Cortical layer 1 and layer 2/3 astrocytes exhibit distinct calcium dynamics in vivo

Cumulative evidence supports bidirectional interactions between astrocytes and neurons, suggesting glial involvement of neuronal information processing in the brain Cytosolic calcium (Ca²⁺) concentration is important for astrocytes as Ca²⁺ surges co-occur with gliotransmission and neurotransmitter reception. Cerebral cortex is organized in layers which are characterized by distinct cytoarchitecture. We asked if astrocyte-dominant layer 1 (L1) of the somatosensory cortex was different from layer 2/3 (L2/3) in spontaneous astrocytic Ca²⁺ activity and if it was influenced by background neural activity. Using a two-photon laser scanning microscope, we compared spontaneous Ca²⁺ activity of astrocytic somata and processes in L1 and L2/3 of anesthetized mature rat somatosensory cortex. We also assessed the contribution of background neural activity to the spontaneous astrocytic Ca²⁺ dynamics by investigating two distinct EEG states ("synchronized" vs. "desynchronized" states). We found that astrocytes in L1 had nearly twice higher Ca²⁺ activity than L2/3. Furthermore, Ca²⁺ fluctuations of processes within an astrocyte were independent in L1 while those in L2/3 were synchronous. Pharmacological blockades of metabotropic receptors for glutamate, ATP, and acetylcholine, as well as suppression of action potentials did not have a significant effect on the spontaneous somatic Ca²⁺ activity. These results suggest that spontaneous astrocytic Ca²⁺ surges occured in large part intrinsically, rather than neural activity-driven. Our findings propose a new functional segregation of layer 1 and 2/3 that is defined by autonomous astrocytic activity.