Sleep- and wake-dependent changes in neuronal activity and reactivity demonstrated in fly neurons using in vivo calcium imaging

Abstract

Sleep in Drosophila shares many features with mammalian sleep, but it remains unknown whether spontaneous and evoked activity of individual neurons change with the sleep/wake cycle in flies as they do in mammals. Here we used calcium imaging to assess how the Kenyon cells in the fly mushroom bodies change their activity and reactivity to stimuli during sleep, wake, and after short or long sleep deprivation. As before, sleep was defined as a period of immobility of >5 min associated with a reduced behavioral response to a stimulus. We found that calcium levels in Kenyon cells decline when flies fall asleep and increase when they wake up. Moreover, calcium transients in response to two different stimuli are larger in awake flies than in sleeping flies. The activity of Kenyon cells is also affected by sleep/wake history: in awake flies, more cells are spontaneously active and responding to stimuli if the last several hours (5-8 h) before imaging were spent awake rather than asleep. By contrast, long wake (≥29 h) reduces both baseline and evoked neural activity and decreases the ability of neurons to respond consistently to the same repeated stimulus. The latter finding may underlie some of the negative effects of sleep deprivation on cognitive performance and is consistent with the occurrence of local sleep during wake as described in behaving rats. Thus, calcium imaging uncovers new similarities between fly and mammalian sleep: fly neurons are more active and reactive in wake than in sleep, and their activity tracks sleep/wake history.T he fundamental features that characterize mammalian sleep also define Drosophila melanogaster sleep (1-3). Most crucially, in both flies and mammals, sleep is distinguished from simple rest (quiet wake) by an increased arousal threshold, i.e., a reduced ability to respond to external stimuli. Moreover, in flies and mammals sleep is controlled homeostatically by the duration as well by the intensity of prior wake, suggesting basic similarities in the mechanisms of sleep regulation across species. Thus, in flies both sleep deprivation and a rich learning experience lead to a sleep rebound characterized by overall increased sleep time, increased arousal threshold, and longer sleep episodes (4-6). As in mammals, overall neuronal activity in flies is also high during wake and low during sleep (6-8). Specifically, a seminal study using local field potential (LFP) recordings from the Drosophila medial protocerebrum found high spike-like potentials that disappeared after the block of synaptic transmission in the mushroom bodies (MBs) (7). This high spike activity was present when flies were moving or had been quiescent for only a few seconds, but disappeared with sleep (i.e., after periods of immobility >5 min), when the overall LFP power in all frequencies also decreased by ∼60% (7). The study concluded that neural activity in the sleeping fly brain, or at least in a central region spanning the MBs, resembles that seen in mammals in several brainstem cell groups including noradrenergic ne...

The Yin and Yang of Sleep and Attention

“…This is already evident in simple animals, such as C. elegans , in which two different types of sensory neurons, ALM and ASH, show reduced excitability during the worm's sleep-like state [26-28] (Figure 2). A similar reduction in intrinsic or evoked excitability has also recently been observed in the Kenyon neurons during Drosophila sleep [29], suggesting that suppression at the individual neuron level is a key characteristic of the sleep state. However, behaviours are not simply produced from individual neurons; they also rely on the coordinated communication between neurons.…”

Propagation of Homeostatic Sleep Signals by Segregated Synaptic Microcircuits of the Drosophila Mushroom Body

“…We thus hypothesized that the MB sleep-control microcircuits are endogenously activated under physiological or environmental conditions that alter sleep pressure, such as sleep deprivation. Interestingly, recent studies reveal changes in KC Ca 2+ levels correlated to quiescent or active states of the fly [31], but the identity of these KCs and their relationship to homeostasis remains uncertain. To test this hypothesis, we compared the electrical activity of sleep-controlling KCs and MBONs in sleep-deprived and – replete conditions, using a standard method for intermittent mechanical perturbation of sleep (see Experimental Procedures).…”

Molecular Mechanisms of Sleep Homeostasis in Flies and Mammals

“…For instance, sleep deprivation impairs learning and memory Seugnet et al 2009;Donlea et al 2011) and reduces aggression, mating success , and neural development (Kayser et al 2014). In flies, the chronic recording of neuronal activity during sleep is possible but remains challenging (Nitz et al 2002;van Swinderen et al 2004;van Alphen et al 2013;Bushey et al 2015). Like in mammals, wake duration is an important factor affecting the sleep rebound in flies (Huber et al 2004b), but wake "intensity," for instance, the exposure to an enriched environment, also increases sleep need (Ganguly-Fitzgerald et al 2006;Donlea et al 2009;Bushey et al 2011), and learning mutants are deficient for experience-dependent increases in sleep (Donlea et al 2009).…”