All-optical mapping of Ca 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 import and Ca 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 transport ϕ ∼ 5.8 μm, a half-life for return to baseline t ∼ 1.7 s, and an effective diffusion coefficient D ∼ 20 μm/s, though there were substantial differences in Ca dynamics between proximal and distal dendrites. At high Ca concentration, distal dendrites showed nonlinear activation of Ca efflux, which we pharmacologically ascribed to the NCX1 antiporter. Genetically encoded tools for all-optical mapping of Ca transport and handling provide a powerful capability for studying this important messenger.