Mouse hippocampal CA1 VIP interneurons detect novelty in the environment and support recognition memory.

In the CA1 hippocampus, vasoactive intestinal polypeptide-expressing interneurons (VIP-INs) play a prominent role in disinhibitory circuit motifs. However, the specific behavioral conditions that lead to circuit disinhibition remain uncertain. To investigate the behavioral relevance of VIP-IN activity, we employed wireless technologies allowing us to monitor and manipulate their function in freely behaving mice. Our findings reveal that, during spatial exploration in new environments, VIP-INs in the CA1 hippocampal region become highly active, facilitating the rapid encoding of novel spatial information. Remarkably, both VIP-INs and pyramidal neurons (PNs) exhibit increased activity when encountering novel changes in the environment, including context- and object-related alterations. Concurrently, somatostatin- and parvalbumin-expressing inhibitory populations show an inverse relationship with VIP-IN and PN activity, revealing circuit disinhibition that occurs on a timescale of seconds. Thus, VIP-IN-mediated disinhibition may constitute a crucial element in the rapid encoding of novelty and the acquisition of recognition memory.

Inhibitory circuits in fear memory and fear-related disorders.

Fear learning and memory rely on dynamic interactions between the excitatory and inhibitory neuronal populations that make up the prefrontal cortical, amygdala, and hippocampal circuits. Whereas inhibition of excitatory principal cells (PCs) by GABAergic neurons restrains their excitation, inhibition of GABAergic neurons promotes the excitation of PCs through a process called disinhibition. Specifically, GABAergic interneurons that express parvalbumin (PV+) and somatostatin (SOM+) provide inhibition to different subcellular domains of PCs, whereas those that express the vasoactive intestinal polypeptide (VIP+) facilitate disinhibition of PCs by inhibiting PV+ and SOM+ interneurons. Importantly, although the main connectivity motifs and the underlying network functions of PV+, SOM+, and VIP+ interneurons are replicated across cortical and limbic areas, these inhibitory populations play region-specific roles in fear learning and memory. Here, we provide an overview of the fear processing in the amygdala, hippocampus, and prefrontal cortex based on the evidence obtained in human and animal studies. Moreover, focusing on recent findings obtained using genetically defined imaging and intervention strategies, we discuss the population-specific functions of PV+, SOM+, and VIP+ interneurons in fear circuits. Last, we review current insights that integrate the region-specific inhibitory and disinhibitory network patterns into fear memory acquisition and fear-related disorders.

Enhanced motor cortex output and disinhibition in asymptomatic female mice with C9orf72 genetic expansion.

Information and action coding by cortical circuits relies on a balanced dialogue between excitation and inhibition. Circuit hyperexcitability is considered a potential pathophysiological mechanism in various brain disorders, but the underlying deficits, especially at early disease stages, remain largely unknown. We report that asymptomatic female mice carrying the chromosome 9 open reading frame 72 (C9orf72) repeat expansion, which represents a high-prevalence genetic abnormality for human amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) spectrum disorder, exhibit abnormal motor cortex output. The number of primary motor cortex (M1) layer 5 pyramidal neurons is reduced in asymptomatic mice, with the surviving neurons receiving a decreased inhibitory drive that results in a higher M1 output, specifically during high-speed animal locomotion. Importantly, using deep-learning algorithms revealed that speed-dependent M1 output predicts the likelihood of C9orf72 genetic expansion. Our data link early circuit abnormalities with a gene mutation in asymptomatic ALS/FTLD carriers.

The role of inhibitory circuits in hippocampal memory processing.

GABAergic inhibitory circuits play an essential role in coordinating various hippocampal functions. Several decades of work dedicated to a thorough characterization of hippocampal inhibitory populations have highlighted how specific types of interneuron can contribute to network activity. Recent studies have used genetically targeted recordings and peturbations of activity during memory-related behaviours to determine how interneurons that inhibit distinct subcellular domains of principal cells or specialize in principal cell disinhibition may sculpt hippocampal memory. These studies highlight unique contributions of distinct interneuron types to the temporal binding of hippocampal ensembles, synaptic plasticity and the acquisition of spatial and contextual information. Here, we review the current state of knowledge around hippocampal inhibition and memory by discussing the multifaceted roles of populations of inhibitory cells at different stages of hippocampal mnemonic processing.

