Sleep and quiet wakefulness signify an idling brain hub for creative insights.

Long-term potentiation of synaptic strength is a fundamental aspect of learning and memory. Memories are believed to be stored within specific populations of neurons known as engram cells, which are subsequently reactivated during sleep, facilitating the consolidation of stored information. However, sleep and offline reactivations are associated not only with past experiences but also with anticipation of future events. During periods of offline reactivation, which occur during sleep and quiet wakefulness, the brain exhibits a capability to form novel connections. This process links various past experiences, often leading to the emergence of qualitatively new information that was not initially available. Brain activity during sleep and quiet wakefulness is referred to as the 'idling brain'. Idling brain activity is believed to play a pivotal role in abstracting essential information, comprehending underlying rules, generating creative ideas and fostering insightful thoughts. In this review, we will explore the current state of research and future directions in understanding how sleep and idling brain activity are interconnected with various cognitive functions, especially creative insights. These insights have profound implications for our daily lives, impacting our ability to process information, make decisions and navigate complex situations effectively. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Memory reactivations during sleep.

Neuronal activities that occur during awake periods are often reactivated again during sleep, to consolidate recently encoded memories, a process known as consolidation. In recent years, advanced tools, specially optical techniques and in-vivo live Ca2+ imaging, have revealed a deeper understanding to the offline periods' neuronal activities and their correspondence to later awake behavioral outputs. Recently, there is a growing consensus that sleep is more of an active process. Sleep has been associated with various functions, memory updating, future imaginations of possible familiar scenarios, decision making and planning by replaying past memories. Also, boosting insightful thoughts, creative thinking and problem solving by forming new associations and connections that were not present in awake states. Sleep activities have been directly associated with many "EUREKA" or "AHA" moments. Here, we describe recent views on memory reactivations during sleep and their implications on learning and memory.

Sevoflurane-induced amnesia is associated with inhibition of hippocampal cell ensemble activity after learning.

General anesthesia could induce amnesia, however the mechanism remains unclear. We hypothesized that suppression of neuronal ensemble activity in the hippocampus by anesthesia during the post-learning period causes retrograde amnesia. To test this hypothesis, two experiments were conducted with sevoflurane anesthesia (2.5%, 30 min): a hippocampus-dependent memory task, the context pre-exposure facilitation effect (CPFE) procedure to measure memory function and in vivo calcium imaging to observe neural activity in hippocampal CA1 during context exploration and sevoflurane/home cage session. Sevoflurane treatment just after context pre-exposure session impaired the CPFE memory, suggesting sevoflurane induced retrograde amnesia. Calcium imaging showed sevoflurane treatment prevented neuronal activity in CA1. Further analysis of neuronal activity with non-negative matrix factorization, which extracts neural ensemble activity based on synchronous activity, showed that sevoflurane treatment reduced the reactivation of neuronal ensembles between during context exploration just before and one day after sevoflurane inhalation. These results suggest that sevoflurane treatment immediately after learning induces amnesia, resulting from suppression of reactivation of neuronal ensembles.

The locus coeruleus as a regulator of memory linking.

A core function of our memory system is the ability to link appropriate events. In this issue of Neuron, Chowdhury et al. (2022) pinpoint a specific neuromodulatory circuit for contextual memory linking, but not formation, in the hippocampus.

Pcdhß deficiency affects hippocampal CA1 ensemble activity and contextual fear discrimination.

Clustered protocadherins (Pcdhs), a large group of adhesion molecules, are important for axonal projections and dendritic spread, but little is known about how they influence neuronal activity. The Pcdhß cluster is strongly expressed in the hippocampus, and in vivo Ca2+ imaging in Pcdhß-deficient mice revealed altered activity of neuronal ensembles but not of individual cells in this region in freely moving animals. Specifically, Pcdhß deficiency increased the number of large-size neuronal ensembles and the proportion of cells shared between ensembles. Furthermore, Pcdhß-deficient mice exhibited reduced repetitive neuronal population activity during exploration of a novel context and were less able to discriminate contexts in a contextual fear conditioning paradigm. These results suggest that one function of Pcdhßs is to modulate neural ensemble activity in the hippocampus to promote context discrimination.

Orchestrated ensemble activities constitute a hippocampal memory engram.

The brain stores and recalls memories through a set of neurons, termed engram cells. However, it is unclear how these cells are organized to constitute a corresponding memory trace. We established a unique imaging system that combines Ca2+ imaging and engram identification to extract the characteristics of engram activity by visualizing and discriminating between engram and non-engram cells. Here, we show that engram cells detected in the hippocampus display higher repetitive activity than non-engram cells during novel context learning. The total activity pattern of the engram cells during learning is stable across post-learning memory processing. Within a single engram population, we detected several sub-ensembles composed of neurons collectively activated during learning. Some sub-ensembles preferentially reappear during post-learning sleep, and these replayed sub-ensembles are more likely to be reactivated during retrieval. These results indicate that sub-ensembles represent distinct pieces of information, which are then orchestrated to constitute an entire memory.

