Evaluating the efficacy of Optoα1AR activation in astrocytes in modulating 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.Significance statementAstrocytes are critical components of synapses and are known to modulate glutamatergic synaptic transmission. However, the extent to which astrocytes modulate basal GABAergic transmission is less clear. Additionally, there is demand for tools that can activate physiologically-relevant signaling pathways in astrocytes with improved temporal precision. Here, we present a novel optogenetic viral vector, AAV8-GFAP-Optoα1AR-eYFP, to stimulate astrocytes with improved temporal control. We report that high-frequency (20 Hz) stimulation of astrocytic Optoα1AR produces changes in inhibitory and excitatory transmission in hippocampal CA1, but low-frequency stimulation (0.5 Hz) is insufficient. These findings suggest that astrocytes are sensitive to the temporal dynamics of optical stimulation, and reinforce the importance of careful consideration of stimulation paradigm when using optogenetic tools to manipulate astrocytic function.

Optogenetic Activation of Astrocytes—Effects on Neuronal Network Function

“…Ionotropic opsins are well studied in different types of cells in the nervous system. Effects of ChR2 activation in astrocytes have been previously described [35][36][37][38]. It was demonstrated that activation of ChR2 in astrocytes triggers a release of glutamate and increase in the frequency of spontaneous excitatory postsynaptic currents in pyramidal neurons [38], in agreement with our findings (Figure 2).…”

“…Effects of ChR2 activation in astrocytes have been previously described [35][36][37][38]. It was demonstrated that activation of ChR2 in astrocytes triggers a release of glutamate and increase in the frequency of spontaneous excitatory postsynaptic currents in pyramidal neurons [38], in agreement with our findings (Figure 2). Activation of ChR2 in astrocytes can also have positive influence on interneurons' excitability and negative influence on pyramidal neuron activity, what reduces their action potential frequency.…”

“…Activation of ChR2 in astrocytes can also have positive influence on interneurons' excitability and negative influence on pyramidal neuron activity, what reduces their action potential frequency. It is possible that different and even opposite effects of astrocytic ChR2 activation may be related to various regimes of illumination [36,38] and to different neuronal activity patterns [39].…”

A perspective on astrocyte regulation of neural circuit function and animal behavior

“…Thorough characterization, including transcriptomic and proteomic profiling, will be necessary to develop a mechanistic understanding of precisely how their activation influences astrocytes, neural circuits, and animal behavior, either directly or indirectly (Figueiredo et al, 2014;Octeau et al, 2019;Philtjens et al, 2021;Yu et al, 2020). Such experiments should also consider how the observed effects depend on stimulation parameters and the approach itself (e.g., light-induced heating-mediated effects; Courtney et al, 2021;Owen et al, 2019;Schmidt & Oheim, 2020).…”

“…Thorough characterization, including transcriptomic and proteomic profiling, will be necessary to develop a mechanistic understanding of precisely how their activation influences astrocytes, neural circuits, and animal behavior, either directly or indirectly ( Figueiredo et al, 2014 ; Octeau et al, 2019 ; Philtjens et al, 2021 ; Yu et al, 2020 ). Such experiments should also consider how the observed effects depend on stimulation parameters and the approach itself (e.g., light-induced heating-mediated effects; Courtney et al, 2021 ; Owen et al, 2019 ; Schmidt & Oheim, 2020 ). Together with established genetic interventions (e.g., CRISPR/Cas9-based approaches to genetically or epigenetically modify astrocytes’ ability to sense, process, or send signals), these new tools will be essential in mapping the interplay between astrocytes and neural circuits in behavior on both short and long timescales.…”