The CA3 "backprojection" to the dentate gyrus

MRI-based axis-referenced morphometric model corresponding to lamellar organization for assessing hippocampal atrophy in dementia

Research on the local hippocampal atrophy for early detection of dementia has gained considerable attention. However, accurately quantifying subtle atrophy remains challenging in existing morphological methods due to the lack of consistent biological correspondence with the complex curving regions like the hippocampal head. Thereby, this article presents an innovative axis-referenced morphometric model (ARMM) that follows the anatomical lamellar organization of the hippocampus, which capture its precise and consistent longitudinal curving trajectory. Specifically, we establish an "axis-referenced coordinate system" based on a 7 T ex vivo hippocampal atlas following its entire curving longitudinal axis and orthogonal distributed lamellae. We then align individual hippocampi by deforming this template coordinate system to target spaces using boundary-guided diffeomorphic transformation, while ensuring that the lamellar vectors adhere to the constraint of medial-axis geometry. Finally, we measure local thickness and curvatures based on the coordinate system and boundary surface reconstructed from vector tips. The morphometric accuracy is evaluated by comparing reconstructed surfaces with those directly extracted from 7 T and 3 T MRI hippocampi. The results demonstrate that ARMM achieves the best performance, particularly in the curving head, surpassing the state-of-the-art morphological models. Additionally, morphological measurements from ARMM exhibit higher discriminatory power in distinguishing early Alzheimer's disease from mild cognitive impairment compared to volume-based measurements. Overall, the ARMM offers a precise morphometric assessment of hippocampal morphology on MR images, and sheds light on discovering potential image markers for neurodegeneration associated with hippocampal impairment.

Context and space coding in mossy cell population activity

The dentate gyrus plays a key role in the discrimination of memories by segregating and storing similar episodes. Whether hilar mossy cells, which constitute a major excitatory principal cell type in the mammalian hippocampus, contribute to this decorrelation function has remained largely unclear. Using two-photon calcium imaging of head-fixed mice performing a spatial virtual reality task, we show that mossy cell populations robustly discriminate between familiar and novel environments. The degree of discrimination depends on the extent of visual cue differences between contexts. A context decoder revealed that successful environmental classification is explained mainly by activity difference scores of mossy cells. By decoding mouse position, we reveal that in addition to place cells, the coordinated activity among active mossy cells markedly contributes to the encoding of space. Thus, by decorrelating context information according to the degree of environmental differences, mossy cell populations support pattern separation processes within the dentate gyrus.

Event Integration and Temporal Differentiation: How Hierarchical Knowledge Emerges in Hippocampal Subfields through Learning

Everyday life is composed of events organized by changes in contexts, with each event containing an unfolding sequence of occurrences. A major challenge facing our memory systems is how to integrate sequential occurrences within events while also maintaining their details and avoiding over-integration across different contexts. We asked if and how distinct hippocampal subfields come to hierarchically and, in parallel, represent both event context and subevent occurrences with learning. Female and male human participants viewed sequential events defined as sequences of objects superimposed on shared color frames while undergoing high-resolution fMRI. Importantly, these events were repeated to induce learning. Event segmentation, as indexed by increased reaction times at event boundaries, was observed in all repetitions. Temporal memory decisions were quicker for items from the same event compared to across different events, indicating that events shaped memory. With learning, hippocampal CA3 multivoxel activation patterns clustered to reflect the event context, with more clustering correlated with behavioral facilitation during event transitions. In contrast, in the dentate gyrus (DG), temporally proximal items that belonged to the same event became associated with more differentiated neural patterns. A computational model explained these results by dynamic inhibition in the DG. Additional similarity measures support the notion that CA3 clustered representations reflect shared voxel populations, while DG's distinct item representations reflect different voxel populations. These findings suggest an interplay between temporal differentiation in the DG and attractor dynamics in CA3. They advance our understanding of how knowledge is structured through integration and separation across time and context.

Dynamic and selective engrams emerge with memory consolidation

Episodic memories are encoded by experience-activated neuronal ensembles that remain necessary and sufficient for recall. However, the temporal evolution of memory engrams after initial encoding is unclear. In this study, we employed computational and experimental approaches to examine how the neural composition and selectivity of engrams change with memory consolidation. Our spiking neural network model yielded testable predictions: memories transition from unselective to selective as neurons drop out of and drop into engrams; inhibitory activity during recall is essential for memory selectivity; and inhibitory synaptic plasticity during memory consolidation is critical for engrams to become selective. Using activity-dependent labeling, longitudinal calcium imaging and a combination of optogenetic and chemogenetic manipulations in mouse dentate gyrus, we conducted contextual fear conditioning experiments that supported our model's predictions. Our results reveal that memory engrams are dynamic and that changes in engram composition mediated by inhibitory plasticity are crucial for the emergence of memory selectivity.

Disruption of TLE epileptiform activity retarded the seizure and reduced pathological HFOs

In temporal lobe epilepsy (TLE), the epileptogenic zones, such as the temporal lobe structure, could generate pathological high-frequency oscillations (pHFOs, 250-500 Hz) before the ictal period. These pHFOs have also been observed during the process of seizures in both TLE patients and animals, exhibiting a critical role as promising biomarkers for TLE seizures. TLE seizures could be modulated via regulating the neural excitability in epileptogenic zones, for that TLE is primarily associated with the excitation-inhibition imbalance. However, whether these kinds of modulations could also impact the pHFOs characteristics during TLE seizures is still unclear. For this purpose, we pharmaco-genetically inhibited the principal cells (PCs) in the mouse CA3 region and tracked the difference in the behavioral and electrophysiological features during LiCl-pilocarpine-induced TLE seizure between the hM4Di+CNO (experimental) mice and mCherry+CNO (control) mice. Delayed latency, decreased averaged duration, and reduced counts of the generalized seizure were observed in the experimental mice. Besides, the electrophysiological characteristics, such as the firing rate of PCs and the count of pHFO, exhibited significant decline in the CA3 and CA1 regions. During TLE seizure, there existed strong phase-coupling between pHFO and PCs spike timing in the control mice, while it was abolished in the experimental mice. In addition, we also found that the counts of pHFO were significantly associated with the behavioral features, indicating the close relationships within them. Collectively, our findings suggested that alterations in pHFO and the retardation of seizures may be attributed to disruptions in neuronal excitability, and the variations of electrophysiological features were related to seizure severity during TLE seizures. These results provide valuable insights into the role of pHFOs in TLE and shed light on the underlying mechanisms involved.

The flow of axonal information among hippocampal sub-regions 2: patterned stimulation sharpens routing of information transmission

The sub-regions of the hippocampal formation are essential for episodic learning and memory formation, yet the spike dynamics of each region contributing to this function are poorly understood, in part because of a lack of access to the inter-regional communicating axons. Here, we reconstructed hippocampal networks confined to four subcompartments in 2D cultures on a multi-electrode array that monitors individual communicating axons. In our novel device, somal, and axonal activity was measured simultaneously with the ability to ascertain the direction and speed of information transmission. Each sub-region and inter-regional axons had unique power-law spiking dynamics, indicating differences in computational functions, with abundant axonal feedback. After stimulation, spiking, and burst rates decreased in all sub-regions, spikes per burst generally decreased, intraburst spike rates increased, and burst duration decreased, which were specific for each sub-region. These changes in spiking dynamics post-stimulation were found to occupy a narrow range, consistent with the maintenance of the network at a critical state. Functional connections between the sub-region neurons and communicating axons in our device revealed homeostatic network routing strategies post-stimulation in which spontaneous feedback activity was selectively decreased and balanced by decreased feed-forward activity. Post-stimulation, the number of functional connections per array decreased, but the reliability of those connections increased. The networks maintained a balance in spiking and bursting dynamics in response to stimulation and sharpened network routing. These plastic characteristics of the network revealed the dynamic architecture of hippocampal computations in response to stimulation by selective routing on a spatiotemporal scale in single axons.

Regional and interhemispheric differences of neuronal representations in dentate gyrus and CA3 inferred from expression of zif268

The hippocampal formation is one of the best studied brain regions for spatial and mnemonic representations. These representations have been reported to differ in their properties for individual hippocampal subregions. One approach that allows the detection of neuronal representations is immediate early gene imaging, which relies on the visualization of genomic responses of activated neuronal populations, so called engrams. This method permits the within-animal comparison of neuronal representations across different subregions. In this work, we have used compartmental analysis of temporal activity by fluorescence in-situ hybridisation (catFISH) of the immediate early gene zif268/erg1 to compare neuronal representations between subdivisions of the dentate gyrus and CA3 upon exploration of different contexts. Our findings give an account of subregion-specific ensemble sizes. We confirm previous results regarding disambiguation abilities in dentate gyrus and CA3 but in addition report novel findings: Although ensemble sizes in the lower blade of the dentate gyrus are significantly smaller than in the upper blade both blades are responsive to environmental change. Beyond this, we show significant differences in the representation of familiar and novel environments along the longitudinal axis of dorsal CA3 and most interestingly between CA3 regions of both hemispheres.

