Parvalbumin - Positive Neurons in the Neocortex: A Review

The calcium binding protein parvalbumin (PV) in the mammalian neocortex is expressed in a subpopulation of cortical GABAergic inhibitory interneurons. PV - producing interneurons represent the largest subpopulation of neocortical inhibitory cells, exhibit mutual chemical and electrical synaptic contacts and are well known to generate gamma oscillation. This review summarizes basic data of the distribution, afferent and efferent connections and physiological properties of parvalbumin expressing neurons in the neocortex. Basic data about participation of PV-positive neurons in cortical microcircuits are presented. Autaptic connections, metabolism and perineuronal nets (PNN) of PV positive neurons are also discussed.

Axo-axonic cells in neuropsychiatric disorders: a systematic review

Imbalance between excitation and inhibition in the cerebral cortex is one of the main theories in neuropsychiatric disorder pathophysiology. Cortical inhibition is finely regulated by a variety of highly specialized GABAergic interneuron types, which are thought to organize neural network activities. Among interneurons, axo-axonic cells are unique in making synapses with the axon initial segment of pyramidal neurons. Alterations of axo-axonic cells have been proposed to be implicated in disorders including epilepsy, schizophrenia and autism spectrum disorder. However, evidence for the alteration of axo-axonic cells in disease has only been examined in narrative reviews. By performing a systematic review of studies investigating axo-axonic cells and axo-axonic communication in epilepsy, schizophrenia and autism spectrum disorder, we outline convergent findings and discrepancies in the literature. Overall, the implication of axo-axonic cells in neuropsychiatric disorders might have been overstated. Additional work is needed to assess initial, mostly indirect findings, and to unravel how defects in axo-axonic cells translates to cortical dysregulation and, in turn, to pathological states.

Erbb4 Deletion From Inhibitory Interneurons Causes Psychosis-Relevant Neuroimaging Phenotypes

BACKGROUND AND HYPOTHESIS

Converging lines of evidence suggest that dysfunction of cortical GABAergic inhibitory interneurons is a core feature of psychosis. This dysfunction is thought to underlie neuroimaging abnormalities commonly found in patients with psychosis, particularly in the hippocampus. These include increases in resting cerebral blood flow (CBF) and glutamatergic metabolite levels, and decreases in ligand binding to GABAA α5 receptors and to the synaptic density marker synaptic vesicle glycoprotein 2A (SV2A). However, direct links between inhibitory interneuron dysfunction and these neuroimaging readouts are yet to be established. Conditional deletion of a schizophrenia susceptibility gene, the tyrosine kinase receptor Erbb4, from cortical and hippocampal inhibitory interneurons leads to synaptic defects, and behavioral and cognitive phenotypes relevant to psychosis in mice.

STUDY DESIGN

Here, we investigated how this inhibitory interneuron disruption affects hippocampal in vivo neuroimaging readouts. Adult Erbb4 conditional mutant mice (Lhx6-Cre;Erbb4F/F, n = 12) and their wild-type littermates (Erbb4F/F, n = 12) were scanned in a 9.4T magnetic resonance scanner to quantify CBF and glutamatergic metabolite levels (glutamine, glutamate, GABA). Subsequently, we assessed GABAA receptors and SV2A density using quantitative autoradiography.

RESULTS

Erbb4 mutant mice showed significantly elevated ventral hippccampus CBF and glutamine levels, and decreased SV2A density across hippocampus sub-regions compared to wild-type littermates. No significant GABAA receptor density differences were identified.

CONCLUSIONS

These findings demonstrate that specific disruption of cortical inhibitory interneurons in mice recapitulate some of the key neuroimaging findings in patients with psychosis, and link inhibitory interneuron deficits to non-invasive measures of brain function and neurochemistry that can be used across species.

Synaptic boutons are smaller in chandelier cell cartridges in autism

Chandelier (Ch) cells are cortical interneurons with axon terminal structures known as cartridges that synapse on the axon initial segment of excitatory pyramidal neurons. Previous studies indicate that the number of Ch cells is decreased in autism, and that GABA receptors are decreased in the Ch cell synaptic target in the prefrontal cortex. To further identify Ch cell alterations, we examined whether the length of cartridges, and the number, density, and size of Ch cell synaptic boutons, differed in the prefrontal cortex of cases with autism versus control cases. We collected samples of postmortem human prefrontal cortex (Brodmann Area (BA) 9, 46, and 47) from 20 cases with autism and 20 age- and sex-matched control cases. Ch cells were labeled using an antibody against parvalbumin, a marker that labeles soma, cartridges, and synaptic boutons. We found no significant difference in the average length of cartridges, or in the total number or density of boutons in control subjects vs. subjects with autism. However, we found a significant decrease in the size of Ch cell boutons in those with autism. The reduced size of Ch cell boutons may result in reduced inhibitory signal transmission and impact the balance of excitation to inhibition in the prefrontal cortex in autism.

Cortical control of chandelier cells in neural codes

Various cortical functions arise from the dynamic interplay of excitation and inhibition. GABAergic interneurons that mediate synaptic inhibition display significant diversity in cell morphology, electrophysiology, plasticity rule, and connectivity. These heterogeneous features are thought to underlie their functional diversity. Emerging attention on specific properties of the various interneuron types has emphasized the crucial role of cell-type specific inhibition in cortical neural processing. However, knowledge is still limited on how each interneuron type forms distinct neural circuits and regulates network activity in health and disease. To dissect interneuron heterogeneity at single cell-type precision, we focus on the chandelier cell (ChC), one of the most distinctive GABAergic interneuron types that exclusively innervate the axon initial segments (AIS) of excitatory pyramidal neurons. Here we review the current understanding of the structural and functional properties of ChCs and their implications in behavioral functions, network activity, and psychiatric disorders. These findings provide insights into the distinctive roles of various single-type interneurons in cortical neural coding and the pathophysiology of cortical dysfunction.

