Bidirectional synaptic changes in deep and superficial hippocampal neurons following in vivo activity

A cortical-hippocampal communication undergoes rebalancing after new learning

The brain's ability to consolidate a wide range of memories while maintaining their distinctiveness across experiences remains poorly understood. Sharp-wave ripples, neural oscillations that occur predominantly within CA1 of the hippocampus during immobility and sleep, have been shown to play a critical role in the consolidation process. More recently, evidence has uncovered functional heterogeneity of pyramidal neurons within distinct sublayers of CA1 that display unique properties during ripples, potentially contributing to memory specificity. Despite this, it remains unclear exactly how ripples shift the activity of CA1 neuronal populations to accommodate the consolidation of specific memories and how sublayer differences manifest. Here, we studied interactions between the anterior cingulate cortex (ACC) and CA1 neurons during ripples and discovered a reorganization of their communication following learning. Notably, this reorganization appeared specifically for CA1 superficial (CA1sup) sublayer neurons. Utilizing a generalized linear model decoder, we demonstrate the pre-existence of ACC-to-CA1sup communication, which is suppressed during new learning and subsequent sleep suggesting that ACC activity may reallocate the contribution of CA1sup neurons during memory acquisition and consolidation. Further supporting this notion, we found that optogenetic stimulations of the ACC preferentially suppressed CA1sup interneurons while activating a unique subset of CA1 interneurons. Overall, these findings highlight a possible role of the ACC in rebalancing CA1 neuronal populations' contribution to ripple contents surrounding learning.

Cutting-edge methodologies for tagging and tracing active neuronal coding in the brain

Decoding the neural substrates that underlie learning and behavior is a fundamental goal in neuroscience. Identifying "key players" at the molecular, cellular, and circuit levels has become possible with recent advancements in molecular technologies offering high spatiotemporal resolution. Immediate-early genes are effective markers of neural activity and plasticity, allowing for the identification of active cells involved in memory-based behavior. A calcium-dependent labeling system coupled with light or biochemical proximity labeling allows characterization of active cell ensembles and circuitry across broader brain regions within short time windows, particularly during transient behaviors. The integration of these systems expands the ability to address diverse research questions across behavioral paradigms. This review examines current molecular systems for activity-dependent labeling, highlighting their applications in identifying specific cell ensembles and circuits relevant to various scientific questions and further discuss their significance, along with future directions for the development of innovative methodologies.

Development of Differential Sublaminar Feedforward Inhibitory Circuits in CA1 Hippocampus Requires Satb2

Pyramidal cells (PCs) in CA1 hippocampus can be classified by their radial position as deep or superficial and organize into subtype-specific circuits necessary for differential information processing. Specifically, superficial PCs receive fewer inhibitory synapses from parvalbumin (PV)-expressing interneurons than deep PCs, resulting in weaker feedforward inhibition of input from CA3 Schaffer collaterals. Using mice, we investigated mechanisms underlying CA1 PC differentiation and the development of this inhibitory circuit motif. We found that the transcriptional regulator SATB2, which is necessary for pyramidal cell differentiation in the neocortex, is selectively expressed in superficial PCs during early postnatal development. To investigate its role in CA1, we conditionally knocked out Satb2 from pyramidal cells during embryonic development using both male and female Emx1IRES-Cre; Satb2flox/flox mice. Loss of Satb2 resulted in increased feedforward inhibition of CA3 Schaffer collateral input to superficial PCs, which matched that observed to deep PCs in control mice. Using paired whole-cell recordings between PCs and PV+ interneurons, we found this was due to an increase in the strength of unitary inhibitory synaptic connections from PV+ interneurons to mutant superficial PCs. Regulation of synapse strength was restricted to inhibitory synapses; excitatory synaptic connections from CA3 to CA1 PCs and CA1 PCs to PV+ interneurons were not affected by loss of Satb2 Finally, we show that SATB2 expression in superficial PCs is necessary to suppress the formation of synapses from PV+ interneurons during synaptogenesis. Thus, early postnatal expression of SATB2 in superficial PCs is necessary for the development of biased feedforward inhibition in CA1.

