The prefrontal cortex (PFC) plays a key role in high-level cognitive functions and emotional behaviors, and PFC alterations correlate with different brain disorders including major depression and anxiety. In mice, the first two postnatal weeks represent a critical period of high sensitivity to environmental changes. In this temporal window, serotonin (5-HT) levels regulate the wiring of PFC cortical neurons. Early-life insults and postnatal exposure to the selective serotonin reuptake inhibitor fluoxetine (FLX) affect PFC development leading to depressive and anxiety-like phenotypes in adult mice. However, the mechanisms responsible for these dysfunctions remain obscure. We found that early postnatal FLX exposure (PNFLX) results in reduced overall firing and high-frequency bursting of putative pyramidal neurons (PNs) of deep layers of the medial PFC of adult mice of both sexes in vivo. Ex vivo, patch-clamp recordings revealed that PNFLX abolished high-frequency firing in a distinct subpopulation of deep-layer mPFC PNs, which transiently express the serotonin transporter SERT during the first 2 postnatal weeks. SERT+ and SERT- PNs exhibit distinct morphofunctional properties. Genetic deletion of 5-HT7Rs and pharmacological 5-HT7R blockade partially rescued both the PNFLX-induced reduction of PN firing in vivo and the altered firing of SERT+ PNs in vitro. This indicates a pivotal role of this 5-HTR subtype in mediating 5-HT-dependent maturation of PFC circuits that are susceptible to early-life insults. Overall, our results suggest potential novel neurobiological mechanisms, underlying detrimental neurodevelopmental consequences induced by early-life alterations of 5-HT levels.

Dendrites critically influence single-neuron computations, but their role in neocortical GABAergic interneurons (INs) remains poorly understood. We found that the two major cortical IN subtypes-somatostatin (SST)- and parvalbumin (PV)-expressing cells-use distinct strategies for distributing and integrating excitatory synaptic inputs along their dendrites. SST-INs exhibit NMDAR-dependent supralinear integration and a uniform distribution of synapses, whereas PV-INs show sublinear integration with a higher density of synapses on proximal dendrites with low NMDAR expression. Compartmental modeling revealed that, while both strategies enhance synaptic efficacy, passive integration and proximally biased inputs enable precise tracking of fast-changing signals in PV-INs, whereas NMDARs in SST-INs promote broader temporal integration, supporting sustained activity tuned to slower input variations. Consistent with these predictions, in vivo measurements showed differentially shaped dynamic visual responses in PV- and SST-INs. Therefore, the heterogeneity of dendritic mechanisms strongly influences the spatiotemporal dynamics of IN-specific inhibition in cortical circuits.

Patient and animal model data suggest a link between the cholinergic neuromodulatory system and the amyloid beta (Aβ) peptide in causing Alzheimer’s disease (AD). But how cholinergic dysfunctions contribute to AD pathology remains controversial. In a mouse model of local amyloid pathology, we show that in the early disease stages, the α7 nicotinic acetylcholine receptor (nAChR) is an important target of Aβ in the prefrontal cortex (PFC). Using in vivo two-photon calcium imaging and in vivo patch-clamp electrophysiology in the PFC of awake mice, we demonstrate that Aβ-mediated disruption of specifically the α7 nAChR subunit, expressed by distinct interneuron subtypes, results in substantial deficits in network activity. This is corroborated by electrophysiology experiments in slices. Combined with computational modeling, we propose that α7 nAChRs are occluded by Aβ early in the disease, whereas the heteropentameric α5 and β2 nAChRs are only partially inactivated and potentially provide novel therapeutic targets for intervention. Accordingly, we show that galantamine, an approved acetylcholine-esterase inhibitor (AChE-I), which acts as a positive allosteric modulator (PAM) of α5-containing nAChRs at low concentrations, reduces neuronal hyperactivity.

