A neural circuit for memory specificity and generalization

Attenuated heterogeneity of hippocampal neuron subsets in response to novelty induced by amyloid-β

Alzheimer's disease (AD) patients exhibited episodic memory impairments including location-object recognition in a spatial environment, which was also presented in animal models with amyloid-β (Aβ) accumulation. A potential cellular mechanism was the unstable representation of spatial information and lack of discrimination ability of novel stimulus in the hippocampal place cells. However, how the firing characteristics of different hippocampal subsets responding to diverse spatial information were interrupted by Aβ accumulation remains unclear. In this study, we observed impaired novel object-location recognition in Aβ-treated Long-Evans rats, with larger receptive fields of place cells in hippocampal CA1, compared with those in the saline-treated group. We identified two subsets of place cells coding object information (ObjCell) and global environment (EnvCell) during the task, with firing heterogeneity in response to introduced novel information. ObjCells displayed a dynamic representation responding to the introduction of novel information, while EnvCells exhibited a stable representation to support the recognition of the familiar environment. However, the dynamic firing patterns of these two subsets of cells were disrupted to present attenuated heterogeneity under Aβ accumulation. The impaired spatial representation novelty information could be due to the disturbed gamma modulation of neural activities. Taken together, these findings provide new evidence for novelty recognition impairments of AD rats with spatial representation dysfunctions of hippocampal subsets.

Generalisation of Placebo and Nocebo Effects: Current Knowledge and Future Directions

BACKGROUND AND OBJECTIVE

Placebo and nocebo effects are beneficial or adverse treatment outcomes upon administration of inert treatment components. These effects have been frequently studied on pain. It remains unclear to what extent generalisation occurs in these effects on pain and other somatic sensations. This review outlines the current knowledge on stimulus generalisation (i.e., generalisation over various stimuli) and response generalisation (i.e., generalisation over various responses) of placebo and nocebo effects on prevalent somatic sensations (i.e., pain, itch, dyspnea, nausea and fatigue).

DATABASES AND DATA TREATMENT

The databases PubMed, Web of Science and PsycINFO were systematically searched for peer-reviewed articles reporting on experimental studies in humans of the induction and generalisation of placebo and nocebo effects on prevalent somatic sensations.

RESULTS

Of 2025 records identified, 23 studies were included. These studies indicated that placebo and nocebo effects can generalise over stimuli (at perceptual, categorical and treatment levels) and over responses within modalities. Most studies investigated pain; fewer studies investigated itch, dyspnea, nausea and fatigue. Generalisation effects tend to be larger when the generalisation stimuli and responses more closely resemble the initial stimulus or response. Generalisation was more likely if a combination of verbal suggestion and conditioning was employed to induce placebo or nocebo effects than if either suggestion or conditioning was employed alone. Response generalisation across modalities remains unclear.

CONCLUSIONS

Placebo and nocebo effects can generalise over stimuli and responses. More experimental and clinical research is warranted to address carryover effects of placebo and nocebo effects.

SIGNIFICANCE

The current review provides an overview of the literature on the generalisation of placebo and nocebo effects to diverse stimuli and responses. This can ultimately benefit healthcare providers to prevent carryover effects of treatment failure and harness carryover effects of treatment success.

Hippocampal ensembles regulate circuit-induced relapse of extinguished fear

Extinction learning is central to behavioral therapies for post-traumatic stress disorder (PTSD), but relapse poses a major challenge to this approach. Recent work has revealed a critical role for the thalamic nucleus reuniens (RE) in the suppression of extinguished fear memories. Silencing the RE yields a relapse of extinguished fear (i.e., "circuit-induced relapse"). Considerable work suggests that RE may contribute to extinction by inhibiting the retrieval of hippocampal (HPC)-dependent fear memories. To test this hypothesis, we first examined whether undermining the formation of contextual fear memories in the HPC would prevent circuit-induced relapse. Intra-hippocampal infusions of the NMDA receptor antagonist, APV, prior to auditory fear conditioning eliminated contextual fear memory and prevented the subsequent relapse of extinguished fear to the auditory conditioned stimulus (CS). In a second experiment, we used an activity-dependent labeling system (AAV-cFos-tTA; AAV-TRE-hM3Dq-mCherry) to express excitatory DREADDs in HPC neurons during fear conditioning. Chemogenetic reactivation of these ensembles after extinction was sufficient to drive relapse of fear to the extinguished CS. Lastly, in a third experiment, we expressed excitatory DREADDs in HPC ensembles captured during extinction learning and found that chemogenetic reactivation of this ensemble was sufficient to inhibit circuit-induced relapse. These results reveal that HPC-dependent ensembles play a critical role in regulating the expression and relapse of extinguished fear.

Exploring the differences in resting state functional magnetic resonance imaging brain activity in patients with chronic low back pain based on ALE meta-analysis

Chronic low back pain (cLBP) is a prevalent condition, yet neuroimaging findings across studies remain inconsistent. This inconsistency may stem from overreliance on a priori regions of interest (ROI) hypotheses in prior studies, which neglected the spatial distribution of true functional abnormalities in cLBP. To address methodological heterogeneity, this study conducted an activation likelihood estimation (ALE) meta-analysis, guided by high-quality neuroimaging meta-analysis guidelines, to identify reliable neural features across 17 eligible cLBP studies, providing more robust conclusions than individual neuroimaging studies. Significantly, the cLBP group exhibited enhanced activity in the bilateral medial frontal gyrus (MFG), right precentral gyrus, and right cingulate gyrus. Based on the data-driven and objective ROI, we further analyzed the functional interactions or connectivity between brain regions by including a cohort of 30 patients with chronic non-specific low back pain (CNLBP) and 30 age- and sex-matched healthy controls. The results revealed decrease functional connectivity in the cortico-limbic circuit in the CNLBP group, particularly between the left MFG and the right hippocampus. Furthermore, effective connectivity from the right to left MFG was significantly reduced in the CNLBP group. Multivariate regression analysis established significant associations between cortico-limbic circuit connectivity alterations and pain catastrophizing. These findings highlight the cortico-limbic circuit as a key biomarker for cLBP, suggesting its targeted modulation could inform novel therapeutic strategies. This study provides novel neurobiological insights into cLBP pathophysiology, while underscoring the necessity for replication in larger cohorts to validate clinically valuable. PERSPECTIVE: This study utilized ALE meta-analysis to investigate abnormal activation regions in cLBP patients, and further analysis revealed functional interactions or connectivity between brain regions. The results highlighted the cortico-limbic circuit as a key biomarker of cLBP, suggesting that modulating this circuit may provide insights into potential therapeutic strategies.

Projection-specific circuits of retrosplenial cortex with differential contributions to spatial cognition

Retrosplenial cortex (RSC) is a brain region involved in neuropsychiatric and neurodegenerative disorders. It has reciprocal connections with a diverse set of cortical and subcortical brain regions, but the afferent structure and behavioral function of circuits defined by its projection-specific sub-populations have yet to be determined. The corticocortical connections between RSC and secondary motor cortex (M2), as well as corticothalamic connections between RSC and anterodorsal thalamus (AD) have been hypothesized to function as semi-independent, but parallel pathways that impact spatial information processing in distinct ways. We used retrograde and anterograde viral tracers and monosynaptic retrograde rabies virus to quantitatively characterize and compare the afferent and efferent distributions of retrosplenial neuron sub-populations projecting to M2 and AD. AD-projecting and M2-projecting RSC neurons overlap in their collateral projections to other brain regions, but not in their projections to M2 and AD, respectively. Compared with AD-projecting RSC neurons, M2-projecting RSC neurons received much greater afferent input from the dorsal subiculum, AD, lateral dorsal and lateral posterior thalamus, and somatosensory cortex. AD-projecting RSC neurons received greater input from the anterior cingulate cortex and medial septum. We performed chemogenetic inhibition of M2- and AD-projecting RSC neurons and examined its impact on object-location memory, object-recognition, open-field exploration, and place-action association. Our findings indicate that inhibition of M2-projecting RSC neurons impairs object location memory as well as place-action association, while the RSC to AD pathway impacts only object-location memory. The findings indicate that RSC is composed of semi-independent circuits distinguishable by their afferent/efferent distributions and differing in the cognitive functions to which they contribute.

