Local circuitry involving parvalbumin-positive basket cells in the CA2 region of the hippocampus

Reduced Cholecystokinin-Expressing Interneuron Input Contributes to Disinhibition of the Hippocampal CA2 Region in a Mouse Model of Temporal Lobe Epilepsy

A significant proportion of temporal lobe epilepsy (TLE) patients experience drug-resistant seizures associated with mesial temporal sclerosis, in which there is extensive cell loss in the hippocampal CA1 and CA3 subfields, with a relative sparing of dentate gyrus granule cells and CA2 pyramidal neurons (PNs). A role for CA2 in seizure generation was suggested based on findings of a reduction in CA2 synaptic inhibition (Williamson and Spencer, 1994) and the presence of interictal-like spike activity in CA2 in resected hippocampal tissue from TLE patients (Wittner et al., 2009). We recently found that in the pilocarpine-induced status epilepticus (PILO-SE) mouse model of TLE there was an increase in CA2 intrinsic excitability associated with a loss of CA2 synaptic inhibition. Furthermore, chemogenetic silencing of CA2 significantly reduced seizure frequency, consistent with a role of CA2 in promoting seizure generation and/or propagation (Whitebirch et al., 2022). In the present study, we explored the cellular basis of this inhibitory deficit using immunohistochemical and electrophysiological approaches in PILO-SE male and female mice. We report a widespread decrease in the density of pro-cholecystokinin-immunopositive (CCK+) interneurons and a functional impairment of CCK+ interneuron-mediated inhibition of CA2 PNs. We also found a disruption in the perisomatic perineuronal net in the CA2 stratum pyramidale. Such pathologic alterations may contribute to an enhanced excitation of CA2 PNs and CA2-dependent seizure activity in the PILO-SE mouse model.SIGNIFICANCE STATEMENT Impaired synaptic inhibition in hippocampal circuits has been identified as a key feature that contributes to the emergence and propagation of seizure activity in human patients and animal models of temporal lobe epilepsy (TLE). Among the hippocampal subfields, the CA2 region is particularly resilient to seizure-associated neurodegeneration and has been suggested to play a key role in seizure activity in TLE. Here we report that perisomatic inhibition of CA2 pyramidal neurons mediated by cholecystokinin-expressing interneurons is selectively reduced in acute hippocampal slices from epileptic mice. Parvalbumin-expressing interneurons, in contrast, appear relatively conserved in epileptic mice. These findings advance our understanding of the cellular mechanisms underlying inhibitory disruption in hippocampal circuits in a mouse model of spontaneous recurring seizures.

Projections of hippocampal CA2 pyramidal neurons: Distinct innervation patterns of CA2 compared to CA3 in rodents

The hippocampus is a center for spatial and episodic memory formation in rodents. Understanding the composition of subregions and circuitry maps of the hippocampus is essential for elucidating the mechanism of memory formation and recall. For decades, the trisynaptic circuit (entorhinal cortex layer II-dentate gyrus - CA3-CA1) has been considered the neural network substrate responsible for learning and memory. Recently, CA2 has emerged as an important area in the hippocampal circuitry, with distinct functions from those of CA3. In this article, we review the historical definition of the hippocampal area CA2 and the differential projection patterns between CA2 and CA3 pyramidal neurons. We provide a concise and comprehensive map of CA2 outputs by comparing (1) ipsi versus contra projections, (2) septal versus temporal projections, and (3) lamellar structures of CA2 and CA3 pyramidal neurons.