Depression is one of the most burdensome psychiatric disorders, affecting hundreds of millions of people worldwide. The disease is characterized not only by severe emotional and affective impairments, but also by disturbed vegetative and cognitive functions. Although many candidate mechanisms have been proposed to cause the disease, the pathophysiology of cognitive impairments in depression remains unclear. In this article, we aim to assess the link between cognitive alterations in depression and possible developmental changes in neuronal circuit wiring during critical periods of susceptibility. We review the existing literature and propose a role of serotonin signaling during development in shaping the functional states of prefrontal neuronal circuits and prefronto-thalamic loops. We discuss how early life insults affecting the serotonergic system could be important in the alterations of these local and long-range circuits, thus favoring the emergence of neurodevelopmental disorders, such as depression.

In the mammalian neocortex, GABAergic interneurons (INs) inhibit cortical networks in profoundly different ways. The extent to which this depends on how different INs process excitatory signals along their dendrites is poorly understood. Here, we reveal that the functional specialization of two major populations of cortical INs is determined by the unique association of different dendritic integration modes with distinct synaptic organization motifs. We found that somatostatin (SST)-INs exhibit NMDAR-dependent dendritic integration and uniform synapse density along the dendritic tree. In contrast, dendrites of parvalbumin (PV)-INs exhibit passive synaptic integration coupled with proximally enriched synaptic distributions. Theoretical analysis shows that these two dendritic configurations result in different strategies to optimize synaptic efficacy in thin dendritic structures. Yet, the two configurations lead to distinct temporal engagement of each IN during network activity. We confirmed these predictions with in vivo recordings of IN activity in the visual cortex of awake mice, revealing a rapid and linear recruitment of PV-INs as opposed to a long-lasting integrative activation of SST-INs. Our work reveals the existence of distinct dendritic strategies that confer distinct temporal representations for the two major classes of neocortical INs and thus dynamics of inhibition.

In the neocortex, fast synaptic inhibition orchestrates both spontaneous and sensory-evoked activity. GABAergic interneurons (INs) inhibit pyramidal neurons (PNs) directly, modulating their output activity and thus contributing to balance cortical networks. Moreover, several IN subtypes also inhibit other INs, forming specific disinhibitory circuits, which play crucial roles in several cognitive functions. Here, we studied a subpopulation of somatostatin-positive INs, the Martinotti cells (MCs) in layer 2/3 of the mouse barrel cortex (both sexes). MCs inhibit the distal portion of PN apical dendrites, thus controlling dendrite electrogenesis and synaptic integration. Yet, it is poorly understood whether MCs inhibit other elements of the cortical circuits, and the connectivity properties with non-PN targets are unknown. We found that MCs have a strong preference for PN dendrites, but they also considerably connect with parvalbumin-positive, vasoactive intestinal peptide-expressing, and layer 1 (L1) INs. Remarkably, GABAergic synapses from MCs exhibited clear cell type-specific short-term plasticity. Moreover, whereas the biophysical properties of MC-PN synapses were consistent with distal dendritic inhibition, MC-IN synapses exhibited characteristics of fast perisomatic inhibition. Finally, MC-PN connections used α5-containing GABAAreceptors (GABAARs), but this subunit was not expressed by the other INs targeted by MCs. We reveal a specialized connectivity blueprint of MCs within different elements of superficial cortical layers. In addition, our results identify α5-GABAARs as the molecular fingerprint of MC-PN dendritic inhibition. This is of critical importance, given the role of α5-GABAARs in cognitive performance and their involvement in several brain diseases.SIGNIFICANCE STATEMENTMartinotti cells (MCs) are a prominent, broad subclass of somatostatin-expressing GABAergic interneurons, specialized in controlling distal dendrites of pyramidal neurons (PNs) and taking part in several cognitive functions. Here we characterize the connectivity pattern of MCs with other interneurons in the superficial layers (L1 and L2/3) of the mouse barrel cortex. We found that the connectivity pattern of MCs with PNs as well as parvalbumin, vasoactive intestinal peptide, and L1 interneurons exhibit target-specific plasticity and biophysical properties. The specificity of α5-GABAARs at MC-PN synapses and the lack or functional expression of this subunit by other cell types define the molecular identity of MC-PN connections and the exclusive involvement of this inhibitory circuits in α5-dependent cognitive tasks.