Cholinergic Modulation of Dendritic Signaling in Hippocampal GABAergic Inhibitory Interneurons.

Dendrites represent the "reception hub" of the neuron as they collect thousands of different inputs and send a coherent response to the cell body. A considerable portion of these signals, especially in vivo, arises from neuromodulatory sources, which affect dendritic computations and cellular activity. In this context, acetylcholine (ACh) exerts a coordinating role of different brain structures, contributing to goal-driven behaviors and sleep-wake cycles. Specifically, cholinergic neurons from the medial septum-diagonal band of Broca complex send numerous projections to glutamatergic principal cells and GABAergic inhibitory neurons in the hippocampus, differentially entraining them during network oscillations. Interneurons display abundant expression of cholinergic receptors and marked responses to stimulation by ACh. Nonetheless, the precise localization of ACh inputs is largely unknown, and evidence for cholinergic modulation of interneuronal dendritic signaling remains elusive. In this article, we review evidence that suggests modulatory effects of ACh on dendritic computations in three hippocampal interneuron subtypes: fast-spiking parvalbumin-positive (PV+) cells, somatostatin-expressing (SOM+) oriens lacunosum moleculare cells and vasoactive intestinal polypeptide-expressing (VIP+) interneuron-selective interneurons. We consider the distribution of cholinergic receptors on these interneurons, including information about their specific somatodendritic location, and discuss how the action of these receptors can modulate dendritic Ca2+ signaling and activity of interneurons. The implications of ACh-dependent Ca2+ signaling for dendritic plasticity are also discussed. We propose that cholinergic modulation can shape the dendritic integration and plasticity in interneurons in a cell type-specific manner, and the elucidation of these mechanisms will be required to understand the contribution of each cell type to large-scale network activity.

Structural analysis of the microglia-interneuron interactions in the CA1 hippocampal area of the APP/PS1 mouse model of Alzheimer's disease.

Microglia can interact with glutamatergic neurons and, through control of synaptic elements, regulate their physiological function. Much less is known about the partnership between microglia and GABAergic inhibitory interneurons. Here, we compared the interactions between microglia and parvalbumin (PV+) and somatostatin (SOM+) expressing interneurons in the CA1 hippocampal area of APP/PS1 transgenic mice that mimic certain aspects of the Alzheimer's disease (AD). We first uncovered a high level of interactions between microglia and two types of interneurons, with 98% of SOM+ and 90% of PV+ cells receiving different types of putative microglial contacts. The latter included the microglia soma to the interneuron soma (SomaMG -to-SomaIN ), the microglia process to the interneuron soma (ProcessMG -to-SomaIN ) and the microglia process to the interneuron dendrite (ProcessMG -to-DendIN ) interactions. Moreover, we found significantly larger areas of interaction for the SomaMG -to-SomaIN and the ProcessMG -to-DendIN type of contacts between microglia and SOM+ cells. In contrast, PV+ cells exhibited larger areas for the ProcessMG -to-SomaIN interactions. Second, in APP/PS1 mice, although the overall microglia interactions with interneurons remained preserved, the fraction of interneurons receiving putative microglia contacts on their dendrites was reduced, and larger areas of interactions were observed for somatic contacts, suggesting a stronger modulation of the interneuron output by microglia in AD. In summary, these results reveal microglia as important partners of hippocampal PV+ and SOM+ GABAergic cells, with interneuron type-specific pattern of interactions. Thus, microglia may play an essential role in the operation of interneurons under normal conditions and their dysfunction in disease.

Cortical disinhibitory circuits: cell types, connectivity and function.