ATLAS: a rationally designed anterograde transsynaptic tracer

Genetically modified rabies virus can map neural circuits retrogradely from genetically determined cells. However, similar tools for anterograde tracing are not available. Here, we describe a method for anterograde transsynaptic tracing from genetically determined neurons based on a rationally designed protein, ATLAS. Expression of ATLAS in neurons causes presynaptic release of a payload composed of an antibody-like protein, AMPA.FingR, which binds to the N terminus of GluA1, and a recombinase. In the synaptic cleft, AMPA.FingR binds to GluA1, causing the payload to be endocytosed into postsynaptic cells and delivered to the nucleus, where it triggers expression of a recombinase-dependent reporter. In mice, ATLAS mediates monosynaptic transneuronal tracing from random or genetically determined cells that is strictly anterograde, synaptic and nontoxic. Moreover, ATLAS-mediated tracing shows activity dependence, suggesting that it can label active circuits underlying specific behaviors. Finally, ATLAS is composed of modular components that can be independently replaced or modified.

Excitatory projections from the nucleus reuniens to the medial prefrontal cortex modulate pain and depression-like behaviors in mice

The medial prefrontal cortex (mPFC) is implicated in emotional processing, cognition, and pain sensation, moreover, its circuitry undergoes neuroplastic changes in chronic pain. Although the nucleus reuniens (RE) of the thalamus provides significant glutamatergic inputs to the mPFC, it remains unclear whether this projection contributes to plasticity changes in the mPFC and pain-related behaviors in chronic pain. Using fiber photometry, we demonstrated that RE neurons responded to pain stimulation and emotional changes. Optogenetic activation of RE neurons and their projections to the mPFC (RE-mPFC projection) elicits hyperalgesia and depression-like behaviors in naïve mice. In a neuropathic pain mouse model, RE neurons were hyperactive, and the RE-mPFC projection was enhanced with a marked preference for the part innervating GABAergic circuits in the mPFC to that controlling mPFC neurons projecting to the ventrolateral periaqueductal gray (vlPAG). Expectedly, optogenetic inhibition of RE neurons and the RE-mPFC projection ameliorated pain-like and depression-like behaviors in neuropathic pain mice. Additionally, chemogenetic inhibition of RE-mPFC neurons conferred analgesia in neuropathic pain mice exposed to both acute and chronic morphine. Our findings highlight the significant role of the RE-mPFC pathway in neuropathic pain comorbid with depression, suggesting its potential as a target for treatment of neuropathic pain.

Brain dynamics alterations induced by partial sleep deprivation: An energy landscape study

Partial sleep deprivation (PSD) alters neural activity of intrinsic brain networks involved in cognitive functions. However, the age-related time-varying properties of large-scale brain functional networks after PSD remain unknown. Our study applied energy landscape analysis to resting-state functional magnetic resonance imaging data to characterize the dominant brain activity patterns in 36 healthy young (19 females, 23.53 ± 2.36 years) and 33 healthy older (18 females, 68.81 ± 2.41 years) adults after full sleep (FS) and PSD. Dynamic properties of these patterns, including appearance probability, duration and transitions, were then calculated. Finally, a 105 steps numerical simulation was performed on each energy landscape. We found that the energy landscapes of the younger and older groups had similar hierarchical structures, including two major states and two minor states. The two major states showed complementary spontaneous activation patterns. But the PSD has altered the temporal evolution of these major brain states in younger participants, manifested by significantly higher appearance frequency of the major states and the direct transitions between major states than FS. These changes were not significant in older participants. Additionally, the weaker functional segregation between two modules assigned by two complementary major states was found during PSD than FS in young group. We further demonstrated that such abnormal brain network functional coordination was associated with the atypical brain dynamics and behaviors. These findings suggested a low-dimensional and restricted dynamic landscape of brain activity in young adults after PSD and provided new insight into understand the neural effects of PSD.

Time- and sex-dependent effects of juvenile social isolation on mouse brain morphology

During early life stages, social isolation disrupts the proper brain growth and brain circuit formation, which is associated with the risk of mental disorders and cognitive deficits in adulthood. Nevertheless, the impact of juvenile social isolation on brain development, particularly regarding variations across age and sex, remains poorly understood. Here, we investigate the effects of social isolation stress (SIS) during early (3-5 weeks old) or late (5-7 weeks old) juvenile period on brain morphology in adult male and female mice using ultra high-field MRI (11.7 T). We found that both early and late SIS in female mice led to volumetric increases in multiple brain regions, such as the medial prefrontal cortex (mPFC) and hippocampus. Correlation tractography revealed that the fiber tracts in the right corpus callosum and right amygdala were positively correlated with SIS in female mice. In male mice, early SIS resulted in small volumetric increases in the isocortex, whereas late SIS led to reductions in the isocortex and hypothalamus. Furthermore, early SIS caused a negative correlation, while late SIS exhibited a positive correlation, with fiber tracts in the corpus callosum and amygdala in male mice. Using a Random Forest classifier, we achieved effective discrimination between socially isolated and control conditions in the brain volume of female mice, with the limbic areas playing a key role in the model's accuracy. Finally, we discovered that SIS led to context fear generalization in a sex-dependent manner. Our findings highlight the importance of considering both the time- and sex-dependent effects of juvenile SIS on brain development and emotional processing, providing new insights into its long-term consequences.

An infralimbic cortex engram encoded during learning attenuates fear generalization

Generalization allows previous experience to adaptively guide behavior when conditions change. The infralimbic (IL) subregion of the ventromedial prefrontal cortex plays a known role in generalization processes, although mechanisms remain unclear. A basic physical unit of memory storage and expression in the brain is a sparse, distributed group of neurons known as an engram. Here, we set out to determine whether an engram established during learning contributes to generalized responses in IL. Generalization was tested in male and female mice by presenting a novel, ambiguous, tone generalization stimulus following Pavlovian defensive (fear) conditioning. The first experiment was designed to test a global role for IL in generalization using chemogenetic manipulations. Results show IL regulates defensive behavior in response to ambiguous stimuli. IL silencing led to a switch in defensive state, from vigilant scanning to generalized freezing, while IL stimulation reduced freezing in favor of scanning. Leveraging activity-dependent "tagging" technology (ArcCreERT2 x eYFP system), an engram, preferentially located in IL Layer 2/3, was associated with the generalization stimulus. Remarkably, in the identical discrete location, fewer reactivated neurons were associated with the generalization stimulus at the remote timepoint (30 days) following learning. When an IL engram established during learning was selectively chemogenetically silenced, freezing increased. Conversely, IL engram stimulation reduced freezing, suggesting attenuated fear generalization. Overall, these data identify a crucial role for IL in suppressing generalized conditioned responses. Further, an IL engram formed during learning functions to later attenuate a conditioned response in the presence of ambiguous threat stimuli.Significance statement Generalization refers to the ability for organisms to use previous experience to guide behavior when environmental conditions change. Despite the immense importance of generalization in adaptive behavior, the precise brain mechanisms remain unknown. Here we identified a small population of neurons, known as an engram, in a discrete region of the frontal cortex that was associated with the expression of generalization related to a threatening situation. When these cells were turned off, generalization increased. When they were turned on, generalization decreased. Considering that over-generalization of threatening stimuli is a known fundamental dimension of both anxiety and post-traumatic stress disorders, these findings have implications not only for our understanding of intrinsic generalization processes but also highly prevalent clinical disorders.

