Distinct physiological and developmental properties of hippocampal CA2 subfield revealed by using anti-Purkinje cell protein 4 (PCP4) immunostaining

Sex- and cycle-dependent changes in spine density and size in hippocampal CA2 neurons

Sex hormones affect structural and functional plasticity in the rodent hippocampus. However, hormone levels not only differ between males and females, but also fluctuate across the female estrous cycle. While sex- and cycle-dependent differences in dendritic spine density and morphology have been found in the rodent CA1 region, but not in the CA3 or the dentate gyrus, comparable structural data on CA2, i.e. the hippocampal region involved in social recognition memory, is so far lacking. In this study, we, therefore, used wildtype male and female mice in diestrus or proestrus to analyze spines on dendritic segments from identified CA2 neurons. In basal stratum oriens, we found no differences in spine density, but a significant shift towards larger spine head areas in male mice compared to females. Conversely, in apical stratum radiatum diestrus females had a significantly higher spine density, and females in either cycle stage had a significant shift towards larger spine head areas as compared to males, with diestrus females showing the larger shift. Our results provide further evidence for the sexual dimorphism of hippocampal area CA2, and underscore the importance of considering not only the sex, but also the stage of the estrous cycle when interpreting morphological data.

Olanzapine Effects on Parvalbumin/GAD67 Cell Numbers in Layers/Subregions of Dorsal Hippocampus of Chronically Socially Isolated Rats

Depression is linked to changes in GABAergic inhibitory neurons, especially parvalbumin (PV) interneurons, which are susceptible to redox dysregulation. Olanzapine (Olz) is an atypical antipsychotic whose mode of action remains unclear. We determined the effect of Olz on PV-positive (+) and glutamate decarboxylase 67 (GAD67) + cell numbers in the layers of dorsal hippocampus (dHIPP) cornu ammonis (CA1-CA3) and dentate gyrus (DG) subregions in rats exposed to chronic social isolation (CSIS), which is an animal model of depression. Antioxidative enzymes and proinflammatory cytokine levels were also examined. CSIS decreased the PV+ cell numbers in the Stratum Oriens (SO) and Stratum Pyramidale (SP) of dCA1 and dDG. It increased interleukin-6 (IL-6), suppressor of cytokine signaling 3 (SOCS3), and copper-zinc superoxide dismutase (CuZnSOD) levels, and it decreased catalase (CAT) protein levels. Olz in CSIS increased the number of GAD67+ cells in the SO and SP layers of dCA1 with no effect on PV+ cells. It reduced the PV+ and GAD67+ cell numbers in the Stratum Radiatum of dCA3 in CSIS. Olz antagonizes the CSIS-induced increase in CuZnSOD, CAT and SOCS3 protein levels with no effect on IL-6. Data suggest that the protective Olz effects in CSIS may be mediated by altering the number of PV+ and GAD67+ cells in dHIPP subregional layers.

Hippocampal representation during collective spatial behaviour in bats

Social animals live and move through spaces shaped by the presence, motion and sensory cues of multiple other individuals1-6. Neural activity in the hippocampus is known to reflect spatial behaviour7-9 yet its study is lacking in such dynamic group settings, which are ubiquitous in natural environments. Here we studied hippocampal activity in groups of bats engaged in collective spatial behaviour. We find that, under spontaneous conditions, a robust spatial structure emerges at the group level whereby behaviour is anchored to specific locations, movement patterns and individual social preferences. Using wireless electrophysiological recordings from both stationary and flying bats, we find that many hippocampal neurons are tuned to key features of group dynamics. These include the presence or absence of a conspecific, but not typically of an object, at landing sites, shared spatial locations, individual identities and sensory signals that are broadcasted in the group setting. Finally, using wireless calcium imaging, we find that social responses are anatomically distributed and robustly represented at the population level. Combined, our findings reveal that hippocampal activity contains a rich representation of naturally emerging spatial behaviours in animal groups that could in turn support the complex feat of collective behaviour.

Hippocampal CA3 inhibitory neurons receive extensive noncanonical synaptic inputs from CA1 and subicular complex

Hippocampal CA3 is traditionally conceptualized as a brain region within a unidirectional feedforward trisynaptic pathway that links major hippocampal subregions. Recent genomic and viral tracing studies indicate that the anatomical connectivity of CA3 and the trisynaptic pathway is more complex than initially expected and suggests that there may be cell type-specific input gradients throughout the three-dimensional hippocampal structure. In several recent studies using multiple viral tracing approaches, we describe subdivisions of the subiculum complex and ventral hippocampal CA1 that show significant back projections to CA1 and CA3 excitatory neurons. These novel connections form "noncanonical" circuits that run in the opposite direction relative to the well-characterized feedforward pathway. Diverse subtypes of GABAergic inhibitory neurons participate within the trisynaptic pathway. In the present study, we have applied monosynaptic retrograde viral tracing to examine noncanonical synaptic inputs from CA1 and subicular complex to the inhibitory neuron in hippocampal CA3. We quantitatively mapped synaptic inputs to CA3 inhibitory neurons to understand how they are connected within and beyond the hippocampus formation. Major brain regions that provide typical inputs to CA3 inhibitory neurons include the medial septum, the dentate gyrus, the entorhinal cortex, and CA3. Noncanonical inputs from ventral CA1 and subicular complex to CA3 inhibitory neurons follow a proximodistal topographic gradient with regard to CA3 subregions. We find novel noncanonical circuit connections between inhibitory CA3 neurons and ventral CA1, subiculum complex, and other brain regions. These results provide a new anatomical connectivity basis to further study the function of CA3 inhibitory neurons.

Development of the hippocampal CA2 region and the emergence of social recognition

Social memories formed in early life, like those for family and unrelated peers, are known to contribute to healthy social interactions throughout life, although how the developing brain supports social memory remains relatively unexplored. The CA2 subregion of the hippocampus is involved in social memory function, but most literature on this subject is restricted to studies of adult rodents. Here, we review the current literature on the embryonic and postnatal development of hippocampal subregion CA2 in mammals, with a focus on the emergence of its unusual molecular and cellular characteristics, including its notably high expression of plasticity-suppressing molecules. We also consider the connectivity of the CA2 with other brain areas, including intrahippocampal regions, such as the dentate gyrus, CA3, and CA1 regions, and extrahippocampal regions, such as the hypothalamus, ventral tegmental area, basal forebrain, raphe nuclei, and the entorhinal cortex. We review developmental milestones of CA2 molecular, cellular, and circuit-level features that may contribute to emerging social recognition abilities for kin and unrelated conspecifics in early life. Lastly, we consider genetic mouse models related to neurodevelopmental disorders in humans in order to survey evidence about whether atypical formation of the CA2 may contribute to social memory dysfunction.

Machine learning-based segmentation of the rodent hippocampal CA2 area from Nissl-stained sections

The hippocampus is a center of learning, memory, and spatial navigation. This region is divided into the CA1, CA2, and CA3 areas, which are anatomically different from each other. Among these divisions, the CA2 area is unique in terms of functional relevance to sociality. The CA2 area is often manually detected based on the size, shape, and density of neurons in the hippocampal pyramidal cell layer, but this manual segmentation relying on cytoarchitecture is impractical to apply to a large number of samples and dependent on experimenters' proficiency. Moreover, the CA2 area has been defined based on expression pattern of molecular marker proteins, but it generally takes days to complete immunostaining for such proteins. Thus, we asked whether the CA2 area can be systematically segmented based on cytoarchitecture alone. Since the expression pattern of regulator of G-protein signaling 14 (RGS14) signifies the CA2 area, we visualized the CA2 area in the mouse hippocampus by RGS14-immunostaining and Nissl-counterstaining and manually delineated the CA2 area. We then established "CAseg," a machine learning-based automated algorithm to segment the CA2 area with the F1-score of approximately 0.8 solely from Nissl-counterstained images that visualized cytoarchitecture. CAseg was extended to the segmentation of the prairie vole CA2 area, which raises the possibility that the use of this algorithm can be expanded to other species. Thus, CAseg will be beneficial for investigating unique properties of the hippocampal CA2 area.

