O Eschenko

The influence of learning on sleep slow oscillations and associated spindles and ripples in humans and rats

The mechanisms underlying off‐line consolidation of memory during sleep are elusive. Learning of hippocampus‐dependent tasks increases neocortical slow oscillation synchrony, and thalamocortical spindle and hippocampal ripple activity during subsequent non‐rapid eye movement sleep. Slow oscillations representing an oscillation between global neocortical states of increased (up‐state) and decreased (down‐state) neuronal firing temporally group thalamic spindle and hippocampal ripple activity, which both occur preferentially during slow oscillation up‐states. Here we examined whether slow oscillations also group learning‐induced increases in spindle and ripple activity, thereby providing time‐frames of facilitated hippocampus‐to‐neocortical information transfer underlying the conversion of temporary into long‐term memories. Learning (word‐pairs in humans, odor–reward associations in rats) increased slow oscillation up‐states and, in humans, shaped the timing of down‐states. Slow oscillations grouped spindle and rat ripple activity into up‐states under basal conditions. Prior learning produced in humans an increase in spindle activity focused on slow oscillation up‐states. In rats, learning induced a distinct increase in spindle and ripple activity that was not synchronized to up‐states. Event‐correlation histograms indicated an increase in spindle activity with the occurrence of ripples. This increase was prolonged after learning, suggesting a direct temporal tuning between ripples and spindles. The lack of a grouping effect of slow oscillations on learning‐induced spindles and ripples in rats, together with the less pronounced effects of learning on slow oscillations, presumably reflects a weaker dependence of odor learning on thalamo‐neocortical circuitry. Slow oscillations might provide an effective temporal frame for hippocampus‐to‐neocortical information transfer only when thalamo‐neocortical systems are already critically involved during learning.

Hippocampal–cortical interaction during periods of subcortical silence

Hippocampal ripples, episodic high-frequency field-potential oscillations primarily occurring during sleep and calmness, have been described in mice, rats, rabbits, monkeys and humans, and so far they have been associated with retention of previously acquired awake experience. Although hippocampal ripples have been studied in detail using neurophysiological methods, the global effects of ripples on the entire brain remain elusive, primarily owing to a lack of methodologies permitting concurrent hippocampal recordings and whole-brain activity mapping. By combining electrophysiological recordings in hippocampus with ripple-triggered functional magnetic resonance imaging, here we show that most of the cerebral cortex is selectively activated during the ripples, whereas most diencephalic, midbrain and brainstem regions are strongly and consistently inhibited. Analysis of regional temporal response patterns indicates that thalamic activity suppression precedes the hippocampal population burst, which itself is temporally bounded by massive activations of association and primary cortical areas. These findings suggest that during off-line memory consolidation, synergistic thalamocortical activity may be orchestrating a privileged interaction state between hippocampus and cortex by silencing the output of subcortical centres involved in sensory processing or potentially mediating procedural learning. Such a mechanism would cause minimal interference, enabling consolidation of hippocampus-dependent memory.

Sustained increase in hippocampal sharp-wave ripple activity during slow-wave sleep after learning

High-frequency oscillations, known as sharp-wave/ripple (SPW-R) complexes occurring in hippocampus during slow-wave sleep (SWS), have been proposed to promote synaptic plasticity necessary for memory consolidation. We recorded sleep for 3 h after rats were trained on an odor-reward association task. Learning resulted in an increased number SPW-Rs during the first hour of post-learning SWS. The magnitude of ripple events and their duration were also elevated for up to 2 h after the newly formed memory. Rats that did not learn the discrimination during the training session did not show any change in SPW-Rs. Successful retrieval from remote memory was likewise accompanied by an increase in SPW-R density and magnitude, relative to the previously recorded baseline, but the effects were much shorter lasting and did not include increases in ripple duration and amplitude. A short-lasting increase of ripple activity was also observed when rats were rewarded for performing a motor component of the task only. There were no increases in ripple activity after habituation to the experimental environment. These experiments show that the characteristics of hippocampal high-frequency oscillations during SWS are affected by prior behavioral experience. Associative learning induces robust and sustained (up to 2 h) changes in several SPW-R characteristics, while after retrieval from remote memory or performance of a well-trained procedural aspect of the task, only transient changes in ripple density were induced.

