Measuring Sharp Waves and Oscillatory Population Activity With the Genetically Encoded Calcium Indicator GCaMP6f.

Pinggan Li,Xinling Geng,Huiyi Jiang,Adam Caccavano,Jian-Young Wu,Stefano Vicini

doi:10.3389/fncel.2019.00274

Abstract

GCaMP6f is among the most widely used genetically encoded calcium indicators for monitoring neuronal activity. Applications are at both the cellular and population levels. Here, we explore two important and under-explored issues. First, we have tested if GCaMP6f is sensitive enough for the detection of population activity with sparse firing, similar to the sensitivity of the local field potential (LFP). Second, we have tested if GCaMP6f is fast enough for the detection of fast network oscillations critical for the encoding and consolidation of memory. We have focused this study on the activity of the hippocampal network including sharp waves (SWs), carbachol-induced theta oscillations, and interictal-like spikes. We compare simultaneous LFP and optical GCaMP6f fluorescent recordings in Thy1-GCaMP6f mouse hippocampal slices. We observe that SWs produce a clear population GCaMP6f signal above noise with an average magnitude of 0.3% ΔF/F. This population signal is highly correlated with the LFP, albeit with a delay of 40.3 ms (SD 10.8 ms). The population GCaMP6f signal follows the LFP evoked by 20 Hz stimulation with high fidelity, while electrically evoked oscillations up to 40 Hz were detectable with reduced amplitude. GCaMP6f and LFP signals showed a large amplitude discrepancy. The amplitude of GCaMP6f fluorescence increased by a factor of 28.9 (SD 13.5) between spontaneous SWs and carbachol-induced theta bursts, while the LFP amplitude increased by a factor of 2.4 (SD 1.0). Our results suggest that GCaMP6f is a useful tool for applications commonly considered beyond the scope of genetically encoded calcium indicators. In particular, population GCaMP6f signals are sensitive enough for detecting synchronous network events with sparse firing and sub-threshold activity, as well as asynchronous events with only a nominal LFP. In addition, population GCaMP6f signals are fast enough for monitoring theta and beta oscillations (<25 Hz). Faster calcium indicators (e.g., GCaMP7) will further improve the frequency response for the detection of gamma band oscillations. The advantage of population optical over LFP recordings are that they are non-contact and free from stimulation artifacts. These features may be particularly useful for high-throughput recordings and applications sensitive to stimulus artifact, such as monitoring responses during continuous stimulation.

Highlights

Fluorescent calcium signals are widely used for monitoring neuronal activity in the brain thanks to the availability of genetically encoded indicators
This is of concern as GCaMP fluorescence appears to primarily reflect supra-threshold somatic Ca2+ influx (Helmchen and Tank, 2015), together with intracellular calcium induced by action potentials (Grienberger and Konnerth, 2012)
These results suggest that optical recordings of population GCaMP6f signals may be useful for detecting network activity complementary to local field potential (LFP) recordings

Summary

Introduction

Fluorescent calcium signals are widely used for monitoring neuronal activity in the brain thanks to the availability of genetically encoded indicators. We ask whether the population GCaMP6f signal is comparable to local field potential (LFP) recordings, i.e., capable of detecting network activity in which only a small fraction of neurons fire action potentials, while the majority of neurons only have subthreshold potentials. This is of concern as GCaMP fluorescence appears to primarily reflect supra-threshold somatic Ca2+ influx (Helmchen and Tank, 2015), together with intracellular calcium induced by action potentials (Grienberger and Konnerth, 2012). While the Ca2+ signals from individual synapses are small, a population signal from the integration of a large number of dendrites in the neuropil might become detectable, given that the sensitivity and fluorescent yield of GCaMP6f are both excellent

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Discussion

Conclusion