Abstract The ventrolateral preoptic nucleus (VLPO) is a key hypothalamic hub for non-rapid eye movement sleep, yet the glial architecture supporting its circuits remains poorly understood. Here, combining genetic labeling, high-resolution imaging and calcium imaging, we uncover unexpected astrocyte diversity in the VLPO. In addition to classical protoplasmic astrocytes, we identify paired “doublet” astrocytes associated with high local proliferative activity, as revealed by EdU incorporation. We further describe a population of long-projection astrocytes extending processes far beyond canonical astrocytic territories and contacting distant cells. These projections challenge the classical territorial organization of astrocytes and resemble morphologies previously thought to be restricted to hominid brains. Notably, VLPO astrocytes display robust spontaneous Ca²⁺ activity and a highly functionally connected network compared to astrocytes in the cortex and hippocampus. Together, these findings reveal specialized astrocyte architectures and enhanced glial network integration within a sleep-promoting nucleus. Reporting summary Bellier et al. identify three astrocyte subtypes in the sleep-promoting VLPO, including long-projection astrocytes with hominid-like morphology. They uncover marked postnatal gliogenesis, distinctive spontaneous Ca²⁺ dynamics, and tightly interconnected astrocytic networks, revealing region-specific astrocyte specialization and enhanced glial communication.

The ventrolateral preoptic nucleus (VLPO) promotes non-rapid eye movement (NREM) sleep. While VLPO neurons often display low-threshold spikes (LTSs), a feature that supports rhythmic activity, rhythmic bursting has never been reported in these neurons. Here, we report that ∼12% of VLPO neurons in a large ex vivo patch-clamp dataset exhibit spontaneous rhythmic bursting. This pattern occurred in putative sleep-promoting neurons (inhibited by noradrenaline, NA), and in wake-active neurons (excited by NA). Unsupervised clustering of 24 bursting neurons using burst parameters, electrophysiological, and morphological features revealed three groups: one putative sleep-promoting subtype and two wake-active subtypes with fast and slow bursting. Strikingly, membrane potential oscillations persisted in tetrodotoxin (TTX), indicating an intrinsic mechanism. Bursts of inhibitory inputs were also recorded on sleep-promoting neurons. These results suggest that intrinsic rhythmic bursting may propagate to the local network, with functional relevance for sleep regulation.

Sensory deprivation leads to extensive cortical plasticity, but the impact of enhanced sensory experience on the mature cortex remains poorly understood. Here, we examine how visually evoked activity in cortical circuits is shaped by repeated exposure to varied stimuli. The most prominent pattern of visually evoked activity in mouse primary visual cortex, beta oscillations (15-30Hz), arises from brief events of neural synchrony with a characteristic pattern of laminar propagation and relies on the activity of somatostatin-expressing (SST) GABAergic interneurons. We find that visually evoked beta activity is initially weak but robustly potentiated by repeated exposure to a diverse visual stimulus set, leading to enhanced recruitment of cortical neurons by these rhythmic network events. Cellular imaging further reveals that visual experience leads to increased visual responses in SST interneurons and suppressed responses in vasoactive intestinal peptide-expressing (VIP) GABAergic interneurons. In association with this rebalancing of inhibitory circuits, visual experience enhances visual selectivity in nearby pyramidal neurons. Visual experience thus selectively reorganizes adult dendrite-targeting inhibitory circuits, promoting network synchrony and enhancing sensory encoding by cortical excitatory projection neurons.

Spatiotemporal patterns of activity in the neocortex are linked to cognitive processes underlying behavior. However, identifying discrete underlying events within highly dynamic cortical network fluctuations remains a critical challenge. Here, we demonstrate a novel analytical method to track network events underlying state-dependent β- (15-30Hz) and γ- (30-80Hz) range activity in mouse primary visual cortex (V1). We find that γ events are selectively associated with enhanced visual encoding by V1 neurons and γ event rate increases prior to visually-cued behavior, accurately predicting single trial visual detection. This relationship between γ events and behavior is sensory modality-specific and rapidly modulated by changes in task objectives. These findings illuminate a distinct role for transient patterns of cortical activity, indicating that γ supports flexible encoding according to behavioral context.

Patients with focal temporal lobe seizures often experience loss of consciousness associated with cortical slow waves, like those in deep sleep. Previous work in rat models suggests that decreased subcortical arousal causes depressed cortical function during focal seizures. However, these studies were performed under light anesthesia, making it impossible to correlate conscious behavior with physiology. We show in an awake mouse model that electrically induced focal seizures in the hippocampus cause impaired behavioral responses to auditory stimuli, cortical slow waves, and reduced mean cortical high-frequency activity. Behavioral responses are related to cortical cholinergic release at two different timescales. Slow state-related decreases in acetylcholine correlate with overall impaired behavior during seizures. Fast phasic acetylcholine release is related to variable spared or impaired behavioral responses with each auditory stimulus. These findings establish a strong relationship between decreased cortical arousal and impaired consciousness in focal seizures, which may help guide future treatment.

