Inhibitory interneurons and their circuit motifs in the many layers of the barrel cortex

Genetic approaches to elucidating cortical and hippocampal GABAergic interneuron diversity

GABAergic interneurons (INs) in the mammalian forebrain represent a diverse population of cells that provide specialized forms of local inhibition to regulate neural circuit activity. Over the last few decades, the development of a palette of genetic tools along with the generation of single-cell transcriptomic data has begun to reveal the molecular basis of IN diversity, thereby providing deep insights into how different IN subtypes function in the forebrain. In this review, we outline the emerging picture of cortical and hippocampal IN speciation as defined by transcriptomics and developmental origin and summarize the genetic strategies that have been utilized to target specific IN subtypes, along with the technical considerations inherent to each approach. Collectively, these methods have greatly facilitated our understanding of how IN subtypes regulate forebrain circuitry via cell type and compartment-specific inhibition and thus have illuminated a path toward potential therapeutic interventions for a variety of neurocognitive disorders.

Modulation of large rhythmic depolarizations in human large basket cells by norepinephrine and acetylcholine

Rhythmic brain activity is critical to many brain functions and is sensitive to neuromodulation, but so far very few studies have investigated this activity on the cellular level in vitro in human brain tissue samples. This study reveals and characterizes a novel rhythmic network activity in the human neocortex. Using intracellular patch-clamp recordings of human cortical neurons, we identify large rhythmic depolarizations (LRDs) driven by glutamate release but not by GABA. These LRDs are intricate events made up of multiple depolarizing phases, occurring at ~0.3 Hz, have large amplitudes and long decay times. Unlike human tissue, rat neocortex layers 2/3 exhibit no such activity under identical conditions. LRDs are mainly observed in a subset of L2/3 interneurons that receive substantial excitatory inputs and are likely large basket cells based on their morphology. LRDs are highly sensitive to norepinephrine (NE) and acetylcholine (ACh), two neuromodulators that affect network dynamics. NE increases LRD frequency through β-adrenergic receptor activity while ACh decreases it via M4 muscarinic receptor activation. Multi-electrode array recordings show that NE enhances and synchronizes oscillatory network activity, whereas ACh causes desynchronization. Thus, NE and ACh distinctly modulate LRDs, exerting specific control over human neocortical activity.

A latent pool of neurons silenced by sensory-evoked inhibition can be recruited to enhance perception

To investigate which activity patterns in sensory cortex are relevant for perceptual decision-making, we combined two-photon calcium imaging and targeted two-photon optogenetics to interrogate barrel cortex activity during perceptual discrimination. We trained mice to discriminate bilateral whisker deflections and report decisions by licking left or right. Two-photon calcium imaging revealed sparse coding of contralateral and ipsilateral whisker input in layer 2/3, with most neurons remaining silent during the task. Activating pyramidal neurons using two-photon holographic photostimulation evoked a perceptual bias that scaled with the number of neurons photostimulated. This effect was dominated by optogenetic activation of non-coding neurons, which did not show sensory or motor-related activity during task performance. Photostimulation also revealed potent recruitment of cortical inhibition during sensory processing, which strongly and preferentially suppressed non-coding neurons. Our results suggest that a pool of non-coding neurons, selectively suppressed by network inhibition during sensory processing, can be recruited to enhance perception.

Synaptic Organization-Function Relationships of Amygdala Interneurons Supporting Associative Learning

Coordinated activity of basolateral amygdala (BLA) GABAergic interneurons (INs) and glutamatergic principal cells (PCs) is critical for associative learning, however the microcircuit organization-function relationships of distinct IN classes remain uncertain. Here, we show somatostatin (SOM) INs provide inhibition onto, and are excited by, local PCs, whereas vasoactive intestinal peptide (VIP) INs are driven by extrinsic afferents. Parvalbumin (PV) INs inhibit PCs and are activated by local and extrinsic inputs. Thus, SOM and VIP INs exhibit complementary roles in feedback and feedforward inhibition, respectively, while PV INs contribute to both microcircuit motifs. Functionally, each IN subtype reveals unique activity patterns across fear- and extinction learning with SOM and VIP INs showing most divergent characteristics, and PV INs display an intermediate phenotype parallelling synaptic data. Finally, SOM and PV INs dynamically track behavioral state transitions across learning. These data provide insight into the synaptic microcircuit organization-function relationships of distinct BLA IN classes.

Inter and intralaminar excitation of parvalbumin interneurons in mouse barrel cortex

Parvalbumin (PV) interneurons are inhibitory fast-spiking cells with essential roles in directing the flow of information through cortical circuits. These neurons set the balance between excitation and inhibition and control rhythmic activity. PV interneurons differ between cortical layers in their morphology, circuitry, and function, but how their electrophysiological properties vary has received little attention. Here we investigate responses of PV interneurons in different layers of primary somatosensory barrel cortex (BC) to different excitatory inputs. With the genetically-encoded hybrid voltage sensor, hVOS, we recorded voltage changes in many L2/3 and L4 PV interneurons simultaneously, with stimulation applied to either L2/3 or L4. A semi-automated procedure was developed to identify small regions of interest corresponding to single responsive PV interneurons. Amplitude, half-width, and rise-time were greater for PV interneurons residing in L2/3 compared to L4. Stimulation in L2/3 elicited responses in both L2/3 and L4 with longer latency compared to stimulation in L4. These differences in latency between layers could influence their windows for temporal integration. Thus, PV interneurons in different cortical layers of BC respond in a layer specific and input specific manner, and these differences have potential roles in cortical computations.

Functional roles of descending projections from the cerebral cortex to the trigeminal spinal subnucleus caudalis in orofacial nociceptive information processing

BACKGROUND

The trigeminal spinal subnucleus caudalis (Sp5C), also known as the medullary dorsal horn, receives orofacial somatosensory inputs, particularly nociceptive inputs, from the trigeminal nerve. In the Sp5C, excitatory and inhibitory neurons, glutamatergic and GABAergic/glycinergic neurons, respectively, form the local circuits. The axons of the glutamatergic neurons in lamina I ascend toward the thalamic and parabrachial nuclei, and this projection is the main pathway of orofacial nociception. Additionally, the axons of the higher brain regions, including the locus coeruleus, dorsal raphe, and cerebral cortex, are sent to the Sp5C.

HIGHLIGHT

Among these descending projections, this review focuses on the functional profiles of the corticotrigeminal projections to the Sp5C, along with their anatomical aspects. The primary and secondary somatosensory and insular cortices are of particular interest.

CONCLUSION

Corticotrigeminal projections from the somatosensory cortex to the Sp5C play a suppressive role in nociceptive information processing, whereas recent studies have demonstrated a facilitative role of the insular cortex in nociceptive information processing at the Sp5C level.