Generalization and extinction of platform-mediated avoidance in male and female rats

Understanding anxiety-related disorders can be advanced by studying the fear learning mechanisms implicated in the transition from adaptive to maladaptive fear. Individuals with anxiety disorders often show impaired fear extinction, pervasive avoidance, and overgeneralization of fear. While these characteristics are usually studied in isolation, their interactions are less understood. We modified the platform-mediated avoidance task to chart avoidance, generalization, and extinction in male and female rats. Male rats acquired avoidance, showed a gradient of generalization, and reduced avoidance and fear under extinction. Female rats also learned avoidance, showed gradual generalization, and extinction of defensive behaviors. Sex differences emerged in extinction learning but were subtler than expected. We present an open-source automated system for processing DeepLabCut and SimBA output to score avoidance and freezing behavior. This task effectively probes avoidance, generalization, and extinction of fear in rats, and our automated scoring approach offers a effective method to quantify defensive behaviors.

Peripheral transcutaneous electrical stimulation to improve cognition: a review of the main effects in healthy humans and in mildly cognitively impaired patient populations

Peripheral nerve stimulation (PNS) is an ancient technique, up to now mainly used for pain management. The least invasive approach for PNS is transcutaneous electrical stimulation (TENS), which is performed by delivering mild electric currents through the skin and, depending on the stimulation pattern, activates the somatosensory Aβ-, Aδ- and C-fibers. In addition to its use for pain relief, accumulating data indicates that TENS can have broad-spectrum cognitive effects through the activation of neuromodulatory brain pathways. This review aims to summarize the current evidence on the cognitive effects of TENS, from healthy participants and mildly cognitively affected patients. Most studies on this topic have investigated the effects of TENS on memory, while fewer studies have explored attention, executive functions, and verbal fluency. Overall, promising evidence suggests that TENS may exert positive effects on specific cognitive functions. Further research is needed to build consensus on the most effective stimulation protocols, for both neurorehabilitation and enhancement, and to better understand the neurobiological mechanisms underlying the cognitive effects of TENS.

Associative coding of conditioned fear in the thalamic nucleus reuniens in rodents and humans

The nucleus reuniens (RE) is a midline thalamic structure interconnecting the medial prefrontal cortex (mPFC) and the hippocampus (HPC). Recent work in both rodents and humans implicates the RE in the adaptive regulation of emotional memories, including the suppression of learned fear. However, the neural correlates of aversive learning in the RE of rodents and humans remains unclear. To address this, we recorded RE activity in humans (BOLD fMRI) and rats (fiber photometry) during Pavlovian fear conditioning and extinction. In both rats and humans, we found that conditioned stimulus (CS)-evoked activity in RE reflects the associative value of the CS. In rats, we additionally found that spontaneous neural activity in RE tracks defensive freezing and shows anticipatory increases in calcium activity that precede the termination of freezing behavior. Single-unit recordings in rats confirmed that individual RE neurons index both the associative value of the CS and defensive behavior transitions. Moreover, distinct neuronal ensembles in the RE encode fear versus extinction memories. These findings suggest a conserved role of the RE across species in modulating defensive states and emotional memory processes, providing a foundation for future translational research on fear-related disorders.

The reuniens nucleus of the thalamus facilitates hippocampo-cortical dialogue during sleep

Memory consolidation during sleep depends on the interregional coupling of slow waves, spindles, and sharp wave-ripples (SWRs), across the cortex, thalamus, and hippocampus. The reuniens nucleus of the thalamus, linking the medial prefrontal cortex (mPFC) and the hippocampus, may facilitate interregional coupling during sleep. To test this hypothesis, we used intracellular, extracellular unit and local field potential recordings in anesthetized and head restrained non-anesthetized cats as well as computational modelling. Electrical stimulation of the reuniens evoked both antidromic and orthodromic intracellular mPFC responses, consistent with bidirectional functional connectivity between mPFC, reuniens and hippocampus in anesthetized state. The major finding obtained from behaving animals is that at least during NREM sleep hippocampo-reuniens-mPFC form a functional loop. SWRs facilitate the triggering of thalamic spindles, which later reach neocortex. In return, transition to mPFC UP states increase the probability of hippocampal SWRs and later modulate spindle amplitude. During REM sleep hippocampal theta activity provides periodic locking of reuniens neuronal firing and strong crosscorrelation at LFP level, but the values of reuniens-mPFC crosscorrelation was relatively low and theta power at mPFC was low. The neural mass model of this network demonstrates that the strength of bidirectional hippocampo-thalamic connections determines the coupling of oscillations, suggesting a mechanistic link between synaptic weights and the propensity for interregional synchrony. Our results demonstrate the presence of functional connectivity in hippocampo-thalamo-cortical network, but the efficacy of this connectivity is modulated by behavioral state.

The midline thalamic nucleus reuniens promotes compulsive-like grooming in rodents

Obsessive-compulsive disorder (OCD), a disabling and notoriously treatment-resistant neuropsychiatric disorder, affects 2-3% of the general population and is characterized by recurring, intrusive thoughts (obsessions) and repetitive, ritualistic behaviors (compulsions). Although long associated with dysfunction within the cortico-striato-thalamic-cortical circuits, the thalamic role in OCD pathogenesis remains highly understudied in the literature. Here, we identified a rat thalamic nucleus - the reuniens (NRe) - that mediates persistent, compulsive self-grooming behavior. Optogenetic activation of this nucleus triggers immediate, excessive grooming with strong irresistibility, increases anxiety, and induces negative affective valence. A thalamic-hypothalamic pathway linking NRe to the dorsal premammillary nucleus (PMd) was discovered to mediate excessive self-grooming behavior and render it a defensive coping response to stress, mirroring the compulsions faced by OCD patients. Given the close resemblance between this self-grooming behavior and the clinical manifestations of OCD, the results from this study highlight the role of NRe in mediating OCD-like compulsive behaviors. This can be attributed to NRe's position at the nexus of an extensive frontal-striatal-thalamic network regulating cognition, emotion, and stress-related behaviors, suggesting NRe as a potential novel target for intervention.

Nucleus reuniens: Modulating emotional overgeneralization in peri-adolescents with anxiety

BACKGROUND

Anxiety affects 4.4-million children in the USA with an onset between childhood and adolescence, a period marked by neural changes that impact emotions and memory. Negative overgeneralization - or responding similarly to innocuous events that share features with past aversive experiences - is common in anxiety but remains mechanistically underspecified. The nucleus reuniens (RE) has been considered a crucial candidate in the modulation of memory specificity. Our study investigated its activation and functional connectivity with the medial prefrontal cortex (mPFC) and hippocampus (HPC) as neurobiological mechanisms of negative overgeneralization in anxious youth.

METHODS

As part of a secondary data analysis, we examined data from 34 participants between 9 and 14 years of age (mean age ± SD, 11.4 ± 2.0 years; 16 females) with varying degrees of anxiety severity. During the Study session participants rated images as negative, neutral, and positive. After 12 h, participants returned for a Test session, where they performed a memory recognition test with repeated (targets) and similar (lures) images. Labeling negative relative to neutral lures as "old" (false alarms) was our operational definition of negative overgeneralization.

RESULTS

Negative relative to neutral false alarmed stimuli displayed elevated RE activation (at Study and Test) and increased functional connectivity with the Cornu Ammonis (CA) 1 (at Test). Elevated anxiety severity was associated with reductions in the RE-mPFC functional coupling for neutral relative to negative stimuli. Exploratory analyses revealed similar patterns in activation and functional connectivity with positive stimuli.

CONCLUSIONS

Our findings demonstrate the importance of the RE in negative overgeneralization and anxiety.

Deciphering consciousness: The role of corticothalamocortical interactions in general anesthesia

General anesthesia is administered to millions of individuals each year, however, the precise mechanism by which it induces unconsciousness remains unclear. While some theories suggest that anesthesia shares similarities with natural sleep, targeting sleep-promoting areas and inhibiting arousal nuclei, recent research indicates a more complex process. Emerging evidence highlights the critical role of corticothalamocortical circuits, which are involved in higher cognitive functions, in controlling arousal states and modulating transitions between different conscious states during anesthesia. The administration of general anesthetics disrupts connectivity within these circuits, resulting in a reversible state of unconsciousness. This review elucidates how anesthetics impair corticothalamocortical interactions, thereby affecting the flow of information across various cortical layers and disrupting higher-order cognitive functions while preserving basic sensory processing. Additionally, the role of the prefrontal cortex in regulating arousal through both top-down and bottom-up pathways was examined. These findings highlight the intricate interplay between the cortical and subcortical networks in maintaining and restoring consciousness under anesthesia, offering potential therapeutic targets for enhancing anesthesia management.