High-resolution CMOS-based biosensor for assessing hippocampal circuit dynamics in experience-dependent plasticity

Experiential richness creates tissue-level changes and synaptic plasticity as patterns emerge from rhythmic spatiotemporal activity of large interconnected neuronal assemblies. Despite numerous experimental and computational approaches at different scales, the precise impact of experience on network-wide computational dynamics remains inaccessible due to the lack of applicable large-scale recording methodology. We here demonstrate a large-scale multi-site biohybrid brain circuity on-CMOS-based biosensor with an unprecedented spatiotemporal resolution of 4096 microelectrodes, which allows simultaneous electrophysiological assessment across the entire hippocampal-cortical subnetworks from mice living in an enriched environment (ENR) and standard-housed (SD) conditions. Our platform, empowered with various computational analyses, reveals environmental enrichment's impacts on local and global spatiotemporal neural dynamics, firing synchrony, topological network complexity, and large-scale connectome. Our results delineate the distinct role of prior experience in enhancing multiplexed dimensional coding formed by neuronal ensembles and error tolerance and resilience to random failures compared to standard conditions. The scope and depth of these effects highlight the critical role of high-density, large-scale biosensors to provide a new understanding of the computational dynamics and information processing in multimodal physiological and experience-dependent plasticity conditions and their role in higher brain functions. Knowledge of these large-scale dynamics can inspire the development of biologically plausible computational models and computational artificial intelligence networks and expand the reach of neuromorphic brain-inspired computing into new applications.

Passive array micro-magnetic stimulation device based on multi-carrier wireless flexible control for magnetic neuromodulation

Objective.The passive micro-magnetic stimulation (µMS) devices typically consist of an external transmitting coil and a single internal micro-coil, which enables a point-to-point energy supply from the external coil to the internal coil and the realization of magnetic neuromodulation via wireless energy transmission. The internal array of micro coils can achieve multi-target stimulation without movement, which improves the focus and effectiveness of magnetic stimulations. However, achieving a free selection of an appropriate external coil to deliver energy to a particular internal array of micro-coils for multiple stimulation targets has been challenging. To address this challenge, this study uses a multi-carrier modulation technique to transmit the energy of the external coil.Approach.In this study, a theoretical model of a multi-carrier resonant compensation network for the arrayµMS is established based on the principle of magnetically coupled resonance. The resonant frequency coupling parameter corresponding to each micro-coil of the arrayµMS is determined, and the magnetic field interference between the external coil and its non-resonant micro-coils is eliminated. Therefore, an effective magnetic stimulation threshold for a micro-coil corresponding to the target is determined, and wireless free control of the internal micro-coil array is achieved by using an external transmitting coil.Main results.The passiveµMS array model is designed using a multi-carrier wireless modulation method, and its synergistic modulation of the magnetic stimulation of synaptic plasticity long-term potentiation in multiple hippocampal regions is investigated using hippocampal isolated brain slices.Significance.The results presented in this study could provide theoretical and experimental bases for implantable micro-magnetic device-targeted therapy, introducing an efficient method for diagnosis and treatment of neurological diseases and providing innovative ideas for in-depth application of micro-magnetic stimulation in the neuroscience field.

Hippocampal network axons respond to patterned theta burst stimulation with lower activity of initially higher spike train similarity from EC to DG and later similarity of axons from CA1 to EC

Objective. Decoding memory functions for each hippocampal subregion involves extensive understanding of how each hippocampal subnetwork processes input stimuli. Theta burst stimulation (TBS) recapitulates natural brain stimuli which potentiates synapses in hippocampal circuits. TBS is typically applied to a bundle of axons to measure the immediate response in a downstream subregion like the cornu ammonis 1 (CA1). Yet little is known about network processing in response to stimulation, especially because individual axonal transmission between subregions is not accessible.Approach. To address these limitations, we reverse engineered the hippocampal network on a micro-electrode array partitioned by a MEMS four-chambered device with interconnecting microfluidic tunnels. The micro tunnels allowed monitoring single axon transmission which is inaccessible in slices orin vivo. The four chambers were plated separately with entorhinal cortex (EC), dentate gyrus (DG), CA1, and CA3 neurons. The patterned TBS was delivered to the EC hippocampal gateway. Evoked spike pattern similarity in each subregions was quantified with Jaccard distance metrics of spike timing.Main results. We found that the network subregion produced unique axonal responses to different stimulation patterns. Single site and multisite stimulations caused distinct information routing of axonal spikes in the network. The most spatially similar output at axons from CA3 to CA1 reflected the auto association within CA3 recurrent networks. Moreover, the spike pattern similarities shifted from high levels for axons to and from DG at 0.2 s repeat stimuli to greater similarity in axons to and from CA1 for repetitions at 10 s intervals. This time-dependent response suggested that CA3 encoded temporal information and axons transmitted the information to CA1.Significance. Our design and interrogation approach provide first insights into differences in information transmission between the four subregions of the structured hippocampal network and the dynamic pattern variations in response to stimulation at the subregional level to achieve probabilistic pattern separation and novelty detection.

Hippocampus: Molecular, Cellular, and Circuit Features in Anxiety

Anxiety disorders are currently a major psychiatric and social problem, the mechanisms of which have been only partially elucidated. The hippocampus serves as a major target of stress mediators and is closely related to anxiety modulation. Yet so far, its complex anatomy has been a challenge for research on the mechanisms of anxiety regulation. Recent advances in imaging, virus tracking, and optogenetics/chemogenetics have permitted elucidation of the activity, connectivity, and function of specific cell types within the hippocampus and its connected brain regions, providing mechanistic insights into the elaborate organization of the hippocampal circuitry underlying anxiety. Studies of hippocampal neurotransmitter systems, including glutamatergic, GABAergic, cholinergic, dopaminergic, and serotonergic systems, have contributed to the interpretation of the underlying neural mechanisms of anxiety. Neuropeptides and neuroinflammatory factors are also involved in anxiety modulation. This review comprehensively summarizes the hippocampal mechanisms associated with anxiety modulation, based on molecular, cellular, and circuit properties, to provide tailored targets for future anxiety treatment.

Aminoprocalcitonin protects against hippocampal neuronal death via preserving oxidative phosphorylation in refractory status epilepticus

Refractory status epilepticus (RSE) is a neurological emergency where sustaining seizure causes severe neuronal death. Currently, there is no available neuroprotectant effective in RSE. Aminoprocalcitonin (NPCT) is a conserved peptide cleaved from procalcitonin, but its distribution and function in the brain remain enigmatic. Survival of neurons relies on sufficient energy supply. Recently, we found that NPCT was extensively distributed in the brain and had potent modulations on neuronal oxidative phosphorylation (OXPHOS), suggesting that NPCT might be involved in neuronal death by regulating energy status. In the present study, combining biochemical and histological methods, high-throughput RNA-sequence, Seahorse XFe analyser, an array of mitochondria function assays, and behavior-electroencephalogram (EEG) monitoring, we investigated the roles and translational values of NPCT in neuronal death after RSE. We found that NPCT was extensively distributed throughout gray matters in rat brain while RSE triggered NPCT overexpression in hippocampal CA3 pyramidal neurons. High-throughput RNA-sequence demonstrated that the influences of NPCT on primary hippocampal neurons were enriched in OXPHOS. Further function assays verified that NPCT facilitated ATP production, enhanced the activities of mitochondrial respiratory chain complexes I, IV, V, and increased neuronal maximal respiration capacity. NPCT exerted multiple neurotrophic effects including facilitating synaptogenesis, neuritogenesis, spinogenesis, and suppression of caspase-3. A polyclonal NPCT immunoneutralization antibody was developed to antagonize NPCT. In the in vitro 0-Mg²⁺ seizure model, immunoneutralization of NPCT caused more neuronal death, while exogenous NPCT supplementation, though did not reverse death outcomes, preserved mitochondrial membrane potential. In rat RSE model, both peripheral and intracerebroventricular immunoneutralization of NPCT exacerbated hippocampal neuronal death and peripheral immunoneutralization increased mortality. Intracerebroventricular immunoneutralization of NPCT further led to more serious hippocampal ATP depletion, and significant EEG power exhaustion. We conclude that NPCT is a neuropeptide regulating neuronal OXPHOS. During RSE, NPCT was overexpressed to protect hippocampal neuronal survival via facilitating energy supply.