Multimodal cortical neuronal cell type classification

Since more than a century, neuroscientists have distinguished excitatory (glutamatergic) neurons with long-distance projections from inhibitory (GABAergic) neurons with local projections and established layer-dependent schemes for the ~ 80% excitatory (principal) cells as well as the ~ 20% inhibitory neurons. Whereas, in the early days, mainly morphological criteria were used to define cell types, later supplemented by electrophysiological and neurochemical properties, nowadays. single-cell transcriptomics is the method of choice for cell type classification. Bringing recent insight together, we conclude that despite all established layer- and area-dependent differences, there is a set of reliably identifiable cortical cell types that were named (among others) intratelencephalic (IT), extratelencephalic (ET), and corticothalamic (CT) for the excitatory cells, which altogether comprise ~ 56 transcriptomic cell types (t-types). By the same means, inhibitory neurons were subdivided into parvalbumin (PV), somatostatin (SST), vasoactive intestinal polypeptide (VIP), and "other (i.e. Lamp5/Sncg)" subpopulations, which altogether comprise ~ 60 t-types. The coming years will show which t-types actually translate into "real" cell types that show a common set of multimodal features, including not only transcriptome but also physiology and morphology as well as connectivity and ultimately function. Only with the better knowledge of clear-cut cell types and experimental access to them, we will be able to reveal their specific functions, a task which turned out to be difficult in a part of the brain being so much specialized for cognition as the cerebral cortex.

Conditioning and pseudoconditioning differently change intrinsic excitability of inhibitory interneurons in the neocortex

Many studies indicate a broad role of various classes of GABAergic interneurons in the processes related to learning. However, little is known about how the learning process affects intrinsic excitability of specific classes of interneurons in the neocortex. To determine this, we employed a simple model of conditional learning in mice where vibrissae stimulation was used as a conditioned stimulus and a tail shock as an unconditioned one. In vitro whole-cell patch-clamp recordings showed an increase in intrinsic excitability of low-threshold spiking somatostatin-expressing interneurons (SST-INs) in layer 4 (L4) of the somatosensory (barrel) cortex after the conditioning paradigm. In contrast, pseudoconditioning reduced intrinsic excitability of SST-LTS, parvalbumin-expressing interneurons (PV-INs), and vasoactive intestinal polypeptide-expressing interneurons (VIP-INs) with accommodating pattern in L4 of the barrel cortex. In general, increased intrinsic excitability was accompanied by narrowing of action potentials (APs), whereas decreased intrinsic excitability coincided with AP broadening. Altogether, these results show that both conditioning and pseudoconditioning lead to plastic changes in intrinsic excitability of GABAergic interneurons in a cell-specific manner. In this way, changes in intrinsic excitability can be perceived as a common mechanism of learning-induced plasticity in the GABAergic system.

Intersectional strategy to study cortical inhibitory parvalbumin-expressing interneurons

Parvalbumin-expressing (PV) interneurons are key neuronal elements to a global excitatory-inhibitory balance in normal cortical functioning. To better understand the circuit functions of PV interneurons, reliable animal models are needed. This study investigated the sensitivity and specificity of the most frequently used PV-Cre/tdTomato mouse line in this regard. The colocalization of the transgene (tdTomato) with the parvalbumin protein, with GAD1 (a conclusive inhibitory cell marker) and Vglut1 (a conclusive excitatory cell marker) as well as with a marker for perineuronal nets (WFA) was assessed and a substantial proportion of layer 5 PV neurons was found to be excitatory and not inhibitory in the PV-Cre/tdTomato mouse. The intersectional transgenic mouse line Vgat-Cre/PV-Flp/tdTomato provided a solution, since no colocalization of tdTomato with the Vglut1 probe was found there. In conclusion, the Vgat-Cre/PV-Flp/tdTomato mouse line seems to be a more reliable animal model for functional studies of GABAergic PV interneurons.

Activation of M1 cholinergic receptors in mouse somatosensory cortex enhances information processing and detection behaviour

To optimise sensory representations based on environmental demands, the activity of cortical neurons is regulated by neuromodulators such as Acetylcholine (ACh). ACh is implicated in cognitive functions including attention, arousal and sleep cycles. However, it is not clear how specific ACh receptors shape the activity of cortical neurons in response to sensory stimuli. Here, we investigate the role of a densely expressed muscarinic ACh receptor M1 in information processing in the mouse primary somatosensory cortex and its influence on the animal's sensitivity to detect vibrotactile stimuli. We show that M1 activation results in faster and more reliable neuronal responses, manifested by a significant reduction in response latencies and the trial-to-trial variability. At the population level, M1 activation reduces the network synchrony, and thus enhances the capacity of cortical neurons in conveying sensory information. Consistent with the neuronal findings, we show that M1 activation significantly improves performances in a vibriotactile detection task.

Synaptic configuration and reconfiguration in the neocortex are spatiotemporally selective

Brain computation relies on the neural networks. Neurons extend the neurites such as dendrites and axons, and the contacts of these neurites that form chemical synapses are the biological basis of signal transmissions in the central nervous system. Individual neuronal outputs can influence the other neurons within the range of the axonal spread, while the activities of single neurons can be affected by the afferents in their somatodendritic fields. The morphological profile, therefore, binds the functional role each neuron can play. In addition, synaptic connectivity among neurons displays preference based on the characteristics of presynaptic and postsynaptic neurons. Here, the author reviews the "spatial" and "temporal" connection selectivity in the neocortex. The histological description of the neocortical circuitry depends primarily on the classification of cell types, and the development of gene engineering techniques allows the cell type-specific visualization of dendrites and axons as well as somata. Using genetic labeling of particular cell populations combined with immunohistochemistry and imaging at a subcellular spatial resolution, we revealed the "spatial selectivity" of cortical wirings in which synapses are non-uniformly distributed on the subcellular somatodendritic domains in a presynaptic cell type-specific manner. In addition, cortical synaptic dynamics in learning exhibit presynaptic cell type-dependent "temporal selectivity": corticocortical synapses appear only transiently during the learning phase, while learning-induced new thalamocortical synapses persist, indicating that distinct circuits may supervise learning-specific ephemeral synapse and memory-specific immortal synapse formation. The selectivity of spatial configuration and temporal reconfiguration in the neural circuitry may govern diverse functions in the neocortex.

Axons of cortical basket cells originating from dendrites develop higher local complexity than axons emerging from basket cell somata

Neuronal differentiation is regulated by neuronal activity. Here, we analyzed dendritic and axonal growth of Basket cells (BCs) and non-Basket cells (non-BCs) using sparse transfection of channelrhodopsin-YFP and repetitive optogenetic stimulation in slice cultures of rat visual cortex. Neocortical interneurons often display axon-carrying dendrites (AcDs). We found that the AcDs of BCs and non-BCs were, on average, the most complex dendrites. Further, the AcD configuration had an influence on BC axonal development. Axons originating from an AcD formed denser arborizations with more terminal endings within the dendritic field of the parent cell. Intriguingly, this occurred already in unstimulated BCs, and complexity was not increased further by optogenetic stimulation. However, optogenetic stimulation exerted a growth-promoting effect on axons emerging from BC somata. The axons of non-BCs neither responded to the AcD configuration nor to the optogenetic stimulation. The results suggest that the formation of locally dense BC plexuses is regulated by spontaneous activity. Moreover, in the AcD configuration, the AcD and the axon it carries mutually support each other's growth.

Recovery kinetics of short-term depression of GABAergic and glutamatergic synapses at layer 2/3 pyramidal cells in the mouse barrel cortex

Introduction

Short-term synaptic plasticity (STP) is a widespread mechanism underlying activity-dependent modifications of cortical networks.