Repeated social defeat in male mice induced unique RNA profiles in projection neurons from the amygdala to the hippocampus

Chronic stress increases the incidence of psychiatric disorders including anxiety, depression, and posttraumatic stress disorder. Repeated Social Defeat (RSD) in mice recapitulates several key physiological, immune, and behavioral changes evident after chronic stress in humans. For instance, neurons in the prefrontal cortex, amygdala, and hippocampus are involved in the interpretation of and response to fear and threatful stimuli after RSD. Therefore, the purpose of this study was to determine how stress influenced the RNA profile of hippocampal neurons and neurons that project into the hippocampus from threat appraisal centers. Here, RSD increased anxiety-like behavior in the elevated plus maze and reduced hippocampal-dependent novel object location memory in male mice. Next, pan-neuronal (Baf53 b-Cre) RiboTag mice were generated to capture ribosomal bound mRNA (i.e., active translation) activated by RSD in the hippocampus. RNAseq revealed that there were 1694 differentially expressed genes (DEGs) in hippocampal neurons after RSD. These DEGs were associated with an increase in oxidative stress, synaptic long-term potentiation, and neuroinflammatory signaling. To further examine region-specific neural circuitry associated with fear and anxiety, a retrograde-adeno-associated-virus (AAV2rg) expressing Cre-recombinase was injected into the hippocampus of male RiboTag mice. This induced expression of a hemagglutinin epitope in neurons that project into the hippocampus. These AAV2rg-RiboTag mice were subjected to RSD and ribosomal-bound mRNA was collected from the amygdala for RNA-sequencing. RSD induced 677 DEGs from amygdala projections. Amygdala neurons that project into the hippocampus had RNA profiles associated with increased synaptogenesis, interleukin-1 signaling, nitric oxide, and reactive oxygen species production. Using a similar approach, there were 1132 DEGs in neurons that project from the prefrontal cortex. These prefrontal cortex neurons had RNA profiles associated with increased synaptogenesis, integrin signaling, and dopamine feedback signaling after RSD. Collectively, there were unique RNA profiles of stress-influenced projection neurons and these profiles were associated with hippocampal-dependent behavioral and cognitive deficits.

Projections from thalamic nucleus reuniens to hippocampal CA1 area participate in context fear extinction by affecting extinction-induced molecular remodeling of excitatory synapses

The ability to extinguish contextual fear in a changing environment is crucial for animal survival. Recent data support the role of the thalamic nucleus reuniens (RE) and its projections to the dorsal hippocampal CA1 area (RE→dCA1) in this process. However, it remains poorly understood how RE impacts dCA1 neurons during contextual fear extinction (CFE). Here, we reveal that the RE→dCA1 pathway contributes to the extinction of contextual fear by affecting CFE-induced molecular remodeling of excitatory synapses. Anatomical tracing and chemogenetic manipulation in mice demonstrate that RE neurons form synapses and regulate synaptic transmission in the stratum oriens (SO) and lacunosum-moleculare (SLM) of the dCA1 area, but not in the stratum radiatum (SR). We also observe CFE-specific structural changes of excitatory synapses and expression of the synaptic scaffold protein, PSD-95, in both strata innervated by RE, but not in SR. Interestingly, only the changes in SLM are specific for the dendrites innervated by RE. To further support the role of the RE→dCA1 projection in CFE, we demonstrate that brief chemogenetic inhibition of the RE→dCA1 pathway during a CFE session persistently impairs the formation of CFE memory and CFE-induced changes of PSD-95 levels in SLM. Thus, our data indicate that RE participates in CFE by regulating CFE-induced molecular remodeling of dCA1 synapses.

An amygdala-cortical circuit for encoding generalized fear memories

Generalized learning is a fundamental process observed across species, contexts, and sensory modalities that enables animals to use past experiences to adapt to changing conditions. Evidence suggests that the prefrontal cortex (PFC) extracts general features of an experience that can be used across multiple situations. The anterior cingulate cortex (ACC), a region of the PFC, is implicated in generalized fear responses to novel contexts. However, the ACC's role in encoding contextual information is poorly understood, especially under increased threat intensity that promotes generalization. Here, we show that synaptic plasticity within the ACC and signaling from amygdala inputs during fear learning are necessary for generalized fear responses to novel encountered contexts. The ACC did not encode specific fear to the training context, suggesting this region extracts general features of a threatening experience rather than specific contextual information. Together with our previous work, our results demonstrate that generalized learning about threatening contexts is encoded, in part, within an ascending amygdala-cortical circuit, whereas descending ACC projections to the amygdala drive generalized fear responses during exposure to novel contexts. Our results further demonstrate that schematic learning can occur in the PFC after single-trial learning, a process typically attributed to learning over many repeated learning episodes.

Mesolimbic dopamine neurons drive infradian rhythms in sleep-wake and heightened activity state

Infradian mood and sleep-wake rhythms with periods of 48 hours and beyond have been observed in patients with bipolar disorder (BD), which even persist in the absence of exogenous timing cues, indicating an endogenous origin. Here, we show that mice exposed to methamphetamine in drinking water develop infradian locomotor rhythms with periods of 48 hours and beyond which extend to sleep length and manic state-associated behaviors in support of a model for cycling in BD. The cycling capacity is abrogated upon genetic disruption of dopamine (DA) production in DA neurons of the ventral tegmental area (VTA) or ablation of nucleus accumbens projecting DA neurons. Furthermore, chemogenetic activation of VTADA neurons including those that project to the nucleus accumbens led to locomotor period lengthening in circadian clock-deficient mice, which was counteracted by antipsychotic treatment. Together, our findings argue that BD cycling relies on infradian rhythm generation that depends on mesolimbic DA neurons.

P2X7 receptors modulate acquisition of cue fear extinction and contextual background memory generalization in male mice

The purinergic P2X7 receptors (P2X7R) are activated by adenosine triphosphate (ATP) in several brain regions, particularly those involved with emotional control and the regulation of fear-related memories. Here, we investigate the role of P2X7R in fear learning memory, specifically in the acquisition and consolidation phases of the cued fear conditioning paradigm. C57Bl/6 wildtype (WT) male mice that received a single i.p. injection of the selective P2X7R antagonist A438079 prior the conditioning session showed generalization of cued fear memory and impaired fear extinction recall in the test session, while those treated prior the extinction session exhibited a similar behavior profile accompanied by resistance in the extinction learning. However, no effects were observed when this drug was administered immediately after the conditioning, extinction, or before the test session. Our results with P2X7R knockout (P2X7 KO) mice showed a behavioral profile that mirrored the collective effects observed across all pharmacological treatment conditions. This suggests that the P2X7R KO model effectively replicates the behavioral changes induced by the pharmacological interventions, demonstrating that we have successfully isolated the role of P2X7R in the fear and extinction phases of memory. These findings highlight the role of P2X7R in the acquisition and recall of extinction memory and supports P2X7R as a promising candidate for controlling abnormal fear processing, with potential applications for stress exposure-related disorders such as post-traumatic stress disorder (PTSD).

The ventral midline thalamus and long-term memory: What consolidation, what retrieval, what plasticity in rodents?

The ventral midline thalamus, including the reuniens and rhomboid (ReRh) nuclei, connects bidirectionally with the medial prefrontal cortex (mPFC) and hippocampus (Hip), both essential for memory processes. This review compiles and discusses studies on a role for the ReRh nuclei in the system consolidation of memories, also considering their potentially limited participation in memory retrieval or early phases of consolidation. It also examines scientific literature on short- and long-term plasticity in ReRh-mPFC and ReRh-Hip connections, emphasizing plasticity's importance in understanding these nuclei's role in memory. The idea that the two nuclei are at the crossroads of information exchange between the mPFC and the Hip is not new, but the relationship between this status and the plasticity of their connections remains elusive. Since this perspective is relatively recent, our concluding section suggests conceptual and practical avenues for future research, aiming perhaps to bring more order to the apparently multi-functional implication of the ventral midline thalamus in cognition.

