Development of pH-responsive fluorescent false neurotransmitters.

We introduce pH-responsive fluorescent false neurotransmitters (pH-responsive FFNs) as novel probes that act as vesicular monoamine transporter (VMAT) substrates and ratiometric fluorescent pH sensors. The development of these agents was achieved by systematic molecular design that integrated several structural elements, including the aminoethyl group (VMAT recognition), halogenated hydroxy-coumarin core (ratiometric optical pH sensing in the desired pH range), and N- or C-alkylation (modulation of lipophilicity). Of 14 compounds that were synthesized, the probe Mini202 was selected based on the highest uptake in VMAT2-transfected HEK cells and desirable optical properties. Using Mini202, we measured the pH of catecholamine secretory vesicles in PC-12 cells (pH approximately 5.9) via two-photon fluorescence microscopy. Incubation with methamphetamine led to an increase in vesicular pH (pH approximately 6.4), consistent with a proposed mechanism of action of this psychostimulant, and eventually to redistribution of vesicular content (including Mini202) from vesicles to cytoplasm. Mini202 is sufficiently bright, photostable, and suitable for two-photon microscopy. This probe will enable fundamental neuroscience and neuroendocrine research as well as drug screening efforts.