Organizing the coactivity structure of the hippocampus from robust to flexible memory

New memories are integrated into prior knowledge of the world. But what if consecutive memories exert opposing demands on the host brain network? We report that acquiring a robust (food-context) memory constrains the mouse hippocampus within a population activity space of highly correlated spike trains that prevents subsequent computation of a flexible (object-location) memory. This densely correlated firing structure developed over repeated mnemonic experience, gradually coupling neurons in the superficial sublayer of the CA1 stratum pyramidale to whole-population activity. Applying hippocampal theta-driven closed-loop optogenetic suppression to mitigate this neuronal recruitment during (food-context) memory formation relaxed the topological constraint on hippocampal coactivity and restored subsequent flexible (object-location) memory. These findings uncover an organizational principle for the peer-to-peer coactivity structure of the hippocampal cell population to meet memory demands.

Circuit dynamics of superficial and deep CA1 pyramidal cells and inhibitory cells in freely moving macaques

Diverse neuron classes in hippocampal CA1 have been identified through the heterogeneity of their cellular/molecular composition. How these classes relate to hippocampal function and the network dynamics that support cognition in primates remains unclear. Here, we report inhibitory functional cell groups in CA1 of freely moving macaques whose diverse response profiles to network states and each other suggest distinct and specific roles in the functional microcircuit of CA1. In addition, pyramidal cells that were grouped by their superficial or deep layer position differed in firing rate, burstiness, and sharp-wave ripple-associated firing. They also showed strata-specific spike-timing interactions with inhibitory cell groups, suggestive of segregated neural populations. Furthermore, ensemble recordings revealed that cell assemblies were preferentially organized according to these strata. These results suggest that hippocampal CA1 in freely moving macaques bears a sublayer-specific circuit organization that may shape its role in cognition.

Brain oscillations: Hippocampal-prefrontal ripples unfolded

Memory consolidation is the process of translating memory traces from the hippocampus to the cortex. Hippocampal ripples are key in driving this transfer. A new study now shows that independent cortical ripples can suppress this communication. What could be the underlying mechanisms?

Circuit dynamics of superficial and deep CA1 pyramidal cells and inhibitory cells in freely-moving macaques

Diverse neuron classes in hippocampal CA1 have been identified through the heterogeneity of their cellular/molecular composition. How these classes relate to hippocampal function and the network dynamics that support cognition in primates remains unclear. Here we report inhibitory functional cell groups in CA1 of freely-moving macaques whose diverse response profiles to network states and each other suggest distinct and specific roles in the functional microcircuit of CA1. In addition, pyramidal cells that were segregated into superficial and deep layers differed in firing rate, burstiness, and sharp-wave ripple-associated firing. They also showed strata-specific spike-timing interactions with inhibitory cell groups, suggestive of segregated neural populations. Furthermore, ensemble recordings revealed that cell assemblies were preferentially organized according to these strata. These results suggest sublayer-specific circuit organization in hippocampal CA1 of the freely-moving macaques that may underlie its role in cognition.

Development of differential sublaminar feedforward inhibitory circuits in CA1 hippocampus requires Satb2

Pyramidal cells (PCs) in CA1 hippocampus can be classified by their radial position as deep or superficial and organize into subtype-specific circuits necessary for differential information processing. Specifically, superficial PCs receive fewer inhibitory synapses from parvalbumin (PV)-expressing interneurons than deep PCs, resulting in weaker feedforward inhibition of input from CA3 Schaffer collaterals. Using mice, we investigated mechanisms underlying PC differentiation and the development of this inhibitory circuit motif. We found that expression of the transcriptional regulator SATB2 is biased towards superficial PCs during early postnatal development and necessary to suppress PV+ interneuron synapse formation. In the absence of SATB2, the number of PV+ interneuron synaptic puncta surrounding superficial PCs increases during development to match deep PCs. This results in equivalent inhibitory current strength observed in paired whole-cell recordings, and equivalent feedforward inhibition of Schaffer collateral input. Thus, SATB2 is necessary for superficial PC differentiation and biased feedforward inhibition in CA1.