Molecular connectomics reveals a glucagon-like peptide 1-sensitive neural circuit for satiety

Liraglutide and other glucagon-like peptide 1 receptor agonists (GLP-1RAs) are effective weight loss drugs, but how they suppress appetite remains unclear. One potential mechanism is by activating neurons that inhibit the hunger-promoting Agouti-related peptide (AgRP) neurons of the arcuate hypothalamus (Arc). To identify these afferents, we developed a method combining rabies-based connectomics with single-nucleus transcriptomics. Here, we identify at least 21 afferent subtypes of AgRP neurons in the mouse mediobasal and paraventricular hypothalamus, which are predicted by our method. Among these are thyrotropin-releasing hormone (TRH)+ Arc (TRHArc) neurons, inhibitory neurons that express the Glp1r gene and are activated by the GLP-1RA liraglutide. Activating TRHArc neurons inhibits AgRP neurons and feeding, probably in an AgRP neuron-dependent manner. Silencing TRHArc neurons causes overeating and weight gain and attenuates liraglutide's effect on body weight. Our results demonstrate a widely applicable method for molecular connectomics, comprehensively identify local inputs to AgRP neurons and reveal a circuit through which GLP-1RAs suppress appetite.

The Spiraling Cognitive-Emotional Brain: Combinatorial, Reciprocal, and Reentrant Macro-organization

This article proposes a framework for understanding the macro-scale organization of anatomical pathways in the mammalian brain. The architecture supports flexible behavioral decisions across a spectrum of spatiotemporal scales. The proposal emphasizes the combinatorial, reciprocal, and reentrant connectivity-called CRR neuroarchitecture-between cortical, BG, thalamic, amygdala, hypothalamic, and brainstem circuits. Thalamic nuclei, especially midline/intralaminar nuclei, are proposed to act as hubs routing the flow of signals between noncortical areas and pFC. The hypothalamus also participates in multiregion circuits via its connections with cortex and thalamus. At slower timescales, long-range behaviors integrate signals across levels of the neuroaxis. At fast timescales, parallel engagement of pathways allows urgent behaviors while retaining flexibility. Overall, the proposed architecture enables context-dependent, adaptive behaviors spanning proximate to distant spatiotemporal scales. The framework promotes an integrative perspective and a distributed, heterarchical view of brain function.

A contextual fear conditioning paradigm in head-fixed mice exploring virtual reality

Contextual fear conditioning is a classical laboratory task that tests associative memory formation and recall. Techniques such as multi-photon microscopy and holographic stimulation offer tremendous opportunities to understand the neural underpinnings of these memories. However, these techniques generally require animals to be head-fixed. There are few paradigms that test contextual fear conditioning in head-fixed mice, and none where the behavioral outcome following fear conditioning is freezing, the most common measure of fear in freely moving animals. To address this gap, we developed a contextual fear conditioning paradigm in head-fixed mice using virtual reality (VR) environments. We designed an apparatus to deliver tail shocks (unconditioned stimulus, US) while mice navigated a VR environment (conditioned stimulus, CS). The acquisition of contextual fear was tested when the mice were reintroduced to the shock-paired VR environment the following day. We tested three different variations of this paradigm and, in all of them, observed an increased conditioned fear response characterized by increased freezing behavior. This was especially prominent during the first trial in the shock-paired VR environment, compared to a neutral environment where the mice received no shocks. Our results demonstrate that head-fixed mice can be fear conditioned in VR, discriminate between a feared and neutral VR context, and display freezing as a conditioned response, similar to freely behaving animals. Furthermore, using a two-photon microscope, we imaged from large populations of hippocampal CA1 neurons before, during, and following contextual fear conditioning. Our findings reconfirmed those from the literature on freely moving animals, showing that CA1 place cells undergo remapping and show narrower place fields following fear conditioning. Our approach offers new opportunities to study the neural mechanisms underlying the formation, recall, and extinction of contextual fear memories. As the head-fixed preparation is compatible with multi-photon microscopy and holographic stimulation, it enables long-term tracking and manipulation of cells throughout distinct memory stages and provides subcellular resolution for investigating axonal, dendritic, and synaptic dynamics in real-time.

Dissociable contributions of the amygdala and ventral hippocampus to stress-induced changes in defensive behavior

Stress can have profound consequences on mental health. While much is known about the neural circuits supporting associative memories of stressful events, our understanding of the circuits underlying the non-associative impacts of stress, such as heightened stress sensitivity and anxiety-related behavior, is limited. Here, we demonstrate that the ventral hippocampus (vHC) and basolateral amygdala (BLA) support distinct non-associative behavioral changes following stress. Inhibiting stress-induced protein synthesis in the BLA blocked subsequent increases in stress sensitivity but not anxiety-related behaviors. Conversely, inhibiting stress-induced protein synthesis in the vHC blocked subsequent increases in anxiety-related behavior but not stress sensitivity. Inhibiting neuronal activity in the BLA and vHC during the assessment of stress sensitivity or anxiety-related behavior recapitulated these structures' dissociable contributions to defensive behavior. Lastly, blocking the associative memory of a stressor had no impact on stress-induced changes in anxiety-related behavior. These findings highlight that multiple memory systems support the long-lasting effects of stress.

Object Feature Memory Is Distorted by Category Structure

Memory systems constantly confront the challenge of capturing both the shared features that connect experiences together and the unique features that distinguish them. Across two experiments, we leveraged a color memory distortion paradigm to investigate how we handle this representational tension when learning new information. Over a thirty-minute period, participants learned shared and unique features of categories of novel objects, where each feature was assigned a particular color. While participants did not differ in how accurately they remembered these features overall, when inaccurate, participants misremembered the color of shared (relative to unique) features as more similar to the category's average color, suggesting more integration of shared features in memory. This same rapid representational warping manifested in a neural network model trained on the same categories. The work reveals how memories for different features are rapidly and differentially warped as a function of their roles in a category.

Prefrontal projections to the bed nuclei of the stria terminalis modulate the specificity of aversive memories

Generalizing aversive memories helps organisms avoid danger, whereas discriminating between dissimilar situations promotes opportunistic behaviors. We identified a novel pathway that controls the contextual specificity of memory consolidation of inhibitory avoidance learning. Optogenetic inhibition of the rostral medial prefrontal cortex (mPFC)-to-anteroventral bed nuclei of the stria terminalis (avBST) pathway after a single footshock exacerbated stress hormonal output, and 2 d later promoted generalization to a novel context. Rostral mPFC-avBST influences were directly mnemonic rather than associated with stress hormone increases, as adrenalectomy did not prevent such influences on generalization. We next observed that fear discrimination between novel and aversive contexts engaged activity along the rostral mPFC and avBST pathway. Finally, post-footshock optogenetic pathway excitation enhanced 2-d discrimination. These findings highlight a prefrontal pathway in which activity immediately after aversive experiences promotes mnemonic discrimination between threatening and non-threatening contexts and may be importance for understanding trauma generalization in psychiatric illnesses.

GLP-1R-positive neurons in the lateral septum mediate the anorectic and weight-lowering effects of liraglutide in mice

Liraglutide, a glucagon-like peptide-1 (GLP-1) analog, is approved for obesity treatment, but the specific neuronal sites that contribute to its therapeutic effects remain elusive. Here, we show that GLP-1 receptor-positive (GLP-1R-positive) neurons in the lateral septum (LSGLP-1R) play a critical role in mediating the anorectic and weight-loss effects of liraglutide. LSGLP-1R neurons were robustly activated by liraglutide, and chemogenetic activation of these neurons dramatically suppressed feeding. Targeted knockdown of GLP-1 receptors within the LS, but not in the hypothalamus, substantially attenuated liraglutide's ability to inhibit feeding and lower body weight. The activity of LSGLP-1R neurons rapidly decreased during naturalistic feeding episodes, while synaptic inactivation of LSGLP-1R neurons diminished the anorexic effects triggered by liraglutide. Together, these findings offer critical insights into the functional role of LSGLP-1R neurons in the physiological regulation of energy homeostasis and delineate their instrumental role in mediating the pharmacological efficacy of liraglutide.

