insular cortex and striatum, innervates almost all cortical regions of the mammalian brain. There is growing evidence that CLA participates in many brain functions, including memory, cognition, and stress response. It is proposed that dysfunction or malfunction of the CLA might be the pathology of some brain diseases, including stress-induced depression and anxiety. However, the role of the CLA in fear memory and anxiety disorders remains largely understudied.

Methods

We evaluated the influences of neurotoxic lesions of the CLA using auditory-cued fear memory and anxiety-like behaviors in rats.

Results

We found that lesions of anterior CLA (aCLA) but not posterior CLA (pCLA) before fear conditioning attenuated fear retrieval, facilitated extinction, and reduced freezing levels during the extinction retention test. Post-learning lesions of aCLA but not pCLA facilitated fear extinction and attenuated freezing behavior during the extinction retention test. Lesions of aCLA or pCLA did not affect anxiety-like behaviors evaluated by the open field test and elevated plus-maze test.

Conclusion

These data suggested that aCLA but not pCLA was involved in fear memory and extinction. Future studies are needed to further investigate the anatomical and functional connections of aCLA subareas that are involved in fear conditioning, which will deepen our understanding of CLA functions.

The functional connectivity of the human claustrum, according to the Human Connectome Project database

The claustrum is an irregular and fine sheet of grey matter in the basolateral telencephalon present in almost all mammals. The claustrum has been the object of several studies using animal models and, more recently, in human beings using neuroimaging. One of the most extended cognitive processes attributed to the claustrum is the salience process, which is also related to the insular cortex. In the same way, studies with human subjects and functional magnetic resonance imaging have reported the coactivation of the claustrum/insular cortex in the integration of sensory signals. This coactivation has been reported in the left claustrum/insular cortex or in the right claustrum/insular cortex. The asymmetry has been reported in task studies and literature related to neurological disorders such as Alzheimer's disease and schizophrenia, relating the severity of delusions with the reduction in left claustral volume. We present a functional connectivity study of the claustrum. Resting-state functional and anatomical MRI data from 100 healthy subjects were analyzed; taken from the Human Connectome Project (HCP, NIH Blueprint: The Human Connectome Project), with 2x2x2 mm3 voxel resolution. We hypothesize that 1) the claustrum is a node involved in different brain networks, 2) the functional connectivity pattern of the claustrum is different from the insular cortex's pattern, and 3) the asymmetry is present in the claustrum's functional connectivity. Our findings include at least three brain networks related to the claustrum. We found functional connectivity between the claustrum, frontoparietal network, and the default mode network as a distinctive attribute. The functional connectivity between the right claustrum with the frontoparietal network and the dorsal attention network supports the hypothesis of claustral asymmetry. These findings provide functional evidence, suggesting that the claustrum is coupled with the frontoparietal network serving together to instantiate new task states by flexibly modulating and interacting with other control and processing networks.

Time to FIRE NORSE: A single acronym for a heterogeneous presentation. Further information from a case series and discussion of the literature

PURPOSE

New-Onset Refractory Status Epilepticus (NORSE) is a rare and severe form of refractory status epilepticus without an apparent underlying cause at presentation or prior history of epilepsy. We aimed to describe the clinical features, etiology, treatment, and outcomes of NORSE in adults in a quaternary-level hospital in Saudi Arabia.

METHODOLOGY

In this retrospective cohort study, inclusion criteria involved patients over 14 years old who met the 2018 consensus definition for NORSE. Patients were identified using a combination of medical record admission labels 'status epilepticus' and 'encephalitis', and continuous EEG reports documenting status epilepticus. Demographic, clinical, and radiological data were collected and then analyzed for factors correlated with specific etiologies, better functional outcomes, and future diagnosis of epilepsy.

RESULTS

We found 24 patients presenting with NORSE between 2010 and 2021. Fever/infectious symptoms were the most common prodrome. Elevated inflammatory serum and cerebrospinal fluid markers in most patients. Brain MRI revealed T2/FLAIR hyperintensity patterns, predominantly affecting limbic and perisylvian structures. The etiology of NORSE varied, with immune-related causes being the most common. Long-term outcomes were poor, with a high mortality rate and most survivors developing drug-resistant epilepsy.

CONCLUSION

This study provides valuable insights into NORSE's clinical characteristics, highlighting the heterogeneity of this condition. The poor outcome is likely related to the progressive nature of the underlying disease, where refractory seizures are a clinical symptom. Thus, we propose to focus future research on the etiology rather than the NORSE acronym.

Anterior cingulate cross-hemispheric inhibition via the claustrum resolves painful sensory conflict

The anterior cingulate cortex (ACC) responds to noxious and innocuous sensory inputs, and integrates them to coordinate appropriate behavioral reactions. However, the role of the projections of ACC neurons to subcortical areas and their influence on sensory processing are not fully investigated. Here, we identified that ACC neurons projecting to the contralateral claustrum (ACC→contraCLA) preferentially respond to contralateral mechanical sensory stimulation. These sensory responses were enhanced during attending behavior. Optogenetic activation of ACC→contraCLA neurons silenced pyramidal neurons in the contralateral ACC by recruiting local circuit fast-spiking interneuron activation via an excitatory relay in the CLA. This circuit activation suppressed withdrawal behavior to mechanical stimuli ipsilateral to the ACC→contraCLA neurons. Chemogenetic silencing showed that the cross-hemispheric circuit has an important role in the suppression of contralateral nociceptive behavior during sensory-driven attending behavior. Our findings identify a cross-hemispheric cortical-subcortical-cortical arc allowing the brain to give attentional priority to competing innocuous and noxious inputs.

Delayed escape behavior requires claustral activity

Animals are known to exhibit innate and learned forms of defensive behaviors, but it is unclear whether animals can escape through methods other than these forms. In this study, we develop the delayed escape task, in which male rats temporarily hold the information required for future escape, and we demonstrate that this task, in which the subject extrapolates from past experience without direct experience of its behavioral outcome, does not fall into either of the two forms of behavior. During the holding period, a subset of neurons in the rostral-to-striatum claustrum (rsCla), only when pooled together, sustain enhanced population activity without ongoing sensory stimuli. Transient inhibition of rsCla neurons during the initial part of the holding period produces prolonged inhibition of the enhanced activity. The transient inhibition also attenuates the delayed escape behavior. Our data suggest that the rsCla activity bridges escape-inducing stimuli to the delayed onset of escape.

