Balanced feedforward inhibition and dominant recurrent inhibition in olfactory cortex

Throughout the brain, the recruitment of feedforward and recurrent inhibition shapes neural responses. However, disentangling the relative contributions of these often-overlapping cortical circuits is challenging. The piriform cortex provides an ideal system to address this issue because the interneurons responsible for feedforward and recurrent inhibition are anatomically segregated in layer (L) 1 and L2/3 respectively. Here we use a combination of optical and electrical activation of interneurons to profile the inhibitory input received by three classes of principal excitatory neuron in the anterior piriform cortex. In all classes, we find that L1 interneurons provide weaker inhibition than L2/3 interneurons. Nonetheless, feedforward inhibitory strength covaries with the amount of afferent excitation received by each class of principal neuron. In contrast, intracortical stimulation of L2/3 evokes strong inhibition that dominates recurrent excitation in all classes. Finally, we find that the relative contributions of feedforward and recurrent pathways differ between principal neuron classes. Specifically, L2 neurons receive more reliable afferent drive and less overall inhibition than L3 neurons. Alternatively, L3 neurons receive substantially more intracortical inhibition. These three features—balanced afferent drive, dominant recurrent inhibition, and differential recruitment by afferent vs. intracortical circuits, dependent on cell class—suggest mechanisms for olfactory processing that may extend to other sensory cortices.The recruitment of inhibition is an essential feature of cortical processing. Feedforward and recurrent inhibitory circuits have been implicated in controlling the timing, strength, and tuning of cortical responses (for review, see ref. 1). In sensory cortices, including the olfactory cortex, neural responses to sensory stimuli depend on the relative balance of inhibition with respect to excitation in both feedforward and recurrent pathways (2–7). Moreover, numerous theoretical studies have suggested that balanced cortical networks underlie the selectivity, sparseness, and correlations of cortical activity (8–15). These studies highlight the importance of quantifying the relationship between excitation and inhibition in cortical networks. However, isolating the contributions of feedforward vs. recurrently evoked inhibition to cortical responses is difficult because these circuits are often coactive and frequently share interneurons (16–18). The piriform cortex is an ideal system to address this issue because the interneurons responsible for feedforward and recurrent inhibition differ by class and laminar location and, thus, are differentially recruited by afferent and intracortical excitation (19–22).The piriform cortex is a trilaminar cortex responsible for processing olfactory stimuli. Principal excitatory neurons are found in layer (L) 2/3 and send dendrites to L1, where they receive odor-related excitation directly from the olfactory bulb via the lateral olfactory tract (LOT) (23). LOT afferents also drive horizontal and neurogliaform inhibitory interneurons within L1, yielding feedforward inhibition of principal neurons (24). Within the cortex, principal neurons send axon collaterals throughout L2/3 and to an intracortical fiber tract in L1b (25, 26). Intracortical excitation recruits a number of interneuron classes within L2/3 that, in turn, provide recurrent inhibition to principal neurons (20, 24, 27, 28). Stimulation of the LOT evokes short- latency feedforward inhibition that targets principal neuron dendrites, as well as long-latency, recurrent inhibition that is somatic (24, 28, 29). These findings are consistent with the different laminar locations of inhibitory interneurons mediating feedforward and recurrent inhibition respectively.Previous studies have shown that electrical stimulation of the LOT as well as odors recruit mixed feedforward and recurrent inhibition in vivo (3, 4, 30, 31). However, in vivo and in vitro studies focusing on different principal neuron classes have led to conflicting reports of the relative contributions of feedforward and feedback inhibition during afferent odor processing (3, 24, 27, 28, 32). Furthermore, because of the disynaptic nature of inhibition, estimates of feedforward or recurrent inhibitory strength depend on the quality of afferent and intracortical excitatory recruitment, which varies with the different stimulation protocols used in each study. Here, we resolve these discrepancies by comparing feedforward and recurrent inhibition evoked by direct optical activation of interneurons that express channelrhodopsin (ChR2) (33), as well as electrical stimulation of excitatory pathways in all three classes of principal neuron in the anterior piriform cortex (APC).In the APC, principal excitatory neuron classes differ in laminar location and in the proportion of afferent vs. intracortical excitatory input received (34–37). Within L2, semilunar cells (SLCs) receive predominantly afferent excitation, whereas superficial pyramidal cells (sPCs) receive weaker afferent and stronger intracortical excitatory drive. In L3, deep pyramidal cells (dPCs) receive minimal afferent, but substantial intracortical excitation. Given these differences in excitatory drive, we hypothesized that inhibition mediated by feedforward and recurrent inhibitory circuits also differs between principal neuron classes. In this study, we find that principal neuron classes are weakly inhibited by L1 interneurons that mediate feedforward inhibition, compared with L2/3 interneurons that provide strong recurrent inhibition. As predicted, feedforward inhibitory strength varies in a manner consistent with the amount of afferent excitation received by each class of principal neuron. In contrast, intracortical stimulation of L3 evokes strong recurrent inhibition that dominates excitation in all classes. Moreover, excitatory and inhibitory profiles differ between SLCs, sPCs, and dPCs. Taken together, our results demonstrate that inhibitory circuits in the piriform cortex provide both balanced feedforward inhibition and dominant recurrent inhibition, as well as segregate principal excitatory neuron classes during cortical processing.     

Nonsensory target-dependent organization of piriform cortex

The piriform cortex (PCX) is the largest component of the olfactory cortex and is hypothesized to be the locus of odor object formation. The distributed odorant representation found in PCX contrasts sharply with the topographical representation seen in other primary sensory cortices, making it difficult to test this view. Recent work in PCX has focused on functional characteristics of these distributed afferent and association fiber systems. However, information regarding the efferent projections of PCX and how those may be involved in odor representation and object recognition has been largely ignored. To investigate this aspect of PCX, we have used the efferent pathway from mouse PCX to the orbitofrontal cortex (OFC). Using double fluorescent retrograde tracing, we identified the output neurons (OPNs) of the PCX that project to two subdivisions of the OFC, the agranular insula and the lateral orbitofrontal cortex (AI-OPNs and LO-OPNs, respectively). We found that both AI-OPNs and LO-OPNs showed a distinct spatial topography within the PCX and fewer than 10% projected to both the AI and the LO as judged by double-labeling. These data revealed that the efferent component of the PCX may be topographically organized. Further, these data suggest a model for functional organization of the PCX in which the OPNs are grouped into parallel output circuits that provide olfactory information to different higher centers. The distributed afferent input from the olfactory bulb and the local PCX association circuits would then ensure a complete olfactory representation, pattern recognition capability, and neuroplasticity in each efferent circuit.The olfactory system creates perceptual odor objects from often complex mixtures of diverse airborne chemicals (1, 2). This formidable job is mainly accomplished by a surprisingly “shallow” three-level pathway, comprising the olfactory epithelium, olfactory bulb, and olfactory cortex (3). The olfactory epithelium accommodates millions of olfactory sensory neurons (OSNs), each of which can be defined by the particular receptor protein selected for expression from the ∼1,000 odor receptor genes in the typical mammalian genome (4, 5). Axons from all OSNs expressing the same odor receptor coalesce into a few glomeruli on the surface of the olfactory bulb (6–8). Each glomerulus is therefore dedicated to a particular receptor. The position of each glomerulus appears to vary only slightly from animal to animal, giving rise to speculation that the glomeruli form a spatial map of odor sensitivities.Within the glomeruli, the incoming OSN axons form synapses with the apical dendrites of second-order neurons and the mitral and tufted cells, providing what would seem to be an anatomical basis for topographical odorant representation (9–11). Each of about a dozen mitral or tufted cells innervating only a single glomerulus send their axons to targets in a number of ventral forebrain areas, collectively termed the olfactory cortex (12).However, this seemingly orderly topography of odorant representation is not maintained in the olfactory cortex. Especially in the largest olfactory area—the piriform cortex (PCX)—odorants are represented by sparse, distributed, and spatially overlapping neural ensembles across the cortex (13–18). This nontopographical representation stems largely from the architecture of the PCX, including distributed afferent inputs (19–21), and a similarly distributed intracortical association fiber system, which links single cortical neurons (pyramidal and semilunar cells) with neighboring and distant neurons (22–25). Furthermore, each cortical neuron receives an apparently random collection of glomerular inputs (26, 27). Therefore, a spatial location of the cortex is not predictive of odorant tuning as neighboring neurons may exhibit distinct receptive ranges (14, 18, 27).Compared with existing data on the afferent and association connections, data on the efferent aspects of PCX are limited. Although previous studies have identified a number of higher centers that are targeted by the PCX output neurons (OPNs), including the orbitofrontal cortex (OFC), hippocampus, hypothalamus, and thalamic nuclei (28–31), information regarding organization and spatial distribution of these neurons (pyramidal and semilunar cells) is lacking. Questions such as how the OPNs projecting to different targets are distributed within PCX and whether that may imply any intrinsic or functional organization of the PCX remain unanswered. Given the complexity of the distributed afferent and association fiber system, data from the efferent system may help to discern organizing principles in the PCX and lend some understanding as to how it processes incoming sensory information.To reach this goal, we focused on the projection from the PCX to the OFC, an important center for odor-guided behaviors (19, 30, 32). We injected different cholera toxin B (CTB) subunit fluorescent conjugates into two subdivisions of the OFC, the agranular insular (AI) and the lateral OFC (LO), in mice and examined the PCX for retrograde labeling (33). We found that the OPNs projecting to the AI and the LO are differentially distributed. Interestingly, both OPNs to the AI and LO exhibited topographically specific distributions in the PCX. In addition, they showed distinct distribution patterns along the anterior–posterior axis of the PCX. These two OPN populations had limited overlap within the anterior PCX (aPCX), as double-labeled neurons were extremely rare. These data suggest that the functional organization of PCX may be better understood through its output circuits, shedding new light on the role of olfactory cortex in central odor processing.     

