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Functions of the neocortex depend on its bidirectional communication with the thalamus, via cortico-thalamo-cortical (CTC) loops. Recent work dissecting the synaptic connectivity in these loops is generating a clearer picture of their cellular organization. Here, we review findings across sensory, motor and cognitive areas, focusing on patterns of cell type-specific synaptic connections between the major types of cortical and thalamic neurons. We outline simple and complex CTC loops, and note features of these loops that appear to be general versus specialized. CTC loops are tightly interlinked with local cortical and corticocortical (CC) circuits, forming extended chains of loops that are probably critical for communication across hierarchically organized cerebral networks. Such CTC–CC loop chains appear to constitute a modular unit of organization, serving as scaffolding for area-specific structural and functional modifications. Inhibitory neurons and circuits are embedded throughout CTC loops, shaping the flow of excitation. We consider recent findings in the context of established CTC and CC circuit models, and highlight current efforts to pinpoint cell type-specific mechanisms in CTC loops involved in consciousness and perception. As pieces of the connectivity puzzle fall increasingly into place, this knowledge can guide further efforts to understand structure–function relationships in CTC loops.The neocortex and the thalamus are connected by looping circuits, enabling cortical function. In this Review, Gordon Shepherd and Naoki Yamawaki examine the patterns of connectivity between the major types of cortical and thalamic neurons and how these cortico-thalamo-cortical loops relate to corticocortical circuits.

Hippocampus, granular retrosplenial cortex (RSCg), and anterior thalamic nuclei (ATN) interact to mediate diverse cognitive functions. To identify cellular mechanisms underlying hippocampo–thalamo–retrosplenial interactions, we investigated the potential circuit suggested by projections to RSCg layer 1 (L1) from GABAergic CA1 neurons and ATN. We find that CA1→RSCg projections stem from GABAergic neurons with a distinct morphology, electrophysiology, and molecular profile. Their long-range axons inhibit L5 pyramidal neurons in RSCg via potent synapses onto apical tuft dendrites in L1. These inhibitory inputs intercept L1-targeting thalamocortical excitatory inputs from ATN to coregulate RSCg activity. Subicular axons, in contrast, excite proximal dendrites in deeper layers. Short-term plasticity differs at each connection. Chemogenetically abrogating CA1→RSCg or ATN→RSCg connections oppositely affects the encoding of contextual fear memory. Our findings establish retrosplenial-projecting CA1 neurons as a distinct class of long-range dendrite-targeting GABAergic neuron and delineate an unusual cortical circuit specialized for integrating long-range inhibition and thalamocortical excitation.Apical tuft dendrites of pyramidal neurons in retrosplenial cortex receive inhibition from a class of CA1 GABAergic neurons with long-range layer 1-targeting axons; this inhibition opposes matrix-type thalamocortical excitation from anterior thalamus.

Sensory-guided limb control relies on communication across sensorimotor loops. For active touch with the hand, the longest loop is the transcortical continuation of ascending pathways, particularly the lemnisco-cortical and corticocortical pathways carrying tactile signals via the cuneate nucleus, ventral posterior lateral (VPL) thalamus, and primary somatosensory (S1) and motor (M1) cortices to reach corticospinal neurons and influence descending activity. We characterized excitatory connectivity along this pathway in the mouse. In the lemnisco-cortical leg, disynaptic cuneate→VPL→S1 connections excited mainly layer (L) 4 neurons. In the corticocortical leg, S1→M1 connections from L2/3 and L5A neurons mainly excited downstream L2/3 neurons, which excite corticospinal neurons. The findings provide a detailed new wiring diagram for the hand/forelimb-related transcortical circuit, delineating a basic but complex set of cell-type-specific feedforward excitatory connections that selectively and extensively engage diverse intratelencephalic projection neurons, thereby polysynaptically linking subcortical somatosensory input to cortical motor output to spinal cord.

The claustrum complex is viewed as fundamental for higher-order cognition; however, the circuit organization and function of its neuroanatomical subregions are not well understood. We demonstrated that some of the key roles of the CLA complex can be attributed to the connectivity and function of a small group of neurons in its ventral subregion, the endopiriform (EN). We identified a subpopulation of EN neurons by their projection to the ventral CA1 (EN vCA1-proj . neurons), embedded in recurrent circuits with other EN neurons and the piriform cortex. Although the EN vCA1-proj. neuron activity was biased toward novelty across stimulus categories, their chemogenetic inhibition selectively disrupted the memory-guided but not innate responses of mice to novelty. Based on our functional connectivity analysis, we suggest that EN vCA1-proj . neurons serve as an essential node for recognition memory through recurrent circuits mediating sustained attention to novelty, and through feed-forward inhibition of distal vCA1 neurons shifting memory-guided behavior from familiarity to novelty.