Elevated Sleep Spindle Density after Learning or after Retrieval in Rats

Non-rapid eye movement sleep has been strongly implicated in consolidation of both declarative and procedural memory in humans. Elevated sleep-spindle density in slow-wave sleep after learning has been shown recently in humans. It has been proposed that sleep spindles, 12–15 Hz oscillations superimposed on slow waves (<1 Hz), in concert with high-frequency hippocampal sharp waves/ripples, promote neural plasticity underlying remote memory formation. The present study reports the first indication of learning-associated increase in spindle density in the rat, providing an animal model to study the role of brain oscillations in memory consolidation during sleep. An odor–reward association task, analogous in many respects to human paired-associate learning, is rapidly learned and leads to robust memory in rats. Rats learned the task over 10 massed trials within a single session, and EEG was monitored for 3 h after learning. Learning-induced increase in spindle density is reliably reproduced in rats in two different learning situations, differing primarily in the behavioral component of the task. This increase in spindle density is also present after reactivation of remote memory and in situations when memory update is required; it is not observed after noncontingent exposure to reward and training context. The latter results substantially extend findings in humans. The magnitude of increase (∼25%) and the time window of maximal effect (∼1 h after sleep onset) were remarkably similar to human data, making this a valid rodent model to study network interactions through the use of simultaneous unit recordings and local field potentials during postlearning sleep.

Hippocampal Sharp Wave/Ripples during Sleep for Consolidation of Associative Memory

The beneficial effect of sleep on memory has been well-established by extensive research on humans, but the neurophysiological mechanisms remain a matter of speculation. This study addresses the hypothesis that the fast oscillations known as ripples recorded in the CA1 region of the hippocampus during slow wave sleep (SWS) may provide a physiological substrate for long term memory consolidation. We trained rats in a spatial discrimination task to retrieve palatable reward in three fixed locations. Hippocampal local field potentials and cortical EEG were recorded for 2 h after each daily training session. There was an increase in ripple density during SWS after early training sessions, in both trained rats and in rats randomly rewarded for exploring the maze. In rats learning the place -reward association, there was a striking further significant increase in ripple density correlated with subsequent improvements in behavioral performance as the rat learned the spatial discrimination aspect of the task. The results corroborate others showing an experience-dependent increase in ripple activity and associated ensemble replay after exploratory activity, but in addition, for the first time, reveal a clear further increase in ripple activity related to associative learning based on spatial discrimination.

Noradrenergic Neurons of the Locus Coeruleus Are Phase Locked to Cortical Up-Down States during Sleep

Nonrapid eye movement (NREM) sleep is characterized by periodic changes in cortical excitability that are reflected in the electroencephalography (EEG) as high-amplitude slow oscillations, indicative of cortical Up/Down states. These slow oscillations are thought to be involved in NREM sleep-dependent memory consolidation. Although the locus coeruleus (LC) noradrenergic system is known to play a role in off-line memory consolidation (that may occur during NREM sleep), cortico-coerulear interactions during NREM sleep have not yet been studied in detail. Here, we investigated the timing of LC spikes as a function of sleep-associated slow oscillations. Cortical EEG was monitored, along with activity of LC neurons recorded extracellularly, in nonanesthetized naturally sleeping rats. LC spike-triggered averaging of EEG, together with phase-locking analysis, revealed preferential firing of LC neurons along the ascending edge of the EEG slow oscillation, correlating with Down-to-Up state transition. LC neurons were locked best when spikes were shifted forward ∼50 ms in time with respect to the EEG slow oscillation. These results suggest that during NREM sleep, firing of LC neurons may contribute to the rising phase of the EEG slow wave by providing a neuromodulatory input that increases cortical excitability, thereby promoting plasticity within these circuits.

Mapping of functional brain activity in freely behaving rats during voluntary running using manganese-enhanced MRI: Implication for longitudinal studies

Magnetic resonance imaging (MRI) is widely used in basic and clinical research to map the structural and functional organization of the brain. An important need of MR research is for contrast agents that improve soft-tissue contrast, enable visualization of neuronal tracks, and enhance the capacity of MRI to provide functional information at different temporal scales. Unchelated manganese can be such an agent, and manganese-enhanced MRI (MEMRI) can potentially be an excellent technique for localization of brain activity (for review see Silva et al., 2004). Yet, the toxicity of manganese presents a major limitation for employing MEMRI in behavioral paradigms. We have tested systematically the voluntary wheel running behavior of rats after systemic application of MnCl(2) in a dose range of 16-80 mg/kg, which is commonly used in MEMRI studies. The results show a robust dose-dependent decrease in motor performance, which was accompanied by weight loss and decrease in food intake. The adverse effects lasted for up to 7 post-injection days. The lowest dose of MnCl(2) (16 mg/kg) produced minimal adverse effects, but was not sufficient for functional mapping. We have therefore evaluated an alternative method of manganese delivery via osmotic pumps, which provide a continuous and slow release of manganese. In contrast to a single systemic injection, the pump method did not produce any adverse locomotor effects, while achieving a cumulative concentration of manganese (80 mg/kg) sufficient for functional mapping. Thus, MEMRI with such an optimized manganese delivery that avoids toxic effects can be safely applied for longitudinal studies in behaving animals.

