Levo-dihydroxyphenylalanine

A little background one of the transmitter heavyweights in the brain is dopamine. Dopamine is called dopamine because it is an amine that comes from an amino acid that is 3,4-dihydroxyphenylalanine and this, in German, is Di-Oxo-Phenyl-Alanine, or DOPA. The levo-optical (or L-) isomer of DOPA has rather cutely been called the punch-drunk Spanish matador, or El Dopa. But that is not part of the story. 

L-Dopa. Dopamine itself cannot penetrate the blood-brain barrier however, its natural precursor, L-dihydroxyphenylalanine (levo-dopa), is effective in replenishing striatal dopamine levels, because it is transported across the blood-brain barrier via an amino acid carrier and is subsequently decarboxy-lated by dopa decarboxylase, present in striatal tissue. Decarboxylation also takes place in peripheral organs where dopamine is not needed and is likely to cause undesirable effects (vomiting hypotension p.116). Extracerebral production of dopamine can be prevented by inhibitors of dopa decarboxylase (carbidopa, benserazide) that do not penetrate the blood-brain barrier, leaving intracerebral decarboxylation unaffected. 

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