Pharmacological difference of L-dopa,apomorphine, and bromocriptine against metoclopramide

Rat prolactin was stimulated by metoclopramide and inhibited by L-dopa, apomorphine and bromocriptine. Depending on the order of administration, the efficiency of bromocriptine deviated from L-dopa and apomorphine in opposite directions. When injected into rats 15 min after metoclopramide, bromocriptine was much less effective than L-dopa and apomorphine in blocking the effect of metoclopramide on serum prolactin. When injected into rats 15 min before metoclopramide, bromocriptine was the most effective antagonist of metoclopramide action. A plausible explanation of these observations is that bromocriptine and metoclopramide are not strictly dopamine agonists and antagonists, while L-dopa and apomorphine act exclusively by a dopamine mechanism.