Cognitive and behavioral processes are often accompanied by changes within well-defined frequency bands of the local field potential (LFP i.e., the voltage induced by neuronal activity). These changes are detectable in the frequency domain using the Fourier transform and are often interpreted as neuronal oscillations. However, aside some well-known exceptions, the processes underlying such changes are difficult to track in time, making their oscillatory nature hard to verify. In addition, many non-periodic neural processes can also have spectra that emphasize specific frequencies. Thus, the notion that spectral changes reflect oscillations is likely too restrictive. In this study, we use a simple yet versatile framework to understand the frequency spectra of neural recordings. Using simulations, we derive the Fourier spectra of periodic, quasi-periodic and non-periodic neural processes having diverse waveforms, illustrating how these attributes shape their spectral signatures. We then show how neural processes sum their energy in the local field potential in simulated and real-world recording scenarios. We find that the spectral power of neural processes is essentially determined by two aspects: (1) the distribution of neural events in time and (2) the waveform of the voltage induced by single neural events. Taken together, this work guides the interpretation of the Fourier spectrum of neural recordings and indicates that power increases in specific frequency bands do not necessarily reflect periodic neural activity.

GABAergic interneurons tend to diversify into similar classes across telencephalic regions. However, it remains unclear whether the electrophysiological and molecular properties commonly used to define these classes are discriminant in the hilus of the dentate gyrus. Here, using patch-clamp combined with single cell RT-PCR, we compare the relevance of commonly used electrophysiological and molecular features for the clustering of GABAergic interneurons sampled from the mouse hilus and primary sensory cortex. While unsupervised clustering groups cortical interneurons into well-established classes, it fails to provide a convincing partition of hilar interneurons. Statistical analysis based on resampling indicates that hilar and cortical GABAergic interneurons share limited homology. While our results do not invalidate the use of classical molecular marker in the hilus, they indicate that classes of hilar interneurons defined by the expression of molecular markers do not exhibit strongly discriminating electrophysiological properties.

Patients with focal temporal lobe seizures often experience loss of consciousness. In humans, this loss of consciousness has been shown to be positively correlated with EEG neocortical slow waves, similar to those seen in non-REM sleep. Previous work in rat models of temporal lobe seizures suggests that decreased activity of subcortical arousal systems cause depressed cortical function during seizures. However, these studies were performed under light anesthesia, making it impossible to correlate behavior, and therefore consciousness, to electrophysiologic data. Further, the genetic and molecular toolkits allowing for precise study of the underlying neural circuitry are much more developed in mice than in rats. Here, we describe an awake-behaving, head-fixed mouse model of temporal lobe seizures with both spared and impaired behavior reflecting level of consciousness. Water-restricted mice were head-fixed on a running wheel and trained to associate an auditory stimulus to the delivery of a drop of water from a dispenser. To investigate the effect of seizures on behavior, seizures were electrically induced by stimulating either the left or right hippocampus via a chronically-implanted electrode, while mice were performing the task. Behavior was measured by monitoring lick responses to the auditory stimulus and running speed on the wheel. Further, local field potentials (LFP) signals were simultaneously recorded from hippocampus and orbitofrontal cortex (OFC). Induced focal seizures were 5-30s in duration, and repeatable for several weeks (n=20 animals). Behavioral responses showed a decrease in lick rate to auditory stimulus, and decreased running speed during seizures (p<0.01, n=20 animals). Interestingly, licking response to sound could vary from being impaired to normal during seizures. We found that behavioral impairment is correlated with large amplitude cortical slow-wave activity in frontal cortex, as seen in patients with temporal lobe seizures. These results suggest that induced focal limbic seizures in the mouse can impair consciousness and that the impaired consciousness is correlated with depressed cortical function resembling slow wave sleep. This novel mouse model has similar characteristics with previously studied rat models and human temporal lobe seizures. By leveraging the genetic and molecular techniques available in the mouse, this model can be used to further uncover fundamental mechanisms for loss of consciousness in focal seizures. ### Competing Interest Statement The authors have declared no competing interest.