Between neurons and networks: investigating mesoscale brain connectivity in neurological and psychiatric disorders

The study of brain connectivity has been a cornerstone in understanding the complexities of neurological and psychiatric disorders. It has provided invaluable insights into the functional architecture of the brain and how it is perturbed in disorders. However, a persistent challenge has been achieving the proper spatial resolution, and developing computational algorithms to address biological questions at the multi-cellular level, a scale often referred to as the mesoscale. Historically, neuroimaging studies of brain connectivity have predominantly focused on the macroscale, providing insights into inter-regional brain connections but often falling short of resolving the intricacies of neural circuitry at the cellular or mesoscale level. This limitation has hindered our ability to fully comprehend the underlying mechanisms of neurological and psychiatric disorders and to develop targeted interventions. In light of this issue, our review manuscript seeks to bridge this critical gap by delving into the domain of mesoscale neuroimaging. We aim to provide a comprehensive overview of conditions affected by aberrant neural connections, image acquisition techniques, feature extraction, and data analysis methods that are specifically tailored to the mesoscale. We further delineate the potential of brain connectivity research to elucidate complex biological questions, with a particular focus on schizophrenia and epilepsy. This review encompasses topics such as dendritic spine quantification, single neuron morphology, and brain region connectivity. We aim to showcase the applicability and significance of mesoscale neuroimaging techniques in the field of neuroscience, highlighting their potential for gaining insights into the complexities of neurological and psychiatric disorders.

Disgust sensitivity and psychopathic behavior: A narrative review

Humans live under constant threat from pathogenic microorganisms and minimizing such threat has been a major evolutionary selective force in shaping human behavior and health. A particular adaptive mechanism against the harm caused by parasites and their infectiousness is disgust sensitivity, which has evolved to detect and avoid poisonous foods as well as bodily secretions harboring virulent microorganisms. This ubiquitous and reflexive behavior requires the integration of several internal and external sensory signals between the brain, the autonomic nervous system (ANS), and the gastrointestinal tract. Although the emotional expression of disgust is experienced by almost all individuals, the neural mechanisms of sensory signals underlying disgust sensitivity may differ in certain psychiatric conditions. Psychopathy, for instance, is a personality disorder in which disgust sensitivity to contagious bodily secretions is apparently absent or downregulated from its atypical personality temperament. In this review, we provide convergent behavioral, anatomical, and cellular evidence to suggest that a fractured experience of disgust sensitivity might be an additional feature of psychopathic behavior. First, we discuss the neural networks of certain brain regions mediating the emotional states of disgust and then discuss the intersection of the ANS and gastrointestinal tract in the processing of disgust and its relevance to aberrant antisocial behavior. Together, this work highlights the interconnections between the brain and the bilateral body plan as an integrated cell network that is relevant for understanding common principles underlying function and dysfunction of disgust levels in psychiatric domains.

Approaches and considerations of studying neuronal ensembles: a brief review

First theorized by Hebb, neuronal ensembles have provided a framework for understanding how the mammalian brain operates, especially regarding learning and memory. Neuronal ensembles are discrete, sparsely distributed groups of neurons that become activated in response to a specific stimulus and are thought to provide an internal representation of the world. Beyond the study of region-wide or projection-wide activation, the study of ensembles offers increased specificity and resolution to identify and target specific memories or associations. Neuroscientists interested in the neurobiology of learning, memory, and motivated behavior have used electrophysiological-, calcium-, and protein-based proxies of neuronal activity in preclinical models to better understand the neurobiology of learned and motivated behaviors. Although these three approaches may be used to pursue the same general goal of studying neuronal ensembles, technical differences lead to inconsistencies in the output and interpretation of data. This mini-review highlights some of the methodologies used in electrophysiological-, calcium-, and protein-based studies of neuronal ensembles and discusses their strengths and weaknesses.