Epileptic baboons have lower numbers of neurons in specific areas of cortex

Epilepsy is characterized by recurrent seizure activity that can induce pathological reorganization and alter normal function in neocortical networks. In the present study, we determined the numbers of cells and neurons across the complete extent of the cortex for two epileptic baboons with naturally occurring seizures and two baboons without epilepsy. Overall, the two epileptic baboons had a 37% average reduction in the number of cortical neurons compared with the two nonepileptic baboons. The loss of neurons was variable across cortical areas, with the most pronounced loss in the primary motor cortex, especially in lateral primary motor cortex, representing the hand and face. Less-pronounced reductions of neurons were found in other parts of the frontal cortex and in somatosensory cortex, but no reduction was apparent in the primary visual cortex and little in other visual areas. The results provide clear evidence that epilepsy in the baboon is associated with considerable reduction in the numbers of cortical neurons, especially in frontal areas of the cortex related to motor functions. Whether or not the reduction of neurons is a cause or an effect of seizures needs further investigation.Epilepsy is associated with structural changes in the cerebral cortex (e.g., refs. 1–6), and partial epilepsies (i.e., seizures originating from a brain region) may lead to loss of neurons (7) and altered connectivity (8). The cerebral cortex is a heterogeneous structure comprised of multiple sensory and motor information-processing systems (e.g., refs. 9 and 10) that vary according to their processing demands, connectivity (e.g., refs. 11 and 12), and intrinsic numbers of cells and neurons (13–16). Chronic seizures have been associated with progressive changes in the region of the epileptic focus and in remote but functionally connected cortical or subcortical structures (3, 17). Because areas of the cortex are functionally and structurally different, they may also differ in susceptibility to pathological changes resulting from epilepsy.The relationship between seizure activity and neuron damage can be difficult to study in humans. Seizure-induced neuronal damage can be convincingly demonstrated in animals using electrically or chemically induced status epilepticus (one continuous seizure episode longer than 5 min) to reveal morphometric (e.g., refs. 18 and 19) or histological changes (e.g., refs. 20 and 21). Subcortical brain regions are often studied for vulnerability to seizure-induced injury (21–27); however, a recent study by Karbowski et al. (28) observed reduction of neurons in cortical layers 5 and 6 in the frontal lobes of rats with seizures. Seizure-induced neuronal damage in the cortex has also been previously demonstrated in baboons with convulsive status epilepticus (29).The goal of the present study was to determine if there is a specific pattern of cell or neuron reduction across the functionally divided areas of the neocortex in baboons with epilepsy. Selected strains of baboons have been studied as a natural primate model of generalized epilepsy (30–36) that is analogous to juvenile myoclonic epilepsy in humans. The baboons demonstrate generalized myoclonic and tonic-clonic seizures, and they have generalized interictal and ictal epileptic discharges on scalp EEG. Because of their phylogenetic proximity to humans, baboons and other Old World monkeys share many cortical areas and other features of cortical organization with humans (e.g., refs. 9 and 10). Cortical cell and neuron numbers were determined using the flow fractionator method (37, 38) in epileptic baboon tissue obtained from the Texas Biomedical Research Institute, where a number of individuals develop generalized epilepsy within a pedigreed baboon colony (31–36). Our results reveal a regionally specific neuron reduction in the cortex of baboons with naturally occurring, generalized seizures.     

Parallel pathways convey olfactory information with opposite polarities in Drosophila

In insects, olfactory information received by peripheral olfactory receptor neurons (ORNs) is conveyed from the antennal lobes (ALs) to higher brain regions by olfactory projection neurons (PNs). Despite the knowledge that multiple types of PNs exist, little is known about how these different neuronal pathways work cooperatively. Here we studied the Drosophila GABAergic mediolateral antennocerebral tract PNs (mlPNs), which link ipsilateral AL and lateral horn (LH), in comparison with the cholinergic medial tract PNs (mPNs). We examined the connectivity of mlPNs in ALs and found that most mlPNs received inputs from both ORNs and mPNs and participated in AL network function by forming gap junctions with other AL neurons. Meanwhile, mlPNs might innervate LH neurons downstream of mPNs, exerting a feedforward inhibition. Using dual-color calcium imaging, which enables a simultaneous monitoring of neural activities in two groups of PNs, we found that mlPNs exhibited robust odor responses overlapping with, but broader than, those of mPNs. Moreover, preferentially down-regulation of GABA in most mlPNs caused abnormal courtship and aggressive behaviors in male flies. These findings demonstrate that in Drosophila, olfactory information in opposite polarities are carried coordinately by two parallel and interacted pathways, which could be essential for appropriate behaviors.In insects, the detection of olfactory cues begins at the peripheral olfactory receptor neurons (ORNs), which transfer the chemical information into neural signals and convey them to the first central relay station—the antennal lobes (ALs) (1–3). After AL local processing, olfactory information is relayed to higher brain regions via different groups of projection neurons (PNs) (4–6). Except some pioneering studies in Hymenopterans (7–11) and Lepidopterans (12), little is known about how these different PNs connect in the olfactory circuit and work physiologically. As the most studied PN type in Drosophila, the cholinergic PNs (mPNs) form the medial antennocerebral tract and convey excitatory signals encoding odor identity and intensity (13–15) that are necessary for the fly to perform appropriate behaviors (16, 17). However, how olfactory information is delivered via pathways mediated by PNs other than mPNs remains to be elucidated. In this study, we focused on the mediolateral antennocerebral tract PNs (mlPNs), which are the second largest PN subset (∼50 mlPNs in each hemisphere) and reported to be largely GABAergic with axons terminating mainly in the lateral horn (LH) (18, 19). Based on the extent of their dendritic arborization, mlPNs can be further categorized into three subtypes: the uniglomerular mlPNs (type 1 mlPNs, mlPN1s); the multiglomerular mlPNs (mlPN2s), which comprise the great majority (>80%) of mlPNs; and the panglomerular mlPNs (mlPN3s) (19). Here we focused on mlPN1s and mlPN2s, which exclusively link ALs with the ipsilateral LH and were labeled by Mz699-Gal4 (hereafter referred to as Mz699-mlPNs, or mlPNs) (20). Using dual whole-cell recordings and optogenetic activation of mlPNs, we examined their connectivity with different groups of AL and LH neurons and identified their main excitatory input neurons and putative output targets. The odor responses of mlPNs were also characterized and compared with those of ORNs and mPNs by dual-color imaging. Finally, we investigated the physiological role of mlPNs at the behavioral level by preferentially silencing the inhibitory output of mlPNs. Our results showed that by conveying olfactory information as inhibitory signals in parallel with the excitatory mPN pathway, the mlPN pathway might play important roles in modulating fly behavior through a novel feedforward inhibition mechansim.     

Nitric oxide mediates local activity-dependent excitatory synapse development

Learning related paradigms play an important role in shaping the development and specificity of synaptic networks, notably by regulating mechanisms of spine growth and pruning. The molecular events underlying these synaptic rearrangements remain poorly understood. Here we identify NO signaling as a key mediator of activity-dependent excitatory synapse development. We find that chronic blockade of NO production in vitro and in vivo interferes with the development of hippocampal and cortical excitatory spine synapses. The effect results from a selective loss of activity-mediated spine growth mechanisms and is associated with morphological and functional alterations of remaining synapses. These effects of NO are mediated by a cGMP cascade and can be reproduced or prevented by postsynaptic expression of vasodilator-stimulated phosphoprotein phospho-mimetic or phospho-resistant mutants. In vivo analyses show that absence of NO prevents the increase in excitatory synapse density induced by environmental enrichment and interferes with the formation of local clusters of excitatory synapses. We conclude that NO plays an important role in regulating the development of excitatory synapses by promoting local activity-dependent spine-growth mechanisms.Neuronal activity and experience critically control the development and organization of synaptic networks by regulating the mechanisms of synapse formation and elimination. Sensory experience, motor training tasks, fear-conditioning, song-learning in birds, or exposure to novel environments in rodents are associated with major structural rearrangements of connectivity (1–8). An interesting aspect of these structural rearrangements is that they are spatially organized: the new spines formed as a result of activity tend to grow in the vicinity of activated synapses and repetitive learning promotes the formation of synaptic clusters (9–11). Despite the importance of these mechanisms for the development of brain circuits, the molecular events underlying these activity-mediated structural rearrangements of connectivity remain still essentially unknown.Here we tested whether the diffusible messenger nitric oxide (NO) could contribute to these mechanisms. NO is produced at excitatory synapses as a result of synaptic activation through the close association of its synthesizing enzyme, neuronal nitric oxide synthase (nNOS), with the postsynaptic density and NMDA receptors (12–14). NO has thus been implicated in several aspects of synaptic function and plasticity (15–18), notably as a retrograde messenger regulating presynaptic properties, such as synaptic vesicle recycling in terminals and growth and remodeling of presynaptic varicosities (19–22). At inhibitory synapses in the ventral tegmental area, NO has even heterosynaptic effects, mediating a form of GABA-mediated long-term potentiation triggered by excitatory synapse activation (23). Studies of nNOS-deficient mice further suggest an important role of NO in cognitive functions and social behavior (24–26) and recent genetic analyses have reported associations between genetic variants of nNOS and schizophrenia (27, 28), suggesting a possible developmental role of NO for brain circuit formation. Our study provides direct evidence for such a role by demonstrating that NO is required for activity-mediated synapse formation.     

