3,4-Dihydroxyphenylacetic acid and homovanillic acid in rat plasma: possible indicators of central dopaminergic activity.

The concentrations of dopamine (DA) metabolites (free and conjugated) was measured in plasma and brain regions of rats by the mass spectrometric method of selected ion monitoring. Experimental treatments which altered the function of central dopamine neurons also induced concomitant changes in plasma 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Stimulation of the nigrostriatal pathway increased plasma DOPAC and HVA whereas lesion of the pathway decreased plasma metabolites. Several drug treatments induced parallel changes in brain and plasma concentrations of DA metabolites. It is suggested that changes in the concentration of DOPAC and HVA in rat brain are reflected by parallel changes in plasma. No conjugated forms of DOPAC and HVA were found in plasma and brain tissue of vervet monkeys (Cercopithecus aethiops).