The concept of a dynamic excitation/inhibition balance tuned by circuit disinhibition, which can shape information flow during complex behavioral tasks, has arisen as an important and conserved information-processing motif. In cortical circuits, different subtypes of GABAergic inhibitory interneurons are connected to each other, offering an anatomical foundation for disinhibitory processes. Moreover, a subpopulation of GABAergic cells that express vasoactive intestinal polypeptide (VIP) preferentially innervates inhibitory interneurons, highlighting their central role in disinhibitory modulation. We discuss inhibitory neuron subtypes involved in disinhibition, with a focus on local circuits and long-range synaptic connections that drive disinhibitory function. We highlight multiple layers of disinhibition across cortical circuits that regulate behavior and serve to maintain an excitation/inhibition balance.

Sex Differences of Microglia and Synapses in the Hippocampal Dentate Gyrus of Adult Mouse Offspring Exposed to Maternal Immune Activation.

Schizophrenia is a psychiatric disorder affecting ∼1% of humans worldwide. It is earlier and more frequently diagnosed in men than woman, and men display more pronounced negative symptoms together with greater gray matter reductions. Our previous findings utilizing a maternal immune activation (mIA) mouse model of schizophrenia revealed exacerbated anxiety-like behavior and sensorimotor gating deficits in adult male offspring that were associated with increased microglial reactivity and inflammation in the hippocampal dentate gyrus (DG). However, both male and female adult offspring displayed stereotypy and impairment of sociability. We hypothesized that mIA may lead to sex-specific alterations in microglial pruning activity, resulting in abnormal synaptic connectivity in the DG. Using the same mIA model, we show in the current study sex-specific differences in microglia and synapses within the DG of adult offspring. Specifically, microglial levels of cluster of differentiation (CD)68 and CD11b were increased in mIA-exposed females. Sex-specific differences in excitatory and inhibitory synapse densities were also observed following mIA. Additionally, inhibitory synaptic tone was increased in DG granule cells of both males and females, while changes in excitatory synaptic transmission occurred only in females with mIA. These findings suggest that phagocytic and complement pathways may together contribute to a sexual dimorphism in synaptic pruning and neuronal dysfunction in mIA, and may propose sex-specific therapeutic targets to prevent schizophrenia-like behaviors.

Alterations in Intrinsic and Synaptic Properties of Hippocampal CA1 VIP Interneurons During Aging.

Learning and memory deficits are hallmarks of the aging brain, with cortical neuronal circuits representing the main target in cognitive deterioration. While GABAergic inhibitory and disinhibitory circuits are critical in supporting cognitive processes, their roles in age-related cognitive decline remain largely unknown. Here, we examined the morphological and physiological properties of the hippocampal CA1 vasoactive intestinal peptide/calretinin-expressing (VIP+/CR+) type 3 interneuron-specific (I-S3) cells across mouse lifespan. Our data showed that while the number and morphological features of I-S3 cells remained unchanged, their firing and synaptic properties were significantly altered in old animals. In particular, the action potential duration and the level of steady-state depolarization were significantly increased in old animals in parallel with a significant decrease in the maximal firing frequency. Reducing the fast-delayed rectifier potassium or transient sodium conductances in I-S3 cell computational models could reproduce the age-related changes in I-S3 cell firing properties. However, experimental data revealed no difference in the activation properties of the Kv3.1 and A-type potassium currents, indicating that transient sodium together with other ion conductances may be responsible for the observed phenomena. Furthermore, I-S3 cells in aged mice received a stronger inhibitory drive due to concomitant increase in the amplitude and frequency of spontaneous inhibitory currents. These age-associated changes in the I-S3 cell properties occurred in parallel with an increased inhibition of their target interneurons and were associated with spatial memory deficits and increased anxiety. Taken together, these data indicate that VIP+/CR+ interneurons responsible for local circuit disinhibition survive during aging but exhibit significantly altered physiological properties, which may result in the increased inhibition of hippocampal interneurons and distorted mnemonic functions.