Persistent Impact of Prior Experience on Spatial Learning

Learning to solve a new problem involves identifying the operating rules, which can be accelerated if known rules generalize in the new context. We ask how prior experience affects learning a new rule that is distinct from known rules. We examined how rats learned a new spatial navigation task after having previously learned tasks with different navigation rules. The new task differed from the previous tasks in spatial layout and navigation rule. We found that experience history did not impact overall performance. However, by examining navigation choice sequences in the new task, we found experience-dependent differences in exploration patterns during early stages of learning, as well as differences in the types of errors made during stable performance. The differences were consistent with the animals adopting experience-dependent memory strategies to discover and implement the new rule. Our results indicate prior experience shapes the strategies for solving novel problems, and the impact of prior experience remains persistent.

Cellular and circuit architecture of the lateral septum for reward processing

The lateral septum (LS) is composed of heterogeneous cell types that are important for various motivated behaviors. However, the transcriptional profiles, spatial arrangement, function, and connectivity of these cell types have not been systematically studied. Using single-nucleus RNA sequencing, we delineated diverse genetically defined cell types in the LS that play distinct roles in reward processing. Notably, we found that estrogen receptor 1 (Esr1)-expressing neurons in the ventral LS (LSEsr1) are key drivers of reward seeking via projections to the ventral tegmental area, and these neurons play an essential role in methamphetamine (METH) reward and METH-seeking behavior. Extended exposure to METH increases the excitability of LSEsr1 neurons by upregulating hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, thereby contributing to METH-induced locomotor sensitization. These insights not only elucidate the intricate molecular, circuit, and functional architecture of the septal region in reward processing but also reveal a neural pathway critical for METH reward and behavioral sensitization.

Inhibitory neurons marked by a connectivity molecule regulate memory precision

The CA3 region is central to hippocampal function during learning and memory and has a unique connectivity. CA3 pyramidal neurons are the targets of huge, excitatory mossy fiber synapses from DG axons and have a high degree of excitatory recurrent connectivity. Thus, inhibition likely plays an outsized importance in constraining excitation and shaping CA3 ensembles during learning and memory. Here, we investigate the function of a never-before studied set of dendrite-targeting, GABAergic neurons defined by expression of the synaptic adhesion molecule, Kirrel3. We discovered that activating Kirrel3-expressing GABAergic neurons specifically impairs memory discrimination and inhibits CA3 pyramidal neurons in novel contexts. Kirrel3 is required for DG-to-GABA synapse formation and variants in Kirrel3 are strong risk factors for neurodevelopmental disorders. Thus, our work suggests that Kirrel3-GABA neurons are a critical source of feed-forward inhibition from DG to CA3 during the encoding and retrieval of contextual memories, a function which may be specifically disrupted in some brain disorders.

Effect of internal and external chaotic stimuli on synchronization of piezoelectric auditory neurons in coupled time-delay systems

Hearing impairment is considered to be related to the damage of hair cells or synaptic terminals, which will cause varying degrees of hearing loss. Numerous studies have shown that cochlear implants can balance this damage. The human ear receives external acoustic signals mostly under complex conditions, and its biophysical mechanisms have important significance for reference in the design of cochlear implants. However, the relevant biophysical mechanisms have not yet been fully determined. Using the characteristics of special acoustoelectric conversion in piezoelectric ceramics, this paper integrates them into the traditional FitzHugh-Nagumo neuron circuit and proposes a comprehensive model with coupled auditory neurons. The model comprehensively considers the effects of synaptic coupling between neurons, information transmission delay, external noise stimulation, and internal chaotic current stimulation on the synchronization of membrane potential signals of two auditory neurons. The experimental results show that coupling strength, delay size, noise intensity, and chaotic current intensity all have a certain regulatory effect on synchronization stability. In particular, when auditory neurons are in a chaotic state, their impact on synchronization stability is sensitive. Numerical results provide a reference for exploring the biophysical mechanisms of auditory neurons. At the same time, we are committed to providing assistance in using sensors to monitor signals and repair hearing impairments.

In relentless pursuit of the white whale: A role for the ventral midline thalamus in behavioral flexibility and adaption?

The reuniens (Re) nucleus is located in the ventral midline thalamus. It has fostered increasing interest, not only for its participation in a variety of cognitive functions (e.g., spatial working memory, systemic consolidation, reconsolidation, extinction of fear or generalization), but also for its neuroanatomical positioning as a bidirectional relay between the prefrontal cortex (PFC) and the hippocampus (HIP). In this review we compile and discuss recent studies having tackled a possible implication of the Re nucleus in behavioral flexibility, a major PFC-dependent executive function controlling goal-directed behaviors. Experiments considered explored a possible role for the Re nucleus in perseveration, reversal learning, fear extinction, and set-shifting. They point to a contribution of this nucleus to behavioral flexibility, mainly by its connections with the PFC, but possibly also by those with the hippocampus, and even with the amygdala, at least for fear-related behavior. As such, the Re nucleus could be a crucial crossroad supporting a PFC-orchestrated ability to cope with new, potentially unpredictable environmental contingencies, and thus behavioral flexibility and adaption.

Orienting of attention and spatial cognition

Humans orient to their sensory world through foveation of target location or through covert shifts of attention. Orienting provides primacy to the selected location and in humans improves the precision of discrimination. Covert orienting appears to arise separately from the mechanisms involved in saccadic eye movements. Covert orienting can serve to prioritize processing the target even increasing its subjective intensity and its acuity. However, this network does not appear to be involved in the operations related to binding and segmentation. Cells exist in the early visual cortex that are activated by both color and form features without attention, however, color and form appear to remain independent even when oriented to the target that is required to be reported. An understanding of the pathways that connect attention networks to memory networks may allow us to understand more complex aspects of spatial cognition and enhance orienting and thus improve spatial cognition.

The cortical amygdala consolidates a socially transmitted long-term memory

Social communication guides decision-making, which is essential for survival. Social transmission of food preference (STFP) is an ecologically relevant memory paradigm in which an animal learns a desirable food odour from another animal in a social context, creating a long-term memory1,2. How food-preference memory is acquired, consolidated and stored is unclear. Here we show that the posteromedial nucleus of the cortical amygdala (COApm) serves as a computational centre in long-term STFP memory consolidation by integrating social and sensory olfactory inputs. Blocking synaptic signalling by the COApm-based circuit selectively abolished STFP memory consolidation without impairing memory acquisition, storage or recall. COApm-mediated STFP memory consolidation depends on synaptic inputs from the accessory olfactory bulb and on synaptic outputs to the anterior olfactory nucleus. STFP memory consolidation requires protein synthesis, suggesting a gene-expression mechanism. Deep single-cell and spatially resolved transcriptomics revealed robust but distinct gene-expression signatures induced by STFP memory formation in the COApm that are consistent with synapse restructuring. Our data thus define a neural circuit for the consolidation of a socially communicated long-term memory, thereby mechanistically distinguishing protein-synthesis-dependent memory consolidation from memory acquisition, storage or retrieval.

Molecular Connectomics Reveals a Glucagon-Like Peptide 1 Sensitive Neural Circuit for Satiety

Liraglutide and other agonists of the glucagon-like peptide 1 receptor (GLP-1RAs) are effective weight loss drugs, but how they suppress appetite remains unclear. One potential mechanism is by activating neurons which inhibit hunger-promoting Agouti-related peptide (AgRP) neurons of the arcuate hypothalamus (Arc). To identify these afferents, we developed a method combining rabies-based connectomics with single-nuclei transcriptomics. Applying this method to AgRP neurons predicted at least 21 afferent subtypes in the mouse mediobasal and paraventricular hypothalamus. Among these are Trh+ Arc neurons, inhibitory neurons which express the Glp1r gene and are activated by the GLP-1RA liraglutide. Activating Trh+ Arc neurons inhibits AgRP neurons and feeding in an AgRP neuron-dependent manner. Silencing Trh+ Arc neurons causes over-eating and weight gain and attenuates liraglutide's effect on body weight. Our results demonstrate a widely applicable method for molecular connectomics, comprehensively identify local inputs to AgRP neurons, and reveal a circuit through which GLP-1RAs suppress appetite.

