Evaluating a genetically encoded optical sensor of neural activity using electrophysiology in intact adult fruit flies

Vivek Jayaraman

doi:10.3389/neuro.04.003.2007

Abstract

Genetically encoded optical indicators hold the promise of enabling non-invasive monitoring of activity in identified neurons in behaving organisms. However, the interpretation of images of brain activity produced using such sensors is not straightforward. Several recent studies of sensory coding used G-CaMP 1.3—a calcium sensor—as an indicator of neural activity; some of these studies characterized the imaged neurons as having narrow tuning curves, a conclusion not always supported by parallel electrophysiological studies. To better understand the possible cause of these conflicting results, we performed simultaneous in vivo 2-photon imaging and electrophysiological recording of G-CaMP 1.3 expressing neurons in the antennal lobe (AL) of intact fruitflies. We find that G-CaMP has a relatively high threshold, that its signal often fails to capture spiking response kinetics, and that it can miss even high instantaneous rates of activity if those are not sustained. While G-CaMP can be misleading, it is clearly useful for the identification of promising neural targets: when electrical activity is well above the sensor's detection threshold, its signal is fairly well correlated with mean firing rate and G-CaMP does not appear to alter significantly the responses of neurons that express it. The methods we present should enable any genetically encoded sensor, activator, or silencer to be evaluated in an intact neural circuit in vivo in Drosophila.

Highlights

The fruit fly has been a mainstay of behavioral genetics for many decades (Benzer, 1967; Hotta and Benzer, 1970; Quinn et al, 1974)
Half of the projection neurons (PNs) population is targeted by this line: G-CaMP-expressing PNs showed strong baseline fluorescence, and PNs showed strong and selective changes in fluorescence in response to odors (Figure 1A)
We found that the faster kinetics of the G-CaMP glomerular responses better represented electrical activity in the PNs (Figures 4E–4H)

Summary

Introduction

The fruit fly has been a mainstay of behavioral genetics for many decades (Benzer, 1967; Hotta and Benzer, 1970; Quinn et al, 1974). Alternative techniques use optical reporters of voltage (Cohen et al, 1978; Cohen and Salzberg, 1978; Grinvald and Hildesheim, 2004; Taylor et al, 2003) or intracellular calcium (Grynkiewicz et al, 1985; Helmchen et al, 1996; Stosiek et al, 2003; Svoboda et al., 1997; Tank et al, 1988; Yuste and Katz, 1991) Some of these indicators are genetically encoded (Miyawaki et al, 1997; Nakai et al, 2001; Persechini et al, 1997); using genetic techniques (Brand and Perrimon, 1993), specific cell groups can be targeted selectively (Fiala et al, 2002; Ng et al, 2002; Schroll et al, 2006; Wang et al, 2003), making optical approaches very powerful. It seems important to evaluate the performance of these sensors in the cells of interest before using them as standalone indicators of neural activity

Methods

Results

Conclusion