Abstract
Although extensively studied, the exact role of sleep in learning and memory is still not very clear. Sleep deprivation has been most frequently used to explore the effects of sleep on learning and memory, but the results from such studies are inevitably complicated by concurrent stress and distress. Furthermore, it is not clear whether there is a strict time-window between sleep and memory consolidation. In the present study we were able to induce time-locked slow-wave sleep (SWS) in mice by optogenetically stimulating GABAergic neurons in the parafacial zone (PZ), providing a direct approach to analyze the influences of SWS on learning and memory with precise time-windows. We found that SWS induced by light for 30 min immediately or 15 min after the training phase of the object-in-place task significantly prolonged the memory from 30 min to 6 h. However, induction of SWS 30 min after the training phase did not improve memory, suggesting a critical time-window between the induction of a brief episode of SWS and learning for memory consolidation. Application of a gentle touch to the mice during light stimulation to prevent SWS induction also failed to improve memory, indicating the specific role of SWS, but not the activation of PZ GABAergic neurons itself, in memory consolidation. Similar influences of light-induced SWS on memory consolidation also occurred for Y-maze spatial memory and contextual fear memory, but not for cued fear memory. SWS induction immediately before the test phase had no effect on memory performance, indicating that SWS does not affect memory retrieval. Thus, by induction of a brief-episode SWS we have revealed a critical time window for the consolidation of hippocampus-dependent memory.
Highlights
Sleep is conserved across species from Caenorhabditis elegans to humans, suggesting that it is critical to survival [1,2,3]
Studies on the roles of different sleep patterns on memory have usually been done by comparing the effect of early retention sleep, which is dominated by slow-wave sleep (SWS), with late retention sleep, which is dominated by rapid eye movement (REM) sleep [8, 9]
To confirm that the expression of ChR2 was restricted to GABAergic neurons, we carried out immunohistochemical detection of GAD67 in the parafacial zone (PZ) of VGAT-ChR2-EYFP transgenic mice (Fig. 1A)
Summary
Sleep is conserved across species from Caenorhabditis elegans to humans, suggesting that it is critical to survival [1,2,3]. Sleep has been associated with many functions, learning and memory [1, 2, 4, 5]. Despite extensive studies, its role in memory processing is still controversial and elusive [5, 6]. Different sleep patterns have been reported to affect different memory processes [2]. Studies on the roles of different sleep patterns on memory have usually been done by comparing the effect of early retention sleep, which is dominated by SWS, with late retention sleep, which is dominated by REM sleep [8, 9]. Early retention sleep includes REM sleep, and late retention sleep includes SWS sleep. It is impossible to selectively eliminate SWS and leave REM sleep undisturbed
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