Scientific Understanding of Consciousness
Sleep and Synaptic Homeostasis
Science 24 June 2011: Vol. 332 no. 6037 pp. 1576-1581
Sleep and Synaptic Homeostasis: Structural Evidence in Drosophila
Daniel Bushey, Giulio Tononi, Chiara Cirelli
Department of Psychiatry, University of Wisconsin, Madison, WI 53719, USA
The functions of sleep remain elusive, but a strong link exists between sleep need and neuronal plasticity. We tested the hypothesis that plastic processes during wake lead to a net increase in synaptic strength and sleep is necessary for synaptic renormalization. We found that, in three Drosophila neuronal circuits, synapse size or number increases after a few hours of wake and decreases only if flies are allowed to sleep. A richer wake experience resulted in both larger synaptic growth and greater sleep need. Finally, we demonstrate that the gene Fmr1 (fragile X mental retardation 1) plays an important role in sleep-dependent synaptic renormalization.
Sleep is present in every species that has been carefully studied, including Drosophila melanogaster, but its functions remain elusive. Increasing evidence points to a link between sleep need and neuronal plasticity. A recent hypothesis suggests that a consequence of staying awake is a progressive increase in synaptic strength, as the awake brain learns and adapts to an ever-changing environment mostly through synaptic potentiation. However, such increase would soon become unsustainable, because stronger synapses consume more energy, occupy more space, require more supplies, and cannot be further potentiated, saturating the ability to learn. Thus, according to the synaptic homeostasis hypothesis, sleep may serve an essential function by promoting a homeostatic reduction in synaptic strength down to sustainable levels. Also, the hypothesis predicts that the more one learns and adapts (i.e., the more intense is the wake experience), the more one needs to sleep. Findings in rodents are consistent with this hypothesis. For instance, molecular and electrophysiological markers of synaptic strength are higher after wake and lower after sleep. Moreover, presynaptic terminals of hypocretin neurons in zebrafish larvae undergo both circadian and sleep-wake–dependent structural changes, the latter consistent with sleep-dependent down-regulation. Finally, in the fly brain, overall levels of synaptic proteins increase after wake and decrease after sleep, and synaptic structural changes have been described after very long sleep deprivation. These results suggest that a role for sleep in synaptic homeostasis may hold in phylogenetically distant species and may thus be of general importance.
The evidence in support of the synaptic homeostasis hypothesis is mainly correlative, and thus it is important to seek direct proof that sleep is necessary for synaptic renormalization and to do so at the level of individual synapses. Moreover, the synaptic homeostasis hypothesis predicts that behavioral paradigms that enhance wake-related plasticity in specific neural circuits should increase synaptic strength in those circuits as well as sleep need, but this prediction has never been tested. Finally, the cellular mechanisms that underlie synaptic and sleep changes remain unexplored. We exploited the power of Drosophila genetics, combined with confocal microscopy and behavioral analysis, to address these questions.
Changes in synaptic strength are often associated with changes in synaptic structure, including synapse number and size, although the link between structural and functional plasticity is complex. In mammals, the diameter and length of synaptic spines correlate with the size of the postsynaptic density and with the magnitude of electric signals transmitted to the dendritic shaft. Moreover, the induction of synaptic potentiation leads to growth of synapses and spines, whereas synaptic depression causes synapses and spines to retract or shrink. Similarly, in Drosophila, synaptic morphology at the neuromuscular junction changes depending on experience, and these changes correlate with synaptic strength. Previous in vivo experiments in mammals and flies measured overall changes in electrophysiological and molecular markers of synaptic strength, without cellular resolution, and without direct evidence for morphological changes in synaptic terminals. We selected three specific cell populations in the fly brain and asked whether sleep and wake affect synaptic density and size
Sleep is perhaps the only major behavior still in search of a function. The results of this study support the hypothesis that plastic processes during wake lead to a net increase in synaptic strength in many brain circuits and that sleep is required for synaptic renormalization. A wake-related increase in synapse number and strength, if unopposed, would lead to a progressive increase in energy expenditure and saturation of learning. A sleep-dependent synaptic homeostasis may explain why sleep is required to maintain cognitive performance. How sleep would bring about a net decrease in synaptic strength remains unknown, but in mammals, potential mechanisms favoring synaptic depression during non–rapid eye movement sleep may require the repeated sequences of depolarization/synchronous firing and hyperpolarization/silence at ~1Hz observed in corticothalamic cells, as well as the low levels of neuromodulators such as noradrenaline and of plasticity-related molecules such as brain-derived neurotrophic factor. To what extent such mechanisms may also apply to flies remains to be determined.
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