Common Principles in Functional Organization of VIP/Calretinin Cell-Driven Disinhibitory Circuits Across Cortical Areas.

In the brain, there is a vast diversity of different structures, circuitries, cell types, and cellular genetic expression profiles. While this large diversity can often occlude a clear understanding of how the brain works, careful analyses of analogous studies performed across different brain areas can hint at commonalities in neuronal organization. This in turn can yield a fundamental understanding of necessary circuitry components that are crucial for how information is processed across the brain. In this review, we outline recent in vivo and in vitro studies that have been performed in different cortical areas to characterize the vasoactive intestinal polypeptide (VIP)- and/or calretinin (CR)-expressing cells that specialize in inhibiting GABAergic interneurons. In doing so, we make the case that, across cortical structures, interneuron-specific cells commonly specialize in the synaptic disinhibition of excitatory neurons, which can ungate the integration and plasticity of external inputs onto excitatory neurons. In line with this, activation of interneuron- specific cells enhances animal performance across a variety of behavioral tasks that involve learning, memory formation, and sensory discrimination, and may represent a key target for therapeutic interventions under different pathological conditions. As such, interneuron-specific cells across different cortical structures are an essential network component for information processing and normal brain function.

Synaptic Mechanisms Underlying the Network State-Dependent Recruitment of VIP-Expressing Interneurons in the CA1 Hippocampus.

Disinhibition is a widespread circuit mechanism for information selection and transfer. In the hippocampus, disinhibition of principal cells is provided by the interneuron-specific interneurons that express the vasoactive intestinal polypeptide (VIP-IS) and innervate selectively inhibitory interneurons. By combining optophysiological experiments with computational models, we determined the impact of synaptic inputs onto the network state-dependent recruitment of VIP-IS cells. We found that VIP-IS cells fire spikes in response to both the Schaffer collateral and the temporoammonic pathway activation. Moreover, by integrating their intrinsic and synaptic properties into computational models, we predicted recruitment of these cells between the rising phase and peak of theta oscillation and during ripples. Two-photon Ca2+-imaging in awake mice supported in part the theoretical predictions, revealing a significant speed modulation of VIP-IS cells and their preferential albeit delayed recruitment during theta-run epochs, with estimated firing at the rising phase and peak of the theta cycle. However, it also uncovered that VIP-IS cells are not activated during ripples. Thus, given the preferential theta-modulated firing of VIP-IS cells in awake hippocampus, we postulate that these cells may be important for information gating during spatial navigation and memory encoding.

Transcriptomic profile of the subiculum-projecting VIP GABAergic neurons in the mouse CA1 hippocampus.

In cortical circuits, the vasoactive intestinal peptide (VIP+)-expressing GABAergic cells represent a heterogeneous but unique group of interneurons that is mainly specialized in network disinhibition. While the physiological properties and connectivity patterns have been elucidated in several types of VIP+ interneurons, little is known about the cell type-specific molecular repertoires important for selective targeting of VIP+ cell types and understanding their functions. Using patch-sequencing approach, we analyzed the transcriptomic profile of anatomically identified subiculum-projecting VIP+ GABAergic neurons that reside in the mouse hippocampal CA1 area, express muscarinic receptor 2 and coordinate the hippocampo-subicular interactions via selective innervation of interneurons in the CA1 area and of interneurons and pyramidal cells in subiculum. We explored the VIP+ cell gene expression within major gene families including ion channels, neurotransmitter receptors, neuromodulators, cell adhesion and myelination molecules. Among others, a large variety of genes involved in neuromodulatory signaling, including acetylcholine (Chrna4), norepinephrin (Adrb1), dopamine (Drd1), serotonin (Htr1d), cannabinoid (Cnr1), opioid (Oprd1, Oprl1) and neuropeptide Y (Npy1r) receptors was detected in these cells. Many genes that were enriched in other local VIP+ cell types, including the interneuron-selective interneurons and the cholecystokinin-coexpressing basket cells, were detected in VIP+ subiculum-projecting cells. In addition, the neuronatin (Nnat) and the Purkinje Cell Protein 4 (Pcp4) genes, which were detected previously in long-range projecting GABAergic neurons, were also common for the subiculum-projecting VIP+ cells. The expression of some genes was validated at the protein level, with proenkephalin being identified as an additional molecular marker of this VIP+ cell type. Together, our data indicate that the VIP+ subiculum-projecting cells share molecular identity with other VIP+ and long-range projecting GABAergic neurons, which can be important for specific function of these cells associated with their local and distant projection patterns.

