L-Dopa Inhibits Prolactin Secretion in Proestrous Rats1

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Abstract
Female rats received an i.p. 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 [.alpha.-methyl-L-dopa-hydrazine], a peripheral inhibitor of aromatic-L-amino acid decarboxylase, did not block the effect of L-dopa on serum prolactin levels. MK-486 pretreatment appeared to prolong the effectiveness of L-dopa. Pretreatment with R04-4602 [N-(DL-seryl)-N''-(2,3,4-trihydroxy-benzyl) hydrazine at a dose sufficient to block central decarboxylase activity did prevent dopa from inhibiting the proestrous surge in serum prolactin. A role for dopamine in the control of prolactin secretion is supported. The mechanism of action of L-dopa apparently does not require peripheral decarboxylation.

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