GluN3A is an atypical glycine-binding subunit of NMDA receptors (NMDARs) whose actions in the brain are mostly unknown. Here, we show that the expression of GluN3A subunits controls the excitability of mouse adult cortical and amygdalar circuits via an unusual signaling mechanism involving the formation of excitatory glycine GluN1/GluN3A receptors (eGlyRs) and their tonic activation by extracellular glycine. eGlyRs are mostly extrasy-naptic and reside in specific neuronal populations, including the principal cells of the basolateral amygdala (BLA) and SST-positive interneurons (SST-INs) of the neocortex. In the BLA, tonic eGlyR currents are sensitive to fear-conditioning protocols, are subject to neuromodulation by the dopaminergic system, and control the sta-bility of fear memories. In the neocortex, eGlyRs control the in vivo spiking of SST-INs and the behavior -depen-dent modulation of cortical activity. GluN3A-containing eGlyRs thus represent a novel and widespread signaling modality in the adult brain, with attributes that strikingly depart from those of conventional NMDARs.

Perisomatic inhibition of pyramidal neurons (PNs) coordinates cortical network activity during sensory processing, and this role is mainly attributed to parvalbumin-expressing basket cells (BCs). However, cannabinoid receptor type 1 (CB1)-expressing interneurons are also BCs, but the connectivity and function of these elusive but prominent neocortical inhibitory neurons are unclear. We find that their connectivity pattern is visual area specific. Persistently active CB1 signaling suppresses GABA release from CB1 BCs in the medial secondary visual cortex (V2M), but not in the primary visual cortex (V1). Accordingly, in vivo, tonic CB1 signaling is responsible for higher but less coordinated PN activity in the V2M than in the V1. These differential firing dynamics in the V1 and V2M can be captured by a computational network model that incorporates visual-area-specific properties. Our results indicate a differential CB1-mediated mechanism controlling PN activity, suggesting an alternative connectivity scheme of a specific GABAergic circuit in different cortical areas.

SummaryPerisomatic inhibition of neocortical pyramidal neurons (PNs) coordinates cortical network activity during sensory processing, and it has been mainly attributed to parvalbumin-expressing basket cells (BCs). However, cannabinoid receptor type 1 (CB1)-expressing interneurons also inhibit the perisomatic region of PNs but the connectivity and function of these elusive – yet prominent – neocortical GABAergic cells is unknown. We found that the connectivity pattern of CB1-positive BCs strongly differs between primary and high-order cortical visual areas. Moreover, persistently active CB1 signaling suppresses GABA release from CB1 BCs in the medial secondary visual cortex (V2M), but not in the primary (V1) visual area. Accordingly, in vivo, tonic CB1 signaling is responsible for higher but less coordinated PN activity in V2M than in V1. Our results indicate a differential CB1-mediated mechanism controlling PN activity, and suggest an alternative connectivity schemes of a specific GABAergic circuit in different cortical areas

Down syndrome (DS) results in various degrees of cognitive deficits. In DS mouse models, recovery of behavioral and neurophysiological deficits using GABA A R antagonists led to hypothesize an excessive activity of inhibitory circuits in this condition. Nonetheless, whether over-inhibition is present in DS and whether this is due to specific alterations of distinct GABAergic circuits is unknown. In the prefrontal cortex of Ts65Dn mice (a well-established DS model), we found that the dendritic synaptic inhibitory loop formed by somatostatin-positive Martinotti cells (MCs) and pyramidal neurons (PNs) was strongly enhanced, with no alteration of their excitability. Conversely, perisomatic inhibition from parvalbumin-positive (PV) interneurons was unaltered, but PV cells of DS mice lost their classical fast-spiking phenotype and exhibited increased excitability. These microcircuit alterations resulted in reduced pyramidal-neuron firing and increased phase locking to cognitive-relevant network oscillations in vivo . These results define important synaptic and circuit mechanisms underlying of cognitive dysfunctions in DS.

