Evaluating a genetically encoded optical sensor of neural activity using electrophysiology in intact adult fruit flies
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Computation and Neural Systems Program, Division of Biology, California Institute of Technology, USA
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.
Keywords:
Drosophila, neural coding, olfaction, antennal lobe, 2-photon imaging, electrophysiology, genetically encoded calcium indicators, G-CaMP
Citation:
Vivek Jayaraman and Gilles Laurent (2007). Evaluating a genetically encoded optical sensor of neural activity using electrophysiology in intact adult fruit flies. Front. Neural Circuits 1:3. doi: 10.3389/neuro.04/003.2007
Received: 10 August 2007;
Paper pending published: 17 September 2007;
Accepted: 15 October 2007;
Published online: 2 November 2007.
Edited by:
Rafael Yuste, Columbia University, New York City, USA
Reviewed by:
Karen Zito, University of California Davis, Davis, United States of America
Rafael Yuste, Columbia University, New York City, USA
Copyright:
© 2007 Jayaraman, Laurent. This is an open-access article subject to an exclusive license agreement between the authors and the Frontiers Research Foundation, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are credited.
*Correspondence:
Gilles Laurent, Computation and Neural Systems Program, Division of Biology, California Institute of Technology, Pasadena, CA 91125, USA. e-mail: laurentg@caltech.edu; or Vivek Jayaraman, e-mail: vivek@janelia.hhmi.org