ABSTRACT
A novel compound for use in magnetic resonance (MR) imaging was created by covalently bonding multiple 19F atom tags to L-DOPA. Tagging L-DOPA permits bypassing the rate-limiting factor in the biosynthesis of dopamine (DA), the conversion of tyrosine into L-DOPA. The next step in the biosynthetic pathway, the removal of the carboxyl group on the molecule by the enzyme L-aromatic acid decarboxylase (AADC), happens rapidly after L-DOPA is taken up into neurons. In order to be useful as a tool in MR imaging, the novel compound and/ or its perfluoro-tagged metabolites must accumulate in vesicles in dopaminergic neurons. We administered L-DOPA with a nine 19F atom tag (250 mg/kg) to mice pups, waited 1.5 or 3 hr, and used high pressure liquid chromatography (HPLC) to examine neural tissue samples for tagged L-DOPA and tagged DA. The isomer of L-DOPA with the tag bonded at the 5 position yielded the highest conversion to tagged DA at 1.5 hr after an i.p. injection. This study provides the first direct evidence that L-DOPA, tagged with nine fluorine atoms, is taken up into mammalian brain dopaminergic neurons where it is converted to perfluro-tagged DA. The use of these tagged compounds may make it feasible to investigate the uptake and conversion of important neurotransmitter in vivo with fluorine imaging.
Subject(s)
Brain/metabolism , Dihydroxyphenylalanine/pharmacokinetics , Animals , Animals, Newborn , Biological Availability , Brain Chemistry , Chromatography, High Pressure Liquid , Dihydroxyphenylalanine/analogs & derivatives , Dihydroxyphenylalanine/metabolism , Dopamine/analogs & derivatives , Dopamine/metabolism , Fluorine , Magnetic Resonance Imaging/methods , MiceABSTRACT
A new tool for magnetic resonance, L-6-heptafluorobutyryl-5-hydroxytryptophan, was synthesized and investigated using an antibody to perfluoroalkyl moieties developed previously. To be useful as an imaging agent, the compound must cross the blood brain barrier and then be concentrated in vesicles in serotonergic neurons in order to accumulate in sufficient quantity for in vivo detection to be possible. The novel imaging compound was administered in ova to domestic chicks (Gallus domestics) to investigate the bioavailability and uptake dynamics of the compound in this model organism. Typical immunoassay methods were ineffective, so a new technique was developed which binds amines and amino acids to the walls of acid-functionalized cuvettes. The first study established the presence of higher quantities of the tags in neural and liver tissue than in heart tissue. A second study investigated regional differences, with the midbrain containing more tagged compounds than the frontal lobe sample, and the frontal lobe sample containing more than the occipital or cerebellum samples. These studies demonstrate that the compound follows the pathway of endogenous serotonin. A third study investigated uptake dynamics of the novel compound. Maximum concentration of the tagged molecule in the brain was achieved three days after injecting Incubation Day 14 eggs, suggesting that it bioaccumulates in vivo. This new immunoassay technique used to detect the novel compound in tissue samples demonstrated good repeatability.