Global remapping in granule cells and mossy cells of the mouse dentate gyrus

Hippocampal place cells exhibit spatially modulated firing, or place fields, which can remap to encode changes in the environment or other variables. Unique among hippocampal subregions, the dentate gyrus (DG) has two excitatory populations of place cells, granule cells and mossy cells, which are among the least and most active spatially modulated cells in the hippocampus, respectively. Previous studies of remapping in the DG have drawn different conclusions about whether granule cells exhibit global remapping and contribute to the encoding of context specificity. By recording granule cells and mossy cells as mice foraged in different environments, we found that by most measures, both granule cells and mossy cells remapped robustly but through different mechanisms that are consistent with firing properties of each cell type. Our results resolve the ambiguity surrounding remapping in the DG and suggest that most spatially modulated granule cells contribute to orthogonal representations of distinct spatial contexts.

Altered connectomes of adult-born granule cells following engraftment of GABAergic progenitors in the mouse hippocampus

Adult neurogenesis occurs in the dentate gyrus (DG) of the rodent hippocampus throughout life, producing new granule cells (GCs) that migrate from a stem cell niche called the subgranular zone (SGZ) into the adjacent granule cell layer (GCL). Seizures associated with temporal lobe epilepsy alter adult neurogenesis and promote the formation of hyperexcitable circuits. Stem cell therapies for treating intractable seizure disorders are based on the premise that transplantation of GABAergic interneurons will strengthen inhibitory connections within the hippocampus and reduce hyperexcitability. Grafts of medial ganglionic eminence (MGE)-derived fetal GABAergic progenitors into the DG of adult mice with pilocarpine-induced TLE have been shown to suppress spontaneous recurrent seizures. In addition, the transplanted cells formed functional inhibitory synaptic connections with hippocampal neurons, including adult-born GCs. However, it is unknown whether MGE grafts change adult-born GC connectivity. To address this question, we compared the first-order monosynaptic inputs to adult-born GCs in TLE mice with or without MGE-derived interneuron grafts. Here we show that TLE increased excitatory inputs from endogenous hippocampal, entorhinal cortex, and medial septum/diagonal band neurons onto adult-born GCs. In contrast, in TLE mice with grafts, these excitatory inputs were reduced, coinciding with transplanted GABAergic interneuron innervation of adult-born GCs. These findings indicate that GABAergic interneuron transplantation into the dentate gyrus may prevent epilepsy-associated alterations in the connectivity of adult-born GCs.

Cell numbers in the reflected blade of CA3 and their relation to other hippocampal principal cell populations across seven species

The hippocampus of many mammals contains a histoarchitectural region that is not present in laboratory mice and rats-the reflected blade of the CA3 pyramidal cell layer. Pyramidal cells of the reflected blade do not extend dendrites into the hippocampal molecular layer, and recent evidence indicates that they, like the proximal CA3 pyramids in laboratory rats and mice, partially integrate functionally with the dentate circuitry in pattern separation. Quantitative assessments of phylogenetic or disease-related changes in the hippocampal structure and function treat the reflected blade heterogeneously. Depending on the ease with which it can be differentiated, it is either assigned to the dentate hilus or to the remainder of CA3. Here, we investigate the impact that the differential assignment of reflected blade neurons may have on the outcomes of quantitative comparisons. We find it to be massive. If reflected blade neurons are treated as a separate entity or pooled with dentate hilar cells, the quantitative makeup of hippocampal cell populations can differentiate between species in a taxonomically sensible way. Assigning reflected blade neurons to CA3 greatly diminishes the differentiating power of all hippocampal principal cell populations, which may point towards a quantitative hippocampal archetype. A heterogeneous assignment results in a differentiation pattern with little taxonomic semblance. The outcomes point towards the reflected blade as either a major potential player in hippocampal functional and structural differentiation or a region that may have cloaked that hippocampi are more similarly organized across species than generally believed.

Glucagon-like peptide-1 receptor differentially controls mossy cell activity across the dentate gyrus longitudinal axis

Understanding the role of dentate gyrus (DG) mossy cells (MCs) in learning and memory has rapidly evolved due to increasingly precise methods for targeting MCs and for in vivo recording and activity manipulation in rodents. These studies have shown MCs are highly active in vivo, strongly remap to contextual manipulation, and that their inhibition or hyperactivation impairs pattern separation and location or context discrimination. Less well understood is how MC activity is modulated by neurohormonal mechanisms, which might differentially control the participation of MCs in cognitive functions during discrete states, such as hunger or satiety. In this study, we demonstrate that glucagon-like peptide-1 (GLP-1), a neuropeptide produced in the gut and the brain that regulates food consumption and hippocampal-dependent mnemonic function, might regulate MC function through expression of its receptor, GLP-1R. RNA-seq demonstrated that most, though not all, Glp1r in hippocampal principal neurons is expressed in MCs, and in situ hybridization revealed strong expression of Glp1r in hilar neurons. Glp1r-ires-Cre mice crossed with Ai14D reporter mice followed by co-labeling for the MC marker GluR2/3 revealed that almost all MCs in the ventral DG expressed Glp1r and that almost all Glp1r-expressing hilar neurons were MCs. However, only ~60% of dorsal DG MCs expressed Glp1r, and Glp1r was also expressed in small hilar neurons that were not MCs. Consistent with this expression pattern, peripheral administration of the GLP-1R agonist exendin-4 (5 μg/kg) increased cFos expression in ventral but not dorsal DG hilar neurons. Finally, whole-cell patch-clamp recordings from ventral MCs showed that bath application of exendin-4 (200 nM) depolarized MCs and increased action potential firing. Taken together, this study adds to known MC activity modulators a neurohormonal mechanism that may preferentially affect ventral DG physiology and may potentially be targetable by several GLP-1R pharmacotherapies already in clinical use.

Sex-biased effects on hippocampal circuit development by perinatal SERT expression in CA3 pyramidal neurons

Neurodevelopmental disorders ranging from autism to intellectual disability display sex-biased prevalence and phenotypical presentations. Despite increasing knowledge about temporospatial cortical map development and genetic variants linked to neurodevelopmental disorders, when and how sex-biased neural circuit derailment may arise in diseased brain remain unknown. Here, we identify in mice that serotonin uptake transporter (SERT) in non-serotonergic neurons - hippocampal and prefrontal pyramidal neurons - confers sex-biased effects specifically during neural circuit development. A set of gradient-patterned CA3 pyramidal neurons transiently express SERT to clear extracellular serotonin, coinciding with hippocampal synaptic circuit establishment. Ablating pyramidal neuron SERT (SERTPyramidΔ) alters dendritic spine developmental trajectory in the hippocampus, and precipitates sex-biased impairments in long-term activity-dependent hippocampal synaptic plasticity and cognitive behaviors. Transcriptomic analyses identify sex-biased alterations in gene sets associated with autism, dendritic spine structure, synaptic function and male-specific enrichment of dysregulated genes in glial cells in early postnatal SERTPyramidΔ hippocampus. Our data suggest that SERT function in these pyramidal neurons underscores a temporal- and brain region-specific regulation of normal sex-dimorphic circuit development and a source for sex-biased vulnerability to cognitive and behavioral impairments. This article has an associated 'The people behind the papers' interview.

Mossy cells of the dentate gyrus: Drivers or inhibitors of epileptic seizures?

Mossy cells (MCs) are glutamatergic cells of the dentate gyrus with an important role in temporal lobe epilepsy. Under physiological conditions MCs can control both network excitations via direct synapses to granule cells and inhibition via connections to GABAergic interneurons innervating granule cells. In temporal lobe epilepsy mossy cell loss is one of the major hallmarks, but whether the surviving MCs drive or inhibit seizure initiation and generalization is still a debate. The aim of the present review is to summarize the latest findings on the role of mossy cells in healthy and overexcited hippocampus.