Sleep deprivation increases dorsal nexus connectivity to the dorsolateral prefrontal cortex in humans

In many patients with major depressive disorder, sleep deprivation, or wake therapy, induces an immediate but often transient antidepressant response. It is known from brain imaging studies that changes in anterior cingulate and dorsolateral prefrontal cortex activity correlate with a relief of depression symptoms. Recently, resting-state functional magnetic resonance imaging revealed that brain network connectivity via the dorsal nexus (DN), a cortical area in the dorsomedial prefrontal cortex, is dramatically increased in depressed patients. To investigate whether an alteration in DN connectivity could provide a biomarker of therapy response and to determine brain mechanisms of action underlying sleep deprivations antidepressant effects, we examined its influence on resting state default mode network and DN connectivity in healthy humans. Our findings show that sleep deprivation reduced functional connectivity between posterior cingulate cortex and bilateral anterior cingulate cortex (Brodmann area 32), and enhanced connectivity between DN and distinct areas in right dorsolateral prefrontal cortex (Brodmann area 10). These findings are consistent with resolution of dysfunctional brain network connectivity changes observed in depression and suggest changes in prefrontal connectivity with the DN as a brain mechanism of antidepressant therapy action.Sleep deprivation has been used for decades as a rapid-acting and effective treatment in patients with major depressive disorder (MDD) (1, 2). Although clinically well established, the mechanisms of action are largely unknown.Brain imaging studies have shown that sleep deprivation in depressed patients is associated with renormalized metabolic activity, mainly in limbic structures including anterior cingulate (ACC) as well as dorsolateral prefrontal cortex (DLPFC) (3–6), and that changes in limbic and DLPFC activity correlated with a relief of depression symptoms (7–9). Recent studies in patients with depression point to a critical importance of altered large-scale brain network connectivity during the resting state (10, 11). Among these networks, the default mode network (DMN), which mainly comprises cortical midline structures including precuneus and medial frontal cortex as well as the inferior parietal lobule (12–15), is most consistently characterized. In functional magnetic resonance imaging (fMRI) studies, the DMN shows the strongest blood oxygenation level–dependent (BOLD) activity during rest and decreased BOLD reactivity during goal-directed task performance. The DMN is anticorrelated with the cognitive control network (CCN), a corresponding task-positive network, which encompasses bilateral fronto-cingulo-parietal structures including lateral prefrontal and superior parietal areas (16). A third system with high relevance for depression—the affective network (AN)—is based in the subgenual and pregenual parts of the ACC [Brodman area (BA) 32] (17). The AN is active during both resting and task-related emotional processing, and forms strong functional and structural connections to other limbic areas such as hypothalamus, amygdala, entorhinal cortex, and nucleus accumbens (18, 19).Increased connectivity of DMN, CCN, and AN with a distinct area in the bilateral dorsomedial prefrontal cortex (DMPFC) was recently found in patients with depression compared with healthy controls (20). This area within the DMPFC was termed dorsal nexus (DN) and was postulated to constitute a converging node of depressive “hot wiring,” which manifests itself in symptoms of emotional, cognitive, and vegetative dysregulation. This led to the hypothesis that a modification in connectivity via the DN would be a potential target for antidepressant treatments (20).Recent studies in healthy subjects reported reduced functional connectivity within DMN and between DMN and CCN in the morning after total (21) and in the evening after partial sleep deprivation (22). However, brain network connectivity via the DN was not examined in these studies. Given the recently proposed role of the DN in mood regulation, here we specifically tested whether sleep deprivation as a well-known antidepressant treatment modality affects connectivity via the DN. Based on our previous findings on network changes by ketamine (23), we hypothesized that sleep deprivation leads to a reduction in connectivity via the DN.     

Elevated hippocampal resting-state connectivity underlies deficient neurocognitive function in aging

The brain is not idle during rest. Functional MRI (fMRI) studies have identified several resting-state networks, including the default mode network (DMN), which contains a set of cortical regions that interact with a hippocampus (HC) subsystem. Age-related alterations in the functional architecture of the DMN and HC may influence memory functions and possibly constitute a sensitive biomarker of forthcoming memory deficits. However, the exact form of DMN–HC alterations in aging and concomitant memory deficits is largely unknown. Here, using both task and resting data from 339 participants (25–80 y old), we have demonstrated age-related decrements in resting-state functional connectivity across most parts of the DMN, except for the HC network for which age-related elevation of connectivity between left and right HC was found along with attenuated HC–cortical connectivity. Elevated HC connectivity at rest, which was partly accounted for by age-related decline in white matter integrity of the fornix, was associated with lower cross-sectional episodic memory performance and declining longitudinal memory performance over 20 y. Additionally, elevated HC connectivity at rest was associated with reduced HC neural recruitment and HC–cortical connectivity during active memory encoding, which suggests that strong HC connectivity restricts the degree to which the HC interacts with other brain regions during active memory processing revealed by task fMRI. Collectively, our findings suggest a model in which age-related disruption in cortico–hippocampal functional connectivity leads to a more functionally isolated HC at rest, which translates into aberrant hippocampal decoupling and deficits during mnemonic processing.The brain is not idle at rest (1). Rather, intrinsic neuronal signaling, which manifests as spontaneous fluctuations in the blood oxygen level-dependent (BOLD) functional MRI (fMRI) signal, is ubiquitous in the human brain and consumes a substantial portion of the brain’s energy (2). Coherent spontaneous activity has been revealed in a hierarchy of networks that span large-scale functional circuits in the brain (3–6). These resting-state networks (RSNs) show moderate-to-high test–retest reliability (7) and replicability (8), and some have been found in the monkey (9) and infant (10) brain. In the adult human brain, RSNs include sensory motor, visual, attention, and mnemonic networks, as well as the default mode network (DMN). There is evidence that the DMN entails interacting subsystems and hubs that are implicated in episodic memory (11–13). One major hub encompasses the posterior cingulate cortex and the retrosplenial cortex. Other hubs include the lateral parietal cortex and the medial prefrontal cortex. In addition, a hippocampus (HC) subsystem is distinct from, yet interrelated with, the major cortical DMN hubs (12, 14).The functional architecture of the DMN and other RSNs is affected by different conditions, such as Alzheimer’s disease (AD), Parkinson’s disease, and head injury, suggesting that measurements of the brain’s intrinsic activity may be a sensitive biomarker and a putative diagnostic tool (for a review, see ref. 15). Alterations of the DMN have also been shown in age-comparative studies (16, 17), but the patterns of alterations are not homogeneous across different DMN components (18). Reduced functional connectivity among major cortical DMN nodes has been reported in aging (16, 17) and also in AD (19) and for asymptomatic APOE e4 carriers at increased risk of developing AD (20). Reduced cortical DMN connectivity has been linked to age-impaired performance on episodic memory (EM) tasks (21, 22). For instance, Wang and colleagues (21) showed that functional connectivity between cortical and HC hubs promoted performance on an EM task and was substantially weaker among low-performing elderly. This and other findings suggest that reductions in the DMN may be a basis for age-related EM impairment. However, elevated connectivity has been observed for the HC in individuals at genetic risk for AD (23, 24) and for elderly with memory complaints (25). Furthermore, a trend toward elevated functional connectivity for the medial temporal lobe (MTL) subsystem was observed in healthy older adults (26). Critically, higher subcortical RSN connectivity was found to correlate negatively with EM performance in an aging sample (27). Moreover, a recent combined fMRI/EEG study observed age increases in HC EEG beta power during rest (28).Thus, the association of aging with components of the DMN is complex, and it has been argued that age-related increases in functional connectivity need further examination (18). Such increases could reflect a multitude of processes, including age-related degenerative effects on the brain’s gray and white matter (18). Additionally, increases in HC functional connectivity may reflect alterations in proteolytic processes, such as amyloid deposition (29). Amyloid deposition is most prominent in posterior cortical regions of the DMN (29). It has been argued that there is a topological relationship between high neural activity over a lifetime within the DMN and amyloid deposition (30). Increased amyloid β protein burden within the posterior cortical DMN may cause cortico–hippocampal functional connectivity disruption (31), leading to a more functionally isolated HC network, which translates into aberrant hippocampal decoupling (30, 32, 33). Correspondingly, a recent model hypothesized that progressively less inhibitory cortical input would cause HC hyperactivity in aging (34).Elevated HC resting-state connectivity might thus be a sign of brain dysfunction, but the evidence remains inconclusive. Here, using data from a population-based sample covering the adult age span (n = 339, 25–80 y old), we tested the hypothesis that aging differentially affects distinct DMN components. A data-driven approach, independent component analysis (ICA), was used to identify DMN subsystems (4). We expected to observe age-related decreases in the connectivity of the cortical DMN. We also examined age-related alterations of HC RSN connectivity, and tested whether such alterations were related to HC volume and white matter integrity. We predicted that if increased HC connectivity was found, it would be accompanied by age-related decreases in internetwork connectivity of the HC RSN with cortical DMN regions. To constrain interpretations of age-related alterations, the DMN components were related to cognitive performance. Elevated HC RSN should negatively correlate with level and longitudinal change in EM performance. Such negative correlations could reflect an inability to flexibly recruit the HC and functionally associated areas during EM task performance due to aberrant hippocampal decoupling (23, 24). We tested this prediction by relating the HC RSN, within-person, to HC recruitment during an EM fMRI task (35, 36).     