Calcium Dynamics in Dendrites of Hippocampal CA1 Interneurons in Awake Mice.

Hippocampal inhibitory interneurons exhibit a large diversity of dendritic Ca2+ mechanisms that are involved in the induction of Hebbian and anti-Hebbian synaptic plasticity. High resolution imaging techniques allowed examining somatic Ca2+ signals and, accordingly, the recruitment of hippocampal interneurons in awake behaving animals. However, little is still known about dendritic Ca2+ activity in interneurons during different behavioral states. Here, we used two-photon Ca2+ imaging in mouse hippocampal CA1 interneurons to reveal Ca2+ signal patterns in interneuron dendrites during animal locomotion and immobility. Despite overall variability in dendritic Ca2+ transients (CaTs) across different cells and dendritic branches, we report consistent behavior state-dependent organization of Ca2+ signaling in interneurons. As such, spreading regenerative CaTs dominated in dendrites during locomotion, whereas both spreading and localized Ca2+ signals were seen during immobility. Thus, these data indicate that while animal locomotion is associated with widespread Ca2+ elevations in interneuron dendrites that may reflect regenerative activity, local CaTs that may be related to synaptic activity become apparent during animal quiet state.

Input-Specific Synaptic Location and Function of the α5 GABAA Receptor Subunit in the Mouse CA1 Hippocampal Neurons.

Hippocampus-dependent learning processes are coordinated via a large diversity of GABAergic inhibitory mechanisms. The α5 subunit-containing GABAA receptor (α5-GABAAR) is abundantly expressed in the hippocampus populating primarily the extrasynaptic domain of CA1 pyramidal cells, where it mediates tonic inhibitory conductance and may cause functional deficits in synaptic plasticity and hippocampus-dependent memory. However, little is known about synaptic expression of the α5-GABAAR and, accordingly, its location site-specific function. We examined the cell- and synapse-specific distribution of the α5-GABAAR in the CA1 stratum oriens/alveus (O/A) using a combination of immunohistochemistry, whole-cell patch-clamp recordings and optogenetic stimulation in hippocampal slices obtained from mice of either sex. In addition, the input-specific role of the α5-GABAAR in spatial learning and anxiety-related behavior was studied using behavioral testing and chemogenetic manipulations. We demonstrate that α5-GABAAR is preferentially targeted to the inhibitory synapses made by the vasoactive intestinal peptide (VIP)- and calretinin-positive terminals onto dendrites of somatostatin-expressing interneurons. In contrast, synapses made by the parvalbumin-positive inhibitory inputs to O/A interneurons showed no or little α5-GABAAR. Inhibiting the α5-GABAAR in control mice in vivo improved spatial learning but also induced anxiety-like behavior. Inhibiting the α5-GABAAR in mice with inactivated CA1 VIP input could still improve spatial learning and was not associated with anxiety. Together, these data indicate that the α5-GABAAR-mediated phasic inhibition via VIP input to interneurons plays a predominant role in the regulation of anxiety while the α5-GABAAR tonic inhibition via this subunit may control spatial learning.SIGNIFICANCE STATEMENT The α5-GABAAR subunit exhibits high expression in the hippocampus, and regulates the induction of synaptic plasticity and the hippocampus-dependent mnemonic processes. In CA1 principal cells, this subunit occupies mostly extrasynaptic sites and mediates tonic inhibition. Here, we provide evidence that, in CA1 somatostatin-expressing interneurons, the α5-GABAAR subunit is targeted to synapses formed by the VIP- and calretinin-expressing inputs, and plays a specific role in the regulation of anxiety-like behavior.

