Search

The neocortex is disconnected from the outside world during sleep, which has been hypothesized to be relevant for synaptic reorganization involved in memory consolidation. Fast network oscillations, such as hippocampal sharp-wave ripples, cortical ripples, and amygdalar high-frequency oscillations, are prominent during non-REM sleep. Although these oscillations are thought to be generated by local circuit mechanisms, their occurrence rates and amplitudes are modulated by thalamocortical spindles and neocortical slow oscillations during non-REM sleep, suggesting that fast network oscillations and slower oscillations cooperatively work to facilitate memory consolidation. This review discusses the recent progress in understanding the generation, coordination, and functional roles of fast network oscillations. Further, it outlines how fast network oscillations in distinct brain regions synergistically support memory consolidation and retrieval by hosting cross-regional coactivation of memory-related neuronal ensembles.

Hippocampal representations that underlie spatial memory undergo continuous refinement following formation during exploration. Understanding the role of sleep in this process has been challenging because of the inaccessibility of place fields when animals are not actively exploring a maze. Here, we used a novel Bayesian learning approach based on the spike-triggered average decoded position in ensemble recordings to track dynamically the spatial tuning of individual neurons during offline states in freely moving rats. Measuring these dynamic tunings, we found spatial representations within hippocampal sharp-wave ripples that were stable for hours during sleep and were strongly aligned with place fields initially observed during maze exploration. These representations were explained by a combination of factors that included the pre-configured structure of firing rates in sleep before exposure to the environment, and representations that emerged during theta oscillations and awake sharp-wave ripples on the maze, revealing the contribution of these events in forming ensembles during sleep. Strikingly, the ripple representations during sleep predicted the future place fields of neurons during re-exposure to the maze, even when those fields deviated from previous place preferences. These observations demonstrate that ripples during sleep drives representational drift observed across maze exposures. In contrast, we observed tunings with poor alignment to maze place fields during other time periods, including in sleep and rest before maze exposure, during rapid eye movement sleep, and following the initial several hours in slow-wave sleep. In sum, the novel decoding approach described here allowed us to infer and characterize the retuning of place fields during offline periods, revealing the rapid emergence of representations following novel exploration and the active role of sleep in the representational dynamics of the hippocampus.

Extracellular recordings in freely moving animals allow the monitoring of brain activity from populations of neurons at single-spike temporal resolution. While state-of-the-art electrophysiological recording devices have been developed in recent years (e.g., µLED and Neuropixels silicon probes), implantation methods for silicon probes in rats and mice have not advanced substantially for a decade. The surgery is complex, takes time to master, and involves handling expensive devices and valuable animal subjects. In addition, chronic silicon neural probes are practically single implant devices due to the current low success rate of probe recovery. To successfully recover silicon probes, improve upon the quality of electrophysiological recording, and make silicon probe recordings more accessible, we have designed a miniature, low cost, and recoverable microdrive system. The addition of a novel 3D-printed skull baseplate makes the surgery less invasive, faster, and simpler for both rats and mice. We provide detailed procedural instructions and print designs, allowing researchers to adapt and flexibly customize our designs to their experimental usage.

New memories are believed to be consolidated over several hours of post-task sleep. The reactivation or “replay” of hippocampal cell assemblies has been proposed to provide a key mechanism for this process. However, previous studies have indicated that such replay is restricted to the first 10–30 min of post-task sleep, suggesting that it has a limited role in memory consolidation. We performed long-duration recordings in sleeping and behaving male rats and applied methods for evaluating the reactivation of neurons in pairs as well as in larger ensembles while controlling for the continued activation of ensembles already present during pre-task sleep (“preplay”). We found that cell assemblies reactivate for up to 10 h, with a half-maximum timescale of ∼6 h, in sleep following novel experience, even when corrected for preplay. We further confirmed similarly prolonged reactivation in post-task sleep of rats in other datasets that used behavior in novel environments. In contrast, we saw limited reactivation in sleep following behavior in familiar environments. Overall, our findings reconcile the duration of replay with the timescale attributed to cellular memory consolidation and provide strong support for an integral role of replay in memory.SIGNIFICANCE STATEMENTNeurons that are active during an experience reactivate again afterward during rest and sleep. This replay of ensembles of neurons has been proposed to help strengthen memories, but it has also been reported that replay occurs only in the first 10–30 min of sleep, suggesting a circumscribed role. We performed long-duration recordings in the hippocampus of rats and found that replay persists for several hours in sleep following novel experience, far beyond the limits found in previous reports based on shorter recordings. These findings reconcile the duration of replay with the hours-long timescales attributed to memory consolidation.