Brain-state-dependent constraints on claustrocortical communication and function

Neural activity in the claustrum has been associated with a range of vigilance states, yet the activity patterns and efficacy of synaptic communication of identified claustrum neurons have not been thoroughly determined. Here, we show that claustrum neurons projecting to the retrosplenial cortex are most active during synchronized cortical states such as non-rapid eye movement (NREM) sleep and are suppressed during increased cortical desynchronization associated with arousal, movement, and REM sleep. The efficacy of claustrocortical signaling is increased during NREM and diminished during movement due in part to increased cholinergic tone. Finally, claustrum activation during NREM sleep enhances memory consolidation through the phase resetting of cortical delta waves. Therefore, claustrocortical communication is constrained to function most effectively during cognitive processes associated with synchronized cortical states, such as memory consolidation.

IntelliCage: the development and perspectives of a mouse- and user-friendly automated behavioral test system

IntelliCage for mice is a rodent home-cage equipped with four corner structures harboring symmetrical double panels for operant conditioning at each of the two sides, either by reward (access to water) or by aversion (non-painful stimuli: air-puffs, LED lights). Corner visits, nose-pokes and actual licks at bottle-nipples are recorded individually using subcutaneously implanted transponders for RFID identification of up to 16 adult mice housed in the same home-cage. This allows for recording individual in-cage activity of mice and applying reward/punishment operant conditioning schemes in corners using workflows designed on a versatile graphic user interface. IntelliCage development had four roots: (i) dissatisfaction with standard approaches for analyzing mouse behavior, including standardization and reproducibility issues, (ii) response to handling and housing animal welfare issues, (iii) the increasing number of mouse models had produced a high work burden on classic manual behavioral phenotyping of single mice. and (iv), studies of transponder-chipped mice in outdoor settings revealed clear genetic behavioral differences in mouse models corresponding to those observed by classic testing in the laboratory. The latter observations were important for the development of home-cage testing in social groups, because they contradicted the traditional belief that animals must be tested under social isolation to prevent disturbance by other group members. The use of IntelliCages reduced indeed the amount of classic testing remarkably, while its flexibility was proved in a wide range of applications worldwide including transcontinental parallel testing. Essentially, two lines of testing emerged: sophisticated analysis of spontaneous behavior in the IntelliCage for screening of new genetic models, and hypothesis testing in many fields of behavioral neuroscience. Upcoming developments of the IntelliCage aim at improved stimulus presentation in the learning corners and videotracking of social interactions within the IntelliCage. Its main advantages are (i) that mice live in social context and are not stressfully handled for experiments, (ii) that studies are not restricted in time and can run in absence of humans, (iii) that it increases reproducibility of behavioral phenotyping worldwide, and (iv) that the industrial standardization of the cage permits retrospective data analysis with new statistical tools even after many years.

A multifaceted architectural framework of the mouse claustrum complex

Accurate anatomical characterizations are necessary to investigate neural circuitry on a fine scale, but for the rodent claustrum complex (CLCX), this has yet to be fully accomplished. The CLCX is generally considered to comprise two major subdivisions, the claustrum (CL) and the dorsal endopiriform nucleus (DEn), but regional boundaries to these areas are debated. To address this, we conducted a multifaceted analysis of fiber- and cytoarchitecture, genetic marker expression, and connectivity using mice of both sexes, to create a comprehensive guide for identifying and delineating borders to CLCX, including an online reference atlas. Our data indicated four distinct subregions within CLCX, subdividing both CL and DEn into two. Additionally, we conducted brain-wide tracing of inputs to CLCX using a transgenic mouse line. Immunohistochemical staining against myelin basic protein (MBP), parvalbumin (PV), and calbindin (CB) revealed intricate fiber-architectural patterns enabling precise delineations of CLCX and its subregions. Myelinated fibers were abundant dorsally in CL but absent ventrally, whereas PV expressing fibers occupied the entire CL. CB staining revealed a central gap within CL, also visible anterior to the striatum. The Nr2f2, Npsr1, and Cplx3 genes expressed specifically within different subregions of the CLCX, and Rprm helped delineate the CL-insular border. Furthermore, cells in CL projecting to the retrosplenial cortex were located within the myelin sparse area. By combining own experimental data with digitally available datasets of gene expression and input connectivity, we could demonstrate that the proposed delineation scheme allows anchoring of datasets from different origins to a common reference framework.

Major Depression and the Perception of Affective Instrumental and Expressive Gestures: An fMRI Investigation

Major depressive disorder (MDD) is associated with biased perception of human movement. Gesture is important for communication and in this study we investigated neural correlates of gesture perception in MDD. We hypothesised different neural activity between individuals with MDD and typical individuals when viewing instrumental and expressive gestures that were negatively or positively valenced. Differences were expected in brain areas associated with gesture perception, including superior temporal, frontal, and emotion processing regions. We recruited 12 individuals with MDD and 12 typical controls matched on age, gender, and handedness. They viewed gestures displayed by stick figures while functional magnetic resonance imaging (fMRI) was performed. Results of a random effects three-way mixed ANOVA indicated that individuals with MDD had greater activity in the right claustrum compared to controls, regardless of gesture type or valence. Additionally, we observed main effects of gesture type and valence, regardless of group. Perceiving instrumental compared to expressive gestures was associated with greater activity in the left cuneus and left superior temporal gyrus, while perceiving negative compared to positive gestures was associated with greater activity in the right precuneus and right lingual gyrus. We also observed a two-way interaction between gesture type and valence in various brain regions.

Pathological claustrum activity drives aberrant cognitive network processing in human chronic pain

Aberrant cognitive network activity and cognitive deficits are established features of chronic pain. However, the nature of cognitive network alterations associated with chronic pain and their underlying mechanisms require elucidation. Here, we report that the claustrum, a subcortical nucleus implicated in cognitive network modulation, is activated by acute painful stimulation and pain-predictive cues in healthy participants. Moreover, we discover pathological activity of the claustrum and a lateral aspect of the right dorsolateral prefrontal cortex (latDLPFC) in migraine patients. Dynamic causal modeling suggests a directional influence of the claustrum on activity in this latDLPFC region, and diffusion weighted imaging (DWI) verifies their structural connectivity. These findings advance understanding of claustrum function during acute pain and provide evidence of a possible circuit mechanism driving cognitive impairments in chronic pain.

The claustrum and consciousness: An update

The seminal paper of Crick and Koch (2005) proposed that the claustrum, an enigmatic and thin grey matter structure that lies beside the insular cortex, may be involved in the processing of consciousness. As a result, this otherwise obscure structure has received ever-increasing interest in the search for neural correlates of consciousness. Here we review theories of consciousness and discuss the possible relationship between the claustrum and consciousness. We review relevant experimental evidence collected since the Crick and Koch (2005) paper and consider whether these findings support or contradict their hypothesis. We also explore how future experimental work can be designed to clarify how consciousness emerges from neural activity and to understand the role of the claustrum in consciousness.

