Deepened sleep makes hippocampal spatial memory more persistent

Sleep Consolidation Potentiates Sensorimotor Adaptation

Contrary to its well-established role in declarative learning, the impact of sleep on motor memory consolidation remains a subject of debate. Current literature suggests that while motor skill learning benefits from sleep, consolidation of sensorimotor adaptation (SMA) depends solely on the passage of time. This has led to the proposal that SMA may be an exception to other types of memories. Here, we addressed this ongoing controversy in humans through three comprehensive experiments using the visuomotor adaptation paradigm (N = 290, 150 females). In Experiment 1, we investigated the impact of sleep on memory retention when the temporal gap between training and sleep was not controlled. In line with the previous literature, we found that memory consolidates with the passage of time. In Experiment 2, we used an anterograde interference protocol to determine the time window during which SMA memory is most fragile and, thus, potentially most sensitive to sleep intervention. Our results show that memory is most vulnerable during the initial hour post-training. Building on this insight, in Experiment 3, we investigated the impact of sleep when it coincided with the critical first hour of memory consolidation. This manipulation unveiled a benefit of sleep (30% memory enhancement) alongside an increase in spindle density and spindle-SO coupling during NREM sleep, two well-established neural markers of sleep consolidation. Our findings reconcile seemingly conflicting perspectives on the active role of sleep in motor learning and point to common mechanisms at the basis of memory formation.

Sleep-dependent decorrelation of hippocampal spatial representations

Neuronal ensembles are crucial for episodic memory and spatial mapping. Sleep, particularly non-REM (NREM), is vital for memory consolidation, as it triggers plasticity mechanisms through brain oscillations that reactivate neuronal ensembles. Here, we assessed their role in consolidating hippocampal spatial representations during sleep. We recorded hippocampus activity in rats performing a spatial object-place recognition (OPR) memory task, during encoding and retrieval periods, separated by intervening sleep. Successful OPR retrieval correlated with NREM duration, during which cortical oscillations decreased in power and density as well as neuronal spiking, suggesting global downregulation of network excitability. However, neurons encoding specific spatial locations (i.e., place cells) or objects during OPR showed stronger synchrony with brain oscillations compared to non-encoding neurons, and the stability of spatial representations decreased proportionally with NREM duration. Our findings suggest that NREM sleep may promote flexible remapping in hippocampal ensembles, potentially aiding memory consolidation and adaptation to novel spatial contexts.

Preoperative recovery sleep ameliorates postoperative cognitive dysfunction aggravated by sleep fragmentation in aged mice by enhancing EEG delta-wave activity and LFP theta oscillation in hippocampal CA1

Sleep fragmentation (SF) is a common sleep problem experienced during the perioperative period by older adults, and is associated with postoperative cognitive dysfunction (POCD). Increasing evidence indicates that delta-wave activity during non-rapid eye movement (NREM) sleep is involved in sleep-dependent memory consolidation and that hippocampal theta oscillations are related to spatial exploratory memory. Recovery sleep (RS), a self-regulated state of sleep homeostasis, enhances delta-wave power and memory performance in sleep-deprived older mice. However, it remains unclear whether RS therapy has a positive effect on cognitive changes following SF in older mouse models. Therefore, this study aimed to explore whether preoperative RS can alleviate cognitive deficits in aged mice with SF. A model of preoperative 24-h SF combined with exploratory laparotomy-induced POCD was established in 18-month-old mice. Aged mice were treated with preoperative 6-h RS following SF and postoperative 6-h RS following surgery, respectively. The changes in hippocampus-dependent cognitive function were investigated using behavioral tests, electroencephalography (EEG), local field potential (LFP), magnetic resonance imaging, and neuromorphology. Mice that underwent 24-h SF combined with surgery exhibited severe spatial memory impairment; impaired cognitive performance could be alleviated by preoperative RS treatment. In addition, preoperative RS increased NREM sleep; enhanced EEG delta-wave activity and LFP theta oscillation in the hippocampal CA1; and improved hippocampal perfusion, microstructural integrity, and neuronal damage. Taken together, these results provide evidence that preoperative RS may ameliorate the severity of POCD aggravated by SF by enhancing delta slow-wave activity and hippocampal theta oscillation, and by ameliorating the reduction in regional cerebral blood flow and white matter microstructure integrity in the hippocampus.