Defining hippocampal area CA2 in the fox (Vulpes vulpes) brain

Since 1959, the Russian Farm-Fox study has bred foxes to be either tame or, more recently, aggressive, and scientists have used them to gain insight into the brain structures associated with these behavioral features. In mice, hippocampal area CA2 has emerged as one of the essential regulators of social aggression, and so to eventually determine whether we could identify differences in CA2 between tame and aggressive foxes, we first sought to identify CA2 in foxes (Vulpes vulpes). As no clearly defined area of CA2 has been described in species such as cats, dogs, or pigs, it was not at all clear whether CA2 could be identified in foxes. In this study, we cut sections of temporal lobes from male and female red foxes, perpendicular to the long axis of the hippocampus, and stained them with markers of CA2 pyramidal cells commonly used in tissue from rats and mice. We observed that antibodies against Purkinje cell protein 4 best stained the pyramidal cells in the area spanning the end of the mossy fibers and the beginning of the pyramidal cells lacking mossy fibers, resembling the pattern seen in rats and mice. Our findings indicate that foxes do have a "molecularly defined" CA2, and further, they suggest that other carnivores like dogs and cats might as well. With this being the case, these foxes could be useful in future studies looking at CA2 as it relates to aggression.

Deciphering tissue heterogeneity from spatially resolved transcriptomics by the autoencoder-assisted graph convolutional neural network

Spatially resolved transcriptomics (SRT) provides an unprecedented opportunity to investigate the complex and heterogeneous tissue organization. However, it is challenging for a single model to learn an effective representation within and across spatial contexts. To solve the issue, we develop a novel ensemble model, AE-GCN (autoencoder-assisted graph convolutional neural network), which combines the autoencoder (AE) and graph convolutional neural network (GCN), to identify accurate and fine-grained spatial domains. AE-GCN transfers the AE-specific representations to the corresponding GCN-specific layers and unifies these two types of deep neural networks for spatial clustering via the clustering-aware contrastive mechanism. In this way, AE-GCN accommodates the strengths of both AE and GCN for learning an effective representation. We validate the effectiveness of AE-GCN on spatial domain identification and data denoising using multiple SRT datasets generated from ST, 10x Visium, and Slide-seqV2 platforms. Particularly, in cancer datasets, AE-GCN identifies disease-related spatial domains, which reveal more heterogeneity than histological annotations, and facilitates the discovery of novel differentially expressed genes of high prognostic relevance. These results demonstrate the capacity of AE-GCN to unveil complex spatial patterns from SRT data.

Simultaneous 3D cellular positioning and apical dendritic morphology of transgenic fluorescent mouse CA3 hippocampal pyramidal neurons

BACKGROUND

Pyramidal neurons throughout hippocampal CA3 are diverse in their dendritic morphology, and CA3 is not homogenous in its structure or function. Nonetheless, few structural studies have captured the precise 3D somatic position and the 3D dendritic morphology of CA3 pyramidal neurons simultaneously.

NEW METHOD

Here, we present a simple approach to reconstruct the apical dendritic morphology of CA3 pyramidal neurons using the transgenic fluorescent Thy1-GFP-M line. The approach simultaneously tracks the dorsoventral, tangential, and radial positions of reconstructed neurons within the hippocampus. It is especially designed for use with transgenic fluorescent mouse lines, which are commonly used in genetic studies of neuronal morphology and development.

RESULTS

We demonstrate how topographic and morphological data are captured from transgenic fluorescent mouse CA3 pyramidal neurons.

COMPARISON WITH EXISTING METHODS

There is no need to select and label CA3 pyramidal neurons with the transgenic fluorescent Thy1-GFP-M line. By taking transverse (not coronal) serial sections, we preserve fine dorsoventral, tangential, and radial somatic positioning of 3D-reconstructed neurons. Because CA2 is well defined by PCP4 immunohistochemistry, we use that technique here to to increase precision in defining tangential position along CA3.

CONCLUSIONS

We developed a method for simultaneously collecting precise somatic positioning as well as 3D morphological data among transgenic fluorescent mouse hippocampal pyramidal neurons. This fluorescent method should be compatible with many other transgenic fluorescent reporter lines and immunohistochemical methods, facilitating the capture of topographic and morphological data from a wide variety of genetic experiments in mouse hippocampus.

Morphological and molecular markers of mouse area CA2 along the proximodistal and dorsoventral hippocampal axes

Hippocampal area CA2 is a molecularly and functionally distinct region of the hippocampus that has classically been defined as the area with large pyramidal neurons lacking input from the dentate gyrus and the thorny excrescences (TEs) characteristic of CA3 neurons. A modern definition of CA2, however, makes use of the expression of several molecular markers that distinguish it from neighboring CA3 and CA1. Using immunohistochemistry, we sought to characterize the staining patterns of commonly used CA2 markers along the dorsal-ventral hippocampal axis and determine how these markers align along the proximodistal axis. We used a region of CA2 that stained for both Regulator of G-protein Signaling 14 (RGS14) and Purkinje Cell Protein 4 (PCP4; "double-labeled zone" [DLZ]) as a reference. Here, we report that certain commonly used CA2 molecular markers may be better suited for drawing distinct boundaries between CA2/3 and CA2/1. For example, RGS14+ and STEP+ neurons showed minimal to no extension into area CA1 while areas stained with VGluT2 and Wisteria Floribunda agglutinin were consistently smaller than the DLZ/CA2 borders by ~100 μ on the CA1 or CA3 sides respectively. In addition, these patterns are dependent on position along the dorsal-ventral hippocampal axis such that PCP4 labeling often extended beyond the distal border of the DLZ into CA1. Finally, we found that, consistent with previous findings, mossy fibers innervate a subset of RGS14 positive neurons (~65%-70%) and that mossy fiber bouton number and relative size in CA2 are less than that of boutons in CA3. Unexpectedly, we did find evidence of some complex spines on apical dendrites in CA2, though much fewer in number than in CA3. Our results indicate that certain molecular markers may be better suited than others when defining the proximal and distal borders of area CA2 and that the presence or absence of complex spines alone may not be suitable as a distinguishing feature differentiating CA3 from CA2 neurons.

Preadolescent exposure to a sexually mature, unrelated male rat reduces postadolescent social recognition memory and CA2 c-Fos labeling

Introduction

Social memory involves social recognition: the ability to discriminate between two or more conspecifics when one has been previously encountered. The CA2 region of the hippocampus has been implicated in social memory, as lesions and dysfunction to this area lead to social memory impairments. A variety of psychogenic manipulations during postnatal sensitive developmental periods are associated with social memory impairments later in life.

Methods

In this study, we exposed preadolescent rats to a sexually, mature unrelated male and examined whether this was associated with changes in postadolescent social memory and c-Fos labeling in the CA2 region. Male and female Long-Evans rats were exposed to a male, adult rat on postnatal days 19-21 (P19-21). Social memory was measured during the postadolescent period and defined as increased interactions towards a novel age-matched rat in contrast to a previously-encountered age-matched rat. After the test, rats were euthanized and brain tissue was then collected to quantify c-Fos labeling within the CA2 region.