Proteomic Profiling Reveals Specific Molecular Hallmarks of the Pig Claustrum

The present study, employing a comparative proteomic approach, analyzes the protein profile of pig claustrum (CLA), putamen (PU), and insula (IN). Pig brain is an interesting model whose key translational features are its similarities with cortical and subcortical structures of human brain. A greater difference in protein spot expression was observed in CLA vs PU as compared to CLA vs IN. The deregulated proteins identified in CLA resulted to be deeply implicated in neurodegenerative (i.e., sirtuin 2, protein disulfide-isomerase 3, transketolase) and psychiatric (i.e., copine 3 and myelin basic protein) disorders in humans. Metascape analysis of differentially expressed proteins in CLA vs PU comparison suggested activation of the α-synuclein pathway and L1 recycling pathway corroborating the involvement of these anatomical structures in neurodegenerative diseases. The expression of calcium/calmodulin-dependent protein kinase and dihydropyrimidinase like 2, which are linked to these pathways, was validated using western blot analysis. Moreover, the protein data set of CLA vs PU comparison was analyzed by Ingenuity Pathways Analysis to obtain a prediction of most significant canonical pathways, upstream regulators, human diseases, and biological functions. Interestingly, inhibition of presenilin 1 (PSEN1) upstream regulator and activation of endocannabinoid neuronal synapse pathway were observed. In conclusion, this is the first study presenting an extensive proteomic analysis of pig CLA in comparison with adjacent areas, IN and PUT. These results reinforce the common origin of CLA and IN and suggest an interesting involvement of CLA in endocannabinoid circuitry, neurodegenerative, and psychiatric disorders in humans.

Origin and development of the claustrum in rhesus macaque

Understanding the claustrum's functions has recently progressed thanks to new anatomical and behavioral studies in rodents, which suggest that it plays an important role in attention, salience detection, slow-wave generation, and neocortical network synchronization. Nevertheless, knowledge about the origin and development of the claustrum, especially in primates, is still limited. Here, we show that neurons of rhesus macaque claustrum primordium are generated between embryonic day E48 and E55 and express some neocortical molecular markers, such as NR4A2, SATB2, and SOX5. However, in the early stages, it lacks TBR1 expression, which separates it from other surrounding telencephalic structures. We also found that two waves of neurogenesis (E48 and E55) in the claustrum, corresponding to the birthdates of layers 6 and 5 of the insular cortex, establish a "core" and "shell" cytoarchitecture, which is potentially a basis for differential circuit formation and could influence information processing underlying higher cognitive functions of the claustrum. In addition, parvalbumin-positive interneurons are the dominant interneuron type in the claustrum in fetal macaque, and their maturation is independent of that in the overlaying neocortex. Finally, our study reveals that the claustrum is likely not a continuance of subplate neurons of the insular cortex, but an independent pallial region, suggesting its potentially unique role in cognitive control.

Claustral neurons projecting to frontal cortex restrict opioid consumption

The synthetic opioid fentanyl is a major contributor to the current opioid addiction crisis. We report that claustral neurons projecting to the frontal cortex limit oral fentanyl self-administration in mice. We found that fentanyl transcriptionally activates frontal-projecting claustrum neurons. These neurons also exhibit a unique suppression of Ca²⁺ activity upon initiation of bouts of fentanyl consumption. Optogenetic stimulation of frontal-projecting claustral neurons, intervening in this suppression, decreased bouts of fentanyl consumption. In contrast, constitutive inhibition of frontal-projecting claustral neurons in the context of a novel, group-housed self-administration procedure increased fentanyl bout consumption. This same manipulation also sensitized conditioned-place preference for fentanyl and enhanced the representation of fentanyl experience in the frontal cortex. Together, our results indicate that claustrum neurons exert inhibitory control over frontal cortical neurons to restrict oral fentanyl intake. Upregulation of activity in the claustro-frontal projection may be a promising strategy for reducing human opioid addiction.

Dopamine receptor 1 on CaMKII-positive neurons within claustrum mediates adolescent cocaine exposure-induced anxiety-like behaviors and electro-acupuncture therapy

Adolescent cocaine exposure (ACE) increases risk of developing psychiatric problems such as anxiety, which may drive relapse in later life, however, its underlying molecular mechanism remains poorly understood. Methods: ACE male mice model were established by exposing to cocaine during adolescent period. Elevated plus maze (EPM) were used to assess anxiety-like behaviors in mice. Within claustrum, local injection of SCH-23390, a specific antagonist for dopamine receptor 1 (D1R), or D1R knocking-down virus were used to regulate D1R function or expression on CaMKII-positive neurons (D1RCaMKII) in vivo. Electro-acupuncture (EA) treatment was performed at acupoints of Baihui and Yintang during withdrawal period. Results: We found that ACE mice exhibited anxiety-like behaviors, along with more activated CaMKII-positive neurons and increased D1RCaMKII levels in claustrum during adulthood. Inhibiting D1R function or knocking-down D1RCaMKII levels in claustrum efficiently reduced claustrum activation, and ultimately suppressed anxiety-like behaviors in ACE mice during adulthood. EA treatment alleviated ACE-evoked claustrum activation and anxiety-like behaviors by suppressing claustrum D1RCaMKII. Conclusion: Our findings identified a novel role of claustrum in ACE-induced anxiety-like behaviors, and put new insight into the D1RCaMKII in the claustrum. The claustrum D1RCaMKII might be a promising pharmacological target, such as EA treatment, to treat drug-induced anxiety-like behaviors.

Temporal origin of mouse claustrum and development of its cortical projections

The claustrum is known for its extensive connectivity with many other forebrain regions, but its elongated shape and deep location have made further study difficult. We have sought to understand when mouse claustrum neurons are born, where they are located in developing brains, and when they develop their widespread connections to the cortex. We established that a well-characterized parvalbumin plexus, which identifies the claustrum in adults, is only present from postnatal day (P) 21. A myeloarchitectonic outline of the claustrum can be derived from a triangular fiber arrangement from P15. A dense patch of Nurr1+ cells is present at its core and is already evident at birth. Bromodeoxyuridine birth dating of forebrain progenitors reveals that the majority of claustrum neurons are born during a narrow time window centered on embryonic day 12.5, which is later than the adjacent subplate and endopiriform nucleus. Retrograde tracing revealed that claustrum projections to anterior cingulate (ACA) and retrosplenial cortex (RSP) follow distinct developmental trajectories. Claustrum-ACA connectivity matures rapidly and reaches adult-like innervation density by P10, whereas claustrum-RSP innervation emerges later over a protracted time window. This work establishes the timeline of claustrum development and provides a framework for understanding how the claustrum is built and develops its unique connectivity.