According to a two-stage memory consolidation model, during waking theta states, afferent activity from the neocortex to the hippocampus induces transient synaptic modification in the hippocampus, where the information is deposited as a labile form of memory trace. During subsequent sharp-wave ripples (SPW-Rs), the newly acquired hippocampal information is transferred to the neocortex and stored as a long-lasting memory trace. Consistent with this hypothesis, waking theta states and SPW-Rs distinctly control information flow in the hippocampal-entorhinal loop. Although both waking theta states and rapid eye movement (REM) sleep are characterized by prominent hippocampal theta oscillations, the two brain states involve distinct temporal coordination and oscillatory coupling in the hippocampal-entorhinal circuit. While distinct brain states have distinct network dynamics, firing rates of individual neurons in the hippocampal-entorhinal circuitry follow lognormal-like distributions in all states. Firing rates of the same neurons are positively correlated across brain states and testing environments, suggesting that memory is allocated in preconfigured, rather than tabula rasa-type, skewed neuronal networks. The fast-firing minority and slow-firing majority neurons, which can support network stability and flexibility, are under distinct homeostatic regulations that are initiated by spindles and SPW-Rs during slow wave sleep and implemented during subsequent REM sleep.

The synchronous activity of populations of neurons in patterns known as sharp-wave ripples (SPW-Rs), which contain fragments of time-compressed neuronal sequences that are replayed from those experienced during waking hours, is thought to mediate the transfer of newly acquired hippocampal information to distributed circuits in support of memory consolidation. Consistent with this hypothesis, it has been shown that the perturbation of neuronal activity during SPW-Rs can lead to memory impairment. Waking theta states and rapid eye movement (REM) sleep involve different kinds of temporal coordination in the hippocampal-entorhinal circuit, reflecting the distinct balance of hippocampal and entorhinal input to the CA1 area. The firing rates of individual neurons follow lognormal-like distributions in all brain states. Fast-firing minority and slow-firing majority neurons, which support system stability and mnemonic functions, are under distinct firing-rate regulation that is initiated by spindles and SPW-Rs during non-REM sleep and implemented during REM sleep.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The medial prefrontal cortex (mPFC) plays an important role in memory. By maintaining a working memory buffer, neurons in prelimbic (PL) mPFC may selectively contribute to learning associations between stimuli that are separated in time, as in trace fear conditioning (TFC). Until now, evidence for this bridging role was largely descriptive. Here we used optogenetics to silence neurons in the PL mPFC of rats during learning in TFC. Memory formation was prevented when mPFC was silenced specifically during the interval separating the cue and shock. Our results provide support for a working memory function for these cells and indicate that associating two noncontiguous stimuli requires bridging activity in PL mPFC.