The motor cortex (M1) is classically considered an agranular area, lacking a distinct layer 4 (L4). Here, we tested the idea that M1, despite lacking a cytoarchitecturally visible L4, nevertheless possesses its equivalent in the form of excitatory neurons with input–output circuits like those of the L4 neurons in sensory areas. Consistent with this idea, we found that neurons located in a thin laminar zone at the L3/5A border in the forelimb area of mouse M1 have multiple L4-like synaptic connections: excitatory input from thalamus, largely unidirectional excitatory outputs to L2/3 pyramidal neurons, and relatively weak long-range corticocortical inputs and outputs. M1-L4 neurons were electrophysiologically diverse but morphologically uniform, with pyramidal-type dendritic arbors and locally ramifying axons, including branches extending into L2/3. Our findings therefore identify pyramidal neurons in M1 with the expected prototypical circuit properties of excitatory L4 neurons, and question the traditional assumption that motor cortex lacks this layer. In 1909, a German scientist called Korbinian Brodmann published the first map of the outer layer of the human brain. After staining neurons with a dye and studying the structures of the cells and how they were organized, he realized that he could divide the cortex into 43 numbered regions. Most Brodmann areas can be divided into a number of horizontal layers, with layer 1 being closest to the surface of the brain. Neurons in the different layers form distinct sets of connections, and the relative thickness of the layers has implications for the function carried out by that area. It is thought, for example, that the motor cortex does not have a layer 4, which suggests that the neural circuitry that controls movement differs from that in charge of vision, hearing, and other functions. Yamawaki et al. now challenge this view by providing multiple lines of evidence for the existence of layer 4 in the motor cortex in mice. Neurons at the border between layer 3 and layer 5A in the motor cortex possess many of the same properties as the neurons in layer 4 in sensory cortex. In particular, they receive inputs from a brain region called the thalamus, and send outputs to neurons in layers 2 and 3. Yamawaki et al. go on to characterize some of the properties of the neurons in the putative layer 4 of the motor cortex, finding that they do not look like the specialized ‘stellate’ cells that are found in some other areas of the cortex. Instead, they resemble the ‘pyramidal’ type of neuron that is found in all layers and areas of the cortex. The discovery that the motor cortex is more similar in its circuit connections to other area of the cortex than previously thought has important implications for our understanding of this region of the brain.

Some stress-related memories are state-dependent: they cannot be retrieved unless the brain is in the same state as during initial encoding. The authors show that hippocampal extrasynaptic GABA A receptors, regulated by miR-33, support state-dependent contextual fear conditioning by altering the processing of context memories within the extended hippocampal circuit. Fear-inducing memories can be state dependent, meaning that they can best be retrieved if the brain states at encoding and retrieval are similar. Restricted access to such memories can present a risk for psychiatric disorders and hamper their treatment. To better understand the mechanisms underlying state-dependent fear, we used a mouse model of contextual fear conditioning. We found that heightened activity of hippocampal extrasynaptic GABA A receptors, believed to impair fear and memory, actually enabled their state-dependent encoding and retrieval. This effect required protein kinase C-βII and was influenced by miR-33, a microRNA that regulates several GABA-related proteins. In the extended hippocampal circuit, extrasynaptic GABA A receptors promoted subcortical, but impaired cortical, activation during memory encoding of context fear. Moreover, suppression of retrosplenial cortical activity, which normally impairs retrieval, had an enhancing effect on the retrieval of state-dependent fear. These mechanisms can serve as treatment targets for managing access to state-dependent memories of stressful experiences.

Prior anatomical studies have suggested that intratelencephalic (IT) and pyramidal tract (PT) cortical neurons project to different populations of striatal spiny projection neurons (SPNs). Here, the authors find using optogenetic stimulation that both IT and PT neurons project to both direct and indirect pathway SPNs. Anatomical studies have led to the assertion that intratelencephalic and pyramidal tract cortical neurons innervate different striatal projection neurons. To test this hypothesis, we measured the responses of mouse striatal neurons to optogenetic activation of intratelencephalic and pyramidal tract axons. Contrary to expectation, direct and indirect pathway striatal spiny projection neurons responded to both intratelencephalic and pyramidal tract activation, arguing that these cortical networks innervate both striatal projection neurons.

