Isolated theta waves originating from the midline thalamus trigger memory reactivation during NREM sleep in mice.

During non-rapid eye movement (NREM) sleep, neural ensembles in the entorhinal-hippocampal circuit responsible for encoding recent memories undergo reactivation to facilitate the process of memory consolidation. This reactivation is widely acknowledged as pivotal for the formation of stable memory and its impairment is closely associated with memory dysfunction. To date, the neural mechanisms driving the reactivation of neural ensembles during NREM sleep remain poorly understood. Here, we show that the neural ensembles in the medial entorhinal cortex (MEC) that encode spatial experiences exhibit reactivation during NREM sleep. Notably, this reactivation consistently coincides with isolated theta waves. In addition, we found that the nucleus reuniens (RE) in the midline thalamus exhibits typical theta waves during NREM sleep, which are highly synchronized with those occurring in the MEC in male mice. Closed-loop optogenetic inhibition of the RE-MEC pathway specifically suppressed these isolated theta waves, resulting in impaired reactivation and compromised memory consolidation following a spatial memory task in male mice. The findings suggest that theta waves originating from the ventral midline thalamus play a role in initiating memory reactivation and consolidation during sleep.

Homeostatic Shrinkage of Dendritic Spines Requires Melatonin Type 3 Receptor Activation During Sleep.

High-frequency oscillatory activity in cognition-related neural circuits during wakefulness consistently induces the growth of dendritic spines and axonal terminals. Although these structural changes are essential for cognitive functions, it is hypothesized that if these newly expanded structures fail to establish functional connections, they may become superfluous. Sleep is believed to facilitate the reduction of such redundant structures to maintain neural homeostasis. However, the mechanisms underlying this pruning process during sleep remain poorly understood. In this study, that melatonin type 3 receptors (MT3Rs) are selectively expressed in the stellate neurons of the medial entorhinal cortex (MEC) is demonstrated, an area where high melatonin levels are detected during sleep. Activation of MT3Rs during sleep initiates the shrinkage of dendritic spines in stellate neurons by downregulating neural network activity and dephosphorylating synaptic proteins in the MEC. This process is disrupted when MT3R expression is knocked down or when MT3Rs are blocked during sleep. Notably, interference with MT3Rs in the MEC during sleep impairs the acquisition of spatial memory but does not affect object memory acquisition following sleep. These findings reveal novel molecular mechanisms involving melatonin and MT3Rs in the regulation of dendritic spine shrinkage during sleep, which is crucial for the acquisition and consolidation of spatial memory.

Melatonin targets the paraventricular thalamus to promote non-rapid eye movement sleep in C3H/HeJ mice.

Melatonin (MLT) is an important circadian signal for sleep regulation, but the neural circuitries underlying the sleep-promoting effects of MLT are poorly understood. The paraventricular thalamus (PVT) is a critical thalamic area for wakefulness control and expresses MLT receptors, raising a possibility that PVT neurons may mediate the sleep-promoting effects of MLT. Here, we found that MLT receptors were densely expressed on PVT neurons and exhibited circadian-dependent variations in C3H/HeJ mice. Application of exogenous MLT decreased the excitability of PVT neurons, resulting in hyperpolarization of membrane potential and reduction of action potential firing. MLT also inhibited the spontaneous activity of PVT neurons at both population and single-neuron levels in freely behaving mice. Furthermore, pharmacological manipulations revealed that local infusion of exogeneous MLT into the PVT promoted non-rapid eye movement (NREM) sleep and increased NREM sleep duration, whereas MLT receptor antagonists decreased NREM sleep. Moreover, we found that selectively knocking down endogenous MLT receptors in the PVT decreased NREM sleep and correspondingly increased wakefulness, with particular changes shortly after the onset of the dark or light phase. Taken together, these results demonstrate that PVT is an important target of MLT for promoting NREM sleep.

A potentiation of REM sleep-active neurons in the lateral habenula may be responsible for the sleep disturbance in depression.

