The Role of Astroglia in Brain State-Dependent Neural Activity

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Awake behavior is accompanied by fluctuations in vigilance shaping the overall activity state of the brain to optimize cellular and circuit activity. The cellular and molecular mechanisms for such brain state-dependent adjustments in neural activity are not well understood. Using a mouse locomotion paradigm we have recently found that the transition from a resting state to active locomotion is associated with release of the neurotransmitter norepinephrine and leads to Ca2+ activation of astroglia. Astroglia are activated simultaneously in brain regions as disparate as the cerebellum and primary visual cortex suggesting that they might play a major role in mediating global brain state-dependent modulation of neural activity. The precise molecular mechanism of locomotion-induced astroglia Ca2+ activation as well as the consequences for neuronal activity in the adult brain are not known. In this project we will test the hypothesis that locomotion- induced noradrenergic modulation of neuronal activity is mediated by astroglia. We will apply a combination of in vivo two-photon Ca2+ imaging and electrophysiology, and acute slice experiments on specific mouse lines that have been genetically modified in a cell type-specific manner, to reveal the cellular and molecular mechanisms of astroglia-mediated, brain state-dependent neuromodulation. The focus of our studies will be on the cerebellar cortex leveraging on a circuit that consists only of a handful of cells. A novel application of a specific Cre mouse line will enable us to selectively manipulate Bergmann glia while leaving velate astrocytes of the cerebellar cortex unperturbed. We will pursue the following aims: (1) Combining immunocytochemistry and functional studies with global and cell type-specific knockout mice we will determine identity and location of receptors required for locomotion-induced Bergmann glia Ca2+ activation. (2) We will investigate locomotion-induced Ca2+ and electrical signals in Purkinje neurons, the principal neurons of the cerebellar cortex. Using genetic elimination of Bergmann glia global Ca2+ elevations we will isolate components of locomotion-induced Purkinje neuron signaling that are caused by prior Bergmann glia Ca2+ activation. (3) Combining pharmacology and cell type-specific knockout in slice experiments and in vivo, we will dissect the molecular mechanism how cerebellar astroglia impact principal neuron activity dependent on the behavioral state. Cerebellar Bergmann glia share many functional properties with velate astrocytes in the remainder of the brain, including locomotion-induced, norepinephrine-dependent global Ca2+ activation. Therefore, we anticipate that our mechanistic studies will be instructive for understanding the role of astroglia in brain state-dependent noradrenergic neuromodulation throughout the brain. This body of work will build the groundwork for future studies on brain state-dependent neural signaling under neurodegenerative and neurobehavioral conditions associated with changes in noradrenergic signaling, such as Alzheimer's disease, Parkinson's disease and autism.