Differentiation of claustrum resting-state functional connectivity in healthy aging, Alzheimer's disease, and Parkinson's disease

The claustrum is a sheet-like of telencephalic gray matter structure whose function is poorly understood. The claustrum is considered a multimodal computing network due to its reciprocal connections with almost all cortical areas as well as subcortical structures. Although the claustrum has been involved in several neurodegenerative diseases, specific changes in connections of the claustrum remain unclear in Alzheimer's disease (AD), and Parkinson's disease (PD). Resting-state fMRI and T1-weighted structural 3D images from healthy elderly (n = 15), AD (n = 16), and PD (n = 12) subjects were analyzed. Seed-based FC analysis was performed using CONN FC toolbox and T1-weighted images were analyzed with the Computational Anatomy Toolbox for voxel-based morphometry analysis. While we observed a decreased FC between the left claustrum and sensorimotor cortex, auditory association cortex, and cortical regions associated with social cognition in PD compared with the healthy control group (HC), no significant difference was found in alterations in the FC of both claustrum comparing the HC and AD groups. In the AD group, high FC of claustrum with regions of sensorimotor cortex and cortical regions related to cognitive control, including cingulate gyrus, supramarginal gyrus, and insular cortex were demonstrated. In addition, the structural results show significantly decreased volume in bilateral claustrum in AD and PD compared with HC. There were no significant differences in the claustrum volumes between PD and AD groups so the FC may offer more precise findings in distinguishing changes for claustrum in AD and PD.

Regional and cell-type-specific afferent and efferent projections of the mouse claustrum

The claustrum (CLA) is a conspicuous subcortical structure interconnected with cortical and subcortical regions. Its regional anatomy and cell-type-specific connections in the mouse remain not fully determined. Using multimodal reference datasets, we confirmed the delineation of the mouse CLA as a single group of neurons embedded in the agranular insular cortex. We quantitatively investigated brain-wide inputs and outputs of CLA using bulk anterograde and retrograde viral tracing data and single neuron tracing data. We found that the prefrontal module has more cell types projecting to the CLA than other cortical modules, with layer 5 IT neurons predominating. We found nine morphological types of CLA principal neurons that topographically innervate functionally linked cortical targets, preferentially the midline cortical areas, secondary motor area, and entorhinal area. Together, this study provides a detailed wiring diagram of the cell-type-specific connections of the mouse CLA, laying a foundation for studying its functions at the cellular level.

Influence of claustrum on cortex varies by area, layer, and cell type

The claustrum is a small subcortical structure with widespread connections to disparate regions of the cortex. However, the impact of the claustrum on cortical activity is not fully understood, particularly beyond frontal areas. Here, using optogenetics and multi-regional Neuropixels recordings from over 15,000 cortical neurons in awake mice, we demonstrate that the effect of claustrum input to the cortex differs depending on brain area, layer, and cell type. Brief claustrum stimulation, producing approximately 1 spike per claustrum neuron, affects many fast spiking (FS; putative inhibitory) but relatively fewer regular-spiking (RS; putative excitatory) cortical neurons and leads to a modest decrease in population activity in frontal cortical areas. Prolonged claustrum stimulation affects many more cortical neurons and can increase or decrease spiking activity. More excitation occurs in posterior regions and superficial layers, while inhibition predominates in frontal regions and deeper layers. These findings suggest that claustro-cortical circuits are organized into functional modules.

The cognitive dysfunction of claustrum on Alzheimer's disease: A mini-review

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases characterized by cognitive deficits and dementia. AD entails predominant pathological characteristics including amyloid beta (Aβ) plaque formation, neurofibrillary entanglements, and brain atrophy, which gradually result in cognitive dysfunctions. Studies showed that these pathological changes are found in a myriad of brain structures, including the claustrum (CLA), a nucleus that penetrates deeply into the brain and is extensively interconnected to various brain structures. The CLA modulates many aspects of cognitive functions, with attention, executive function, visuospatial ability, language, and memory in particular. It is also implicated in multiple neuropsychiatric disorders, of which one worthy of particular attention is AD-related cognitive impairments. To inspire novel AD treatment strategies, this review has summarized the CLA functionality in discriminative cognitive dysfunctions in AD. And then propose an array of potential mechanisms that might contribute to the cognitive impairments caused by an abnormal CLA physiology. We advocate that the CLA might be a new promising therapeutic target in combination with existing anti-AD drugs and brain stimulation approaches for future AD treatment.

Claustral MeCP2 Regulates Methamphetamine-induced Conditioned Place Preference in Cynomolgus Monkey

The claustrum, a brain nucleus located between the cortex and the striatum, has recently been highlighted in drug-related reward processing. Methyl CpG-binding protein-2 (MeCP2) is a transcriptional regulator that represses or activates the expression of the target gene and has been known to have an important role in the regulation of drug addiction in the dopaminergic reward system. The claustrum is an important region for regulating reward processing where most neurons receive dopamine input; additionally, in this region, MeCP2 is also abundantly expressed. Therefore, here, we hypothesized that MeCP2 would be involved in drug addiction control in the Claustrum as well and investigated how claustral MeCP2 regulates drug addiction. To better understand the function of human claustral MeCP2, we established a non-human primate model of methamphetamine (METH) - induced conditioned place preference (CPP). After a habituation of two days and conditioning of ten days, the CPP test was conducted for three days. Interestingly, we confirmed that virus-mediated overexpression of MECP2 in the claustrum showed a significant reduction of METH-induced CPP in the three consecutive days during the testing period. Moreover, they showed a decrease in visit scores (frequency for visit) for the METH-paired room compared to the control group although the scores were statistically marginal. Taken together, we suggest that the claustrum is an important brain region associated with drug addiction, in which MeCP2 may function as a mediator in regulating the response to addictive drugs.

The mouse claustrum synaptically connects cortical network motifs

Spatially distant areas of the cerebral cortex coordinate their activity into networks that are integral to cognitive processing. A common structural motif of cortical networks is co-activation of frontal and posterior cortical regions. The neural circuit mechanisms underlying such widespread inter-areal cortical coordination are unclear. Using a discovery based functional magnetic resonance imaging (fMRI) approach in mouse, we observe frontal and posterior cortical regions that demonstrate significant functional connectivity with the subcortical nucleus, the claustrum. Examining whether the claustrum synaptically supports such frontoposterior cortical network architecture, we observe cortico-claustro-cortical circuits reflecting the fMRI data: significant trans-claustral synaptic connectivity from frontal cortices to posteriorly lying sensory and sensory association cortices contralaterally. These data reveal discrete cortical pathways through the claustrum that are positioned to support cortical network motifs central to cognitive control functions and add to the canon of major extended cortico-subcortico-cortical systems in the mammalian brain.

