Abstract

Estimation of dopamine (DA) synthesis in the rat striatum was performed by measuring, as a function of time, both the ³H-H₂O which was formed as a result of L-3,5-³H-tyrosine . hydroxylation to ³H-dopa, and ³H-DA accumulation. Striatal slices were incubated for various time periods up to 30 minutes with L-3,5-³H-tyrosine and the "total" ³H-H₂O and ³H-DA accumulated in slices and incubating medium were measured. The rate of ³H-H₂O formation paralleled the rate of ³H-DA accumulation in normal rats indicating that ³H-DA newly synthesized is protected from enzymatic inactivation. α-Methyl-p-tyrosine inhibited similarly ³H-H₂O and ³H-DA accumulation. Reserpine pretreatment inhibited storage as well as synthesis of DA; ³H-DA and ³H-H₂O accumulation were diminished by 70% and 35%, respectively. In striatal slices of rats pretreated with a neuroleptic, thioproperazin, ³H-H₂O and ³H-DA accumulation were both increased by 70%. On the other hand, amphetamine administered in vivo or in vitro decreased both ³H-H₂O and ³H-DA (40%). Initial rates of DA synthesis were estimated for increasing concentrations of l-3,5-³H tyrosine in the incubating medium. K_m and V_max for DA synthesis in striatal slices of normal and amphetamine-pretreated rats were determined by graphic analysis with the method of Lineweaver and Burk. This kinetic analysis revealed that amphetamine inhibited DA synthesis by noncompetitive inhibition for tyrosine. This study demonstrated that changes in biosynthetic regulation of DA in the striatum induced by psychotropic drugs can be estimated accurately and distinguished from effects on other DA metabolic processes.