Normalized unitary synaptic signaling of the hippocampus and entorhinal cortex predicted by deep learning of experimental recordings

Biologically realistic computer simulations of neuronal circuits require systematic data-driven modeling of neuron type-specific synaptic activity. However, limited experimental yield, heterogeneous recordings conditions, and ambiguous neuronal identification have so far prevented the consistent characterization of synaptic signals for all connections of any neural system. We introduce a strategy to overcome these challenges and report a comprehensive synaptic quantification among all known neuron types of the hippocampal-entorhinal network. First, we reconstructed >2600 synaptic traces from ∼1200 publications into a unified computational representation of synaptic dynamics. We then trained a deep learning architecture with the resulting parameters, each annotated with detailed metadata such as recording method, solutions, and temperature. The model learned to predict the synaptic properties of all 3,120 circuit connections in arbitrary conditions with accuracy approaching the intrinsic experimental variability. Analysis of data normalized and completed with the deep learning model revealed that synaptic signals are controlled by few latent variables associated with specific molecular markers and interrelating conductance, decay time constant, and short-term plasticity. We freely release the tools and full dataset of unitary synaptic values in 32 covariate settings. Normalized synaptic data can be used in brain simulations, and to predict and test experimental hypothesis.

A deep learning model trained on roughly 2,600 synaptic traces from hippocampal electrophysiology datasets demonstrates how specific covariates influence synaptic signals.

The mechanisms shaping CA2 pyramidal neuron action potential bursting induced by muscarinic acetylcholine receptor activation

Recent studies have revealed that hippocampal area CA2 plays an important role in hippocampal network function. Disruption of this region has been implicated in neuropsychiatric disorders. It is well appreciated that cholinergic input to the hippocampus plays an important role in learning and memory. While the effect of elevated cholinergic tone has been well studied in areas CA1 and CA3, it remains unclear how changes in cholinergic tone impact synaptic transmission and the intrinsic properties of neurons in area CA2. In this study, we applied the cholinergic agonist carbachol and performed on-cell, whole-cell, and extracellular recordings in area CA2. We observed that under conditions of high cholinergic tone, CA2 pyramidal neurons depolarized and rhythmically fired bursts of action potentials. This depolarization depended on the activation of M1 and M3 cholinergic receptors. Furthermore, we examined how the intrinsic properties and action-potential firing were altered in CA2 pyramidal neurons treated with 10 µM carbachol. While this intrinsic burst firing persisted in the absence of synaptic transmission, bursts were shaped by synaptic inputs in the intact network. We found that both excitatory and inhibitory synaptic transmission were reduced upon carbachol treatment. Finally, we examined the contribution of different channels to the cholinergic-induced changes in neuronal properties. We found that a conductance from K_v7 channels partially contributed to carbachol-induced changes in resting membrane potential and membrane resistance. We also found that D-type potassium currents contributed to controlling several properties of the bursts, including firing rate and burst kinetics. Furthermore, we determined that T-type calcium channels and small conductance calcium-activated potassium channels play a role in regulating bursting activity.

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.

Adult vitamin D deficiency disrupts hippocampal-dependent learning and structural brain connectivity in BALB/c mice

Converging evidence from human and animal studies support an association between vitamin D deficiency and cognitive impairment. Previous studies have shown that hippocampal volume is reduced in adults with vitamin D deficiency as well as in a range of disorders, such as schizophrenia. The aim of the current study was to examine the effect of adult vitamin D (AVD) deficiency on hippocampal-dependent spatial learning, and hippocampal volume and connectivity in healthy adult mice. Ten-week-old male BALB/c mice were fed a control (vitamin D 1500 IU/kg) or vitamin D-depleted (vitamin D 0 IU/kg) diet for a minimum of 10 weeks. The mice were then tested for hippocampal-dependent spatial learning using active place avoidance (APA) and on tests of muscle and motor coordination (rotarod and grip strength). The mice were perfused and brains collected to acquire ex vivo structural and diffusion-weighted images using a 16.4 T MRI scanner. We also performed immunohistochemistry to quantify perineuronal nets (PNNs) and parvalbumin (PV) interneurons in various brain regions. AVD-deficient mice had a lower latency to enter the shock zone on APA, compared to control mice, suggesting impaired hippocampal-dependent spatial learning. There were no differences in rotarod or grip strength, indicating that AVD deficiency did not have an impact on muscle or motor coordination. AVD deficiency did not have an impact on hippocampal volume. However, AVD-deficient mice displayed a disrupted network centred on the right hippocampus with abnormal connectomes among 29 nodes. We found a reduction in PNN positive cells, but no change in PV, centred on the hippocampus. Our results provide compelling evidence to show that AVD deficiency in otherwise healthy adult mice may play a key role in hippocampal-dependent learning and memory formation. We suggest that the spatial learning deficits could be due to the disruption of right hippocampal structural connectivity.