Acute social defeat stress activated neurons project to the claustrum and basolateral amygdala

We recently reported that a neuronal population in the claustrum (CLA) identified under exposure to psychological stressors plays a key role in stress response processing. Upon stress exposure, the main inputs to the CLA come from the basolateral amygdala (BLA); however, the upstream brain regions that potentially regulate both the CLA and BLA during stressful experiences remain unclear. Here by combining activity-dependent viral retrograde labeling with whole brain imaging, we analyzed neurons projecting to the CLA and BLA activated by exposure to social defeat stress. The labeled CLA projecting neurons were mostly ipsilateral, excluding the prefrontal cortices, which had a distinctly labeled population in the contralateral hemisphere. Similarly, the labeled BLA projecting neurons were predominantly ipsilateral, aside from the BLA in the opposite hemisphere, which also had a notably labeled population. Moreover, we found co-labeled double-projecting single neurons in multiple brain regions such as the ipsilateral ectorhinal/perirhinal cortex, entorhinal cortex, and the contralateral BLA. These results suggest that CLA and BLA receive inputs from neuron collaterals in various brain regions during stress, which may regulate the CLA and BLA forming in a stress response circuitry.

Functional properties of eyelid conditioned responses and involved brain centers

For almost a century the classical conditioning of nictitating membrane/eyelid responses has been used as an excellent and feasible experimental model to study how the brain organizes the acquisition, storage, and retrieval of new motor abilities in alert behaving mammals, including humans. Lesional, pharmacological, and electrophysiological approaches, and more recently, genetically manipulated animals have shown the involvement of numerous brain areas in this apparently simple example of associative learning. In this regard, the cerebellum (both cortex and nuclei) has received particular attention as a putative site for the acquisition and storage of eyelid conditioned responses, a proposal not fully accepted by all researchers. Indeed, the acquisition of this type of learning implies the activation of many neural processes dealing with the sensorimotor integration and the kinematics of the acquired ability, as well as with the attentional and cognitive aspects also involved in this process. Here, we address specifically the functional roles of three brain structures (red nucleus, cerebellar interpositus nucleus, and motor cortex) mainly involved in the acquisition and performance of eyelid conditioned responses and three other brain structures (hippocampus, medial prefrontal cortex, and claustrum) related to non-motor aspects of the acquisition process. The main conclusion is that the acquisition of this motor ability results from the contribution of many cortical and subcortical brain structures each one involved in specific (motor and cognitive) aspects of the learning process.

A role for the claustrum in cognitive control

Early hypotheses of claustrum function were fueled by neuroanatomical data and yielded suggestions that the claustrum is involved in processes ranging from salience detection to multisensory integration for perceptual binding. While these hypotheses spurred useful investigations, incompatibilities inherent in these views must be reconciled to further conceptualize claustrum function amid a wealth of new data. Here, we review the varied models of claustrum function and synthesize them with developments in the field to produce a novel functional model: network instantiation in cognitive control (NICC). This model proposes that frontal cortices direct the claustrum to flexibly instantiate cortical networks to subserve cognitive control. We present literature support for this model and provide testable predictions arising from this conceptual framework.

Missing pieces in decoding the brain oxytocin puzzle: Functional insights from mouse brain wiring diagrams

The hypothalamic neuropeptide, oxytocin (Oxt), has been the focus of research for decades due to its effects on body physiology, neural circuits, and various behaviors. Oxt elicits a multitude of actions mainly through its receptor, the Oxt receptor (OxtR). Despite past research to understand the central projections of Oxt neurons and OxtR- coupled signaling pathways in different brain areas, it remains unclear how this nonapeptide exhibits such pleiotropic effects while integrating external and internal information. Most reviews in the field either focus on neuroanatomy of the Oxt-OxtR system, or on the functional effects of Oxt in specific brain areas. Here, we provide a review by integrating brain wide connectivity of Oxt neurons and their downstream circuits with OxtR expression in mice. We categorize Oxt connected brain regions into three functional modules that regulate the internal state, somatic visceral, and cognitive response. Each module contains three neural circuits that process distinct behavioral effects. Broad innervations on functional circuits (e.g., basal ganglia for motor behavior) enable Oxt signaling to exert coordinated modulation in functionally inter-connected circuits. Moreover, Oxt acts as a neuromodulator of neuromodulations to broadly control the overall state of the brain. Lastly, we discuss the mismatch between Oxt projections and OxtR expression across various regions of the mouse brain. In summary, this review brings forth functional circuit-based analysis of Oxt connectivity across the whole brain in light of Oxt release and OxtR expression and provides a perspective guide to future studies.

The Genetic and Neural Substrates of Externalizing Behavior

Background

To gain more insight into the biological factors that mediate vulnerability to display externalizing behaviors, we leveraged genome-wide association study summary statistics on 13 externalizing phenotypes.

Methods

After data classification based on genetic resemblance, we performed multivariate genome-wide association meta-analyses and conducted extensive bioinformatic analyses, including genetic correlation assessment with other traits, Mendelian randomization, and gene set and gene expression analyses.

Results

The genetic data could be categorized into disruptive behavior (DB) and risk-taking behavior (RTB) factors, and subsequent genome-wide association meta-analyses provided association statistics for DB and RTB (N _eff = 523,150 and 1,506,537, respectively), yielding 50 and 257 independent genetic signals. The statistics of DB, much more than RTB, signaled genetic predisposition to adverse cognitive, mental health, and personality outcomes. We found evidence for bidirectional causal influences between DB and substance use behaviors. Gene set analyses implicated contributions of neuronal cell development (DB/RTB) and synapse formation and transcription (RTB) mechanisms. Gene-brain mapping confirmed involvement of the amygdala and hypothalamus and highlighted other candidate regions (cerebellar dentate, cuneiform nucleus, claustrum, paracentral cortex). At the cell-type level, we noted enrichment of glutamatergic neurons for DB and RTB.

Conclusions

This bottom-up, data-driven study provides new insights into the genetic signals of externalizing behaviors and indicates that commonalities in genetic architecture contribute to the frequent co-occurrence of different DBs and different RTBs, respectively. Bioinformatic analyses supported the DB versus RTB categorization and indicated relevant biological mechanisms. Generally similar gene-brain mappings indicate that neuroanatomical differences, if any, escaped the resolution of our methods.

Footshock-induced activation of the claustrum-entorhinal cortical pathway in freely moving mice

Footshock is frequently used as an unconditioned stimulus in fear conditioning behavior studies. The medial entorhinal cortex (MEC) contributes to fear learning and receives neuronal inputs from the claustrum. However, whether footshocks can induce a neuronal response in claustrum-MEC (CLA-MEC) projection remains unknown. Here, we combined fiber-based Ca2+ recordings with a retrograde AAV labeling method to investigate neuronal responses of MEC-projecting claustral neurons to footshock stimulation in freely moving mice. We achieved successful Ca2+ recordings in both anesthetized and freely exploring mice. We found that footshock stimulation reliably induced neuronal responses to MEC-projecting claustral neurons. Therefore, the footshock-induced response detected in the CLA-MEC projection suggests its potential role in fear processin.

