All-optical mapping of Ca²⁺ 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 Ca²⁺ import and Ca²⁺ 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 Ca²⁺ 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 μm²/s, though there were substantial differences in Ca²⁺ dynamics between proximal and distal dendrites. At high Ca²⁺ concentration, distal dendrites showed nonlinear activation of Ca²⁺ efflux, which we pharmacologically ascribed to the NCX1 antiporter. Genetically encoded tools for all-optical mapping of Ca²⁺ transport and handling provide a powerful capability for studying this important messenger.