Methods

To investigate how STP influences excitatory and inhibitory synapses in layer 2/3 of mouse barrel cortex, we combined whole-cell patch-clamp recordings from visually identified pyramidal neurons (PyrN) and parvalbumin-positive interneurons (PV-IN) of cortical layer 2/3 in acute slices with electrical stimulation of afferent fibers in layer 4 and optogenetic activation of PV-IN.

Results

These experiments revealed that electrical burst stimulation (10 pulses at 10 Hz) of layer 4 afferents to layer 2/3 neurons induced comparable short-term depression (STD) of glutamatergic postsynaptic currents (PSCs) in PyrN and in PV-IN, while disynaptic GABAergic PSCs in PyrN showed a stronger depression. Burst-induced depression of glutamatergic PSCs decayed within <4 s, while the decay of GABAergic PSCs required >11 s. Optogenetically-induced GABAergic PSCs in PyrN also demonstrated STD after burst stimulation, with a decay of >11 s. Excitatory postsynaptic potentials (EPSPs) in PyrN were unaffected after electrical burst stimulation, while a selective optogenetic STD of GABAergic synapses caused a transient increase of electrically evoked EPSPs in PyrN.

Discussion

In summary, these results demonstrate substantial short-term plasticity at all synapses investigated and suggest that the prominent STD observed in GABAergic synapses can moderate the functional efficacy of glutamatergic STD after repetitive synaptic stimulations. This mechanism may contribute to a reliable information flow toward the integrative layer 2/3 for complex time-varying sensory stimuli.

Id2 GABAergic interneurons comprise a neglected fourth major group of cortical inhibitory cells

Cortical GABAergic interneurons (INs) represent a diverse population of mainly locally projecting cells that provide specialized forms of inhibition to pyramidal neurons and other INs. Most recent work on INs has focused on subtypes distinguished by expression of Parvalbumin (PV), Somatostatin (SST), or Vasoactive Intestinal Peptide (VIP). However, a fourth group that includes neurogliaform cells (NGFCs) has been less well characterized due to a lack of genetic tools. Here, we show that these INs can be accessed experimentally using intersectional genetics with the gene Id2. We find that outside of layer 1 (L1), the majority of Id2 INs are NGFCs that express high levels of neuropeptide Y (NPY) and exhibit a late-spiking firing pattern, with extensive local connectivity. While much sparser, non-NGFC Id2 INs had more variable properties, with most cells corresponding to a diverse group of INs that strongly expresses the neuropeptide CCK. In vivo, using silicon probe recordings, we observed several distinguishing aspects of NGFC activity, including a strong rebound in activity immediately following the cortical down state during NREM sleep. Our study provides insights into IN diversity and NGFC distribution and properties, and outlines an intersectional genetics approach for further study of this underappreciated group of INs.

Dynamics of neocortical networks: connectivity beyond the canonical microcircuit

The neocortical network consists of two types of excitatory neurons and a variety of GABAergic inhibitory interneurons, which are organized in distinct microcircuits providing feedforward, feedback, lateral inhibition, and disinhibition. This network is activated by layer- and cell-type specific inputs from first and higher order thalamic nuclei, other subcortical regions, and by cortico-cortical projections. Parallel and serial information processing occurs simultaneously in different intracortical subnetworks and is influenced by neuromodulatory inputs arising from the basal forebrain (cholinergic), raphe nuclei (serotonergic), locus coeruleus (noradrenergic), and ventral tegmentum (dopaminergic). Neocortical neurons differ in their intrinsic firing pattern, in their local and global synaptic connectivity, and in the dynamics of their synaptic interactions. During repetitive stimulation, synaptic connections between distinct neuronal cell types show short-term facilitation or depression, thereby activating or inactivating intracortical microcircuits. Specific networks are capable to generate local and global activity patterns (e.g., synchronized oscillations), which contribute to higher cognitive function and behavior. This review article aims to give a brief overview on our current understanding of the structure and function of the neocortical network.

Inter and Intralaminar Excitation of Parvalbumin Interneurons in Mouse Barrel Cortex

Parvalbumin (PV) interneurons are inhibitory fast-spiking cells with essential roles in directing the flow of information through cortical circuits. These neurons set the balance between excitation and inhibition, control rhythmic activity, and have been linked to disorders including autism spectrum and schizophrenia. PV interneurons differ between cortical layers in their morphology, circuitry, and function, but how their electrophysiological properties vary has received little attention. Here we investigate responses of PV interneurons in different layers of primary somatosensory barrel cortex (BC) to different excitatory inputs. With the genetically-encoded hybrid voltage sensor, hVOS, we recorded voltage changes simultaneously in many L2/3 and L4 PV interneurons to stimulation in either L2/3 or L4. Decay-times were consistent across L2/3 and L4. Amplitude, half-width, and rise-time were greater for PV interneurons residing in L2/3 compared to L4. Stimulation in L2/3 elicited responses in both L2/3 and L4 with longer latency compared to stimulation in L4. These differences in latency between layers could influence their windows for temporal integration. Thus PV interneurons in different cortical layers of BC show differences in response properties with potential roles in cortical computations.

Spatial Distribution of Inhibitory Innervations of Excitatory Pyramidal Cells by Major Interneuron Subtypes in the Auditory Cortex

Mental disorders, characterized by the National Institute of Mental Health as disruptions in neural circuitry, currently account for 13% of the global incidence of such disorders. An increasing number of studies suggest that imbalances between excitatory and inhibitory neurons in neural networks may be a crucial mechanism underlying mental disorders. However, the spatial distribution of inhibitory interneurons in the auditory cortex (ACx) and their relationship with excitatory pyramidal cells (PCs) remain elusive. In this study, we employed a combination of optogenetics, transgenic mice, and patch-clamp recording on brain slices to investigate the microcircuit characteristics of different interneurons (PV, SOM, and VIP) and the spatial pattern of inhibitory inhibition across layers 2/3 to 6 in the ACx. Our findings revealed that PV interneurons provide the strongest and most localized inhibition with no cross-layer innervation or layer specificity. Conversely, SOM and VIP interneurons weakly regulate PC activity over a broader range, exhibiting distinct spatial inhibitory preferences. Specifically, SOM inhibitions are preferentially found in deep infragranular layers, while VIP inhibitions predominantly occur in upper supragranular layers. PV inhibitions are evenly distributed across all layers. These results suggest that the input from inhibitory interneurons to PCs manifests in unique ways, ensuring that both strong and weak inhibitory inputs are evenly dispersed throughout the ACx, thereby maintaining a dynamic excitation-inhibition balance. Our findings contribute to understanding the spatial inhibitory characteristics of PCs and inhibitory interneurons in the ACx at the circuit level, which holds significant clinical implications for identifying and targeting abnormal circuits in auditory system diseases.