Pharmacological, neural, and psychological mechanisms underlying psychedelics: A critical review

This paper provides a critical review of several possible mechanisms at different levels of analysis underlying the effects and therapeutic potential of psychedelics. At the (1) biochemical level, psychedelics primarily affect the 5-HT_2A receptor, increase neuroplasticity, offer a critical period for social reward learning, and have anti-inflammatory properties. At the (2) neural level, psychedelics have been associated with reduced efficacy of thalamo-cortical filtering, the loosening of top-down predictive signaling and an increased sensitivity to bottom-up prediction errors, and activation of the claustro-cortical-circuit. At the (3) psychological level, psychedelics have been shown to induce altered and affective states, they affect cognition, induce belief change, exert social effects, and can result in lasting changes in behavior. We outline the potential for a unifying account of the mechanisms underlying psychedelics and contrast this with a model of pluralistic causation. Ultimately, a better understanding of the specific mechanisms underlying the effects of psychedelics could allow for a more targeted therapeutic approach. We highlight current challenges for psychedelic research and provide a research agenda to foster insight in the causal-mechanistic pathways underlying the efficacy of psychedelic research and therapy.

Bidirectionally connected cores in a mouse connectome: towards extracting the brain subnetworks essential for consciousness

Where in the brain consciousness resides remains unclear. It has been suggested that the subnetworks supporting consciousness should be bidirectionally (recurrently) connected because both feed-forward and feedback processing are necessary for conscious experience. Accordingly, evaluating which subnetworks are bidirectionally connected and the strength of these connections would likely aid the identification of regions essential to consciousness. Here, we propose a method for hierarchically decomposing a network into cores with different strengths of bidirectional connection, as a means of revealing the structure of the complex brain network. We applied the method to a whole-brain mouse connectome. We found that cores with strong bidirectional connections consisted of regions presumably essential to consciousness (e.g. the isocortical and thalamic regions, and claustrum) and did not include regions presumably irrelevant to consciousness (e.g. cerebellum). Contrarily, we could not find such correspondence between cores and consciousness when we applied other simple methods that ignored bidirectionality. These findings suggest that our method provides a novel insight into the relation between bidirectional brain network structures and consciousness.

Brain-wide screen of prelimbic cortex inputs reveals a functional shift during early fear memory consolidation

Memory formation and storage rely on multiple interconnected brain areas, the contribution of which varies during memory consolidation. The medial prefrontal cortex, in particular the prelimbic cortex (PL), was traditionally found to be involved in remote memory storage, but recent evidence points toward its implication in early consolidation as well. Nevertheless, the inputs to the PL governing these dynamics remain unknown. Here, we first performed a brain-wide, rabies-based retrograde tracing screen of PL engram cells activated during contextual fear memory formation in male mice to identify relevant PL input regions. Next, we assessed the specific activity pattern of these inputs across different phases of memory consolidation, from fear memory encoding to recent and remote memory recall. Using projection-specific chemogenetic inhibition, we then tested their functional role in memory consolidation, which revealed a hitherto unknown contribution of claustrum to PL inputs at encoding, and of insular cortex to PL inputs at recent memory recall. Both of these inputs further impacted how PL engram cells were reactivated at memory recall, testifying to their relevance for establishing a memory trace in the PL. Collectively, these data identify a spatiotemporal shift in PL inputs important for early memory consolidation, and thereby help to refine the working model of memory formation.

Topologically Organized Networks in the Claustrum Reflect Functional Modularization

Using genetic strategies and viral-based directional tracers, we investigated the topological location and output networks of claustrum (CLA) neuron populations projecting to either the retrosplenial cortex, primary motor cortex, or basolateral amygdala. We found that all three CLA neuron populations clearly reside in distinct topological locations within the CLA complex and project broadly to multiple downstream targets. Each neuron population projects to different targets, suggesting that each CLA subzone coordinates a unique set of brain-wide functions. Our findings establish that the claustrum complex encompasses at least three minimally overlapping networks that are compartmentalized into different topological subzones. Such modularity is likely to be important for CLA function.

Ca2+-based neural activity recording for rapidly screening behavioral correlates of the claustrum in freely behaving mice

The claustrum has been hypothesized to participate in high-order brain functions, but experimental studies to demonstrate these functions are currently lacking. Neural activity recording of the claustrum in freely-behaving animals allows for correlating claustral activities with specific behaviors. However, previously utilized methods for studying the claustrum make it difficult to monitor neural activity patterns of freely-behaving animals in real time. Here we applied fiber photometry to monitor Ca²⁺ activity in the claustrum of freely-behaving mice. Using this method, we were able to achieve Ca²⁺ activity recording in both anesthetized and freely-behaving mice. We found that the dynamics of Ca²⁺ activity depended on anesthesia levels. As compared to the use of genetically encoded Ca²⁺ indicators that requires a few weeks of virus-dependent expression, we used a synthetic fluorescent Ca²⁺-sensitive dye, Oregon green 488 BAPTA-1, that allows for rapidly screening neural activity of interest within a few hours that relates to certain behaviors. In this way, we found the correlation between Ca²⁺ activity and specific behaviors, such as approaching an object. Our work offers an effective method for recording neural activity in the claustrum and thus for rapidly screening any behavioral relevance of the claustrum in freely-behaving mice.

Human lesions and animal studies link the claustrum to perception, salience, sleep and pain

The claustrum is the most densely interconnected region in the human brain. Despite the accumulating data from clinical and experimental studies, the functional role of the claustrum remains unknown. Here, we systematically review claustrum lesion studies and discuss their functional implications. Claustral lesions are associated with an array of signs and symptoms, including changes in cognitive, perceptual and motor abilities; electrical activity; mental state; and sleep. The wide range of symptoms observed following claustral lesions do not provide compelling evidence to support prominent current theories of claustrum function such as multisensory integration or salience computation. Conversely, the lesions studies support the hypothesis that the claustrum regulates cortical excitability. We argue that the claustrum is connected to, or part of, multiple brain networks that perform both fundamental and higher cognitive functions. As a multifunctional node in numerous networks, this may explain the manifold effects of claustrum damage on brain and behaviour.

