Morphological diversity and connectivity of hippocampal interneurons

Oxidative Stress-mediated Loss of Hippocampal Parvalbumin Interneurons Contributes to Memory Precision Decline After Acute Sleep Deprivation

Sleep is pivotal to memory consolidation, and sleep deprivation (SD) after learning can impede this process, leading to memory disorders. In the present study, we aimed to explore the effects of acute sleep deprivation (ASD) on memory disorders and the underlying mechanisms. ASD model was induced by subjecting the mice to 6 h of SD following fear conditioning training. Different cohorts were used for behavioral, biochemical, and electrophysiological tests. Here, we showed that memory precision decline was induced by ASD, concomitant with a notable elevation in oxidative stress within PV interneurons, loss of PV, and disturbed neuronal oscillation in the CA1 region. Notably, chemogenetic activation of PV interneurons effectively ameliorated abnormal gamma oscillation and memory precision decline observed in ASD mice. Meanwhile, chemogenetic inhibition of PV interneurons successfully mimicked the abnormal brain oscillations and memory precision decline observed in ASD mice. Additionally, prior administration of the antioxidant medication N-acetylcysteine effectively reversed memory precision decline and mitigated PV loss and abnormal oscillation triggered by ASD. Collectively, our findings indicated that ASD increased oxidative stress in PV interneurons, thereby disrupting neural oscillation in the CA1 and ultimately leading to memory precision decline.

Transcriptome signatures of the medial prefrontal cortex underlying GABAergic control of resilience to chronic stress exposure

Analyses of postmortem human brains and preclinical studies of rodents have identified somatostatin (SST)-positive, dendrite-targeting GABAergic interneurons as key elements that regulate the vulnerability to stress-related psychiatric disorders. Conversely, genetically induced disinhibition of SST neurons (induced by Cre-mediated deletion of the γ2 GABAA receptor subunit gene selectively from SST neurons, SSTCre:γ2f/f mice) results in stress resilience. Similarly, chronic chemogenetic activation of SST neurons in the medial prefrontal cortex (mPFC) results in stress resilience but only in male and not in female mice. Here, we used RNA sequencing of the mPFC of SSTCre:γ2f/f mice to characterize the transcriptome changes underlying GABAergic control of stress resilience. We found that stress resilience of male but not female SSTCre:γ2f/f mice is characterized by resilience to chronic stress-induced transcriptome changes in the mPFC. Interestingly, the transcriptome of non-stressed SSTCre:γ2f/f (stress-resilient) male mice resembled that of chronic stress-exposed SSTCre (stress-vulnerable) mice. However, the behavior and the serum corticosterone levels of non-stressed SSTCre:γ2f/f mice showed no signs of physiological stress. Most strikingly, chronic stress exposure of SSTCre:γ2f/f mice was associated with an almost complete reversal of their chronic stress-like transcriptome signature, along with pathway changes suggesting stress-induced enhancement of mRNA translation. Behaviorally, the SSTCre:γ2f/f mice were not only resilient to chronic stress-induced anhedonia - they also showed an inversed, anxiolytic-like behavioral response to chronic stress exposure that mirrored the chronic stress-induced reversal of the chronic stress-like transcriptome signature. We conclude that GABAergic dendritic inhibition by SST neurons exerts bidirectional control over behavioral vulnerability and resilience to chronic stress exposure that is mirrored in bidirectional changes in the expression of putative stress resilience genes, through a sex-specific brain substrate.

Integrating endocannabinoid signaling, CCK interneurons, and hippocampal circuit dynamics in behaving animals

The brain's endocannabinoid signaling system modulates a diverse range of physiological phenomena and is also involved in various psychiatric and neurological disorders. The basic components of the molecular machinery underlying endocannabinoid-mediated synaptic signaling have been known for decades. However, limitations associated with the short-lived nature of endocannabinoid lipid signals had made it challenging to determine the spatiotemporal specificity and dynamics of endocannabinoid signaling in vivo. Here, we discuss how novel technologies have recently enabled unprecedented insights into endocannabinoid signaling taking place at specific synapses in behaving animals. In this review, we primarily focus on cannabinoid-sensitive inhibition in the hippocampus in relation to place cell properties to illustrate the potential of these novel methodologies. In addition, we highlight implications of these approaches and insights for the unraveling of cannabinoid regulation of synapses in vivo in other brain circuits in both health and disease.

GABAergic Progenitor Cell Graft Rescues Cognitive Deficits in Fragile X Syndrome Mice

Fragile X syndrome (FXS) is an inherited neurodevelopmental disorder characterized by a range of clinical manifestations with no effective treatment strategy to date. Here, transplantation of GABAergic precursor cells from the medial ganglionic eminence (MGE) is demonstrated to significantly improve cognitive performance in Fmr1 knockout (KO) mice. Within the hippocampus of Fmr1-KO mice, MGE-derived cells from wild-type donor mice survive, migrate, differentiate into functionally mature interneurons, and form inhibitory synaptic connections with host pyramidal neurons. MGE cell transplantation restores Ras-PKB signaling in pyramidal neurons, enhances AMPA receptor trafficking, rescues synaptic plasticity, and corrects abnormal hippocampal neural oscillations. These findings highlight the potential of GABAergic precursor cell transplantation as a promising therapeutic strategy for FXS.

Fast-spiking parvalbumin-positive interneurons: new perspectives of treatment and future challenges in dementia

Central nervous system parvalbumin-positive interneurons (PV-INs) are crucial and highly vulnerable to various stressors. They also play a significant role in the pathological processes of many neuropsychiatric diseases, especially those associated with cognitive impairment, such as Alzheimer's disease (AD), vascular dementia (VD), Lewy body dementia, and schizophrenia. Although accumulating evidence suggests that the loss of PV-INs is associated with memory impairment in dementia, the precise molecular mechanisms remain elusive. In this review, we delve into the current evidence regarding the physiological properties of PV-INs and summarize the latest insights into how their loss contributes to cognitive decline in dementia, particularly focusing on AD and VD. Additionally, we discuss the influence of PV-INs on brain development, the variations in their characteristics across different types of dementia, and how their loss affects the etiology and progression of cognitive impairments. Ultimately, our goal is to provide a comprehensive overview of PV-INs and to consider their potential as novel therapeutic targets in dementia treatment.

Supralinear dendritic integration in murine dendrite-targeting interneurons

Non-linear summation of synaptic inputs to the dendrites of pyramidal neurons has been proposed to increase the computation capacity of neurons through coincidence detection, signal amplification, and additional logic operations such as XOR. Supralinear dendritic integration has been documented extensively in principal neurons, mediated by several voltage-dependent conductances. It has also been reported in parvalbumin-positive hippocampal basket cells, in dendrites innervated by feedback excitatory synapses. Whether other interneurons, which support feed-forward or feedback inhibition of principal neuron dendrites, also exhibit local non-linear integration of synaptic excitation is not known. Here, we use patch-clamp electrophysiology, and two-photon calcium imaging and glutamate uncaging, to show that supralinear dendritic integration of near-synchronous spatially clustered glutamate-receptor mediated depolarization occurs in NDNF-positive neurogliaform cells and oriens-lacunosum moleculare interneurons in the mouse hippocampus. Supralinear summation was detected via recordings of somatic depolarizations elicited by uncaging of glutamate on dendritic fragments, and, in neurogliaform cells, by concurrent imaging of dendritic calcium transients. Supralinearity was abolished by blocking NMDA receptors (NMDARs) but resisted blockade of voltage-gated sodium channels. Blocking L-type calcium channels abolished supralinear calcium signalling but only had a minor effect on voltage supralinearity. Dendritic boosting of spatially clustered synaptic signals argues for previously unappreciated computational complexity in dendrite-projecting inhibitory cells of the hippocampus.

NMDA receptors regulate the firing rate set point of hippocampal circuits without altering single-cell dynamics

Understanding how neuronal circuits stabilize their activity is a fundamental yet poorly understood aspect of neuroscience. Here, we show that hippocampal network properties, such as firing rate distribution and dimensionality, are actively regulated, despite perturbations and single-cell drift. Continuous inhibition of N-methyl-D-aspartate receptors (NMDARs) ex vivo lowers the excitation/inhibition ratio and network firing rates while preserving resilience to perturbations. This establishes a new network firing rate set point via NMDAR-eEF2K signaling pathway. NMDARs' capacity to modulate and stabilize network firing is mediated by excitatory synapses and the intrinsic excitability of parvalbumin-positive neurons, respectively. In behaving mice, continuous NMDAR blockade in CA1 reduces network firing without altering single-neuron drift or triggering a compensatory response. These findings expand NMDAR function beyond their canonical role in synaptic plasticity and raise the possibility that some NMDAR-dependent behavioral effects are mediated by their unique regulation of population activity set points.