CA2 neuronal activity controls hippocampal low gamma and ripple oscillations

Hippocampal oscillations arise from coordinated activity among distinct populations of neurons and are associated with cognitive functions. Much progress has been made toward identifying the contribution of specific neuronal populations in hippocampal oscillations, but less is known about the role of hippocampal area CA2, which is thought to support social memory. Furthermore, the little evidence on the role of CA2 in oscillations has yielded conflicting conclusions. Therefore, we sought to identify the contribution of CA2 to oscillations using a controlled experimental system. We used excitatory and inhibitory DREADDs to manipulate CA2 neuronal activity and studied resulting hippocampal-prefrontal cortical network oscillations. We found that modification of CA2 activity bidirectionally regulated hippocampal and prefrontal cortical low-gamma oscillations and inversely modulated hippocampal ripple oscillations in mice. These findings support a role for CA2 in low-gamma generation and ripple modulation within the hippocampus and underscore the importance of CA2 in extrahippocampal oscillations.

Oxytocin Transforms Firing Mode of CA2 Hippocampal Neurons

Oxytocin is an important neuromodulator in the mammalian brain that increases information salience and circuit plasticity, but its signaling mechanisms and circuit effect are not fully understood. Here we report robust oxytocinergic modulation of intrinsic properties and circuit operations in hippocampal area CA2, a region of emerging importance for hippocampal function and social behavior. Upon oxytocin receptor activation, CA2 pyramidal cells depolarize and fire bursts of action potentials, a consequence of phospholipase C signaling to modify two separate voltage-dependent ionic processes. A reduction of potassium current carried by KCNQ-based M channels depolarizes the cell; protein kinase C activity attenuates spike rate of rise and overshoot, dampening after-hyperpolarizations. These actions, in concert with activation of fast-spiking interneurons, promote repetitive firing and CA2 bursting; bursting then governs short-term plasticity of CA2 synaptic transmission onto CA1 and, thus, efficacy of information transfer in the hippocampal network.

Morphological diversity and connectivity of hippocampal interneurons

The mammalian forebrain is constructed from ensembles of neurons that form local microcircuits giving rise to the exquisite cognitive tasks the mammalian brain can perform. Hippocampal neuronal circuits comprise populations of relatively homogenous excitatory neurons, principal cells and exceedingly heterogeneous inhibitory neurons, the interneurons. Interneurons release GABA from their axon terminals and are capable of controlling excitability in every cellular compartment of principal cells and interneurons alike; thus, they provide a brake on excess activity, control the timing of neuronal discharge and provide modulation of synaptic transmission. The dendritic and axonal morphology of interneurons, as well as their afferent and efferent connections within hippocampal circuits, is central to their ability to differentially control excitability, in a cell-type- and compartment-specific manner. This review aims to provide an up-to-date compendium of described hippocampal interneuron subtypes, with respect to their morphology, connectivity, neurochemistry and physiology, a full understanding of which will in time help to explain the rich diversity of neuronal function.

