Several studies suggest that neurons from the lateral region of the SuM (SuML) innervating the dorsal dentate gyrus (DG) display a dual GABAergic and glutamatergic transmission and are specifically activated during paradoxical (REM) sleep (PS). The objective of the present study is to characterize the anatomical, neurochemical and electrophysiological properties of the SuML-DG projection neurons and to determine how they control DG oscillations and neuronal activation during PS and other vigilance states. For this purpose, we combine structural connectivity techniques using neurotropic viral vectors (rabies virus, AAV), neurochemical anatomy (immunohistochemistry, in situ hybridization) and imaging (light, electron and confocal microscopy) with in vitro (patch clamp) and in vivo (LFP, EEG) optogenetic and electrophysiological recordings performed in transgenic VGLUT2-cre male mice. At the cellular level, we show that the SuML-DG neurons co-release GABA and glutamate on dentate granule cells and increase the activity of a subset of DG granule cells. At the network level, we show that activation of the SuML-DG pathway increases theta power and frequency during PS as well as gamma power during PS and waking in the DG. At the behavioral level, we show that the activation of this pathway does not change animal behavior during PS, induces awakening during slow wave sleep and increases motor activity during waking. These results suggest that the SuML-DG pathway is capable of supporting the increase of theta and gamma power in the DG observed during PS and plays an important modulatory role of DG network activity during this state.

Several studies suggest that neurons from the lateral region of the SuM (SuML) innervating the dorsal dentate gyrus (DG) display a dual GABAergic and glutamatergic transmission and are specifically activated during paradoxical (REM) sleep (PS). The objective of the present study is to fully characterize the anatomical, neurochemical and electrophysiological properties of the SuML-DG projection neurons and to determine how they control DG oscillations and neuronal activation during PS and other vigilance states. For this purpose, we combine structural connectivity techniques using neurotropic viral vectors (rabies virus, AAV), neurochemical anatomy (immunohistochemistry, in situ hybridization) and imaging (light, electron and confocal microscopy) with in vitro (patch clamp) and in vivo (LFP, EEG) optogenetic and electrophysiological recordings performed in transgenic VGLUT2-cre male mice. At the cellular level, we show that the SuML-DG neurons co-release GABA and glutamate on dentate granule cells and increase the activity of a subset of DG granule cells. At the network level, we show that activation of the SuML-DG pathway increases theta power and frequency during PS as well as gamma power during PS and waking in the DG. At the behavioral level, we show that the activation of this pathway does not change animal behavior during PS, induces awakening during slow wave sleep and increases motor activity during waking. These results suggest that the SuML-DG pathway is capable of supporting the increase of theta and gamma power in the DG observed during PS and plays an important modulatory role of DG network activity during this state.Significant statementAn increase of theta and gamma power in the dentate gyrus (DG) is an hallmark of paradoxical (REM) sleep (PS) and is suggested to promote learning and memory consolidation by synchronizing hippocampal networks and increasing its outputs to cortical targets. However the neuronal networks involved in such control of DG activity during PS are poorly understood. The present study identifies a population of GABA/Glutamate neurons in the lateral supramammllary nucleus (SuML) innervating the DG that could support such control during PS. Indeed, we show that activation of these SuML-DG projections increase theta power and frequency as well as gamma power in the DG specifically during PS and modulate activity of a subset of DG granule cells.