Genetically Defined Subtypes of Somatostatin-Containing Cortical Interneurons

Inhibitory interneurons play a crucial role in proper development and function of the mammalian cerebral cortex. Of the different inhibitory subclasses, dendritic-targeting, somatostatin-containing (SOM) interneurons may be the most diverse. Earlier studies used transgenic mouse lines to identify and characterize subtypes of SOM interneurons by morphological, electrophysiological and neurochemical properties. More recently, large-scale studies classified SOM interneurons into 13 morpho-electro-transcriptomic (MET) types. It remains unclear, however, how these various classification schemes relate to each other, and experimental access to MET types has been limited by the scarcity of type-specific mouse driver lines. To begin to address these issues we crossed Flp and Cre driver mouse lines and a dual-color combinatorial reporter, allowing experimental access to genetically defined SOM subsets. Brains from adult mice of both sexes were retrogradely dye-labeled from the pial surface to identify layer 1-projecting neurons, and immunostained against several marker proteins, allowing correlation of genetic label, axonal target and marker protein expression in the same neurons. Using whole-cell recordings ex-vivo, we compared electrophysiological properties between intersectional and transgenic SOM subsets. We identified two layer 1-targeting intersectional subsets with non-overlapping marker protein expression and electrophysiological properties which, together with a previously characterized layer 4-targeting subtype, account for about half of all layer 5 SOM cells and >40% of all SOM cells, and appear to map onto 5 of the 13 MET types. Genetic access to these subtypes will allow researchers to determine their synaptic inputs and outputs and uncover their roles in cortical computations and animal behavior.

SIGNIFICANCE STATEMENT

Inhibitory neurons are critically important for proper development and function of the cerebral cortex. Although a minority population, they are highly diverse, which poses a major challenge to investigating their contributions to cortical computations and animal and human behavior. As a step towards understanding this diversity we crossed genetically modified mouse lines to allow detailed examination of genetically-defined groups of the most diverse inhibitory subtype, somatostatin-containing interneurons. We identified and characterized three somatostatin subtypes in the deep cortical layers with distinct combinations of anatomical, neurochemical and electrophysiological properties. Future studies could now use these genetic tools to examine how these different subtypes are integrated into the cortical circuit and what roles they play during sensory, cognitive or motor behavior.

Histamine Release in the Prefrontal Cortex Excites Fast-Spiking Interneurons while GABA Released from the Same Axons Inhibits Pyramidal Cells

We studied how histamine and GABA release from axons originating from the hypothalamic tuberomammillary nucleus (TMN) and projecting to the prefrontal cortex (PFC) influence circuit processing. We optostimulated histamine/GABA from genetically defined TMN axons that express the histidine decarboxylase gene (TMN_HDC axons). Whole-cell recordings from PFC neurons in layer 2/3 of prelimbic, anterior cingulate, and infralimbic regions were used to monitor excitability before and after optostimulated histamine/GABA release in male and female mice. We found that histamine-GABA release influences the PFC through actions on distinct neuronal types: the histamine stimulates fast-spiking interneurons; and the released GABA enhances tonic (extrasynaptic) inhibition on pyramidal cells (PyrNs). For fast-spiking nonaccommodating interneurons, histamine released from TMN_HDC axons induced additive gain changes, which were blocked by histamine H1 and H2 receptor antagonists. The excitability of other fast-spiking interneurons in the PFC was not altered. In contrast, the GABA released from TMN_HDC axons predominantly produced divisive gain changes in PyrNs, increasing their resting input conductance, and decreasing the slope of the input-output relationship. This inhibitory effect on PyrNs was not blocked by histamine receptor antagonists but was blocked by GABA_A receptor antagonists. Across the adult life span (from 3 to 18 months of age), the GABA released from TMN_HDC axons in the PFC inhibited PyrN excitability significantly more in older mice. For individuals who maintain cognitive performance into later life, the increases in TMN_HDC GABA modulation of PyrNs during aging could enhance information processing and be an adaptive mechanism to buttress cognition.SIGNIFICANCE STATEMENT The hypothalamus controls arousal state by releasing chemical neurotransmitters throughout the brain to modulate neuronal excitability. Evidence is emerging that the release of multiple types of neurotransmitters may have opposing actions on neuronal populations in key cortical regions. This study demonstrates for the first time that the neurotransmitters histamine and GABA are released in the prefrontal cortex from axons originating from the tuberomammillary nucleus of the hypothalamus. This work demonstrates how hypothalamic modulation of neuronal excitability is maintained throughout adult life, highlighting an unexpected aspect of the aging process that may help maintain cognitive abilities.

Effects of optogenetic inhibition of a small fraction of parvalbumin-positive interneurons on the representation of sensory stimuli in mouse barrel cortex

Inhibitory interneurons play central roles in the modulation of spontaneous network activity and in processing of neuronal information. In sensory neocortical areas, parvalbumin-positive (PV+) GABAergic interneurons control the representation and processing of peripheral sensory inputs. We studied the functional role of PV+ interneurons in the barrel cortex of anesthetized adult PVCre mice by combining extracellular multi-electrode recordings with optogenetic silencing of a small fraction of PV+ interneurons. In all cortical layers, optogenetic inhibition caused an increase in spontaneous network activity from theta to gamma frequencies. The spatio-temporal representation of sensory inputs was studied by stimulating one or two whiskers at different intervals and analyzing the resulting local field potential (LFP) and single unit (SU) response. Silencing PV+ interneurons caused an increase in LFP response to sensory stimulation and a decrease in temporal discrimination of consecutive whisker deflections. The combined effect of whisker deflection and optogenetic inhibition was highly similar to the linear sum of the individual effects of these two manipulations. SU recordings revealed that optogenetic silencing reduced stimulus detectability by increasing stimulus-evoked firing rate by a constant offset, suggesting that PV+ interneurons improve signal-to-noise ratio by reducing ongoing spiking activity, thereby sharpening the spatio-temporal representation of sensory stimuli.

Effect of whole-hand water flow stimulation on the neural balance between excitation and inhibition in the primary somatosensory cortex

Sustained peripheral somatosensory stimulations, such as high-frequency repetitive somatosensory stimulation (HF-RSS) and vibrated stimulation, are effective in altering the balance between excitation and inhibition in the somatosensory cortex (S1) and motor cortex (M1). A recent study reported that whole-hand water flow (WF) stimulation induced neural disinhibition in the M1. Based on previous results, we hypothesized that whole-hand WF stimulation would lead to neural disinhibition in the S1 because there is a strong neural connection between M1 and S1 and aimed to examine whether whole-hand WF stimulation would change the neural balance between excitation and inhibition in the S1. Nineteen healthy volunteers were studied by measuring excitation and inhibition in the S1 before and after each of the four 15-min interventions. The excitation and inhibition in the S1 were assessed using somatosensory evoked potentials (SEPs) and paired-pulse inhibition (PPI) induced by single- and paired-pulse stimulations, respectively. The four interventions were as follows: control, whole-hand water immersion, whole-hand WF, and HF-RSS. The results showed no significant changes in SEPs and PPI following any intervention. However, changes in PPI with an interstimulus interval (ISI) of 30 ms were significantly correlated with the baseline value before whole-hand WF. Thus, the present findings indicated that the whole-hand WF stimulation had a greater decreased neural inhibition in participants with higher neural inhibition in the S1 at baseline. Considering previous results on M1, the present results possibly show that S1 has lower plasticity than M1 and that the duration (15 min) of each intervention may not have been enough to alter the balance of excitation and inhibition in the S1.

