Monitoring Brain Activity with Protein Voltage and Calcium Sensors

Abstract

Understanding the roles of different cell types in the behaviors generated by neural circuits requires protein indicators that report neural activity with high spatio-temporal resolution. Genetically encoded fluorescent protein (FP) voltage sensors, which optically report the electrical activity in distinct cell populations, are, in principle, ideal candidates. Here we demonstrate that the FP voltage sensor ArcLight reports odor-evoked electrical activity in the in vivo mammalian olfactory bulb in single trials using both wide-field and 2-photon imaging. ArcLight resolved fast odorant-responses in individual glomeruli, and distributed odorant responses across a population of glomeruli. Comparisons between ArcLight and the protein calcium sensors GCaMP3 and GCaMP6f revealed that ArcLight had faster temporal kinetics that more clearly distinguished activity elicited by individual odorant inspirations. In contrast, the signals from both GCaMPs were a saturating integral of activity that returned relatively slowly to the baseline. ArcLight enables optical electrophysiology of mammalian neuronal population activity in vivo.

Genetically expressed voltage sensor ArcLight for imaging large scale cortical activity in the anesthetized and awake mouse

“…These observations of ArcLight neuronal membrane expression are highly consistent with recent work in the olfactory bulb under similar conditions with the hsyn1 promoter. 18 The histology highlights the ability of ArcLight, under hsyn1 promoter, to genetically target all neural membranes, which offers higher selectivity than traditional VSDs that bind to all cellular membranes (neuronal and glial).…”

“…2, we developed several analytical tools that are described in-depth here. ArcLight has been shown to be very photostable over long periods of excitation in vitro 12,13,18 and therefore is ideal for long imaging experiments. We also found this to be the case here, where ArcLight showed only a small, slow linear decay of fluorescence over time.…”

“…The high-frequency 40-Hz stimulation is at the upper limit of the innate capabilities of ArcLight based on in vitro studies. 2 These in vitro ArcLight experiments demonstrate fast temporal dynamics with a 10-to 20-ms rise time, 12,18 and ∼20 ms decay, which limits the fluorophore's ability to represent high-frequency information greater than 40 Hz. Note that few studies have specifically examined the frequency encoding in mice under isoflurane anesthesia, so the limitations from a coding perspective are presently unclear.…”

Genetically Encoded Voltage Indicators: Opportunities and Challenges

“…For VSD-based indicators, sensitivity can be improved by mutating the fluorescent protein , the VSD Piao et al, 2015;Abdelfattah et al, 2016), or the linkers coupling the VSD to the fluorescent protein St-Pierre et al, 2014;Jung et al, 2015;Sung et al, 2015;Abdelfattah et al, 2016;Yang et al, 2016). Replacing the VSD or the fluorescent proteins with homologs can also affect sensitivity Han et al, 2013;St-Pierre et al, 2014).…”

“…Some fast GEVIs were developed from previous variants by replacing their VSDs or opsin domains with orthologs Han et al, 2013; or by replacing a section of the VSD with a homologous fragment from a fast voltage-gated potassium channel (Mishina et al, 2012(Mishina et al, , 2014. Faster kinetics were also achieved by rational (Gong et al, 2013;Piao et al, 2015;Treger et al, 2015) or random (Abdelfattah et al, 2016) mutagenesis and by optimizing coupling between the VSD and fluorescent protein(s) (Jung et al, 2015;Sung et al, 2015).…”