Protein synthesis is involved in the consolidation of short-term memory into long-term memory. Previous electrophysiological data concerning LTP in CA3 suggest that protein synthesis in that region might also be necessary for short-term memory. We tested this hypothesis by locally injecting the protein synthesis inhibitor anisomycin in hippocampal area CA1 or CA3 immediately after contextual fear conditioning. As previously shown, injections in CA1 impaired long-term memory but spared short-term memory. Conversely, injections in CA3 impaired both long-term and short-term memories. We conclude that early steps of experience-induced plasticity occurring in CA3 and underlying short-term memory require protein synthesis.Memory formation is conventionally described by the succession of two steps. First comes a short-term memory that lasts a few hours and is independent of de novo protein synthesis; then comes a long-term memory, requiring the synthesis of new proteins (Davis and Squire 1984). Long-term potentiation (LTP) is a key mechanism for memory formation at the synaptic level (Bliss et al. 2006). Strikingly, LTP also displays a short-lived phase (termed early LTP) independent of protein synthesis, and a late LTP requiring de novo protein synthesis that lasts for months (Frey et al. 1993). Short-term memory is thus thought to rely on early LTP, while long-term memory formation would stand on late LTP (Kandel 2001). Importantly, the distinction between early and late LTP on the basis of the requirement for protein synthesis mainly relies on observations of LTP occurring at the Schaffer collateral synapses, corresponding to the CA3 pyramidal terminals onto area CA1 of the hippocampus (Malenka and Bear 2004). However, the mechanisms responsible for LTP can vary depending on the type of synapse considered. The properties of LTP occurring at the mossy fiber synapses, which are axon terminals given off by dentate gyrus granule cells and projecting to CA3 pyramidal neurons, are very different from the canonical LTP occurring at the Schaffer collaterals (Malenka and Bear 2004). In particular, some studies claim that LTP at mossy fiber synapses requires de novo protein synthesis from its initiation (BareaRodriguez et al. 2000;Hagena and Manahan-Vaughan 2013; but see Huang et al. 1994). Thus, protein synthesis seems to be required for LTP at the mossy fiber synapses, at least during its first hour. Given the putative aforementioned causal relationship between early LTP establishment and short-term memory formation, these observations indicate that protein synthesis should be required for the formation of any short-term memory relying on the plasticity of the mossy fibers. Thus, the inhibition of protein synthesis in CA3 should impair short-term memory.Contextual fear conditioning is a very common learning paradigm in which rodents have to quickly build a conjunctive representation of a context and to associate this context with an electric footshock. In this kind of declarative memory, the representation of the context i...

Study Objectives: It is commonly accepted that sleep is beneficial to memory processes, but it is still unclear if this benefit originates from improved memory consolidation or enhanced information processing. It has thus been proposed that sleep may also promote forgetting of undesirable and non-essential memories, a process required for optimization of cognitive resources. We tested the hypothesis that non-rapid eye movement sleep (NREMS) promotes forgetting of irrelevant information, more specifically when processing information in working memory (WM), while REM sleep (REMS) facilitates the consolidation of important information. Methods: We recorded sleep patterns of rats trained in a radial maze in three different tasks engaging either the long-term or short-term storage of information, as well as a gradual level of interference. Results: We observed a transient increase in REMS amount on the day the animal learned the rule of a long-term/reference memory task (RM), and, in contrast, a positive correlation between the performance of rats trained in a WM task involving an important processing of interference and the amount of NREMS or slow wave activity. Various oscillatory events were also differentially modulated by the type of training involved. Notably, NREMS spindles and REMS rapid theta increase with RM training, while sharp-wave ripples increase with all types of training. Conclusions: These results suggest that REMS, but also rapid oscillations occurring during NREMS would be specifically implicated in the long-term memory in RM, whereas NREMS and slow oscillations could be involved in the forgetting of irrelevant information required for WM.

Synaptic changes play a major role in memory processes. Modulation of synaptic responses by brain states remains however poorly understood in hippocampal networks, even in basal conditions. We recorded evoked synaptic responses at five hippocampal pathways in freely moving male rats. We showed that, at the perforant path to dentate gyrus (PP-DG) synapse, responses increase during wakefulness compared to sleep. At the Schaffer collaterals to CA1 (SC-CA1) synapse, responses increase during non-REM sleep (NREM) compared to the other states. During REM sleep (REM), responses decreased at the PP-DG and SC-CA1 synapses compared to NREM, while they increased at the fornix to nucleus accumbens synapse (Fx-NAc) during REM compared to the other states. In contrast, responses at the fornix to medial prefrontal cortex synapse (Fx-PFC) and at the fornix to amygdala synapse (Fx-Amy) were weakly modulated by vigilance states. Extended sleep periods led to synaptic changes at PP-DG and Fx-Amy synapses but not at the other synapses. Synaptic responses were also linked to local oscillations and were highly correlated between Fx-PFC and Fx-NAc but not between Fx-Amy and these synapses. These results reveal synapse specific modulations that may contribute to memory consolidation during the sleep-wake cycle.SIGNIFICANCE STATEMENT:Surprisingly, the cortical network dynamics remains poorly known at the synaptic level. We tested the hypothesis that brain states would modulate synaptic changes in the same way at different cortical connections. To tackle this issue, we implemented an approach to explore the synaptic behavior of five connections upstream and downstream the rat hippocampus. Our study reveals that synaptic responses are modulated in a highly synapse-specific manner by wakefulness and sleep states as well as by local oscillations at these connections. Moreover, we found rapid synaptic changes during wake and sleep transitions as well as synaptic down and upregulations after extended periods of sleep. These synaptic changes are likely related to the mechanisms of sleep dependent memory consolidation.