Psychiatric disorders with dysfunction of the lateral habenula (LHb) show sleep disturbance, especially a disinhibition of rapid eye movement (REM) sleep in major depression. However, the role of LHb in physiological sleep control and how LHb contributes to sleep disturbance in major depression remain elusive. Here, we found that functional manipulations of LHb glutamatergic neurons bidirectionally modulated both non-REM (NREM) sleep and REM sleep. Activity recording revealed heterogeneous activity patterns of LHb neurons across sleep/wakefulness cycles, but LHb neurons were preferentially active during REM sleep. Using an activity-dependent tagging method, we selectively labeled a population of REM sleep-active LHb neurons and demonstrated that these neurons specifically promoted REM sleep. Neural circuit studies showed that LHb neurons regulated REM sleep via projections to the ventral tegmental area but not to the rostromedial tegmental nucleus. Furthermore, we found that the increased REM sleep in a depression mouse model was associated with a potentiation of REM sleep-active LHb neurons, including an increased proportion, elevated spike firing, and altered activity mode. Importantly, inhibition of REM sleep-active LHb neurons not only attenuated the increased REM sleep but also alleviated depressive-like behaviors in a depression mouse model. Thus, our results demonstrated that REM sleep-active LHb neurons selectively promoted REM sleep, and a potentiation of these neurons contributed to depression-associated sleep disturbance.

A midbrain GABAergic circuit constrains wakefulness in a mouse model of stress.

Enhancement of wakefulness is a prerequisite for adaptive behaviors to cope with acute stress, but hyperarousal is associated with impaired behavioral performance. Although the neural circuitries promoting wakefulness in acute stress conditions have been extensively identified, less is known about the circuit mechanisms constraining wakefulness to prevent hyperarousal. Here, we found that chemogenetic or optogenetic activation of GAD2-positive GABAergic neurons in the midbrain dorsal raphe nucleus (DRNGAD2) decreased wakefulness, while inhibition or ablation of these neurons produced an increase in wakefulness along with hyperactivity. Surprisingly, DRNGAD2 neurons were paradoxically wakefulness-active and were further activated by acute stress. Bidirectional manipulations revealed that DRNGAD2 neurons constrained the increase of wakefulness and arousal level in a mouse model of stress. Circuit-specific investigations demonstrated that DRNGAD2 neurons constrained wakefulness via inhibition of the wakefulness-promoting paraventricular thalamus. Therefore, the present study identified a wakefulness-constraining role DRNGAD2 neurons in acute stress conditions.

Spatial memory requires hypocretins to elevate medial entorhinal gamma oscillations.

The hypocretin (Hcrt) (also known as orexin) neuropeptidic wakefulness-promoting system is implicated in the regulation of spatial memory, but its specific role and mechanisms remain poorly understood. In this study, we revealed the innervation of the medial entorhinal cortex (MEC) by Hcrt neurons in mice. Using the genetically encoded G-protein-coupled receptor activation-based Hcrt sensor, we observed a significant increase in Hcrt levels in the MEC during novel object-place exploration. We identified the function of Hcrt at presynaptic glutamatergic terminals, where it recruits fast-spiking parvalbumin-positive neurons and promotes gamma oscillations. Bidirectional manipulations of Hcrt neurons' projections from the lateral hypothalamus (LHHcrt) to MEC revealed the essential role of this pathway in regulating object-place memory encoding, but not recall, through the modulation of gamma oscillations. Our findings highlight the significance of the LHHcrt-MEC circuitry in supporting spatial memory and reveal a unique neural basis for the hypothalamic regulation of spatial memory.

A Sleep-Specific Midbrain Target for Sevoflurane Anesthesia.

Sevoflurane has been the most widely used inhaled anesthetics with a favorable recovery profile; however, the precise mechanisms underlying its anesthetic action are still not completely understood. Here the authors show that sevoflurane activates a cluster of urocortin 1 (UCN1+ )/cocaine- and amphetamine-regulated transcript (CART+ ) neurons in the midbrain involved in its anesthesia. Furthermore, growth hormone secretagogue receptor (GHSR) is highly enriched in sevoflurane-activated UCN1+ /CART+ cells and is necessary for sleep induction. Blockade of GHSR abolishes the excitatory effect of sevoflurane on UCN1+ /CART+ neurons and attenuates its anesthetic effect. Collectively, their data suggest that anesthetic action of sevoflurane necessitates the GHSR activation in midbrain UCN1+ /CART+ neurons, which provides a novel target including the nucleus and receptor in the field of anesthesia.

A paraventricular thalamus to central amygdala neural circuit modulates acute stress-induced heightened wakefulness.

