Hugin+ neurons provide a link between sleep homeostat and circadian clock neurons

Sleep is controlled by homeostatic mechanisms, which drive sleep after wakefulness, and a circadian clock, which confers the 24-h rhythm of sleep. These processes interact with each other to control the timing of sleep in a daily cycle as well as following sleep deprivation. However, the mechanisms by which they interact are poorly understood. We show here that hugin+ neurons, previously identified as neurons that function downstream of the clock to regulate rhythms of locomotor activity, are also targets of the sleep homeostat. Sleep deprivation decreases activity of hugin+ neurons, likely to suppress circadian-driven activity during recovery sleep, and ablation of hugin+ neurons promotes sleep increases generated by activation of the homeostatic sleep locus, the dorsal fan-shaped body (dFB). Also, mutations in peptides produced by the hugin+ locus increase recovery sleep following deprivation. Transsynaptic mapping reveals that hugin+ neurons feed back onto central clock neurons, which also show decreased activity upon sleep loss, in a Hugin peptide–dependent fashion. We propose that hugin+ neurons integrate circadian and sleep signals to modulate circadian circuitry and regulate the timing of sleep.

Sleep promoting neurons remodel their response properties to calibrate sleep drive with environmental demands.

Falling asleep at the wrong time can place an individual at risk of immediate physical harm. However, not sleeping degrades cognition and adaptive behavior. To understand how animals match sleep need with environmental demands, we used live-brain imaging to examine the physiological response properties of the Drosophila sleep homeostat (dFB) following interventions that modify sleep (sleep deprivation, starvation, time-restricted feeding, memory consolidation). We report that dFB neurons can distinguish between different types of waking and can change their physiological response-properties accordingly. That is, dFB neurons are not simply passive components of a hard-wired circuit. Rather, the dFB neurons themselves can determine their response to the activity from upstream circuits. Finally, we show that the dFB appears to contain a memory trace of prior exposure to metabolic challenges induced by starvation or time-restricted feeding. Together these data highlight that the sleep homeostat is plastic and suggests an underlying mechanism.

Circadian programming of the ellipsoid body sleep homeostat in Drosophila

Homeostatic and circadian processes collaborate to appropriately time and consolidate sleep and wake. To understand how these processes are integrated, we scheduled brief sleep deprivation at different times of day in Drosophila and find elevated morning rebound compared to evening. These effects depend on discrete morning and evening clock neurons, independent of their roles in circadian locomotor activity. In the R5 ellipsoid body sleep homeostat, we identified elevated morning expression of activity dependent and presynaptic gene expression as well as the presynaptic protein BRUCHPILOT consistent with regulation by clock circuits. These neurons also display elevated calcium levels in response to sleep loss in the morning, but not the evening consistent with the observed time-dependent sleep rebound. These studies reveal the circuit and molecular mechanisms by which discrete circadian clock neurons program a homeostatic sleep center.

Estradiol Influences Adenosinergic Signaling and NREM Sleep Need in Adult Female Rats

Studies report estradiol (E2) suppresses sleep in females; however, the mechanisms of E2 action remain largely undetermined. Our previous findings suggest that the median preoptic nucleus (MnPO) is a key nexus for E2 action on sleep. Here, using behavioral, neurochemical and pharmacological approaches, we investigated whether E2 influenced the sleep homeostat as well as adenosinergic signaling in the MnPO of adult female rats. During the Light Phase, where rats accumulate the majority of sleep, E2 markedly reduced NREM-SWA (a measure of the homeostatic sleep need). Following 6-hours of sleep deprivation, levels of NREM-SWA were significantly increased compared to baseline sleep. However, the NREM-SWA levels were not different between E2 and control treatment despite a significant increase in wake at the expense of NREM sleep. Analysis of NREM-SWA differences between baseline and recovery sleep following sleep deprivation demonstrated that E2 induced a 2-fold increase in delta power compared to controls suggesting that E2 significantly expanded the dynamic range for the sleep homeostat. Correlated with E2-induced changes in physiological markers of homeostatic sleep was a marked increase in extracellular adenosine (a molecular marker of homeostatic sleep need) during unrestricted and recovery sleep following a 6-hour deprivation. Additionally, E2 blocked the ability of an adenosine A2A receptor agonist (CGS-21680) to increase NREM sleep compared to controls. Thus, taken together, the findings that E2 increased extracellular adenosine content, while blocking A2A signaling in the MnPO suggests a potential mechanism for how estrogens impact sleep in the female brain.

Sleep duration, sleep variability, and impairments of visual attention

Attentional networks are sensitive to sleep deprivation. However, variation in attentional performance as a function of normal sleep parameters is understudied. We examined whether attentional performance is influenced by (a) individual differences in sleep duration, (b) sleep duration variability, and/or (c) their interaction. A total of 57 healthy participants (61.4% female, Mage = 32.37 years, SD = 8.68) completed questionnaires, wore wrist actigraphy for 1 week, and subsequently completed the attention network test. Sleep duration and sleep duration variability did not predict orienting score, executive control score, or error rates. Sleep duration variability appeared to moderate the association between sleep duration with overall reaction time (β = –.34, t = –2.13, p = .04) and alerting scores (β = .43, t = 2.94, p = .01), though further inspection of the data suggested that these were spurious findings. Time of testing was a significant predictor of alerting score (β = .35, t = 2.96, p = .01), chronotype of orienting (β = .31, t = 2.28, p = .03), and age of overall reaction time (β = .35, t = 2.70, p = .01). Our results highlight the importance of examining the associations between variations in sleep–wake patterns and attentional networks in samples with greater variation in sleep, as well as the importance of rigorously teasing apart mechanisms of the sleep homeostat from those related to the circadian rhythm in studies examining cognition.

A role for astroglial calcium in mammalian sleep

Mammalian sleep is characterized by dramatic changes in neuronal activity, and waking neuronal activity is thought to increase sleep need. Changes in other brain cells (glia) across the natural sleep-wake cycle and their role in sleep regulation are comparatively unexplored. We show that sleep is also accompanied by large changes in astroglial activity as measured by intracellular calcium concentrations in unanesthetized mice. These changes in calcium vary across different vigilance states and are most pronounced in distal astroglial processes. We find that reducing intracellular calcium in astrocytes impaired the homeostatic response to sleep deprivation. Thus, astroglial calcium changes dynamically across vigilance states and is a component of the sleep homeostat.One Sentence SummaryAstroglial calcium concentrations vary with sleep and wake, change after sleep deprivation, and mediate sleep need.

Differential regulation of the Drosophila sleep homeostat by circadian and arousal inputs

One output arm of the sleep homeostat in Drosophila appears to be a group of neurons with projections to the dorsal fan-shaped body (dFB neurons) of the central complex in the brain. However, neurons that regulate the sleep homeostat remain poorly understood. Using neurogenetic approaches combined with Ca2+ imaging, we characterized synaptic connections between dFB neurons and distinct sets of upstream sleep-regulatory neurons. One group of the sleep-promoting upstream neurons is a set of circadian pacemaker neurons that activates dFB neurons via direct glutaminergic excitatory synaptic connections. Opposing this population, a group of arousal-promoting neurons downregulates dFB axonal output with dopamine. Co-activating these two inputs leads to frequent shifts between sleep and wake states. We also show that dFB neurons release the neurotransmitter GABA and inhibit octopaminergic arousal neurons. We propose that dFB neurons integrate synaptic inputs from distinct sets of upstream sleep-promoting circadian clock neurons, and arousal neurons.