Reactivation of encoding ensembles in the prelimbic cortex supports temporal associations

Fear conditioning is encoded by strengthening synaptic connections between the neurons activated by a conditioned stimulus (CS) and those activated by an unconditioned stimulus (US), forming a memory engram, which is reactivated during memory retrieval. In temporal associations, activity within the prelimbic cortex (PL) plays a role in sustaining a short-term, transient memory of the CS, which is associated with the US after a temporal gap. However, it is unknown whether the PL has only a temporary role, transiently representing the CS, or is part of the neuronal ensembles that support the retrieval, i.e., whether PL neurons support both transient, short-term memories and stable, long-term memories. We investigated neuronal ensembles underlying temporal associations using fear conditioning with a 5-s interval between the CS and US (CFC-5s). Controls were trained in contextual fear conditioning (CFC), in which the CS-US overlaps. We used Robust Activity Marking (RAM) to selectively manipulate PL neurons activated by CFC-5s learning and Targeted Recombination in Active Populations (TRAP2) mice to label neurons activated by CFC-5s learning and reactivated by memory retrieval in the amygdala, medial prefrontal cortex, hippocampus, perirhinal cortices (PER) and subiculum. We also computed their co-reactivation to generate correlation-based networks. The optogenetic reactivation or silencing of PL encoding ensembles either promoted or impaired the retrieval of CFC-5s but not CFC. CFC-5s retrieval reactivated encoding ensembles in the PL, PER, and basolateral amygdala. The engram network of CFC-5s had higher amygdala and PER centralities and interconnectivity. The same PL neurons support learning and stable associative memories.

An ultra-short-acting benzodiazepine in thalamic nucleus reuniens undermines fear extinction via intermediation of hippocamposeptal circuits

Benzodiazepines, commonly used for anxiolytics, hinder conditioned fear extinction, and the underlying circuit mechanisms are unclear. Utilizing remimazolam, an ultra-short-acting benzodiazepine, here we reveal its impact on the thalamic nucleus reuniens (RE) and interconnected hippocamposeptal circuits during fear extinction. Systemic or RE-specific administration of remimazolam impedes fear extinction by reducing RE activation through A type GABA receptors. Remimazolam enhances long-range GABAergic inhibition from lateral septum (LS) to RE, underlying the compromised fear extinction. RE projects to ventral hippocampus (vHPC), which in turn sends projections characterized by feed-forward inhibition to the GABAergic neurons of the LS. This is coupled with long-range GABAergic projections from the LS to RE, collectively constituting an overall positive feedback circuit construct that promotes fear extinction. RE-specific remimazolam negates the facilitation of fear extinction by disrupting this circuit. Thus, remimazolam in RE disrupts fear extinction caused by hippocamposeptal intermediation, offering mechanistic insights for the dilemma of combining anxiolytics with extinction-based exposure therapy.

Dissociable contributions of the amygdala and ventral hippocampus to stress-induced changes in defensive behavior

BACKGROUND

Severe stress can produce multiple persistent changes in defensive behavior relevant to psychiatric illness. While much is known about the circuits supporting stress-induced associative fear, how stress-induced circuit plasticity supports non-associative changes in defensive behavior remains unclear.

METHODS

Mice were exposed to an acute severe stressor, and subsequently, both associative and non-associative defensive behavioral responses were assessed. A mixture of local protein synthesis inhibition, pan-neuronal chemogenetic inhibition, and projection-specific chemogenetic inhibition were utilized to isolate the roles of the basolateral amygdala (BLA) and ventral hippocampus (vHC) to the induction and expression of associative and non-associative defensive behavioral changes.

RESULTS

Stress-induced protein synthesis in the BLA was necessary for enhancements in stress sensitivity but not enhancements in anxiety-related behaviors, whereas protein synthesis in the vHC was necessary for enhancements in anxiety-related behavior but not enhancements in stress sensitivity. Like protein synthesis, neuronal activity of the BLA and vHC were found to differentially support the expression of these same defensive behaviors. Additionally, projection-specific inhibition of BLA-vHC connections failed to alter these behaviors, indicating that these defensive behaviors are regulated by distinct BLA and vHC circuits. Lastly, contributions of the BLA and vHC to stress sensitivity and anxiety-related behavior were independent of their contributions to associative fear.

CONCLUSIONS

Stress-induced plasticity in the BLA and vHC were found to support dissociable non-associative behavioral changes, with BLA supporting enhancements in stress sensitivity and vHC supporting increased anxiety-related behavior. These findings demonstrate that independent BLA and vHC circuits are critical for stress-induced defensive behaviors, and that differential targeting of BLA and vHC circuits may be needed in disease treatment.

Disconnecting prefrontal cortical neurons from the ventral midline thalamus: Loss of specificity due to progressive neural toxicity of an AAV-Cre in the rat thalamus

BACKGROUND

The thalamic reuniens (Re) and rhomboid (Rh) nuclei are bidirectionally connected with the medial prefrontal cortex (mPFC) and the hippocampus (Hip). Fiber-sparing N-methyl-D-aspartate lesions of the ReRh disrupt cognitive functions, including persistence of certain memories. Because such lesions irremediably damage neurons interconnecting the ReRh with the mPFC and the Hip, it is impossible to know if one or both pathways contribute to memory persistence. Addressing such an issue requires selective, pathway-restricted and direction-specific disconnections.

NEW METHOD

A recent method associates a retrograde adeno-associated virus (AAV) expressing Cre recombinase with an anterograde AAV expressing a Cre-dependent caspase, making such disconnection feasible by caspase-triggered apoptosis when both constructs meet intracellularly. We injected an AAVrg-Cre-GFP into the ReRh and an AAV5-taCasp into the mPFC. As expected, part of mPFC neurons died, but massive neurotoxicity of the AAVrg-Cre-GFP was found in ReRh, contrasting with normal density of DAPI staining. Other stainings demonstrated increasing density of reactive astrocytes and microglia in the neurodegeneration site.

COMPARISON WITH EXISTING METHODS

Reducing the viral titer (by a 4-fold dilution) and injection volume (to half) attenuated toxicity substantially, still with evidence for partial disconnection between mPFC and ReRh.

CONCLUSIONS

There is an imperative need to verify potential collateral damage inherent in this type of approach, which is likely to distort interpretation of experimental data. Therefore, controls allowing to distinguish collateral phenotypic effects from those linked to the desired disconnection is essential. It is also crucial to know for how long neurons expressing the Cre-GFP protein remain operational post-infection.

Beyond hippocampus: Thalamic and prefrontal contributions to an evolving memory

The hippocampus has long been at the center of memory research, and rightfully so. However, with emerging technological capabilities, we can increasingly appreciate memory as a more dynamic and brain-wide process. In this perspective, our goal is to begin developing models to understand the gradual evolution, reorganization, and stabilization of memories across the brain after their initial formation in the hippocampus. By synthesizing studies across the rodent and human literature, we suggest that as memory representations initially form in hippocampus, parallel traces emerge in frontal cortex that cue memory recall, and as they mature, with sustained support initially from limbic then diencephalic then cortical circuits, they become progressively independent of hippocampus and dependent on a mature cortical representation. A key feature of this model is that, as time progresses, memory representations are passed on to distinct circuits with progressively longer time constants, providing the opportunity to filter, forget, update, or reorganize memories in the process of committing to long-term storage.