Non-linear calcium signalling and synaptic plasticity in interneurons.

Understanding of how intracellular calcium (Ca2+) signals regulate the efficacy of transmission at excitatory and inhibitory synapses in the central nervous system (CNS) has been a focus of intense investigation. This review discusses recent findings on how Ca2+ signals are integrated in dendrites of inhibitory interneurons to regulate their synapses. In particular, Ca2+ signaling through intracellular Ca2+ release plays an essential role in synaptic signal transduction and experience-dependent plasticity in dendrites of interneurons. Understanding the alternative pathways of Ca2+ signaling in the absence of canonical voltage-gated Ca2+ mechanisms is beginning to shed light on how their regulation can contribute to interneuron function and dysfunction in disease.

Calcium extrusion mechanisms in dendrites of mouse hippocampal CA1 inhibitory interneurons.

Local circuit GABAergic inhibitory interneurons control the integration and transfer of information in many brain regions. Several different forms of plasticity reported at interneuron excitatory synapses are triggered by cell- and synapse-specific postsynaptic calcium (Ca2+) mechanisms. To support this function, the spatiotemporal dynamics of dendritic Ca2+ elevations must be tightly regulated. While the dynamics of postsynaptic Ca2+ signaling through activation of different Ca2+ sources has been explored, the Ca2+ extrusion mechanisms that operate in interneuron dendrites during different patterns of activity remain largely unknown. Using a combination of whole-cell patch-clamp recordings and two-photon Ca2+ imaging in acute mouse hippocampal slices, we characterized the Ca2+ extrusion mechanisms activated by Ca2+ transients (CaTs) associated with backpropagating action potentials (bAPs) in dendrites of hippocampal CA1 stratum radiatum interneurons. Our data showed that Ca2+ clearance increased as a function of activity, pointing to an activity-dependent recruitment of specific Ca2+ extrusion mechanisms. bAP-CaTs were significantly prolonged in the presence of the plasma membrane Ca2+ ATPase (PMCA) and Na+/Ca2+ exchanger (NCX) inhibitors as well as the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) and the mitochondria Ca2+ uniporter (MCU) blockers. While PMCA, NCX and SERCA pumps cooperated in the cytosolic Ca2+ removal at a wide range of concentrations, the MCU was only activated at higher Ca2+ loads produced by repetitive interneuron firing. These results identify a division of labor between distinct Ca2+ extrusion mechanisms shaping dendritic Ca2+ dynamics and possibly contributing to activity-dependent regulation of synaptic inputs in interneurons. In addition, the MCU activated by larger Ca2+ levels may be involved in the activity-dependent ATP production or interneuron-selective vulnerability associated with cytosolic Ca2+ overloads under pathological conditions.

Connectivity and network state-dependent recruitment of long-range VIP-GABAergic neurons in the mouse hippocampus.

GABAergic interneurons in the hippocampus provide for local and long-distance coordination of neurons in functionally connected areas. Vasoactive intestinal peptide-expressing (VIP+) interneurons occupy a distinct niche in circuitry as many of them specialize in innervating GABAergic cells, thus providing network disinhibition. In the CA1 hippocampus, VIP+ interneuron-selective cells target local interneurons. Here, we discover a type of VIP+ neuron whose axon innervates CA1 and also projects to the subiculum (VIP-LRPs). VIP-LRPs show specific molecular properties and target interneurons within the CA1 area but both interneurons and pyramidal cells within subiculum. They are interconnected through gap junctions but demonstrate sparse spike coupling in vitro. In awake mice, VIP-LRPs decrease their activity during theta-run epochs and are more active during quiet wakefulness but not coupled to sharp-wave ripples. Together, the data provide evidence for VIP interneuron molecular diversity and functional specialization in controlling cell ensembles along the hippocampo-subicular axis.