5/ Huge thanks to Paul Soden for intrepid and creative problem solving, @crystianmassengill.bsky.social/Oren Amsalem/Marta Porniece for major help along the way, Yulong Li’s lab for sharing several new GRAB sensors, and Maria Lehtinen & @markandermann.bsky.social for generous support and guidance!

4/ Using calcium and GRAB sensor imaging in the choroid plexus epithelium, we demonstrated that CSF serotonin likely impacts choroid plexus secretion. MORSE can thus capture dynamic composition of brain fluids with sufficient sensitivity to detect biologically relevant signals.

3/ In awake mouse cerebrospinal fluid, we tracked simultaneous dynamics of multiple molecules delivered into the blood or directly into the brain. We also estimated quantitative CSF concentrations of the molecules in real time and identified surprising spontaneous serotonin waves.

2/ In acute brain slices, we used the probes to quantify serotonin and norepinephrine release in response to depolarization. We were also able to unmix the catecholamine crosstalk at the norepinephrine and dopamine GRAB sensors by recording both sensors at the same time.

1/ We expressed 16 GRAB sensors one by one in cultured cells and embedded all cells in a tiny 40 nanoliter hydrogel glued to the front of a GRIN lens. We then calibrated the probe by robotic dipping into fluids containing each sensor cell’s ligand while imaging through the lens.