Significance of the anterior cingulate cortex in neurogenesis plasticity: Connections, functions, and disorders across postnatal and adult stages

The anterior cingulate cortex (ACC) is a complex and continually evolving brain region that remains a primary focus of research due to its multifaceted functions. Various studies and analyses have significantly advanced our understanding of how the ACC participates in a wide spectrum of memory and cognitive processes. However, despite its strong connections to brain areas associated with hippocampal and olfactory neurogenesis, the functions of the ACC in regulating postnatal and adult neurogenesis in these regions are still insufficiently explored. Investigating the intricate involvement of the ACC in neurogenesis could enhance our comprehension of essential aspects of brain plasticity. This involvement stems from its complex circuitry with other relevant brain regions, thereby exerting both direct and indirect impacts on the neurogenesis process. This review sheds light on the promising significance of the ACC in orchestrating postnatal and adult neurogenesis in conditions related to memory, cognitive behavior, and associated disorders.

A prefrontal-thalamic circuit encodes social information for social recognition

Social recognition encompasses encoding social information and distinguishing unfamiliar from familiar individuals to form social relationships. Although the medial prefrontal cortex (mPFC) is known to play a role in social behavior, how identity information is processed and by which route it is communicated in the brain remains unclear. Here we report that a ventral midline thalamic area, nucleus reuniens (Re) that has reciprocal connections with the mPFC, is critical for social recognition in male mice. In vivo single-unit recordings and decoding analysis reveal that neural populations in both mPFC and Re represent different social stimuli, however, mPFC coding capacity is stronger. We demonstrate that chemogenetic inhibitions of Re impair the mPFC-Re neural synchronization and the mPFC social coding. Projection pathway-specific inhibitions by optogenetics reveal that the reciprocal connectivity between the mPFC and the Re is necessary for social recognition. These results reveal an mPFC-thalamic circuit for social information processing.

Neural circuits for the adaptive regulation of fear and extinction memory

The regulation of fear memories is critical for adaptive behaviors and dysregulation of these processes is implicated in trauma- and stress-related disorders. Treatments for these disorders include pharmacological interventions as well as exposure-based therapies, which rely upon extinction learning. Considerable attention has been directed toward elucidating the neural mechanisms underlying fear and extinction learning. In this review, we will discuss historic discoveries and emerging evidence on the neural mechanisms of the adaptive regulation of fear and extinction memories. We will focus on neural circuits regulating the acquisition and extinction of Pavlovian fear conditioning in rodent models, particularly the role of the medial prefrontal cortex and hippocampus in the contextual control of extinguished fear memories. We will also consider new work revealing an important role for the thalamic nucleus reuniens in the modulation of prefrontal-hippocampal interactions in extinction learning and memory. Finally, we will explore the effects of stress on this circuit and the clinical implications of these findings.

Amelioration of the brain structural connectivity is accompanied with changes of gut microbiota in a tuberous sclerosis complex mouse model

Tuberous sclerosis complex (TSC) is a genetic disease that causes benign tumors and dysfunctions in many organs, including the brain. Aside from the brain malformations, many individuals with TSC exhibit neuropsychiatric symptoms. Among these symptoms, autism spectrum disorder (ASD) is one of the most common co-morbidities, affecting up to 60% of the population. Past neuroimaging studies strongly suggested that the impairments in brain connectivity contribute to ASD, whether or not TSC-related. Specifically, the tract-based diffusion tensor imaging (DTI) analysis provides information on the fiber integrity and has been used to study the neuropathological changes in the white matter of TSC patients with ASD symptoms. In our previous study, curcumin, a diet-derived mTOR inhibitor has been shown to effectively mitigate learning and memory deficits and anxiety-like behavior in Tsc2+/- mice via inhibiting astroglial proliferation. Recently, gut microbiota, which is greatly influenced by the diet, has been considered to play an important role in regulating several components of the central nervous system, including glial functions. In this study, we showed that the abnormal social behavior in the Tsc2+/- mice can be ameliorated by the dietary curcumin treatment. Second, using tract-based DTI analysis, we found that the Tsc2+/- mice exhibited altered fractional anisotropy, axial and radial diffusivities of axonal bundles connecting the prefrontal cortex, nucleus accumbens, hypothalamus, and amygdala, indicating a decreased brain network. Third, the dietary curcumin treatment improved the DTI metrics, in accordance with changes in the gut microbiota composition. At the bacterial phylum level, we showed that the abundances of Actinobacteria, Verrucomicrobia, and Tenericutes were significantly correlated with the DTI metrics FA, AD, and RD, respectively. Finally, we revealed that the expression of myelin-associated proteins, myelin bassic protein (MBP) and proteolipid protein (PLP) was increased after the treatment. Overall, we showed a strong correlation between structural connectivity alterations and social behavioral deficits, as well as the diet-dependent changes in gut microbiota composition.

PREFRONTAL CORRELATES OF FEAR GENERALIZATION DURING ENDOCANNABINOID DEPLETION

Maladaptive fear generalization is one of the hallmarks of trauma-related disorders. The endocannabinoid 2-arachidonoylglycerol (2-AG) is crucial for modulating anxiety, fear, and stress adaptation but its role in balancing fear discrimination versus generalization is not known. To address this, we used a combination of plasma endocannabinoid measurement and neuroimaging from a childhood maltreatment exposed and non-exposed mixed population combined with human and rodent fear conditioning models. Here we show that 2-AG levels are inversely associated with fear generalization at the behavioral level in both mice and humans. In mice, 2-AG depletion increases the proportion of neurons, and the similarity between neuronal representations, of threat-predictive and neutral stimuli within prelimbic prefrontal cortex ensembles. In humans, increased dorsolateral prefrontal cortical-amygdala resting state connectivity is inversely correlated with fear generalization. These data provide convergent cross-species evidence that 2-AG is a key regulator of fear generalization and suggest 2-AG deficiency could represent a trauma-related disorder susceptibility endophenotype.

Sex-specific prefrontal-hypothalamic control of behavior and stress responding

Depression and cardiovascular disease are both augmented by daily life stress. Yet, the biological mechanisms that translate psychological stress into affective and physiological outcomes are unknown. Previously, we demonstrated that stimulation of the ventromedial prefrontal cortex (vmPFC) has sexually divergent outcomes on behavior and physiology. Importantly, the vmPFC does not innervate the brain regions that initiate autonomic or neuroendocrine stress responses; thus, we hypothesized that intermediate synapses integrate cortical information to regulate stress responding. The posterior hypothalamus (PH) directly innervates stress-effector regions and receives substantial innervation from the vmPFC. In the current studies, circuit-specific approaches examined whether vmPFC synapses in the PH coordinate stress responding. Here we tested the effects of optogenetic vmPFC-PH circuit stimulation in male and female rats on social and motivational behaviors as well as physiological stress responses. Additionally, an intersectional genetic approach was used to knock down synaptobrevin in PH-projecting vmPFC neurons. Our collective results indicate that male vmPFC-PH circuitry promotes positive motivational valence and is both sufficient and necessary to reduce sympathetic-mediated stress responses. In females, the vmPFC-PH circuit does not affect social or preference behaviors but is sufficient and necessary to elevate neuroendocrine stress responses. Altogether, these data suggest cortical regulation of stress reactivity and behavior is mediated, in part, by projections to the hypothalamus that function in a sex-specific manner.

Memory Storage in Distributed Engram Cell Ensembles

One of the most fascinating aspects of the brain is its ability to acquire new information from experience and retain it over time as memory. The search for physical correlates of memory, the memory engram, has been a longstanding priority in modern neurobiology. Advanced genetic approaches have led to the localization of engram cells in a few brain regions, including the hippocampus and cortex. Additionally, engram cells exhibit learning-induced, persistent modifications and have at least two states, active and silent. However, it has been hypothesized that engrams for a specific memory are distributed among multiple brain regions that are functionally connected, referred to as a unified engram complex. Recent tissue-clearing techniques have permitted high-throughput analyses of intact brain samples, which have been used to obtain a map of the engram complex for a contextual fear memory. Careful examination of these engram complex maps has revealed a potentially underappreciated contribution of subcortical regions, specifically thalamic nuclei, to memory function. These more holistic studies support the unified engram complex hypothesis for memory storage and have important implications for understanding dysfunctional engrams in the context of human disease.