Evidence for sleep-dependent synaptic renormalization in mouse pups

Abstract

In adolescent and adult brains several molecular, electrophysiological, and ultrastructural measures of synaptic strength are higher after wake than after sleep [1, 2]. These results support the proposal that a core function of sleep is to renormalize the increase in synaptic strength associated with ongoing learning during wake, to reestablish cellular homeostasis and avoid runaway potentiation, synaptic saturation, and memory interference [2, 3]. Before adolescence however, when the brain is still growing and many new synapses are forming, sleep is widely believed to promote synapse formation and growth. To assess the role of sleep on synapses early in life, we studied 2-week-old mouse pups (both sexes) whose brain is still undergoing significant developmental changes, but in which sleep and wake are easy to recognize. In two strains (CD-1, YFP-H) we found that pups spend ~50% of the day asleep and show an immediate increase in total sleep duration after a few hours of enforced wake, indicative of sleep homeostasis. In YFP-H pups we then used serial block-face electron microscopy to examine whether the axon-spine interface (ASI), an ultrastructural marker of synaptic strength, changes between wake and sleep. We found that the ASI of cortical synapses (layer 2, motor cortex) was on average 33.9% smaller after sleep relative to after extended wake and the differences between conditions were consistent with multiplicative scaling. Thus, the need for sleep-dependent synaptic renormalization may apply also to the young, pre-weaned cerebral cortex, at least in the superficial layers of the primary motor area.

Sleep–wake regulation in preterm and term infants

“…There are a few reports that sleep duration is a relatively insensitive index of sleep pressure early in development 45,108 . However, our interpretation is strengthened by the converging evidence that pre-term infants' sleep is also resilient to nociceptive perturbations, a separate index of sleep pressure validated by experiments in neonatal non-human primates, rats and mice [45][46][47][48] . A second limitation is that the recording length was insufficient to capture multiple sleep-wake cycles.…”

“…Having demonstrated that active sleep pressure was highest in very pre-term infants, as indexed by its extended duration, we sought to confirm this in another way by testing whether their sleep was more resilient to disturbance. To do this we examined whether awakenings from sleep were less likely to be evoked by a nociceptive or somatosensory perturbation in younger infants [45][46][47][48] .…”

Effects of sleep and waking on the synaptic ultrastructure

“…(a) Examples of dendritic segments from primary motor cortex (M1) in P30 mice (left, lighter blue, S group) and in P13 pups (right, darker blue, S group) and lists of some of their structural differences. Results are described in detail in the original publications: primary motor cortex P30 [11]; primary motor cortex P13 [19]. (b) 2D images of M1 synapses in P13 pups (layer 2).…”

“…In summary, sleep/waking pattern and sleep homeostatic regulation in adolescent (P30) YFP-H mice highly resemble those described in adult mice, the most notable difference being the lack of a rebound in SWA after acute sleep loss. More recently we also characterized sleep/waking behaviour in YFP-H pups aged P13 [19]. EEG patterns are not informative at this age owing to the immaturity of the cortex, and thus sleep and waking were distinguished solely based on behaviour.…”

“…It was not known, however, whether sleep would lead to a similar decrease in synapse size during early development, when the wiring of cortical circuits still occurs at significant pace and spine turnover is high. To address this question, we used YFP-H pups, aged P13 [19]. Their brain is still undergoing significant developmental changes and sleep and waking can be distinguished reliably using behavioural criteria.…”