Optogenetic activation of G_q signaling in astrocytes yields stimulation-specific effects on basal hippocampal synaptic excitation and inhibition

Abstract

Astrocytes play active roles at synapses and can monitor, respond, and adapt to local synaptic activity. Although there is growing evidence that astrocytes modulate synaptic excitation, the extent to which astrocytes modulate inhibition remains unknown. Additionally, tools that can selectively activate native G protein signaling pathways in astrocytes with both spatial and temporal precision are needed. Here, we present AAV8-GFAP-Optoα1AR-eYFP (Optoα1AR), an astrocyte-specific viral vector that activates the Gq-mediated intracellular cascade via light-sensitive α1-adrenergic receptors. To determine if stimulation of Optoα1AR in astrocytes modulates hippocampal synaptic transmission, whole-cell recordings were made in CA1 pyramidal cells in slices with surrounding astrocytes expressing either Optoα1AR, channelrhodopsin (ChR2), or control green fluorescent protein (GFP). CA1 astrocytes were exposed to either low-frequency (0.5 Hz, 1-s pulses at increasing 1, 5, and 10 mW intensities, 90 s/intensity) or high-frequency (20 Hz, 45-ms light pulses, 5 mW, 5 min) blue light stimulation. Low-frequency stimulation of astrocytic Optoα1AR was insufficient to modulate the frequency or strength of either inhibitory or excitatory spontaneous postsynaptic currents (sIPSCs/sEPSCs), whereas the same stimulation of astrocytic ChR2 produced increases in sIPSC frequency and sEPSC frequency and amplitude. By contrast, 20 Hz stimulation of astrocytic Optoα1AR increased frequency of both miniature IPSCs and EPSCs, and the miniature IPSC frequency effect was largely reversible within 20 min after light stimulation. These data demonstrate that Optoα1AR 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.