Beta frequency oscillations (10-35 Hz) in motor regions of cerebral cortex play an important role in stabilising and suppressing unwanted movements, and become intensified during the pathological akinesia of Parkinson's Disease. We have used a cortical slice preparation of rat brain, combined with concurrent intracellular and field recordings from the primary motor cortex (M1), to explore the cellular basis of the persistent beta frequency (27-30 Hz) oscillations manifest in local field potentials (LFP) in layers II and V of M1 produced by continuous perfusion of kainic acid (100 nM) and carbachol (5 µM). Spontaneous depolarizing GABA-ergic IPSPs in layer V cells, intracellularly dialyzed with KCl and IEM1460 (to block glutamatergic EPSCs), were recorded at -80 mV. IPSPs showed a highly significant (P< 0.01) beta frequency component, which was highly significantly coherent with both the Layer II and V LFP oscillation (which were in antiphase to each other). Both IPSPs and the LFP beta oscillations were abolished by the GABAA antagonist bicuculline. Layer V cells at rest fired spontaneous action potentials at sub-beta frequencies (mean of 7.1+1.2 Hz; n = 27) which were phase-locked to the layer V LFP beta oscillation, preceding the peak of the LFP beta oscillation by some 20 ms. We propose that M1 beta oscillations, in common with other oscillations in other brain regions, can arise from synchronous hyperpolarization of pyramidal cells driven by synaptic inputs from a GABA-ergic interneuronal network (or networks) entrained by recurrent excitation derived from pyramidal cells. This mechanism plays an important role in both the physiology and pathophysiology of control of voluntary movement generation.

As hippocampal neurons respond to diverse types of information 1 , a subset assembles into microcircuits representing a memory 2 . Those neurons typically undergo energy-intensive molecular adaptations, occasionally resulting in transient DNA damage 3 – 5 . Here we found discrete clusters of excitatory hippocampal CA1 neurons with persistent double-stranded DNA (dsDNA) breaks, nuclear envelope ruptures and perinuclear release of histone and dsDNA fragments hours after learning. Following these early events, some neurons acquired an inflammatory phenotype involving activation of TLR9 signalling and accumulation of centrosomal DNA damage repair complexes 6 . Neuron-specific knockdown of Tlr9 impaired memory while blunting contextual fear conditioning-induced changes of gene expression in specific clusters of excitatory CA1 neurons. Notably, TLR9 had an essential role in centrosome function, including DNA damage repair, ciliogenesis and build-up of perineuronal nets. We demonstrate a novel cascade of learning-induced molecular events in discrete neuronal clusters undergoing dsDNA damage and TLR9-mediated repair, resulting in their recruitment to memory circuits. With compromised TLR9 function, this fundamental memory mechanism becomes a gateway to genomic instability and cognitive impairments implicated in accelerated senescence, psychiatric disorders and neurodegenerative disorders. Maintaining the integrity of TLR9 inflammatory signalling thus emerges as a promising preventive strategy for neurocognitive deficits. Learning results in persistent double-stranded DNA breaks, nuclear rupture and release of DNA fragments and histones within hippocampal CA1 neurons that, following TLR9-mediated DNA damage repair, results in their recruitment to memory circuits.

The claustrum complex is viewed as fundamental for higher-order cognition; however, the circuit organization and function of its neuroanatomical subregions are not well understood. We demonstrated that some of the key roles of the CLA complex can be attributed to the connectivity and function of a small group of neurons in its ventral subregion, the endopiriform (EN). We identified a subpopulation of EN neurons by their projection to the ventral CA1 (EN vCA1-proj . neurons), embedded in recurrent circuits with other EN neurons and the piriform cortex. Although the EN vCA1-proj. neuron activity was biased toward novelty across stimulus categories, their chemogenetic inhibition selectively disrupted the memory-guided but not innate responses of mice to novelty. Based on our functional connectivity analysis, we suggest that EN vCA1-proj . neurons serve as an essential node for recognition memory through recurrent circuits mediating sustained attention to novelty, and through feed-forward inhibition of distal vCA1 neurons shifting memory-guided behavior from familiarity to novelty.