Abstract
To understand the circuitry of the brain, it is essential to clarify the functional connectivity among distinct neuronal populations. For this purpose, neuronal activity imaging using genetically-encoded calcium sensors such as GCaMP has been a powerful approach due to its cell-type specificity. However, calcium (Ca2+) is an indirect measure of neuronal activity. A more direct approach would be to use genetically encoded voltage indicators (GEVIs) to observe subthreshold, synaptic activities. The GEVI, ArcLight, which exhibits large fluorescence transients in response to voltage, was expressed in excitatory neurons of the mouse CA1 hippocampus. Fluorescent signals in response to the electrical stimulation of the Schaffer collateral axons were observed in brain slice preparations. ArcLight was able to map both excitatory and inhibitory inputs projected to excitatory neurons. In contrast, the Ca2+ signal detected by GCaMP6f, was only associated with excitatory inputs. ArcLight and similar voltage sensing probes are also becoming powerful paradigms for functional connectivity mapping of brain circuitry.
Highlights
Neuronal circuit activity is a delicate balance between activation and inhibition
Two weeks after the associated adeno virus (AAV) injection, ArcLight and GCaMP6f showed broad expression in the entire CA1 region., The expression pattern of the membrane bound genetically encoded voltage indicators (GEVIs) versus the cytosolic Genetically-encoded calcium indicators (GECIs) resulted in near negative images of one another (Fig. 1B)
This study has demonstrated the ability of the GEVI, ArcLight, to map multiple activities of the neuronal circuits in the CA1 region of mouse hippocampus
Summary
Neuronal circuit activity is a delicate balance between activation and inhibition. For example, Parkinson’s Disease results from a shift towards inhibition in the output of the Basal Ganglia circuit, while a shift towards activation can result in Tourette Syndrome[1, 2]. Even though VSFP-Butterfly’s voltage-dependent change in fluorescence was small (0.5% ΔR/R), clear optical signals could be seen demonstrating the usefulness of GEVIs in cortical brain slices[44] For these reasons, we chose brain slice to assess the ability of the GEVI, ArcLight[40], to report different types of neuronal activities from a population of cells in the CA1 region of the mouse hippocampus. We chose brain slice to assess the ability of the GEVI, ArcLight[40], to report different types of neuronal activities from a population of cells in the CA1 region of the mouse hippocampus This region of the hippocampus is an excellent testing ground for reporting both depolarizations and hyperpolarizations of neuronal plasma membranes since the CA1 circuitry exhibits feed-forward inhibition as a consequence of excitatory synaptic inputs rapidly followed by inhibitory inputs[45] while ArcLight has a broad voltage range that can respond to both hyperpolarization and depolarization of the plasma membrane
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