Neurobiology Select

Sleep is one of the most fundamental, yet least understood, facets of animal physiology—its universality reflected in the saying, ''life is one long process of getting tired.'' This issue's Neurobiology Select describes the latest progress toward understanding sleep, including its role in maintaining synaptic homeostasis, its role in long-term memory formation, and the causes and consequences of sleep disruption. If attending a party full of strangers leaves you exhausted, you may have something in common with the fruit fly Drosophila. In their latest work, Donlea et al. (2009) have uncovered a neural basis for the increased need for sleep that fruit flies display after social enrichment. The authors first identify mutant flies that do not have an increased need for sleep after social enrichment, in this case housing flies in groups rather than in isolation. These mutant flies include those lacking the rutabaga gene (involved in memory formation), period (a core regulator of the circadian cycle), and blistered (implicated in long-term potentiation at synapses). The authors then sought to define which population of neurons requires rutabaga, period, and blistered for this behavior and show that reestablishing expression of these genes in ventral lateral neurons restores the increased need for sleep after social enrichment. Ventral lateral neurons are a group of clock neurons that control circadian behavior in flies. Close examination reveals that social enrichment enhances the number of synapses between ventral lateral neurons and neurons of the medulla. Remarkably, sleep reduced the number of these synaptic connections, and this reduction is blocked by sleep deprivation. These findings suggest that this synaptic plasticity in ventral lateral neurons underlies the need for sleep, and provides a compelling example of sleep being required for synaptic homeostasis. A different means by which sleep maintains synaptic homeostasis is reported by Gilestro et al. (2009), who examined the expression of synaptic proteins in Drosophila at intervals throughout the day to analyze the potential impact of sleep on synaptic function. They observe a consistent pattern—the levels of synaptic proteins are lowest after sleep. This finding suggests that individual synapses build up protein levels during the day as a consequence of neuronal activity, and clean house at night in preparation for the following day. This reduction in synaptic protein levels is not simply a feature of the circadian cycle, given that sleep deprivation blocks the reduction. This work raises many interesting questions, including whether the reduction in synaptic …