Inverse-response Ca 2+ indicators for optogenetic visualization of neuronal inhibition

We have developed a series of yellow genetically encoded Ca2+ indicators for optical imaging (Y-GECOs) with inverted responses to Ca2+ and apparent dissociation constants (Kd′) ranging from 25 to 2400 nM. To demonstrate the utility of this affinity series of Ca2+ indicators, we expressed the four highest affinity variants (Kd′s = 25, 63, 121, and 190 nM) in the Drosophila medulla intrinsic neuron Mi1. Hyperpolarization of Mi1 by optogenetic stimulation of the laminar monopolar neuron L1 produced a decrease in intracellular Ca2+ in layers 8–10, and a corresponding increase in Y-GECO fluorescence. These experiments revealed that lower Kd′ was associated with greater increases in fluorescence, but longer delays to reach the maximum signal change due to slower off-rate kinetics.