Optoα1AR activation in astrocytes modulates basal hippocampal synaptic excitation and inhibition in a stimulation-specific manner

Astrocytes play active roles at synapses and can monitor, respond, and adapt to local synaptic activity. While there is abundant evidence that astrocytes modulate excitatory transmission in the hippocampus, evidence for astrocytic modulation of hippocampal synaptic inhibition remains more limited. Furthermore, to better investigate roles for astrocytes in modulating synaptic transmission, more tools that can selectively activate native G protein signaling pathways in astrocytes with both spatial and temporal precision are needed. Here, we utilized AAV8-GFAP-Opto alpha 1AR-eYFP (Opto alpha 1AR), a viral vector that enables activation of G(q) signaling in astrocytes via light-sensitive alpha 1-adrenergic receptors. To determine if stimulating astrocytic Opto alpha 1AR modulates hippocampal synaptic transmission, recordings were made in CA1 pyramidal cells with surrounding astrocytes expressing Opto alpha 1AR, channelrhodopsin (ChR2), or GFP. Both high-frequency (20 Hz, 45-ms light pulses, 5 mW, 5 min) and low-frequency (0.5 Hz, 1-s pulses at increasing 1, 5, and 10 mW intensities, 90 s per intensity) blue light stimulation were tested. 20 Hz Opto alpha 1AR stimulation increased both inhibitory and excitatory postsynaptic current (IPSC and EPSC) frequency, and the effect on miniature IPSCs (mIPSCs) was largely reversible within 20 min. However, low-frequency stimulation of Opto alpha 1AR did not modulate either IPSCs or EPSCs, suggesting that astrocytic Gq-dependent modulation of basal synaptic transmission in the hippocampus is stimulation-dependent. By contrast, low-frequency stimulation of astrocytic ChR2 was effective in increasing both synaptic excitation and inhibition. Together, these data demonstrate that Opto alpha 1AR activation in astrocytes changes basal GABAergic and glutamatergic transmission, but only following high-frequency stimulation, highlighting the importance of temporal dynamics when using optical tools to manipulate astrocyte function.