Neuropeptides and small-molecule amine transmitters: cooperative signaling in the nervous system

Neuropeptides are expressed in cell-specific patterns throughout mammalian brain. Neuropeptide gene expression has been useful for clustering neurons by phenotype, based on single-cell transcriptomics, and for defining specific functional circuits throughout the brain. How neuropeptides function as first messengers in inter-neuronal communication, in cooperation with classical small-molecule amine transmitters (SMATs) is a current topic of systems neurobiology. Questions include how neuropeptides and SMATs cooperate in neurotransmission at the molecular, cellular and circuit levels; whether neuropeptides and SMATs always co-exist in neurons; where neuropeptides and SMATs are stored in the neuron, released from the neuron and acting, and at which receptors, after release; and how neuropeptides affect 'classical' transmitter function, both directly upon co-release, and indirectly, via long-term regulation of gene transcription and neuronal plasticity. Here, we review an extensive body of data about the distribution of neuropeptides and their receptors, their actions after neuronal release, and their function based on pharmacological and genetic loss- and gain-of-function experiments, that addresses these questions, fundamental to understanding brain function, and development of neuropeptide-based, and potentially combinatorial peptide/SMAT-based, neurotherapeutics.

Seizure-induced strengthening of a recurrent excitatory circuit in the dentate gyrus is proconvulsant

Epilepsy is a devastating brain disorder for which effective treatments are very limited. There is growing interest in early intervention, which requires a better mechanistic understanding of the early stages of this disorder. While diverse brain insults can lead to epileptic activity, a common cellular mechanism relies on uncontrolled recurrent excitatory activity. In the dentate gyrus, excitatory mossy cells (MCs) project extensively onto granule cells (GCs) throughout the hippocampus, thus establishing a recurrent MC-GC-MC excitatory loop. MCs are implicated in temporal lobe epilepsy, a common form of epilepsy, but their role during initial seizures (i.e., before the characteristic MC loss that occurs in late stages) is unclear. Here, we show that initial seizures acutely induced with an intraperitoneal kainic acid (KA) injection in adult mice, a well-established model that leads to experimental epilepsy, not only increased MC and GC activity in vivo but also triggered a brain-derived neurotrophic factor (BDNF)-dependent long-term potentiation (LTP) at MC-GC excitatory synapses. Moreover, in vivo induction of MC-GC LTP using MC-selective optogenetic stimulation worsened KA-induced seizures. Conversely, Bdnf genetic removal from GCs, which abolishes LTP, and selective MC silencing were both anticonvulsant. Thus, initial seizures are associated with MC-GC synaptic strengthening, which may promote later epileptic activity. Our findings reveal a potential mechanism of epileptogenesis that may help in developing therapeutic strategies for early intervention.

The fibro- and cyto-architecture demarcating the border between the dentate gyrus and CA3 in sheep (Ovis aries) and domestic pig (Sus scrofa domesticus)

The hippocampal formation is essential for spatial navigation and episodic memory. The anatomical structure is largely similar across mammalian species, apart from the deep polymorphic layer of the dentate gyrus and the adjacent part of cornu ammonis 3 (CA3) which feature substantial variations. In rodents, the polymorphic layer has a triangular cross‐section abutting on the end of the CA3 pyramidal layer, while in primates it is long and band‐shaped capping the expanded CA3 end, which here lacks a distinct pyramidal layer. This structural variation has resulted in a confusing nomenclature and unclear anatomical criteria for the definition of the dentate‐ammonic border. Seeking to clarify the border, we present here a light microscopic investigation based on Golgi‐impregnated and Timm–thionin‐stained sections of the Artiodactyla sheep and domestic pig, in which the dentate gyrus and CA3 end have some topographical features in common with primates. In short, the band‐shaped polymorphic layer coincides with the Timm‐positive mossy fiber collateral plexus and the Timm‐negative subgranular zone. While the soma and excrescence‐covered proximal dendrites of the mossy cells are localized within the plexus, the peripheral mossy cell dendrites extend outside the plexus, both into the granular and molecular layers, and the CA3. The main mossy fibers leave the collateral plexus in a scattered formation to converge gradually through the CA3 end in between the dispersed pyramidal cells, which are of three subtypes, as in monkey, with the classical apical subtype dominating near the hidden blade, the nonapical subtype near the exposed blade, and the dentate subtype being the only pyramidal cells that extend dendrites into the dentate gyrus. In agreement with our previous study in mink, the findings show that the border between the dentate gyrus and the CA3 end can be more accurately localized by the mossy fiber system than by cyto‐architecture alone.

UP-DOWN states and ripples differentially modulate membrane potential dynamics across DG, CA3, and CA1 in awake mice

Hippocampal ripples are transient population bursts that structure cortico-hippocampal communication and play a central role in memory processing. However, the mechanisms controlling ripple initiation in behaving animals remain poorly understood. Here we combine multisite extracellular and whole-cell recordings in awake mice to contrast the brain state and ripple modulation of subthreshold dynamics across hippocampal subfields. We find that entorhinal input to the dentate gyrus (DG) exhibits UP and DOWN dynamics with ripples occurring exclusively in UP states. While elevated cortical input in UP states generates depolarization in DG and CA1, it produces persistent hyperpolarization in CA3 neurons. Furthermore, growing inhibition is evident in CA3 throughout the course of the ripple buildup, while DG and CA1 neurons exhibit depolarization transients 100 ms before and during ripples. These observations highlight the importance of CA3 inhibition for ripple generation, while pre-ripple responses indicate a long and orchestrated ripple initiation process in the awake state.

The Polysaccharides from the Aerial Parts of Bupleurum chinense DC Attenuate Epilepsy-Like Behavior through Oxidative Stress Signaling Pathways

Bupleurum chinense DC. is a traditional Chinese medicine with a long medicinal history and is often used as the main ingredient in prescription drugs for epilepsy. The aerial parts of B. chinense DC. have similar efficacy and composition to B. chinense DC. Therefore, we speculated that the aerial parts of B. chinense DC. could be used in the treatment of epilepsy. Polysaccharides from the aerial parts of B. chinense DC. were selected to explore their therapeutic effects on epilepsy and their potential mechanism of action. The study is aimed at clarifying the antiepileptic effects of the polysaccharides from the aerial parts of B. chinense DC. and their potential underlying mechanisms. The chemical profile of the aerial parts of B. chinense DC. polysaccharides (ABP) was characterized by FT-IR spectrum and HPLC chromatogram. To determine the therapeutic effects of ABPs on epilepsy, we established a kainic acid- (KA-) induced rat model of epilepsy, and through H&E staining, Nissl staining, immunohistochemistry, biochemical analysis, ELISA, and Western blot analysis, we explored the mechanisms underlying the therapeutic effects of ABPs on epilepsy. The monosaccharide content of ABP included galacturonic acid (45.19%), galactose (36.63%), arabinose rhamnose (12.13%), and mannose (6.05%). Moreover, the average molecular weight of ABP was 1.38 × 10³ kDa. ABP could improve hippocampal injuries and neuronal function in the KA-induced epilepsy rat model. ABP significantly inhibited oxidative stress in the hippocampus of KA-induced rats. More importantly, ABP could regulate TREM2 activation in the PI3K/Akt/GSK-3β pathway to inhibit neuronal apoptosis, including increasing the expression of superoxide dismutase and lactate dehydrogenase and decreasing the expression of malondialdehyde. The current study defined the potential role of ABP in inhibiting the development of epilepsy, indicating that ABP could upregulate TREM2 to alleviate neuronal apoptosis, by activating the PI3K/Akt/GSK-3β pathway and oxidative stress in epilepsy.

Flexible encoding of objects and space in single cells of the dentate gyrus

The hippocampus is involved in the formation of memories that require associations among stimuli to construct representations of space and the items and events within that space. Neurons in the dentate gyrus (DG), an initial input region of the hippocampus, have robust spatial tuning, but it is unclear how nonspatial information may be integrated with spatial activity in this region. We recorded from the DG of 21 adult mice as they foraged for food in an environment that contained discrete objects. We found DG cells with multiple firing fields at a fixed distance and direction from objects (landmark vector cells) and cells that exhibited localized changes in spatial firing when objects in the environment were manipulated. By classifying recorded DG cells into putative dentate granule cells and mossy cells, we examined how the addition or displacement of objects affected the spatial firing of these DG cell types. Object-related activity was detected in a significant proportion of mossy cells. Although few granule cells with responses to object manipulations were recorded, likely because of the sparse nature of granule cell firing, there was generally no significant difference in the proportion of granule cells and mossy cells with object responses. When mice explored a second environment with the same objects, DG spatial maps completely reorganized, and a different subset of cells responded to object manipulations. Together, these data reveal the capacity of DG cells to detect small changes in the environment while preserving a stable spatial representation of the overall context.

