Dihydroxyphenylacetic acid conjugate: natural occurrence and demonstration of probenecid-induced accumulation in rat striatum, olfactory tubercles and frontal cortex.

Methods for the synthesis of 14C-dihydroxyphenylacetic acid (DOPAC) conjugate and for the fluorometric determination of both free and conjugated DOPAC in the same tissue sample are described. Both free and conjugated DOPAC were demonstrated to occur endogenously in the rat corpus striatum, olfactoy tubercles and frontal cortical area, and the ratio of conjugated DOPAC to free DOPAC was 2-3 times greater in the olfactory tubercles and frontal cortical area than in the striatum. Probenecid administration (200 mg/kg, i.p., 4 and 2h before sacrificing) significantly increased the levels of DOPAC conjugate in all 3 brain areas studied. The levels of free DOPAC were also increased in the olfactory tubercles and frontal cortex by the probenecid treatment, but this increase was much less than that seen for DOPAC conjugate in these regions. Free DOPAC levels in the striatum were unaffected by the probenecid treatment. In all 3 brain areas studied, therefore, probenecid treatment resulted in a significant accumulation of conjugated DOPAC relative to free DOPAC. The magnitude of this effect varied, and was most marked in the frontal cortex. These results suggest that, in order for DOPAC to be transported from the central nervous system via a probenecid-sensitive transport system, it must first be conjugated. Additionally, it appears that the rates of synthesis, metabolism, and transport for both free and conjugated DOPAC may vary greatly among different dopamine-containing brain regions.