Quantitative analysis of the GABAergic innervation of the soma and axon initial segment of pyramidal cells in the human and mouse neocortex

Perisomatic GABAergic innervation in the cerebral cortex is carried out mostly by basket and chandelier cells, which differentially participate in the control of pyramidal cell action potential output and synchronization. These cells establish multiple synapses with the cell body (and proximal dendrites) and the axon initial segment (AIS) of pyramidal neurons, respectively. Using multiple immunofluorescence, confocal microscopy and 3D quantification techniques, we have estimated the number and density of GABAergic boutons on the cell body and AIS of pyramidal neurons located through cortical layers of the human and mouse neocortex. The results revealed, in both species, that there is clear variability across layers regarding the density and number of perisomatic GABAergic boutons. We found a positive linear correlation between the surface area of the soma, or the AIS, and the number of GABAergic terminals in apposition to these 2 neuronal domains. Furthermore, the density of perisomatic GABAergic boutons was higher in the human cortex than in the mouse. These results suggest a selectivity for the GABAergic innervation of the cell body and AIS that might be related to the different functional attributes of the microcircuits in which neurons from different layers are involved in both human and mouse.

Origin, Development, and Synaptogenesis of Cortical Interneurons

The mammalian cerebral cortex represents one of the most recent and astonishing inventions of nature, responsible of a large diversity of functions that range from sensory processing to high-order cognitive abilities, such as logical reasoning or language. Decades of dedicated study have contributed to our current understanding of this structure, both at structural and functional levels. A key feature of the neocortex is its outstanding richness in cell diversity, composed by multiple types of long-range projecting neurons and locally connecting interneurons. In this review, we will describe the great diversity of interneurons that constitute local neocortical circuits and summarize the mechanisms underlying their development and their assembly into functional networks.

Pyramidal cell axon initial segment in Alzheimer´s disease

The axon initial segment (AIS) is a region of the neuron that is critical for action potential generation as well as for the regulation of neural activity. This specialized structure-characterized by the expression of different types of ion channels as well as adhesion, scaffolding and cytoskeleton proteins-is subjected to morpho-functional plastic changes in length and position upon variations in neural activity or in pathological conditions. In the present study, using immunocytochemistry with the AT8 antibody (phospho-tau S202/T205) and 3D confocal microscopy reconstruction techniques in brain tissue from Alzheimer's disease patients, we found that around half of the cortical pyramidal neurons with hyperphosphorylated tau showed changes in AIS length and position in comparison with AT8-negative neurons from the same cortical layers. We observed a wide variety of AIS alterations in neurons with hyperphosphorylated tau, although the most common changes were a proximal shift or a lengthening of the AISs. Similar results were found in neocortical tissue from non-demented cases with neurons containing hyperphosphorylated tau. These findings support the notion that the accumulation of phospho-tau is associated with structural alterations of the AIS that are likely to have an impact on normal neuronal activity, which might contribute to neuronal dysfunction in AD.

Parvalbumin and parvalbumin chandelier interneurons in autism and other psychiatric disorders

Parvalbumin (PV) is a calcium binding protein expressed by inhibitory fast-spiking interneurons in the cerebral cortex. By generating a fast stream of action potentials, PV+ interneurons provide a quick and stable inhibitory input to pyramidal neurons and contribute to the generation of gamma oscillations in the cortex. Their fast-firing rates, while advantageous for regulating cortical signaling, also leave them vulnerable to metabolic stress. Chandelier (Ch) cells are a type of PV+ interneuron that modulate the output of pyramidal neurons and synchronize spikes within neuron populations by directly innervating the pyramidal axon initial segment. Changes in the morphology and/or function of PV+ interneurons, mostly of Ch cells, are linked to neurological disorders. In ASD, the number of PV+ Ch cells is decreased across several cortical areas. Changes in the morphology and/or function of PV+ interneurons have also been linked to schizophrenia, epilepsy, and bipolar disorder. Herein, we review the role of PV and PV+ Ch cell alterations in ASD and other psychiatric disorders.

Neuronal Circuits in Barrel Cortex for Whisker Sensory Perception

The array of whiskers on the snout provides rodents with tactile sensory information relating to the size, shape and texture of objects in their immediate environment. Rodents can use their whiskers to detect stimuli, distinguish textures, locate objects and navigate. Important aspects of whisker sensation are thought to result from neuronal computations in the whisker somatosensory cortex (wS1). Each whisker is individually represented in the somatotopic map of wS1 by an anatomical unit named a 'barrel' (hence also called barrel cortex). This allows precise investigation of sensory processing in the context of a well-defined map. Here, we first review the signaling pathways from the whiskers to wS1, and then discuss current understanding of the various types of excitatory and inhibitory neurons present within wS1. Different classes of cells can be defined according to anatomical, electrophysiological and molecular features. The synaptic connectivity of neurons within local wS1 microcircuits, as well as their long-range interactions and the impact of neuromodulators, are beginning to be understood. Recent technological progress has allowed cell-type-specific connectivity to be related to cell-type-specific activity during whisker-related behaviors. An important goal for future research is to obtain a causal and mechanistic understanding of how selected aspects of tactile sensory information are processed by specific types of neurons in the synaptically connected neuronal networks of wS1 and signaled to downstream brain areas, thus contributing to sensory-guided decision-making.

Shedding Light on Chandelier Cell Development, Connectivity, and Contribution to Neural Disorders

Chandelier cells (ChCs) are a unique type of GABAergic interneuron that selectively innervate the axon initial segment (AIS) of excitatory pyramidal neurons; the subcellular domain where action potentials are initiated. The proper genesis and maturation of ChCs is critical for regulating neural ensemble firing in the neocortex throughout development and adulthood. Recently, genetic and molecular studies have shed new light on the complex innerworkings of ChCs in health and disease. This review presents an overview of recent studies on the developmental origins, migratory properties, and morphology of ChCs. In addition, attention is given to newly identified molecules regulating ChC morphogenesis and connectivity as well as recent work linking ChC dysfunction to neural disorders, including schizophrenia, epilepsy, and autism spectrum disorder (ASD).