Increased Excitability and Reduced Excitatory Synaptic Input Into Fast-Spiking CA2 Interneurons After Enzymatic Attenuation of Extracellular Matrix

The neural extracellular matrix (ECM) is enriched with hyaluronic acid, chondroitin sulfate proteoglycans (CSPGs) and the glycoprotein tenascin-R, which play important roles in synaptic plasticity, as shown by studies of the CA1 region of the hippocampus. However, ECM molecules are strongly expressed in the CA2 region, which harbors a high number of fast-spiking interneurons (FSIs) surrounded by a particularly condensed form of ECM, perineuronal nets. Despite this intriguing peculiarity, the functional role of ECM in the CA2 region is mostly unknown. Here, we investigate the acute and delayed effects of chondroitinase ABC (ChABC), an enzyme that digests chondroitin sulfate side chains of CSPGs and greatly attenuates neural ECM, on neuronal excitability and excitatory transmission in the CA2 region. Whole-cell patch clamp recordings of CA2 pyramidal cells (PCs) and FSIs in hippocampal slices revealed that 7 days after injection of ChABC into the CA2 region in vivo, there are alterations in excitability of FSIs and PCs. FSIs generated action potentials with larger amplitudes and longer durations in response to less depolarizing currents compared to controls. PCs were excited at less depolarized membrane potentials, resulted in lower latency of spike generation. The frequency of excitatory postsynaptic currents in FSIs was selectively reduced, while the frequency of inhibitory postsynaptic currents was selectively increased. Acute treatment of hippocampal slices with ChABC did not result in any of these effects. This increase in excitability and changes in synaptic inputs to FSIs after attenuation of ECM suggests a crucial role for perineuronal nets associated with FSIs in regulation of synaptic and electrical properties of these cells.

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.

Target Cell Type-Dependent Differences in Ca2+ Channel Function Underlie Distinct Release Probabilities at Hippocampal Glutamatergic Terminals

Target cell type-dependent differences in presynaptic release probability (P_r ) and short-term plasticity are intriguing features of cortical microcircuits that increase the computational power of neuronal networks. Here, we tested the hypothesis that different voltage-gated Ca²⁺ channel densities in presynaptic active zones (AZs) underlie different P_r values. Two-photon Ca²⁺ imaging, triple immunofluorescent labeling, and 3D electron microscopic (EM) reconstruction of rat CA3 pyramidal cell axon terminals revealed ∼1.7-1.9 times higher Ca²⁺ inflow per AZ area in high P_r boutons synapsing onto parvalbumin-positive interneurons (INs) than in low P_r boutons synapsing onto mGluR1α-positive INs. EM replica immunogold labeling, however, demonstrated only 1.15 times larger Cav2.1 and Cav2.2 subunit densities in high P_r AZs. Our results indicate target cell type-specific modulation of voltage-gated Ca²⁺ channel function or different subunit composition as possible mechanisms underlying the functional differences. In addition, high P_r synapses are also characterized by a higher density of docked vesicles, suggesting that a concerted action of these mechanisms underlies the functional differences.SIGNIFICANCE STATEMENT Target cell type-dependent variability in presynaptic properties is an intriguing feature of cortical synapses. When a single cortical pyramidal cell establishes a synapse onto a somatostatin-expressing interneuron (IN), the synapse releases glutamate with low probability, whereas the next bouton of the same axon has high release probability when its postsynaptic target is a parvalbumin-expressing IN. Here, we used combined molecular, imaging, and anatomical approaches to investigate the mechanisms underlying these differences. Our functional experiments implied an approximately twofold larger Ca²⁺ channel density in high release probability boutons, whereas freeze-fracture immunolocalization demonstrated only a 15% difference in Ca²⁺ channel subunit densities. Our results point toward a postsynaptic target cell type-dependent regulation of Ca²⁺ channel function or different subunit composition as the underlying mechanism.

Interneuronal mechanisms of hippocampal theta oscillations in a full-scale model of the rodent CA1 circuit

The hippocampal theta rhythm plays important roles in information processing; however, the mechanisms of its generation are not well understood. We developed a data-driven, supercomputer-based, full-scale (1:1) model of the rodent CA1 area and studied its interneurons during theta oscillations. Theta rhythm with phase-locked gamma oscillations and phase-preferential discharges of distinct interneuronal types spontaneously emerged from the isolated CA1 circuit without rhythmic inputs. Perturbation experiments identified parvalbumin-expressing interneurons and neurogliaform cells, as well as interneuronal diversity itself, as important factors in theta generation. These simulations reveal new insights into the spatiotemporal organization of the CA1 circuit during theta oscillations.