Recent work suggests an important role for cortical-subcortical networks in seizure-related loss of consciousness. Temporal lobe seizures disrupt subcortical arousal systems, which may lead to depressed cortical function and loss of consciousness. Extracellular recordings show ictal neocortical slow waves at about 1 Hz, but it is not known whether these simply represent seizure propagation or alternatively deep sleep-like activity, which should include cortical neuronal Up and Down states. In this study, using in vivo whole-cell recordings in a rat model of focal limbic seizures, we directly examine the electrophysiological properties of cortical neurons during seizures and deep anesthesia. We found that during seizures, the membrane potential of frontal cortical secondary motor cortex layer 5 neurons fluctuates between Up and Down states, with decreased input resistance and increased firing rate in Up states when compared to Down states. Importantly, Up and Down states in seizures are not significantly different from those in deep anesthesia, in terms of membrane potential, oscillation frequency, firing rate, and input resistance. By demonstrating these fundamental similarities in cortical electrophysiology between deep anesthesia and seizures, our results support the idea that a state of decreased cortical arousal may contribute to mechanisms of loss of consciousness during seizures.

The activity of the cerebral cortex patterns into recurring dynamic motifs. In the present issue of Neuron, Senzai et al. (2019) elucidate how these motifs recruit excitatory and inhibitory neurons across cortical layers and how brain state modulates laminar interactions.

Cortical gamma activity (30-80 Hz) is believed to play important functions in neural computation and arises from the interplay of parvalbumin-expressing interneurons (PV) and pyramidal cells (PYRs). However, the subthreshold dynamics underlying its emergence in the cortex of awake animals remain unclear. Here, we characterized the intracellular dynamics of PVs and PYRs during spontaneous and visually evoked gamma activity in layers 2/3 of V1 of awake mice using targeted patch-clamp recordings and synchronous local field potentials (LFPs). Strong gamma activity patterned in short bouts (one to three cycles), occurred when PVs and PYRs were depolarizing and entrained their membrane potential dynamics regardless of the presence of visual stimulation. PV firing phase locked unconditionally to gamma activity. However, PYRs only phase locked to visually evoked gamma bouts. Taken together, our results indicate that gamma activity corresponds to short pulses of correlated background synaptic activity synchronizing the output of cortical neurons depending on external sensory drive.

The in situ hybridization Allen Mouse Brain Atlas was mined for proteases expressed in the somatosensory cerebral cortex. Among the 480 genes coding for protease/peptidases, only four were found enriched in cortical interneurons: Reln coding for reelin; Adamts8 and Adamts15 belonging to the class of metzincin proteases involved in reshaping the perineuronal net (PNN) and Mme encoding for Neprilysin, the enzyme degrading amyloid β-peptides. The pattern of expression of metalloproteases (MPs) was analyzed by single-cell reverse transcriptase multiplex PCR after patch clamp and was compared with the expression of 10 canonical interneurons markers and 12 additional genes from the Allen Atlas. Clustering of these genes by K-means algorithm displays five distinct clusters. Among these five clusters, two fast-spiking interneuron clusters expressing the calcium-binding protein Pvalb were identified, one co-expressing Pvalb with Sst (PV-Sst) and another co-expressing Pvalb with three metallopeptidases Adamts8, Adamts15 and Mme (PV-MP). By using Wisteria floribunda agglutinin, a specific marker for PNN, PV-MP interneurons were found surrounded by PNN, whereas the ones expressing Sst, PV-Sst, were not.

Neocortical layer VI modulates the thalamocortical transfer of information and has a significant impact on sensory processing. This function implicates local γ-aminobutyric acidergic (GABAergic) interneurons that have only been partly described at the present time. Here, we characterized 85 layer VI GABAergic interneurons in acute slices of mouse somatosensory barrel cortex, using whole-cell current-clamp recordings, single-cell reverse transcription-polymerase chain reaction, and biocytin labeling followed by Neurolucida reconstructions. Unsupervised clustering based on electrophysiological molecular and morphological properties disclosed 4 types of interneurons. The 2 major classes were fast-spiking cells transcribing parvalbumin (PV) (51%) and adapting interneurons transcribing somatostatin (SOM) (26%). The third population (18%) transcribed neuropeptide Y (NPY) and appeared very similar to neurogliaform cells. The last class (5%) was constituted by well-segregated GABAergic interneurons transcribing vasoactive intestinal peptide (VIP). Using transgenic mice expressing GFP under the control of the glutamic acid decarboxylase 67k (GAD67) promoter, we investigated the densities of GABAergic cells immunolabeled against PV, SOM, VIP, and NPY through the depth of layer VI. This analysis revealed that PV and NPY translating interneurons concentrate in the upper and lower parts of layer VI, respectively. This study provides an extensive characterization of the properties of layer VI interneurons.