From childhood adversity to latent stress vulnerability in adulthood: the mediating roles of sleep disturbances and HPA axis dysfunction

Childhood adversity is a prominent predisposing risk factor for latent stress vulnerability, expressed as an elevated likelihood of developing stress-related psychopathology upon subsequent exposure to trauma in adulthood. Sleep disturbances have emerged as one of the most pronounced maladaptive behavioral outcomes of childhood adversity and are also a highly prevalent core feature of stress-related psychopathology, including post-traumatic stress disorder (PTSD). After reviewing the extensive literature supporting these claims, the current review addresses the notion that childhood adversity-induced sleep disturbances may play a causal role in elevating individuals' stress vulnerability in adulthood. Corroborating this, sleep disturbances that predate adult trauma exposure have been associated with an increased likelihood of developing stress-related psychopathology post-exposure. Furthermore, novel empirical evidence suggests that sleep disturbances, including irregularity of the sleep-wake cycle, mediate the link between childhood adversity and stress vulnerability in adulthood. We also discuss cognitive and behavioral mechanisms through which such a cascade may evolve, highlighting the putative role of impaired memory consolidation and fear extinction. Next, we present evidence to support the contribution of the hypothalamic-pituitary-adrenal (HPA) axis to these associations, stemming from its critical role in stress and sleep regulatory pathways. Childhood adversity may yield bi-directional effects within the HPA stress and sleep axes in which sleep disturbances and HPA axis dysfunction reinforce each other, leading to elevated stress vulnerability. To conclude, we postulate a conceptual path model from childhood adversity to latent stress vulnerability in adulthood and discuss the potential clinical implications of these notions, while highlighting directions for future research.

Sleep is required to consolidate odor memory and remodel olfactory synapses

Animals with complex nervous systems demand sleep for memory consolidation and synaptic remodeling. Here, we show that, although the Caenorhabditis elegans nervous system has a limited number of neurons, sleep is necessary for both processes. In addition, it is unclear if, in any system, sleep collaborates with experience to alter synapses between specific neurons and whether this ultimately affects behavior. C. elegans neurons have defined connections and well-described contributions to behavior. We show that spaced odor-training and post-training sleep induce long-term memory. Memory consolidation, but not acquisition, requires a pair of interneurons, the AIYs, which play a role in odor-seeking behavior. In worms that consolidate memory, both sleep and odor conditioning are required to diminish inhibitory synaptic connections between the AWC chemosensory neurons and the AIYs. Thus, we demonstrate in a living organism that sleep is required for events immediately after training that drive memory consolidation and alter synaptic structures.

Effect of different doses of almorexant on learning and memory in 8-month-old APP/PS1 (AD) mice

OBJECTIVE

To explore the effects of different doses of almorexant (an dual orexin receptor antagonist) on learning and memory in Alzheimer's disease (AD) mice.

METHODS

Forty-four APP/PS1 (model of Alzheimer's disease; AD) mice were randomly divided into 4 groups: the control group (CON) and those that received 10mg/kg almorexant (low dose; LOW), 30mg/kg almorexant (medium dose; MED) and 60mg/kg almorexant (high dose; HIGH). During the 28-day intervention period, mice received an intraperitoneal injection at the beginning of the light period (6:00 am). The effects of different doses of almorexant on learning and memory and 24-hour sleep-wake behaviour were assessed by immunohistochemical staining. The above continuous variables are expressed as the mean ± standard deviation (SD), and then univariate regression analysis and generalized estimating equations were performed to compare the groups; these results are expressed as the mean difference (MD) and 95% confidence interval (CI). The statistical software used STATA 17.0 MP.

RESULTS

Forty-one mice completed the experiment (3 died: 2 mice in the HIGH group and 1 mouse in the CON group). Compared with the CON group, the LOW group (MD=6,803s, 95% CI: 4,470 to 9,137s), MED group (MD=14,473s, 95% CI: 12,140 to 16,806s) and the HIGH group (MD=24,505s, 95% CI: 22,052 to 26,959s) had significantly longer sleep durations. The Y maze results showed that LOW group (MD=0.14,95%CI: 0.078 to 0.20) and MED group (MD=0.14,95%CI = 0.074 to 0.20) mice compared to the CON group, and the low-medium dose of Almorexant did not damage the short-term learning and memory performance of APP / PS1 (AD) mice.Compared with the CON, LOW, and MED groups, the HIGH group exhibited a significant decrease in the Aβ plaque-positive area in the cortex (MD= -0.030, 95% CI: -0.035 to -0.025; MD=-0.049, 95% CI: -0.054 to -0.044; and MD=-0.07, 95% CI: -0.076 to -0.066, respectively).

CONCLUSION

The moderate dose of almorexant (30mg/kg) prolonged the sleep duration of APP/PS1 (AD) mice to a greater extent than the low dose (10mg/kg) without altering learning and memory. The MED mice showed a good sleep response and a small residual effect on the next day. High-dose (60mg / kg) almorexant impaired behavioral learning and memory performance in mice.Compared to the CON group and the LOW group, the MED group exhibited improved working memory. Thus, treatment with almorexant may reduce β-amyloid deposition in AD, slowing neurodegeneration. Additional studies are needed to determine the mechanism of action.

Sleep-A brain-state serving systems memory consolidation

Although long-term memory consolidation is supported by sleep, it is unclear how it differs from that during wakefulness. Our review, focusing on recent advances in the field, identifies the repeated replay of neuronal firing patterns as a basic mechanism triggering consolidation during sleep and wakefulness. During sleep, memory replay occurs during slow-wave sleep (SWS) in hippocampal assemblies together with ripples, thalamic spindles, neocortical slow oscillations, and noradrenergic activity. Here, hippocampal replay likely favors the transformation of hippocampus-dependent episodic memory into schema-like neocortical memory. REM sleep following SWS might balance local synaptic rescaling accompanying memory transformation with a sleep-dependent homeostatic process of global synaptic renormalization. Sleep-dependent memory transformation is intensified during early development despite the immaturity of the hippocampus. Overall, beyond its greater efficacy, sleep consolidation differs from wake consolidation mainly in that it is supported, rather than impaired, by spontaneous hippocampal replay activity possibly gating memory formation in neocortex.