LTP of inhibition at PV interneuron output synapses requires developmental BMP signaling

Parvalbumin (PV)-expressing interneurons (PV-INs) mediate well-timed inhibition of cortical principal neurons, and plasticity of these interneurons is involved in map remodeling of primary sensory cortices during critical periods of development. To assess whether bone morphogenetic protein (BMP) signaling contributes to the developmental acquisition of the synapse- and plasticity properties of PV-INs, we investigated conditional/conventional double KO mice of BMP-receptor 1a (BMPR1a; targeted to PV-INs) and 1b (BMPR1a/1b (c)DKO mice). We report that spike-timing dependent LTP at the synapse between PV-INs and principal neurons of layer 4 in the auditory cortex was absent, concomitant with a decreased paired-pulse ratio (PPR). On the other hand, baseline synaptic transmission at this connection, and action potential (AP) firing rates of PV-INs were unchanged. To explore possible gene expression targets of BMP signaling, we measured the mRNA levels of the BDNF receptor TrkB and of P/Q-type Ca2+ channel α-subunits, but did not detect expression changes of the corresponding genes in PV-INs of BMPR1a/1b (c)DKO mice. Our study suggests that BMP-signaling in PV-INs during and shortly after the critical period is necessary for the expression of LTP at PV-IN output synapses, involving gene expression programs that need to be addressed in future work.

The impact of silencing feed-forward parvalbumin-expressing inhibitory interneurons in the cortico-thalamocortical network on seizure generation and behaviour

Feed-forward inhibition (FFI) is an essential mechanism within the brain, to regulate neuronal firing and prevent runaway excitation. In the cortico-thalamocortical (CTC) network, fast spiking parvalbumin-expressing (PV+) inhibitory interneurons regulate the firing of pyramidal cells in the cortex and relay neurons in the thalamus. PV+ interneuron dysfunction has been implicated in several neurological disorders, including epilepsy. Previously, we demonstrated that loss of excitatory AMPA-receptors, specifically at synapses on PV+ interneurons in CTC feedforward microcircuits, occurs in the stargazer mouse model of absence epilepsy. These mice present with absence seizures characterized by spike and wave discharges (SWDs) on electroencephalogram (EEG) and concomitant behavioural arrest, similar to childhood absence epilepsy. The aim of the current study was to investigate the impact of loss of FFI within the CTC on absence seizure generation and behaviour using new Designer Receptor Exclusively Activated by Designer Drug (DREADD) technology. We crossed PV-Cre mice with Cre-dependent hM4Di DREADD strains of mice, which allowed Cre-recombinase-mediated restricted expression of inhibitory G_i-DREADDs in PV+ interneurons. We then tested the impact of global and focal (within the CTC network) silencing of PV+ interneurons. CNO mediated silencing of all PV+ interneurons by intraperitoneal injection caused the impairment of motor control, decreased locomotion and increased anxiety in a dose-dependent manner. Such silencing generated pathological oscillations similar to absence-like seizures. Focal silencing of PV+ interneurons within cortical or thalamic feedforward microcircuits, induced SWD-like oscillations and associated behavioural arrest. Epileptiform activity on EEG appeared significantly sooner after focal injection compared to peripheral injection of CNO. However, the mean duration of each oscillatory burst and spike frequency was similar, irrespective of mode of CNO delivery. No significant changes were observed in vehicle-treated or non-DREADD wild-type control animals. These data suggest that dysfunctional feed-forward inhibition in CTC microcircuits may be an important target for future therapy strategies for some patients with absence seizures. Additionally, silencing of PV+ interneurons in other brain regions may contribute to anxiety related neurological and psychiatric disorders.

Development and Functional Diversification of Cortical Interneurons

In the cerebral cortex, GABAergic interneurons have evolved as a highly heterogeneous collection of cell types that are characterized by their unique spatial and temporal capabilities to influence neuronal circuits. Current estimates suggest that up to 50 different types of GABAergic neurons may populate the cerebral cortex, all derived from progenitor cells in the subpallium, the ventral aspect of the embryonic telencephalon. In this review, we provide an overview of the mechanisms underlying the generation of the distinct types of interneurons and their integration in cortical circuits. Interneuron diversity seems to emerge through the implementation of cell-intrinsic genetic programs in progenitor cells, which unfold over a protracted period of time until interneurons acquire mature characteristics. The developmental trajectory of interneurons is also modulated by activity-dependent, non-cell-autonomous mechanisms that influence their ability to integrate in nascent circuits and sculpt their final distribution in the adult cerebral cortex.

Genetic disruption of ankyrin-G in adult mouse forebrain causes cortical synapse alteration and behavior reminiscent of bipolar disorder

Genome-wide association studies have implicated the ANK3 locus in bipolar disorder, a major human psychotic illness. ANK3 encodes ankyrin-G, which organizes the neuronal axon initial segment (AIS). We generated a mouse model with conditional disruption of ANK3 in pyramidal neurons of the adult forebrain (Ank-G cKO). This resulted in the expected loss of pyramidal neuron AIS voltage-gated sodium and potassium channels. There was also dramatic loss of markers of afferent GABAergic cartridge synapses, resembling the cortical microcircuitry changes in brains from psychotic patients, and suggesting disinhibition. Expression of c-fos was increased in cortical pyramidal neurons, consistent with increased neuronal activity due to disinhibition. The mice showed robust behavioral phenotypes reminiscent of aspects of human mania, ameliorated by antimania drugs lithium and valproate. Repeated social defeat stress resulted in repeated episodes of dramatic behavioral changes from hyperactivity to "depression-like" behavior, suggestive of some aspects of human bipolar disorder. Overall, we suggest that this Ank-G cKO mouse model recapitulates some of the core features of human bipolar disorder and indicates that cortical microcircuitry alterations during adulthood may be involved in pathogenesis. The model may be useful for studying disease pathophysiology and for developing experimental therapeutics.