Neuronal diversity and stereotypy at multiple scales through whole brain morphometry

We conducted a large-scale whole-brain morphometry study by analyzing 3.7 peta-voxels of mouse brain images at the single-cell resolution, producing one of the largest multi-morphometry databases of mammalian brains to date. We registered 204 mouse brains of three major imaging modalities to the Allen Common Coordinate Framework (CCF) atlas, annotated 182,497 neuronal cell bodies, modeled 15,441 dendritic microenvironments, characterized the full morphology of 1876 neurons along with their axonal motifs, and detected 2.63 million axonal varicosities that indicate potential synaptic sites. Our analyzed six levels of information related to neuronal populations, dendritic microenvironments, single-cell full morphology, dendritic and axonal arborization, axonal varicosities, and sub-neuronal structural motifs, along with a quantification of the diversity and stereotypy of patterns at each level. This integrative study provides key anatomical descriptions of neurons and their types across a multiple scales and features, contributing a substantial resource for understanding neuronal diversity in mammalian brains.

Dysregulation of neuropilin-2 expression in inhibitory neurons impairs hippocampal circuit development and enhances risk for autism-related behaviors and seizures

Dysregulation of development, migration, and function of interneurons, collectively termed interneuronopathies, have been proposed as a shared mechanism for autism spectrum disorders (ASDs) and childhood epilepsy. Neuropilin-2 (Nrp2), a candidate ASD gene, is a critical regulator of interneuron migration from the median ganglionic eminence (MGE) to the pallium, including the hippocampus. While clinical studies have identified Nrp2 polymorphisms in patients with ASD, whether selective dysregulation of Nrp2-dependent interneuron migration contributes to pathogenesis of ASD and enhances the risk for seizures has not been evaluated. We tested the hypothesis that the lack of Nrp2 in MGE-derived interneuron precursors disrupts the excitation/inhibition balance in hippocampal circuits, thus predisposing the network to seizures and behavioral patterns associated with ASD. Embryonic deletion of Nrp2 during the developmental period for migration of MGE derived interneuron precursors (iCKO) significantly reduced parvalbumin, neuropeptide Y, and somatostatin positive neurons in the hippocampal CA1. Consequently, when compared to controls, the frequency of inhibitory synaptic currents in CA1 pyramidal cells was reduced while frequency of excitatory synaptic currents was increased in iCKO mice. Although passive and active membrane properties of CA1 pyramidal cells were unchanged, iCKO mice showed enhanced susceptibility to chemically evoked seizures. Moreover, iCKO mice exhibited selective behavioral deficits in both preference for social novelty and goal-directed learning, which are consistent with ASD-like phenotype. Together, our findings show that disruption of developmental Nrp2 regulation of interneuron circuit establishment, produces ASD-like behaviors and enhanced risk for epilepsy. These results support the developmental interneuronopathy hypothesis of ASD epilepsy comorbidity.

Sex-specific GABAergic microcircuits that switch vulnerability into resilience to stress and reverse the effects of chronic stress exposure

Clinical and preclinical studies have identified somatostatin (SST)-positive interneurons as critical elements that regulate the vulnerability to stress-related psychiatric disorders. Conversely, disinhibition of SST neurons in mice results in resilience to the behavioral effects of chronic stress. Here, we established a low-dose chronic chemogenetic protocol to map these changes in positively and negatively motivated behaviors to specific brain regions. AAV-hM3Dq-mediated chronic activation of SST neurons in the prelimbic cortex (PLC) had antidepressant drug-like effects on anxiety- and anhedonia-like motivated behaviors in male but not female mice. Analogous manipulation of the ventral hippocampus (vHPC) had such effects in female but not male mice. Moreover, the activation of SST neurons in the PLC of male mice and the vHPC of female mice resulted in stress resilience. Activation of SST neurons in the PLC reversed prior chronic stress-induced defects in motivated behavior in males but was ineffective in females. Conversely, activation of SST neurons in the vHPC reversed chronic stress-induced behavioral alterations in females but not males. Quantitation of c-Fos+ and FosB+ neurons in chronic stress-exposed mice revealed that chronic activation of SST neurons leads to a paradoxical increase in pyramidal cell activity. Collectively, these data demonstrate that GABAergic microcircuits driven by dendrite targeting interneurons enable sex- and brain-region-specific neural plasticity that promotes stress resilience and reverses stress-induced anxiety- and anhedonia-like motivated behavior. The data provide a rationale for the lack of antidepressant efficacy of benzodiazepines and superior efficacy of dendrite-targeting, low-potency GABAA receptor agonists, independent of sex and despite striking sex differences in the relevant brain substrates.

Neuron specific quantitation of Gαolf expression and signaling in murine brain tissue

The heterotrimeric G-protein α subunit, Gαolf, acts to transduce extracellular signals through G-protein coupled receptors (GPCRs) and stimulates adenylyl cyclase mediated production of the second messenger cyclic adenosine monophosphate. Numerous mutations in the GNAL gene, which encodes Gαolf, have been identified as causative for an adult-onset dystonia. These mutations disrupt GPCR signaling cascades in in vitro assays through several mechanisms, and this disrupted signaling is hypothesized to lead to dystonic motor symptoms in patients. However, the cells and circuits that mutations in GNAL corrupt are not well understood. Published patterns of Gαolf expression outside the context of the striatum are sparse, conflicting, often lack cell type specificity, and may be confounded by expression of the close GNAL homolog of GNAS. Here, we use RNAScope in-situ hybridization to quantitatively characterize Gnal mRNA expression in brain tissue from wildtype C57BL/6J adult mice. We observed widespread expression of Gnal puncta throughout the brain, suggesting Gαolf is expressed in more brain structures and neuron types than previously accounted for. We quantify transcripts at a single cell level, and use neuron type specific markers to further classify and understand patterns of GNAL expression. Our data suggests that brain regions classically associated with motor control, initiation, and regulation show the highest expression of GNAL, with Purkinje Cells of the cerebellum showing the highest expression of any neuron type examined. Subsequent conditional Gnal knockout in Purkinje cells led to markedly decreased intracellular cAMP levels and downstream cAMP-dependent enzyme activation. Our work provides a detailed characterization of Gnal expression throughout the brain and the biochemical consequences of loss of Gαolf signaling in vivo in neurons that highly express Gnal.

HCN1 hyperpolarization-activated cyclic nucleotide-gated channels enhance evoked GABA release from parvalbumin-positive interneurons

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels generate the cationic Ih current in neurons and regulate the excitability of neuronal networks. The function of HCN channels depends, in part, on their subcellular localization. Of the four HCN isoforms (HCN1-4), HCN1 is strongly expressed in the dendrites of pyramidal neurons (PNs) in hippocampal area CA1 but also in presynaptic terminals of parvalbumin-positive interneurons (PV+ INs), which provide strong inhibitory control over hippocampal activity. Yet, little is known about how HCN1 channels in these cells regulate the evoked release of the inhibitory transmitter GABA from their axon terminals. Here, we used genetic, optogenetic, electrophysiological, and imaging techniques to investigate how the electrophysiological properties of PV+ INs are regulated by HCN1, including how HCN1 activity at presynaptic terminals regulates the release of GABA onto PNs in CA1. We found that application of HCN1 pharmacological blockers reduced the amplitude of the inhibitory postsynaptic potential recorded from CA1 PNs in response to selective optogenetic stimulation of PV+ INs. Homozygous HCN1 knockout mice also show reduced IPSCs in postsynaptic cells. Finally, two-photon imaging using genetically encoded fluorescent calcium indicators revealed that HCN1 blockers reduced the probability that an extracellular electrical stimulating pulse evoked a Ca2+ response in individual PV+ IN presynaptic boutons. Taken together, our results show that HCN1 channels in the axon terminals of PV+ interneurons facilitate GABAergic transmission in the hippocampal CA1 region.

Erythropoietin restrains the inhibitory potential of interneurons in the mouse hippocampus

Severe psychiatric illnesses, for instance schizophrenia, and affective diseases or autism spectrum disorders, have been associated with cognitive impairment and perturbed excitatory-inhibitory balance in the brain. Effects in juvenile mice can elucidate how erythropoietin (EPO) might aid in rectifying hippocampal transcriptional networks and synaptic structures of pyramidal lineages, conceivably explaining mitigation of neuropsychiatric diseases. An imminent conundrum is how EPO restores synapses by involving interneurons. By analyzing ~12,000 single-nuclei transcriptomic data, we generated a comprehensive molecular atlas of hippocampal interneurons, resolved into 15 interneuron subtypes. Next, we studied molecular alterations upon recombinant human (rh)EPO and saw that gene expression changes relate to synaptic structure, trans-synaptic signaling and intracellular catabolic pathways. Putative ligand-receptor interactions between pyramidal and inhibitory neurons, regulating synaptogenesis, are altered upon rhEPO. An array of in/ex vivo experiments confirms that specific interneuronal populations exhibit reduced dendritic complexity, synaptic connectivity, and changes in plasticity-related molecules. Metabolism and inhibitory potential of interneuron subgroups are compromised, leading to greater excitability of pyramidal neurons. To conclude, improvement by rhEPO of neuropsychiatric phenotypes may partly owe to restrictive control over interneurons, facilitating re-connectivity and synapse development.