Connectivity concepts in neuronal network modeling

Sustainable research on computational models of neuronal networks requires published models to be understandable, reproducible, and extendable. Missing details or ambiguities about mathematical concepts and assumptions, algorithmic implementations, or parameterizations hinder progress. Such flaws are unfortunately frequent and one reason is a lack of readily applicable standards and tools for model description. Our work aims to advance complete and concise descriptions of network connectivity but also to guide the implementation of connection routines in simulation software and neuromorphic hardware systems. We first review models made available by the computational neuroscience community in the repositories ModelDB and Open Source Brain, and investigate the corresponding connectivity structures and their descriptions in both manuscript and code. The review comprises the connectivity of networks with diverse levels of neuroanatomical detail and exposes how connectivity is abstracted in existing description languages and simulator interfaces. We find that a substantial proportion of the published descriptions of connectivity is ambiguous. Based on this review, we derive a set of connectivity concepts for deterministically and probabilistically connected networks and also address networks embedded in metric space. Beside these mathematical and textual guidelines, we propose a unified graphical notation for network diagrams to facilitate an intuitive understanding of network properties. Examples of representative network models demonstrate the practical use of the ideas. We hope that the proposed standardizations will contribute to unambiguous descriptions and reproducible implementations of neuronal network connectivity in computational neuroscience.

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.

A subpopulation of cortical VIP-expressing interneurons with highly dynamic spines

Structural synaptic plasticity may underlie experience and learning-dependent changes in cortical circuits. In contrast to excitatory pyramidal neurons, insight into the structural plasticity of inhibitory neurons remains limited. Interneurons are divided into various subclasses, each with specialized functions in cortical circuits. Further knowledge of subclass-specific structural plasticity of interneurons is crucial to gaining a complete mechanistic understanding of their contribution to cortical plasticity overall. Here, we describe a subpopulation of superficial cortical multipolar interneurons expressing vasoactive intestinal peptide (VIP) with high spine densities on their dendrites located in layer (L) 1, and with the electrophysiological characteristics of bursting cells. Using longitudinal imaging in vivo, we found that the majority of the spines are highly dynamic, displaying lifetimes considerably shorter than that of spines on pyramidal neurons. Using correlative light and electron microscopy, we confirmed that these VIP spines are sites of excitatory synaptic contacts, and are morphologically distinct from other spines in L1.

Neurophysiology of the Developing Cerebral Cortex: What We Have Learned and What We Need to Know

This review article aims to give a brief summary on the novel technologies, the challenges, our current understanding, and the open questions in the field of the neurophysiology of the developing cerebral cortex in rodents. In the past, in vitro electrophysiological and calcium imaging studies on single neurons provided important insights into the function of cellular and subcellular mechanism during early postnatal development. In the past decade, neuronal activity in large cortical networks was recorded in pre- and neonatal rodents in vivo by the use of novel high-density multi-electrode arrays and genetically encoded calcium indicators. These studies demonstrated a surprisingly rich repertoire of spontaneous cortical and subcortical activity patterns, which are currently not completely understood in their functional roles in early development and their impact on cortical maturation. Technological progress in targeted genetic manipulations, optogenetics, and chemogenetics now allow the experimental manipulation of specific neuronal cell types to elucidate the function of early (transient) cortical circuits and their role in the generation of spontaneous and sensory evoked cortical activity patterns. Large-scale interactions between different cortical areas and subcortical regions, characterization of developmental shifts from synchronized to desynchronized activity patterns, identification of transient circuits and hub neurons, role of electrical activity in the control of glial cell differentiation and function are future key tasks to gain further insights into the neurophysiology of the developing cerebral cortex.

Theta activity paradoxically boosts gamma and ripple frequency sensitivity in prefrontal interneurons

Fast oscillations in cortical circuits critically depend on GABAergic interneurons. Which interneuron types and populations can drive different cortical rhythms, however, remains unresolved and may depend on brain state. Here, we measured the sensitivity of different GABAergic interneurons in prefrontal cortex under conditions mimicking distinct brain states. While fast-spiking neurons always exhibited a wide bandwidth of around 400 Hz, the response properties of spike-frequency adapting interneurons switched with the background input's statistics. Slowly fluctuating background activity, as typical for sleep or quiet wakefulness, dramatically boosted the neurons' sensitivity to gamma and ripple frequencies. We developed a time-resolved dynamic gain analysis and revealed rapid sensitivity modulations that enable neurons to periodically boost gamma oscillations and ripples during specific phases of ongoing low-frequency oscillations. This mechanism predicts these prefrontal interneurons to be exquisitely sensitive to high-frequency ripples, especially during brain states characterized by slow rhythms, and to contribute substantially to theta-gamma cross-frequency coupling.

Synchronous inhibitory synaptic inputs to layer II/III pyramidal neurons in the murine barrel cortex

The barrel cortex exhibits obvious columnar organization. Although GABAergic inhibition plays a critical role in regulating neural excitation in response to mechanical stimuli applied to whiskers, the profiles of synchronous events for inhibitory synaptic transmission in intracolumnar and transcolumnar pyramidal neurons remain unknown. To explore a functional mechanism of synchronous inhibition of pyramidal neurons, we performed paired whole-cell patch-clamp recordings and recorded spontaneous inhibitory postsynaptic currents (sIPSCs) from layer II/III pyramidal neurons. A cross-correlogram of sIPSCs (1 ms bin) was used to detect synchronous sIPSCs. Synchronous neuron pairs were defined as those whose peak number of sIPSCs between -3 and 3 ms exceeded the mean + 2 SD of the number of sIPSCs in the period of -50 to 50 ms minus the number in that of -3 to 3 ms period. In the recording of pyramidal neurons located in the same column (intracolumn), 61.5% of neuron pairs were classified as synchronous neuron pairs, while 52.6% of pyramidal neuron pairs in adjacent columns (transcolumn) were defined as synchronous neuron pairs. The amplitude of synchronous sIPSCs was comparable to that of asynchronous sIPSCs in asynchronous neuron pairs, whereas that of synchronous sIPSCs was larger than that of asynchronous sIPSCs in synchronous neuron pairs. Synchronicity of sIPSCs did not depend on the distance of neuron pairs. These results suggest that layer II/III pyramidal neurons receive synchronous inhibitory synaptic inputs generated by a certain type of GABAergic interneuron that induces large IPSCs in pyramidal neurons, likely to be fast-spiking cells.

Structural changes in the neocortex as correlates of variations in EEG spectra and seizure susceptibility in rat brains with different degrees of dysplasia

Disturbances of the early stages of neurogenesis lead to irreversible changes in the structure of the mature brain and its functional impairment, including increased excitability, which may be the basis for drug‐resistant epilepsy. The range of possible clinical symptoms is as wide as the different stages of disturbed neurogenesis may be. In this study, we used a quadruple model of brain dysplasia by comparing structural and functional disorders in animals whose neurogenesis was disturbed with a single dose of 1 Gy of gamma rays at one of the four stages of neurogenesis, that is, on days 13, 15, 17, or 19 of prenatal development.

When reached adulthood, the prenatally irradiated rats received EEG teletransmitter implantation. Thereafter, pilocarpine was administered and significant differences in susceptibility to seizure behavioral symptoms were detected depending on the degree of brain dysplasia. Before, during, and after the seizures significant correlations were found between the density of parvalbumin‐immunopositive neurons located in the cerebral cortex and the intensity of behavioral seizure symptoms or increases in the power of particular EEG bands. Neurons expressing calretinin or NPY showed also dysplasia‐related increases without, however, correlations with parameters of seizure intensity. The results point to significant roles of parvalbumin‐expressing interneurons, and also to expression of NPY—an endogenous anticonvulsant and neuroprotectant reducing susceptibility to seizures and supporting neuronal survival.