Coordinated hippocampal-thalamic-cortical communication crucial for engram dynamics underneath systems consolidation

Systems consolidation refers to the time-dependent reorganization of memory representations or engrams across brain regions. Despite recent advancements in unravelling this process, the exact mechanisms behind engram dynamics and the role of associated pathways remain largely unknown. Here we propose a biologically-plausible computational model to address this knowledge gap. By coordinating synaptic plasticity timescales and incorporating a hippocampus-thalamus-cortex circuit, our model is able to couple engram reactivations across these regions and thereby reproduce key dynamics of cortical and hippocampal engram cells along with their interdependencies. Decoupling hippocampal-thalamic-cortical activity disrupts systems consolidation. Critically, our model yields testable predictions regarding hippocampal and thalamic engram cells, inhibitory engrams, thalamic inhibitory input, and the effect of thalamocortical synaptic coupling on retrograde amnesia induced by hippocampal lesions. Overall, our results suggest that systems consolidation emerges from coupled reactivations of engram cells in distributed brain regions enabled by coordinated synaptic plasticity timescales in multisynaptic subcortical-cortical circuits.

How connectivity rules and synaptic properties shape the efficacy of pattern separation in the entorhinal cortex-dentate gyrus-CA3 network

Pattern separation is a fundamental brain computation that converts small differences in input patterns into large differences in output patterns. Several synaptic mechanisms of pattern separation have been proposed, including code expansion, inhibition and plasticity; however, which of these mechanisms play a role in the entorhinal cortex (EC)-dentate gyrus (DG)-CA3 circuit, a classical pattern separation circuit, remains unclear. Here we show that a biologically realistic, full-scale EC-DG-CA3 circuit model, including granule cells (GCs) and parvalbumin-positive inhibitory interneurons (PV+-INs) in the DG, is an efficient pattern separator. Both external gamma-modulated inhibition and internal lateral inhibition mediated by PV+-INs substantially contributed to pattern separation. Both local connectivity and fast signaling at GC-PV+-IN synapses were important for maximum effectiveness. Similarly, mossy fiber synapses with conditional detonator properties contributed to pattern separation. By contrast, perforant path synapses with Hebbian synaptic plasticity and direct EC-CA3 connection shifted the network towards pattern completion. Our results demonstrate that the specific properties of cells and synapses optimize higher-order computations in biological networks and might be useful to improve the deep learning capabilities of technical networks.

New Insights and Methods for Recording and Imaging Spontaneous Spreading Depolarizations and Seizure-Like Events in Mouse Hippocampal Slices

Spreading depolarization (SD) is a sudden, large, and synchronous depolarization of principal cells which also involves interneurons and astrocytes. It is followed by depression of neuronal activity, and it slowly propagates across brain regions like cortex or hippocampus. SD is considered to be mechanistically relevant to migraine, epilepsy, and traumatic brain injury (TBI), but there are many questions about its basic neurophysiology and spread. Research into SD in hippocampus using slices is often used to gain insight and SD is usually triggered by a focal stimulus with or without an altered extracellular buffer. Here, we optimize an in vitro experimental model allowing us to record SD without focal stimulation, which we call spontaneous. This method uses only an altered extracellular buffer containing 0 mM Mg2+ and 5 mM K+ and makes it possible for simultaneous patch and extracellular recording in a submerged chamber plus intrinsic optical imaging in slices of either sex. We also add methods for quantification and show the quantified optical signal is much more complex than imaging alone would suggest. In brief, acute hippocampal slices were prepared with a chamber holding a submerged slice but with flow of artificial cerebrospinal fluid (aCSF) above and below, which we call interface-like. As soon as slices were placed in the chamber, aCSF with 0 Mg2+/5 K+ was used. Most mouse slices developed SD and did so in the first hour of 0 Mg2+/5 K+ aCSF exposure. In addition, prolonged bursts we call seizure-like events (SLEs) occurred, and the interactions between SD and SLEs suggest potentially important relationships. Differences between rats and mice in different chambers are described. Regarding optical imaging, SD originated in CA3 and the pattern of spread to CA1 and the dentate gyrus was similar in some ways to prior studies but also showed interesting differences. In summary, the methods are easy to use, provide new opportunities to study SD, new insights, and are inexpensive. They support previous suggestions that SD is diverse, and also suggest that participation by the dentate gyrus merits greater attention.

Direct synaptic excitation between hilar mossy cells revealed with a targeted voltage sensor

The dentate gyrus not only gates the flow of information into the hippocampus, it also integrates and processes this information. Mossy cells (MCs) are a major type of excitatory neuron strategically located in the hilus of the dentate gyrus where they can contribute to this processing through networks of synapses with inhibitory neurons and dentate granule cells. Some prior work has suggested that MCs can form excitatory synapses with other MCs, but the role of these synapses in the network activity of the dentate gyrus has received little attention. Here, we investigated synaptic inputs to MCs in mouse hippocampal slices using a genetically encoded hybrid voltage sensor (hVOS) targeted to MCs by Cre-lox technology. This enabled optical recording of voltage changes from multiple MCs simultaneously. Stimulating granule cells and CA3 pyramidal cells activated well-established inputs to MCs and elicited synaptic responses as expected. However, the weak blockade of MC responses to granule cell layer stimulation by DCG-IV raised the possibility of another source of excitation. To evaluate synapses between MCs as this source, single MCs were stimulated focally. Stimulation of one MC above its action potential threshold evoked depolarizing responses in neighboring MCs that depended on glutamate receptors. Short latency responses of MCs to other MCs did not depend on release from granule cell axons. However, granule cells did contribute to the longer latency responses of MCs to stimulation of other MCs. Thus, MCs transmit their activity to other MCs both through direct synaptic coupling and through polysynaptic coupling with dentate granule cells. MC-MC synapses can redistribute information entering the dentate gyrus and thus shape and modulate the electrical activity underlying hippocampal functions such as navigation and memory, as well as excessive excitation during seizures.

Epileptiform activity in mouse hippocampal slices induced by moderate changes in extracellular Mg2+, Ca2+, and K. BMC Neurosci 2021;22:46. [PMID: 34301200 PMCID: PMC8305515 DOI: 10.1186/s12868-021-00650-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

BACKGROUND

Rodent brain slices-particularly hippocampal slices-are widely used in experimental investigations of epileptiform activity. Oxygenated artificial cerebrospinal fluid (ACSF) is used to maintain slices in vitro. Physiological or standard ACSF containing 3-3.5 mM K⁺, 1-2 mM Mg²⁺, and 1-3 mM Ca²⁺ generally does not induce population epileptiform activity, which can be induced by ACSF with high K⁺ (8-10 mM), low Mg²⁺, or low Ca²⁺ alone or in combination. While low-Mg²⁺ ACSF without intentionally added Mg salt but with contaminating Mg²⁺ (≤ 50-80 µM) from other salts can induce robust epileptiform activity in slices, it is unclear whether such epileptiform activity can be achieved using ACSF with moderately decreased Mg²⁺. To explore this issue, we examined the effects of moderately modified (m)ACSF with 0.8 mM Mg²⁺, 1.3 mM Ca²⁺, and 5.7 mM K⁺ on induction of epileptiform discharges in mouse hippocampal slices.

RESULTS

Hippocampal slices were prepared from young (21-28 days old), middle-aged (13-14 months old), and aged (24-26 months old) C57/BL6 mice. Conventional thin (0.4 mm) and thick (0.6 mm) slices were obtained using a vibratome and pretreated with mACSF at 35-36 °C for 1 h prior to recordings. During perfusion with mACSF at 35-36 °C, spontaneous or self-sustained epileptiform field potentials following high-frequency stimulation were frequently recorded in slices pretreated with mACSF but not in those without the pretreatment. Seizure-like ictal discharges were more common in thick slices than in thin slices.

CONCLUSIONS

Prolonged exposure to mACSF by pretreatment and subsequent perfusion can induce epileptiform field potentials in mouse hippocampal slices.

Epistemic Autonomy: Self-supervised Learning in the Mammalian Hippocampus

Biological cognition is based on the ability to autonomously acquire knowledge, or epistemic autonomy. Such self-supervision is largely absent in artificial neural networks (ANN) because they depend on externally set learning criteria. Yet training ANN using error backpropagation has created the current revolution in artificial intelligence, raising the question of whether the epistemic autonomy displayed in biological cognition can be achieved with error backpropagation-based learning. We present evidence suggesting that the entorhinal-hippocampal complex combines epistemic autonomy with error backpropagation. Specifically, we propose that the hippocampus minimizes the error between its input and output signals through a modulatory counter-current inhibitory network. We further discuss the computational emulation of this principle and analyze it in the context of autonomous cognitive systems.