Neurons fire at highly variable innate rates and recent evidence suggests that low and high firing rate neurons display different plasticity and dynamics. Furthermore, recent publications imply possibly differing rate-dependent effects in hippocampus versus neocortex, but those analyses were carried out separately and with possibly important differences. To more effectively synthesize these questions, we analyzed the firing rate dynamics of populations of neurons in both hippocampal CA1 and frontal cortex under one framework that avoids pitfalls of previous analyses and accounts for regression-to-the-mean. We observed remarkably consistent effects across these regions. While rapid eye movement (REM) sleep was marked by decreased hippocampal firing and increased neocortical firing, in both regions firing rates distributions widened during REM due to differential changes in high-firing versus low-firing cells in parallel with increased interneuron activity. In contrast, upon non-REM (NREM) sleep, firing rate distributions narrowed while interneuron firing decreased. Interestingly, hippocampal interneuron activity closely followed the patterns observed in neocortical principal cells rather than the hippocampal principal cells, suggestive of long-range interactions. Following these undulations in variance, the net effect of sleep was a decrease in firing rates. These decreases were greater in lower-firing hippocampal neurons but higher-firing frontal cortical neurons, suggestive of greater plasticity in these cell groups. Our results across two different regions and with statistical corrections indicate that the hippocampus and neocortex show a mixture of differences and similarities as they cycle between sleep states with a unifying characteristic of homogenization of firing during NREM and diversification during REM.Significance StatementMiyawaki and colleagues analyze firing patterns across low-firing and high-firing neurons in the hippocampus and the frontal cortex throughout sleep in a framework that accounts for regression-to-the-mean. They find that in both regions REM sleep activity is relatively dominated by high-firing neurons and increased inhibition, resulting in a wider distribution of firing rates. On the other hand, NREM sleep produces lower inhibition, and results in a more homogenous distribution of firing rates. Integration of these changes across sleep results in net decrease of firing rates with largest drops in low-firing hippocampal pyramidal neurons and high-firing neocortical principal neurons. These findings provide insights into the effects and functions of different sleep stages on cortical neurons.

Study Objectives: To better understand the distinct activity patterns of the brain during sleep, we observed and investigated periods of diminished oscillatory and population spiking activity lasting for seconds during non-rapid eye movement (non-REM) sleep, which we call “LOW” activity sleep. Methods: We analyzed spiking and local field potential (LFP) activity of hippocampal CA1 region alongside neocortical electroencephalogram (EEG) and electromyogram (EMG) in 19 sessions from four male Long-Evans rats (260–360 g) during natural wake/sleep across the 24-hr cycle as well as data from other brain regions obtained from http://crcns.org. Results: LOW states lasted longer than OFF/DOWN states and were distinguished by a subset of “LOW-active” cells. LOW activity sleep was preceded and followed by increased sharp-wave ripple activity. We also observed decreased slow-wave activity and sleep spindles in the hippocampal LFP and neocortical EEG upon LOW onset, with a partial rebound immediately after LOW. LOW states demonstrated activity patterns consistent with sleep but frequently transitioned into microarousals and showed EMG and LFP differences from small-amplitude irregular activity during quiet waking. Their likelihood decreased within individual non-REM epochs yet increased over the course of sleep. By analyzing data from the entorhinal cortex of rats,1 as well as the hippocampus, the medial prefrontal cortex, the postsubiculum, and the anterior thalamus of mice,2 obtained from http://crcns.org, we confirmed that LOW states corresponded to markedly diminished activity simultaneously in all of these regions. Conclusions: We propose that LOW states are an important microstate within non-REM sleep that provide respite from high-activity sleep and may serve a restorative function.

It has been hypothesized that waking leads to higher-firing neurons, with increased energy expenditure, and that sleep serves to return activity to baseline levels. Oscillatory activity patterns during different stages of sleep may play specific roles in this process, but consensus has been missing. To evaluate these phenomena in the hippocampus, we recorded from region CA1 neurons in rats across the 24-hr cycle, and we found that their firing increased upon waking and decreased 11% per hour across sleep. Waking and sleeping also affected lower- and higher-firing neurons differently. Interestingly, the incidences of sleep spindles and sharp-wave ripples (SWRs), typically associated with cortical plasticity, were predictive of ensuing firing changes and were more robustly predictive than other oscillatory events. Spindles and SWRs were initiated during non-REM sleep, yet the changes were incorporated in the network over the following REM sleep epoch. These findings indicate an important role for spindles and SWRs and provide novel evidence of a symbiotic relationship between non-REM and REM stages of sleep in the homeostatic regulation of neuronal activity.

