Conjugated dopamine in superfusates of slices of rat striatum

An acid-hydrolyzable conjugate of 3,4-dihydroxyphenylethylamine (dopamine, DA) was detected in superfusates from slices from rat striatum. The concentrations of endogenous free and conjugated DA, and of the acid metabolites (3,4-dihydroxyphenylacetic acid [DOPAC] and homovanillic acid [HVA]) in superfusates were measured using HPLC with electrochemical detection. Conjugated DA in superfusates represented 10-20% of the free DA under basal conditions and during release evoked by p-tyramine (5 X 10(-6) M to 5 X 10(-4) M); much smaller amounts of conjugated DA overflowed into superfusate when DA was released by equimolar concentrations of beta-phenylethylamine. Surprisingly, inhibition of monoamine oxidase by the inhibitors N-methyl-N-propargyl-3-(2,4-dichlorophenoxy)propylamine hydrochloride (clorgyline) or N-methyl-N-2-propynylbenylamine (pargyline) had little effect on the amounts of conjugated DA present in superfusate. Under basal conditions, the amounts of conjugated DA in superfusate were always less than the amounts of DOPAC but quite similar to the amounts of HVA. However, during release of DA evoked by p-tyramine the concentrations of conjugated DA in superfusate showed much more pronounced increases than those of the acidic metabolites.