Voltage and Calcium Imaging of Brain Activity

Abstract

Sensors for imaging brain activity have been under development for almost 50 years. The development of some of these tools is relatively mature, whereas qualitative improvements of others are needed and are actively pursued. In particular, genetically encoded voltage indicators are just now starting to be used to answer neurobiological questions and, at the same time, more than 10 laboratories are working to improve them. In this Biophysical Perspective, we attempt to discuss the present state of the art and indicate areas of active development.

Figure 1

Schematic structures of three types of GEVls. (A) Given here are mosaic sensors combining the voltage-sensitive domain of a voltage-sensitive phosphatase and a fluorescent protein. (B) Given here are sensors based on the voltage sensitivity of a microbial rhodopsin. (C) Given here are sensors that are a combination of two separate molecules. (B) Modified from (61). (C) Modified from (32).

Figure 2

In vivo population measurements. Comparing GEVls and GECls. (A) Shown here are in vivo population measurements using ArcLight, a single FP mosaic GEVI. The mitral-tufted cells were transduced by infection with an AAV1 virus containing ArcLight DNA. An overlay of the responses to six repeated trials from a single glomerulus to the odorant ethyl tiglate presented during seven breaths. There was little change from trial to trial. (B) Given here is a comparison of ArcLight and GCaMP3 (B1) or GCaMP6f (B2) signals from opposite olfactory bulbs in the same preparations over response to odorant presentations lasting two breaths. The ArcLight responses are faster. Modified from (12).

Figure 3

Imaging neural activity in current clamp-dissociated hippocampal cultures. ASAP1 followed a spontaneous AP train in a cultured hippocampal neuron (ΔF/F = −6.2 ± 0.5% mean ± SE, n = 10 APs). ASAP1 follows the membrane potential changes with high fidelity. Modified from (22).

Figure 4

The Effect of monomeric mutations at the dimerization site of a single FP mosaic GEVI. (A) The GEVI triple mutant with 206A-221L-223F that favors dimerization has the largest signal (orange). Individual mutations favoring the monomer at the three sites reduced the signal (green and purple); double mutations cause a further reduction (red and black). (B) Given here is a schematic model of a triple mutant dimer. Modified from (54).