By the time a patient with Parkinson’s disease presents symptoms, about half of the dopaminergic neurons in the substantia nigra have died. In addition to regulating motor activity, dopamine is also involved in learning, reward, and addiction. Therefore, the ability to monitor dopamine storage and release could provide not only a diagnostic tool for Parkinson’s disease, but also inform the basic research of this and other neurological and psychiatric disorders. We currently use fluorescent false neurotransmitters (FFNs), substrates for dopamine active transporter (DAT) and vesicular monoamine transporter (VMAT2), in mice to monitor neurotransmitter storage and release in slice and in vivo. We propose extending the FFN strategy into MRI/S, by relying on fluorine’s absence in the brain, as well as its wide range of chemical shifts. By designing false neurotransmitters with fluorine atoms vicinal to a pH sensitive moiety, we can differentiate FN stored in acidic synaptic vesicles from that in the cytoplasm. FFN103 and FFN270, respectively fluorescent markers for dopaminergic and noradrenergic transmission, serve as a starting point. Both compounds consist of a coumarin scaffold with an ethylamine group to facilitate active transport into the cell and loading into vesicles, and a fluorine positioned near a hydroxy group with a pKa lower than physiological pH. Another approach focuses on 19F-dopamine. Once in an acidic environment, the hydroxy group is protonated, causing a change in the nearby fluorine atom’s chemical shift. First, these compounds are analyzed by NMR in solution over a wide pH range. In-cell 19F-NMR using VMAT2-DAT HEK293 cells should yield two distinct peaks corresponding to both cytoplasmic and vesicular populations of FN. Comparing these peaks to the previously determined pH-δ curve can quantify the pH of the vesicle, and will also inform future micro MRS imaging in mice.

Tracking dopamine with 19F-MRI/S using pH-sensitive false neurotransmitters

256th National Meeting (2018)

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