Interneuronal mechanisms of hippocampal theta oscillations in a full-scale model of the rodent CA1 circuit

The hippocampal theta rhythm plays important roles in information processing; however, the mechanisms of its generation are not well understood. We developed a data-driven, supercomputer-based, full-scale (1:1) model of the rodent CA1 area and studied its interneurons during theta oscillations. Theta rhythm with phase-locked gamma oscillations and phase-preferential discharges of distinct interneuronal types spontaneously emerged from the isolated CA1 circuit without rhythmic inputs. Perturbation experiments identified parvalbumin-expressing interneurons and neurogliaform cells, as well as interneuronal diversity itself, as important factors in theta generation. These simulations reveal new insights into the spatiotemporal organization of the CA1 circuit during theta oscillations.

DOI:http://dx.doi.org/10.7554/eLife.18566.001

Role of Hippocampal CA2 Region in Triggering Sharp-Wave Ripples

Sharp-wave ripples (SPW-Rs) in the hippocampus are implied in memory consolidation, as shown by observational and interventional experiments. However, the mechanism of their generation remains unclear. Using two-dimensional silicon probe arrays, we investigated the propagation of SPW-Rs across the hippocampal CA1, CA2, and CA3 subregions. Synchronous activation of CA2 ensembles preceded SPW-R-related population activity in CA3 and CA1 regions. Deep CA2 neurons gradually increased their activity prior to ripples and were suppressed during the population bursts of CA3-CA1 neurons (ramping cells). Activity of superficial CA2 cells preceded the activity surge in CA3-CA1 (phasic cells). The trigger role of the CA2 region in SPW-R was more pronounced during waking than sleeping. These results point to the CA2 region as an initiation zone for SPW-Rs.

The Diversity of Cortical Inhibitory Synapses

The most typical and well known inhibitory action in the cortical microcircuit is a strong inhibition on the target neuron by axo-somatic synapses. However, it has become clear that synaptic inhibition in the cortex is much more diverse and complicated. Firstly, at least ten or more inhibitory non-pyramidal cell subtypes engage in diverse inhibitory functions to produce the elaborate activity characteristic of the different cortical states. Each distinct non-pyramidal cell subtype has its own independent inhibitory function. Secondly, the inhibitory synapses innervate different neuronal domains, such as axons, spines, dendrites and soma, and their inhibitory postsynaptic potential (IPSP) size is not uniform. Thus, cortical inhibition is highly complex, with a wide variety of anatomical and physiological modes. Moreover, the functional significance of the various inhibitory synapse innervation styles and their unique structural dynamic behaviors differ from those of excitatory synapses. In this review, we summarize our current understanding of the inhibitory mechanisms of the cortical microcircuit.

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.

Inhibitory Plasticity Permits the Recruitment of CA2 Pyramidal Neurons by CA3

Area CA2 is emerging as an important region for hippocampal memory formation. However, how CA2 pyramidal neurons (PNs) are engaged by intrahippocampal inputs remains unclear. Excitatory transmission between CA3 and CA2 is strongly inhibited and is not plastic. We show in mice that different patterns of activity can in fact increase the excitatory drive between CA3 and CA2. We provide evidence that this effect is mediated by a long-term depression at inhibitory synapses (iLTD), as it is evoked by the same protocols and shares the same pharmacology. In addition, we show that the net excitatory drive of distal inputs is also increased after iLTD induction. The disinhibitory increase in excitatory drive is sufficient to allow CA3 inputs to evoke action potential firing in CA2 PNs. Thus, these data reveal that the output of CA2 PNs can be gated by the unique activity-dependent plasticity of inhibitory neurons in area CA2.

