Research Article

Regulation of sleep homeostasis mediator adenosine by basal forebrain glutamatergic neurons

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Science  04 Sep 2020:
Vol. 369, Issue 6508, eabb0556
DOI: 10.1126/science.abb0556

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Sleep and basal forebrain activity

Different patterns of neural activity in the brain control the sleep-wake cycle. However, how this activity contributes to sleep homeostasis remains largely unknown. Adenosine in the basal forebrain is a prominent physiological mediator of sleep homeostasis. Using a newly developed indicator, Peng et al. monitored adenosine concentration in the mouse basal forebrain. There was a clear correlation with wake state and REM sleep. Activity-dependent release of adenosine could also be elicited after optogenetic stimulation of basal forebrain glutamatergic, but not cholinergic, neurons. These findings offer new insights into how neuronal activity during wakefulness contributes to sleep pressure through the release of sleep-inducing factors.

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Structured Abstract

INTRODUCTION

Sleep homeostasis, the balance between the duration of sleep and wakefulness, is a fundamental feature of the sleep-wake cycle. During wakefulness, sleep-promoting somnogenic factors accumulate and cause an increase in sleep pressure or our need for sleep. Decades of research have identified many genes, molecules, and biochemical processes involved in the regulation of sleep homeostasis. Among various processes implicated in sleep homeostasis, adenosine—a critical component of the cell metabolic pathway—is a prominent physiological mediator of sleep homeostasis. Adenosine released in the basal forebrain (BF), a brain region that plays a critical role in regulating the sleep-wake cycle, can suppress neural activity mediated by the A1 receptor and increase sleep pressure. In addition, the sleep-wake cycle is controlled by different patterns of neural activity in the brain, but how this neural activity contributes to sleep homeostasis remains mostly unclear. In this study, we examine the neural control of sleep homeostasis by investigating in detail the mechanisms underlying adenosine increase in the BF.

RATIONALE

Because the traditional microdialysis measurement of adenosine concentration has a poor temporal resolution, we first designed a genetically encoded G protein–coupled receptor (GPCR)–activation-based (GRAB) sensor for adenosine (GRABAdo), in which the amount of extracellular adenosine is indicated by the intensity of fluorescence produced by green fluorescent protein (GFP) (see the figure, panel A). Using the GRABAdo, we first measured the dynamics of extracellular adenosine concentrations during the sleep-wake cycle in the mouse BF. We then used a simultaneous optical recording of the Ca2+ activity in different BF neurons and the change in adenosine concentrations to examine the correlation between adenosine increase and neural activity. We further studied the ability of different BF neurons in controlling the adenosine release using optogenetic activation. Finally, we used cell type–specific lesion to confirm the contribution of BF neurons in controlling the increase in adenosine concentrations and examine its contribution to the sleep homeostasis regulation.

RESULTS

We found that the amount of extracellular adenosine was high during wakefulness and low during non–rapid eye movement (NREM) sleep. Benefiting from the high temporal resolution of the GRABAdo, we also found a prominent increase in adenosine during REM sleep and revealed rapid changes in adenosine concentrations during brain state transitions. Simultaneous fiber photometry recording of the Ca2+ activity in different BF neurons and the change in extracellular adenosine concentrations showed that both cholinergic neurons and glutamatergic neurons had highly correlated activity with changes in the adenosine concentration (see the figure, panel A). In examining the time course of the two signals, we found that neural activity always preceded changes in adenosine dynamics by tens of seconds. When we measured the evoked adenosine release by optogenetic activation of these two types of neurons using their physiological firing frequencies, we found that the activation of BF cholinergic neurons only produced a moderate increase in extracellular adenosine; by contrast, the activation of BF glutamatergic neurons caused a large and robust increase (see the figure, panel B). Finally, we selectively ablated BF glutamatergic neurons and found a significantly reduced increase in the amounts of extracellular adenosine. Also, mice with a selective lesion of BF glutamatergic neurons showed impaired sleep homeostasis regulation, with significantly increased wakefulness during the active period (see the figure, panel C).

CONCLUSION

Here, we report the design and characterization of a genetically encoded adenosine sensor with high sensitivity and specificity, and high temporal resolution; using the sensor, in combination with fiber photometry recording, optogenetic activation, and cell type–specific lesion, we demonstrate a neural activity–dependent rapid dynamics of the extracellular adenosine concentration during the sleep-wake cycle in the mouse BF and uncover a critical role of the BF glutamatergic neurons in controlling adenosine dynamics and sleep homeostasis. These findings suggest that cell type–specific neural activity during wakefulness can contribute to the increase in sleep pressure by stimulating the release of somnogenic factors.

Neural control of rapid adenosine dynamics and sleep homeostasis.

(A) Simultaneous optical recording of the Ca2+ activity and adenosine concentration using GCaMP and GRABAdo reveals neural activity–dependent rapid adenosine dynamics in the mouse basal forebrain (BF) during the sleep-wake cycle. (B) Optogenetic activation of BF glutamatergic neurons evokes a robust increase of extracellular adenosine. (C) Cell type–specific lesion of BF glutamatergic neurons significantly increases wakefulness.

Abstract

Sleep and wakefulness are homeostatically regulated by a variety of factors, including adenosine. However, how neural activity underlying the sleep-wake cycle controls adenosine release in the brain remains unclear. Using a newly developed genetically encoded adenosine sensor, we found an activity-dependent rapid increase in the concentration of extracellular adenosine in mouse basal forebrain (BF), a critical region controlling sleep and wakefulness. Although the activity of both BF cholinergic and glutamatergic neurons correlated with changes in the concentration of adenosine, optogenetic activation of these neurons at physiological firing frequencies showed that glutamatergic neurons contributed much more to the adenosine increase. Mice with selective ablation of BF glutamatergic neurons exhibited a reduced adenosine increase and impaired sleep homeostasis regulation. Thus, cell type–specific neural activity in the BF dynamically controls sleep homeostasis.

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