p-Tyramine, a natural amine, inhibits prolactin release in vivo

p-Tyramine, an endogenous amine with sympathomimetic action, is found in the mammalian hypothalamus. When injected ip, p-tyramine reduced serum PRL without altering LH and TSH serum titers in adult orchidectomized rats and rats subjected to ether or immobilization stress. The hyperprolactinemia achieved by this last procedure was inhibited by both tyramine and dopamine; tyramine produced the same effect as dopamine at a dose 5 times greater. When PRL levels were increased by pretreatment with alpha-methyl-p-tyrosine or haloperidol, once again both tyramine and dopamine lowered PRL titers. The hypoprolactinemic effect of p-tyramine was also observed in median eminence-lesioned animals, suggesting a pituitary site of action. These results show that low amounts of tyramine, a naturally occurring amine, can inhibit in vivo increases in PRL levels achieved physiologically or pharmacologically.