Postsynaptic GABAB-receptor mediated currents in diverse dentate gyrus interneuron types

The processing of rich synaptic information in the dentate gyrus (DG) relies on a diverse population of inhibitory GABAergic interneurons to regulate cellular and circuit activity, in a layer-specific manner. Metabotropic GABAB-receptors (GABABRs) provide powerful inhibition to the DG circuit, on timescales consistent with behavior and learning, but their role in controlling the activity of interneurons is poorly understood with respect to identified cell types. We hypothesize that GABABRs display cell type-specific heterogeneity in signaling strength, which will have direct ramifications for signal processing in DG networks. To test this, we perform in vitro whole-cell patch-clamp recordings from identified DG principal cells and interneurons, followed by GABABR pharmacology, photolysis of caged GABA, and extracellular stimulation of endogenous GABA release to classify the cell type-specific inhibitory potential. Based on our previous classification of DG interneurons, we show that postsynaptic GABABR-mediated currents are present on all interneuron types albeit at different amplitudes, dependent largely on soma location and synaptic targets. GABABRs were coupled to inwardly-rectifying K+ channels that strongly reduced the excitability of those interneurons where large currents were observed. These data provide a systematic characterization of GABABR signaling in the rat DG to provide greater insight into circuit dynamics.

Presubicular VIP expressing interneurons receive facilitating excitation from anterior thalamus

The presubiculum is part of the parahippocampal cortex and plays a fundamental role for orientation in space. Many principal neurons of the presubiculum signal head direction, and show persistent firing when the head of an animal is oriented in a specific preferred direction. GABAergic neurons of the presubiculum control the timing, sensitivity and selectivity of head directional signals from the anterior thalamic nuclei. However, the role of vasoactive intestinal peptide (VIP) expressing interneurons in the presubicular microcircuit has not yet been addressed. Here, we examined the intrinsic properties of VIP interneurons as well as their input connectivity following photostimulation of anterior thalamic axons. We show that presubicular VIP interneurons are more densely distributed in superficial than in deep layers. They are highly excitable. Three groups emerged from the unsupervised cluster analysis of their electrophysiological properties. We demonstrate a frequency dependent recruitment of VIP cells by thalamic afferences and facilitating synaptic input dynamics. Our data provide initial insight into the contribution of VIP interneurons for the integration of thalamic head direction information in the presubiculum.

Distinct ventral hippocampal inhibitory microcircuits regulating anxiety and fear behaviors

In emotion research, anxiety and fear have always been interconnected, sharing overlapping brain structures and neural circuitry. Recent investigations, however, have unveiled parallel long-range projection pathways originating from the ventral hippocampus, shedding light on their distinct roles in anxiety and fear. Yet, the mechanisms governing the emergence of projection-specific activity patterns to mediate different negative emotions remain elusive. Here, we show a division of labor in local GABAergic inhibitory microcircuits of the ventral hippocampus, orchestrating the activity of subpopulations of pyramidal neurons to shape anxiety and fear behaviors in mice. These findings offer a comprehensive insight into how distinct inhibitory microcircuits are dynamically engaged to encode different emotional states.

Dysregulation of Neuropilin-2 Expression in Inhibitory Neurons Impairs Hippocampal Circuit Development and Enhances Risk for Autism-Related Behaviors and Seizures

Dysregulation of development, migration, and function of interneurons, collectively termed interneuronopathies, have been proposed as a shared mechanism for autism spectrum disorders (ASDs) and childhood epilepsy. Neuropilin-2 (Nrp2), a candidate ASD gene, is a critical regulator of interneuron migration from the median ganglionic eminence (MGE) to the pallium, including the hippocampus. While clinical studies have identified Nrp2 polymorphisms in patients with ASD, whether selective dysregulation of Nrp2-dependent interneuron migration contributes to pathogenesis of ASD and enhances the risk for seizures has not been evaluated. We tested the hypothesis that the lack of Nrp2 in MGE-derived interneuron precursors disrupts the excitation/inhibition balance in hippocampal circuits, thus predisposing the network to seizures and behavioral patterns associated with ASD. Embryonic deletion of Nrp2 during the developmental period for migration of MGE derived interneuron precursors (iCKO) significantly reduced parvalbumin, neuropeptide Y, and somatostatin positive neurons in the hippocampal CA1. Consequently, when compared to controls, the frequency of inhibitory synaptic currents in CA1 pyramidal cells was reduced while frequency of excitatory synaptic currents was increased in iCKO mice. Although passive and active membrane properties of CA1 pyramidal cells were unchanged, iCKO mice showed enhanced susceptibility to chemically evoked seizures. Moreover, iCKO mice exhibited selective behavioral deficits in both preference for social novelty and goal-directed learning, which are consistent with ASD-like phenotype. Together, our findings show that disruption of developmental Nrp2 regulation of interneuron circuit establishment, produces ASD-like behaviors and enhanced risk for epilepsy. These results support the developmental interneuronopathy hypothesis of ASD epilepsy comorbidity.

Interneurons in the CA1 stratum oriens expressing αTTP may play a role in the delayed-ageing Pol μ mouse model

Neurodegeneration associated with ageing is closely linked to oxidative stress (OS) and disrupted calcium homeostasis. Some areas of the brain, like the hippocampus - particularly the CA1 region - have shown a high susceptibility to age-related changes, displaying early signs of pathology and neuronal loss. Antioxidants such as α-tocopherol (αT) have been effective in mitigating the impact of OS during ageing. αT homeostasis is primarily regulated by the α-tocopherol transfer protein (αTTP), which is widely distributed throughout the brain - where it plays a crucial role in maintaining αT levels within neuronal cells. This study investigates the distribution of αTTP in the hippocampus of 4- and 24-month-old Pol μ knockout mice (Pol μ-/-), a delayed-ageing model, and the wild type (Pol μ+/+). We also examine the colocalisation in the stratum oriens (st.or) of CA1 region with the primary interneuron populations expressing calcium-binding proteins (CBPs) (calbindin (CB), parvalbumin (PV), and calretinin (CR)). Our findings reveal that αTTP immunoreactivity (-IR) in the st.or of Pol μ mice is significantly reduced. The density of PV-expressing interneurons (INs) increased in aged mice in both Pol μ genotypes (Pol μ-/- and Pol μ+/+), although the density of PV-positive INs was lower in the aged Pol μ-/- mice compared to wild-type mice. By contrast, CR- and CB-positive INs in Pol μ mice remained unchanged during ageing. Furthermore, double immunohistochemistry reveals the colocalisation of αTTP with CBPs in INs of the CA1 st.or. Our study also shows that the PV/αTTP-positive IN population remains unchanged in all groups. A significant decrease of CB/αTTP-positive INs in young Pol μ-/- mice has been detected, as well as a significant increase in CR/αTTP-IR in older Pol μ-/- animals. These results suggest that the differential expression of αTTP and CBPs could have a crucial effect in aiding the survival and maintenance of the different IN populations in the CA1 st.or, and their coexpression could contribute to the enhancement of their resistance to OS-related damage and neurodegeneration associated with ageing.

GABA system in the prefrontal cortex involved in psychostimulant addiction

Drug addiction is a chronic and relapse brain disorder. Psychostimulants such as cocaine and amphetamine are highly addictive drugs. Abuse drugs target various brain areas in the nervous system. Recent studies have shown that the prefrontal cortex (PFC) plays a key role in regulating addictive behaviors. The PFC is made up of excitatory glutamatergic cells and gamma-aminobutyric acid (GABAergic) interneurons. Recently, studies showed that GABA level was related with psychostimulant addiction. In this review, we will introduce the role and mechanism of GABA and γ-aminobutyric acid receptors (GABARs) of the PFC in regulating drug addiction, especially in psychostimulant addiction.