GABAergic interneurons are local integrators of cortical activity that have been reported to be involved in the control of cerebral blood flow (CBF) through their ability to produce vasoactive molecules and their rich innervation of neighboring blood vessels. They form a highly diverse population among which the serotonin 5-hydroxytryptamine 3A receptor (5-HT3A)-expressing interneurons share a common developmental origin, in addition to the responsiveness to serotonergic ascending pathway. We have recently shown that these neurons regroup two distinct subpopulations within the somatosensory cortex: Neuropeptide Y (NPY)-expressing interneurons, displaying morphological properties similar to those of neurogliaform cells and Vasoactive Intestinal Peptide (VIP)-expressing bipolar/bitufted interneurons. The aim of the present study was to determine the role of these neuronal populations in the control of vascular tone by monitoring blood vessels diameter changes, using infrared videomicroscopy in mouse neocortical slices. Bath applications of 1-(3-Chlorophenyl)biguanide hydrochloride (mCPBG), a 5-HT3R agonist, induced both constrictions (30%) and dilations (70%) of penetrating arterioles within supragranular layers. All vasoconstrictions were abolished in the presence of the NPY receptor antagonist (BIBP 3226), suggesting that they were elicited by NPY release. Vasodilations persisted in the presence of the VIP receptor antagonist VPAC1 (PG-97-269), whereas they were blocked in the presence of the neuronal Nitric Oxide (NO) Synthase (nNOS) inhibitor, L-NNA. Altogether, these results strongly suggest that activation of neocortical 5-HT3A-expressing interneurons by serotoninergic input could induces NO mediated vasodilatations and NPY mediated vasoconstrictions.

In the neocortex, neuronal nitric oxide (NO) synthase (nNOS) is essentially expressed in two classes of GABAergic neurons: type I neurons displaying high levels of expression and type II neurons displaying weaker expression. Using immunocytochemistry in mice expressing GFP under the control of the glutamic acid decarboxylase 67k (GAD67) promoter, we studied the distribution of type I and type II neurons in the barrel cortex and their expression of parvalbumin (PV), somatostatin (SOM), and vasoactive intestinal peptide (VIP). We found that type I neurons were predominantly located in deeper layers and expressed SOM (91.5%) while type II neurons were concentrated in layer II/III and VI and expressed PV (17.7%), SOM (18.7%), and VIP (10.2%). We then characterized neurons expressing nNOS mRNA (n = 42 cells) ex vivo, using whole-cell recordings coupled to single-cell reverse transcription-PCR and biocytin labeling. Unsupervised cluster analysis of this sample disclosed four classes. One cluster (n = 7) corresponded to large, deep layer neurons, displaying a high expression of SOM (85.7%) and was thus very likely to correspond to type I neurons. The three other clusters were identified as putative type II cells and corresponded to neurogliaform-like interneurons (n = 19), deep layer neurons expressing PV or SOM (n = 9), and neurons expressing VIP (n = 7). Finally, we performed nNOS immunohistochemistry on mouse lines in which GFP labeling revealed the expression of two specific developmental genes (Lhx6 and 5-HT3A). We found that type I neurons expressed Lhx6 but never 5-HT3A, indicating that they originate in the medial ganglionic eminence (MGE). Type II neurons expressed Lhx6 (63%) and 5-HT3A (34.4%) supporting their derivation either from the MGE or from the caudal ganglionic eminence (CGE) and the entopeduncular and dorsal preoptic areas. Together, our results in the barrel cortex of mouse support the view that type I neurons form a specific class of SOM-expressing neurons while type II neurons comprise at least three classes.

The introduction of a reporter gene into bacterial artificial chromosome (BAC) constructs allows a rapid identification of the cell type expressing the gene of interest. Here we used BAC transgenic mice expressing a tau‐sapphire green fluorescent protein (GFP) under the transcriptional control of the neuropeptide Y (NPY) genomic sequence to characterize morphological and electrophysiological properties of NPY‐GFP interneurons of the mouse juvenile primary somatosensory cortex. Electrophysiological whole‐cell recordings and biocytin injections were performed to allow the morphological reconstruction of the recorded neurons in three dimensions. Ninety‐six recorded NPY‐GFP interneurons were compared with 39 wild‐type (WT) NPY interneurons, from which 23 and 19 were reconstructed, respectively. We observed that 91% of the reconstructed NPY‐GFP interneurons had developed an atypical axonal swelling from which emerge numerous ramifications. These abnormalities were very heterogeneous in shape and size. They were immunoreactive for the microtubule‐associated protein tau and the lysosomal‐associated membrane protein 1 (LAMP1). Moreover, an electron microscopic analysis revealed the accumulation of numerous autophagic and lysosomal vacuoles in swollen axons. Morphological analyses of NPY‐GFP interneurons also indicated that their somata were smaller, their entire dendritic tree was thickened and presented a restricted spatial distribution in comparison with WT NPY interneurons. Finallly, the morphological defects observed in NPY‐GFP interneurons appeared to be associated with alterations of their electrophysiological intrinsic properties. Altogether, these results demonstrate that NPY‐GFP interneurons developed dystrophic axonal swellings and severe morphological and electrophysiological defects that could be due to the overexpression of tau‐coupled reporter constructs. © 2009 Wiley‐Liss, Inc.