Role of purinergic receptor P2Y1 in spatiotemporal Ca ²⁺ dynamics in astrocytes

Fine processes of astrocytes enwrap synapses and are well positioned to sense neuronal information via synaptic transmission. In rodents, astrocyte processes sense synaptic transmission via Gq-protein coupled receptors (GqPCR), including the P2Y1 receptor (P2Y1R), to generate Ca ²⁺ signals. Astrocytes display numerous spontaneous microdomain Ca ²⁺ signals; however, it is not clear whether such signals are due to local synaptic transmission and/or in what timeframe astrocytes sense local synaptic transmission. To ask whether GqPCRs mediate microdomain Ca ²⁺ signals, we engineered mice (both sexes) to specifically overexpress P2Y1Rs in astrocytes, and we visualized Ca ²⁺ signals via a genetically encoded Ca ²⁺ indicator, GCaMP6f, in astrocytes from adult mice. Astrocytes overexpressing P2Y1Rs showed significantly larger Ca ²⁺ signals in response to exogenously applied ligand and to repetitive electrical stimulation of axons compared with controls. However, we found no evidence of increased microdomain Ca ²⁺ signals. Instead, Ca ²⁺ waves appeared and propagated to occupy areas that were up to 80-fold larger than microdomain Ca ²⁺ signals. These Ca ²⁺ waves accounted for only 2% of total Ca ²⁺ events, but they were 1.9-fold larger and 2.9-fold longer in duration than microdomain Ca ²⁺ signals at processes. Ca ²⁺ waves did not require action potentials for their generation and occurred in a probenecid-sensitive manner, indicating that the endogenous ligand for P2Y1R is elevated independently of synaptic transmission. Our data suggest that spontaneous microdomain Ca ²⁺ signals occur independently of P2Y1R activation and that astrocytes may not encode neuronal information in response to synaptic transmission at a point source of neurotransmitter release.