Somatostatin Interneurons Recruit Pre- and Postsynaptic GABAB Receptors in the Adult Mouse Dentate Gyrus

The integration of spatial information in the mammalian dentate gyrus (DG) is critical to navigation. Indeed, DG granule cells (DGCs) rely upon finely balanced inhibitory neurotransmission in order to respond appropriately to specific spatial inputs. This inhibition arises from a heterogeneous population of local GABAergic interneurons (INs) that activate both fast, ionotropic GABAA receptors (GABAAR) and slow, metabotropic GABAB receptors (GABABR), respectively. GABABRs in turn inhibit pre- and postsynaptic neuronal compartments via temporally long-lasting G-protein-dependent mechanisms. The relative contribution of each IN subtype to network level GABABR signal setting remains unknown. However, within the DG, the somatostatin (SSt) expressing IN subtype is considered crucial in coordinating appropriate feedback inhibition on to DGCs. Therefore, we virally delivered channelrhodopsin 2 to the DG in order to obtain control of this specific SSt IN subpopulation in male and female adult mice. Using a combination of optogenetic activation and pharmacology, we show that SSt INs strongly recruit postsynaptic GABABRs to drive greater inhibition in DGCs than GABAARs at physiological membrane potentials. Furthermore, we show that in the adult mouse DG, postsynaptic GABABR signaling is predominantly regulated by neuronal GABA uptake and less so by astrocytic mechanisms. Finally, we confirm that activation of SSt INs can also recruit presynaptic GABABRs, as has been shown in neocortical circuits. Together, these data reveal that GABABR signaling allows SSt INs to control DG activity and may constitute a key mechanism for gating spatial information flow within hippocampal circuits.

Tracing nerve fibers with volume electron microscopy to quantitatively analyze brain connectivity

The highly complex structure of the brain requires an approach that can unravel its connectivity. Using volume electron microscopy and a dedicated software we can trace and measure all nerve fibers present within different samples of brain tissue. With this software tool, individual dendrites and axons are traced, obtaining a simplified "skeleton" of each fiber, which is linked to its corresponding synaptic contacts. The result is an intricate meshwork of axons and dendrites interconnected by a cloud of synaptic junctions. To test this methodology, we apply it to the stratum radiatum of the hippocampus and layers 1 and 3 of the somatosensory cortex of the mouse. We find that nerve fibers are densely packed in the neuropil, reaching up to 9 kilometers per cubic mm. We obtain the number of synapses, the number and lengths of dendrites and axons, the linear densities of synapses established by dendrites and axons, and their location on dendritic spines and shafts. The quantitative data obtained through this method enable us to identify subtle traits and differences in the synaptic organization of the samples, which might have been overlooked in a qualitative analysis.

Modification of temporal pattern sensitivity for inputs from medial entorhinal cortex by lateral inputs in hippocampal granule cells

The medial dendrites (MDs) of granule cells (GCs) receive spatial information through the medial entorhinal cortex (MEC) from the entorhinal cortex in the rat hippocampus while the distal dendrites (DDs) of GCs receive non-spatial information (sensory inputs) through the lateral entorhinal cortex (LEC). However, it is unclear how information processing through the two pathways is managed in GCs. In this study, we investigated associative information processing between two independent inputs to MDs and DDs. First, in physiological experiments, to compare response characteristics between MDs and DDs, electrical stimuli comprising five pulses were applied to the MPP or LPP in rat hippocampal slices. These stimuli transiently decreased the excitatory postsynaptic potentials (EPSPs) of successive input pulses to MDs, whereas EPSPs to DDs showed sustained responses. Next, in computational experiments using a local network model obtained by fitting of the physiological experimental data, we compared associative information processing between DDs and MDs. The results showed that the temporal pattern sensitivity for burst inputs to MDs depended on the frequency of the random pulse inputs applied to DDs. On the other hand, with lateral inhibition to GCs from interneurons, the temporal pattern sensitivity for burst inputs to MDs was enhanced or tuned up according to the frequency of the random pulse inputs to the other cells. Thus, our results suggest that the temporal pattern sensitivity of spatial information depends on the non-spatial inputs to GCs.

Extracellular ATP inhibits excitatory synaptic input on parvalbumin positive interneurons and attenuates gamma oscillations via P2X4 receptors

BACKGROUND AND PURPOSE

P2X4 receptors (P2X4R) are ligand gated cation channels that are activated by extracellular ATP released by neurons and glia. The receptors are widely expressed in the brain and have fractional calcium currents comparable with NMDA receptors. Although P2X4Rs have been reported to modulate synaptic transmission and plasticity, their involvement in shaping neuronal network activity remains to be elucidated.

EXPERIMENTAL APPROACH

We investigated the effects of P2X receptors at network and synaptic level using local field potential electrophysiology, whole cell patch clamp recordings and calcium imaging in fast spiking parvalbumin positive interneurons (PVINs) in rat and mouse hippocampal slices. The stable ATP analogue ATPγS, selective antagonists and P2X4R knockout mice were used.

KEY RESULTS

The P2XR agonist ATPγS reversibly decreased the power of gamma oscillations. This inhibition could be antagonized by the selective P2X4R antagonist PSB-12062 and was not observed in P2X4-/- mice. The phasic excitatory inputs of CA3 PVINs were one of the main regulators of the gamma power. Associational fibre compound excitatory postsynaptic currents (cEPSCs) in CA3 PVINs were inhibited by P2X4R activation. This effect was reversible, dependent on intracellular calcium and dynamin-dependent internalization of AMPA receptors.

CONCLUSIONS AND IMPLICATIONS

The results indicate that P2X4Rs are an important source of dendritic calcium in CA3 PVINs, thereby regulating excitatory synaptic inputs onto the cells and presumably the state of gamma oscillations in the hippocampus. P2X4Rs represent an effective target to modulate hippocampal network activity in pathophysiological conditions such as Alzheimer's disease and schizophrenia.

LRP1 in GABAergic neurons is a key link between obesity and memory function

OBJECTIVE

Low-density lipoprotein receptor-related protein-1 (LRP1) regulates energy homeostasis, blood-brain barrier integrity, and metabolic signaling in the brain. Deficiency of LRP1 in inhibitory gamma-aminobutyric acid (GABA)ergic neurons causes severe obesity in mice. However, the impact of LRP1 in inhibitory neurons on memory function and cognition in the context of obesity is poorly understood.

METHODS

Mice lacking LRP1 in GABAergic neurons (Vgat-Cre; LRP1loxP/loxP) underwent behavioral tests for locomotor activity and motor coordination, short/long-term and spatial memory, and fear learning/memory. This study evaluated the relationships between behavior and metabolic risk factors and followed the mice at 16 and 32 weeks of age.

RESULTS

Deletion of LRP1 in GABAergic neurons caused a significant impairment in memory function in 32-week-old mice. In the spatial Y-maze test, Vgat-Cre; LRP1loxP/loxP mice exhibited decreased travel distance and duration in the novel arm compared with controls (LRP1loxP/loxP mice). In addition, GABAergic neuron-specific LRP1-deficient mice showed a diminished capacity for performing learning and memory tasks during the water T-maze test. Moreover, reduced freezing time was observed in these mice during the contextual and cued fear conditioning tests. These effects were accompanied by increased neuronal necrosis and satellitosis in the hippocampus. Importantly, the distance and duration in the novel arm, as well as the performance of the reversal water T-maze test, negatively correlated with metabolic risk parameters, including body weight, serum leptin, insulin, and apolipoprotein J. However, in 16-week-old Vgat-Cre; LRP1loxP/loxP mice, there were no differences in the behavioral tests or correlations between metabolic parameters and cognition.

CONCLUSIONS

Our findings demonstrate that LRP1 from GABAergic neurons is important in regulating normal learning and memory. Metabolically, obesity caused by GABAergic LRP1 deletion negatively regulates memory and cognitive function in an age-dependent manner. Thus, LRP1 in GABAergic neurons may play a crucial role in maintaining normal excitatory/inhibitory balance, impacting memory function, and reinforcing the potential importance of LRP1 in neural system integrity.

Functional specialization of hippocampal somatostatin-expressing interneurons

Hippocampal somatostatin-expressing (Sst) GABAergic interneurons (INs) exhibit considerable anatomical and functional heterogeneity. Recent single-cell transcriptome analyses have provided a comprehensive Sst-IN subpopulations census, a plausible molecular ground truth of neuronal identity whose links to specific functionality remain incomplete. Here, we designed an approach to identify and access subpopulations of Sst-INs based on transcriptomic features. Four mouse models based on single or combinatorial Cre- and Flp- expression differentiated functionally distinct subpopulations of CA1 hippocampal Sst-INs that largely tiled the morpho-functional parameter space of the Sst-INs superfamily. Notably, the Sst;;Tac1 intersection revealed a population of bistratified INs that preferentially synapsed onto fast-spiking interneurons (FS-INs) and were sufficient to interrupt their firing. In contrast, the Ndnf;;Nkx2-1 intersection identified a population of oriens lacunosum-moleculare INs that predominantly targeted CA1 pyramidal neurons, avoiding FS-INs. Overall, our results provide a framework to translate neuronal transcriptomic identity into discrete functional subtypes that capture the diverse specializations of hippocampal Sst-INs.