Distribution of interneurons in the CA2 region of the rat hippocampus

The CA2 region of the mammalian hippocampus is a unique region with its own distinctive properties, inputs and pathologies. Disruption of inhibitory circuits in this region appears to be linked with the pathology of specific psychiatric disorders, promoting interest in its local circuitry, its role in hippocampal function and its dysfunction in disease. In previous studies, CA2 interneurons, including a novel subclass of CA2 dendrite-preferring interneurons that has not been identified in other CA regions, have been shown to display physiological, synaptic and morphological properties unique to this sub-field and may therefore play a crucial role in the hippocampal circuitry. The distributions of immuno-labeled interneurons in dorsal CA2 were studied and compared with those of interneurons in CA1 and CA3. Like those in CA1 and CA3, the somata of CA2 parvalbumin-immunoperoxidase-labeled interneurons were located primarily in Stratum Pyramidale (SP) and Stratum Oriens (SO), with very few cells in Stratum Radiatum (SR) and none in Stratum Lacunosum Moleculare (SLM). There was, however, a greater proportion of GAD-positive cells were immunopositive for PV in SP in CA2 than in CA1 or CA3. CA2 SP also contained a larger density of somatostatin-, calbindin-, and VIP-immunopositive somata than CA1 and/or CA3. Like those in CA1 and CA3, CCK-immunopositive somata in CA2 were mostly located in SR. Reelin- and NPY- immunolabeled cell bodies were located in all layers of the three CA regions. However, a higher density of Reelin-positive somata was found in SP and SR of CA2 than in CA1 or CA3.

Highly energized inhibitory interneurons are a central element for information processing in cortical networks

Gamma oscillations (∼30 to 100 Hz) provide a fundamental mechanism of information processing during sensory perception, motor behavior, and memory formation by coordination of neuronal activity in networks of the hippocampus and neocortex. We review the cellular mechanisms of gamma oscillations about the underlying neuroenergetics, i.e., high oxygen consumption rate and exquisite sensitivity to metabolic stress during hypoxia or poisoning of mitochondrial oxidative phosphorylation. Gamma oscillations emerge from the precise synaptic interactions of excitatory pyramidal cells and inhibitory GABAergic interneurons. In particular, specialized interneurons such as parvalbumin-positive basket cells generate action potentials at high frequency ('fast-spiking') and synchronize the activity of numerous pyramidal cells by rhythmic inhibition ('clockwork'). As prerequisites, fast-spiking interneurons have unique electrophysiological properties and particularly high energy utilization, which is reflected in the ultrastructure by enrichment with mitochondria and cytochrome c oxidase, most likely needed for extensive membrane ion transport and γ-aminobutyric acid metabolism. This supports the hypothesis that highly energized fast-spiking interneurons are a central element for cortical information processing and may be critical for cognitive decline when energy supply becomes limited ('interneuron energy hypothesis'). As a clinical perspective, we discuss the functional consequences of metabolic and oxidative stress in fast-spiking interneurons in aging, ischemia, Alzheimer's disease, and schizophrenia.

Delta-opioid receptors mediate unique plasticity onto parvalbumin-expressing interneurons in area CA2 of the hippocampus

Inhibition is critical for controlling information transfer in the brain. However, the understanding of the plasticity and particular function of different interneuron subtypes is just emerging. Using acute hippocampal slices prepared from adult mice, we report that in area CA2 of the hippocampus, a powerful inhibitory transmission is acting as a gate to prevent CA3 inputs from driving CA2 neurons. Furthermore, this inhibition is highly plastic, and undergoes a long-term depression following high-frequency 10 Hz or theta-burst induction protocols. We describe a novel form of long-term depression at parvalbumin-expressing (PV+) interneuron synapses that is dependent on delta-opioid receptor (DOR) activation. Additionally, PV+ interneuron transmission is persistently depressed by DOR activation in area CA2 but only transiently depressed in area CA1. These results provide evidence for a differential temporal modulation of PV+ synapses between two adjacent cortical circuits, and highlight a new function of PV+ cells in controlling information transfer.