Results

Compared to home cage controls and controls not exposed to the adult male, male and female rats exposed to the unrelated adult during preadolescence were unable to discriminate between a novel and previously encountered conspecific during the postadolescent test showing social memory deficits. The groups that showed social recognition deficits also had significantly fewer c-Fos-positive cells within the CA2 region compared to the control groups.

Discussion

These findings indicate that threatening psychogenic encounters during preadolescence can have detrimental long-term effects on social memory potentially via disrupted activity in the CA2 hippocampal region.

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.

CA2 physiology underlying social memory

In recent years, convergent evidence has emerged in support of the idea of social brain networks, specific brain regions that are interconnected and support social behaviors. One of these regions is the CA2 area of the hippocampus, a small region strongly connected with cortical and subcortical areas implicated in social behaviors. Furthermore, CA2 area is enriched in receptors for several neuromodulators that are related to various aspects of social behaviors, suggesting that this area could be a key component of social information processing in the brain. In this review, recent findings related to the physiological mechanisms underlying the role of CA2 in social memory are discussed.

Atypical perineuronal nets in the CA2 region interfere with social memory in a mouse model of social dysfunction

Social memory dysfunction is an especially devastating symptom of many neuropsychiatric disorders, which makes understanding the cellular and molecular processes that contribute to such abnormalities important. Evidence suggests that the hippocampus, particularly the CA2 region, plays an important role in social memory. We sought to identify potential mechanisms of social memory dysfunction in the hippocampus by investigating features of neurons, glia, and the extracellular matrix (ECM) of BTBR mice, an inbred mouse strain with deficient social memory. The CA2 is known to receive inputs from dentate gyrus adult-born granule cells (abGCs), neurons known to participate in social memory, so we examined this cell population and found fewer abGCs, as well as fewer axons from abGCs in the CA2 of BTBR mice compared to controls. We also found that BTBR mice had fewer pyramidal cell dendritic spines, in addition to fewer microglia and astrocytes, in the CA2 compared to controls. Along with diminished neuronal and glial elements, we found atypical perineuronal nets (PNNs), specialized ECM structures that regulate plasticity, in the CA2 of BTBR mice. By diminishing PNNs in the CA2 of BTBR mice to control levels, we observed a partial restoration of social memory. Our findings suggest that the CA2 region of BTBR mice exhibits multiple cellular and extracellular abnormalities and identify atypical PNNs as one mechanism producing social memory dysfunction, although the contribution of reduced abGC afferents, pyramidal cell dendritic spine, and glial cell numbers remains unexplored.

Functionally-distinct pyramidal cell subpopulations during gamma oscillations in mouse hippocampal area CA3

Gamma oscillations (γ-oscillations) in hippocampal area CA3 are essential for memory function. Particularly, CA3 is involved in the memory related process pattern completion, which is linked with the γ-oscillations in human hippocampus. Recent studies suggest that heterogeneity in the functional properties of pyramidal cells (PCs) in CA3 plays an important role in hippocampal function. By performing concomitant recordings of PC activity and network γ-oscillations in CA3 we found three functionally-different PC subpopulations. PCs with high spike-frequency adaptation (hA_PC) have the strongest action potential gamma phase-coupling, PCs with low adaptation (lA_PC) show lower phase-coupling and PCs displaying a burst-firing pattern (B_PC) remained quiescent. In addition, we discovered that hA_PC display the highest excitatory/inhibitory drive, followed by lA_PC, and lastly B_PC. In conclusion, our data advance the hypothesis that PCs in CA3 are organized into subpopulations with distinct functional roles for cognition-relevant network dynamics and provide new insights in the physiology of hippocampus.

Noncanonical projections to the hippocampal CA3 regulate spatial learning and memory by modulating the feedforward hippocampal trisynaptic pathway

The hippocampal formation (HF) is well documented as having a feedforward, unidirectional circuit organization termed the trisynaptic pathway. This circuit organization exists along the septotemporal axis of the HF, but the circuit connectivity across septal to temporal regions is less well described. The emergence of viral genetic mapping techniques enhances our ability to determine the detailed complexity of HF circuitry. In earlier work, we mapped a subiculum (SUB) back projection to CA1 prompted by the discovery of theta wave back propagation from the SUB to CA1 and CA3. We reason that this circuitry may represent multiple extended noncanonical pathways involving the subicular complex and hippocampal subregions CA1 and CA3. In the present study, multiple retrograde viral tracing approaches produced robust mapping results, which supports this prediction. We find significant noncanonical synaptic inputs to dorsal hippocampal CA3 from ventral CA1 (vCA1), perirhinal cortex (Prh), and the subicular complex. Thus, CA1 inputs to CA3 run opposite the trisynaptic pathway and in a temporal to septal direction. Our retrograde viral tracing results are confirmed by anterograde-directed viral mapping of projections from input mapped regions to hippocampal dorsal CA3 (dCA3). We find that genetic inactivation of the projection of vCA1 to dCA3 impairs object-related spatial learning and memory but does not modulate anxiety-related behaviors. Our data provide a circuit foundation to explore novel functional roles contributed by these noncanonical hippocampal circuit connections to hippocampal circuit dynamics and learning and memory behaviors.

Postnatal Developmental Expression Profile Classifies the Indusium Griseum as a Distinct Subfield of the Hippocampal Formation

The indusium griseum (IG) is a cortical structure overlying the corpus callosum along its anterior–posterior extent. It has been classified either as a vestige of the hippocampus or as an extension of the dentate gyrus via the fasciola cinerea, but its attribution to a specific hippocampal subregion is still under debate. To specify the identity of IG neurons more precisely, we investigated the spatiotemporal expression of calbindin, secretagogin, Necab2, PCP4, and Prox1 in the postnatal mouse IG, fasciola cinerea, and hippocampus. We identified the calcium-binding protein Necab2 as a first reliable marker for the IG and fasciola cinerea throughout postnatal development into adulthood. In contrast, calbindin, secretagogin, and PCP4 were expressed each with a different individual time course during maturation, and at no time point, IG or fasciola cinerea principal neurons expressed Prox1, a transcription factor known to define dentate granule cell fate. Concordantly, in a transgenic mouse line expressing enhanced green fluorescent protein (eGFP) in dentate granule cells, neurons of IG and fasciola cinerea were eGFP-negative. Our findings preclude that IG neurons represent dentate granule cells, as earlier hypothesized, and strongly support the view that the IG is an own hippocampal subfield composed of a distinct neuronal population.

Adenosine A1 Receptor-Mediated Synaptic Depression in the Developing Hippocampal Area CA2

Immunolabeling for adenosine A₁ receptors (A₁Rs) is high in hippocampal area CA2 in adult rats, and the potentiating effects of caffeine or other A₁R-selective antagonists on synaptic responses are particularly robust at Schaffer collateral synapses in CA2. Interestingly, the pronounced staining for A₁Rs in CA2 is not apparent until rats are 4 weeks old, suggesting that developmental changes other than receptor distribution underlie the sensitivity of CA2 synapses to A₁R antagonists in young animals. To evaluate the role of A₁R-mediated postsynaptic signals at these synapses, we tested whether A₁R agonists regulate synaptic transmission at Schaffer collateral inputs to CA2 and CA1. We found that the selective A₁R agonist CCPA caused a lasting depression of synaptic responses in both CA2 and CA1 neurons in slices obtained from juvenile rats (P14), but that the effect was observed only in CA2 in slices prepared from adult animals (~P70). Interestingly, blocking phosphodiesterase activity with rolipram inhibited the CCPA-induced depression in CA1, but not in CA2, indicative of robust phosphodiesterase activity in CA1 neurons. Likewise, synaptic responses in CA2 and CA1 differed in their sensitivity to the adenylyl cyclase activator, forskolin, in that it increased synaptic transmission in CA2, but had little effect in CA1. These findings suggest that the A₁R-mediated synaptic depression tracks the postnatal development of immunolabeling for A₁Rs and that the enhanced sensitivity to antagonists in CA2 at young ages is likely due to robust adenylyl cyclase activity and weak phosphodiesterase activity rather than to enrichment of A₁Rs.