Heightened wakefulness in response to stressors is essential for survival but can also lead to sleep disorders like insomnia. The paraventricular thalamus (PVT) is both a critical thalamic area for wakefulness and a stress-sensitive brain region. However, whether the PVT and its neural circuitries are involved in controlling wakefulness in stress conditions remains unknown. Here, we find that PVT neurons projecting to the central amygdala (CeA) are activated by different stressors. These neurons are wakefulness-active and increase their activities upon sleep to wakefulness transitions. Optogenetic activation of the PVT-CeA circuit evokes transitions from sleep to wakefulness, whereas selectively silencing the activity of this circuit decreases time spent in wakefulness. Specifically, chemogenetic inhibition of CeA-projecting PVT neurons not only alleviates stress responses but also attenuates the acute stress-induced increase of wakefulness. Thus, our results demonstrate that the PVT-CeA circuit controls physiological wakefulness and modulates acute stress-induced heightened wakefulness.

REM sleep-active hypothalamic neurons may contribute to hippocampal social-memory consolidation.

The hippocampal CA2 region plays a key role in social memory. The encoding of such memory involves afferent activity from the hypothalamic supramammillary nucleus (SuM) to CA2. However, the neuronal circuits required for consolidation of freshly encoded social memory remain unknown. Here, we used circuit-specific optical and single-cell electrophysiological recordings in mice to explore the role of sleep in social memory consolidation and its underlying circuit mechanism. We found that SuM neurons projecting to CA2 were highly active during rapid-eye-movement (REM) sleep but not during non-REM sleep or quiet wakefulness. REM-sleep-selective optogenetic silencing of these neurons impaired social memory. By contrast, the silencing of another group of REM sleep-active SuM neurons that projects to the dentate gyrus had no effect on social memory. Therefore, we provide causal evidence that the REM sleep-active hypothalamic neurons that project to CA2 are specifically required for the consolidation of social memory.

The paraventricular thalamus is a critical thalamic area for wakefulness.

Clinical observations indicate that the paramedian region of the thalamus is a critical node for controlling wakefulness. However, the specific nucleus and neural circuitry for this function remain unknown. Using in vivo fiber photometry or multichannel electrophysiological recordings in mice, we found that glutamatergic neurons of the paraventricular thalamus (PVT) exhibited high activities during wakefulness. Suppression of PVT neuronal activity caused a reduction in wakefulness, whereas activation of PVT neurons induced a transition from sleep to wakefulness and an acceleration of emergence from general anesthesia. Moreover, our findings indicate that the PVT-nucleus accumbens projections and hypocretin neurons in the lateral hypothalamus to PVT glutamatergic neurons' projections are the effector pathways for wakefulness control. These results demonstrate that the PVT is a key wakefulness-controlling nucleus in the thalamus.

Superficial Layer-Specific Histaminergic Modulation of Medial Entorhinal Cortex Required for Spatial Learning.

The medial entorhinal cortex (MEC) plays a crucial role in spatial learning and memory. Whereas the MEC receives a dense histaminergic innervation from the tuberomamillary nucleus of the hypothalamus, the functions of histamine in this brain region remain unclear. Here, we show that histamine acts via H1Rs to directly depolarize the principal neurons in the superficial, but not deep, layers of the MEC when recording at somata. Moreover, histamine decreases the spontaneous GABA, but not glutamate, release onto principal neurons in the superficial layers by acting at presynaptic H3Rs without effect on synaptic release in the deep layers. Histamine-induced depolarization is mediated via inhibition of Kir channels and requires the activation of protein kinase C, whereas the inhibition of spontaneous GABA release by histamine depends on voltage-gated Ca(2+) channels and extracellular Ca(2+). Furthermore, microinjection of the H1R or H3R, but not H2R, antagonist respectively into the superficial, but not deep, layers of MEC impairs rat spatial learning as assessed by water maze tasks but does not affect the motor function and exploratory activity in an open field. Together, our study indicates that histamine plays an essential role in spatial learning by selectively regulating neuronal excitability and synaptic transmission in the superficial layers of the MEC.

Rhynchophylline Protects Against the Amyloid ß-Induced Increase of Spontaneous Discharges in the Hippocampal CA1 Region of Rats.

