While the circadian rhythm provides the temporal framework for sleep-wake cycles, sleep homeostasis ensures that the body’s need for sleep increases with the duration of prior wakefulness. For years, the neural basis of sleep homeostasis was primarily attributed to neurons.
However, a groundbreaking discovery has expanded our understanding by revealing that glial astrocytes play a pivotal role in sleep homeostasis as well.
Astrocytes: Multifunctional Guardians of the Brain
Astrocytes exhibit receptors for neuromodulators that influence wakefulness, such as noradrenaline, and they are capable of releasing sleep-promoting substances like ATP and adenosine.
Moreover, inhibiting gliotransmission in astrocytes has been shown to dampen sleep drive, implying their vital contribution to the sleep homeostatic process.
The Astrocyte Revelation
Until recently, the scientific community was predominantly focused on understanding the neuronal mechanisms behind sleep homeostasis. However, this paradigm shifted when it was demonstrated that glial astrocytes are integral players in this intricate dance of sleep regulation.
Research on transgenic mice revealed that conditional inhibition of gliotransmission in astrocytes led to reduced sleep drive, indicating that astrocytes have a direct impact on the accumulation of sleep need.
Astrocytic Responses to Neuronal Signals
The interaction between astrocytes and neurons is a cornerstone of sleep homeostasis. Astrocytes appear to respond to extracellular signals originating from neurons, such as classic neurotransmitters or neuromodulators. Through a cascade of molecular events, astrocytes process these signals, modulating neuronal activity and influencing sleep-wake behavior. Experiments involving the manipulation of key receptors on astrocyte membranes, as well as intracellular calcium regulating proteins, have further elucidated their role in sleep homeostasis.
Unlocking the Gq Pathway
To delve deeper into the cellular mechanisms underlying astrocytic involvement in sleep regulation, researchers focused on a crucial signaling pathway: the Gq pathway.
This pathway is activated by membrane-bound receptors, such as muscarinic acetylcholine receptors, group I metabotropic glutamate receptors, and histamine H1 receptors. By selectively expressing designer receptors exclusively activated by designer drugs (DREADDs) coupled to the Gq pathway, researchers were able to manipulate these native pathways in astrocytes. This approach allowed them to investigate the impact of astroglial Gq-DREADD activation on sleep expression.
Novel Insights from Basal Forebrain Astrocytes
The basal forebrain (BF) emerged as a region of interest due to its diverse classes of neurons that influence both sleep and wake time, as well as sleep homeostasis. This region had not been extensively studied in terms of astrocytic involvement in sleep regulation. The researchers found that chemogenetically activating the Gq pathway in BF astrocytes resulted in prolonged periods of wakefulness, defying the expected compensatory changes in sleep duration and intensity.
Conclusion
In summary, the traditional view that sleep homeostasis is solely a product of neuronal activity has been challenged by groundbreaking research demonstrating the pivotal role of astrocytes in this intricate process. Astrocytes, with their receptive membranes and intricate signaling pathways, respond to neuronal signals and exert direct influence on sleep-wake behavior.
The revelation that astrocytes contribute significantly to sleep homeostasis opens up new avenues for understanding sleep disorders and developing innovative therapeutic approaches. As we unlock the secrets of astrocytic involvement in sleep regulation, the broader landscape of sleep research is destined for remarkable evolution.
reference link :https://www.biorxiv.org/content/10.1101/2023.01.09.523360v1.full