Specialization and integration of functional thalamocortical connectivity in the human infant

Connections between the thalamus and cortex develop rapidly before birth, and aberrant cerebral maturation during this period may underlie a number of neurodevelopmental disorders. To define functional thalamocortical connectivity at the normal time of birth, we used functional MRI (fMRI) to measure blood oxygen level-dependent (BOLD) signals in 66 infants, 47 of whom were at high risk of neurocognitive impairment because of birth before 33 wk of gestation and 19 of whom were term infants. We segmented the thalamus based on correlation with functionally defined cortical components using independent component analysis (ICA) and seed-based correlations. After parcellating the cortex using ICA and segmenting the thalamus based on dominant connections with cortical parcellations, we observed a near-facsimile of the adult functional parcellation. Additional analysis revealed that BOLD signal in heteromodal association cortex typically had more widespread and overlapping thalamic representations than primary sensory cortex. Notably, more extreme prematurity was associated with increased functional connectivity between thalamus and lateral primary sensory cortex but reduced connectivity between thalamus and cortex in the prefrontal, insular and anterior cingulate regions. This work suggests that, in early infancy, functional integration through thalamocortical connections depends on significant functional overlap in the topographic organization of the thalamus and that the experience of premature extrauterine life modulates network development, altering the maturation of networks thought to support salience, executive, integrative, and cognitive functions.The formation of topographically organized neural connections between cerebral cortex and thalamus is necessary for normal cortical morphogenesis (1), and development of these connections requires thalamocortical projections to synapse transiently in the temporary cortical subplate before penetrating the cortical plate (2–4). In humans, the subplate is at maximal extent in the last trimester of gestation (5), a time of rapid growth for thalamocortical fibers and the cortical dendritic tree, particularly in heteromodal cortex (6, 7). This process has been shown to be disrupted by preterm birth (8). Premature delivery is associated with increased risk of neurocognitive impairment, and it is widely hypothesized that abnormal development of brain structure during this period is the cause of these problems and may also underlie the development of autistic spectrum disorders and attention deficit disorders in genetically predisposed individuals.During the last trimester of pregnancy, functional MRI (fMRI) detects the emergence of coordinated, spontaneous fluctuations in the blood oxygen level-dependent (BOLD) signals, which are closely linked with the development of electroencephalographic activity (9–11) and develop into a near-facsimile of the mature adult resting-state network architecture by the normal age of birth at 38–42 wk gestational age (12). However, little is known about the growth of functional connectivity between the thalamus and cortex during this period.Anatomical studies in animals and postmortem adult human subjects have defined the thalamic microstructure and described a corticotopic parcellation of the thalamus with precise connectivity to specific cortical regions (13, 14). Diffusion tensor imaging studies have described a similar pattern of structural thalamocortical connectivity (15, 16), with evidence in adults that some thalamocortical circuits share common thalamic territory, giving the potential for integrative functions (17). Functional connectivity MRI analysis between the thalamus and the cortex has also shown corticotopic organization in the thalamus (18, 19).It is not known, however, when this thalamocortical mapping develops or how it might be disrupted during development. We, therefore, used connectivity fMRI to address a series of questions. First, is the pattern of dominant thalamocortical connectivity at the time of normal birth already similar to the mature adult pattern? Second, in addition to the dominant thalamocortical correlations, is there a pattern of overlapping cortical representations in the neonatal thalamus that might reflect developing integration of functional cortical regions? Third, does the experience of preterm delivery and premature extrauterine life affect the development of thalamocortical connectivity, and is the effect more marked in rapidly developing heteromodal cortex than in more mature primary cortex?     

Functional feedback from mushroom bodies to antennal lobes in the Drosophila olfactory pathway

Feedback plays important roles in sensory processing. Mushroom bodies are believed to be involved in olfactory learning/memory and multisensory integration in insects. Previous cobalt-labeling studies have suggested the existence of feedback from the mushroom bodies to the antennal lobes in the honey bee. In this study, the existence of functional feedback from Drosophila mushroom bodies to the antennal lobes was investigated through ectopic expression of the ATP receptor P2X₂ in the Kenyon cells of mushroom bodies. Activation of Kenyon cells induced depolarization in projection neurons and local interneurons in the antennal lobes in a nicotinic receptor-dependent manner. Activation of Kenyon cell axons in the βγ-lobes in the mushroom body induced more potent responses in the antennal lobe neurons than activation of Kenyon cell somata. Our results indicate that functional feedback from Kenyon cells to projection neurons and local interneurons is present in Drosophila and is likely mediated by the βγ-lobes. The presence of this functional feedback from the mushroom bodies to the antennal lobes suggests top-down modulation of olfactory information processing in Drosophila.In the fruit fly, the olfactory system is important for identifying food sources, avoiding predators, and recognizing mating partners (1). The primary Drosophila olfactory neurons are the olfactory receptor neurons (ORNs) present in the two olfactory organs of the fruit fly, the antennae and the maxillary palps (2, 3). Chemical stimuli detected by the olfactory receptors in the ORNs are converted into electrical signals, which are transmitted to the secondary neurons, the projection neurons (PNs), in the antennal lobes. The antennal lobes are important olfactory coding centers that consist of PNs as well as inhibitory and excitatory local interneurons (LNs) (4 –6). The PNs receive signals from the primary ORNs and also receive lateral inhibitory/excitatory inputs from the LNs (7 –10). After processing in the antennal lobes, olfactory information is relayed by PNs to the mushroom bodies and the protocerebrum region of the fly brain (11 –13).Feedback is important in sensory processing. For example, in the mammalian thalamocortical system, a large number of thalamus neurons are modulated by cortical feedback mechanisms (14). Such top-down cortical feedback regulation is critical for visual perception (15). Anatomical and functional studies of the mammalian olfactory system indicate that there is also functional feedback from the cortex to the olfactory bulb (16, 17). Several lines of evidence also suggest the existence of feedback in the insect olfactory pathway. Olsen and Wilson (8) showed that spontaneous excitatory postsynaptic activity in PNs is suppressed presynaptically via lateral inhibition by LNs in antennal lobes, suggesting modulation of the primary ORNs by the secondary LNs. In addition, Tanaka et al. (18) demonstrated that odor stimulation can induce spikes and subthreshold membrane potential oscillations in PNs that are phase-locked to odor-elicited local field-potential oscillations in mushroom bodies. These results indicate the existence of feedback within the antennal lobe of Drosophila. Finally, by injecting cobalt into the α-lobe of the mushroom bodies of the honey bee, Rybak and Menzel identified a projection from the α-lobe to the antennal lobe (19). A neuron (the antennal lobe feedback neurons, ALF-1) with similar projection pattern was reported by Kirschner et al. (20). The results of these studies suggest feedback from the mushroom body to the antennal lobe in the honey bee. Based on these observations, we investigated whether functional feedback from mushroom bodies to antennal lobes is present in Drosophila. Drosophila mushroom bodies are predominately composed of Kenyon cells (KCs) (21). Because of the small size of KCs, it is difficult to stimulate these cells using conventional electrophysiological methods. Taking advantage of fly genetics, and the absence of the ionotropic ATP receptor P2X₂ gene in the Drosophila genome (22), Zemelman et al. established a UAS-P2X₂ system for precise activation of specific neurons in the fly brain using exogenous ATP (23). In this study, we have used the UAS-P2X₂ system for activation of KCs in combination with the patch-clamp method for recording of PN and LN activity. Using these techniques, we showed in this study the existence of functional feedback from KCs in the mushroom bodies to PNs and LNs in the antennal lobes.     