GABAergic Interneurons with Nonlinear Dendrites: From Neuronal Computations to Memory Engrams

GABAergic interneurons (INs) are a highly diverse class of neurons in the mammalian brain with a critical role in orchestrating multiple cognitive functions and maintaining the balance of excitation/inhibition across neuronal circuitries. In this perspective, we discuss recent findings regarding the ability of some IN subtypes to integrate incoming inputs in nonlinear ways within their dendritic branches. These recently discovered features may endow the specific INs with advanced computing capabilities, whose breadth and functional contributions remain an open question. Along these lines, we discuss theoretical and experimental evidence regarding the potential role of nonlinear IN dendrites in advancing single neuron computations and contributing to memory formation.

Interneuron Dysfunction and Inhibitory Deficits in Autism and Fragile X Syndrome

The alteration of excitatory–inhibitory (E–I) balance has been implicated in various neurological and psychiatric diseases, including autism spectrum disorder (ASD). Fragile X syndrome (FXS) is a single-gene disorder that is the most common known cause of ASD. Understanding the molecular and physiological features of FXS is thought to enhance our knowledge of the pathophysiology of ASD. Accumulated evidence implicates deficits in the inhibitory circuits in FXS that tips E–I balance toward excitation. Deficits in interneurons, the main source of an inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), have been reported in FXS, including a reduced number of cells, reduction in intrinsic cellular excitability, or weaker synaptic connectivity. Manipulating the interneuron activity ameliorated the symptoms in the FXS mouse model, which makes it reasonable to conceptualize FXS as an interneuronopathy. While it is still poorly understood how the developmental profiles of the inhibitory circuit go awry in FXS, recent works have uncovered several developmental alterations in the functional properties of interneurons. Correcting disrupted E–I balance by potentiating the inhibitory circuit by targeting interneurons may have a therapeutic potential in FXS. I will review the recent evidence about the inhibitory alterations and interneuron dysfunction in ASD and FXS and will discuss the future directions of this field.

Mechanisms Underlying Target Selectivity for Cell Types and Subcellular Domains in Developing Neocortical Circuits

The cerebral cortex contains numerous neuronal cell types, distinguished by their molecular identity as well as their electrophysiological and morphological properties. Cortical function is reliant on stereotyped patterns of synaptic connectivity and synaptic function among these neuron types, but how these patterns are established during development remains poorly understood. Selective targeting not only of different cell types but also of distinct postsynaptic neuronal domains occurs in many brain circuits and is directed by multiple mechanisms. These mechanisms include the regulation of axonal and dendritic guidance and fine-scale morphogenesis of pre- and postsynaptic processes, lineage relationships, activity dependent mechanisms and intercellular molecular determinants such as transmembrane and secreted molecules, many of which have also been implicated in neurodevelopmental disorders. However, many studies of synaptic targeting have focused on circuits in which neuronal processes target different lamina, such that cell-type-biased connectivity may be confounded with mechanisms of laminar specificity. In the cerebral cortex, each cortical layer contains cell bodies and processes from intermingled neuronal cell types, an arrangement that presents a challenge for the development of target-selective synapse formation. Here, we address progress and future directions in the study of cell-type-biased synaptic targeting in the cerebral cortex. We highlight challenges to identifying developmental mechanisms generating stereotyped patterns of intracortical connectivity, recent developments in uncovering the determinants of synaptic target selection during cortical synapse formation, and current gaps in the understanding of cortical synapse specificity.

Neuromodulation Leads to a Burst-Tonic Switch in a Subset of VIP Neurons in Mouse Primary Somatosensory (Barrel) Cortex

Neocortical GABAergic interneurons expressing vasoactive intestinal polypeptide (VIP) contribute to sensory processing, sensorimotor integration, and behavioral control. In contrast to other major subpopulations of GABAergic interneurons, VIP neurons show a remarkable diversity. Studying morphological and electrophysiological properties of VIP cells, we found a peculiar group of neurons in layer II/III of mouse primary somatosensory (barrel) cortex, which showed a highly dynamic burst firing behavior at resting membrane potential that switched to tonic mode at depolarized membrane potentials. Furthermore, we demonstrate that burst firing depends on T-type calcium channels. The burst-tonic switch could be induced by acetylcholine (ACh) and serotonin. ACh mediated a depolarization via nicotinic receptors whereas serotonin evoked a biphasic depolarization via ionotropic and metabotropic receptors in 48% of the population and a purely monophasic depolarization via metabotropic receptors in the remaining cells. These data disclose an electrophysiologically defined subpopulation of VIP neurons that via neuromodulator-induced changes in firing behavior is likely to regulate the state of cortical circuits in a profound manner.

GABAergic neuron-specific whole-brain transduction by AAV-PHP.B incorporated with a new GAD65 promoter

GABAergic interneurons play a critical role in tuning neural networks in the central nervous system, and their defects are associated with neuropsychiatric disorders. Currently, the mDlx enhancer is solely used for adeno-associated virus (AAV) vector-mediated transgene delivery into cortical interneurons. Here, we developed a new inhibitory neuron-specific promoter (designated as the mGAD65 promoter), with a length of 2.5 kb, from a mouse genome upstream of exon 1 of the Gad2 gene encoding glutamic acid decarboxylase (GAD) 65. Intravenous infusion of blood-brain barrier-penetrating AAV-PHP.B expressing an enhanced green fluorescent protein under the control of the mGAD65 promoter transduced the whole brain in an inhibitory neuron-specific manner. The specificity and efficiency of the mGAD65 promoter for GABAergic interneurons, which was assessed at the motor cortex, were almost identical to or slightly higher than those of the mDlx enhancer. Immunohistochemical analysis revealed that the mGAD65 promoter preferentially transduced parvalbumin (PV)-expressing interneurons. Notably, the mGAD65 promoter transduced chandelier cells more efficiently than the mDlx enhancer and robustly labeled their synaptic boutons, called the cartridge, targeting the axon initial segments of excitatory pyramidal neurons. To test the ability of the mGAD65 promoter to express a functional molecule, we virally expressed G-CaMP, a fluorescent Ca2+ indicator, in the motor cortex, and this enabled us to monitor spontaneous and drug-induced Ca2+ activity in GABAergic inhibitory neurons. These results suggest that the mGAD65 promoter is useful for AAV-mediated targeting and manipulation of GABAergic neurons with the dominance of cortical PV-expressing neurons, including chandelier cells.