Accumulated soluble amyloid ß (Aß)-induced aberrant neuronal network activity has been recognized as a key causative factor leading to cognitive deficits which are the most outstanding characteristic of Alzheimer's disease (AD). As an important structure associated with learning and memory, the hippocampus is one of the brain regions that are impaired very early in AD, and the hippocampal CA1 region is selectively vulnerable to soluble Aß oligomers. Our recent study showed that soluble Aß1-42 oligomers induced hyperactivity and perturbed the firing patterns in hippocampal neurons. Rhynchophylline (RIN) is an important active tetracyclic oxindole alkaloid isolated from Uncaria rhynchophylla which is a traditional Chinese medicine and often used to treat central nervous system illnesses such as hypertension, convulsions, tremor, stroke etc. Previous evidence showed that RIN possessed neuroprotective effects of improving the cognitive function of mice with Alzheimer-like symptoms. In the present study, we aimed to investigate the protective effect of RIN against soluble Aß1-42 oligomers-induced hippocampal hyperactivity. The results showed that (1) the mean frequency of spontaneous discharge was increased by the local application of 3 µM soluble Aß1-42 oligomers; (2) 30 µM RIN did not exert any obvious effects on basal physiological discharges; and (3) treatment with RIN effectively inhibited the soluble Aß1-42 oligomers-induced enhancement of spontaneous discharge, in a concentration-dependent manner with an IC50 = 9.0 µM. These in vivo electrophysiological results indicate that RIN can remold the spontaneous discharges disturbed by Aß and counteract the deleterious effect of Aß1-42 on neural circuit. The experimental findings provide further evidence to affirm the potential of RIN as a worthy candidate for further development into a therapeutic agent for AD.

Riluzole prevents soluble Aß1-42 oligomers-induced perturbation of spontaneous discharge in the hippocampal CA1 region of rats.

Abnormal accumulation of soluble amyloid beta (Aß) is believed to cause malfunction of neurons in Alzheimer's disease (AD). The hippocampus is one of the earliest affected brain regions in AD. However, little effort has been made to investigate the effects of soluble Aß1-42 oligomers on discharge properties of hippocampal neurons in vivo. This study was designed to examine the effects of soluble Aß1-42 oligomers on the discharge properties of hippocampal CA1 neurons using extracellular single-unit recordings in vivo. The protective effects of riluzole (RLZ) were also investigated for the prevention of soluble oligomers of Aß1-42-induced alterations in the spontaneous discharge of hippocampal neurons. The results showed that (1) the mean frequency of spontaneous discharge was increased by the local application of 100 µM Aß1-42 oligomers; (2) Aß1-42 oligomers also induced alterations of the neuronal firing patterns in the hippocampal CA1 region; and (3) pretreatment with 20 µM RLZ effectively inhibited the Aß1-42-induced enhancement of spontaneous discharge and alterations of neuronal firing patterns in CA1 neurons. Our study suggested that Aß1-42 oligomers induced hyperactivity and perturbed the firing patterns in hippocampal neurons. RLZ may provide neuroprotective effects on the Aß1-42-induced perturbation of neuronal activities in the hippocampal region of rats.

Gastrodin suppresses the amyloid ß-induced increase of spontaneous discharge in the entorhinal cortex of rats.

Accumulated soluble amyloid beta- (Aß-) induced aberrant neuronal network activity may directly contribute to cognitive deficits, which are the most outstanding characteristics of Alzheimer's disease (AD). The entorhinal cortex (EC) is one of the earliest affected brain regions in AD. Impairments of EC neurons are responsible for the cognitive deficits in AD. However, little effort has been made to investigate the effects of soluble Aß on the discharge properties of EC neurons in vivo. The present study was designed to examine the effects of soluble Aß(1-42) on the discharge properties of EC neurons, using in vivo extracellular single unit recordings. The protective effects of gastrodin (GAS) were also investigated against Aß(1-42)-induced alterations in EC neuronal activities. The results showed that the spontaneous discharge of EC neurons was increased by local application of soluble Aß(1-42) and that GAS can effectively reverse Aß(1-42)-induced facilitation of spontaneous discharge in a concentration-dependent manner. Moreover, whole-cell patch clamp results indicated that the protective function of GAS on abnormal hyperexcitability may be partially mediated by its inhibitory action on Aß(1-42)-elicited inward currents in EC neurons. Our study suggested that GAS may provide neuroprotective effects on Aß(1-42)-induced hyperactivity in EC neurons of rats.