Learning-Dependent, Transient Increase of Activity in Noradrenergic Neurons of Locus Coeruleus during Slow Wave Sleep in the Rat: Brain Stem–Cortex Interplay for Memory Consolidation?

Memory consolidation during sleep is regaining attention due to a wave of recent reports of memory improvements after sleep or deficits after sleep disturbance. Neuromodulators have been proposed as possible players in this putative off-line memory processing, without much experimental evidence. We recorded neuronal activity in the rat noradrenergic nucleus locus coeruleus (LC) using chronically implanted movable microelectrodes while monitoring the behavioral state via electrocorticogram and online video recording. Extracellular recordings of physiologically identified noradrenergic neurons of LC were made in freely behaving rats for 3 h before and after olfactory discrimination learning. On subsequent days, if LC recording remained stable, additional learning sessions were made within the olfactory discrimination protocol, including extinction, reversals, learning new odors. Contrary to the long-standing dogma about the quiescence of noradrenergic neurons of LC, we found a transient increase in LC activity in trained rats during slow wave sleep (SWS) 2 h after learning. The discovery of learning-dependent engagement of LC neurons during SWS encourages exploration of brain stem-cortical interaction during this delayed phase of memory consolidation and should bring new insights into mechanisms underlying memory formation.

Memory Influences on Hippocampal and Striatal Neural Codes: Effects of a Shift Between Task Rules

Interactions with neocortical memory systems may facilitate flexible information processing by hippocampus. We sought direct evidence for such memory influences by recording hippocampal neural responses to a change in cognitive strategy. Well-trained rats switched (within a single recording session) between the use of place and response strategies to solve a plus maze task. Maze and extramaze environments were constant throughout testing. Place fields demonstrated (in-field) firing rate and location-based reorganization [Leutgeb, S., Leutgeb, J. K., Barnes, C. A., Moser, E. I., McNaughton, B. L., & Moser, M. B. (2005). Independent codes for spatial and episodic memory in hippocampal neuronal ensembles. Science, 309, 619-623] after a task switch, suggesting that hippocampus encoded each phase of testing as a different context, or episode. The task switch also resulted in qualitative and quantitative changes to discharge that were correlated with an animals velocity or acceleration of movement. Thus, the effects of a strategy switch extended beyond the spatial domain, and the movement correlates were not passive reflections of the current behavioral state. To determine whether hippocampal neural responses were unique, striatal place and movement-correlated neurons were simultaneously recorded with hippocampal neurons. Striatal place and movement cells exhibited a response profile that was similar, but not identical, to that observed for hippocampus after a strategy switch. Thus, retrieval of a different memory led both neural systems to represent a different context. However, hippocampus may play a special (though not exclusive) role in flexible spatial processing since correlated firing amongst cell pairs was highest when rats successfully switched between two spatial tasks. Correlated firing by striatal cell pairs increased following any strategy switch, supporting the view that striatum codes change in reinforcement contingencies.

Behavioral, electrophysiological and histopathological consequences of systemic manganese administration in MEMRI.

Manganese (Mn(2+))-enhanced magnetic resonance imaging (MEMRI) offers the possibility to generate longitudinal maps of brain activity in unrestrained and behaving animals. However, Mn(2+) is a metabolic toxin and a competitive inhibitor for Ca(2+), and therefore, a yet unsolved question in MEMRI studies is whether the concentrations of metal ion used may alter brain physiology. In the present work we have investigated the behavioral, electrophysiological and histopathological consequences of MnCl(2) administration at concentrations and dosage protocols regularly used in MEMRI. Three groups of animals were sc injected with saline, 0.1 and 0.5 mmol/kg MnCl(2), respectively. In vivo electrophysiological recordings in the hippocampal formation revealed a mild but detectable decrease in both excitatory postsynaptic potentials (EPSP) and population spike (PS) amplitude under the highest MnCl(2) dose. The EPSP to PS ratio was preserved at control levels, indicating that neuronal excitability was not affected. Experiments of pair pulse facilitation demonstrated a dose dependent increase in the potentiation of the second pulse, suggesting presynaptic Ca(2+) competition as the mechanism for the decreased neuronal response. Tetanization of the perforant path induced a long-term potentiation of synaptic transmission that was comparable in all groups, regardless of treatment. Accordingly, the choice accuracy tested on a hippocampal-dependent learning task was not affected. However, the response latency in the same task was largely increased in the group receiving 0.5 mmol/kg of MnCl(2). Immunohistological examination of the hippocampus at the end of the experiments revealed no sign of neuronal toxicity or glial reaction. Although we show that MEMRI at 0.1 mmol/Kg MnCl(2) may be safely applied to the study of cognitive networks, a detailed assessment of toxicity is strongly recommended for each particular study and Mn(2+) administration protocol.