Traumatic Brain Injury and Neuronal Functionality Changes in Sensory Cortex

Traumatic brain injury (TBI), caused by direct blows to the head or inertial forces during relative head-brain movement, can result in long-lasting cognitive and motor deficits which can be particularly consequential when they occur in young people with a long life ahead. Much is known of the molecular and anatomical changes produced in TBI but much less is known of the consequences of these changes to neuronal functionality, especially in the cortex. Given that much of our interior and exterior lives are dependent on responsiveness to information from and about the world around us, we have hypothesized that a significant contributor to the cognitive and motor deficits seen after TBI could be changes in sensory processing. To explore this hypothesis, and to develop a model test system of the changes in neuronal functionality caused by TBI, we have examined neuronal encoding of simple and complex sensory input in the rat’s exploratory and discriminative tactile system, the large face macrovibrissae, which feeds to the so-called “barrel cortex” of somatosensory cortex. In this review we describe the short-term and long-term changes in the barrel cortex encoding of whisker motion modeling naturalistic whisker movement undertaken by rats engaged in a variety of tasks. We demonstrate that the most common form of TBI results in persistent neuronal hyperexcitation specifically in the upper cortical layers, likely due to changes in inhibition. We describe the types of cortical inhibitory neurons and their roles and how selective effects on some of these could produce the particular forms of neuronal encoding changes described in TBI, and then generalize to compare the effects on inhibition seen in other forms of brain injury. From these findings we make specific predictions as to how non-invasive extra-cranial electrophysiology can be used to provide the high-precision information needed to monitor and understand the temporal evolution of changes in neuronal functionality in humans suffering TBI. Such detailed understanding of the specific changes in an individual patient’s cortex can allow for treatment to be tailored to the neuronal changes in that particular patient’s brain in TBI, a precision that is currently unavailable with any technique.

Inhibitory Circuits in Cortical Layer 5

Inhibitory neurons play a fundamental role in cortical computation and behavior. Recent technological advances, such as two photon imaging, targeted in vivo recording, and molecular profiling, have improved our understanding of the function and diversity of cortical interneurons, but for technical reasons most work has been directed towards inhibitory neurons in the superficial cortical layers. Here we review current knowledge specifically on layer 5 (L5) inhibitory microcircuits, which play a critical role in controlling cortical output. We focus on recent work from the well-studied rodent barrel cortex, but also draw on evidence from studies in primary visual cortex and other cortical areas. The diversity of both deep inhibitory neurons and their pyramidal cell targets make this a challenging but essential area of study in cortical computation and sensory processing.

Chandelier Cells in Functional and Dysfunctional Neural Circuits

Chandelier cells (ChCs; also called axo-axonic cells) are a specialized GABAergic interneuron subtype that selectively innervates pyramidal neurons at the axon initial segment (AIS), the site of action potential generation. ChC connectivity allows for powerful yet precise modulation of large populations of pyramidal cells, suggesting ChCs have a critical role in brain functions. Dysfunctions in ChC connectivity are associated with brain disorders such as epilepsy and schizophrenia; however, whether this is causative, contributory or compensatory is not known. A likely stumbling block toward mechanistic discoveries and uncovering potential therapeutic targets is the apparent lack of rudimentary understanding of ChCs. For example, whether cortical ChCs are inhibitory or excitatory remains unresolved, and thus whether altered ChC activity results in altered inhibition or excitation is not clear. Recent studies have shed some light onto this excitation-inhibition controversy. In addition, new findings have identified preferential cell-type connectivities established by cortical ChCs, greatly expanding our understanding of the role of ChCs in the cortical microcircuit. Here we aim to bring more attention to ChC connectivity to better understand its role in neural circuits, address whether ChCs are inhibitory or excitatory in light of recent findings and discuss ChC dysfunctions in brain disorders.

Differential Pharmacological Regulation of Sensorimotor Gating Deficit in CB1 Knockout Mice and Associated Neurochemical and Histological Alterations

The endocannabinoid system has been widely involved in the pathophysiology of sensorimotor gating deficits. This study aimed to evaluate the pharmacological modulation of the sensorimotor gating impairment induced by cannabinoid CB1 receptor (CB1r) deletion. For this purpose, the prepulse inhibition (PPI) paradigm was used to evaluate the effect of two antipsychotics drugs (risperidone and haloperidol) and a psychostimulant (methylphenidate) on the preattentional deficit presented by CB1KO mice. Furthermore, the effects of the CB1r antagonist AM251 on PPI were evaluated in WT mice. Real-time PCR and immunohistochemical studies were carried out to analyze dopamine transporter (DAT) and α-2C adrenergic receptor (ADRA2C) gene expressions and the distribution of parvalbumin (PV) and cholecystokinin-8 (CCK) immunoreactive (ir) cortical neurons, respectively. Neither risperidone nor haloperidol significantly modified the PPI of WT and CB1KO mice, whereas methylphenidate improved the preattentional deficit of CB1KO mice. In addition, treatment with AM251 (3 mg/kg; i.p.) significantly decreased the PPI of WT animals. The administration of methylphenidate increased DAT and ADRA2C gene expressions in CB1KO mice without producing any effect in WT animals. Immunohistochemical studies revealed that there were no significant changes in CCK immunolabeling between WT and CB1KO mice, whereas the radial distribution of PV-ir neurons was abnormal in CB1KO mice. These data further support the important role of CB1r in sensorimotor gating regulation and the therapeutic usefulness of methylphenidate for the treatment of psychiatric disorders with associated preattentional deficits.