The Role of Tenascin-C on the Structural Plasticity of Perineuronal Nets and Synaptic Expression in the Hippocampus of Male Mice

Neuronal plasticity is a crucial mechanism for an adapting nervous system to change. It is shown to be regulated by perineuronal nets (PNNs), the condensed forms of the extracellular matrix (ECM) around neuronal bodies. By assessing the changes in the number, intensity, and structure of PNNs, the ultrastructure of the PNN mesh, and the expression of inhibitory and excitatory synaptic inputs on these neurons, we aimed to clarify the role of an ECM glycoprotein, tenascin-C (TnC), in the dorsal hippocampus. To enhance neuronal plasticity, TnC-deficient (TnC-/-) and wild-type (TnC+/+) young adult male mice were reared in an enriched environment (EE) for 8 weeks. Deletion of TnC in TnC-/- mice showed an ultrastructural reduction of the PNN mesh and an increased inhibitory input in the dentate gyrus (DG), and an increase in the number of PNNs with a rise in the inhibitory input in the CA2 region. EE induced an increased inhibitory input in the CA2, CA3, and DG regions; in DG, the change was also followed by an increased intensity of PNNs. No changes in PNNs or synaptic expression were found in the CA1 region. We conclude that the DG and CA2 regions emerged as focal points of alterations in PNNs and synaptogenesis with EE as mediated by TnC.

G α olf Regulates Biochemical Signaling in Neurons Associated with Movement Control and Initiation

The heterotrimeric G-protein α subunit, Gα olf , acts to transduce extracellular signals through G-protein coupled receptors (GPCRs) and stimulates adenylyl cyclase mediated production of the second messenger cyclic adenosine monophosphate. Numerous mutations in the GNAL gene, which encodes Gα olf , have been identified as causative for an adult-onset dystonia. These mutations disrupt GPCR signaling cascades in in vitro assays through several mechanisms, and this disrupted signaling is hypothesized to lead to dystonic motor symptoms in patients. However, the cells and circuits that mutations in GNAL corrupt are not well understood. Published patterns of Gα olf expression outside the context of the striatum are sparse, conflicting, often lack cell type specificity, and may be confounded by expression of the close GNAL homolog of GNAS . Here, we use RNAScope in-situ hybridization to quantitatively characterize Gnal mRNA expression in brain tissue from wildtype C57BL/6J adult mice. We observed widespread expression of Gnal puncta throughout the brain, suggesting Gα olf is expressed in more brain structures and neuron types than previously accounted for. We quantify transcripts at a single cell level, and use neuron type specific markers to further classify and understand patterns of GNAL expression. Our data suggests that brain regions classically associated with motor control, initiation, and regulation show the highest expression of GNAL , with Purkinje Cells of the cerebellum showing the highest expression of any neuron type examined. Subsequent conditional Gnal knockout in Purkinje cells led to markedly decreased intracellular cAMP levels and downstream cAMP-dependent enzyme activation. Our work provides a detailed characterization of Gnal expression throughout the brain and the biochemical consequences of loss of Gα olf signaling in vivo in neurons that highly express Gnal .

Mystery of the memory engram: History, current knowledge, and unanswered questions

The quest to understand the memory engram has intrigued humans for centuries. Recent technological advances, including genetic labelling, imaging, optogenetic and chemogenetic techniques, have propelled the field of memory research forward. These tools have enabled researchers to create and erase memory components. While these innovative techniques have yielded invaluable insights, they often focus on specific elements of the memory trace. Genetic labelling may rely on a particular immediate early gene as a marker of activity, optogenetics may activate or inhibit one specific type of neuron, and imaging may capture activity snapshots in a given brain region at specific times. Yet, memories are multifaceted, involving diverse arrays of neuronal subpopulations, circuits, and regions that work in concert to create, store, and retrieve information. Consideration of contributions of both excitatory and inhibitory neurons, micro and macro circuits across brain regions, the dynamic nature of active ensembles, and representational drift is crucial for a comprehensive understanding of the complex nature of memory.

Synaptic and dendritic architecture of different types of hippocampal somatostatin interneurons

GABAergic inhibitory neurons fundamentally shape the activity and plasticity of cortical circuits. A major subset of these neurons contains somatostatin (SOM); these cells play crucial roles in neuroplasticity, learning, and memory in many brain areas including the hippocampus, and are implicated in several neuropsychiatric diseases and neurodegenerative disorders. Two main types of SOM-containing cells in area CA1 of the hippocampus are oriens-lacunosum-moleculare (OLM) cells and hippocampo-septal (HS) cells. These cell types show many similarities in their soma-dendritic architecture, but they have different axonal targets, display different activity patterns in vivo, and are thought to have distinct network functions. However, a complete understanding of the functional roles of these interneurons requires a precise description of their intrinsic computational properties and their synaptic interactions. In the current study we generated, analyzed, and make available several key data sets that enable a quantitative comparison of various anatomical and physiological properties of OLM and HS cells in mouse. The data set includes detailed scanning electron microscopy (SEM)-based 3D reconstructions of OLM and HS cells along with their excitatory and inhibitory synaptic inputs. Combining this core data set with other anatomical data, patch-clamp electrophysiology, and compartmental modeling, we examined the precise morphological structure, inputs, outputs, and basic physiological properties of these cells. Our results highlight key differences between OLM and HS cells, particularly regarding the density and distribution of their synaptic inputs and mitochondria. For example, we estimated that an OLM cell receives about 8,400, whereas an HS cell about 15,600 synaptic inputs, about 16% of which are GABAergic. Our data and models provide insight into the possible basis of the different functionality of OLM and HS cell types and supply essential information for more detailed functional models of these neurons and the hippocampal network.

Plexin-B1 and Plexin-B2 play non-redundant roles in GABAergic synapse formation

Synapse formation in the mammalian brain is a complex and dynamic process requiring coordinated function of dozens of molecular families such as cell adhesion molecules (CAMs) and ligand-receptor pairs (Ephs/Ephrins, Neuroligins/Neurexins, Semaphorins/Plexins). Due to the large number of molecular players and possible functional redundancies within gene families, it is challenging to determine the precise synaptogenic roles of individual molecules, which is key to understanding the consequences of mutations in these genes for brain function. Furthermore, few molecules are known to exclusively regulate either GABAergic or glutamatergic synapses, and cell and molecular mechanisms underlying GABAergic synapse formation in particular are not thoroughly understood. We previously demonstrated that Semaphorin-4D (Sema4D) regulates GABAergic synapse development in the mammalian hippocampus while having no effect on glutamatergic synapse development, and this effect occurs through binding to its high affinity receptor, Plexin-B1. In addition, we demonstrated that RNAi-mediated Plexin-B2 knock-down decreases GABAergic synapse density suggesting that both receptors function in this process. Here, we perform a structure-function study of the Plexin-B1 and Plexin-B2 receptors to identify the protein domains in each receptor which are required for its synaptogenic function. Further, we examine whether Plexin-B2 is required in the presynaptic neuron, the postsynaptic neuron, or both to regulate GABAergic synapse formation. Our data reveal that Plexin-B1 and Plexin-B2 function non-redundantly to regulate GABAergic synapse formation and suggest that the transmembrane domain may underlie functional distinctions. We also provide evidence that Plexin-B2 expression in presynaptic GABAergic interneurons, as well as postsynaptic pyramidal cells, regulates GABAergic synapse formation in hippocampus. These findings lay the groundwork for future investigations into the precise signaling pathways required for synapse formation downstream of Plexin-B receptor signaling.

Dysregulation of Neuropilin-2 Expression in Inhibitory Neurons Impairs Hippocampal Circuit Development Leading to Autism-Epilepsy Phenotype

Dysregulation of development, migration, and function of interneurons, collectively termed interneuronopathies, have been proposed as a shared mechanism for autism spectrum disorders (ASDs) and childhood epilepsy. Neuropilin-2 (Nrp2), a candidate ASD gene, is a critical regulator of interneuron migration from the median ganglionic eminence (MGE) to the pallium, including the hippocampus. While clinical studies have identified Nrp2 polymorphisms in patients with ASD, whether dysregulation of Nrp2-dependent interneuron migration contributes to pathogenesis of ASD and epilepsy has not been tested. We tested the hypothesis that the lack of Nrp2 in MGE-derived interneuron precursors disrupts the excitation/inhibition balance in hippocampal circuits, thus predisposing the network to seizures and behavioral patterns associated with ASD. Embryonic deletion of Nrp2 during the developmental period for migration of MGE derived interneuron precursors (iCKO) significantly reduced parvalbumin, neuropeptide Y, and somatostatin positive neurons in the hippocampal CA1. Consequently, when compared to controls, the frequency of inhibitory synaptic currents in CA1 pyramidal cells was reduced while frequency of excitatory synaptic currents was increased in iCKO mice. Although passive and active membrane properties of CA1 pyramidal cells were unchanged, iCKO mice showed enhanced susceptibility to chemically evoked seizures. Moreover, iCKO mice exhibited selective behavioral deficits in both preference for social novelty and goal-directed learning, which are consistent with ASD-like phenotype. Together, our findings show that disruption of developmental Nrp2 regulation of interneuron circuit establishment, produces ASD-like behaviors and enhanced risk for epilepsy. These results support the developmental interneuronopathy hypothesis of ASD epilepsy comorbidity.