Models of psychedelic drug action: modulation of cortical-subcortical circuits

Classic psychedelic drugs such as psilocybin and lysergic acid diethylamide (LSD) have recaptured the imagination of both science and popular culture, and may have efficacy in treating a wide range of psychiatric disorders. Human and animal studies of psychedelic drug action in the brain have demonstrated the involvement of the serotonin 2A (5-HT2A) receptor and the cerebral cortex in acute psychedelic drug action, but different models have evolved to try to explain the impact of 5-HT2A activation on neural systems. Two prominent models of psychedelic drug action (the cortico-striatal thalamo-cortical, or CSTC, model and relaxed beliefs under psychedelics, or REBUS, model) have emphasized the role of different subcortical structures as crucial in mediating psychedelic drug effects. We describe these models and discuss gaps in knowledge, inconsistencies in the literature and extensions of both models. We then introduce a third circuit-level model involving the claustrum, a thin strip of grey matter between the insula and the external capsule that densely expresses 5-HT2A receptors (the cortico-claustro-cortical, or CCC, model). In this model, we propose that the claustrum entrains canonical cortical network states, and that psychedelic drugs disrupt 5-HT2A-mediated network coupling between the claustrum and the cortex, leading to attenuation of canonical cortical networks during psychedelic drug effects. Together, these three models may explain many phenomena of the psychedelic experience, and using this framework, future research may help to delineate the functional specificity of each circuit to the action of both serotonergic and non-serotonergic hallucinogens.

Differential distribution of inhibitory neuron types in subregions of claustrum and dorsal endopiriform nucleus of the short-tailed fruit bat

Few brain regions have such wide-ranging inputs and outputs as the claustrum does, and fewer have posed equivalent challenges in defining their structural boundaries. We studied the distributions of three calcium-binding proteins-calretinin, parvalbumin, and calbindin-in the claustrum and dorsal endopiriform nucleus of the fruit bat, Carollia perspicillata. The proportionately large sizes of claustrum and dorsal endopiriform nucleus in Carollia brain afford unique access to these structures' intrinsic anatomy. Latexin immunoreactivity permits a separation of claustrum into core and shell subregions and an equivalent separation of dorsal endopiriform nucleus. Using latexin labeling, we found that the claustral shell in Carollia brain can be further subdivided into at least four distinct subregions. Calretinin and parvalbumin immunoreactivity reinforced the boundaries of the claustral core and its shell subregions with diametrically opposite distribution patterns. Calretinin, parvalbumin, and calbindin all colocalized with GAD67, indicating that these proteins label inhibitory neurons in both claustrum and dorsal endopiriform nucleus. Calretinin, however, also colocalized with latexin in a subset of neurons. Confocal microscopy revealed appositions that suggest synaptic contacts between cells labeled for each of the three calcium-binding proteins and latexin-immunoreactive somata in claustrum and dorsal endopiriform nucleus. Our results indicate significant subregional differences in the intrinsic inhibitory connectivity within and between claustrum and dorsal endopiriform nucleus. We conclude that the claustrum is structurally more complex than previously appreciated and that claustral and dorsal endopiriform nucleus subregions are differentially modulated by multiple inhibitory systems. These findings can also account for the excitability differences between claustrum and dorsal endopiriform nucleus described previously.

Premotor activity in the claustrum

In this issue of Neuron, Chevée et al. (2022) performed extracellular electrophysiological recordings from claustrum neurons during a sensory selection task. They found that neural activity in the claustrum reflected future motor output rather than sensory inputs and that chemogenetic suppression of claustrum activity reduced motor impulsivity in this task.

Neural activity in the mouse claustrum in a cross-modal sensory selection task

The claustrum, a subcortical nucleus forming extensive connections with the neocortex, has been implicated in sensory selection. Sensory-evoked claustrum activity is thought to modulate the neocortex's context-dependent response to sensory input. Recording from claustrum neurons while mice performed a tactile-visual sensory-selection task, we found that neurons in the anterior claustrum, including putative optotagged claustrocortical neurons projecting to the primary somatosensory cortex (S1), were rarely modulated by sensory input. Rather, they exhibited different types of direction-tuned motor responses. Furthermore, we found that claustrum neurons encoded upcoming movement during intertrial intervals and that pairs of claustrum neurons exhibiting synchronous firing were enriched for pairs preferring contralateral lick directions, suggesting that the activity of specific ensembles of similarly tuned claustrum neurons may modulate cortical activity. Chemogenetic inhibition of claustrocortical neurons decreased lick responses to inappropriate sensory stimuli. Altogether, our data indicate that the claustrum is integrated into higher-order premotor circuits recently implicated in decision-making.

Layer 6A Pyramidal Cell Subtypes Form Synaptic Microcircuits with Distinct Functional and Structural Properties

Neocortical layer 6 plays a crucial role in sensorimotor co-ordination and integration through functionally segregated circuits linking intracortical and subcortical areas. We performed whole-cell recordings combined with morphological reconstructions to identify morpho-electric types of layer 6A pyramidal cells (PCs) in rat barrel cortex. Cortico-thalamic (CT), cortico-cortical (CC), and cortico-claustral (CCla) PCs were classified based on their distinct morphologies and have been shown to exhibit different electrophysiological properties. We demonstrate that these three types of layer 6A PCs innervate neighboring excitatory neurons with distinct synaptic properties: CT PCs establish weak facilitating synapses onto other L6A PCs; CC PCs form synapses of moderate efficacy, while synapses made by putative CCla PCs display the highest release probability and a marked short-term depression. For excitatory-inhibitory synaptic connections in layer 6, both the presynaptic PC type and the postsynaptic interneuron type govern the dynamic properties of the respective synaptic connections. We have identified a functional division of local layer 6A excitatory microcircuits which may be responsible for the differential temporal engagement of layer 6 feed-forward and feedback networks. Our results provide a basis for further investigations on the long-range CC, CT, and CCla pathways.

Psychedelics and Consciousness: Distinctions, Demarcations, and Opportunities

Psychedelic substances produce unusual and compelling changes in conscious experience that have prompted some to propose that psychedelics may provide unique insights explaining the nature of consciousness. At present, psychedelics, like other current scientific tools and methods, seem unlikely to provide information relevant to the so-called "hard problem of consciousness," which involves explaining how first-person experience can emerge. However, psychedelics bear on multiple "easy problems of consciousness," which involve relations between subjectivity, brain function, and behavior. In this review, we discuss common meanings of the term "consciousness" when used with regard to psychedelics and consider some models of the effects of psychedelics on the brain that have also been associated with explanatory claims about consciousness. We conclude by calling for epistemic humility regarding the potential for psychedelic research to aid in explaining the hard problem of consciousness while pointing to ways in which psychedelics may advance the study of many specific aspects of consciousness.

Five Breakthroughs: A First Approximation of Brain Evolution From Early Bilaterians to Humans

Retracing the evolutionary steps by which human brains evolved can offer insights into the underlying mechanisms of human brain function as well as the phylogenetic origin of various features of human behavior. To this end, this article presents a model for interpreting the physical and behavioral modifications throughout major milestones in human brain evolution. This model introduces the concept of a "breakthrough" as a useful tool for interpreting suites of brain modifications and the various adaptive behaviors these modifications enabled. This offers a unique view into the ordered steps by which human brains evolved and suggests several unique hypotheses on the mechanisms of human brain function.