Chronic ketamine produces altered distribution of parvalbumin-positive cells in the hippocampus of adult rats

The underlying mechanisms of schizophrenia pathogenesis are not well understood. Increasing evidence supports the glutamatergic hypothesis that posits a hypofunction of the N-methyl D-aspartate (NMDA) receptor on specific gamma amino-butyric acid (GABA)-ergic neurons may be responsible for the disorder. Alterations in the GABAergic system have been observed in schizophrenia, most notably a change in the expression of parvalbumin (PV) in the cortex and hippocampus. Several reports also suggest abnormal neuronal migration may play a role in the etiology of schizophrenia. The current study examined the positioning and distribution of PV-positive cells in the hippocampus following chronic treatment with the NMDA receptor antagonist ketamine. A robust increase was found in the number of PV-positive interneurons located outside the stratum oriens (SO), the layer where most of these cells are normally localized, as well as an overall numerical increase in CA3 PV cells. These results suggest ketamine leads to an abnormal distribution of PV-positive cells, which may be indicative of aberrant migratory activity and possibly related to the Morris water maze deficits observed. These findings may also be relevant to alterations observed in schizophrenia populations.

Distinct dendritic arborization and in vivo firing patterns of parvalbumin-expressing basket cells in the hippocampal area CA3

Hippocampal CA3 area generates temporally structured network activity such as sharp waves and gamma and theta oscillations. Parvalbumin-expressing basket cells, making GABAergic synapses onto cell bodies and proximal dendrites of pyramidal cells, control pyramidal cell activity and participate in network oscillations in slice preparations, but their roles in vivo remain to be tested. We have recorded the spike timing of parvalbumin-expressing basket cells in areas CA2/3 of anesthetized rats in relation to CA3 putative pyramidal cell firing and activity locally and in area CA1. During theta oscillations, CA2/3 basket cells fired on the same phase as putative pyramidal cells, but, surprisingly, significantly later than downstream CA1 basket cells. This indicates a distinct modulation of CA3 and CA1 pyramidal cells by basket cells, which receive different inputs. We observed unexpectedly large dendritic arborization of CA2/3 basket cells in stratum lacunosum moleculare (33% of length, 29% surface, and 24% synaptic input from a total of ∼35,000), different from the dendritic arborizations of CA1 basket cells. Area CA2/3 basket cells fired phase locked to both CA2/3 and CA1 gamma oscillations, and increased firing during CA1 sharp waves, thus supporting the role of CA3 networks in the generation of gamma oscillations and sharp waves. However, during ripples associated with sharp waves, firing of CA2/3 basket cells was phase locked only to local but not CA1 ripples, suggesting the independent generation of fast oscillations by basket cells in CA1 and CA2/3. The distinct spike timing of basket cells during oscillations in CA1 and CA2/3 suggests differences in synaptic inputs paralleled by differences in dendritic arborizations.

Input-specific learning rules at excitatory synapses onto hippocampal parvalbumin-expressing interneurons

Hippocampal parvalbumin-expressing interneurons (PV INs) provide fast and reliable GABAergic signalling to principal cells and orchestrate hippocampal ensemble activities. Precise coordination of principal cell activity by PV INs relies in part on the efficacy of excitatory afferents that recruit them in the hippocampal network. Feed-forward (FF) inputs in particular from Schaffer collaterals influence spike timing precision in CA1 principal cells whereas local feedback (FB) inputs may contribute to pacemaker activities. Although PV INs have been shown to undergo activity-dependent long term plasticity, how both inputs are modulated during principal cell firing is unknown. Here we show that FF and FB synapses onto PV INs are endowed with distinct postsynaptic glutamate receptors which set opposing long-term plasticity rules. Inward-rectifying AMPA receptors (AMPARs) expressed at both FF and FB inputs mediate a form of anti-Hebbian long term potentiation (LTP), relying on coincident membrane hyperpolarization and synaptic activation. In contrast, FF inputs are largely devoid of NMDA receptors (NMDARs) which are more abundant at FB afferents and confer on them an additional form of LTP with Hebbian properties. Both forms of LTP are expressed with no apparent change in presynaptic function. The specific endowment of FF and FB inputs with distinct coincidence detectors allow them to be differentially tuned upon high frequency afferent activity. Thus, high frequency (>20 Hz) stimulation specifically potentiates FB, but not FF afferents. We propose that these differential, input-specific learning rules may allow PV INs to adapt to changes in hippocampal activity while preserving their precisely timed, clockwork operation.

