Dopamine > 3-methoxytyramine > HVA
FIGURE 1 Peripheral and central metabolism of levodopa. Abbreviations: COMT, catechol-O-methyltransferase; 3-OMD, 3-O-methyldopa; AAAD, aromatic amino acid decarboxylase; MAO, monoamine oxidase; DOPAC, 3,4-dioxyphenylacetic acid; HVA, homovanillic acid.
diminished, indicating a similar effect at the BBB as well (22). Upon entering the CNS, levodopa is taken up by dopaminergic neurons and converted to dopamine. Glial cells have the same potential as they contain dopa decarboxylase (23). The dopamine generated in the neuron is stored and released in a more physiological manner in early PD, but as the disease progresses and neurons die, glial cells take over the metabolic activity and release occurs in large bursts paralleling the peaks in plasma levels in a pattern that is nonphysiologic.
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