Strategically positioned inhibitory synapses of axo-axonic cells potently control principal neuron spiking in the basolateral amygdala

Axo-axonic cells (AACs) in cortical regions selectively innervate the axon initial segments (AISs) of principal cells (PCs), where the action potentials are generated. These GABAergic interneurons can alter the activity of PCs, but how the efficacy of spike control correlates with the number of output synapses remains unclear. Moreover, the relationship between the spatial distribution of GABAergic synapses and the action potential initiation site along the AISs is not well defined. Using paired recordings obtained in the mouse basolateral amygdala, we found that AACs powerfully inhibited or delayed the timing of PC spiking by 30 ms, if AAC output preceded PC spiking with no more than 80 ms. By correlating the number of synapses and the probability of spiking, we revealed that larger numbers of presynaptic AAC boutons giving rise to larger postsynaptic responses provided more effective inhibition of PC spiking. At least 10-12 AAC synapses, which could originate from 2-3 AACs on average, were necessary to veto the PC firing under our recording conditions. Furthermore, we determined that the threshold for the action potential generation along PC axons is the lowest between 20 and 40 μm from soma, which axonal segment received the highest density of GABAergic inputs. Single AACs preferentially innervated this narrow portion of the AIS where action potentials were generated with the highest likelihood, regardless of the number of synapses forming a given connection. Our results uncovered a fine organization of AAC innervation maximizing their inhibitory efficacy by strategically positioning synapses along the AISs.

Molecular identity of axonal sodium channels in human cortical pyramidal cells

Studies in rodents revealed that selective accumulation of Na⁺ channel subtypes at the axon initial segment (AIS) determines action potential (AP) initiation and backpropagation in cortical pyramidal cells (PCs); however, in human cortex, the molecular identity of Na⁺ channels distributed at PC axons, including the AIS and the nodes of Ranvier, remains unclear. We performed immunostaining experiments in human cortical tissues removed surgically to cure brain diseases. We found strong immunosignals of Na⁺ channels and two channel subtypes, Na_V1.2 and Na_V1.6, at the AIS of human cortical PCs. Although both channel subtypes were expressed along the entire AIS, the peak immunosignals of Na_V1.2 and Na_V1.6 were found at proximal and distal AIS regions, respectively. Surprisingly, in addition to the presence of Na_V1.6 at the nodes of Ranvier, Na_V1.2 was also found in a subpopulation of nodes in the adult human cortex, different from the absence of Na_V1.2 in myelinated axons in rodents. Na_V1.1 immunosignals were not detected at either the AIS or the nodes of Ranvier of PCs; however, they were expressed at interneuron axons with different distribution patterns. Further experiments revealed that parvalbumin-positive GABAergic axon cartridges selectively innervated distal AIS regions with relatively high immunosignals of Na_V1.6 but not the proximal Na_V1.2-enriched compartments, suggesting an important role of axo-axonic cells in regulating AP initiation in human PCs. Together, our results show that both Na_V1.2 and Na_V1.6 (but not Na_V1.1) channel subtypes are expressed at the AIS and the nodes of Ranvier in adult human cortical PCs, suggesting that these channel subtypes control neuronal excitability and signal conduction in PC axons.

Thorough GABAergic innervation of the entire axon initial segment revealed by an optogenetic 'laserspritzer'

GABAergic terminals of chandelier cells exclusively innervate the axon initial segment (AIS) of excitatory neurons. Although the anatomy of these synapses has been well-studied in several brain areas, relatively little is known about their physiological properties. Using vesicular γ-aminobutyric acid transporter-channelrhodopsin 2-enhanced yellow fluorescence protein (VGAT-ChR2-YFP)-expressing mice and a novel fibreoptic 'laserspritzer' approach that we developed, we investigated the physiological properties of axo-axonic synapses (AASs) in brain slices from the piriform cortex (PC) of mice. AASs were in close proximity to voltage-gated Na(+) (NaV) channels located at the AIS. AASs were selectively activated by a 5 μm laserspritzer placed in close proximity to the AIS. Under a minimal laser stimulation condition and using whole-cell somatic voltage-clamp recordings, the amplitudes and kinetics of IPSCs mediated by AASs were similar to those mediated by perisomatic inhibitions. Results were further validated with channelrhodopsin 2-assisted circuit mapping (CRACM) of the entire inhibitory inputs map. For the first time, we revealed that the laserspritzer-induced AAS-IPSCs persisted in the presence of TTX and TEA but not 4-AP. Next, using gramicidin-based perforated patch recordings, we found that the GABA reversal potential (EGABA) was -73.6 ± 1.2 mV when induced at the AIS and -72.8 ± 1.1 mV when induced at the perisomatic site. Our anatomical and physiological results lead to the novel conclusions that: (1) AASs innervate the entire length of the AIS, as opposed to forming a highly concentrated cartridge, (2) AAS inhibition suppresses action potentials and epileptiform activity more robustly than perisomatic inhibitions, and (3) AAS activation alone can be sufficient to inhibit action potential generation and epileptiform activities in vitro.

Spatial distribution of neurons innervated by chandelier cells

Chandelier (or axo-axonic) cells are a distinct group of GABAergic interneurons that innervate the axon initial segments of pyramidal cells and are thus thought to have an important role in controlling the activity of cortical circuits. To examine the circuit connectivity of chandelier cells (ChCs), we made use of a genetic targeting strategy to label neocortical ChCs in upper layers of juvenile mouse neocortex. We filled individual ChCs with biocytin in living brain slices and reconstructed their axonal arbors from serial semi-thin sections. We also reconstructed the cell somata of pyramidal neurons that were located inside the ChC axonal trees and determined the percentage of pyramidal neurons whose axon initial segments were innervated by ChC terminals. We found that the total percentage of pyramidal neurons that were innervated by a single labeled ChC was 18–22 %. Sholl analysis showed that this percentage peaked at 22–35 % for distances between 30 and 60 µm from the ChC soma, decreasing to lower percentages with increasing distances. We also studied the three-dimensional spatial distribution of the innervated neurons inside the ChC axonal arbor using spatial statistical analysis tools. We found that innervated pyramidal neurons are not distributed at random, but show a clustered distribution, with pockets where almost all cells are innervated and other regions within the ChC axonal tree that receive little or no innervation. Thus, individual ChCs may exert a strong, widespread influence on their local pyramidal neighbors in a spatially heterogeneous fashion.

