Astrocytic chloride is brain state dependent and regulates cortical neuronal activity

Abstract Information transfer within neuronal circuits depends on the balance and recurrent activity of excitatory and inhibitory neurotransmission. Chloride (Cl-) is the major CNS anion mediating inhibitory neurotransmission and thus essential for balanced brain function. Sustained activation of inhibitory synapses leads to a significant reduction in the amplitude of IPSPs over time, reflecting a collapse of the Cl- gradient, due to Cl- influx into neurons through GABAA receptors. Astrocytes are key homeostatic glial cells populating all parts of the central nervous system (CNS). Despite astrocytic contribution to brain state transitions, the role of these cells in regulating inhibition remains unexplored. Here we show, for the first time, that astrocytes act as a dynamic Cl- reservoir regulating Cl- homeostasis in awake mice. Intracellular chloride concentration ([Cl-]i) in astrocytes is high and stable during sleep. In awake mice astrocytic [Cl-]i is lower and fluctuates in response to both sensory input and motor activity. Stimulation of astrocytic GABAA receptors induces Cl- efflux, providing an additional source of Cl- for inhibitory transmission. Increasing astrocytic [Cl-]i by optogenetic manipulations alleviates neuronal activity induced by locomotion or whisker stimulation. Our analysis shows that astrocytes serve as a dynamic source of extracellular Cl- available for GABAergic transmission in awake mice, which represents a novel mechanism for modulation of the inhibitory tone.