All-optical mapping of Ca ²⁺ transport and homeostasis in dendrites

Calcium mediates many important signals in dendrites. However, the basic transport properties of calcium in dendrites have been difficult to measure: how far and how fast does a local influx of calcium propagate? We developed an all-optical system for simultaneous targeted Ca ²⁺ import and concentration mapping. We co-expressed a blue light-activated calcium selective channelrhodopsin, CapChR2, with a far-red calcium sensor, FR-GECO1c, in cultured rat hippocampal neurons, and used patterned optogenetic stimulation to introduce calcium into cells with user-defined patterns of space and time. We determined a mean steady-state length constant for Ca ²⁺ transport ϕ ∼ 5.8 μm, a half-life for return to baseline t _1/2 ∼ 1.7 s, and an effective diffusion coefficient D ∼ 20 μm ² /s, though there were substantial differences in Ca ²⁺ dynamics between proximal and distal dendrites. At high Ca ²⁺ concentration, distal dendrites showed nonlinear activation of Ca ²⁺ efflux, which we pharmacologically ascribed to the NCX1 antiporter. Genetically encoded tools for all-optical study of Ca ²⁺ transport and handling provide a powerful capability for studying this important messenger.