An enhanced role and expanded developmental origins for gamma-aminobutyric acidergic interneurons in the human cerebral cortex

Human beings have considerably expanded cognitive abilities compared with all other species and they also have a relatively larger cerebral cortex compared with their body size. But is a bigger brain the only reason for higher cognition or have other features evolved in parallel? Humans have more and different types of GABAergic interneurons, found in different places, than our model species. Studies are beginning to show differences in function. Is this expanded repertoire of functional types matched by an evolution of their developmental origins? Recent studies support the idea that generation of interneurons in the ventral telencephalon may be more complicated in primates, which have evolved a large and complex outer subventricular zone in the ganglionic eminences. In addition, proportionally more interneurons appear to be produced in the caudal ganglionic eminence, the majority of which populate the superficial layers of the cortex. Whether or not the cortical proliferative zones are a source of interneurogenesis, and to what extent and of what significance, is a contentious issue. As there is growing evidence that conditions such as autism, schizophrenia and congenital epilepsy may have developmental origins in the failure of interneuron production and migration, it is important we understand fully the similarities and differences between human development and our animal models.

The chandelier cell, form and function

Among γ-aminobutyric acid (GABA) interneurons, the chandelier cell (ChC) has captured the interest of neuroscientists for a very long time as a subtype not described by Ramon y Cajal. ChCs feature an axonal arborization that selectively innervates the axon initial segments of pyramidal cells. Recent studies involving transgenic mice have identified intriguing features of ChCs, including a remarkably specific spatial and temporal origins, their capacity to have either excitatory or inhibitory influences on pyramidal neurons, and their synaptic alterations in schizophrenia. This review explores these and other developmental and functional aspects of this fascinating cortical neuronal subtype.

Genetic dissection of GABAergic neural circuits in mouse neocortex

Diverse and flexible cortical functions rely on the ability of neural circuits to perform multiple types of neuronal computations. GABAergic inhibitory interneurons significantly contribute to this task by regulating the balance of activity, synaptic integration, spiking, synchrony, and oscillation in a neural ensemble. GABAergic interneurons display a high degree of cellular diversity in morphology, physiology, connectivity, and gene expression. A considerable number of subtypes of GABAergic interneurons diversify modes of cortical inhibition, enabling various types of information processing in the cortex. Thus, comprehensively understanding fate specification, circuit assembly, and physiological function of GABAergic interneurons is a key to elucidate the principles of cortical wiring and function. Recent advances in genetically encoded molecular tools have made a breakthrough to systematically study cortical circuitry at the molecular, cellular, circuit, and whole animal levels. However, the biggest obstacle to fully applying the power of these to analysis of GABAergic circuits was that there were no efficient and reliable methods to express them in subtypes of GABAergic interneurons. Here, I first summarize cortical interneuron diversity and current understanding of mechanisms, by which distinct classes of GABAergic interneurons are generated. I then review recent development in genetically encoded molecular tools for neural circuit research, and genetic targeting of GABAergic interneuron subtypes, particularly focusing on our recent effort to develop and characterize Cre/CreER knockin lines. Finally, I highlight recent success in genetic targeting of chandelier cells, the most unique and distinct GABAergic interneuron subtype, and discuss what kind of questions need to be addressed to understand development and function of cortical inhibitory circuits.

Regulation of chandelier cell cartridge and bouton development via DOCK7-mediated ErbB4 activation

Chandelier cells (ChCs), typified by their unique axonal morphology, are the most distinct interneurons present in cortical circuits. Via their distinctive axonal terminals, called cartridges, these cells selectively target the axon initial segment of pyramidal cells and control action potential initiation; however, the mechanisms that govern the characteristic ChC axonal structure have remained elusive. Here, by employing an in utero electroporation-based method that enables genetic labeling and manipulation of ChCs in vivo, we identify DOCK7, a member of the DOCK180 family, as a molecule essential for ChC cartridge and bouton development. Furthermore, we present evidence that DOCK7 functions as a cytoplasmic activator of the schizophrenia-associated ErbB4 receptor tyrosine kinase and that DOCK7 modulates ErbB4 activity to control ChC cartridge and bouton development. Thus, our findings define DOCK7 and ErbB4 as key components of a pathway that controls the morphological differentiation of ChCs, with implications for the pathogenesis of schizophrenia.

Erbb4 deletion from fast-spiking interneurons causes schizophrenia-like phenotypes

Genetic variation in neuregulin and its ErbB4 receptor has been linked to schizophrenia, although little is known about how they contribute to the disease process. Here, we have examined conditional Erbb4 mouse mutants to study how disruption of specific inhibitory circuits in the cerebral cortex may cause large-scale functional deficits. We found that deletion of ErbB4 from the two main classes of fast-spiking interneurons, chandelier and basket cells, causes relatively subtle but consistent synaptic defects. Surprisingly, these relatively small wiring abnormalities boost cortical excitability, increase oscillatory activity, and disrupt synchrony across cortical regions. These functional deficits are associated with increased locomotor activity, abnormal emotional responses, and impaired social behavior and cognitive function. Our results reinforce the view that dysfunction of cortical fast-spiking interneurons might be central to the pathophysiology of schizophrenia.

