IN-VIVO AND IN-VITRO STUDIES ON THE NEUROTOXIC POTENTIAL OF 6-HYDROXYDOPAMINE ANALOGS

In an attempt to determine which physical and biological properties co uld best be correlated with neurotoxic potential, seven analogs of 1-( 2,4,5-trihydroxyphenyl)-2-aminoethane (1), better known as B-hydroxydo pamine, were synthesized and compared to 1 in a variety of ways both i n vivo and in vitro. The analogs, in combination with-the standard 1, include all eight of the 2,4,5-trisubstituted-phenyl derivatives of ph enethylamine and alpha-methylphenethylamine in which the substitution is of the trihydroxy or aminodihydroxy form. Low (60 nmol) and high (3 00 nmol) intracerebroventricular doses of all analogs produced long-te rm (7 day) reduction of mouse whole brain norepinephrine (NE) and less er depletions of dopamine (DA), and effects on serotonin were varied. The analog 1-(5-amino-2,4-dihydroxyphenyl)-2-aminopropane (8) was both more complete and more selective than the standard 1 in depleting NE. Using a histofluorometric glyoxylic acid method and Fink-Heimer silve r degeneration stain, it was determined that overt neural degeneration was produced by 8. In vitro, the ease of oxidation of the eight analo gs was found to be represented by a formal potential range of -130 to -212 mV vs SCE. However, there was no obvious relationship between eas e of oxidation and the extent of monoamine depletion from mouse brain. Using kinetic analysis of synaptosomal accumulation of [H-3]NE and [H -3]DA, it was found that the standard 1 is more potent in its interact ion with the DA uptake site (K-i = 12 +/- 0 mu M) than the NE uptake s ite (K-i = 51 +/- 1 mu M). A correlation analysis was used to determin e that differences in NE and DA depletion by each analog could not be explained by differences in potency for in vitro uptake blockade. Howe ver, there was a correlation between the K-i for [H-3]NE uptake blocka de and the EC(50) for synaptosomal release of preloaded [H-3]NE for th e eight analogs (R(2) = 0.96; for log:log plot, R(2) = 0.54), indicati ng that the results for these two in vitro tests both reflect interact ion with the same NE neuronal membrane transport site. A similar corre lation between K-i and EC(50) was shown for all eight analogs using [H -3]DA (R(2) = 0.92; for log:log plot, R(2) = 0.52), indicating interac tion with the same DA neuronal membrane transport site. These findings demonstrate that there is no single property that can account for sel ectivity of action and/or potency of catecholamine neurotoxins related to 6-hydroxydopamine.