Stoichiometric alteration of PMP22 protein determines the phenotype of hereditary neuropathy with liability to pressure palsies.

BACKGROUND: Hereditary neuropathy with liability to pressure palsies (HNPP) is caused by a 1.4-megabase deletion at chromosome 17p11.2, which bears the PMP22 gene and other genes. However, whether other genes besides PMP22 contribute to the phenotype is unknown. Whether any mutation within the coding region of the PMP22 gene ultimately causes HNPP by reducing the amount of peripheral myelin protein 22 (PMP22) expressed in myelin is also unknown. OBJECTIVE: To determine whether affected patients develop a phenotype identical to that found in HNPP and whether the leucine 7 frameshift (Leu7fs) mutation reduces PMP22 levels in myelin. DESIGN: We evaluated affected family members by neurological examination, electrophysiology, and skin biopsies. We identified a large family with a Leu7fs mutation of PMP22 (11 affected members across 3 generations) that predicts truncation of the protein prematurely and eliminates PMP22 expression from the mutant allele. RESULTS: We found that PMP22 levels were reduced in peripheral nerve myelin in dermal skin biopsies in patients with an Leu7fs mutation. Through clinical and electrophysiological evaluation, we also found that patients with the Leu7fs mutation were indistinguishable from patients with HNPP caused by deletion. We also found that a length-dependent axonal loss became pronounced in elderly patients with Leu7fs mutations, similar to what has been described in heterozygous knockout mice (pmp22 +/-). CONCLUSIONS: Taken together, these results confirm that the phenotypic expression is identical in patients with Leu7fs mutation and patients with HNPP caused by chromosome 17p11.2 deletion. They also demonstrate that reduction of PMP22 is sufficient to cause the full HNPP phenotype.

Effect of an R69C mutation in the myelin protein zero gene on myelination and ion channel subtypes.

BACKGROUND: Most mutations in the myelin protein zero gene (MPZ) typically cause a severe demyelinating/dysmyelinating neuropathy that begins in infancy or an adult-onset axonal neuropathy. Axonal degeneration in the late-onset H10P mutation may be caused by the disruption of axoglial interaction. OBJECTIVE: To evaluate sural nerve biopsy samples from a patient with early-onset Charcot-Marie-Tooth disease type 1B caused by an arg69-to-cys (R69C) mutation. DESIGN AND PARTICIPANTS: Biopsies of sural nerves were performed 20 years apart in a patient with an R69C mutation (early onset). In addition, peripheral nerves were obtained from autopsy material from a patient with a T95M mutation (late onset). These nerves were analyzed using light microscopy of semithin sections, teased nerve fiber immunohistochemical analysis, electron microscopy, and immunologic electron microscopy. MAIN OUTCOME MEASURES: Pathological changes in sural nerve. RESULTS: Both R69C biopsy samples showed prominent demyelination and onion bulb formation, unlike the late-onset T95M mutation, which showed primarily axonal degeneration with no onion bulbs. The sural biopsy sample obtained 20 years earlier from the R69C patient showed minimal difference from the present sample, consistent with the lack of clinical progression during the 2 decades. Teased fiber immunohistochemical analysis of R69C revealed voltage-gated sodium channel subtype 1.8 expressions at the nodes of Ranvier around the areas of segmental demyelination. Internodal length in all R69C nerve fibers was invariably short (>94% of all internodes are <150 mum). CONCLUSIONS: Morphologic abnormalities in this early-onset R69C neuropathy were severe in childhood but progressed very slowly after adolescence. The switch to voltage-gated sodium channel subtype 1.8 expression at the nodes may provide clues into the pathogenesis of this case of early-onset neuropathy, and the short internodes may contribute to the extremely slowed conduction velocities in this case (<10 m/s).

Skin biopsies in myelin-related neuropathies: bringing molecular pathology to the bedside.

Skin biopsy is a minimally invasive procedure and has been used in the evaluation of non-myelinated, but not myelinated nerve fibres, in sensory neuropathies. We therefore evaluated myelinated nerves in skin biopsies from normal controls and patients with Charcot-Marie-Tooth (CMT) disease caused by mutations in myelin proteins. Light microscopy, electron microscopy and immunohistochemistry routinely identified myelinated dermal nerves in glabrous skin that appeared similar to myelinated fibres in sural and sciatic nerve. Myelin abnormalities were observed in all patients with CMT. Moreover, skin biopsies detected potential pathogenic abnormalities in the axolemmal molecular architecture previously undetected in human neuropathies. Finally, myelin gene expression at both mRNA and protein levels was evaluated by real-time PCR and immunoelectron microscopy. Peripheral myelin protein 22 (PMP22) was increased in CMT1A (PMP22 duplication) and decreased in patients with hereditary neuropathy with liability to pressure palsies (PMP22 deletion). Taken together, our data suggest that skin biopsy may in certain circumstances replace the more invasive sural nerve biopsy in the morphological and molecular evaluation of inherited and other demyelinating neuropathies.