Causal relationship of CA3 back-projection to the dentate gyrus and its role in CA1 fast ripple generation

BACKGROUND

Pathophysiological evidence from temporal lobe epilepsy models highlights the hippocampus as the most affected structure due to its high degree of neuroplasticity and control of the dynamics of limbic structures, which are necessary to encode information, conferring to it an intrinsic epileptogenicity. A loss in this control results in observable oscillatory perturbations called fast ripples, in epileptic rats those events are found in CA1, CA3, and the dentate gyrus (DG), which are the principal regions of the trisynaptic circuit of the hippocampus. The present work used Granger causality to address which relationships among these three regions of the trisynaptic circuit are needed to cause fast ripples in CA1 in an in vivo model. For these purposes, male Wistar rats (210-300 g) were injected with a single dose of pilocarpine hydrochloride (2.4 mg/2 µl) into the right lateral ventricle and video-monitored 24 h/day to detect spontaneous and recurrent seizures. Once detected, rats were implanted with microelectrodes in these regions (fixed-recording tungsten wire electrodes, 60-μm outer diameter) ipsilateral to the pilocarpine injection. A total of 336 fast ripples were recorded and probabilistically characterized, from those fast ripples we made a subset of all the fast ripple events associated with sharp-waves in CA1 region (n = 40) to analyze them with Granger Causality.

RESULTS

Our results support existing evidence in vitro in which fast ripple events in CA1 are initiated by CA3 multiunit activity and describe a general synchronization in the theta band across the three regions analyzed DG, CA3, and CA1, just before the fast ripple event in CA1 have begun.

CONCLUSION

This in vivo study highlights the causal participation of the CA3 back-projection to the DG, a connection commonly overlooked in the trisynaptic circuit, as a facilitator of a closed-loop among these regions that prolongs the excitatory activity of CA3. We speculate that the loss of inhibitory drive of DG and the mechanisms of ripple-related memory consolidation in which also the CA3 back-projection to DG has a fundamental role might be underlying processes of the fast ripples generation in CA1.

Development of the mammalian cortical hem and its derivatives: the choroid plexus, Cajal-Retzius cells and hippocampus

The dorsal medial region of the developing mammalian telencephalon plays a central role in the patterning of the adjacent brain regions. This review describes the development of this specialized region of the vertebrate brain, called the cortical hem, and the formation of the various cells and structures it gives rise to, including the choroid plexus, Cajal–Retzius cells and the hippocampus. We highlight the ontogenic processes that create these different forebrain derivatives from their shared embryonic origin and discuss the key signalling pathways and molecules that influence the patterning of the cortical hem. These include BMP, Wnt, FGF and Shh signalling pathways acting with Homeobox factors to carve the medial telencephalon into district progenitor regions, which in turn give rise to the choroid plexus, dentate gyrus and hippocampus. We then link the formation of the lateral ventricle choroid plexus with embryonic and postnatal neurogenesis in the hippocampus.

Region-specific Effects of Early-life Status Epilepticus on the Adult Hippocampal CA3 - Medial Entorhinal Cortex Circuitry In vitro: Focus on Interictal Spikes and Concurrent High-frequency Oscillations

Convulsive status epilepticus (SE) in immature life is often associated with lasting neurobiological changes. We provoked SE by pentylenetetrazole in postnatal day 20 rat pups and examined communication modalities between the temporal hippocampus and medial entorhinal cortex (mEC) in vitro. After a minimum of 40 days post-SE, we prepared combined temporal hippocampal - medial entorhinal cortex (mEC) slices from conditioned (SE) and naïve (N) adult rats and recorded 4-aminopyridine-induced spontaneous epileptiform interictal-like discharges (IED) simultaneously from CA3 and mEC layer V-VI. We analyzed IED frequency and high frequency oscillations (HFOs) in intact slices and after surgical separation of hippocampus from mEC, by two successive incisions (Schaffer collateral cut, Parasubiculum cut). In all slices, IED frequency was higher in CA3 vs mEC (5N, 4SE) and Raster plots indicated no temporal coincidence between them either in intact or in CA1-cut slices (4N, 4SE). IED frequency was significantly higher in SE mEC, but similar in SE and N CA3, independently of connectivity state. Ripples (R) and Fast Ripples (FR) coincided with IEDs and their power differed between SE and N intact slices (22N, 12SE), both in CA3 and mEC. CA3 FR/R ratios were higher in the absence of mEC (14N, 8SE). Moreover, SE (vs N) slices showed significantly higher FR/R ratios independently of the presence of mEC. Taken together, these findings suggest lasting effects of immature SE in network dynamics governing hippocampal-entorhinal communication which may impact adult cognitive, behavioral, and/or seizure threshold sequalae.

Dorsal and ventral mossy cells differ in their axonal projections throughout the dentate gyrus of the mouse hippocampus

Glutamatergic hilar mossy cells (MCs) have axons that terminate both near and far from their cell body but stay within the DG, making synapses primarily in the molecular layer. The long-range axons are considered the primary projection, and extend throughout the DG ipsilateral to the soma, and project to the contralateral DG. The specificity of MC axons for the inner molecular layer (IML) has been considered to be a key characteristic of the DG. In the present study, we made the surprising finding that dorsal MC axons are an exception to this rule. We used two mouse lines that allow for Cre-dependent viral labeling of MCs and their axons: dopamine receptor D2 (Drd2-Cre) and calcitonin receptor-like receptor (Crlr-Cre). A single viral injection into the dorsal DG to label dorsal MCs resulted in labeling of MC axons in both the IML and middle molecular layer (MML). Interestingly, this broad termination of dorsal MC axons occurred throughout the septotemporal DG. In contrast, long-range axons of ventral MCs terminated in the IML, consistent with the literature. Taken together, these results suggest that dorsal and ventral MCs differ significantly in their axonal projections. Since MC projections in the ML are thought to terminate primarily on GCs, the results suggest a dorsal-ventral difference in MC activation of GCs. The surprising difference in dorsal and ventral MC projections should therefore be considered when evaluating dorsal-ventral differences in DG function.

Functional differentiation in the transverse plane of the hippocampus: An update on activity segregation within the DG and CA3 subfields

Decades of neuroscience research in rodents have established an essential role of the hippocampus in the processing of episodic memories. Based on accumulating evidence of functional segregation in the hippocampus along the longitudinal axis, this role has been primarily ascribed to the dorsal hippocampus. More recent findings, however, demonstrate that functional segregation also occurs along transverse axis of the hippocampus, within the hippocampal subfields CA1, CA2, CA3, and the dentate gyrus (DG). Because the functional heterogeneity within CA1 has been addressed in several recent articles, here we discuss behavioral findings and putative mechanisms supporting generation of asymmetrical activity patterns along the transverse axis of DG and CA3. While transverse subnetworks appear to discretely contribute to the processing of spatial, non-spatial, temporal, and social components of episodic memories, integration of these components also occurs, especially in the CA3 subfield and possibly downstream, in the cortical targets of the hippocampus.

Evaluation of the hippocampal immunoreactivity of the serotonin 5-HT1A, 5-HT2 and 5-HT7 receptors in a pilocarpine temporal lobe epilepsy rat model with fast ripples

Fast ripples (FRs) are found in the hippocampus of epileptic brains, and this fast electrical activity has been described as a biomarker of the epileptogenic process itself. Results from our laboratory, such as the observation of decreased seizure rates and FR incidence at a specific citalopram dose, have suggested that serotonin (5-HT) may play a key role in the FR generation process. Therefore, to gather more details about the state of the serotoninergic system in the hippocampus under an epileptogenic process, we studied the immunoreactivity of three 5-HT receptors (5-HT1A, 5-HT2 and 5-HT7) as well as the extracellular levels of 5-HT in the hippocampal tissue of epileptic rats with FR. Wistar rats (210-300 g) were injected with a single dose of pilocarpine hydrochloride (2.4 mg/2 µl) in the right lateral ventricle and video-monitored 24 h/d to detect spontaneous and recurrent seizures; microelectrodes were implanted in the dentate gyrus (DG) and CA3 and CA1 regions of these rats ipsilateral to the pilocarpine injection site 1 day after the first spontaneous seizure was observed, and only rats who suffered FR events were used in this work. Thirty-three days after the first spontaneous seizure, an immunostaining procedure and high performance liquid chromatography were performed to measure the 5-HT levels. A general depletion of the 5-HT and 5-HIIA levels in hippocampal tissue from epileptic animals compared with those in controls was observed; in addition, a general decrease in immunoreactivity for the three receptors was found, especially in the DG, which may support the establishment of an excitatory/inhibitory imbalance in the trisynaptic circuit that underlies the FR generation process.