Behavioral response inhibition and maturation of goal representation in prefrontal cortex after puberty

Executive functions including behavioral response inhibition mature after puberty, in tandem with structural changes in the prefrontal cortex. Little is known about how activity of prefrontal neurons relates to this profound cognitive development. To examine this, we tracked neuronal responses of the prefrontal cortex in monkeys as they transitioned from puberty into adulthood and compared activity at different developmental stages. Performance of the antisaccade task greatly improved in this period. Among neural mechanisms that could facilitate it, reduction of stimulus-driven activity, increased saccadic activity, or enhanced representation of the opposing goal location, only the latter was evident in adulthood. Greatly accentuated in adults, this neural correlate of vector inversion may be a prerequisite to the formation of a motor plan to look away from the stimulus. Our results suggest that the prefrontal mechanisms that underlie mature performance on the antisaccade task are more strongly associated with forming an alternative plan of action than with suppressing the neural impact of the prepotent stimulus.Behavioral response inhibition, and cognitive task performance more generally, improves substantially between the time of puberty and adulthood (1–4). Risky decision-making peaks in adolescence, the time period between puberty and adulthood that is most closely linked to delinquent behavior in humans (5–7). Performance in tasks that assay response inhibition, such as the antisaccade task, improves into adulthood, reflecting the progressive development of behavioral control (3). This period of cognitive enhancement parallels the maturation of the prefrontal cortex (8–11). Anatomical changes in the prefrontal cortex continue during adolescence, involving gray and white matter volumes and myelination of axon fibers within the prefrontal cortex and between the prefrontal cortex and other areas (8–15). Changes in prefrontal activation, including increases (12, 16–20) and decreases (21, 22), have been documented in imaging studies for tasks that require inhibition of prepotent behavioral responses and filtering of distractors.Much less is known about how the physiological properties of prefrontal neurons develop after puberty. Similar to the human pattern of development, the monkey prefrontal cortex undergoes anatomical maturation in adolescence and early adulthood (23, 24). Male monkeys enter puberty at ∼3.5 y of age and reach full sexual maturity at 5 y, approximately equivalent to the human ages of 11 y and 16 y, respectively (25, 26). By some accounts, biochemical and anatomical changes characteristic of adolescence in humans occur at an earlier, prepubertal age in the monkey prefrontal cortex (27, 28), so it is not known if cognitive maturation or neurophysiological changes occur in monkeys after puberty. The contribution of prefrontal cortex to antisaccade performance has also been a matter of debate, with contrasting views favoring mechanisms of inhibiting movement toward the visual stimulus or enhancing movement away from it (29–31). Potential maturation of behavioral response inhibition may therefore be associated with a more efficient suppression of the stimulus representation in neural activity (weaker visual responses to stimuli inside the receptive field), stronger motor responses (higher activity to saccades), or enhancement of the appropriate goal representation (stronger activity for planning a saccade away from the stimulus). To examine the mechanisms that facilitate the mature ability to resist generating a response toward a salient stimulus, we used developmental markers to track transition from puberty to adulthood in monkeys and sought to identify neural correlates of changes in antisaccade performance within the visual and saccade-related activations of prefrontal neurons.     

Olfactory learning promotes input-specific synaptic plasticity in adult-born neurons

The production of new neurons in the olfactory bulb (OB) through adulthood is a major mechanism of structural and functional plasticity underlying learning-induced circuit remodeling. The recruitment of adult-born OB neurons depends not only on sensory input but also on the context in which the olfactory stimulus is received. Among the multiple steps of adult neurogenesis, the integration and survival of adult-born neurons are both strongly influenced by olfactory learning. Conversely, optogenetic stimulation of adult-born neurons has been shown to specifically improve olfactory learning and long-term memory. However, the nature of the circuit and the synaptic mechanisms underlying this reciprocal influence are not yet known. Here, we showed that olfactory learning increases the spine density in a region-restricted manner along the dendritic tree of adult-born granule cells (GCs). Anatomical and electrophysiological analysis of adult-born GCs showed that olfactory learning promotes a remodeling of both excitatory and inhibitory inputs selectively in the deep dendritic domain. Circuit mapping revealed that the malleable dendritic portion of adult-born neurons receives excitatory inputs mostly from the regions of the olfactory cortex that project back to the OB. Finally, selective optogenetic stimulation of olfactory cortical projections to the OB showed that learning strengthens these inputs onto adult-born GCs. We conclude that learning promotes input-specific synaptic plasticity in adult-born neurons, which reinforces the top-down influence from the olfactory cortex to early stages of olfactory information processing.Within the framework of Hebbian theory, information processing, learning, and memory all depend on dramatic changes in the synaptic weights throughout life. Recent progress in microscopy has extended this notion to structural synaptic plasticity, as synaptic networks were reported to be highly dynamic because of ongoing mechanisms that encompass synapse formation, stabilization, and elimination (1). Therefore, when studying synaptic plasticity today, one has to take into account both the functional and structural changes that lead to continuous remodeling of a given synaptic network.Developing cortical networks are molded by experience or activity patterns during a “sensitive period” in development (2). Similarly, newly formed neurons in the adult olfactory bulb (OB) may be subject to plastic changes during a restricted time window after generation (3). Identifying the neural mechanisms and the nature of signals that trigger changes in developing and adult brain circuits during critical periods is a matter of intense debate (4). To address this fundamental question of circuit plasticity, we used the adult OB circuit, which receives about 30,000 new neurons per day in rodents, as a model system. As in embryonic cell development, adult-born neuron integration is under a strong selection process in which half of the young neuronal population is eliminated (5). Sensory experience might promote cell survival during a specific critical window, when new neurons receive synaptic inputs from preexisting circuits (6–9) and exhibit long-term potentiation (LTP) (10). During this window, an odor-reward association task (but not a mere odor exposure) promotes cell survival (11, 12) and specific activation of adult-born neurons monitored through immediate early gene labeling (13, 14). As a result, olfactory memory is impaired when adult neurogenesis is compromised (13, 15). Conversely, the selective stimulation of adult-born neurons improves olfactory learning and memory (16), suggesting that these neurons are part of the olfactory memory engram (17). Although recent transsynaptic strategies have revealed the presynaptic connectivity of adult-born neurons (18, 19), little is known about how learning affects their structural and synaptic plasticity or which circuits and presynaptic cells are functionally recruited with them during associative learning. By demonstrating that olfactory learning triggers both selective structural rearrangements and changes in synaptic transmission onto adult-born neurons, this study represents a first attempt to link associative learning to functional plasticity of circuits endowed with adult neurogenesis.     

Neurobiological basis of head motion in brain imaging

Individual differences in brain metrics, especially connectivity measured with functional MRI, can correlate with differences in motion during data collection. The assumption has been that motion causes artifactual differences in brain connectivity that must and can be corrected. Here we propose that differences in brain connectivity can also represent a neurobiological trait that predisposes to differences in motion. We support this possibility with an analysis of intra- versus intersubject differences in connectivity comparing high- to low-motion subgroups. Intersubject analysis identified a correlate of head motion consisting of reduced distant functional connectivity primarily in the default network in individuals with high head motion. Similar connectivity differences were not found in analysis of intrasubject data. Instead, this correlate of head motion was a stable property in individuals across time. These findings suggest that motion-associated differences in brain connectivity cannot fully be attributed to motion artifacts but rather also reflect individual variability in functional organization.Head motion has long been known as a confounding factor in brain imaging including MRI (1, 2), PET (3, 4), single-photon emission computerized tomography (5, 6), and near infrared spectroscopy (7), but has raised particular concerns recently following the growing prominence of resting-state functional connectivity MRI. Studies found that head motion can vary considerably across individuals and often demonstrates systematic group effects when contrasting different populations, especially in neurodevelopmental (8–10), aging (11, 12), and neuropsychiatric studies (13). Some recent work reported that head motion augmented local coupling of the blood oxygenation level-dependent (BOLD) signal but reduced distant coupling (14–16). These correlations between connectivity measures and head motion have raised appropriate concern that previously observed differences in connectivity are due to artifact induced by differences in head motion. For example, developmental changes in functional connectivity might also be predicted by head motion (15). The assumption has been that head motion causes distorted connectivity measurements that must be addressed through improved motion-correction techniques (15). However, this correlation could be driven by causal factors in the other direction. Specifically, individual differences in brain connectivity could determine how well a subject can lie still in the scanner. This is not unreasonable as individual differences in structural connectivity can predict trait anxiety and can be related to attention deficits (17, 18) and individual differences in resting-state functional MRI (fMRI) measures may relate to various behavioral differences, including impulsivity (19–22). In such a scenario, certain intersubject differences in connectivity measures could persist even after the most rigorous motion correction, as has been suggested in several earlier studies (23, 24).To explore the relation between head motion and brain connectivity, we examined functional connectivity in different subject groups selected on the basis of head motion parameters from a large database of 3,000+ participants, many of whom were scanned multiple times. These cohorts allowed us to compare intersubject and intrasubject differences in connectivity in high- versus low-motion scans. If motion causes connectivity differences, these should be similar both inter- and intrasubject. However, if connectivity differences include a stable trait that predisposes to head motion, then these differences should be present between subjects but not within subjects.     