Persistent sodium currents contribute to Aß1-42-induced hyperexcitation of hippocampal CA1 pyramidal neurons.

Patients with Alzheimer's disease (AD) have elevated incidence of epilepsy. Moreover, neuronal hyperexcitation occurs in transgenic mouse models overexpressing amyloid precursor protein and its pathogenic product, amyloid ß protein (Aß). However, the cellular mechanisms of how Aß causes neuronal hyperexcitation are largely unknown. We hypothesize that the persistent sodium current (INaP), a subthreshold sodium current that can increase neuronal excitability, may in part account for the Aß-induced neuronal hyperexcitation. The present study was designed to evaluate the involvement of INaP in Aß-induced hyperexcitation of hippocampal CA1 pyramidal neurons using a whole-cell patch-clamp recording technique. Our results showed that bath application of soluble Aß1-42 increased neuronal excitability in a concentration-dependent manner. Soluble Aß1-42 also increased the amplitude of INaP without significantly affecting its activation properties. In the presence of riluzole (RLZ), an antagonist of INaP, the Aß1-42-induced neuronal hyperexcitation and INaP augmentation were significantly inhibited. These findings suggest that soluble Aß1-42 may induce neuronal hyperexcitation by increasing the amplitude of INaP and that RLZ can inhibit the Aß1-42-induced abnormal neuronal activity.

Synaptic mechanisms underlying thalamic activation-induced plasticity in the rat auditory cortex.

Electrical stimulation of ventral division of medial geniculate body (MGBv) neurons evokes a shift of the frequency-tuning curves of auditory cortical (AC) neurons toward the best frequency (BF) of the stimulated MGBv neurons (frequency-specific plasticity). The shift of BF is induced by inhibition of responses at the BF of the recorded AC neuron, with coincident facilitation of responses at the BF of the stimulated MGBv neuron. However, the synaptic mechanisms are not yet understood. We hypothesize that activation of thalamocortical synaptic transmission and receptor function may contribute to MGBv stimulation-induced frequency-specific auditory plasticity and the shift of BF. To test this hypothesis, we measured changes in the excitatory postsynaptic currents in pyramidal neurons of layer III/IV in the auditory cortex following high-frequency stimulation (HFS) of the MGBv, using whole cell recordings in an auditory thalamocortical slice. Our data showed that in response to the HFS of the MGBv the excitatory postsynaptic currents of AC neurons showed long-term bidirectional synaptic plasticity and long-term potentiation and depression. Pharmacological studies indicated that the long-term synaptic plasticity was induced through the activation of different sets of N-methyl-d-aspartate-type glutamatergic receptors, γ-aminobutyric acid-type receptors, and type 5 metabotropic glutamate receptors. Our data further demonstrated that blocking of different receptors with specific antagonists significantly inhibited MGBv stimulation-induced long-term plasticity as well as the shift of BF. These data indicate that these receptors have an important role in mediating frequency-specific auditory cortical plasticity.

Frequency-specific plasticity of the auditory cortex elicited by thalamic stimulation in the rat.

In a process known as frequency-specific plasticity, electrical stimulation of the ventral division of the medial geniculate body (MGBv) in the thalamus evokes a shift in the frequency-tuning curves of auditory cortical (AC) neurons toward the best frequency (BF) of stimulated MGBv neurons. However, the underlying synaptic mechanisms of this process are uncharacterized. To investigate whether this dynamic change depends on thalamocortical (TC) synaptic plasticity, we studied frequency-specific changes in synaptic transmission efficacy in TC pathways evoked by thalamic stimulation. Specifically, we induced cortical plasticity by repetitive focal electrical stimulation of the MGBv in rats and measured receptive field shifts and local field potentials in AC neurons. Our data show that focal electrical stimulation of the MGBv induced receptive field shifts as well as long-term potentiation or depression of the local field potentials in AC neurons. The evoked potentiation and depression depended on the frequency of the electrical stimulation of the MGBv synchronized with the BF of MGBv and AC neurons. Receptive field shifts were produced by inhibition of responses at the BF of the recorded AC neurons and facilitation of responses at the BF of the stimulated MGBv neurons. These results suggest that MGBv neurons play a decisive role in the expression of AC synaptic plasticity and that activation of different frequency-specific TC pathways may be the synaptic mechanism underlying this plasticity.