Molecular identification of wide-field amacrine cells in mouse retina that encode stimulus orientation

Visual information processing is sculpted by a diverse group of inhibitory interneurons in the retina called amacrine cells. Yet, for most of the >60 amacrine cell types, molecular identities and specialized functional attributes remain elusive. Here, we developed an intersectional genetic strategy to target a group of wide-field amacrine cells (WACs) in mouse retina that co-express the transcription factor Bhlhe22 and the Kappa Opioid Receptor (KOR; B/K WACs). B/K WACs feature straight, unbranched dendrites spanning over 0.5 mm (∼15° visual angle) and produce non-spiking responses to either light increments or decrements. Two-photon dendritic population imaging reveals Ca 2+ signals tuned to the physical orientations of B/K WAC dendrites, signifying a robust structure-function alignment. B/K WACs establish divergent connections with multiple retinal neurons, including unexpected connections with non-orientation-tuned ganglion cells and bipolar cells. Our work sets the stage for future comprehensive investigations of the most enigmatic group of retinal neurons: WACs.

Fast-spiking parvalbumin-positive interneurons in brain physiology and Alzheimer's disease

Fast-spiking parvalbumin (PV) interneurons are inhibitory interneurons with unique morphological and functional properties that allow them to precisely control local circuitry, brain networks and memory processing. Since the discovery in 1987 that PV is expressed in a subset of fast-spiking GABAergic inhibitory neurons, our knowledge of the complex molecular and physiological properties of these cells has been expanding. In this review, we highlight the specific properties of PV neurons that allow them to fire at high frequency and with high reliability, enabling them to control network oscillations and shape the encoding, consolidation and retrieval of memories. We next discuss multiple studies reporting PV neuron impairment as a critical step in neuronal network dysfunction and cognitive decline in mouse models of Alzheimer's disease (AD). Finally, we propose potential mechanisms underlying PV neuron dysfunction in AD and we argue that early changes in PV neuron activity could be a causal step in AD-associated network and memory impairment and a significant contributor to disease pathogenesis.

Mapping mesoscale connectivity within the human hippocampus

The connectivity of the hippocampus is essential to its functions. To gain a whole system view of intrahippocampal connectivity, ex vivo mesoscale (100 μm isotropic resolution) multi-shell diffusion MRI (11.7T) and tractography were performed on entire post-mortem human right hippocampi. Volumetric measurements indicated that the head region was largest followed by the body and tail regions. A unique anatomical organization in the head region reflected a complex organization of the granule cell layer (GCL) of the dentate gyrus. Tractography revealed the volumetric distribution of the perforant path, including both the tri-synaptic and temporoammonic pathways, as well as other well-established canonical connections, such as Schaffer collaterals. Visualization of the perforant path provided a means to verify the borders between the pro-subiculum and CA1, as well as between CA1/CA2. A specific angularity of different layers of fibers in the alveus was evident across the whole sample and allowed a separation of afferent and efferent connections based on their origin (i.e. entorhinal cortex) or destination (i.e. fimbria) using a cluster analysis of streamlines. Non-canonical translamellar connections running along the anterior-posterior axis were also discerned in the hilus. In line with "dentations" of the GCL, mossy fibers were bunching together in the sagittal plane revealing a unique lamellar organization and connections between these. In the head region, mossy fibers projected to the origin of the fimbria, which was distinct from the body and tail region. Mesoscale tractography provides an unprecedented systems view of intrahippocampal connections that underpin cognitive and emotional processing.

Plexin-B1 and Plexin-B2 play non-redundant roles in GABAergic synapse formation

Synapse formation in the mammalian brain is a complex and dynamic process requiring coordinated function of dozens of molecular families such as cell adhesion molecules (CAMs) and ligand-receptor pairs (Ephs/Ephrins, Neuroligins/Neurexins, Semaphorins/Plexins). Due to the large number of molecular players and possible functional redundancies within gene families, it is challenging to determine the precise synaptogenic roles of individual molecules, which is key to understanding the consequences of mutations in these genes for brain function. Furthermore, few molecules are known to exclusively regulate either GABAergic or glutamatergic synapses, and cell and molecular mechanisms underlying GABAergic synapse formation in particular are not thoroughly understood. However, we previously demonstrated that Semaphorin-4D (Sema4D) regulates GABAergic synapse development in the mammalian hippocampus while having no effect on glutamatergic synapse development, and this effect occurs through binding to its high affinity receptor, Plexin-B1. Furthermore, Plexin-B2 contributes to GABAergic synapse formation as well but is not required for GABAergic synapse formation induced by binding to Sema4D. Here, we perform a structure-function study of the Plexin-B1 and Plexin-B2 receptors to identify the protein domains in each receptor that are required for its synaptogenic function. We also provide evidence that Plexin-B2 expression in presynaptic parvalbumin-positive interneurons is required for formation of GABAergic synapses onto excitatory pyramidal neurons in CA1. Our data reveal that Plexin-B1 and Plexin-B2 function non-redundantly to regulate GABAergic synapse formation and suggest that the transmembrane domain may underlie these functional distinctions. These findings lay the groundwork for future investigations into the precise signaling pathways required for synapse formation downstream of Plexin-B receptor signaling.

Hippocampal GABAergic interneurons and memory

One of the most captivating questions in neuroscience revolves around the brain's ability to efficiently and durably capture and store information. It must process continuous input from sensory organs while also encoding memories that can persist throughout a lifetime. What are the cellular-, subcellular-, and network-level mechanisms that underlie this remarkable capacity for long-term information storage? Furthermore, what contributions do distinct types of GABAergic interneurons make to this process? As the hippocampus plays a pivotal role in memory, our review focuses on three aspects: (1) delineation of hippocampal interneuron types and their connectivity, (2) interneuron plasticity, and (3) activity patterns of interneurons during memory-related rhythms, including the role of long-range interneurons and disinhibition. We explore how these three elements, together showcasing the remarkable diversity of inhibitory circuits, shape the processing of memories in the hippocampus.

Abnormal accumulation of extracellular vesicles in hippocampal dystrophic axons and regulation by the primary cilia in Alzheimer's disease

Dystrophic neurites (DNs) are abnormal axons and dendrites that are swollen or deformed in various neuropathological conditions. In Alzheimer's disease (AD), DNs play a crucial role in impairing neuronal communication and function, and they may also contribute to the accumulation and spread of amyloid beta (Aβ) in the brain of AD patients. However, it is still a challenge to understand the DNs of specific neurons that are vulnerable to Aβ in the pathogenesis of AD. To shed light on the development of radiating DNs, we examined enriched dystrophic hippocampal axons in a mouse model of AD using a three-dimensional rendering of projecting neurons. We employed the anterograde spread of adeno-associated virus (AAV)1 and conducted proteomic analysis of synaptic compartments obtained from hippocampo-septal regions. Our findings revealed that DNs were formed due to synaptic loss at the axon terminals caused by the accumulation of extracellular vesicle (EV). Abnormal EV-mediated transport and exocytosis were identified in association with primary cilia, indicating their involvement in the accumulation of EVs at presynaptic terminals. To further address the regulation of DNs by primary cilia, we conducted knockdown of the Ift88 gene in hippocampal neurons, which impaired EV-mediated secretion of Aβ and promoted accumulation of axonal spheroids. Using single-cell RNA sequencing, we identified the septal projecting hippocampal somatostatin neurons (SOM) as selectively vulnerable to Aβ with primary cilia dysfunction and vesicle accumulation. Our study suggests that DNs in AD are initiated by the ectopic accumulation of EVs at the neuronal axon terminals, which is affected by neuronal primary cilia.

Comparison of three common inbred mouse strains reveals substantial differences in hippocampal GABAergic interneuron populations and in vitro network oscillations

A major challenge in neuroscience is to pinpoint neurobiological correlates of specific cognitive and neuropsychiatric traits. At the mesoscopic level, promising candidates for establishing such connections are brain oscillations that can be robustly recorded as local field potentials with varying frequencies in the hippocampus in vivo and in vitro. Inbred mouse strains show natural variation in hippocampal synaptic plasticity (e.g. long-term potentiation), a cellular correlate of learning and memory. However, their diversity in expression of different types of hippocampal network oscillations has not been fully explored. Here, we investigated hippocampal network oscillations in three widely used inbred mouse strains: C57BL/6J (B6J), C57BL/6NCrl (B6N) and 129S2/SvPasCrl (129) with the aim to identify common oscillatory characteristics in inbred mouse strains that show aberrant emotional/cognitive behaviour (B6N and 129) and compare them to "control" B6J strain. First, we detected higher gamma oscillation power in the hippocampal CA3 of both B6N and 129 strains. Second, higher incidence of hippocampal sharp wave-ripple (SPW-R) transients was evident in these strains. Third, we observed prominent differences in the densities of distinct interneuron types and CA3 associative network activity, which are indispensable for sustainment of mesoscopic network oscillations. Together, these results add further evidence to profound physiological differences among inbred mouse strains commonly used in neuroscience research.