Mossy Cells in the Dorsal and Ventral Dentate Gyrus Differ in Their Patterns of Axonal Projections

Mossy cells (MCs) of the dentate gyrus (DG) are a major group of excitatory hilar neurons that are important for regulating activity of dentate granule cells. MCs are particularly intriguing because of their extensive longitudinal connections within the DG. It has generally been assumed that MCs in the dorsal and ventral DG have similar patterns of termination in the inner one-third of the dentate molecular layer. Here, we demonstrate that axonal projections of MCs in these two regions are considerably different. MCs in dorsal and ventral regions were labeled selectively with Cre-dependent eYFP or mCherry, using two transgenic mouse lines (including both sexes) that express Cre-recombinase in MCs. At four to six weeks following unilateral labeling of MCs in the ventral DG, a dense band of fibers was present in the inner one-fourth of the molecular layer and extended bilaterally throughout the rostral-caudal extent of the DG, replicating the expected distribution of MC axons. In contrast, following labeling of MCs in the dorsal DG, the projections were more diffusely distributed. At the level of transfection, fibers were present in the inner molecular layer, but they progressively expanded into the middle molecular layer and, most ventrally, formed a distinct band in this region. Optical stimulation of these caudal fibers expressing ChR2 demonstrated robust EPSCs in ipsilateral granule cells and enhanced the effects of perforant path stimulation in the ventral DG. These findings suggest that MCs in the dorsal and ventral DG differ in the distribution of their axonal projections and possibly their function.SIGNIFICANCE STATEMENT Mossy cells (MCs), a major cell type in the hilus of the dentate gyrus (DG), are unique in providing extensive longitudinal and commissural projections throughout the DG. Although it has been assumed that all MCs have similar patterns of termination in the inner molecular layer of the DG, we discovered that the axonal projections of dorsal and ventral MCs differ. While ventral MC projections exhibit the classical pattern, with dense innervation in the inner molecular layer, dorsal MCs have a more diffuse distribution and expand into the middle molecular layer where they overlap and interact with innervation from the perforant path. These distinct locations and patterns of axonal projections suggest that dorsal and ventral MCs may have different functional roles.

Bidirectional Regulation of Cognitive and Anxiety-like Behaviors by Dentate Gyrus Mossy Cells in Male and Female Mice

The dentate gyrus (DG) of the hippocampus is important for cognition and behavior. However, the circuits underlying these functions are unclear. DG mossy cells (MCs) are potentially important because of their excitatory synapses on the primary cell type, granule cells (GCs). However, MCs also activate GABAergic neurons, which inhibit GCs. We used viral delivery of designer receptors exclusively activated by designer drugs (DREADDs) in mice to implement a gain- and loss-of-function study of MCs in diverse behaviors. Using this approach, manipulations of MCs could bidirectionally regulate behavior. The results suggest that inhibiting MCs can reduce anxiety-like behavior and improve cognitive performance. However, not all cognitive or anxiety-related behaviors were influenced, suggesting specific roles of MCs in some, but not all, types of cognition and anxiety. Notably, several behaviors showed sex-specific effects, with females often showing more pronounced effects than the males. We also used the immediate early gene c-Fos to address whether DREADDs bidirectionally regulated MC or GC activity. We confirmed excitatory DREADDs increased MC c-Fos. However, there was no change in GC c-Fos, consistent with MC activation leading to GABAergic inhibition of GCs. In contrast, inhibitory DREADDs led to a large increase in GC c-Fos, consistent with a reduction in MC excitation of GABAergic neurons, and reduced inhibition of GCs. Together, these results suggest that MCs regulate anxiety and cognition in specific ways. We also raise the possibility that cognitive performance may be improved by reducing anxiety.SIGNIFICANCE STATEMENT The dentate gyrus (DG) has many important cognitive roles as well as being associated with affective behavior. This study addressed how a glutamatergic DG cell type called mossy cells (MCs) contributes to diverse behaviors, which is timely because it is known that MCs regulate the activity of the primary DG cell type, granule cells (GCs), but how MC activity influences behavior is unclear. We show, surprisingly, that activating MCs can lead to adverse behavioral outcomes, and inhibiting MCs have an opposite effect. Importantly, the results appeared to be task-dependent and showed that testing both sexes was important. Additional experiments indicated what MC and GC circuitry was involved. Together, the results suggest how MCs influence behaviors that involve the DG.

Behavioral role of PACAP signaling reflects its selective distribution in glutamatergic and GABAergic neuronal subpopulations

The neuropeptide PACAP, acting as a co-transmitter, increases neuronal excitability, which may enhance anxiety and arousal associated with threat conveyed by multiple sensory modalities. The distribution of neurons expressing PACAP and its receptor, PAC1, throughout the mouse nervous system was determined, in register with expression of glutamatergic and GABAergic neuronal markers, to develop a coherent chemoanatomical picture of PACAP role in brain motor responses to sensory input. A circuit role for PACAP was tested by observing Fos activation of brain neurons after olfactory threat cue in wild-type and PACAP knockout mice. Neuronal activation and behavioral response, were blunted in PACAP knock-out mice, accompanied by sharply downregulated vesicular transporter expression in both GABAergic and glutamatergic neurons expressing PACAP and its receptor. This report signals a new perspective on the role of neuropeptide signaling in supporting excitatory and inhibitory neurotransmission in the nervous system within functionally coherent polysynaptic circuits.

Bilateral histone deacetylase 1 and 2 activity and enrichment at unique genes following induction of long-term potentiation in vivo

Long-term potentiation (LTP) is a synaptic plasticity mechanism critical to long-term memory. LTP induced in vivo is characterized by altered transcriptional activity, including a period of upregulation of gene expression which is followed by a later dominant downregulation. This temporal shift to downregulated gene expression is predicted to be partly mediated by epigenetic inhibitors of gene expression, such as histone deacetylases (HDACs). Further, pharmacological inhibitors of HDAC activity have previously been shown to enhance LTP persistence in vitro. To explore the contribution of HDACs to the persistence of LTP in vivo, we examined HDAC1 and HDAC2 activity over a 24 hr period following unilateral LTP induction in the dentate gyrus of freely moving rats. Surprisingly, we found significant changes in HDAC1 and HDAC2 activity in both the stimulated as well as the unstimulated hemispheres, with the largest increase in activity occurring bilaterally, 20 min after LTP stimulation. During this time point of heightened activity, chromatin immunoprecipitation assays showed that both HDAC1 and HDAC2 were enriched at distinct sets of genes within each hemispheres. Further, the HDAC inhibitor Trichostatin A enhanced an intermediate phase of LTP lasting days, which has not previously been associated with altered transcription. The inhibitor had no effect on the persistence of LTP lasting weeks. Together, these data suggest that HDAC activity early after the induction of LTP may negatively regulate plasticity-related gene expression that is involved in the initial stabilization of LTP, but not its long-term maintenance.

Role of Hippocampal Neurogenesis in Alcohol Withdrawal Seizures

Chronic alcohol consumption results in alcohol use disorder (AUD). Interestingly, however, sudden alcohol withdrawal (AW) after chronic alcohol exposure also leads to a devastating series of symptoms, referred to as alcohol withdrawal syndromes. One key feature of AW syndromes is to produce phenotypes that are opposite to AUD. For example, while the brain is characterized by a hypoactive state in the presence of alcohol, AW induces a hyperactive state, which is manifested as seizure expression. In this review, we discuss the idea that hippocampal neurogenesis and neural circuits play a key role in neuroadaptation and establishment of allostatic states in response to alcohol exposure and AW. The intrinsic properties of dentate granule cells (DGCs), and their contribution to the formation of a potent feedback inhibitory loop, endow the dentate gyrus with a "gate" function, which can limit the entry of excessive excitatory signals from the cortex into the hippocampus. We discuss the possibility that alcohol exposure and withdrawal disrupts structural development and circuitry integration of hippocampal newborn neurons, and that this altered neurogenesis impairs the gate function of the hippocampus. Failure of this gate function is expected to alter the ratio of excitatory to inhibitory (E/I) signals in the hippocampus and to induce seizure expression during AW. Recent functional studies have shown that specific activation and inhibition of hippocampal newborn DGCs are both necessary and sufficient for the expression of AW-associated seizures, further supporting the concept that neurogenesis-induced neuroadaptation is a critical target to understand and treat AUD and AW-associated seizures.