Effect of a dual orexin receptor antagonist on Alzheimer's disease: Sleep disorders and cognition

Orexin is a neuropeptide produced by the lateral hypothalamus that plays an important role in regulating the sleep-wake cycle. The overexpression of the orexinergic system may be related to the pathology of sleep/wakefulness disorders in Alzheimer's disease (AD). In AD patients, the increase in cerebrospinal fluid orexin levels is associated with parallel sleep deterioration. Dual orexin receptor antagonist (DORA) can not only treat the sleep-wakefulness disorder of AD but also improve the performance of patients with cognitive behavior disorder. It is critical to clarify the role of the orexin system in AD, study its relationship with cognitive decline in AD, and evaluate the safety and efficacy of DORA.

Two distinct ways to form long-term object recognition memory during sleep and wakefulness

Memory consolidation is promoted by sleep. However, there is also evidence for consolidation into long-term memory during wakefulness via processes that preferentially affect nonhippocampal representations. We compared, in rats, the effects of 2-h postencoding periods of sleep and wakefulness on the formation of long-term memory for objects and their associated environmental contexts. We employed a novel-object recognition (NOR) task, using object exploration and exploratory rearing as behavioral indicators of these memories. Remote recall testing (after 1 wk) confirmed significant long-term NOR memory under both conditions, with NOR memory after sleep predicted by the occurrence of EEG spindle-slow oscillation coupling. Rats in the sleep group decreased their exploratory rearing at recall testing, revealing successful recall of the environmental context. By contrast, rats that stayed awake after encoding showed equally high levels of rearing upon remote testing as during encoding, indicating that context memory was lost. Disruption of hippocampal function during the postencoding interval (by muscimol administration) suppressed long-term NOR memory together with context memory formation when animals slept, but enhanced NOR memory when they were awake during this interval. Testing remote recall in a context different from that during encoding impaired NOR memory in the sleep condition, while exploratory rearing was increased. By contrast, NOR memory in the wake rats was preserved and actually superior to that after sleep. Our findings indicate two distinct modes of long-term memory formation: Sleep consolidation is hippocampus dependent and implicates event-context binding, whereas wake consolidation is impaired by hippocampal activation and strengthens context-independent representations.

Nap effects on preschool children’s learning of letter‐sound mappings

This study explored whether a daytime nap aids children's acquisition of letter‐sound knowledge, which is a fundamental component for learning to read. Thirty‐two preschool children in Sydney, Australia (M_age = 4 years;3 months) were taught letter‐sound mappings in two sessions: one followed by a nap and the other by a wakeful period. Learning was assessed by explicit letter‐sound mappings (“Which sound does this letter make?”) and knowledge generalization tasks (“Here's Tav and Cav, which one is /kav/?”). Results from the knowledge generalization task showed better performance after a nap than after wake. However, no nap benefit was found for explicit letter‐sound knowledge. This study provides initial evidence that naps could be beneficial for preschool children's learning of letter‐sound mappings.

Sleep and conditioning of the siphon withdrawal reflex in Aplysia

Sleep is essential for memory consolidation after learning as shown in mammals and invertebrates such as bees and flies. Aplysia californica displays sleep, and sleep in this mollusk was also found to support memory for an operant conditioning task. Here, we investigated whether sleep in Aplysia is also required for memory consolidation in a simpler type of learning, i.e. the conditioning of the siphon withdrawal reflex. Two groups of animals (Wake, Sleep, each n=11) were conditioned on the siphon withdrawal reflex, with the training following a classical conditioning procedure where an electrical tail shock served as the unconditioned stimulus (US) and a tactile stimulus to the siphon as the conditioned stimulus (CS). Responses to the CS were tested before (pre-test), and 24 and 48 h after training. While Wake animals remained awake for 6 h after training, Sleep animals had undisturbed sleep. The 24 h test in both groups was combined with extinction training, i.e. the extended presentation of the CS alone over two blocks. At the 24 h test, siphon withdrawal duration in response to the CS was distinctly enhanced in both Sleep and Wake groups with no significant difference between groups, consistent with the view that consolidation of a simple conditioned reflex response does not require post-training sleep. Surprisingly, extinction training did not reverse the enhancement of responses to the CS. On the contrary, at the 48 h test, withdrawal duration in response to the CS was even further enhanced across both groups. This suggests that processes of sensitization, an even simpler non-associative type of learning, contributed to the withdrawal responses. Our study provides evidence for the hypothesis that sleep preferentially benefits consolidation of more complex learning paradigms than conditioning of simple reflexes.