In the neocortex, synaptic inhibition shapes all forms of spontaneous and sensory-evoked activity. Importantly, inhibitory transmission is highly plastic, but the functional role of inhibitory synaptic plasticity is unknown. In the mouse barrel cortex, activation of layer 2/3 PNs elicited strong feed-forward perisomatic inhibition (FFI) onto layer 5 PNs. We found that FFI involving PV cells was strongly potentiated by postsynaptic PN burst firing. FFI plasticity modified PN excitation-to-inhibition (E/I) ratio, strongly modulated PN gain and altered information transfer across cortical layers. Moreover, our LTPi-inducing protocol modified the firing of layer 5 PNs and altered the temporal association of PN spikes to γ-oscillations both in vitro and in vivo. All these effects were captured by unbalancing the E/I ratio in a feed-forward inhibition circuit model. Altogether, our results indicate that activity-dependent modulation of perisomatic inhibitory strength effectively influences the participation of single principal cortical neurons to cognitive-relevant network activity. Impact Statement Long-term potentiation of feed-forward perisomatic inhibition effectively alters the computational properties of single layer 5 pyramidal neurons and their association to network activity.

The neocortex plays a crucial role in all basic and abstract cognitive functions. Conscious mental processes are achieved through a correct flow of information within and across neocortical networks, whose particular activity state results from a tight balance between excitation and inhibition. The proper equilibrium between these indissoluble forces is operated with multiscale organization: along the dendro–somatic axis of single neurons and at the network level. Fast synaptic inhibition is assured by a multitude of inhibitory interneurons. During cortical activities, these cells operate a finely tuned division of labor that is epitomized by their detailed connectivity scheme. Recent results combining the use of mouse genetics, cutting-edge optical and neurophysiological approaches have highlighted the role of fast synaptic inhibition in driving cognition-related activity through a canonical cortical circuit, involving several major interneuron subtypes and principal neurons. Here we detail the organization of this cortical blueprint and we highlight the crucial role played by different neuron types in fundamental cortical computations. In addition, we argue that this canonical circuit is prone to many variations on the theme, depending on the resolution of the classification of neuronal types, and the cortical area investigated. Finally, we discuss how specific alterations of distinct inhibitory circuits can underlie several devastating brain diseases.

Down syndrome (DS) results in various degrees of cognitive deficits. In DS mouse models, recovery of behavioral and neurophysiological deficits using GABA A R antagonists led to hypothesize an excessive activity of inhibitory circuits in this condition. Nonetheless, whether over-inhibition is present in DS and whether this is due to specific alterations of distinct GABAergic circuits is unknown. In the prefrontal cortex of Ts65Dn mice (a well-established DS model), we found that the dendritic synaptic inhibitory loop formed by somatostatin-positive Martinotti cells (MCs) and pyramidal neurons (PNs) was strongly enhanced, with no alteration in their excitability. Conversely, perisomatic inhibition from parvalbumin-positive (PV) interneurons was unaltered, but PV cells of DS mice lost their classical fast-spiking phenotype and exhibited increased excitability. These microcircuit alterations resulted in reduced pyramidal-neuron firing and increased phase locking to cognitive-relevant network oscillations in vivo. These results define important synaptic and circuit mechanisms underlying cognitive dysfunctions in DS.