Synapses in the mammalian central nervous system show substantial diversity in their physiological and morphological properties. However, the correlations among them have remained elusive. Here, we tried to clarify the correlations by establishing a method to record excitatory postsynaptic currents (EPSCs) at individual synapses and also to observe the morphology at the same time. A pair of pre- and postsynaptic neurons were labeled with different fluorescent dyes, and a presynaptic varicosity was selectively stimulated with a θ-tube glass electrode under conditions in which action potential generation was suppressed. Two representative types of excitatory glutamatergic synapses, one on hippocampal pyramidal neurons and the other on cerebellar Purkinje neurons, were studied. The correlations between the properties of quantal EPSCs (qEPSCs) and those of synaptic morphology were analyzed in rat primary culture preparations. The amplitude and the decay time of qEPSC were correlated with the size of the postsynaptic spine only at hippocampal synapses. In contrast, the size of the presynaptic varicosity was correlated with the size of the postsynaptic spine and the quantal content of evoked EPSCs only at granule neuron–Purkinje neuron synapses in the cerebellum. These results suggest that the interaction between pre- and postsynaptic structures and the coupling of postsynaptic responsiveness and the spine morphology differ between cerebellar and hippocampal glutamatergic synapses. Synapse, 2010. © 2010 Wiley-Liss, Inc.

We have purified to apparent homogeneity and characterized a molecular chaperonin GroEL homologue (hpGroEL) from a moderately halophilic eubacterium, Pseudomonas sp. #43. Although this halophilic bacterium requires 1-2 M NaCl for growth, hpGroEL did not require a high concentration of salt for its stability, ATPase activity and refold-promoting activity for denatured protein. The ATPase activity was even more halo-sensitive than that of GroEL from Escherichia coli. The hpGroEL protein promotes Mg(2+)-ATP-dependent refolding of urea-denatured alpha-glucosidase in the presence of E. coli-GroES, indicating that chaperonins 60 and 10 isolated from halophilic and nonhalophilic eubacteria, respectively, can cooperate with each other.

Purkinje cells are the sole output neurons of the cerebellar cortex and their dysfunction causes severe ataxia. We found that Purkinje cells could be robustly generated from mouse embryonic stem (ES) cells by recapitulating the self-inductive signaling microenvironments of the isthmic organizer. The cell-surface marker Neph3 enabled us to carry out timed prospective selection of Purkinje cell progenitors, which generated morphologically characteristic neurons with highly arborized dendrites that expressed mature Purkinje cell-specific markers such as the glutamate receptor subunit GluRδ2. Similar to mature Purkinje cells, these neurons also showed characteristic spontaneous and repeated action potentials and their postsynaptic excitatory potentials were generated exclusively through nonNMDA glutamate receptors. Fetal transplantation of precursors isolated by fluorescence-activated cell sorting showed orthotopic integration of the grafted neurons into the Purkinje cell layer with their axons extending to the deep cerebellar nuclei and dendrites receiving climbing and parallel fibers. This selective preparation of bona fide Purkinje cells should aid future investigation of this important neuron.

Mongolian gerbil, Meriones unguiculatus, communicate with others by various sounds. About 80% of those sounds range over 20 kHz. A threshold of hearing in this range, however, was about 20 dB higher than their most sensitive frequency (1 to 16 kHz). We proposed a hypothesis that the auditory sensitivity heightened when gerbils communicated with others. In order to test the idea, a cochlear microphonics (CM) was recorded under various behavioral context as a measure of auditory sensitivity. Under paired condition, a subject was set with another gerbil. The condition simulated the subject gerbils in their group, and the subject was considered to pay attention to sounds of the company. The CM response under the situation was higher than under single situation. Then, we investigated if the CM increase was specific under the situation. Subject was trained to pay attention to a sound of conditioned stimulus by electric shock as a negative reinforcer. The sound worked as an alarm sound for gerbils. The CM response increased while the gerbil was paying attention to the sound. Those results suggest that the sensitivity of auditory periphery is raised by attention in various behavioral contexts.

