Intravenous l-DOPA plus carbidopa in depressed patients: average evoked response, learning, and behavioral changes.
The peripheral decarboxylase inhibitor carbidopa (L-alphamethyl-dopa-hydrazine) allowed safe intravenous administration of L-DOPA in amounts sufficient to alter cortical average evoked response (AER) and learning function in 13 depressed patients. The apparently rapid conversion of L-DOPA to dopamine, as reported from studies in animals, is consistent with the 20-30 min onset of effects seen in our study. Unipolar and bipolar depressed patients responded differently to the alterations in brain biogenic amines and also to the nonspecific stress of the experiment. Intravenous L-DOPA given acutely had effects on the AER that were similar to those documented with oral dopa given chronically--an augmentation of amplitude--intensity slopes in unipolar patients and a relative reduction of slopes in bipolar patients. In contrast, intravenous L-DOPA did not enhance verbal learning as did chronic oral treatment, but rather was associated with reduced learning compared with placebo infusions. Different neurochemical changes following L-DOPA given in single intravenous doses may be responsible for the different learning and behavioral changes form those found previously with oral dopa administered chronically.