Local field potential (LFP) recording is a very useful electrophysiological method to study brain processes. However, this method is criticized for recording low frequency activity in a large area of extracellular space potentially contaminated by distal activity. Here, we theoretically and experimentally compare ground-referenced (RR) with differential recordings (DR). We analyze electrical activity in the rat cortex with these two methods. Compared with RR, DR reveals the importance of local phasic oscillatory activities and their coherence between cortical areas. Finally, we show that DR provides a more faithful assessment of functional connectivity caused by an increase in the signal to noise ratio, and of the delay in the propagation of information between two cortical structures. Introduction 1 LFP recording of cortical structures constitutes a powerful tool to detect functional 2 signatures of cognitive processes. However, several studies have suggested that 3 recording methods suffer of major caveats due to the recording of activity in distant 4 neural populations [1-4]. Thus, theta oscillations (6-10Hz) during active wake seem to 5 propagate from the hippocampus to the frontal cortical areas [5]. Despite these 6 important studies, LFP recording has revealed important features of cortical 7 organizations [6, 7]. For example, cortical slow wave oscillations of NREM sleep, which 8 constitute a prominent feature of this vigilance state, contribute moderately to 9 coherence between cortical areas [7]. In contrast, weak slow wave oscillations during 10 active wake contribute to a relatively high level of coherence between cortical 11 areas [6, 7]. LFPs are mainly generated by post-synaptic response to pre-synaptic 12 activity of neurons [8-11] and constitutes a natural integrator of action potentials 13coming from a given cortical region [12][13][14]. In its usual description, LFP recording 14 appears to be less local than multi-unit activity recordings. Indeed, the usual 15 PLOS 1/15 recording mode of LFP consists in implanting a single electrode in the investigated 16 cortical region and a second one in a supposed neutral site. This simple recording 17 configuration, called monopolar or referential recording (RR) mode, is well adapted to 18 evaluate a global brain state. Unlike single and multi-unit probe, the impedance of the 19 standard electrode used for LFP recording is usually low in order to record neural 20 activity of a larger area. However, this method may detect activities from distant 21 cortical areas located between the recording and the reference electrode [1, 13-19], a 22phenomenon called volume conduction. We propose here to compare monopolar or RR 23 mode to bipolar or differential recording (DR), which consists in setting a pair of 24 electrodes in the same cortical area and measuring the voltage difference between 25 them. The main historical reasons why RR is widely used [7,20] are: 1) its simplicity 26 because of the low number of wires that needs to be implanted (contributing to the 27 ...

Local field potential (LFP) recording is a very useful electrophysiological method to study brain processes. However, this method is criticized for recording low frequency activity in a large area of extracellular space potentially contaminated by distal activity. Here, we theoretically and experimentally compare ground-referenced (RR) with differential recordings (DR). We analyze electrical activity in the rat cortex with these two methods. Compared with RR, DR reveals the importance of local phasic oscillatory activities and their coherence between cortical areas. Finally, we show that DR provides a more faithful assessment of functional connectivity caused by an increase in the signal to noise ratio, and of the delay in the propagation of information between two cortical structures.

Notice of Republication This article was republished on January 9, 2020, to correct errors in the author byline and citation affecting all author names. Please download this article again to view the correct version. The originally published, uncorrected article and the republished, corrected articles are provided here for reference.