Dense and overlapping innervation of pyramidal neurons by chandelier cells

Chandelier (or axo-axonic) cells are a distinct group of GABAergic interneurons that innervate the axon initial segments of pyramidal cells and thus could have an important role controlling the activity of cortical circuits. To understand their connectivity, we labeled upper layers chandelier cells (ChCs) from mouse neocortex with a genetic strategy and studied how their axons contact local populations of pyramidal neurons, using immunohistochemical detection of axon initial segments. We studied ChCs located in the border of layers 1 and 2 from primary somatosensory cortex and found that practically all ChC axon terminals contact axon initial segments, with an average of three to five boutons per cartridge. By measuring the number of putative GABAergic synapses in initial segments, we estimate that each pyramidal neuron is innervated, on average, by four ChCs. Additionally, each individual ChC contacts 35-50% of pyramidal neurons within the areas traversed by its axonal arbor, with pockets of very high innervation density. Finally, ChCs have similar innervation patterns at different postnatal ages (P18-P90), with only relatively small lateral expansions of their arbor and increases in the total number of their cartridges during the developmental period analyzed. We conclude that ChCs innervate neighboring pyramidal neurons in a dense and overlapping manner, a connectivity pattern that could enable ChCs to exert a widespread influence on their local circuits.

Functional diversity of supragranular GABAergic neurons in the barrel cortex

Although the neocortex forms a distributed system comprised of several functional areas, its vertical columnar organization is largely conserved across areas and species, suggesting the existence of a canonical neocortical microcircuit. In order to elucidate the principles governing the organization of such a cortical diagram, a detailed understanding of the dynamics binding different types of cortical neurons into a coherent algorithm is essential. Within this complex circuitry, GABAergic interneurons, while forming approximately only 15–20% of all cortical neurons, appear critical in maintaining a dynamic balance between excitation and inhibition. Despite their importance, cortical GABAergic neurons have not been extensively studied in vivo and their precise role in shaping the local microcircuit sensory response still remains to be determined. Their paucity, combined with their molecular, anatomical, and physiological diversity, has made it difficult to even establish a consensual nomenclature. However, recent technological advances in microscopy and mouse genetics have fostered a renewed interest in neocortical interneurons by putting them within “visible” reach of experimenters. The anatomically well-defined whisker-to-barrel pathway of the rodent is particularly amenable to studies attempting to link cortical circuit dynamics to behavior. To each whisker corresponds a discrete cortical unit equivalent to a single column, specialized in the encoding and processing of the sensory information it receives. In this review, we will focus on the functional role that each subtype of supragranular GABAergic neuron embedded within such a single neocortical unit may play in shaping the dynamics of the local circuit during somatosensory integration.

Characterization of axo-axonic synapses in the piriform cortex of Mus musculus

Previous anatomical and physiological studies have established major glutamatergic and GABAergic neuronal subtypes within the piriform cortical circuits. However, quantitative information regarding axo-axonic inhibitory synapses mediated by chandelier cells across major cortical subdivisions of piriform cortex is lacking. Therefore, we examined the properties of these synapses across the entire piriform cortex. Our results show the following. 1) γ-Aminobutyric acid membrane transporter 1-positive varicosities, whose appearance resembles chandelier cartridges, are found around the initial segments of axons of glutamatergic cells across layers II and III. 2) Both the density of axo-axonic cartridges and the degree of γ-aminobutyric acid membrane transporter 1 innervation in each axo-axonic synapse are significantly higher in the piriform cortex than in the neocortex. 3) Glutamate decarboxylase 67, vesicular GABA transporter, and parvalbumin, but not calbindin, are colocalized with the presynaptic varicosities, whereas gephyrin, Na-K-2Cl cotransporter 1, and GABA(A) receptor α1 subunit, but not K-Cl cotransporter 2, are colocalized at the presumed postsynaptic sites. 4) The axo-axonic cartridges innervate the majority of excitatory neurons and are distributed more frequently in putative centrifugal cells and posterior piriform cortex. We further describe the morphology of chandelier cells by using parvalbumin-immunoreactivity and single-cell labeling. In summary, our results demonstrate that a small population of chandelier cells mediates abundant axo-axonic synapses across the entire piriform cortex. Because of the critical location of these inhibitory synapses in relation to action potential regulation, our results highlight a critical role of axo-axonic synapses in regulating information flow and olfactory-related oscillations within the piriform cortex in vivo.

State-dependent function of neocortical chandelier cells

Chandelier (axoaxonic) cells (ChCs) are a distinct group of GABAergic interneurons that innervate the axon initial segments of pyramidal cells. However, their circuit role and the function of their clearly defined anatomical specificity remain unclear. Recent work has demonstrated that chandelier cells can produce depolarizing GABAergic PSPs, occasionally driving postsynaptic targets to spike. On the other hand, other work suggests that ChCs are hyperpolarizing and may have an inhibitory role. These disparate functional effects may reflect heterogeneity among ChCs. Here, using brain slices from transgenic mouse strains, we first demonstrate that, across different neocortical areas and genetic backgrounds, upper Layer 2/3 ChCs belong to a single electrophysiologically and morphologically defined population, extensively sampling Layer 1 inputs with asymmetric dendrites. Consistent with being a single cell type, we find electrical coupling between ChCs. We then investigate the effect of chandelier cell activation on pyramidal neuron spiking in several conditions, ranging from the resting membrane potential to stimuli designed to approximate in vivo membrane potential dynamics. We find that under quiescent conditions, chandelier cells are capable of both promoting and inhibiting spike generation, depending on the postsynaptic membrane potential. However, during in vivo-like membrane potential fluctuations, the dominant postsynaptic effect was a strong inhibition. Thus, neocortical chandelier cells, even from within a homogeneous population, appear to play a dual role in the circuit, helping to activate quiescent pyramidal neurons, while at the same time inhibiting active ones.