Structural Correlates of CA2 and CA3 Pyramidal Cell Activity in Freely-Moving Mice

Plasticity within hippocampal circuits is essential for memory functions. The hippocampal CA2/CA3 region is thought to be able to rapidly store incoming information by plastic modifications of synaptic weights within its recurrent network. High-frequency spike-bursts are believed to be essential for this process, by serving as triggers for synaptic plasticity. Given the diversity of CA2/CA3 pyramidal neurons, it is currently unknown whether and how burst activity, assessed in vivo during natural behavior, relates to principal cell heterogeneity. To explore this issue, we juxtacellularly recorded the activity of single CA2/CA3 neurons from freely-moving male mice, exploring a familiar environment. In line with previous work, we found that spatial and temporal activity patterns of pyramidal neurons correlated with their topographical position. Morphometric analysis revealed that neurons with a higher proportion of distal dendritic length displayed a higher tendency to fire spike-bursts. We propose that the dendritic architecture of pyramidal neurons might determine burst-firing by setting the relative amount of distal excitatory inputs from the entorhinal cortex.SIGNIFICANCE STATEMENT High-frequency spike-bursts are thought to serve fundamental computational roles within neural circuits. Within hippocampal circuits, spike-bursts are believed to serve as potent instructive signals, which increase the efficiency of information transfer and induce rapid modifications of synaptic efficacies. In the present study, by juxtacellularly recording and labeling single CA2/CA3 neurons in freely-moving mice, we explored whether and how burst propensity relates to pyramidal cell heterogeneity. We provide evidence that, within the CA2/CA3 region, neurons with higher proportion of distal dendritic length display a higher tendency to fire spike-bursts. Thus, the relative amount of entorhinal inputs, arriving onto the distal dendrites, might determine the burst propensity of individual CA2/CA3 neurons in vivo during natural behavior.

Proximodistal Organization of the CA2 Hippocampal Area

The proximodistal axis is considered a major organizational principle of the hippocampus. At the interface between the hippocampus and other brain structures, CA2 apparently breaks this rule. The region is involved in social, temporal, and contextual memory function, but mechanisms remain elusive. Here, we reveal cell-type heterogeneity and a characteristic expression gradient of the transcription factor Sox5 within CA2 in the rat. Using intracellular and extracellular recordings followed by neurochemical identification of single cells, we find marked proximodistal trends of synaptic activity, subthreshold membrane potentials, and phase-locked firing coupled to theta and gamma oscillations. Phase-shifting membrane potentials and opposite proximodistal correlations with theta sinks and sources at different layers support influences from different current generators. CA2 oscillatory activity and place coding of rats running in a linear maze reflect proximodistal state-dependent trends. We suggest that the structure and function of CA2 are distributed along the proximodistal hippocampal axis.

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The CA2 region is organized around the limit of the mossy fibers

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Heterogeneous pyramidal cell types populate the proximal and distal CA2 region

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Responses to intra- and extra-hippocampal inputs segregate along this axis

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CA2 oscillatory activity and spatial coding change proximodistally

Hippocampal CA2 Organizes CA1 Slow and Fast γ Oscillations during Novel Social and Object Interaction

A key goal in hippocampal research is to understand how neuronal activity is generated and organized across hippocampal subregions to enable memory formation and retrieval. Neuronal activity in CA2 is regulated by spatial and social investigation as well as by novelty (Mankin et al., 2015; Alexander et al., 2016), and CA2 activity controls population oscillatory activity in the slow γ and ripple ranges within hippocampus (Kay et al., 2016; Oliva et al., 2016; Boehringer et al., 2017; Alexander et al., 2018). CA2 neurons are also required for social recognition memory (Stevenson and Caldwell, 2012; Hitti and Siegelbaum, 2014; Smith et al., 2016). Because CA1 exhibits layer-specific organization (Scheffer-Teixeira et al., 2012; Lasztóczi and Klausberger, 2014, 2016) reflective of its inputs (Fernández-Ruiz et al., 2012; Schomburg et al., 2014), and because CA2 activity controls CA1 slow γ (Alexander et al., 2018), we hypothesized that silencing CA2 would affect CA1 slow γ in a layer-specific manner during investigation of a novel social stimulus. While recording from CA1, we leveraged molecular tools to selectively target and inhibit CA2 pyramidal cells using inhibitory DREADDs while subject mice investigated novel animals or objects. We found that CA2 inhibition reduced slow γ power during investigation of a novel animal and fast γ power during both novel object and animal investigation in a manner reflective of the CA2 axonal projection zones within CA1. Our results suggest that CA2 contributes to CA1 slow and fast γ oscillations in a stimulus-specific manner.

A qualitative solution with quantitative potential for the mouse hippocampal cortex flatmap problem

The hippocampal formation (HPF) is a focus of intense experimental investigation, particularly because of its roles in conscious memory consolidation, spatial navigation, emotion, and motivated behaviors. However, the HPF has a complex three-dimensional geometry resulting from extreme curvature of its layers, and this presents a challenge for investigators seeking to decipher hippocampal structure and function at cellular and molecular scales (neuronal circuitry, gene expression, and other properties). Previously, this problem was solved qualitatively for the rat by constructing a physical surface model of the HPF based on histological sections, and then deriving from the model a flatmap. Its usefulness is exemplified by previous studies that used it to display topological relationships between different components of intrahippocampal circuitry derived from experimental pathway-tracing experiments. Here the rat HPF flatmap was used as a starting point to construct an analogous flatmap for the mouse, where the great majority of experimental hippocampal research is currently performed. A detailed account of underlying knowledge and principles is provided, including for hippocampal terminology, and development from an embryonic nonfolded sheet into differentiated multiple adjacent cortical areas, giving rise to the adult shape. To demonstrate its utility, the mouse flatmap was used to display the results of pathway-tracing experiments showing the dentate gyrus mossy fiber projection, and its relationship to the intrahippocampal Purkinje cell protein 4 gene-expression pattern. Finally, requirements for constructing a computer graphics quantitative intrahippocampal flatmap, with accompanying intrahippocampal coordinate system, are presented; they should be applicable to all mammals, including human.

Neuronal diversity and reciprocal connectivity between the vertebrate hippocampus and septum

A hallmark of the nervous system is the precision with which myriad cell types are integrated into functional networks that control complex behaviors. The limbic system governs evolutionarily conserved processes essential for survival. The septum and the hippocampus are central to the limbic system, and control not only emotion-related behaviors but also learning and memory. Here, we provide a developmental and evolutionary perspective of the hippocampus and septum and highlight the neuronal diversity and circuitry that connects these two central components of the limbic system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development Nervous System Development > Vertebrates: General Principles Comparative Development and Evolution > Regulation of Organ Diversity.