Exposure to stress during early life (infancy/childhood) has long-term effects on the structure and function of the prefrontal cortex (PFC), and increases the risk for adult depression and anxiety disorders. However, little is known about the molecular and cellular mechanisms of these effects. Here, we focused on changes induced by chronic maternal separation during the first 2 weeks of postnatal life. Unbiased mRNA expression profiling in the medial PFC (mPFC) of maternally separated (MS) pups identified an increased expression of myelin-related genes and a decreased expression of immediate early genes. Oligodendrocyte lineage markers and birthdating experiments indicated a precocious oligodendrocyte differentiation in the mPFC at P15, leading to a depletion of the oligodendrocyte progenitor pool in MS adults. We tested the role of neuronal activity in oligodendrogenesis, using designed receptors exclusively activated by designed drugs (DREADDs) techniques. hM4Di or hM3Dq constructs were transfected into mPFC neurons using fast-acting AAV8 viruses. Reduction of mPFC neuron excitability during the first 2 postnatal weeks caused a premature differentiation of oligodendrocytes similar to the MS pups, while chemogenetic activation normalised it in the MS animals. Bidirectional manipulation of neuron excitability in the mPFC during the P2–P14 period had long lasting effects on adult emotional behaviours and on temporal object recognition: hM4Di mimicked MS effects, while hM3Dq prevented the pro-depressive effects and short-term memory impairment of MS. Thus, our results identify neuronal activity as a critical target of early-life stress and demonstrate its function in controlling both postnatal oligodendrogenesis and adult mPFC-related behaviours.

Parvalbumin (PV)-positive interneurons modulate cortical activity through highly specialized connectivity patterns onto excitatory pyramidal neurons (PNs) and other inhibitory cells. PV cells are autoconnected through powerful autapses, but the contribution of this form of fast disinhibition to cortical function is unknown. We found that autaptic transmission represents the most powerful inhibitory input of PV cells in neocortical layer V. Autaptic strength was greater than synaptic strength onto PNs as a result of a larger quantal size, whereas autaptic and heterosynaptic PV-PV synapses differed in the number of release sites. Overall, single-axon autaptic transmission contributed to approximately 40% of the global inhibition (mostly perisomatic) that PV interneurons received. The strength of autaptic transmission modulated the coupling of PV-cell firing with optogenetically induced γ-oscillations, preventing high-frequency bursts of spikes. Autaptic self-inhibition represents an exceptionally large and fast disinhibitory mechanism, favoring synchronization of PV-cell firing during cognitive-relevant cortical network activity.

In the neocortex, critical periods (CPs) of plasticity are closed following the accumulation of perineuronal nets (PNNs) around parvalbumin (PV)-positive inhibitory interneurons. However, how PNNs tune cortical function and plasticity is unknown. We found that PNNs modulated the gain of visual responses and γ-oscillations in the adult mouse visual cortex in vivo, consistent with increased interneuron function. Removal of PNNs in adult V1 did not affect GABAergic neurotransmission from PV cells, nor neuronal excitability in layer 4. Importantly, PNN degradation coupled to sensory input potentiated glutamatergic thalamic synapses selectively onto PV cells. In the absence of PNNs, increased thalamic PV-cell recruitment modulated feed-forward inhibition differently on PV cells and pyramidal neurons. These effects depended on visual input, as they were strongly attenuated by monocular deprivation in PNN-depleted adult mice. Thus, PNNs control visual processing and plasticity by selectively setting the strength of thalamic recruitment of PV cells.

SummaryIn the neocortex, the closure of critical periods (CPs) of plasticity is paralleled by the accumulation of perineuronal nets (PNNs) around parvalbumin (PV)-positive inhibitory interneurons. Accordingly, PNN degradation in adult mammals re-opens cortical plasticity. However, how PNNs tune cortical function and plasticity is unknown. We found that PNNs modulated the gain of visual responses in the adult mouse visual cortex in vivo. Removal of PNNs in adult V1 strongly increased thalamic neurotransmission selectively on layer 4 PV cells. This produced a differential gating of feed-forward inhibition on principal neurons and other PV cells, with no alterations of unitary inhibitory synaptic transmission and neuronal excitability. These effects depended on visual input, as they were strongly attenuated by monocular deprivation in PNN-depleted adult mice. Thus, PNNs control visual processing and plasticity by selectively setting the strength of thalamic recruitment of PV cells. We conclude that PNN accumulation during circuit maturation likely prevents excessive thalamic excitation of PV cells at the expense of cortical plasticity.