Sensory processing is regulated by the coordinated excitation and inhibition of neurons in neuronal circuits. The analysis of neuronal activities has greatly benefited from the recent development of genetically encoded Ca2+ indicators (GECIs). These molecules change their fluorescence intensities or colours in response to changing levels of Ca2+ and can, therefore, be used to sensitively monitor intracellular Ca2+ concentration, which enables the detection of neuronal excitation, including action potentials. These GECIs were developed to monitor increases in Ca2+ concentration; therefore, neuronal inhibition cannot be sensitively detected by these GECIs. To overcome this difficulty, we hypothesised that an inverse-type of GECI, whose fluorescence intensity increases as Ca2+ levels decrease, could sensitively monitor reducing intracellular Ca2+ concentrations. We, therefore, developed a Ca2+ indicator named inverse-pericam 2.0 (IP2.0) whose fluorescent intensity decreases 25-fold upon Ca2+ binding in vitro. Using IP2.0, we successfully detected putative neuronal inhibition by monitoring the decrease in intracellular Ca2+ concentration in AWCON and ASEL neurons in Caenorhabditis elegans. Therefore, IP2.0 is a useful tool for studying neuronal inhibition and for the detailed analysis of neuronal activities in vivo.
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