Overexpression of human wtTDP-43 causes impairment in hippocampal plasticity and behavioral deficits in CAMKII-tTa transgenic mouse model

AIMS

The current study utilizes the adeno-associated viral gene transfer system in the CAMKIIα-tTA mouse model to overexpress human wild type TDP-43 (wtTDP-43) and α-synuclein (α-Syn) proteins. The co-existence of these proteins is evident in the pathology of neurodegenerative disorders such as frontotemporal lobar degeneration (FTLD), Parkinson disease (PD), and dementia with Lewy bodies (DLB).

METHODS

The novel bicistronic recombinant adeno-associated virus (rAAV) serotype 9 drives wtTDP-43 and α-Syn expression in the hippocampus via "TetO" CMV promoter. Behavior, electrophysiology, and biochemical and histological assays were used to validate neuropathology.

RESULTS

We report that overexpression of wtTDP-43 but not α-Syn contributes to hippocampal CA2-specific pyramidal neuronal loss and overall hippocampal atrophy. Further, we report a reduction of hippocampal long-term potentiation and decline in learning and memory performance of wtTDP-43 expressing mice. Elevated wtTDP-43 levels induced selective degeneration of Purkinje cell protein 4 (PCP-4) positive neurons while both wtTDP-43 and α-Syn expression reduced subsets of the glutamate receptor expression in the hippocampus.

CONCLUSIONS

Overall, our findings suggest the significant vulnerability of hippocampal neurons toward elevated wtTDP-43 levels possibly via PCP-4 and GluR-dependent calcium signaling pathways. Further, we report that wtTDP-43 expression induced selective CA2 subfield degeneration, contributing to the deterioration of the hippocampal-dependent cognitive phenotype.

Social Memory and the Role of the Hippocampal CA2 Region

The CA2 region of the hippocampus is a somewhat obscure area lacking in an understanding of its form and function. Until recently, the CA2 has been thought of as merely an extension of the CA3, with some referring to it as the CA3a region. Recent investigations into this area have not only delineated the CA2, but also defined it as a region distinct from both CA1 and CA3, with a unique set of cortical inputs and outputs and contributions to cognitive processes. One such process that has been shown to depend on the CA2 is the ability to recognize a novel or familiar conspecific, known as social recognition memory. Here, we review these findings and discuss the parallels between CA2 dysfunction and social impairments.

Clozapine increased c-Fos protein expression in several brain subregions of socially isolated rats

Chronic social stress and/or pharmacological treatments differentially modulate the expression of c-Fos, a marker of neuronal activity, in subregions of the rat brain. Here, we examined the effect of the atypical antipsychotic Clozapine (Clz) (20 mg/kg/day for 3 weeks) on the neuronal activation pattern of c-Fos protein expression in stress-relevant brain subregions of adult male Wistar rats exposed to chronic social isolation (CSIS: 3 weeks), an animal model of depression and schizophrenia, and controls. The protein expression of c-Fos was also used to map neuronal populations in brain subregions activated by CSIS alone. Subregions which showed significantly increased c-Fos protein expression following CSIS included the retrosplenial cortex (RSC), (subregions:RSC granular cortex, c region (RSGc) and dysgranular (RSD)), dentate gyrus, dorsal (DGd), paraventricular thalamic nucleus, posterior part (PVP), lateral (LA)/basolateral (BL) complex of amygdala, caudate putamen (CPu) and accumbens nucleus, shell (AcbSh). Increases in c-Fos protein expression in the RSGc, RSD, DGd, PVP, LA/BL complex of amygdala and striatum (CPu, Acb Core (AcbC) and AcbSh) following Clz treatment in controls were found. Clz applied simultaneously with CSIS modulated neuronal activity in CPu, AcbC and AcbSh subregions compared to CSIS alone, increasing c-Fos protein expression. Furthermore, Clz revealed synergistic effects with CSIS in the CA1d and PVP. These identified neural circuits reflect brain subregions activated following CSIS and/or Clz administration. These data further contribute to the understanding of the effectiveness of Clz in the modulation of brain subregion activation in response to CSIS.

Specific Proteomes of Hippocampal Regions CA2 and CA1 Reveal Proteins Linked to the Unique Physiology of Area CA2

The hippocampus is well established as an essential brain center for learning and memory. Within the hippocampus, recent studies show that area CA2 is important for social memory and is an anomaly compared to its better-understood neighboring region, CA1. Unlike CA1, CA2 displays a lack of typical synaptic plasticity, enhanced calcium buffering and extrusion, and resilience to cell death following injury. Although recent studies have identified multiple molecular markers of area CA2, the proteins that mediate the unique physiology, signaling, and resilience of this region are unknown. Using a transgenic GFP-reporter mouse line that expresses eGFP in CA2, we were able to perform targeted dissections of area CA2 and CA1 for proteomic analysis. We identified over 100 proteins with robustly enriched expression in area CA2 compared to CA1. Many of these proteins, including RGS14 and NECAB2, have already been shown to be enriched in CA2 and important for its function, while many more merit further study in the context of enhanced expression in this enigmatic brain region. Furthermore, we performed a comprehensive analysis of the entire data set (>2300 proteins) using a weighted protein co-expression network analysis. This identified eight distinct co-expressed patterns of protein co-enrichment associated with increased expression in area CA2 tissue (compared to CA1). The novel data set we present here reveals a specific CA2 hippocampal proteome, laying the groundwork for future studies and a deeper understanding of area CA2 and the proteins mediating its unique physiology and signaling.

GABAergic Medial Septal Neurons with Low-Rhythmic Firing Innervating the Dentate Gyrus and Hippocampal Area CA3

The medial septum implements cortical theta oscillations, a 5–12 Hz rhythm associated with locomotion and paradoxical sleep reflecting synchronization of neuronal assemblies such as place cell sequence coding. Highly rhythmic burst-firing parvalbumin-positive GABAergic medial septal neurons are strongly coupled to theta oscillations and target cortical GABAergic interneurons, contributing to coordination within one or several cortical regions. However, a large population of medial septal neurons of unidentified neurotransmitter phenotype and with unknown axonal target areas fire with a low degree of rhythmicity. We investigated whether low-rhythmic-firing neurons (LRNs) innervated similar or different cortical regions to high-rhythmic-firing neurons (HRNs) and assessed their temporal dynamics in awake male mice. The majority of LRNs were GABAergic and parvalbumin-immunonegative, some expressing calbindin; they innervated interneurons mostly in the dentate gyrus (DG) and CA3. Individual LRNs showed several distinct firing patterns during immobility and locomotion, forming a parallel inhibitory stream for the modulation of cortical interneurons. Despite their fluctuating firing rates, the preferred firing phase of LRNs during theta oscillations matched the highest firing probability phase of principal cells in the DG and CA3. In addition, as a population, LRNs were markedly suppressed during hippocampal sharp-wave ripples, had a low burst incidence, and several of them did not fire on all theta cycles. Therefore, CA3 receives GABAergic input from both HRNs and LRNs, but the DG receives mainly LRN input. We propose that distinct GABAergic LRNs contribute to changing the excitability of the DG and CA3 during memory discrimination via transient disinhibition of principal cells.

