L-Dopa inhibits prolactin secretion in proestrous rats

Female rats received an ip injection of L-dopa on the afternoon of proestrus. L-Dopa reduced serum prolactin concentrations within 1 h, whether administered just prior to, or during, the normal surge in serum hormone level. This inhibition lasted for 2-3 h, after which serum prolactin concentrations rose substantially. Pretreatment of proestrous rats with MK-486, a peripheral inhibitor of aromatic-L-amino acid decarboxylase, did not block the effect of L-dopa on serum prolactin levels. In fact, MK-486 pretreatment appeared to prolong the effectiveness of L-dopa. Pretreatment with RO4-4602 at a dose sufficient to block central decarboxylase activity, however, did prevent dopa from inhibiting the proestrous surge in serum prolactin. These data are consistent with a role for dopamine in the control of prolactin secretion and suggest that the mechanism of action of L-dopa apparently does not require peripheral decarboxylation.