Vasopressin inhibits LTP in the CA2 mouse hippocampal area

Growing evidence points to vasopressin (AVP) as a social behavior regulator modulating various memory processes and involved in pathologies such as mood disorders, anxiety and depression. Accordingly, AVP antagonists are actually envisaged as putative treatments. However, the underlying mechanisms are poorly characterized, in particular the influence of AVP on cellular or synaptic activities in limbic brain areas involved in social behavior. In the present study, we investigated AVP action on the synapse between the entorhinal cortex and CA2 hippocampal pyramidal neurons, by using both field potential and whole-cell recordings in mice brain acute slices. Short application (1 min) of AVP transiently reduced the synaptic response, only following induction of long-term potentiation (LTP) by high frequency stimulation (HFS) of afferent fibers. The basal synaptic response, measured in the absence of HFS, was not affected. The Schaffer collateral-CA1 synapse was not affected by AVP, even after LTP, while the Schaffer collateral-CA2 synapse was inhibited. Although investigated only recently, this CA2 hippocampal area appears to have a distinctive circuitry and a peculiar role in controlling episodic memory. Accordingly, AVP action on LTP-increased synaptic responses in this limbic structure may contribute to the role of this neuropeptide in controlling memory and social behavior.

SP-SR interneurones: a novel class of neurones of the CA2 region of the hippocampus

The CA2 region of the hippocampus has distinctive properties and inputs and may be linked with the pathology of specific psychiatric and neurological disorders. It is, therefore, important to understand CA2 circuitry and its involvement in the circuitry of the hippocampus. Properties of CA2 basket cells have been reported. However, other classes of CA2 interneurones with cell bodies located in stratum pyramidale remained to be described. In this study, the unusual axonal arbors of a novel subclass of dendrite-preferring CA2 interneurones whose somata are located in the pyramidal cell layer was revealed following intracellular recordings and biocytin labeling. One to four apical dendrites emerged from the soma, branched in stratum radiatum (SR) forming a tuft, but rarely penetrated stratum lacunosum-moleculare (SLM). One or two basal dendrites branched close to the soma, the branches extended through stratum oriens (SO) and often reached the alveus. Unlike CA2 bistratified cells, the axons of these cells arborized almost exclusively in SR with few, if any, branches extending to stratum pyramidale (SP), SO, or SLM. These interneurones again, unlike bistratified cells, were immunonegative for parvalbumin and cholecystokinin. Electrophysiologically, they were similar to some CA2 basket and bistratified cells in that they presented a "sag" in response to hyperpolarizing current injections and displayed spike frequency adaptation. They targeted the apical dendrites of neighboring CA2 pyramidal cells and received inputs from them.

Synaptic integration by different dendritic compartments of hippocampal CA1 and CA2 pyramidal neurons

Pyramidal neurons have a complex dendritic arbor containing tens of thousands of synapses. In order for the somatic/axonal membrane potential to reach action potential threshold, concurrent activation of multiple excitatory synapses is required. Frequently, instead of a simple algebraic summation of synaptic potentials in the soma, different dendritic compartments contribute to the integration of multiple inputs, thus endowing the neuron with a powerful computational ability. Most pyramidal neurons share common functional properties. However, different and sometimes contrasting dendritic integration rules are also observed. In this review, we focus on the dendritic integration of two neighboring pyramidal neurons in the hippocampus: the well-characterized CA1 and the much less understood CA2. The available data reveal that the dendritic integration of these neurons is markedly different even though they are targeted by common inputs at similar locations along their dendrites. This contrasting dendritic integration results in different routing of information flow and generates different corticohippocampal loops.