Identity-specific coding of future rewards in the human orbitofrontal cortex

Nervous systems must encode information about the identity of expected outcomes to make adaptive decisions. However, the neural mechanisms underlying identity-specific value signaling remain poorly understood. By manipulating the value and identity of appetizing food odors in a pattern-based imaging paradigm of human classical conditioning, we were able to identify dissociable predictive representations of identity-specific reward in orbitofrontal cortex (OFC) and identity-general reward in ventromedial prefrontal cortex (vmPFC). Reward-related functional coupling between OFC and olfactory (piriform) cortex and between vmPFC and amygdala revealed parallel pathways that support identity-specific and -general predictive signaling. The demonstration of identity-specific value representations in OFC highlights a role for this region in model-based behavior and reveals mechanisms by which appetitive behavior can go awry.Predictive representations of future outcomes are critical for guiding adaptive behavior. To choose different types of rewards, such as food, shelter, and mates, it is essential that predictive signals contain specific information about the identity of those outcomes. Food rewards differ dramatically in their nutritional composition, and identity-specific cues allow differential foraging depending on current needs of the organism. The absence of precise mappings between predictive reward signals and their intended outcomes would have devastating effects on food-based decisions.Despite the ecological relevance of outcome-specific predictive coding, which can be observed even in Drosophila (1), most research in human and nonhuman primates has focused on “common currency” signals of economic values in the orbitofrontal cortex (OFC) (2, 3) and ventromedial prefrontal cortex (vmPFC) (4–8). These signals, which by definition are independent of the specific nature of the reward, can be used to compare and choose between alternative outcomes, but are unable to inform expectations about the specific identity of the outcome. For this, identity-specific representations that conjointly represent information about both affective value (how good is it?) and outcome identity (what is it?) are necessary. Recent data suggest that the OFC is involved in signaling information about specific outcomes (9–14). For instance, many OFC neurons signal both the value and the identity of the predicted outcome (12), and OFC lesions diminish the effects of outcome identity (but not general affective value) on conditioned behavior (13).Recent imaging work has also begun to address how the human brain encodes predictive information about rewarding outcomes. One study (9) used a functional magnetic resonance imaging (fMRI) adaptation paradigm to provide evidence for identity-based codes for reward in the OFC. Another investigation (4) used fMRI data from a willingness to pay auction combined with decoding techniques to reveal category-dependent and -independent value codes in vmPFC and lateral OFC, respectively. However, neither of these studies varied value independently of identity, and they were therefore unable to test for the presence of identity-specific and -general value codes in the OFC.Here, we combined an olfactory paradigm of classical conditioning with fMRI pattern-based approaches to test the hypothesis that the human OFC simultaneously encodes both the value and the identity of an expected rewarding outcome. Critically, we took advantage of two unique properties of appetizing food odors to reveal identity-specific value representations. First, food odors act as potent rewards (15–17), with pleasantness that can scale with odor intensity (18, 19). Second, different food odors vary widely in identity (e.g., chocolate cake vs. pizza) but may still hold similar value. These distinct features enabled us to systematically manipulate outcome value and identity independently within the same stimulus space.     

Progressive maturation of silent synapses governs the duration of a critical period

During critical periods, all cortical neural circuits are refined to optimize their functional properties. The prevailing notion is that the balance between excitation and inhibition determines the onset and closure of critical periods. In contrast, we show that maturation of silent glutamatergic synapses onto principal neurons was sufficient to govern the duration of the critical period for ocular dominance plasticity in the visual cortex of mice. Specifically, postsynaptic density protein-95 (PSD-95) was absolutely required for experience-dependent maturation of silent synapses, and its absence before the onset of critical periods resulted in lifelong juvenile ocular dominance plasticity. Loss of PSD-95 in the visual cortex after the closure of the critical period reinstated silent synapses, resulting in reopening of juvenile-like ocular dominance plasticity. Additionally, silent synapse-based ocular dominance plasticity was largely independent of the inhibitory tone, whose developmental maturation was independent of PSD-95. Moreover, glutamatergic synaptic transmission onto parvalbumin-positive interneurons was unaltered in PSD-95 KO mice. These findings reveal not only that PSD-95–dependent silent synapse maturation in visual cortical principal neurons terminates the critical period for ocular dominance plasticity but also indicate that, in general, once silent synapses are consolidated in any neural circuit, initial experience-dependent functional optimization and critical periods end.Immature cortical neural networks, which are formed primarily under genetic control (1), require experience and training to shape and optimize their functional properties. This experience-dependent refinement is considered to be a general developmental process for all functional cortical domains and typically peaks during their respective critical periods (CPs) (2, 3). Known examples for CPs span functional domains as diverse as filial imprinting and courtship song learning in birds (4, 5); cognitive functions, such as linguistic or musical skills in humans (6, 7); and likely best studied, the different features of sensory modalities (3). CPs are characterized by the absolute requirement for experience in a restricted time window for neural network optimization. Lack of visual experience during the CP for visual cortex refinements can, for example, cause irreversible visual impairment (8). Refinements during the CP play an essential role (9). Although some functions can be substantially ameliorated after the CP, they are rarely optimally restored.It is believed that the neural network refinement is based on synapse stabilization and elimination (10–12) and includes forms of long-term synaptic plasticity to remodel excitatory synapses of principal neurons (13, 14). Although long-term plasticity at these excitatory synapses is instructive for shaping neural networks for functional output and their expression coincides with CPs, it is not known whether the remodeling itself governs the duration of CPs. In contrast, only permissive mechanisms have been shown to terminate CPs. Among these, the developmental increase of local inhibition appears to be the dominating mechanism to regulate cortical plasticity and CPs (15–17). Additionally, extracellular matrix remodeling is involved, as well as receptors of immune signaling, such as paired Ig-like receptor B (PirB), or axon pathfinding, such as Nogo (18–21). However, a specific function to directly regulate synapse remodeling during initial neural network optimization is not known and a potential instructive function of PirB was described for adult cortical plasticity but not plasticity of the initial synapse remodeling during CPs (22).AMPA receptor-silent synapses have been proposed to be efficient plasticity substrates during early cortical network refinements (13, 23, 24). Silent synapses are thought to be immature, still-developing excitatory synapses containing only NMDA receptors (NMDARs) but lacking AMPA receptors (AMPARs) (23, 24). They are functionally dormant but can evolve into fully transmitting synapses by experience-dependent insertion of AMPARs, a plasticity process thought to occur frequently in developing cortices (10). Although they appear as the ideal synaptic substrate for CP plasticity and their maturation correlates with sensory experience (10, 25), it has not been experimentally tested whether maturation of silent synapses indeed causes the termination of critical periods. This conceptual model contrasts with the current view that increased local inhibition and the expression of plasticity brakes ends critical periods (18–20, 26). We hypothesize that experience-dependent unsilencing of silent synapses, which results in strengthening and maturation of excitatory synapses, governs network stabilization and refinement during critical periods, and that the progressive decrease of silent synapses leads to the closure of critical periods.Experience-dependent cortical plasticity is classically tested with ocular dominance (OD) plasticity (ODP) in the primary visual cortex (V1), induced by monocular deprivation (MD). In the binocular region of mouse V1, neurons respond to sensory inputs from both eyes, but activity is dominated by afferents from the contralateral eye. During the critical period, a brief MD induces an OD shift of visually evoked responses in V1 toward the open eye (27–29). This juvenile ODP is mediated by a reduction of deprived eye responses in V1 and is temporally confined to a critical period (30, 31).A molecular candidate regulating the cellular basis of critical period plasticity is postsynaptic density protein-95 (PSD-95), whose expression in the visual cortex increases on eye opening and thus the onset of visual experience (32). PSD-95 promotes the maturation of AMPA receptor-silent excitatory synapses in hippocampal neurons and is required for activity-driven synapse stabilization (33–35). In juvenile PSD-95 KO mice, ODP displays the same features as in WT mice (36). However, as adult PSD-95 KO mice have not yet been analyzed, it is unknown whether PSD-95 is essential for the closure of critical periods. Thus, PSD-95 appeared to be the ideal molecular candidate to test our conceptual model that progressive silent synapse maturation marks the closure of critical periods.     