SIGNIFICANCE STATEMENT For the encoding and recall of episodic memories, nerve cells in the cerebral cortex are activated in precisely timed sequences. Rhythmicity facilitates the coordination of neuronal activity and these rhythms are detected as oscillations of different frequencies such as 5–12 Hz theta oscillations. Degradation of these rhythms, such as through neurodegeneration, causes memory deficits. The medial septum, a part of the basal forebrain that innervates the hippocampal formation, contains high- and low-rhythmic-firing neurons (HRNs and LRNs, respectively), which may contribute differentially to cortical neuronal coordination. We discovered that GABAergic LRNs preferentially innervate the dentate gyrus and the CA3 area of the hippocampus, regions important for episodic memory. These neurons act in parallel with the HRNs mostly via transient inhibition of inhibitory neurons.

Tianeptine antagonizes the reduction of PV+ and GAD67 cells number in dorsal hippocampus of socially isolated rats

Adult male rats exposed to chronic social isolation (CSIS) show depressive- and anxiety-like behaviors and reduce the numbers of parvalbumin-positive (PV+) interneurons in the dorsal hippocampus. We aimed to determine whether tianeptine (Tian), administered during the last three weeks of a six-week-social isolation (10 mg/kg/day), may reverse CSIS-induced behavioral changes and antagonize the CSIS-induced reduction in the number of PV+ interneurons. We also studied whether Tian affects the GABA-producing enzyme GAD67+ cells, in Stratum Oriens (SO), Stratum Pyramidale (SP), Stratum Radiatum (SR) and Stratum Lacunosum Moleculare (LM) of CA1-3, as well as in molecular layer-granule cell layer (ML-GCL) and Hilus (H) of the dentate gyrus (DG). CSIS-induced reduction in the number of PV+ cells was layer/subregion-specific with the greatest decrease in SO of CA2. Reduction in the number of PV+ cells was significantly higher than GAD67+ cells, indicating that PV+ cells are the main target following CSIS. Tian reversed CSIS-induced behavior phenotype and antagonized the reduction in the number of PV+ and GAD67+ cells in all subregions. In controls, Tian led to an increase in the number of PV+ and GAD67+ cells in SP of all subregions and PV+ interneurons in ML-GCL of DG, while treatment during CSIS, compared to CSIS alone, resulted with an increase of PV+ interneurons in SO and SP CA1, SP CA2/CA3 and ML-GCL DG with simultaneous increase in GAD67+ cells in all CA1, LM CA2, SO/SR/LM CA3. Data show that Tian offers protection from CSIS via modulation of the dorsal hippocampal GABAergic system.

Brain Sub/Region-Specific Effects of Olanzapine on c-Fos Expression of Chronically Socially Isolated Rats

Olanzapine (Olz) is an atypical antipsychotic used to treat depression, anxiety and schizophrenia, which can be caused by chronic psychosocial stress. c-Fos protein expression has been used as an indirect marker of neuronal activity in response to various forms of stress or pharmacological treatments. We examined the effects of a 3-week treatment of Olz (7.5 mg/kg/day) on c-Fos protein expression in stress-relevant brain sub/regions, its relationship with isolation-induced behavioral changes, and potential sites of Olz action on control and male rats exposed to 6 weeks of chronic social isolation (CSIS), an animal model of depression. Olz treatment reversed depression- and anxiety-like behaviors induced by CSIS and suppressed a CSIS-induced increase in the number of c-Fos-positive cells in subregions of the dorsal hippocampus, ventral (v) DG, retrosplenial cortex, and medial prefrontal cortex. In contrast, no change in c-Fos expression was seen in the CA3v, amygdala and thalamic, hypothalamic or striatal subregions in Olz-treated CSIS rats, suggesting different brain sub/regions' susceptibility to Olz. An increased number of c-Fos-positive cells in the CA1v, amygdala and thalamic, hypothalamic and striatal subregions in controls as well as in the CA1v and subregion of the hypothalamus and nucleus accumbens in Olz-treated CSIS rats was found. Results suggest the activation of brain sub/regions following CSIS that may be involved in depressive and anxiety-like behaviors. Olz treatment showed region-specific effects on neuronal activation. Our data contribute to a better understanding of the mechanisms underlying the CSIS response and potential brain targets of Olz in socially isolated rats.

Prenatal one-carbon metabolism dysregulation programs schizophrenia-like deficits

The methionine-folate cycle-dependent one-carbon metabolism is implicated in the pathophysiology of schizophrenia. Since schizophrenia is a developmental disorder, we examined the effects that perturbation of the one-carbon metabolism during gestation has on mice progeny. Pregnant mice were administered methionine equivalent to double their daily intake during the last week of gestation. Their progeny (MET mice) exhibited schizophrenia-like social deficits, cognitive impairments and elevated stereotypy, decreased neurogenesis and synaptic plasticity, and abnormally reduced local excitatory synaptic connections in CA1 neurons. Neural transcript expression of only one gene, encoding the Npas4 transcription factor, was >twofold altered (downregulated) in MET mice; strikingly, similar Npas4 downregulation occurred in the prefrontal cortex of human patients with schizophrenia. Finally, therapeutic actions of typical (haloperidol) and atypical (clozapine) antipsychotics in MET mice mimicked effects in human schizophrenia patients. Our data support the validity of MET mice as a model for schizophrenia, and uncover methionine metabolism as a potential preventive and/or therapeutic target.

Hippocampal area CA2: properties and contribution to hippocampal function

This review focuses on area CA2 of the hippocampus, as recent results have revealed the unique properties and surprising role of this region in encoding social, temporal and contextual aspects of memory. Originally identified and described by Lorente de No, in 1934, this region of the hippocampus has unique intra-and extra-hippocampal connectivity, sending and receiving input to septal and hypothalamic regions. Recent in vivo studies have indicated that CA2 pyramidal neurons encode spatial information during immobility and play an important role in the generation of sharp-wave ripples. Furthermore, CA2 neurons act to control overall excitability in the hippocampal network and have been found to be consistently altered in psychiatric diseases, indicating that normal function of this region is necessary for normal cognition. With its unique role, area CA2 has a unique molecular profile, interneuron density and composition. Furthermore, this region has an unusual manifestation of synaptic plasticity that does not occur post-synaptically at pyramidal neuron dendrities but through the local network of inhibitory neurons. While much progress has recently been made in understanding the large contribution of area CA2 to social memory formation, much still needs to be learned.

Cornu Ammonis Regions-Antecedents of Cortical Layers?

Studying neocortex and hippocampus in parallel, we are struck by the similarities. All three to four layered allocortices and the six layered mammalian neocortex arise in the pallium. All receive and integrate multiple cortical and subcortical inputs, provide multiple outputs and include an array of neuronal classes. During development, each cell positions itself to sample appropriate local and distant inputs and to innervate appropriate targets. Simpler cortices had already solved the need to transform multiple coincident inputs into serviceable outputs before neocortex appeared in mammals. Why then do phylogenetically more recent cortices need multiple pyramidal cell layers? A simple answer is that more neurones can compute more complex functions. The dentate gyrus and hippocampal CA regions-which might be seen as hippocampal antecedents of neocortical layers-lie side by side, albeit around a tight bend. Were the millions of cells of rat neocortex arranged in like fashion, the surface area of the CA pyramidal cell layers would be some 40 times larger. Even if evolution had managed to fold this immense sheet into the space available, the distances between neurones that needed to be synaptically connected would be huge and to maintain the speed of information transfer, massive, myelinated fiber tracts would be needed. How much more practical to stack the "cells that fire and wire together" into narrow columns, while retaining the mechanisms underlying the extraordinary precision with which circuits form. This demonstrably efficient arrangement presents us with challenges, however, not the least being to categorize the baffling array of neuronal subtypes in each of five "pyramidal layers." If we imagine the puzzle posed by this bewildering jumble of apical dendrites, basal dendrites and axons, from many different pyramidal and interneuronal classes, that is encountered by a late-arriving interneurone insinuating itself into a functional circuit, we can perhaps begin to understand why definitive classification, covering every aspect of each neurone's structure and function, is such a challenge. Here, we summarize and compare the development of these two cortices, the properties of their neurones, the circuits they form and the ordered, unidirectional flow of information from one hippocampal region, or one neocortical layer, to another.