Characterization of Feedback Neurons in the High-Level Visual Cortical Areas That Project Directly to the Primary Visual Cortex in the Cat

Previous studies indicate that top-down influence plays a critical role in visual information processing and perceptual detection. However, the substrate that carries top-down influence remains poorly understood. Using a combined technique of retrograde neuronal tracing and immunofluorescent double labeling, we characterized the distribution and cell type of feedback neurons in cat's high-level visual cortical areas that send direct connections to the primary visual cortex (V1: area 17). Our results showed: (1) the high-level visual cortex of area 21a at the ventral stream and PMLS area at the dorsal stream have a similar proportion of feedback neurons back projecting to the V1 area, (2) the distribution of feedback neurons in the higher-order visual area 21a and PMLS was significantly denser than in the intermediate visual cortex of area 19 and 18, (3) feedback neurons in all observed high-level visual cortex were found in layer II-III, IV, V, and VI, with a higher proportion in layer II-III, V, and VI than in layer IV, and (4) most feedback neurons were CaMKII-positive excitatory neurons, and few of them were identified as inhibitory GABAergic neurons. These results may argue against the segregation of ventral and dorsal streams during visual information processing, and support "reverse hierarchy theory" or interactive model proposing that recurrent connections between V1 and higher-order visual areas constitute the functional circuits that mediate visual perception. Also, the corticocortical feedback neurons from high-level visual cortical areas to the V1 area are mostly excitatory in nature.

Layer-Specific Inhibitory Microcircuits of Layer 6 Interneurons in Rat Prefrontal Cortex

GABAergic interneurons in different cortical areas play important roles in diverse higher-order cognitive functions. The heterogeneity of interneurons is well characterized in different sensory cortices, in particular in primary somatosensory and visual cortex. However, the structural and functional properties of the medial prefrontal cortex (mPFC) interneurons have received less attention. In this study, a cluster analysis based on axonal projection patterns revealed four distinct clusters of L6 interneurons in rat mPFC: Cluster 1 interneurons showed axonal projections similar to Martinotti-like cells extending to layer 1, cluster 2 displayed translaminar projections mostly to layer 5, and cluster 3 interneuron axons were confined to the layer 6, whereas those of cluster 4 interneurons extend also into the white matter. Correlations were found between neuron location and axonal distribution in all clusters. Moreover, all cluster 1 L6 interneurons showed a monotonically adapting firing pattern with an initial high-frequency burst. All cluster 2 interneurons were fast-spiking, while neurons in cluster 3 and 4 showed heterogeneous firing patterns. Our data suggest that L6 interneurons that have distinct morphological and physiological characteristics are likely to innervate different targets in mPFC and thus play differential roles in the L6 microcircuitry and in mPFC-associated functions.

Modification of oxygen consumption and blood flow in mouse somatosensory cortex by cell-type-specific neuronal activity

Gamma activity arising from the interplay between pyramidal neurons and fast-spiking parvalbumin (PV) interneurons is an integral part of higher cognitive functions and is assumed to contribute significantly to brain metabolic responses. Cerebral metabolic rate of oxygen (CMRO₂) responses were evoked by optogenetic stimulation of cortical PV interneurons and pyramidal neurons. We found that CMRO₂ responses depended on neuronal activation, but not on the power of gamma activity induced by optogenetic stimulation. This implies that evoked gamma activity per se is not energy demanding. Optogenetic stimulation of PV interneurons during somatosensory stimulation reduced excitatory neuronal activity but did not potentiate O₂ consumption as previously hypothesized. In conclusion, our data suggest that activity-driven CMRO₂ responses depend on neuronal excitation rather than the cerebral rhythmic activity they induce. Excitation of both excitatory and inhibitory neurons requires energy, but inhibition of cortical excitatory neurons by interneurons does not potentiate activity-driven energy consumption.

Feed-forward recruitment of electrical synapses enhances synchronous spiking in the mouse cerebellar cortex

In the cerebellar cortex, molecular layer interneurons use chemical and electrical synapses to form subnetworks that fine-tune the spiking output of the cerebellum. Although electrical synapses can entrain activity within neuronal assemblies, their role in feed-forward circuits is less well explored. By combining whole-cell patch-clamp and 2-photon laser scanning microscopy of basket cells (BCs), we found that classical excitatory postsynaptic currents (EPSCs) are followed by GABA_A receptor-independent outward currents, reflecting the hyperpolarization component of spikelets (a synapse-evoked action potential passively propagating from electrically coupled neighbors). FF recruitment of the spikelet-mediated inhibition curtails the integration time window of concomitant excitatory postsynaptic potentials (EPSPs) and dampens their temporal integration. In contrast with GABAergic-mediated feed-forward inhibition, the depolarizing component of spikelets transiently increases the peak amplitude of EPSPs, and thus postsynaptic spiking probability. Therefore, spikelet transmission can propagate within the BC network to generate synchronous inhibition of Purkinje cells, which can entrain cerebellar output for driving temporally precise behaviors.

The Synaptic Organization of Layer 6 Circuits Reveals Inhibition as a Major Output of a Neocortical Sublamina

The canonical cortical microcircuit has principally been defined by interlaminar excitatory connections among the six layers of the neocortex. However, excitatory neurons in layer 6 (L6), a layer whose functional organization is poorly understood, form relatively rare synaptic connections with other cortical excitatory neurons. Here, we show that the vast majority of parvalbumin inhibitory neurons in a sublamina within L6 send axons through the cortical layers toward the pia. These interlaminar inhibitory neurons receive local synaptic inputs from both major types of L6 excitatory neurons and receive stronger input from thalamocortical afferents than do neighboring pyramidal neurons. The distribution of these interlaminar interneurons and their synaptic connectivity further support a functional subdivision within the standard six layers of the cortex. Positioned to integrate local and long-distance inputs in this sublayer, these interneurons generate an inhibitory interlaminar output. These findings call for a revision to the canonical cortical microcircuit.

Cellular Mechanisms of Conscious Processing

Recent breakthroughs in neurobiology indicate that the time is ripe to understand how cellular-level mechanisms are related to conscious experience. Here, we highlight the biophysical properties of pyramidal cells, which allow them to act as gates that control the evolution of global activation patterns. In conscious states, this cellular mechanism enables complex sustained dynamics within the thalamocortical system, whereas during unconscious states, such signal propagation is prohibited. We suggest that the hallmark of conscious processing is the flexible integration of bottom-up and top-down data streams at the cellular level. This cellular integration mechanism provides the foundation for Dendritic Information Theory, a novel neurobiological theory of consciousness.

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.

Biphasic Impact of Prenatal Inflammation and Macrophage Depletion on the Wiring of Neocortical Inhibitory Circuits

The etiology of neurodevelopmental disorders is linked to defects in parvalbumin (PV)-expressing cortical interneurons and to prenatal immune challenges. Mouse models of maternal immune activation (MIA) and microglia deficits increase the postnatal density of PV interneurons, raising the question of their functional integration. Here, we show that MIA and embryonic depletion of macrophages including microglia have a two-step impact on PV interneurons wiring onto their excitatory target neurons in the barrel cortex. In adults, both challenges reduced the inhibitory drive from PV interneurons, as reported in neurodevelopmental disorders. In juveniles, however, we found an increased density of PV neurons, an enhanced strength of unitary connections onto excitatory cells, and an aberrant horizontal inhibition with a reduced lateral propagation of sensory inputs in vivo. Our results provide a comprehensive framework for understanding the impact of prenatal immune challenges onto the developmental trajectory of inhibitory circuits that leads to pathological brain wiring.