Age-dependent changes in prefrontal intrinsic connectivity

The prefrontal cortex continues to mature after puberty and into early adulthood, mirroring the time course of maturation of cognitive abilities. However, the way in which prefrontal activity changes during peri- and postpubertal cortical maturation is largely unknown. To address this question, we evaluated the developmental stage of peripubertal rhesus monkeys with a series of morphometric, hormonal, and radiographic measures, and conducted behavioral and neurophysiological tests as the monkeys performed working memory tasks. We compared firing rate and the strength of intrinsic functional connectivity between neurons in peripubertal vs. adult monkeys. Notably, analyses of spike train cross-correlations demonstrated that the average magnitude of functional connections measured between neurons was lower overall in the prefrontal cortex of peripubertal monkeys compared with adults. The difference resulted because negative functional connections (indicative of inhibitory interactions) were stronger and more prevalent in peripubertal compared with adult monkeys, whereas the positive connections showed similar distributions in the two groups. Our results identify changes in the intrinsic connectivity of prefrontal neurons, particularly that mediated by inhibition, as a possible substrate for peri- and postpubertal advances in cognitive capacity.The prefrontal cortex, the brain area associated with the highest-level cognitive operations, is known to undergo a protracted period of development (1–3). A virtually linear increase in performance with age has been observed in tasks that assess visuospatial working memory, executive control, and resistance to distraction, a process that continues well after puberty and into early adulthood (4, 5). The accrual of cognitive capacities during this period parallels structural changes of the prefrontal cortex in humans and nonhuman primates (6–10). Imaging studies in humans suggest that patterns of brain activation associated with working memory tasks undergo distinct changes between childhood and adulthood, supporting the idea of prolonged prefrontal maturation (11–14). However, how the patterns of prefrontal activation change during cortical maturation remains unclear. A possible mechanism that could account for variations in prefrontal responses—and which could have a significant functional impact (15)—is an overall change in the distribution of intrinsic functional connections, i.e., those between neurons within the prefrontal cortex. The intrinsic connectivity of a network is directly related to the correlation structure of neuronal responses, and this determines in a fundamental way the information-coding properties of the network and its ability to sustain activity on its own (16–18). In this study, we sought to determine if the strengths of functional connections inferred from multisite neurophysiological recordings differ between peripubertal and adult monkeys.     

Functional hierarchy underlies preferential connectivity disturbances in schizophrenia

Schizophrenia may involve an elevated excitation/inhibition (E/I) ratio in cortical microcircuits. It remains unknown how this regulatory disturbance maps onto neuroimaging findings. To address this issue, we implemented E/I perturbations within a neural model of large-scale functional connectivity, which predicted hyperconnectivity following E/I elevation. To test predictions, we examined resting-state functional MRI in 161 schizophrenia patients and 164 healthy subjects. As predicted, patients exhibited elevated functional connectivity that correlated with symptom levels, and was most prominent in association cortices, such as the fronto-parietal control network. This pattern was absent in patients with bipolar disorder (n = 73). To account for the pattern observed in schizophrenia, we integrated neurobiologically plausible, hierarchical differences in association vs. sensory recurrent neuronal dynamics into our model. This in silico architecture revealed preferential vulnerability of association networks to E/I imbalance, which we verified empirically. Reported effects implicate widespread microcircuit E/I imbalance as a parsimonious mechanism for emergent inhomogeneous dysconnectivity in schizophrenia.Schizophrenia (SCZ) is a disabling psychiatric disease associated with widespread neural disturbances. These involve abnormal neurodevelopment (1–3), neurochemistry (4–7), neuronal gene expression (8–11), and altered microscale neural architecture (2). Such deficits are hypothesized to impact excitation-inhibition (E/I) balance in cortical microcircuits (12). Clinically, SCZ patients display a wide range of symptoms, including delusions, hallucinations (13, 14), higher-level cognitive deficits (15, 16), and lower-level sensory alterations (17). This display is consistent with a widespread neuropathology (18), such as the E/I imbalance suggested by the NMDA receptor (NMDAR) hypofunction model (19–21). However, emerging resting-state functional magnetic resonance imaging (rs-fMRI) studies implicate more network-specific abnormalities in SCZ. Typically, these alterations are localized to higher-order association regions, such as the fronto-parietal control network (FPCN) (18, 22) and the default mode network (DMN) (23, 24), with corresponding disturbances in thalamo-cortical circuits connecting to association regions (25, 26). It remains unknown how to reconcile widespread cellular-level neuropathology in SCZ (20, 21, 27, 28) with preferential association network disruptions (29, 30).Currently a tension exists between two competing frameworks: global versus localized neural dysfunction in SCZ. Association network alterations in SCZ, identified via neuroimaging, may arise from a localized dysfunction (3, 9, 31, 32). Alternatively, they may represent preferential abnormalities arising emergently from a nonspecific global microcircuit disruption (20, 33). Mechanistically, an emergent preferential effect could occur because of intrinsic differences between cortical areas in the healthy brain, leading to differential vulnerability toward a widespread homogenous neuropathology. For example, histological studies of healthy primate brains show interregional variation in cortical cytoarchitectonics (34–38). Additional studies reveal differences in microscale organization and activity timescales for neuronal populations in higher-order association cortex compared with lower-order sensory regions (38–40). However, these well-established neuroanatomical and neurophysiological hierarchies have yet to be systematically applied to inform network-level neuroimaging disturbances in SCZ. In this study, we examined the neuroimaging consequences of cortical hierarchy as defined by neurophysiological criteria (i.e., functional) rather than anatomical or structural criteria.One way to link cellular-level neuropathology hypotheses with neuroimaging is via biophysically based computational models (18, 41). Although these models have been applied to SCZ, none have integrated cortical hierarchy into their architecture. Here we initially implemented elevated E/I ratio within our well-validated computational model of resting-state neural activity (18, 42, 43) without assuming physiological differences between brain regions, but maintaining anatomical differences. The model predicted widespread elevated functional connectivity as a consequence of elevated E/I ratio. In turn, we tested this connectivity prediction across 161 SCZ patients and 164 matched healthy comparison subjects (HCS). However, we discovered an inhomogeneous spatial pattern of elevated connectivity in SCZ generally centered on association cortices.To capture the observed inhomogeneity, we hypothesized that pre-existing intrinsic regional differences between association and lower-order cortical regions may give rise to preferential network-level vulnerability to elevated E/I. Guided by primate studies examining activity timescale differences across the cortical hierarchy (39, 44), we incorporated physiological differentiation across cortical regions in the model. Specifically, we tested whether pre-existing stronger recurrent excitation in “association” networks (39, 40) would preferentially increase their functional connectivity in response to globally elevated E/I. Indeed, modeling simulations predicted preferential effects of E/I elevation in association networks, which could not be explained by structural connectivity differences alone.Finally, we empirically tested all model-derived predictions by examining network-specific disruptions in SCZ. To investigate diagnostic specificity of SCZ effects, we examined an independent sample of bipolar disorder (BD) patients (n = 73) that did not follow model-derived predictions. These results collectively support a parsimonious theoretical framework whereby emergent preferential association network disruptions in SCZ can arise from widespread and nonspecific E/I elevations at the microcircuit level. This computational psychiatry study (45) illustrates the productive interplay between biologically grounded modeling and clinical effects, which may inform refinement of neuroimaging markers and ultimately rational development of treatments for SCZ.     

Rapid homeostasis by disinhibition during whisker map plasticity

How homeostatic processes contribute to map plasticity and stability in sensory cortex is not well-understood. Classically, sensory deprivation first drives rapid Hebbian weakening of spiking responses to deprived inputs, which is followed days later by a slow homeostatic increase in spiking responses mediated by excitatory synaptic scaling. Recently, more rapid homeostasis by inhibitory circuit plasticity has been discovered in visual cortex, but whether this process occurs in other brain areas is not known. We tested for rapid homeostasis in layer 2/3 (L2/3) of rodent somatosensory cortex, where D-row whisker deprivation drives Hebbian weakening of whisker-evoked spiking responses after an unexplained initial delay, but no homeostasis of deprived whisker responses is known. We hypothesized that the delay reflects rapid homeostasis through disinhibition, which masks the onset of Hebbian weakening of L2/3 excitatory input. We found that deprivation (3 d) transiently increased whisker-evoked spiking responses in L2/3 single units before classical Hebbian weakening (≥5 d), whereas whisker-evoked synaptic input was reduced during both periods. This finding suggests a transient homeostatic increase in L2/3 excitability. In whole-cell recordings from L2/3 neurons in vivo, brief deprivation decreased whisker-evoked inhibition more than excitation and increased the excitation–inhibition ratio. In contrast, synaptic scaling and increased intrinsic excitability were absent. Thus, disinhibition is a rapid homeostatic plasticity mechanism in rodent somatosensory cortex that transiently maintains whisker-evoked spiking in L2/3, despite the onset of Hebbian weakening of excitatory input.During deprivation-induced sensory map plasticity in cerebral cortex, changes in sensory input trigger both homeostatic plasticity mechanisms that maintain stable cortical firing rates and Hebbian mechanisms, in which inactive inputs lose (and active inputs gain) representation in sensory maps (1). Diverse mechanisms for homeostasis exist, including synaptic scaling (2–4), plasticity of intrinsic excitability (5, 6), and changes in sensory-evoked inhibition and excitation–inhibition (E-I) ratio (7–11). How homeostatic and Hebbian mechanisms interact to control map stability and plasticity remains unclear.One key unknown is the relative dynamics of homeostatic and Hebbian plasticity. Homeostasis mediated by synaptic scaling is slow, occurring over hours in vitro and days in vivo. This process is evident in visual cortex, where eyelid closure during the critical period classically drives Hebbian weakening of closed eye spiking responses (after 2 d of deprivation) followed several days later by a slower homeostatic increase in visual responses (12, 13) mediated by excitatory synaptic scaling (3, 14, 15). We investigated whether more rapid forms of homeostasis also exist that shape the earliest stages of cortical plasticity. Recent results in visual cortex show that eyelid closure rapidly weakens inhibitory circuits (within 1 d), and this process increases network excitability and, therefore, is an initial homeostatic response to deprivation (10, 16). This disinhibition correlates with rapid structural plasticity in inhibitory axons and dendrites (17) and is mediated by a reduction in excitatory drive to parvalbumin-positive interneurons (10). Whether rapid homeostasis by disinhibition or other mechanisms is a general feature of cortical plasticity outside the visual cortex is unknown. Theoretical work shows that rapid homeostasis by inhibitory and/or intrinsic plasticity can guide development of realistic sensory tuning and sparse sensory coding in cortical networks, suggesting broad relevance (18).We tested for rapid homeostasis during the onset of whisker map plasticity in the rodent primary somatosensory (S1) cortex, a major model of cortical plasticity. Each cortical column in the S1 whisker map corresponds to one facial whisker, termed its principal whisker (PW). Trimming or plucking a subset of whiskers in young adults weakens spiking responses to deprived PWs in layer 2/3 (L2/3) of deprived columns (19, 20). This process is mediated by Hebbian synaptic weakening at L4–L2/3 and L2/3–L2/3 excitatory synapses (21–23). No homeostatic restoration or strengthening of deprived whisker responses is known. However, PW response weakening is often preceded by an unexplained initial delay of ∼7 d, in which deprived whisker-evoked spiking responses remain stable (24, 25). We hypothesized that this initial delay reflects not a lack of plasticity but rapid homeostasis that (i) masks initial Hebbian weakening of L2/3 excitatory input and (ii) is mediated by loss of inhibition and/or increased intrinsic excitability in L2/3 neurons. Such rapid homeostasis would be a unique component of whisker map plasticity.     