Juvenile Hippocampal CA2 Region Expresses Aggrecan

Perineuronal nets (PNNs) are distributed primarily around inhibitory interneurons in the hippocampus, such as parvalbumin-positive interneurons. PNNs are also present around excitatory neurons in some brain regions and prevent plasticity in these neurons. A recent study demonstrated that PNNs also exist around mouse hippocampal pyramidal cells, which are the principle type of excitatory neurons, in the CA2 subregion and modulate the excitability and plasticity of these neurons. However, the development of PNNs in the CA2 region during postnatal maturation was not fully investigated. This study found that a main component of PNNs, aggrecan, existed in the pyramidal cell layer of the putative CA2 subarea prior to the appearance of the CA2 region, which was defined by the CA2 marker protein regulator of G protein signaling 14 (RGS14). We also found that aggrecan immunoreactivity was more evident in the anterior sections of the CA2 area than the posterior sections, which suggests that the function of CA2 PNNs varies along the anterior-posterior axis.

Rediscovering area CA2: unique properties and functions

Hippocampal area CA2 has several features that distinguish it from CA1 and CA3, including a unique gene expression profile, failure to display long-term potentiation and relative resistance to cell death. A recent increase in interest in the CA2 region, combined with the development of new methods to define and manipulate its neurons, has led to some exciting new discoveries on the properties of CA2 neurons and their role in behaviour. Here, we review these findings and call attention to the idea that the definition of area CA2 ought to be revised in light of gene expression data.

Noncanonical connections between the subiculum and hippocampal CA1

The hippocampal formation is traditionally viewed as having a feedforward, unidirectional circuit organization that promotes propagation of excitatory processes. While the substantial forward projection from hippocampal CA1 to the subiculum has been very well established, accumulating evidence supports the existence of a significant backprojection pathway comprised of both excitatory and inhibitory elements from the subiculum to CA1. Based on these recently updated anatomical connections, such a backprojection could serve to modulate information processing in hippocampal CA1. Here we review the published anatomical and physiological studies on the subiculum to CA1 backprojection, and present recent conclusive anatomical evidence for the presence of noncanonical subicular projections to CA1. New insights into this understudied pathway will improve our understanding of reciprocal CA1-subicular connections and guide future studies on how the subiculum interacts with CA1 to regulate hippocampal circuit activity and learning and memory behaviors. J. Comp. Neurol. 524:3666-3673, 2016. © 2016 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.

High-resolution and cell-type-specific photostimulation mapping shows weak excitatory vs. strong inhibitory inputs in the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is a key component of the extended amygdala and has been implicated in anxiety and addiction. As individual neurons function within neural circuits, it is important to understand local microcircuits and larger network connections of identified neuronal types and understand how maladaptive changes in the BNST neural networks are induced by stress and drug abuse. However, due to limitations of classic anatomical and physiological methods, the local circuit organization of synaptic inputs to specific BNST neuron types is not well understood. In this study, we report on the application of high-resolution and cell-type-specific photostimulation methodology developed in our laboratory to local circuit mapping in the BNST. Under calibrated experimental conditions, laser photostimulation via glutamate uncaging or channelrhodopsin-2 photoactivation evokes spiking of BNST neurons perisomatically, without activating spikes from axons of passage or distal dendrites. Whole cell recordings, combined with spatially restricted photostimulation of presynaptic neurons at many different locations over a large region, allow high-resolution mapping of presynaptic input sources to single recorded neurons in the BNST. We constructed maps of synaptic inputs impinging onto corticotrophin-releasing hormone-expressing (CRH+) BNST neurons in the dorsolateral BNST and found that the CRH+ neurons receive predominant local inhibitory synaptic connections with very weak excitatory connections. Through cell-type-specific optogenetic stimulation mapping, we generated maps of somatostatin-expressing neuron-specific inhibitory inputs to BNST neurons. Taken together, the photostimulation-based techniques offer us powerful tools for determining the functional organization of local circuits of specific BNST neuron types.

Genetic cell targeting uncovers specific neuronal types and distinct subregions in the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) plays an important role in fear, stress, and anxiety. It contains a collection of subnuclei delineated by gross cytoarchitecture features; however, there has yet to be a systematic examination of specific BNST neuronal types and their associated neurochemical makeup. The present study focuses on improved characterization of the anterior BNST based on differing molecular and chemical expression aided by mouse genetics. Specific Cre driver lines crossed with a fluorescent reporter line were used for genetic cell targeting and immunochemical staining. Using this new approach, we were able to robustly identify specific excitatory and inhibitory cell types in the BNST. The presence and distribution of excitatory neurons were firmly established; glutamatergic neurons in the anterior BNST accounted for about 14% and 31% of dorsal and ventral BNST cells, respectively. GABAergic neurons expressing different isoforms of glutamic acid decarboxylase were found to have differential subregional distributions. Almost no parvalbumin-expressing cells were found in the BNST, while somatostatin-expressing cells and calretinin-expressing cells account for modest proportions of BNST cells. In addition, vasoactive intestinal peptide-expressing axonal plexuses were prominent in the oval and juxtacapsular subregions. In addition, we discovered that corticotropin-releasing hormone-expressing cells contain GABAergic and glutamatergic subpopulations. Together, this study reveals new information on excitatory and inhibitory neurons in the BNST, which will facilitate genetic dissection and functional studies of BNST subregions. J. Comp. Neurol. 524:2379-2399, 2016. © 2016 Wiley Periodicals, Inc.

Social and novel contexts modify hippocampal CA2 representations of space

The hippocampus supports a cognitive map of space and is critical for encoding declarative memory (who, what, when and where). Recent studies have implicated hippocampal subfield CA2 in social and contextual memory but how it does so remains unknown. Here we find that in adult male rats, presentation of a social stimulus (novel or familiar rat) or a novel object induces global remapping of place fields in CA2 with no effect on neuronal firing rate or immediate early gene expression. This remapping did not occur in CA1, suggesting this effect is specific for CA2. Thus, modification of existing spatial representations might be a potential mechanism by which CA2 encodes social and novel contextual information.

Neural Activity Patterns Underlying Spatial Coding in the Hippocampus

The hippocampus is well known as a central site for memory processing-critical for storing and later retrieving the experiences events of daily life so they can be used to shape future behavior. Much of what we know about the physiology underlying hippocampal function comes from spatial navigation studies in rodents, which have allowed great strides in understanding how the hippocampus represents experience at the cellular level. However, it remains a challenge to reconcile our knowledge of spatial encoding in the hippocampus with its demonstrated role in memory-dependent tasks in both humans and other animals. Moreover, our understanding of how networks of neurons coordinate their activity within and across hippocampal subregions to enable the encoding, consolidation, and retrieval of memories is incomplete. In this chapter, we explore how information may be represented at the cellular level and processed via coordinated patterns of activity throughout the subregions of the hippocampal network.

