On the Significance of Tyrosine for the Synthesis and Catabolism of Dopamine in Rat Brain: Evaluation by HPLC with Electrochemical Detection

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Abstract
A chemical assay of tyrosine (Tyr) in nervous tissue is described. The method is based on a rapidly performed isolation of Tyr on small Sephadex G 10 columns, followed by reverse-phase HPLC in conjunction with amperometric detection. The method permitted the additional quantification of 3,4-dihydroxyphenylalanine, dopamine (DA), and its acidic metabolites. The method was applied to a study of the effects of γ-butyrolactone, haloperidol, haloperidol in combination with amfonelic acid, morphine, NSD 1015, and tyrosine methylester on the concentration of Tyr in the striatum, frontal cortex, hypothalamus, and cerebellum of rat brain. The effect of tyrosine methylester on DA and its acidic metabolites was investigated in the striatum and frontal cortex. Morphine and NSD 1015 were found to increase Tyr levels. γ-Butyrolactone, haloperidol, and haloperidol combined with amfonelic acid decreased the Tyr content in a manner related to their stimulatory effect on DA biosynthesis. These effects were restricted to DA-rich brain areas. It was concluded that during conditions of increased DA biosynthesis, the Tyr pool still possesses a considerable reserve capacity. The results bring into question the concept that brain Tyr is an important additional factor controlling catechol synthesis during increased tyrosine hydroxylase activity.