Contribution of the ventrolateral thalamus to the locomotion-related activity of motor cortex

The activity of motor cortex is necessary for accurate stepping on a complex terrain. How this activity is generated remains unclear. The goal of this study was to clarify the contribution of signals from the ventrolateral thalamus (VL) to formation of locomotion-related activity of motor cortex during vision-independent and vision-dependent locomotion. In two cats, we recorded the activity of neurons in layer V of motor cortex as cats walked on a flat surface and a horizontal ladder. We reversibly inactivated ~10% of the VL unilaterally with the glutamatergic transmission antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and analyzed how this affected the activity of motor cortex neurons. We examined neuronal subpopulations with somatosensory receptive fields on different segments of the forelimb and pyramidal tract projecting neurons (PTNs). We found that the VL contribution to the locomotion-related activity of motor cortex is very powerful and has both excitatory and inhibitory components. The magnitudes of both the excitatory and inhibitory contributions fluctuate over the step cycle and depend on locomotion task. On a flat surface, the VL contributes more excitation to the shoulder- and elbow-related neurons than the wrist/paw-related cells. The VL excites the shoulder-related group the most during the transition from stance to swing phase, while most intensively exciting the elbow-related group during the transition from swing to stance. The VL contributes more excitation for the fast- than slow-conducting PTNs. Upon transition to vision-dependent locomotion on the ladder, the VL contribution increases more for the wrist/paw-related neurons and slow-conducting PTNs.NEW & NOTEWORTHY How the activity of motor cortex is generated and the roles that different inputs to motor cortex play in formation of response properties of motor cortex neurons during movements remain unclear. This is the first study to characterize the contribution of the input from the ventrolateral thalamus (VL), the main subcortical input to motor cortex, to the activity of motor cortex neurons during vision-independent and vision-dependent locomotion.

Prelimbic cortical targets of ventromedial thalamic projections include inhibitory interneurons and corticostriatal pyramidal neurons in the rat

Ventromedial thalamic axons innervate cortical layer I and make contacts onto the apical dendritic tuft of pyramidal neurons. Optical stimulation of ventromedial thalamic axon terminals in prefrontal cortical areas in mouse brain slices evokes responses in corticocortical, corticothalamic and layer I inhibitory interneurons. Using anterograde tracing techniques and immunohistochemistry in male Sprague–Dawley rats, we provide anatomical evidence that ventromedial thalamic axon terminals in prelimbic cortex make contacts onto pyramidal neurons and, in particular, onto corticostriatal neurons as well as layer I inhibitory interneurons. Using stereology, we made quantitative estimates of contacts in uppermost prelimbic layer I onto dendrites of pyramidal neurons, corticostriatal neurons and layer I inhibitory interneurons. Prefrontal cortex has long been associated with decision making. Specifically, corticostriatal neurons in rat prelimbic cortex play an important role in cost–benefit decision making. Although recent experiments have detailed the physiology of this area in thalamocortical circuits, the extent of the impact of ventromedial thalamic input on corticostriatal neurons or layer I inhibitory interneurons has not been explored. Our quantitative anatomical results provide evidence that most ventromedial thalamic input to pyramidal neurons is provided to corticostriatal neurons and that overall more contacts are made onto the population of excitatory than onto the population of inhibitory neurons.

Interneuron Types as Attractors and Controllers

Cortical interneurons display striking differences in shape, physiology, and other attributes, challenging us to appropriately classify them. We previously suggested that interneuron types should be defined by their role in cortical processing. Here, we revisit the question of how to codify their diversity based upon their division of labor and function as controllers of cortical information flow. We suggest that developmental trajectories provide a guide for appreciating interneuron diversity and argue that subtype identity is generated using a configurational (rather than combinatorial) code of transcription factors that produce attractor states in the underlying gene regulatory network. We present our updated three-stage model for interneuron specification: an initial cardinal step, allocating interneurons into a few major classes, followed by definitive refinement, creating subclasses upon settling within the cortex, and lastly, state determination, reflecting the incorporation of interneurons into functional circuit ensembles. We close by discussing findings indicating that major interneuron classes are both evolutionarily ancient and conserved. We propose that the complexity of cortical circuits is generated by phylogenetically old interneuron types, complemented by an evolutionary increase in principal neuron diversity. This suggests that a natural neurobiological definition of interneuron types might be derived from a match between their developmental origin and computational function.

Histological assessment of optogenetic tools to study fronto-visual and fronto-parietal cortical networks in the rhesus macaque

Optogenetics offers unprecedented possibilities to investigate cortical networks. Yet, the number of successful optogenetic applications in non-human primates is still low, and the consequences of opsin expression in the primate brain are not well documented. We assessed histologically if we can target cerebrocortical networks with three common optogenetic constructs (AAV2/5-CaMKIIα-eNpHR3.0-mCherry, -ChR2-eYFP, -C1V1-mCherry). The frontal eye field or the dorsal premotor area of rhesus macaques were virally injected, and the resulting transduction spread, expression specificity, and opsin trafficking into axons projecting to parietal and visual areas were examined. After variable periods (2–24 months), expression was robust for all constructs at the injection sites. The CaMKIIα promoter driven-expression was predominant, but not exclusive, in excitatory neurons. In the case of eNpHR3.0-mCherry and ChR2-eYFP, opsins were present in axonal projections to target areas, in which sparse, retrogradely transduced neurons could also be found. Finally, the intracellular distribution of opsins differed: ChR2-eYFP had almost exclusive membrane localization, while eNpHR3.0-mCherry and C1V1-mCherry showed additional intracellular accumulations, which might affect neuronal survival in the long-term. Results indicate that all three constructs can be used for local neuronal modulation, but axonal stimulation and long-term use require additional considerations of construct selection and verification.

Cholinergic and Adenosinergic Modulation of Synaptic Release

In this review we will discuss the effect of two neuromodulatory transmitters, acetylcholine (ACh) and adenosine, on the synaptic release probability and short-term synaptic plasticity. ACh and adenosine differ fundamentally in the way they are released into the extracellular space. ACh is released mostly from synaptic terminals and axonal bouton of cholinergic neurons in the basal forebrain (BF). Its mode of action on synaptic release probability is complex because it activate both ligand-gated ion channels, so-called nicotinic ACh receptors and G-protein coupled muscarinic ACh receptors. In contrast, adenosine is released from both neurons and glia via nucleoside transporters or diffusion over the cell membrane in a non-vesicular, non-synaptic fashion; its receptors are exclusively G-protein coupled receptors. We show that ACh and adenosine effects are highly specific for an identified synaptic connection and depend mostly on the presynaptic but also on the postsynaptic receptor type and discuss the functional implications of these differences.

Up and Down States and Memory Consolidation Across Somatosensory, Entorhinal, and Hippocampal Cortices

In the course of a day, brain states fluctuate, from conscious awake information-acquiring states to sleep states, during which previously acquired information is further processed and stored as memories. One hypothesis is that memories are consolidated and stored during "offline" states such as sleep, a process thought to involve transfer of information from the hippocampus to other cortical areas. Up and Down states (UDS), patterns of activity that occur under anesthesia and sleep states, are likely to play a role in this process, although the nature of this role remains unclear. Here we review what is currently known about these mechanisms in three anatomically distinct but interconnected cortical areas: somatosensory cortex, entorhinal cortex, and the hippocampus. In doing so, we consider the role of this activity in the coordination of "replay" during sleep states, particularly during hippocampal sharp-wave ripples. We conclude that understanding the generation and propagation of UDS may provide key insights into the cortico-hippocampal dialogue linking archi- and neocortical areas during memory formation.