Impact of Developmental Changes of GABAA Receptors on Interneuron-NG2 Glia Transmission in the Hippocampus

NG2 glia receive synaptic input from neurons, but the functional impact of this glial innervation is not well understood. In the developing cerebellum and somatosensory cortex the GABAergic input might regulate NG2 glia differentiation and myelination, and a switch from synaptic to extrasynaptic neuron-glia signaling was reported in the latter region. Myelination in the hippocampus is sparse, and most NG2 glia retain their phenotype throughout adulthood, raising the question of the properties and function of neuron-NG2 glia synapses in that brain region. Here, we compared spontaneous and evoked GABAA receptor-mediated currents of NG2 glia in juvenile and adult hippocampi of mice of either sex and assessed the mode of interneuron-glial signaling changes during development. With patch-clamp and pharmacological analyses, we found a decrease in innervation of hippocampal NG2 glia between postnatal days 10 and 60. At the adult stage, enhanced activation of extrasynaptic receptors occurred, indicating a spillover of GABA. This switch from synaptic to extrasynaptic receptor activation was accompanied by downregulation of γ2 and upregulation of the α5 subunit. Molecular analyses and high-resolution expansion microscopy revealed mechanisms of glial GABAA receptor trafficking and clustering. We found that gephyrin and radixin are organized in separate clusters along glial processes. Surprisingly, the developmental loss of γ2 and postsynaptic receptors were not accompanied by altered glial expression of scaffolding proteins, auxiliary receptor subunits or postsynaptic interaction proteins. The GABAergic input to NG2 glia might contribute to the release of neurotrophic factors from these cells and influence neuronal synaptic plasticity.

Voltage-Gated Na+ Channels in Alzheimer's Disease: Physiological Roles and Therapeutic Potential

Alzheimer's disease (AD) is the most common cause of dementia and is classically characterized by two major histopathological abnormalities: extracellular plaques composed of amyloid beta (Aβ) and intracellular hyperphosphorylated tau. Due to the progressive nature of the disease, it is of the utmost importance to develop disease-modifying therapeutics that tackle AD pathology in its early stages. Attenuation of hippocampal hyperactivity, one of the earliest neuronal abnormalities observed in AD brains, has emerged as a promising strategy to ameliorate cognitive deficits and abate the spread of neurotoxic species. This aberrant hyperactivity has been attributed in part to the dysfunction of voltage-gated Na+ (Nav) channels, which are central mediators of neuronal excitability. Therefore, targeting Nav channels is a promising strategy for developing disease-modifying therapeutics that can correct aberrant neuronal phenotypes in early-stage AD. This review will explore the role of Nav channels in neuronal function, their connections to AD pathology, and their potential as therapeutic targets.

Full-Spectrum Neuronal Diversity and Stereotypy through Whole Brain Morphometry

We conducted a large-scale study of whole-brain morphometry, analyzing 3.7 peta-voxels of mouse brain images at the single-cell resolution, producing one of the largest multi-morphometry databases of mammalian brains to date. We spatially registered 205 mouse brains and associated data from six Brain Initiative Cell Census Network (BICCN) data sources covering three major imaging modalities from five collaborative projects to the Allen Common Coordinate Framework (CCF) atlas, annotated 3D locations of cell bodies of 227,581 neurons, modeled 15,441 dendritic microenvironments, characterized the full morphology of 1,891 neurons along with their axonal motifs, and detected 2.58 million putative synaptic boutons. Our analysis covers six levels of information related to neuronal populations, dendritic microenvironments, single-cell full morphology, sub-neuronal dendritic and axonal arborization, axonal boutons, and structural motifs, along with a quantitative characterization of the diversity and stereotypy of patterns at each level. We identified 16 modules consisting of highly intercorrelated brain regions in 13 functional brain areas corresponding to 314 anatomical regions in CCF. Our analysis revealed the dendritic microenvironment as a powerful method for delineating brain regions of cell types and potential subtypes. We also found that full neuronal morphologies can be categorized into four distinct classes based on spatially tuned morphological features, with substantial cross-areal diversity in apical dendrites, basal dendrites, and axonal arbors, along with quantified stereotypy within cortical, thalamic and striatal regions. The lamination of somas was found to be more effective in differentiating neuron arbors within the cortex. Further analysis of diverging and converging projections of individual neurons in 25 regions throughout the brain reveals branching preferences in the brain-wide and local distributions of axonal boutons. Overall, our study provides a comprehensive description of key anatomical structures of neurons and their types, covering a wide range of scales and features, and contributes to our understanding of neuronal diversity and its function in the mammalian brain.

GSK-3β orchestrates the inhibitory innervation of adult-born dentate granule cells in vivo

Adult hippocampal neurogenesis enhances brain plasticity and contributes to the cognitive reserve during aging. Adult hippocampal neurogenesis is impaired in neurological disorders, yet the molecular mechanisms regulating the maturation and synaptic integration of new neurons have not been fully elucidated. GABA is a master regulator of adult and developmental neurogenesis. Here we engineered a novel retrovirus encoding the fusion protein Gephyrin:GFP to longitudinally study the formation and maturation of inhibitory synapses during adult hippocampal neurogenesis in vivo. Our data reveal the early assembly of inhibitory postsynaptic densities at 1 week of cell age. Glycogen synthase kinase 3 Beta (GSK-3β) emerges as a key regulator of inhibitory synapse formation and maturation during adult hippocampal neurogenesis. GSK-3β-overexpressing newborn neurons show an increased number and altered size of Gephyrin+ postsynaptic clusters, enhanced miniature inhibitory postsynaptic currents, shorter and distanced axon initial segments, reduced synaptic output at the CA3 and CA2 hippocampal regions, and impaired pattern separation. Moreover, GSK-3β overexpression triggers a depletion of Parvalbumin+ interneuron perineuronal nets. These alterations might be relevant in the context of neurological diseases in which the activity of GSK-3β is dysregulated.

Differential expression of GABAA receptor subunits δ and α6 mediates tonic inhibition in parvalbumin and somatostatin interneurons in the mouse hippocampus

Inhibitory γ-aminobutyric acid (GABA)-ergic interneurons mediate inhibition in neuronal circuitry and support normal brain function. Consequently, dysregulation of inhibition is implicated in various brain disorders. Parvalbumin (PV) and somatostatin (SST) interneurons, the two major types of GABAergic inhibitory interneurons in the hippocampus, exhibit distinct morpho-physiological properties and coordinate information processing and memory formation. However, the molecular mechanisms underlying the specialized properties of PV and SST interneurons remain unclear. This study aimed to compare the transcriptomic differences between these two classes of interneurons in the hippocampus using the ribosome tagging approach. The results revealed distinct expressions of genes such as voltage-gated ion channels and GABAA receptor subunits between PV and SST interneurons. Gabrd and Gabra6 were identified as contributors to the contrasting tonic GABAergic inhibition observed in PV and SST interneurons. Moreover, some of the differentially expressed genes were associated with schizophrenia and epilepsy. In conclusion, our results provide molecular insights into the distinct roles of PV and SST interneurons in health and disease.

mTORC1-mediated acquisition of reward-related representations by hippocampal somatostatin interneurons

Plasticity of principal cells and inhibitory interneurons underlies hippocampal memory. Bidirectional modulation of somatostatin cell mTORC1 activity, a crucial translational control mechanism in synaptic plasticity, causes parallel changes in hippocampal CA1 somatostatin interneuron (SOM-IN) long-term potentiation and hippocampus-dependent memory, indicating a key role in learning. However, SOM-IN activity changes and behavioral correlates during learning, and the role of mTORC1 in these processes, remain ill-defined. To address these questions, we used two-photon Ca²⁺ imaging from SOM-INs during a virtual reality goal-directed spatial memory task in head-fixed control mice (SOM-IRES-Cre mice) or in mice with conditional knockout of Rptor (SOM-Rptor-KO mice) to block mTORC1 activity in SOM-INs. We found that control mice learn the task, but SOM-Raptor-KO mice exhibit a deficit. Also, SOM-IN Ca²⁺ activity became increasingly related to reward during learning in control mice but not in SOM-Rptor-KO mice. Four types of SOM-IN activity patterns related to reward location were observed, "reward off sustained", "reward off transient", "reward on sustained" and "reward on transient", and these responses showed reorganization after reward relocation in control but not SOM-Rptor-KO mice. Thus, SOM-INs develop mTORC1-dependent reward- related activity during learning. This coding may bi-directionally interact with pyramidal cells and other structures to represent and consolidate reward location.