Spatially patterned excitatory neuron subtypes and projections of the claustrum

The claustrum is a functionally and structurally complex brain region, whose very spatial extent remains debated. Histochemical-based approaches typically treat the claustrum as a relatively narrow anatomical region that primarily projects to the neocortex, whereas circuit-based approaches can suggest a broader claustrum region containing projections to the neocortex and other regions. Here, in the mouse, we took a bottom-up and cell-type-specific approach to complement and possibly unite these seemingly disparate conclusions. Using single-cell RNA-sequencing, we found that the claustrum comprises two excitatory neuron subtypes that are differentiable from the surrounding cortex. Multicolor retrograde tracing in conjunction with 12-channel multiplexed in situ hybridization revealed a core-shell spatial arrangement of these subtypes, as well as differential downstream targets. Thus, the claustrum comprises excitatory neuron subtypes with distinct molecular and projection properties, whose spatial patterns reflect the narrower and broader claustral extents debated in previous research. This subtype-specific heterogeneity likely shapes the functional complexity of the claustrum.

The mystery of claustral neural circuits and recent updates on its role in neurodegenerative pathology

INTRODUCTION

The claustrum is a structure involved in formation of several cortical and subcortical neural microcircuits which may be involved in such functions as conscious sensations and rewarding behavior. The claustrum is regarded as a multi-modal information processing network. Pathology of the claustrum is seen in certain neurological disorders. To date, there are not enough comprehensive studies that contain accurate information regarding involvement of the claustrum in development of neurological disorders.

OBJECTIVE

Our review aims to provide an update on claustrum anatomy, ontogenesis, cytoarchitecture, neural networks and their functional relation to the incidence of neurological diseases.

MATERIALS AND METHODS

A literature review was conducted using the Google Scholar, PubMed, NCBI MedLine, and eLibrary databases.

RESULTS

Despite new methods that have made it possible to study the claustrum at the molecular, genetic and epigenetic levels, its functions and connectivity are still poorly understood. The anatomical location, relatively uniform cytoarchitecture, and vast network of connections suggest a divergent role of the claustrum in integration and processing of input information and formation of coherent perceptions. Several studies have shown changes in the appearance, structure and volume of the claustrum in neurodegenerative diseases, such as Parkinson's disease (PD), Alzheimer's disease (AD), autism, schizophrenia, and depressive disorders. Taking into account the structure, ontogenesis, and functions of the claustrum, this literature review offers insight into understanding the crucial role of this structure in brain function and behavior.

Organization of feedback projections to mouse primary visual cortex

Top-down, context-dependent modulation of visual processing has been a topic of wide interest, including in mouse primary visual cortex (V1). However, the organization of feedback projections to V1 is relatively unknown. Here, we investigated inputs to mouse V1 by injecting retrograde tracers. We developed a software pipeline that maps labeled cell bodies to corresponding brain areas in the Allen Reference Atlas. We identified more than 24 brain areas that provide inputs to V1 and quantified the relative strength of their projections. We also assessed the organization of the projections, based on either the organization of cell bodies in the source area (topography) or the distribution of projections across V1 (bias). Projections from most higher visual and some nonvisual areas to V1 showed both topography and bias. Such organization of feedback projections to V1 suggests that parts of the visual field are differentially modulated by context, which can be ethologically relevant for a navigating animal.

Changing the Cortical Conductor's Tempo: Neuromodulation of the Claustrum

The claustrum is a thin sheet of neurons that is densely connected to many cortical regions and has been implicated in numerous high-order brain functions. Such brain functions arise from brain states that are influenced by neuromodulatory pathways from the cholinergic basal forebrain, dopaminergic substantia nigra and ventral tegmental area, and serotonergic raphe. Recent revelations that the claustrum receives dense input from these structures have inspired investigation of state-dependent control of the claustrum. Here, we review neuromodulation in the claustrum-from anatomical connectivity to behavioral manipulations-to inform future analyses of claustral function.

Neuroscience: A Role for the Claustrum in Drug Reward

The claustrum is a poorly understood but intriguing part of the brain: a new study has found that it plays an important role in drug reward by providing incentive salience to the location where the drug is administered.

Claustral Input to the Macaque Medial Posterior Parietal Cortex (Superior Parietal Lobule and Adjacent Areas)

The projections from the claustrum to cortical areas within and adjacent to the superior parietal lobule were studied in 10 macaque monkeys, using retrograde tracers, computerized reconstructions, and quantitative methods. In contrast with the classical view that posterior parietal areas receive afferents primarily from the dorsal and posterior regions of the claustrum, we found that these areas receive more extensive projections, including substantial afferents from the anterior and ventral regions of the claustrum. Moreover, our findings uncover a previously unsuspected variability in the precise regions of the claustrum that originate the projections, according to the target areas. For example, areas dominated by somatosensory inputs for control of body movements tend to receive most afferents from the dorsal-posterior claustrum, whereas those which also receive significant visual inputs tend to receive more afferents from the ventral claustrum. In addition, different areas within these broadly defined groups differ in terms of quantitative emphasis in the origin of projections. Overall, these results argue against a simple model whereby adjacency in the cortex determines adjacency in the sectors of claustral origin of projections and indicate that subnetworks defined by commonality of function may be an important factor in defining claustrocortical topography.

Topographic gradients define the projection patterns of the claustrum core and shell in mice

The claustrum is densely connected to the cortex and participates in brain functions such as attention and sleep. Although some studies have reported the widely divergent organization of claustrum projections, others describe parallel claustrocortical connections to different cortical regions. Therefore, the details underlying how claustrum neurons broadcast information to cortical networks remain incompletely understood. Using multicolor retrograde tracing we determined the density, topography, and co-projection pattern of 14 claustrocortical pathways, in mice. We spatially registered these pathways to a common coordinate space and found that the claustrocortical system is topographically organized as a series of overlapping spatial modules, continuously distributed across the dorsoventral claustrum axis. The claustrum core projects predominantly to frontal-midline cortical regions, whereas the dorsal and ventral shell project to the cortical motor system and temporal lobe, respectively. Anatomically connected cortical regions receive common input from a subset of claustrum neurons shared by neighboring modules, whereas spatially separated regions of cortex are innervated by different claustrum modules. Therefore, each output module exhibits a unique position within the claustrum and overlaps substantially with other modules projecting to functionally related cortical regions. Claustrum inhibitory cells containing parvalbumin, somatostatin, and neuropeptide Y also show unique topographical distributions, suggesting different output modules are controlled by distinct inhibitory circuit motifs. The topographic organization of excitatory and inhibitory cell types may enable parallel claustrum outputs to independently coordinate distinct cortical networks.