Astrocytes in the Dentate Gyrus Shift the Equilibrium from Pattern Separation to Pattern Completion and Inhibit Fear Extinction
Pattern separation and pattern completion are core hippocampal computations linked to memory interference and overgeneralization in many neuropsychiatric disorders and to mnemonic discrimination deficits in neurodegenerative conditions. Astrocytes are increasingly recognized as important integrators and modulators of neuronal activity as many studies have shown changes in animal behavior and neural circuits induced by manipulation of astrocytic activity. The role of astrocytes in the dentate gyrus for learning and memory is, however, largely unknown. To investigate the functional significance of astrocytic activity in the dentate gyrus, we expressed the calcium extruding pump hPMCA2w/b (CalEx), resulting in inhibition of astrocytic calcium activity, the Gq-coupled receptor hM3Dq, or the Gi-coupled receptor hM4Di, allowing chronic activation of Gq- and Gi-dependent pathways by a designer drug, in dentate gyrus astrocytes. Our main finding is that inhibition of astrocytic calcium activity by CalEx resulted in improved pattern separation and impaired pattern completion. In -vivo calcium imaging revealed that CalEx expression in astrocytes altered the neuronal activity pattern in dentate gyrus neurons with an overall inhibitory effect. This suggests that in the dentate gyrus astrocytes promote the activity of these neurons. Furthermore, chronic activation of hM4Di resulted in impaired between-session fear extinction while chronic activation of hM3Dq resulted in impaired within-session fear extinction and improved pattern separation learning over multiple days, suggesting that activation of astrocytic activity enhances mechanistically separable forms of fear extinction in a Gq- or Gi-pathway-specific manner. Our findings reveal that dentate gyrus astrocytic calcium shifts the balance between pattern separation and pattern completion towards pattern completion and thus modulates an essential neuronal functional output of the dentate gyrus. Chronic activation of astrocytes inhibits fear extinction, and thus also affects management of memory interference. Our findings imply that astrocytic modulation may be beneficial to treat cognitive deficits in disorders in which overgeneralization is a problem.