natal period of rat ontogeny when rats have no blood–brain barrier. We developed a specific pharmacological
model to inhibit DA synthesis in the brain and maintain its constant level on the periphery using
methyl-p-tyrosine (
MPT), an inhibitor of the key enzyme of DA synthesis tyrosine hydroxylase. On the basis of prelim-inary systemic administration of
MPT (200, 100, 80, and 50 
g), we selected a dose of the inhibitor of 50 g, which excluded its effects on DA metabolism in peripheral organs. In subsequent experiments,
MPT was ste-reotaxically administered into the lateral brain ventricles of three-day-old rats at the selected dose. After this,
we measured the concentration of catecholamines and metabolites using high performance liquid chromatog-
raphy with electrochemical detection in the brain, Zuckerkandl’s organ, kidneys, adrenals, and plasma. We
found that in 4 h after administration of the inhibitor, the DA concentration decreased in the brain by 54% and
in the plasma by 74%, whereas in the peripheral organs it remained unchanged. Thus, we directly showed that
DA is secreted from the brain in the general blood circulation before the formation of the BBB.