Local generation of multineuronal spike sequences in the hippocampal CA1 region

Sequential activity of multineuronal spiking can be observed during theta and high-frequency ripple oscillations in the hippocampal CA1 region and is linked to experience, but the mechanisms underlying such sequences are unknown. We compared multineuronal spiking during theta oscillations, spontaneous ripples, and focal optically induced high-frequency oscillations (“synthetic” ripples) in freely moving mice. Firing rates and rate modulations of individual neurons, and multineuronal sequences of pyramidal cell and interneuron spiking, were correlated during theta oscillations, spontaneous ripples, and synthetic ripples. Interneuron spiking was crucial for sequence consistency. These results suggest that participation of single neurons and their sequential order in population events are not strictly determined by extrinsic inputs but also influenced by local-circuit properties, including synapses between local neurons and single-neuron biophysics.A hypothesized hallmark of cognition is self-organized sequential activation of neuronal assemblies (1). Self-organized neuronal sequences have been observed in several cortical structures (2–5) and neuronal models (6–7). In the hippocampus, sequential activity of place cells (8) may be induced by external landmarks perceived by the animal during spatial navigation (9) and conveyed to CA1 by the upstream CA3 region or layer 3 of the entorhinal cortex (10). Internally generated sequences have been also described in CA1 during theta oscillations in memory tasks (4, 11), raising the possibility that a given neuronal substrate is responsible for generating sequences at multiple time scales. The extensive recurrent excitatory collateral system of the CA3 region has been postulated to be critical in this process (4, 7, 12, 13).The sequential activity of place cells is “replayed” during sharp waves (SPW) in a temporally compressed form compared with rate modulation of place cells (14–20) and may arise from the CA3 recurrent excitatory networks during immobility and slow wave sleep. The SPW-related convergent depolarization of CA1 neurons gives rise to a local, fast oscillatory event in the CA1 region (“ripple,” 140–180 Hz; refs. 8 and 21). Selective elimination of ripples during or after learning impairs memory performance (22–24), suggesting that SPW ripple-related replay assists memory consolidation (12, 13). Although the local origin of the ripple oscillations is well demonstrated (25, 26), it has been tacitly assumed that the ripple-associated, sequentially ordered firing of CA1 neurons is synaptically driven by the upstream CA3 cell assemblies (12, 15), largely because excitatory recurrent collaterals in the CA1 region are sparse (27). However, sequential activity may also emerge by local mechanisms, patterned by the different biophysical properties of CA1 pyramidal cells and their interactions with local interneurons, which discharge at different times during a ripple (28–30). A putative function of the rich variety of interneurons is temporal organization of principal cell spiking (29–32). We tested the “local-circuit” hypothesis by comparing the probability of participation and sequential firing of CA1 neurons during theta oscillations, natural spontaneous ripple events, and “synthetic” ripples induced by local optogenetic activation of pyramidal neurons.     

AMPA receptor inhibition by synaptically released zinc

The vast amount of fast excitatory neurotransmission in the mammalian central nervous system is mediated by AMPA-subtype glutamate receptors (AMPARs). As a result, AMPAR-mediated synaptic transmission is implicated in nearly all aspects of brain development, function, and plasticity. Despite the central role of AMPARs in neurobiology, the fine-tuning of synaptic AMPA responses by endogenous modulators remains poorly understood. Here we provide evidence that endogenous zinc, released by single presynaptic action potentials, inhibits synaptic AMPA currents in the dorsal cochlear nucleus (DCN) and hippocampus. Exposure to loud sound reduces presynaptic zinc levels in the DCN and abolishes zinc inhibition, implicating zinc in experience-dependent AMPAR synaptic plasticity. Our results establish zinc as an activity-dependent, endogenous modulator of AMPARs that tunes fast excitatory neurotransmission and plasticity in glutamatergic synapses.The development, function, and experience-dependent plasticity of the mammalian brain depend on the refined neuronal interactions that occur in synapses. In the majority of excitatory synapses, the release of the neurotransmitter glutamate from presynaptic neurons opens transmembrane ion channels in postsynaptic neurons, the ionotropic glutamate receptors, thereby generating the flow of excitatory signaling in the brain. As a result, these receptors play a fundamental role in normal function and development of the brain, and they are also involved in many brain disorders (1).The ionotropic glutamate receptor family consists of AMPA, kainate, and NMDA receptors (NMDARs). Although kainate receptor-mediated excitatory postsynaptic responses occur in a few central synapses (2), AMPA receptors (AMPARs) and NMDARs are localized in the postsynaptic density of the vast majority of glutamatergic synapses in the brain, mediating most of excitatory neurotransmission (1). NMDAR function is regulated by a wide spectrum of endogenous allosteric neuromodulators that fine-tune synaptic responses (3–5); however, much less is known about endogenous AMPAR neuromodulators [(1, 5), but see refs. 6 and 7]. Recent structural studies revealed that the amino terminal domain (ATD) and ligand-binding domain (LBD) are tightly packed in NMDARs but not AMPARs (8–10). These structural differences explain some of the functional differences in allosteric modulation between AMPARs and NMDARs, such as why the ATD of NMDARs, unlike that of AMPARs, modulates function and contains numerous binding sites for allosteric regulators. Nonetheless, given the importance of fine-tuning both synaptic AMPAR and NMDAR responses for brain function, it is puzzling that there is not much evidence for endogenous, extracellular AMPAR modulation. The discovery and establishment of endogenous AMPAR modulators is crucial both for understanding ionotropic glutamate receptor signaling and for developing therapeutic agents for the treatment of AMPAR-related disorders, such as depression, cognitive dysfunctions associated with Alzheimer’s disease, and schizophrenia (1, 11).Free, or readily chelatable, zinc is an endogenous modulator of synaptic and extrasynaptic NMDARs (12–15). Free zinc is stored in glutamatergic vesicles in many excitatory synapses in the cerebral cortex, limbic, and brainstem nuclei (16). In some brain areas, such as in the hippocampus, 50% of boutons synapsing onto CA1 neurons and all mossy fibers synapsing onto CA3 neurons contain synaptic free zinc (17). Whereas earlier studies demonstrated that exogenous zinc inhibits AMPARs (18–21), more recent work suggests that endogenously released synaptic zinc does not modulate AMPARs in central synapses (14, 22). This conclusion was derived from the inability to efficiently chelate and quantify synaptic zinc with the zinc-selective chelators and probes used (15), in apparent support of the hypothesized low levels of released zinc during synaptic stimulation (14).Recent work in our laboratories used new chemical tools that allowed us to intercept and visualize mobile zinc efficiently (15). These studies revealed modulation of extrasynaptic NMDARs by zinc and led us to reinvestigate whether synaptically released zinc might be an endogenous modulator of AMPARs as well. In the present study, we applied these same tools in electrophysiological, laser-based glutamate uncaging and in imaging experiments using wild type and genetically modified mice that lack synaptic zinc.