1. 1. The present study deals with the metabolism of β-sitosterol in the mussel, Mytilus edulis , and the snail, Omphalius pfeifferi , after injection of radioactive β-sitosterol. 2. 2. In both the molluscs, β-sitosterol was undergone esterification and converted to some Δ 5,7 -sterols. 3. 3. The mussel also converted β-sitosterol to 5α-stigmastan-3β-ol but did not dealkylate β-sitosterol to cholesterol. 4. 4. The snail was able to dealkylate β-sitosterol to cholesterol but incapable of converting β-sitosterol to Δ 0 - and Δ 7 -sterols. 5. 5. These data are discussed in relation to the sterol components of both molluscs.

When serotonin was intravenously injected into adreno-demedullated, glucoseu-14C administered rats, glycogen specific activity as well as content was increased in heart, and lowered in liver, 10 mins. after the injection. By shortening of the time interval of glucose-u-14C administration and serotonin injection, increased glycogen specific activity was observed not only in heart but in diaphragm, and the serotonin effect was also detected in gastrocnemius muscle 30 mins. after the injection.‘Central’ blood glucose level was elevated in the serotonin treated rats, but the increment was not large enough to explain the increased incorporation of glucose-14C into muscle glycogen by serotonin. When glucose-1-14C was given to the control and the serotonin treated rats, all of radioactivity in cardiac and diaphragmatic glycogen was distributed in position-1 of the glucose. These data indicate that serotonin stimulates the incorporation of glucose into muscle glycogen rather than the resynthesis of glycogen from smaller fragments.

1. In urine of rats dosed with N-(alpha-methylbenzyl)linoleamide (MBLA), three dicarboxylic acid monoamides, N-(alpha-methylbenzyl)succinic acid monoamide, N-(alpha-methylbenzyl)glutaric acid monoamide and N-(alpha-methylbenzyl)adipic acid monoamide, were identified. Conjugated alpha-methylbenzylamine, hippuric acid and conjugates of the dicarboxylic acid monoamides were also found in the urine. N-(alpha-Methylbenzyl)succinic acid monoamide was the main metabolite in rats. 2. Biliary excretion of radioactivity was studied in rats, cannulated for collection of bile and duodenal infusion, after oral administration of N-(alpha-methylbenzyl)[1-14C]linoleamide. With constant duodenal infusion of bile, about 7% of the dose was excreted in the bile, while excretion of radioactivity was negligible without bile infusion. 3. The g.l.c. analysis of human urine after oral administration of MBLA revealed that two dicarboxylic acid monoamides were present and N-(alpha-methylbenzyl)succinic acid monoamide was the main metabolite. 4. MBLA was excreted unchanged in the faeces of men who received MBLA to the extent of about 53% dose in 3 days. 5. MBLA was not detected (less than 1 mug/ml) in the serum of a volunteer who had been taking an oral daily dose of 1500 mg of MBLA for 3 months.

1. Two metabolites, isolated from the urine of rats dosed with bencyclane fumarate, were characterized as cis-1-benzyl-1 -(3-N,N-dimethylaminopropoxy)-4-hydroxycycloheptane (metabolite I) and 1-benzyl-1-(3-N,N-dimethylaminopropoxy)-4-oxocycloheptane (metabolite II).2. Bencyclane and the two metabolites were determined in the urine of rats and volunteers by g.l.c. Metabolite I was a major metabolite in men, being excreted in urine to the extent of 23·5% dose in the first 24 h.

It has been reported that N-(α-methylbenzyl) linoleamide (MBLA) had a remarkable cholesterol-lowering effect on the experimental hypercholesterolemia in rabbits and rats.There are three optical isomers of MBLA: D-, L- and DL-MBLA because of an asymmetric carbon atom in the amine moiety of MBLA. And the potency of the cholesterol-lowering effect was found to be the following order: D->DL->L-MBLA.Fukushima also found that the respiratory excretion of 14C after oral administration of carboxy 14C-labeled L-MBLA to rats was significantly greater than that of D- or DL-MBLA, whereas the ratio of urinary 14C excretion of D-MBLA was higher than that of L- or DL-MBLA. These results suggest that the difference of the cholesterol-lowering potency observed among the MBLA isomers might be closely related with the difference of biological disposition of the isomers.The present paper describes the results of in vitro studies on the enzymatic hydrolysis of the amide linkage of MBLA isomers.