Astrocytes Regulate Brain State Transitions.

We celebrate the life of our close friend and collaborator, Arne Schousboe, by writing this prose of the role of astrocytes in distinct aspects of arousal. Most animals exhibit cyclic behavioural transitions between sleep and wakefulness. Highly interconnected and complex networks of neurones, which release neurotransmitters, particularly noradrenaline, that target astrocytes by volume transmission, support the arousal system. Astrocyte noradrenergic signalling pathways are intricately connected to energy metabolism, whereby noradrenaline stimulates metabolism and leads to changes in cellular morphology, which is consistent with the maturation, territorial reach and complexity of these glial cells. We briefly discuss historic hypotheses contributing to the ever-going notion that cellular morphology and function affect each other. The message is that astrocytes contribute to sleep-wake transitions through the regulation of homeostatic control; these glial cells are responsible for ionostasis, metabolism, biosynthesis and degradation of neurotransmitters and the regulation of microcirculation and interstitial fluid flow. By regulating brain homeostasis, astrocytes in turn affect neuronal activity in the context of sleep-arousal regulation. "It's complicated."® (Arne Schousboe, Denmark).