Excitatory and inhibitory imbalances in the trisynaptic pathway in the hippocampus in schizophrenia: a postmortem ultrastructural study

BACKGROUND

A preponderance of evidence suggests that the hippocampus is a key region of dysfunction in schizophrenia. Neuroimaging and other studies indicate a relationship between hippocampal dysfunction and the degree of psychosis. Clinical data indicate hyperactivity in the hippocampus that precedes the onset of psychosis, and is correlated with symptom severity. In this study, we sought to identify circuitry at the electron microscopic level that could contribute to region-specific imbalances in excitation and inhibition in the hippocampus in schizophrenia. We used postmortem tissue from the anterior hippocampus from patients with schizophrenia and matched controls. Using stereological techniques, we counted and measured synapses, postsynaptic densities (PSDs), and evaluated size, number and optical density of mitochondria and parvalbumin-containing interneurons in key nodes of the trisynaptic pathway. Compared to controls, the schizophrenia group had decreased numbers of inhibitory synapses in CA3 and increased numbers of excitatory synapses in CA1; together, this indicates deficits in inhibition and an increase in excitation. The thickness of the PSD was larger in excitatory synapses in CA1, suggesting greater synaptic strength. In the schizophrenia group, there were fewer mitochondria in the dentate gyrus and a decrease in the optical density, a measure of functional integrity, in CA1. The number and optical density of parvalbumin interneurons were lower in CA3. The results suggest region-specific increases in excitatory circuitry, decreases in inhibitory neurotransmission and fewer or damaged mitochondria. These results are consistent with the hyperactivity observed in the hippocampus in schizophrenia in previous studies.

Inhibitory control of sharp-wave ripple duration during learning in hippocampal recurrent networks

Recurrent excitatory connections in hippocampal regions CA3 and CA2 are thought to play a key role in the generation of sharp-wave ripples (SWRs), electrophysiological oscillations tightly linked with learning and memory consolidation. However, it remains unknown how defined populations of inhibitory interneurons regulate these events during behavior. Here, we use large-scale, three-dimensional calcium imaging and retrospective molecular identification in the mouse hippocampus to characterize molecularly identified CA3 and CA2 interneuron activity during SWR-associated memory consolidation and spatial navigation. We describe subtype- and region-specific responses during behaviorally distinct brain states and find that SWRs are preceded by decreased cholecystokinin-expressing interneuron activity and followed by increased parvalbumin-expressing basket cell activity. The magnitude of these dynamics correlates with both SWR duration and behavior during hippocampal-dependent learning. Together these results assign subtype- and region-specific roles for inhibitory circuits in coordinating operations and learning-related plasticity in hippocampal recurrent circuits.

Functional specialization of hippocampal somatostatin-expressing interneurons

Hippocampal somatostatin-expressing (Sst) GABAergic interneurons (INs) exhibit considerable anatomical and functional heterogeneity. Recent single cell transcriptome analyses have provided a comprehensive Sst-IN subtype census, a plausible molecular ground truth of neuronal identity whose links to specific functionality remain incomplete. Here, we designed an approach to identify and access subpopulations of Sst-INs based on transcriptomic features. Four mouse models based on single or combinatorial Cre- and Flp- expression differentiated functionally distinct subpopulations of CA1 hippocampal Sst-INs that largely tiled the morpho-functional parameter space of the Sst-INs superfamily. Notably, the Sst;;Tac1 intersection revealed a population of bistratified INs that preferentially synapsed onto fast-spiking interneurons (FS-INs) and were both necessary and sufficient to interrupt their firing. In contrast, the Ndnf;;Nkx2-1 intersection identified a population of oriens lacunosum-moleculare (OLM) INs that predominantly targeted CA1 pyramidal neurons, avoiding FS-INs. Overall, our results provide a framework to translate neuronal transcriptomic identity into discrete functional subtypes that capture the diverse specializations of hippocampal Sst-INs.

Organization and Plasticity of Inhibition in Hippocampal Recurrent Circuits

Excitatory-inhibitory interactions structure recurrent network dynamics for efficient cortical computations. In the CA3 area of the hippocampus, recurrent circuit dynamics, including experience-induced plasticity at excitatory synapses, are thought to play a key role in episodic memory encoding and consolidation via rapid generation and flexible selection of neural ensembles. However, in vivo activity of identified inhibitory motifs supporting this recurrent circuitry has remained largely inaccessible, and it is unknown whether CA3 inhibition is also modifiable upon experience. Here we use large-scale, 3-dimensional calcium imaging and retrospective molecular identification in the mouse hippocampus to obtain the first comprehensive description of molecularly-identified CA3 interneuron dynamics during both spatial navigation and sharp-wave ripple (SWR)-associated memory consolidation. Our results uncover subtype-specific dynamics during behaviorally distinct brain-states. Our data also demonstrate predictive, reflective, and experience-driven plastic recruitment of specific inhibitory motifs during SWR-related memory reactivation. Together these results assign active roles for inhibitory circuits in coordinating operations and plasticity in hippocampal recurrent circuits.

LINCs Are Vulnerable to Epileptic Insult and Fail to Provide Seizure Control via On-Demand Activation

Temporal lobe epilepsy (TLE) is notoriously pharmacoresistant, and identifying novel therapeutic targets for controlling seizures is crucial. Long-range inhibitory neuronal nitric oxide synthase-expressing cells (LINCs), a population of hippocampal neurons, were recently identified as a unique source of widespread inhibition in CA1, able to elicit both GABAA-mediated and GABAB-mediated postsynaptic inhibition. We therefore hypothesized that LINCs could be an effective target for seizure control. LINCs were optogenetically activated for on-demand seizure intervention in the intrahippocampal kainate (KA) mouse model of chronic TLE. Unexpectedly, LINC activation at 1 month post-KA did not substantially reduce seizure duration in either male or female mice. We tested two different sets of stimulation parameters, both previously found to be effective with on-demand optogenetic approaches, but neither was successful. Quantification of LINCs following intervention revealed a substantial reduction of LINC numbers compared with saline-injected controls. We also observed a decreased number of LINCs when the site of initial insult (i.e., KA injection) was moved to the amygdala [basolateral amygdala (BLA)-KA], and correspondingly, no effect of light delivery on BLA-KA seizures. This indicates that LINCs may be a vulnerable population in TLE, regardless of the site of initial insult. To determine whether long-term circuitry changes could influence outcomes, we continued testing once a month for up to 6 months post-KA. However, at no time point did LINC activation provide meaningful seizure suppression. Altogether, our results suggest that LINCs are not a promising target for seizure inhibition in TLE.

Identifying foetal forebrain interneurons as a target for monogenic autism risk factors and the polygenic 16p11.2 microdeletion

BACKGROUND

Autism spectrum condition or 'autism' is associated with numerous genetic risk factors including the polygenic 16p11.2 microdeletion. The balance between excitatory and inhibitory neurons in the cerebral cortex is hypothesised to be critical for the aetiology of autism making improved understanding of how risk factors impact on the development of these cells an important area of research. In the current study we aim to combine bioinformatics analysis of human foetal cerebral cortex gene expression data with anatomical and electrophysiological analysis of a 16p11.2^+/- rat model to investigate how genetic risk factors impact on inhibitory neuron development.

METHODS

We performed bioinformatics analysis of single cell transcriptomes from gestational week (GW) 8-26 human foetal prefrontal cortex and anatomical and electrophysiological analysis of 16p11.2^+/- rat cerebral cortex and hippocampus at post-natal day (P) 21.

RESULTS

We identified a subset of human interneurons (INs) first appearing at GW23 with enriched expression of a large fraction of risk factor transcripts including those expressed from the 16p11.2 locus. This suggests the hypothesis that these foetal INs are vulnerable to mutations causing autism. We investigated this in a rat model of the 16p11.2 microdeletion. We found no change in the numbers or position of either excitatory or inhibitory neurons in the somatosensory cortex or CA1 of 16p11.2^+/- rats but found that CA1 Sst INs were hyperexcitable with an enlarged axon initial segment, which was not the case for CA1 pyramidal cells.

LIMITATIONS

The human foetal gene expression data was acquired from cerebral cortex between gestational week (GW) 8 to 26. We cannot draw inferences about potential vulnerabilities to genetic autism risk factors for cells not present in the developing cerebral cortex at these stages. The analysis 16p11.2^+/- rat phenotypes reported in the current study was restricted to 3-week old (P21) animals around the time of weaning and to a single interneuron cell-type while in human 16p11.2 microdeletion carriers symptoms likely involve multiple cell types and manifest in the first few years of life and on into adulthood.

CONCLUSIONS

We have identified developing interneurons in human foetal cerebral cortex as potentially vulnerable to monogenic autism risk factors and the 16p11.2 microdeletion and report interneuron phenotypes in post-natal 16p11.2^+/- rats.