Conditions and Constraints for Astrocyte Calcium Signaling in the Hippocampal Mossy Fiber Pathway

Highlights

• Astrocyte branches/territories were studied using optical and pharmacogenetic tools

• Spontaneous astrocyte Ca²⁺ signals were not due to action potentials or glutamate

• Evoked astrocyte Ca²⁺ signals were slow, territorial and triggered by spike trains

• Astrocyte Ca²⁺ signaling was tightly gated by neurotransmitter clearance

Summary

The spatiotemporal activities of astrocyte Ca²⁺ signaling in mature neuronal circuits remain unclear. We used genetically encoded Ca²⁺ and glutamate indicators as well as pharmacogenetic and electrical control of neurotransmitter release to explore astrocyte activity in the hippocampal mossy fiber pathway. Our data revealed numerous localized, spontaneous Ca²⁺ signals in astrocyte branches and territories, but these were not driven by neuronal activity or glutamate. Moreover, evoked astrocyte Ca²⁺ signaling changed linearly with the number of mossy fiber action potentials. Under these settings, astrocyte responses were global, suppressed by neurotransmitter clearance, and mediated by glutamate and GABA. Thus, astrocyte engagement in the fully developed mossy fiber pathway was slow and territorial, contrary to that frequently proposed for astrocytes within microcircuits. We show that astrocyte Ca²⁺ signaling functionally segregates large volumes of neuropil and that these transients are not suited for responding to, or regulating, single synapses in the mossy fiber pathway.