Integration of spatial and non-spatial information by heterogeneous dentate gyrus granule cells

The hippocampus is the key site for learning and memory and for processing of spatial information in the brain. It is divided into three main subregions: the dentate gyrus (DG), the CA3 area, and the CA1 region, which are linearly interconnected to form a so-called trisynaptic circuit. Thus, the DG sits in a strategic position to gate the flow of information from the neocortex into the hippocampal network. The granule cells (GCs), the main cell type in the DG, receive 'where' and 'what' information from the medial and lateral entorhinal cortex, respectively. How they process this mixed information remains enigmatic. By characterizing the spatial information encoded by the excitatory postsynaptic potentials (EPSPs) in GCs, we demonstrated that the majority of GCs received spatially tuned synaptic input. However, only a minority of GCs successfully converted spatially tuned input to spatially tuned output. Furthermore, we found that mature GCs were highly heterogeneous in terms of their dendritic morphology and intrinsic excitability, which contributes to the sparse and heterogeneous firing of GCs. Finally, we discuss the possible origin of this neural heterogeneity and its potential role in enlarging the computational power of the DG, facilitating pattern separation in this network.

Properties of primary cilia in melanin-concentrating hormone receptor 1-bearing hippocampal neurons in vivo and in vitro

The primary cilium is a solitary organelle that organizes a sensitive signaling hub in a highly ordered microenvironment. Cilia are plastic structures, changing their length in response to bioactive substances, and ciliary length may be regulated to ensure efficient signaling capacity. Mammalian brain neurons possess primary cilia that are enriched in a set of G protein-coupled receptors (GPCRs), including the feeding-related melanin-concentrating hormone (MCH) receptor 1 (MCHR1). We previously demonstrated a novel biological phenomenon, ciliary MCHR1-mediated cilia length shortening through Gi/o and Akt signaling, using a simple cell culture model of human retinal pigmented epithelial RPE1 cells exogenously expressing MCHR1. In the present study, we characterized the properties of endogenous MCHR1-expressing primary cilia in hippocampal neurons in rodents. Using cultured dissociated rat hippocampal neurons in vitro, we showed that MCH triggered cilia length reduction involved in MCHR1-Gi/o and -Akt signaling. In rat hippocampal slice cultures with preservation of the cytoarchitecture and cell populations, ciliary MCHR1 was abundantly located in the CA1 and CA3 regions, but not in the dentate gyrus. Notably, treatment of slice cultures with MCH induced Gi/o- and Akt-dependent cilia shortening in the CA1 region without influencing cilia length in the CA3 region. Regarding the in vivo mouse brain, we observed higher levels of ciliary MCHR1 in the CA1 and CA3 regions as well as in slice cultures. In the starved state mice, a marked increase in MCH mRNA expression was detected in the lateral hypothalamus. Furthermore, MCHR1-positive cilia length in the hippocampal CA1 region was significantly shortened in fasted mice compared with fed mice. The present findings focused on the hippocampus provide a potential approach to investigate how MCHR1-driven cilia shortening regulates neuronal activity and physiological function toward feeding and memory tasks.

Hippocampal GABAergic interneurons and their co-localized neuropeptides in stress vulnerability and resilience

Studies in humans and rodents suggest a critical role for the hippocampal formation in cognition and emotion, but also in the adaptation to stressful events. Successful stress adaptation promotes resilience, while its failure may lead to stress-induced psychopathologies such as depression and anxiety disorders. Hippocampal architecture and physiology is shaped by its strong control of activity via diverse classes of inhibitory interneurons that express typical calcium binding proteins and neuropeptides. Celltype-specific opto- and chemogenetic intervention strategies that take advantage of these biochemical markers have bolstered our understanding of the distinct role of different interneurons in anxiety, fear and stress adaptation. Moreover, some of the signature proteins of GABAergic interneurons have a potent impact on emotion and cognition on their own, making them attractive targets for interventions. In particular, neuropeptide Y is a promising endogenous agent for mediating resilience against severe stress. In this review, we evaluate the role of the major types of interneurons across hippocampal subregions in the adaptation to chronic and acute stress and to emotional memory formation.

Parallel processing streams in the hippocampus

The hippocampus performs two complementary processes, pattern separation and pattern completion, to minimize interference and maximize the storage capacity of memories. Classic computational models have suggested that the dentate gyrus (DG) supports pattern separation and the putative attractor circuitry in CA3 supports pattern completion. However, recent evidence of functional heterogeneity along the CA3 transverse axis of the hippocampus suggests that the DG and proximal CA3 work as a functional unit for pattern separation, while distal CA3 forms an autoassociative network for pattern completion. We propose that the outputs of these functional circuits, combined with direct projections from entorhinal cortex to CA1, form interconnected, parallel processing circuits to support accurate memory storage and retrieval.

Dentate Gyrus Sharp Waves, a Local Field Potential Correlate of Learning in the Dentate Gyrus of Mice

The hippocampus plays an essential role in learning. Each of the three major hippocampal subfields, dentate gyrus (DG), CA3, and CA1, has a unique function in memory formation and consolidation, and also exhibit distinct local field potential (LFP) signatures during memory consolidation processes in non-rapid eye movement (NREM) sleep. The classic LFP events of the CA1 region, sharp-wave ripples (SWRs), are induced by CA3 activity and considered to be an electrophysiological biomarker for episodic memory. In LFP recordings along the dorsal CA1-DG axis from sleeping male mice, we detected and classified two types of LFP events in the DG: high-amplitude dentate spikes (DSs), and a novel event type whose current source density (CSD) signature resembled that seen during CA1 SWR, but which, most often, occurred independently of them. Because we hypothesize that this event type is similarly induced by CA3 activity, we refer to it as dentate sharp wave (DSW). We show that both DSWs and DSs differentially modulate the electrophysiological properties of SWR and multiunit activity (MUA). Following two hippocampus-dependent memory tasks, DSW occurrence rates, ripple frequencies, and ripple and sharp wave (SW) amplitudes were increased in both, while SWR occurrence rates in dorsal CA1 increased only after the spatial task. Our results suggest that DSWs, like SWRs, are induced by CA3 activity and that DSWs complement SWRs as a hippocampal LFP biomarker of memory consolidation.SIGNIFICANCE STATEMENT Awake experience is consolidated into long-term memories during sleep. Memory consolidation crucially depends on sharp-wave ripples (SWRs), which are local field potential (LFP) patterns in hippocampal CA1 that increase after learning. The dentate gyrus (DG) plays a central role in the process of memory formation, prompting us to cluster sharp waves (SWs) in the DG [dentate SWs (DSWs)] during sleep. We show that both DSW coupling to CA1 SWRs, and their occurrence rates, robustly increase after learning trials. Our results suggest that the DG is directly affected by memory consolidation processes. DSWs may thus complement SWRs as a sensitive electrophysiological biomarker of memory consolidation in mice.

Dentate gyrus circuits for encoding, retrieval and discrimination of episodic memories

The dentate gyrus (DG) has a key role in hippocampal memory formation. Intriguingly, DG lesions impair many, but not all, hippocampus-dependent mnemonic functions, indicating that the rest of the hippocampus (CA1-CA3) can operate autonomously under certain conditions. An extensive body of theoretical work has proposed how the architectural elements and various cell types of the DG may underlie its function in cognition. Recent studies recorded and manipulated the activity of different neuron types in the DG during memory tasks and have provided exciting new insights into the mechanisms of DG computational processes, particularly for the encoding, retrieval and discrimination of similar memories. Here, we review these DG-dependent mnemonic functions in light of the new findings and explore mechanistic links between the cellular and network properties of, and the computations performed by, the DG.

Pten loss results in inappropriate excitatory connectivity

Pten mutations are associated with autism spectrum disorder. Pten loss of function in neurons increases excitatory synaptic connectivity, contributing to an imbalance between excitation and inhibition. We aimed to determine whether Pten loss results in aberrant connectivity in neural circuits. We compared postnatally generated wild-type and Pten knockout granule neurons integrating into the dentate gyrus using a variety of methods to examine their connectivity. We found that postsynaptic Pten loss provides an advantage to dendritic spines in competition over a limited pool of presynaptic boutons. Retrograde monosynaptic tracing with rabies virus reveals that this results in synaptic contact with more presynaptic partners. Using independently excitable opsins to interrogate multiple inputs onto a single neuron, we found that excess connectivity is established indiscriminately from among glutamatergic afferents. Therefore, Pten loss results in inappropriate connectivity whereby neurons are coupled to a greater number of synaptic partners.