GABAergic complex basket formations in the human neocortex

Certain GABAergic interneurons in the cerebral cortex, basket cells, establish multiple connections with cell bodies that typically outline the somata and proximal dendrites of pyramidal cells. During studies into the distribution of the vesicular GABA transporter (VGAT) in the human cerebral cortex, we were struck by the presence of a very dense, pericellular arrangement of multiple VGAT-immunoreactive (-ir) terminals in certain cortical areas. We called these terminals "Complex basket formations" (Cbk-formations) to distinguish them from the simpler and more typical pericellular GABAergic innervations of most cortical neurons. Here we examined the distribution of these VGAT-ir Cbk-formations in various cortical areas, including the somatosensory (area 3b), visual (areas 17 and 18), motor (area 4), associative frontal (dorsolateral areas 9, 10, 45, 46, and orbital areas 11, 12, 13, 14, 47), associative temporal (areas 20, 21, 22, and 38), and limbic cingulate areas (areas 24, 32). Furthermore, we used dual or triple staining techniques to study the chemical nature of the innervated cells. We found that VGAT-ir Cbk-formations were most frequently found in area 4 followed by areas 3b, 13, and 18. In addition, they were mostly observed in layer III, except in area 17, where they were most dense in layer IV. We also found that 70% of the innervated neurons were pyramidal cells, while the remaining 30% were multipolar cells. Most of these multipolar cells expressed the calcium-binding protein parvalbumin and the lectin Vicia villosa agglutinin.

Chandelier and interfascicular neurons in the adult mouse piriform cortex

The structure of two neuron types native to the adult mouse piriform cortex (PC) is described. The first cell, termed an interfascicular neuron (IFN), lies between the axon fascicles of layer I. The IFN axon divides dichotomously and daughter fibrils run horizontally in the domain of layer Ia. The frequent apposition of the IFN axon to distal dendrites of the underlying pyramidal cells suggests an en passage synaptic interaction with them. A second neuron observed in layer II, or less frequently in layer III, matched in most respects the structure of the chandelier cell (CC) described elsewhere in the neo- and archi-cortex. In the PC, chandelier cells (PC-CC) display the following peculiarities. First, the PC-CC axonal field distributes in the neuropil of layers II and III and candlesticks are in close apposition to the initial axonal segment of the pyramidal cell, although somatic interactions cannot be rule out. Second, the PC-CC ascending dendrites pierce layer I, receiving short collaterals and boutons en passage from the olfactory axons therein. The possible role of IFN's and PC-CC and their interactions with the adjacent cells is discussed in the broad context of the cellular organization of the PC.

Local circuits targeting parvalbumin-containing interneurons in layer IV of rat barrel cortex

Interactions between inhibitory interneurons and excitatory spiny neurons and also other inhibitory cells represent fundamental network properties which cause the so-called thalamo-cortical response transformation and account for the well-known receptive field differences of cortical layer IV versus thalamic neurons. We investigated the currently largely unknown morphological basis of these interactions utilizing acute slice preparations of barrel cortex in P19-21 rats. Layer IV spiny (spiny stellate, star pyramidal and pyramidal) neurons or inhibitory (basket and bitufted) interneurons were electrophysiologically characterized and intracellularly biocytin-labeled. In the same slice, we stained parvalbumin-immunoreactive (PV-ir) interneurons as putative target cells after which the tissue was subjected to confocal image acquisition. Parallel experiments confirmed the existence of synaptic contacts in these types of connection by correlated light and electron microscopy. The axons of the filled neurons differentially targeted barrel PV-ir interneurons: (1) The relative number of all contacted PV-ir cells within the axonal sphere was 5-17% for spiny (n = 10), 32 and 58% for basket (n = 2) and 12 and 13% for bitufted (n = 2) cells. (2) The preferential subcellular site which was contacted on PV-ir target cells was somatic for four and dendritic for five spiny cells; for basket cells, there was a somatic and for bitufted cells a dendritic preference in each examined case. (3) The highest number of contacts on a single PV-ir cell was 9 (4 somatic and 5 dendritic) for spiny neurons, 15 (10 somatic and 5 dendritic) for basket cells and 4 (1 somatic and 3 dendritic) for bitufted cells. These patterns suggest a cell type-dependent communication within layer IV microcircuits in which PV-ir interneurons provide not only feed-forward but also feedback inhibition thus triggering the thalamo-cortical response transformation.

Cell-type-dependent molecular composition of the axon initial segment

The exact site of initiation and shape of action potentials vary among different neuronal types. The reason for this variability is largely unknown, but the subunit composition, density and distribution of voltage-gated sodium (Nav) and potassium (Kv) channels within the axon initial segment (AIS) are likely to play a key role. Here, we asked how heterogeneous are the density and distribution of Nav and Kv channels within the AISs of a variety of excitatory and inhibitory neurons. Most of the studied cell types expressed a high density of Nav1.6, Kv1.1, and Kv1.2 subunits in their AIS, but the Nav1.1 subunit could only be detected in GABAergic interneurons. A proximo-distal gradient in the density of these subunits was observed within the AIS of certain nerve cells but not in others. For example, a gradual increase of the Nav1.6 subunit was observed in cortical layer 2/3 and hippocampal CA1 pyramidal cell (PC) AISs, whereas its density was rather uniform in layer 5 PC AISs. The Nav1.1 subunit was distributed evenly along the AIS of short-axon cells of the main olfactory bulb but was restricted to the proximal part of the AIS in cortical and cerebellar interneurons. Our results reveal a cell type-dependent expression of sodium and potassium channel subunits with varying densities along the proximo-distal axis of the AISs. This precise arrangement is likely to contribute to the diversity of firing properties observed among central neurons.

Of mice and men, and chandeliers

How does the human neocortex reliably propagate information through neural circuits? One mechanism appears to involve relying on strong connections from pyramidal neurons to interneurons and a depolarizing action of cortical chandelier cells.