Some factors affecting the concentrations of para-hydroxyphenylacetic acid and meta-hydroxyphenylacetic acid in the mouse caudate nucleus.

Acid hydrolysis of mouse caudate homogenates results in a significantly increased concentration of para-hydroxyphenylacetic acid (p-HPAA). Thus p-HPAA appears to be significantly present as an acid-labile conjugate in the mouse caudate nucleus. Saline injection also increased the concentration of the acid metabolite of the tyramines. The subcutaneous injection of either meta-hydroxyphenylacetic acid (m-HPAA) or p-HPAA or both produced at 0.5 hr and 2.0 hr large increases in the mouse caudate nucleus concentrations of m- and/or p-HPAA. Similarly at 2.0 hr the injection of a combination of para- and meta-tyramine caused large increases in the acid metabolite concentrations. Probenecid, at a dose of 500 mg/kg, increased striatal p-HPAA and m-HPAA concentrations. Probenecid, injection prior to the administration of p-HPAA and m-HPAA, did not reduce the increased concentrations of these two acid metabolites previously observed after the administration of the acid metabolites alone. It thus appears that p-HPAA and m-HPAA are transported from the brain by a probenecid-sensitive mechanism, but in the presence of high blood concentrations, p-HPAA and m-HPAA may enter the brain by a mechanism that is not affected by probenecid.