Mossy fiber sprouting and pyramidal cell dispersion in the hippocampal CA2 region in a mouse model of temporal lobe epilepsy

Dentate granule cells and the hippocampal CA2 region are resistant to cell loss associated with mesial temporal lobe epilepsy (MTLE). It is known that granule cells undergo mossy fiber sprouting in the dentate gyrus which contributes to a recurrent, proepileptogenic circuitry in the hippocampus. Here it is shown that mossy fiber sprouting also targets CA2 pyramidal cell somata and that the CA2 region undergoes prominent structural reorganization under epileptic conditions. Using the intrahippocampal kainate mouse model for MTLE and the CA2-specific markers Purkinje cell protein 4 (PCP4) and regulator of G-Protein signaling 14 (RGS14), it was found that during epileptogenesis CA2 neurons survive and disperse in direction of CA3 and CA1 resulting in a significantly elongated CA2 region. Using transgenic mice that express enhanced green fluorescent protein (eGFP) in granule cells and mossy fibers, we show that the recently described mossy fiber projection to CA2 undergoes sprouting resulting in aberrant large, synaptoporin-expressing mossy fiber boutons which surround the CA2 pyramidal cell somata. This opens up the potential for altered synaptic transmission that might contribute to epileptic activity in CA2. Indeed, intrahippocampal recordings in freely moving mice revealed that epileptic activity occurs concomitantly in the dentate gyrus and in CA2. Altogether, the results call attention to CA2 as a region affected by MTLE-associated pathological restructuring.

Determinants of different deep and superficial CA1 pyramidal cell dynamics during sharp-wave ripples

Sharp-wave ripples represent a prominent synchronous activity pattern in the mammalian hippocampus during sleep and immobility. GABAergic interneuronal types are silenced or fire during these events, but the mechanism of pyramidal cell (PC) participation remains elusive. We found opposite membrane polarization of deep (closer to stratum oriens) and superficial (closer to stratum radiatum) rat CA1 PCs during sharp-wave ripples. Using sharp and multi-site recordings in combination with neurochemical profiling, we observed a predominant inhibitory drive of deep calbindin (CB)-immunonegative PCs that contrasts with a prominent depolarization of superficial CB-immunopositive PCs. Biased contribution of perisomatic GABAergic inputs, together with suppression of CA2 PCs, may explain the selection of CA1 PCs during sharp-wave ripples. A deep-superficial gradient interacted with behavioral and spatial effects to determine cell participation during sleep and awake sharp-wave ripples in freely moving rats. Thus, the firing dynamics of hippocampal PCs are exquisitely controlled at subcellular and microcircuit levels in a cell type-selective manner.

Regulator of G Protein Signaling 14: A Molecular Brake on Synaptic Plasticity Linked to Learning and Memory

The regulators of G protein signaling (RGS) proteins are a diverse family of proteins that function as central components of G protein and other signaling pathways. In the brain, regulator of G protein signaling 14 (RGS14) is enriched in neurons in the hippocampus where the mRNA and protein are highly expressed. This brain region plays a major role in processing learning and forming new memories. RGS14 is an unusual RGS protein that acts as a multifunctional scaffolding protein to integrate signaling events and pathways essential for synaptic plasticity, including conventional and unconventional G protein signaling, mitogen-activated protein kinase, and, possibly, calcium signaling pathways. Within the hippocampus of primates and rodents, RGS14 is predominantly found in the enigmatic CA2 subfield. Principal neurons within the CA2 subfield differ from neighboring hippocampal regions in that they lack a capacity for long-term potentiation (LTP) of synaptic transmission, which is widely viewed as the cellular substrate of learning and memory formation. RGS14 was recently identified as a natural suppressor of LTP in hippocampal CA2 neurons as well as forms of learning and memory that depend on the hippocampus. Although CA2 has only recently been studied, compelling recent evidence implicates area CA2 as a critical component of hippocampus circuitry with functional roles in mediating certain types of learning and memory. This review will highlight the known functions of RGS14 in cell signaling and hippocampus physiology, and discuss potential roles for RGS14 in human cognition and disease.

Distribution of PCP4 protein in the forebrain of adult mice

Purkinje-cell protein 4 (PCP4) is a small calmodulin (CaM)-binding protein that has been discovered to be selectively expressed by cerebellar Purkinje cells in the adult rodent brain. In addition, expression of PCP4 mRNA has been detected in the hippocampus and in the cortex. In order to determine the expression of PCP4 protein in the brain, we performed an immunohistochemical analysis using adult mice. We could demonstrate that PCP4 is expressed in neocortical structures, especially in the deep layers, as well as in other cortical structures and parts of the hippocampal formation. Moreover, PCP4 protein is highly expressed in the olfactory bulb and caudate putamen. PCP4 positive cells were also detected in specific areas of the amygdala, thalamus (especially dorsal lateral geniculate nucleus) and hypothalamus. By performing double-labeling experiments together with NeuN (a neuronal marker), we could demonstrate that PCP4 expressing cells in the brain are of neuronal origin.

Cell-type-specific circuit connectivity of hippocampal CA1 revealed through Cre-dependent rabies tracing

We developed and applied a Cre-dependent, genetically modified rabies-based tracing system to map direct synaptic connections to specific CA1 neuron types in the mouse hippocampus. We found common inputs to excitatory and inhibitory CA1 neurons from CA3, CA2, the entorhinal cortex (EC), the medial septum (MS), and, unexpectedly, the subiculum. Excitatory CA1 neurons receive inputs from both cholinergic and GABAergic MS neurons, whereas inhibitory neurons receive a great majority of inputs from GABAergic MS neurons. Both cell types also receive weaker input from glutamatergic MS neurons. Comparisons of inputs to CA1 PV+ interneurons versus SOM+ interneurons showed similar strengths of input from the subiculum, but PV+ interneurons received much stronger input than SOM+ neurons from CA3, the EC, and the MS. Thus, rabies tracing identifies hippocampal circuit connections and maps how the different input sources to CA1 are distributed with different strengths on each of its constituent cell types.

Adult neurogenesis modifies excitability of the dentate gyrus

Adult-born dentate granule neurons contribute to memory encoding functions of the dentate gyrus (DG) such as pattern separation. However, local circuit-mechanisms by which adult-born neurons partake in this process are poorly understood. Computational, neuroanatomical and electrophysiological studies suggest that sparseness of activation in the granule cell layer (GCL) is conducive for pattern separation. A sparse coding scheme is thought to facilitate the distribution of similar entorhinal inputs across the GCL to decorrelate overlapping representations and minimize interference. Here we used fast voltage-sensitive dye (VSD) imaging combined with laser photostimulation and electrical stimulation to examine how selectively increasing adult DG neurogenesis influences local circuit activity and excitability. We show that DG of mice with more adult-born neurons exhibits decreased strength of neuronal activation and more restricted excitation spread in GCL while maintaining effective output to CA3c. Conversely, blockade of adult hippocampal neurogenesis changed excitability of the DG in the opposite direction. Analysis of GABAergic inhibition onto mature dentate granule neurons in the DG of mice with more adult-born neurons shows a modest readjustment of perisomatic inhibitory synaptic gain without changes in overall inhibitory tone, presynaptic properties or GABAergic innervation pattern. Retroviral labeling of connectivity in mice with more adult-born neurons showed increased number of excitatory synaptic contacts of adult-born neurons onto hilar interneurons. Together, these studies demonstrate that adult hippocampal neurogenesis modifies excitability of mature dentate granule neurons and that this non-cell autonomous effect may be mediated by local circuit mechanisms such as excitatory drive onto hilar interneurons. Modulation of DG excitability by adult-born dentate granule neurons may enhance sparse coding in the GCL to influence pattern separation.